FIBER COMMUNICATION SYSTEMS AND METHODS
An injection locked transmitter for an optical communication network includes a master seed laser source input substantially confined to a single longitudinal mode, an input data stream, and a laser injected modulator including at least one slave laser having a resonator frequency that is injection locked to a frequency of the single longitudinal mode of the master seed laser source. The laser injected modulator is configured to receive the master seed laser source input and the input data stream, and output a laser modulated data stream.
This application is a continuation in part of U.S. application Ser. No. 16/460,964, filed on Jul. 2, 2019. U.S. application Ser. No. 16/460,964 is a continuation in part of U.S. application Ser. No. 16/408,285, filed on May 9, 2019. U.S. application Ser. No. 16/408,285 is a continuation in part of U.S. application Ser. No. 15/861,303, filed on Jan. 3, 2018, now U.S. Pat. No. 10,447,404, issued Oct. 15, 2019. U.S. application Ser. No. 15/861,303 is a continuation of U.S. application Ser. No. 15/283,632, filed on Oct. 3, 2016, now U.S. Pat. No. 9,912,409, issued Mar. 6, 2018. U.S. application Ser. No. 15/283,632 further claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/321,211, filed Apr. 12, 2016. U.S. application Ser. No. 16/408,285 further claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/669,035, filed May 9, 2018, to U.S. Provisional Patent Application Ser. No. 62/671,270, filed May 14, 2018, and to U.S. Provisional Patent Application Ser. No. 62/678,110, filed May 30, 2018. U.S. application Ser. No. 16/460,964 further claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/693,035, filed Jul. 2, 2018. The present application further claims the benefit of and priority to U.S. Provisional Patent Application Ser. No. 62/744,498, filed Oct. 11, 2018, and to U.S. Provisional Patent Application Ser. No. 62/785,016, filed Dec. 26, 2018. All of these applications are incorporated herein by reference in their entireties.
BACKGROUNDThe field of the disclosure relates generally to fiber communication networks, and more particularly, to optical networks utilizing wavelength division multiplexing.
Telecommunications networks include an access network through which end user subscribers connect to a service provider. Bandwidth requirements for delivering high-speed data and video services through the access network are rapidly increasing to meet growing consumer demands. At present, data delivery over the access network is growing by gigabits(Gb)/second for residential subscribers, and by multi-Gb/s for business subscribers. Present access networks are based on passive optical network (PON) access technologies, which have become the dominant system architecture to meet the growing high capacity demand from end users.
Gigabit PON and Ethernet PON architectures are conventionally known, and presently provide about 2.5 Gb/s data rates for downstream transmission and 1.25 Gb/s for upstream transmission (half of the downstream rate). 10 Gb/s PON (XG-PON or IEEE 10G-EPON) has begun to be implemented for high-bandwidth applications, and a 40 Gb/s PON scheme, which is based on time and wavelength division multiplexing (TWDM and WDM) has recently been standardized. A growing need therefore exists to develop higher/faster data rates per-subscriber to meet future bandwidth demand, and also increase the coverage for services and applications, but while also minimizing the capital expenditures (CAPEX) and operational expenditures (OPEX) necessary to deliver higher capacity and performance access networks.
One known solution to increase the capacity of a PON is the use of WDM technology to send a dedicated wavelength signal to end users. Current detection scheme WDM technology, however, is limited by its low receiver sensitivity, and also by the few options available to upgrade and scale the technology, particularly with regard to use in conjunction with the lower-quality legacy fiber environment. The legacy fiber environment requires operators to squeeze more capacity out of the existing fiber infrastructure to avoid costs associated with having to retrench new fiber installment. Conventional access networks typically include six fibers per node, servicing as many as 500 end users, such as home subscribers. Conventional nodes cannot be split further and do not typically contain spare (unused) fibers, and thus there is a need to utilize the limited fiber availability in a more efficient and cost-effective manner.
Coherent technology has been proposed as one solution to increase both receiver sensitivity and overall capacity for WDM-PON optical access networks, in both brown and green field deployments. Coherent technology offers superior receiver sensitivity and extended power budget, and high frequency selectivity that provides closely-spaced dense or ultra-dense WDM without the need for narrow band optical filters. Moreover, a multi-dimensional recovered signal experienced by coherent technology provides additional benefits to compensate for linear transmission impairments such as chromatic dispersion (CD) and polarization-mode dispersion (PMD), and to efficiently utilize spectral resources to benefit future network upgrades through the use of multi-level advanced modulation formats. Long distance transmission using coherent technology, however, requires elaborate post-processing, including signal equalizations and carrier recovery, to adjust for impairments experienced along the transmission pathway, thereby presenting significant challenges by significantly increasing system complexity.
Coherent technology in longhaul optical systems typically requires significant use of high quality discrete photonic and electronic components, such as digital-to-analog converters (DAC), analog-to-digital converters (ADC), and digital signal processing (DSP) circuitry such as an application-specific integrated circuit (ASIC) utilizing CMOS technology, to compensate for noise, frequency drift, and other factors affecting the transmitted channel signals over the long distance optical transmission. Coherent pluggable modules for metro solution have gone through C Form-factor pluggable (CFP) to CFP2 and future CFP4 via multi-source agreement (MSA) standardization to reduce their footprint, to lower costs, and also to lower power dissipation. However, these modules still require significant engineering complexity, expense, size, and power to operate, and therefore have not been efficient or practical to implement in access applications.
Furthermore, according to Nielsen's Law, if current trends continue, high-end end users are expected to require as much as 10 Gb/s by 2023, and 100 Gb/s by 2029. For the new and upcoming generations of communication systems performing under these requirements, the data speed will also need to be matched for upstream communications. However, conventional PONs seeking to approach the 100 Gb/s aggregating data rate suffer from several limitations, due to the reliance on traditional direct detection techniques, that render 100 Gb/s technically and economically infeasible for these PONs. The conventional direct detection PONs, for example, are known to have poor receiver sensitivity, to experience power fading due to chromatic dispersion at high symbol rates and long transmission distances, and to utilize bandwidth- and power-inefficient modulation. Besides frequency selectivity and linear detection, Coherent for PONs demonstrates superior receiver sensitivity, which can be translated to extend reach and split ratio.
In the downlink of conventional PONs, the complexity limits on the transceiver in an optical line terminal (OLT) at the headend, central office, and/or hub are less stringent than the limits placed on a receiver in an optical network unit (ONU), since the cost of the OLT transceiver, which sends and receives data to and from multiple ONUs, is shared by all end users supported in the respective network. In contrast, the cost of each ONU is born solely by the respective end user. Accordingly, lower costs and lower complexities will more significantly impact the ONU than the OLT. For this reason, the complexity and high cost of conventional coherent transceivers has been limited to point to point (P2P) applications, but prevented from implementation in point to multipoint (P2MP) PON applications. That is, despite the significant advantages offered by digital coherent technology, the complexity and high cost of conventional coherent transceivers has not been economically feasible for individual ONUs at the home location of each subscriber end-user.
P2P and P2MP applications differ in that they P2P connection provides a link between one transmitter and one receiver, whereas a P2MP application provides a link between one transmitter and multiple receivers. Accordingly, in the coherent paradigm, only two coherent transceivers may be needed in a P2P link, whereas the number of coherent transceivers needed in the P2MP link (i.e., one coherent transceiver for each ONU) may be significant (as many as 500, in the example above).
Therefore, the laser source is of critical importance for the realization of such coherent optical transmission systems. That is, one type of laser may not simply be substituted for another type without significantly affecting the network. Additionally, the frequency and phase noise of the laser will also significantly affect the performance of conventional optical coherent transceivers, and impairments therefrom have to be mitigated by carrier-phase recovery (CPR) techniques, since frequency and phase noise are directly related to each other, and are closely related to the linewidth of the laser.
Furthermore, the modulation speed and transmission distance of the network also will strongly depend on the spectral linewidth of the laser. That is, narrower linewidths are required for higher modulation speeds (data rates) and longer distance transmissions. Some conventional coherent transceivers use an external cavity laser (ECL). From the performance perspective, ECLs have demonstrated superior performance capabilities for coherent systems, sufficient for present long haul and metro distance sensitivity requirements. However, within the access environment, ECLs are considered prohibitively expensive if deployed at each ONU at an end user's home location. In contrast, Fabry-Perot laser diodes (FP-LD) and weak-resonant-cavity laser diode (WRC-FPLD) based transmitters are considerably less expensive than the costly externally tunable lasers such as ECLs or distributed feedback (DFB)/distributed Bragg reflector (DBR) lasers. However, use of these relatively lower-cost, simpler FP lasers is limited by transmission bandwidth and capacity, particularly in direct-detection systems, and is not applicable for coherent systems in the conventional use form.
BRIEF SUMMARYIn one aspect, an injection locked transmitter for an optical communication network includes a master seed laser source input substantially confined to a single longitudinal mode, an input data stream, and a laser injected modulator including at least one slave laser having a resonator frequency that is injection locked to a frequency of the single longitudinal mode of the master seed laser source. The laser injected modulator is configured to receive the master seed laser source input and the input data stream, and output a laser modulated data stream.
In another aspect, an optical network communication system includes, an input signal source, an optical frequency comb generator configured to receive the input signal source and output a plurality of phase synchronized coherent tone pairs. Each of the plurality of phase synchronized coherent tone pairs includes a first unmodulated signal and a second unmodulated signal. The system further include a first transmitter configured to receive the first unmodulated signal of a selected one of the plurality of phase synchronized coherent tone pairs as a seed source and to output a first modulated data stream, and a first receiver configured to receive the first modulated data stream from the first transmitter and receive the second unmodulated signal of the selected one of the plurality of phase synchronized coherent tone pairs as a local oscillator source.
In yet another aspect, an optical network communication system includes an optical hub including an optical frequency comb generator configured to output at least one phase synchronized coherent tone pair having a first unmodulated signal and a second unmodulated signal, and a downstream transmitter configured to receive the first unmodulated signal as a seed source and to output a downstream modulated data stream. The system further includes a fiber node and an end user including a downstream receiver configured to receive the downstream modulated data stream from the downstream transmitter and receive the second unmodulated signal as a local oscillator source.
In a still further aspect, a method of optical network processing includes steps of generating at least one pair of first and second unmodulated phase synchronized coherent tones, transmitting the first unmodulated phase synchronized coherent tone to a first transmitter as a seed signal, adhering downstream data, in the first transmitter, to the first unmodulated phase synchronized coherent tone to generate a first modulated data stream signal, optically multiplexing the first modulated data stream signal and the second unmodulated phase synchronized coherent tone together within a hub optical multiplexer, and communicating the multiplexed first modulated data stream signal and the second unmodulated phase synchronized coherent tone to a first receiver, by way of fiber optics, for downstream heterodyne detection.
In an aspect, an optical communication network includes an optical hub. The optical hub includes at least one master laser source and at least one hub transceiver configured to transmit a downstream signal of the at least one master laser source. The downstream signal includes a plurality of spaced wavelength channels. The network further includes an optical transport medium configured to carry the downstream signal from the optical hub, and a plurality of distributed modem devices operably coupled to the optical transport medium. The modem is (i) configured to receive at least one channel of the plurality of spaced wavelength channels, and (ii) including at least one child laser source injection locked to the master laser source.
In an embodiment, an optical injection locking based coherent optical transmitter is provided for a coherent optical communications network. The coherent optical transmitter includes a master laser source configured to provide a low linewidth frequency channel as a master laser signal, and a coherent optical injection locking (COIL) subsystem. The COIL subsystem includes (i) a first slave laser configured for COIL with a master frequency of the master laser signal, (ii) an optical circulator configured to inject the master laser signal into a cavity of the first slave laser, and (iii) a full-field modulator configured to output a first modulated optical signal based on an output of the first slave laser routed through the optical circulator.
In an embodiment, an optical communication network includes an optical hub, a receiver, and an optical transport medium. The optical hub includes a master laser source and a plurality of full-field coherent optical transmitters disposed proximate the optical hub. Each full-field transmitter of the plurality of full-field coherent optical transmitters (i) is configured to transmit a downstream coherent optical signal using a center frequency of a master signal from the master laser source, and (ii) includes a first slave laser injection locked to the center frequency of the master signal. The optical transport medium operably connects the plurality of full-field coherent optical transmitters to the receiver.
These and other features, aspects, and advantages of the present disclosure will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
Unless otherwise indicated, the drawings provided herein are meant to illustrate features of embodiments of this disclosure. These features are believed to be applicable in a wide variety of systems including one or more embodiments of this disclosure. As such, the drawings are not meant to include all conventional features known by those of ordinary skill in the art to be required for the practice of the embodiments disclosed herein.
DETAILED DESCRIPTIONIn the following specification and the claims, reference will be made to a number of terms, which shall be defined to have the following meanings.
The singular forms “a,” “an,” and “the” include plural references unless the context clearly dictates otherwise.
“Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” “approximately,” and “substantially,” are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged; such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise.
As used herein, the terms “processor” and “computer” and related terms, e.g., “processing device”, “computing device”, and “controller” are not limited to just those integrated circuits referred to in the art as a computer, but broadly refers to a microcontroller, a microcomputer, a programmable logic controller (PLC), an application specific integrated circuit (ASIC), and other programmable circuits, and these terms are used interchangeably herein. In the embodiments described herein, memory may include, but is not limited to, a computer-readable medium, such as a random access memory (RAM), and a computer-readable nonvolatile medium, such as flash memory. Alternatively, a floppy disk, a compact disc-read only memory (CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc (DVD) may also be used. Also, in the embodiments described herein, additional input channels may be, but are not limited to, computer peripherals associated with an operator interface such as a mouse and a keyboard. Alternatively, other computer peripherals may also be used that may include, for example, but not be limited to, a scanner. Furthermore, in the exemplary embodiment, additional output channels may include, but not be limited to, an operator interface monitor.
Further, as used herein, the terms “software” and “firmware” are interchangeable, and include computer program storage in memory for execution by personal computers, workstations, clients, and servers.
As used herein, the term “non-transitory computer-readable media” is intended to be representative of any tangible computer-based device implemented in any method or technology for short-term and long-term storage of information, such as, computer-readable instructions, data structures, program modules and sub-modules, or other data in any device. Therefore, the methods described herein may be encoded as executable instructions embodied in a tangible, non-transitory, computer readable medium, including, without limitation, a storage device and a memory device. Such instructions, when executed by a processor, cause the processor to perform at least a portion of the methods described herein. Moreover, as used herein, the term “non-transitory computer-readable media” includes all tangible, computer-readable media, including, without limitation, non-transitory computer storage devices, including, without limitation, volatile and nonvolatile media, and removable and non-removable media such as a firmware, physical and virtual storage, CD-ROMs, DVDs, and any other digital source such as a network or the Internet, as well as yet to be developed digital means, with the sole exception being a transitory, propagating signal.
Furthermore, as used herein, the term “real-time” refers to at least one of the time of occurrence of the associated events, the time of measurement and collection of predetermined data, the time for a computing device (e.g., a processor) to process the data, and the time of a system response to the events and the environment. In the embodiments described herein, these activities and events occur substantially instantaneously.
As used herein, “modem termination system” (MTS) refers to a termination unit including one or more of an Optical Network Terminal (ONT), an optical line termination (OLT), a network termination unit, a satellite termination unit, a cable modem termination system (CMTS), and/or other termination systems which may be individually or collectively referred to as an MTS.
As used herein, “modem” refers to a modem device, including one or more a cable modem (CM), a satellite modem, an optical network unit (ONU), a DSL unit, etc., which may be individually or collectively referred to as modems.
As described herein, a “PON” generally refers to a passive optical network or system having components labeled according to known naming conventions of similar elements that are used in conventional PON systems. For example, an OLT may be implemented at an aggregation point, such as a headend/hub, and multiple ONUs may be disposed and operable at a plurality of end user, customer premises, or subscriber locations. Accordingly, an “uplink transmission” refers to an upstream transmission from an end user to a headend/hub, and a “downlink transmission” refers to a downstream transmission from a headend/hub to the end user, which may be presumed to be generally broadcasting continuously (unless in a power saving mode, or the like).
Optical hub 102 communicates with fiber node 104 by way of downstream fiber 108. Optionally, where upstream communication is desired along system 100, optical hub 102 further connects with fiber node 104 by way of upstream fiber 110. In operation, downstream fiber 108 and upstream fiber 110 are typically 30 km or shorter. However, according to the embodiments presented herein, greater lengths are contemplated, such as between 100 km and 1000 km. In an exemplary embodiment, fiber node 104 connects with end user 106 by way of fiber optics 112. Alternatively, fiber node 104 and end user 106 may be integrated as a single device, such as a virtualized cable modem termination system (vCMTS), which may be located at a customer premises. Where fiber node 104 and end user 106 are separate devices, fiber optics 112 typically spans a distance of approximately 5000 feet or less.
Optical hub 102 includes an optical frequency comb generator 114, which is configured to receive a high quality source signal 116 from an external laser 118 and thereby generate multiple coherent tones 120(1), 120(1′), . . . 120(N), 120(N′). Optical frequency comb generator 114 utilizes, for example, a mode-locked laser, a gain-switched laser, or electro-optic modulation, and is constructed such that multiple coherent tones 120 are generated as simultaneous low-linewidth wavelength channels of known and controllable spacing. This advantageous aspect of the upstream input signal into system 100 allows a simplified architecture throughout the entire downstream portion of system 100, as described further below.
Generated coherent tones 120 are fed into an amplifier 122, and the amplified signal therefrom is input into a first hub optical demultiplexer 124. In an exemplary embodiment, amplifier 122 is an erbium-doped fiber amplifier (EDFA). Optical hub 102 further includes a downstream transmitter 126 and a hub optical multiplexer 128. In an embodiment, optical hub 102 optionally includes a hub optical splitter 130, an upstream receiver 132, and a second hub optical demultiplexer 134.
Downstream transmitter 126 includes a downstream optical circulator 136 and a downstream modulator 138. In an exemplary embodiment, downstream modulator 138 is an injection locked laser modulator. Upstream receiver 132 includes an upstream integrated coherent receiver (ICR) 140, an upstream analog to digital converter (ADC) 142, and an upstream digital signal processor (DSP) 144. In the exemplary embodiment, fiber node 104 includes a node optical demultiplexer 146. In an alternative embodiment, where upstream transmission is desired, fiber node 104 further includes a node optical multiplexer 148. In the exemplary embodiment, node optical demultiplexer 146 and node optical multiplexer 148 are passive devices.
End user 106 further includes a downstream receiver 150. In an exemplary embodiment, downstream receiver 150 has a similar architecture to upstream receiver 132, and includes a downstream ICR 152, a downstream ADC 154, and a downstream DSP 156. For upstream transmission, end user 106 optionally includes end user optical splitter 158, which may be located within downstream receiver 150 or separately, and an upstream transmitter 160. In an exemplary embodiment, upstream transmitter 160 has a similar architecture to downstream transmitter 126, and includes an upstream optical circulator 162, and an upstream modulator 164.
In operation, system 100 utilizes optical frequency comb generator 114 and amplifier 122 convert the input high quality source signal 116 into multiple coherent tones 120 (e.g., 32 tones, 64 tones, etc.), which are then input to first hub optical demultiplexer 124. In an exemplary embodiment, high quality source signal 116 is of sufficient amplitude and a narrow bandwidth such that a selected longitudinal mode of signal 116 is transmitted into optical frequency comb generator 114 without adjacent longitudinal modes, which are suppressed prior to processing by comb generator 114. First hub optical demultiplexer 124 then outputs a plurality of phase synchronized coherent tone pairs 166(1), 166(2), . . . 166(N). That is, the generated coherent frequency tones 120 are amplified by amplifier 122 to enhance optical power, and then demultiplexed into multiple separate individual phased synchronized coherent tone source pairs 166. For simplicity of discussion, the following description pertains only to coherent tone pair 166(1) corresponding to the synchronized pair signal for the first channel output, which includes a first unmodulated signal 168 for Ch1 and a second unmodulated signal 170 for Ch1′, and their routing through system 100.
With source signal 116 of a high quality, narrow band, and substantially within a single longitudinal mode, coherent tone pair 166(1), including first unmodulated signal 168 (Ch1) and second unmodulated signal 170 (Ch1′), is output as a high quality, narrowband signal, which then serves as both a source of seed and local oscillator (LO) signals for both downstream and upstream transmission and reception directions of system 100. That is, by an exemplary configuration, the architecture of optical frequency comb generator 114 advantageously produces high quality continuous wave (CW) signals. Specifically, first unmodulated signal 168 (Ch1) may function as a downstream seed and upstream LO throughout system 100, while second unmodulated signal 170 (Ch1′) concurrently may function as an upstream seed and downstream LO for system 100.
According to the exemplary embodiment, within optical hub 102, first unmodulated signal 168 (Ch1) is divided by hub optical splitter 130 and is separately input to both downstream transmitter 126 and upstream receiver 132 as a “pure” signal, and i.e., substantially low amplitude, narrow bandwidth continuous wave does not include adhered data. First unmodulated signal 168 (Ch1) thus becomes a seed signal for downstream transmitter 126 and an LO signal for upstream receiver 132. In an exemplary embodiment, within downstream transmitter 126, first unmodulated signal 168 (Ch1) passes through downstream optical circulator 136 into downstream modulator 138, in which one or more laser diodes (not shown in
In an exemplary embodiment, downstream optical circulator 136 is within downstream transmitter 126. Alternatively, downstream optical circulator 136 may be physically located separately from downstream transmitter 126, or else within the confines of downstream modulator 138. Downstream modulated data stream 172 (Ch1) is then combined in hub optical multiplexer 128 with the plurality of modulated/unmodulated data stream pairs from other channels (not shown) and transmitted over downstream fiber 108, to a node optical demultiplexer 174 in fiber node 104, which then separates the different channel stream pairs for transmission to different respective end users 106. At end user 106, because the data stream pair 170, 172 entering downstream receiver 150 is phase synchronized, digital signal processing at downstream DSP 156 is greatly simplified, as described below with respect to
Where upstream reception is optionally sought at optical hub 102, second unmodulated signal 170 (Ch1′) is divided, within end user 106, by end user optical splitter 158 and is separately input to both downstream receiver 150 and upstream transmitter 160 as a “pure” unmodulated signal for Ch1′. In this alternative embodiment, second unmodulated signal 170 (Ch1′) thus functions a seed signal for upstream transmitter 160 and a “pseudo LO signal” for downstream receiver 150 for the coherent detection of Ch1. For purposes of this discussion, second unmodulated signal 170 (Ch1′) is referred to as a “pseudo LO signal” because it uses an LO signal from a remote source (output from first hub optical demultiplexer 124), and is not required to produce an LO signal locally at end user 106. This particular configuration further significantly reduces cost and complexity of the architecture of the system 100 by the reduction of necessary electronic components.
For upstream transmission, in an exemplary embodiment, a similar coherent detection scheme is implemented for upstream transmitter 160 as is utilized for downstream transmitter 126. That is, second unmodulated signal 170 (Ch1′) is input to upstream optical circulator 162 and modulated by upstream modulator 164 to adhere symmetric or asymmetric data (not shown, described below with respect to
By this exemplary configuration, multiple upstream channels from different end users 106 can be multiplexed at fiber node 104 (or a remote node) and sent back to optical hub 102. Thus, within optical hub 102, the same coherent detection scheme may be used at upstream receiver 132 as is used with downstream receiver 150, except that upstream receiver 132 utilizes first unmodulated signal 168 (Ch1) as the LO and upstream modulated data stream 176 (Ch1′) to carry data, whereas downstream receiver 150 utilizes the data stream pair (Ch1, Ch1′) in reverse. That is, downstream receiver 150 utilizes second unmodulated signal 170 (Ch1′) as the LO and downstream modulated data stream 172 (Ch1) to carry data.
Implementation of the embodiments described herein are useful for migrating hybrid fiber-coaxial (HFC) architectures towards other types of fiber architectures, as well as deeper fiber architectures. Typical HFC architectures tend to have very few fiber strands available from fiber node to hub (e.g. fibers 108, 110), but many fiber strands could be deployed to cover the shorter distances that are typical from legacy HFC nodes to end users (e.g., fiber optics 112). In the exemplary embodiments described herein, two fibers (i.e., fibers 108, 110) are illustrated between optical hub 102 and fiber node 104, which can be a legacy HFC fiber node. That is, one fiber (i.e., downstream fiber 108) is utilized for downstream signal and upstream seed/downstream LO, and another fiber (i.e., upstream fiber 110) is utilized for upstream signal. Additionally, three fibers (i.e., fiber optics 112A-C) are illustrated for each end user from fiber node 104 (e.g., legacy HFC fiber node) to end user 106. By utilization of the advantageous configurations herein, fiber deeper or all-fiber migration schemes can utilize an HFC fiber node as an optical fiber distribution node, thereby greatly minimizing the need for fiber retrenching from an HFC node to an optical hub.
The architecture described herein, by avoiding the need for conventional compensation hardware, can therefore be structured as a significantly less expensive and more compact physical device than conventional devices. This novel and advantageous system and subsystem arrangement allows for multi-wavelength emission with simplicity, reliability, and low cost. Implementation of optical frequency comb generator 114, with high quality input source signal 116, further allows simultaneous control of multiple sources that are not realized by conventional discrete lasers. According to the embodiments herein, channel spacing, for example, may be 25 GHz, 12.5 GHz, or 6.25 GHz, based on available signal bandwidth occupancy.
The embodiments described herein realize still further advantages by utilizing a comb generator (i.e., optical frequency comb generator 114) that maintains a constant wavelength spacing, thereby avoiding optical beat interference (OBI) that may be prevalent in cases with simultaneous transmissions over a single fiber. In the exemplary embodiment illustrated in
As illustrated and described herein, system 100 may utilize an architecture of coherent DWDM-PON incorporate novel solutions to meet the unique requirements of access environment, but with cost-efficient structures not seen in conventional hardware systems. Optical frequency comb generator 114 produces a plurality of simultaneous narrow width wavelength channels with controlled spacing, thereby allowing simplified tuning of the entire wavelength comb. This centralized comb light source in optical hub 102 therefore provides master seeding sources and LO signals for both downstream and upstream directions in heterodyne detection configurations in order to reuse the optical sources throughout the entirety of system 100. This advantageous configuration realizes significant cost savings and reduction in hardware complexity over intradyne detection schemes in long-haul systems, for example.
In operation, downstream transmitter 200 performs the same general functions as downstream transmitter 126 (
More specifically, first unmodulated signal 168 (Ch1) exiting hub optical splitter 130 is input to downstream optical circulator 136, which then excites laser diode 204, that is, laser diode 204 emits light at a specified modulation rate. Laser injected modulator 202 adheres data 206 to the excited Ch1 signal, and the resultant modulated Ch1 signal with adhered data is output from downstream optical circulator 136 as downstream modulated data stream 172 (Ch1). According to this exemplary embodiment, first unmodulated signal 168 (Ch1) is input to downstream transmitter 126 as an unmodulated, low amplitude, narrow bandwidth, low noise “pure” source, and is modulated by laser diode 204, which is a high amplitude, wide bandwidth device, and resultant downstream modulated data stream 172 (Ch1) is a high amplitude, narrow bandwidth, low noise “pure” signal that can be transmitted throughout system 100 without the need for further conventional compensation means (hardware and programming). Suppression of adjacent longitudinal modes from laser diode 204, for example, is not necessary because of the exciting source signal (i.e., signal 168) is of such high quality and narrow bandwidth that output downstream modulated data stream 172 (Ch1) is substantially amplified only within the narrow bandwidth of external laser 118. In the exemplary embodiment illustrated in
Optical injection locking as described herein thus improves upon the performance of the relatively less expensive, multi-longitudinal slave laser source (i.e., laser diode 204) in terms of spectral bandwidth and noise properties. With respect to heterodyne coherent detection, incoming signals (upstream or downstream) can be combined with the LO or pseudo-LO and brought to an intermediate frequency (IF) for electronic processing. According to this exemplary configuration, part of the LO/pseudo-LO optical power can also be employed as the master/seed laser for the reverse transmission direction, at both optical hub 102, and at end user 106 (described below with respect to
Downstream transmitter 300 includes downstream optical circulator 136 (see
In operation, downstream transmitter 300 is similar to downstream transmitter 200 with respect to the implementation of direct modulation, and master/slave laser injection locking. Downstream transmitter 300 though, alternatively implements dual-polarization from the splitter portion of PBS/PBC 304, which splits first unmodulated signal 168 (Ch1) into its x-polarization component P1 and y-polarization component P2, which separately excite first laser diode 306 and second laser diode 310, respectively. Similar to downstream transmitter 200 (
In an exemplary embodiment, the polarized light components received by first and second laser diodes 306, 310 are orthogonal (90 degrees and/or noninteractive). That is, first laser diode 306 and second laser diode 310 are optimized as slave lasers to lock onto the same wavelength as external laser 118 (master), but with perpendicular polarization directions. By this configuration, large data packets (e.g., first data 308 and second data 312) can be split and simultaneously sent along separate pathways before recombination as downstream modulated data stream 172 (Ch1). Alternatively, first data 308 and second data 312 may come from two (or more) separate unrelated sources. The orthogonal split prevents data interference between the polarized signal components. However, one of ordinary skill in the art will appreciate that, according to the embodiment of
According to this exemplary embodiment, master continuous wave signal for Ch1, namely, first unmodulated signal 168, is received from optical frequency comb generator 114 and is split to be used, in the first part, as the LO for upstream receiver 132, and in the second part, to synchronize two slave lasers (i.e., first laser diode 306 and second laser diode 310) by the respective x-polarization and y-polarization light portions such that both slave lasers oscillate according to the wavelength of the master laser (i.e., external laser 118). Data (i.e., first data 308 and second data 312) is directly modulated onto the two slave lasers, respectively. This injection locking technique thus further allows for frequency modulation (FM) noise spectrum control from the master laser to the slave laser, and is further able to realize significant improvements in FM noise/phase jitter suppression and emission linewidth reduction.
As described herein, utilization of optical injection with a dual-polarization optical transmitter (i.e., downstream transmitter 300) by direct modulation may advantageously implement relatively lower-cost lasers to perform the functions of conventional lasers that are considerably more costly. According to this configuration of a dual-polarization optical transmitter by direct modulation of semiconductor laser together with coherent detection, the present embodiments are particular useful for short-reach applications in terms of its lower cost and architectural compactness. Similar advantages may be realized for long reach applications.
In this exemplary embodiment, downstream transmitter 400 performs the same general functions as downstream transmitter 126 (
Downstream transmitter 500 includes downstream optical circulator 136 (see
In operation, downstream transmitter 500 implements dual-polarization from the splitter portion of PBS/PBC 504, which splits first unmodulated signal 168 (Ch1) into its x-polarization component (P1) and y-polarization component (P2). Each polarization component P1, P2 is then input to first non-polarized optical splitter/combiner 522 and second non-polarized optical splitter/combiner 524, respectively. First and second optical splitters/combiners 522, 524 each then further split their respective polarization components P1, P2 into their I-signals 526, 528, respectively, and also into their Q-signals 530, 532, respectively. Generated I-signals 526, 528 then directly excite laser diodes 506, 514, respectively. Before directly communicating with laser diodes 510, 518, respectively, generated Q-signals 530, 532 first pass through first and second quadrature phase shift elements 534, 536, respectively, each of which shifts the Q-signal by 45 degrees in each direction, such that the respective Q-signal is offset by 90 degrees from its respective I-signal when recombined at splitters/combiners 522, 524.
The resultant modulated Ch1 signal, with adhered data, is output from downstream optical circulator 136 of downstream transmitter 500 as downstream modulated data stream 172 (Ch1), and as a polarized, multiplexed QAM signal. According to this exemplary embodiment, utilization of a photonic integrated circuit allows for directly modulated polarization of a multiplexed coherent system, but at significantly lower cost hardware configurations than could be realized by conventional architectures. In an exemplary embodiment, laser diodes 506, 510, 514, 516 are PAM-4 modulated laser diodes capable of generating 16-QAM polarization multiplexed signals.
Upstream transmitter 600 is thus nearly identical to downstream transmitter 300 (
In this example, upstream transmitter 600 is illustrated to substantially mimic the architecture of downstream transmitter 300 (
The significant cost savings according to the present embodiments are thus best realized when considering that as many as 512 downstream transmitters (e.g., downstream transmitter 126,
First and second data stream pairs 700, 702 the multiplexed phase synchronized pairs modulated/unmodulated of optical signals that are converted into analog electrical signals by ICR 140 and ICR 152, respectively. The respective analog signals are then converted into digital domain by ADC 142 and ADC 154, for digital signal processing by DSP 144 and DSP 156. In an exemplary embodiment, digital signal processing may be performed by a CMOS ASIC employing very large quantities of gate arrays. A conventional CMOS ASIC, for example, can utilize as many as 70 million gates to process incoming digitized data streams. In the conventional systems, modulated data streams for Ch1 and Ch1′ are processed independently, which requires significant resources to estimate frequency offset, drift, and digital down conversion compensation factors (e.g., e{circumflex over ( )}-jωt, where ω represents the frequency difference between first unmodulated signal 168 and upstream modulated data stream 176, and w is held constant for coherent tone pair 166, as extended throughout system 100).
According to the exemplary embodiments disclosed herein, on the other hand, the modulated and unmodulated signals from Ch1 and Ch1′ are phase synchronized together such that the difference between w of the signal pair is always known, and phase synchronized to maintain a constant relationship. In contrast, conventional systems are required to constantly estimate the carrier phase to compensate for factors such as draft which requires considerable processing resources, as discussed above. According to the present embodiments though, since Ch1 and Ch1′ are synchronized together as first and second data stream pairs 700, 702, the offset ω between the pairs 700, 702 need not be estimated, since it may be instead easily derived by a simplified subtraction process in DSP 144 and DSP 156 because the signal pairs will drift together by the same amount in a constant relationship. By this advantageous configuration and process, digital signal processing by a CMOS ASIC can be performed utilizing as few as one million gates, thereby greatly improving the processing speed of the respective DSP, and/or reducing the number of physical chips required to perform the processing (or similarly increasing the amount of separate processing that may be performed by the same chip). At present, implementation of the embodiments described herein may improve downstream and upstream data transmission speeds by as much as 5000 times faster than conventional systems.
In step 804, first unmodulated signal 168 (Ch1) is input to an optical splitter, e.g., optical splitter 130,
Step 810 is an optional step, where polarization division multiplexing is desired. In step 810, first unmodulated signal 168 (Ch1) is split into its x-component and y-component parts P1, P2, respectively (e.g., by PBS/PBC 304,
In step 816, second unmodulated signal 170 (Ch1′) and downstream modulated data stream 172 (Ch1) are optically multiplexed, i.e., by hub optical multiplexer 128,
Where an end user (e.g., end user 106) further includes upstream transmission capability, process 800 further includes optional steps 824 and 826. In step 824, and prior to downstream reception in step 822, second unmodulated signal 170 (Ch1′) is optically split (e.g., by end user optical splitter 158,
In step 904, second unmodulated signal 170 (Ch1′), or its polarized components if optional step 902 is implemented, is injection locked to the master source laser (e.g., external laser 118,
In step 910, upstream modulated data stream 176 (Ch1′) is optically multiplexed, i.e., by node optical multiplexer 178,
As illustrated in the exemplary embodiment, a difference between upstream and downstream signal transmission is that an entire synchronized modulated/unmodulated channel pair (e.g., second data stream pair 702,
According to the present disclosure, utilization of dual-polarization optical transmitters, and by direct modulation of semiconductor lasers with coherent detection, is particularly beneficial for not only longhaul applications, but also for shortreach applications to reduce the cost of electronic hardware, while also rendering the overall network system architecture more compact. The present systems and methods further solve the conventional problem of synchronizing two laser sources over a long period of time. Utilization of the phase synchronized data stream pairs and slave lasers herein allows continual synchronization of the various laser sources throughout the system during its entire operation. These solutions can be implemented within coherent DWDM-PON system architectures for access networks in a cost-efficient manner.
Utilization of the high quality optical comb source at the front end of the system thus further allows a plurality of simultaneous narrow bandwidth wavelength channels to be generated with easily controlled spacing, and therefore also simplified tuning of the entire wavelength comb. This centralized comb light source in the optical hub provides master seeding sources and LO signals that can be reused throughout the system, and for both downstream and upstream transmission. The implementation of optical injection, as described herein, further improves the performance of low-cost multi-longitudinal slave laser sources in terms of spectral bandwidth and noise properties. Access networks according to the present systems and methods thus achieve more efficient transmission of wavelengths through optical fibers, thereby increasing the capacity of transmitted data, but at lower power, increased sensitivity, lower hardware cost, and a reduction in dispersion, DSP compensation, and error correction.
Injection Locking Systems and MethodsAs described above, the innovative access network architectures of the present embodiments that implement coherent optics technology form the foundation of achieving transport speeds of 100G and beyond in the access network paradigm. The coherent modulation schemes described above advantageously enable an access network to drive down cost and complexity due to the spectral efficiency of coherent optics technology, as well as the management simplicity of the present systems and methods.
In a typical access network, much of the cost per bit of the network is fixed (e.g., by the fiber, line systems, facilities, management system, etc.), and largely independent of the data rate. Thus, the cost per bit in the access network will be most significantly lowered by achieving a higher data rate. In a practical implementation of the embodiments described above, the present inventors have already achieved a single-direction capacity of over 8,000 Gb/s on a single fiber, which represents approximately 5,000 times the capacity of access networks employing analog optics technology. This capacity is projected to increase to approximately 25,000 Gb/s per fiber within the next year, 50,000 Gb/s within the following few years, and greater within the next decade. However, the bulk of the capacity improvements has been primarily directed toward P2P configurations; conventional direct detection techniques have not been designed to reach residential end users in their homes.
The following systems and methods describe further improvements to the novel coherent PON (CPON) embodiments, described above, that implement injection locking technology from a master laser source. In exemplary embodiments, the present injection locking techniques may be implemented with respect to a master laser source generating a plurality of distinct coherent CW signals, or with respect to a master laser source generating a plurality of phase synchronized coherent tone pairs for each wavelength (e.g., phased synchronized coherent tone source pairs 166,
In an exemplary embodiment, a novel CPON architecture implements optical injection locking technology that results in a significant reduction to the structural cost and operational complexity of an ONU. In some embodiments, homodyne detection techniques are implemented at the ONU utilizing downlink signals for both a master laser and a local oscillator. In other embodiments, heterodyne detection techniques are implemented at the ONU using remote delivery for the master laser and the local oscillator. According to these innovative the methods, the present CPON configurations advantageously overcome the significant limitations of the conventional direct detection systems described above.
In an embodiment, CPON 1000 may be a coherent optical network configured to transmit a downstream coherent optical signal over transport medium 1006, from headend/hub 1002 to a splitter 1008, which splits the downstream coherent optical signal into a plurality of optical wavelengths, for further transport, over a plurality of short fibers 1010, to respective ONUs of end users 1004. In this example, the OLT of headend/hub 1002 may include at least one downstream coherent optical transmitter (not separately shown), and each ONU of an end-user 1004 may include a counterpart downstream coherent optical receiver. In an embodiment, splitter 1008 may represent one or more of an optical combiner, an optical splitter, a wavelength multiplexer, a wavelength demultiplexer, an optical coupler, and/or combinations thereof.
In a practical implementation of P2MP CPON 1000, the present inventors have successfully demonstrated a 120 Gb/s capacity, shared among 512 subscriber end users 1004, per wavelength over 80 km of fiber 1006. The present embodiments therefore realize significant improvements to the downstream portion of the CPON solution, thereby providing a technically feasible approach for both an extended reach, and also an ultra-high split ratio, for beyond the target 100G data rate capacity.
More particularly, the downstream solutions described herein have been successfully proven to advantageously increase the downstream capacity reach and split ratio, and for both conventional communication networks as well as emerging full duplex coherent optical systems (i.e., simultaneous bidirectional communication over the same fiber and same wavelength through coherent optics technology). The present systems and methods further advantageously enable full utilization of a novel coherent upstream burst transceiver, as well as all of the coherent optics injection locking techniques described herein, both individually and in combination, while also providing for use of a significantly low-cost laser at the customer premises of an end user. The novel coherent upstream burst transceiver is the subject of different application, whereas the following embodiments focus in greater detail on customer premises equipment, such as the ONU, which represents one of the most cost-sensitive portions of the network. As described further below, the systems and methods herein advantageously enable the use of significantly less expensive lasers that simplify the coherent transceiver configuration, which results in an ultra-low cost coherent transceiver that is relatively affordable for implementation at each subscriber residential home location, in comparison with conventional direct detection communication systems.
Nevertheless, reduction of the cost and complexity of the laser source has been a significant challenge to conventional solutions. As described above, high-quality laser sources are known, but are too expensive to be practically implemented at the location of each end user in a coherent P2MP network. The following embodiments thus demonstrate an innovative solution for inexpensively providing a high-quality, less complex, laser source at each downstream receiver/upstream transmitter of an end user (e.g., the ONU).
According to the hybrid solution depicted in
Accordingly, utilizing this COIL technique, the cost of expensive laser 1202 may be shared among and buying the many end users. In this example, in the case of 512 end users for a single expensive (i.e., approximately $1500 US) laser 1202, the expense of deploying an individual low-cost laser 1210 for each end user is approximately $40 US per residential home location implementing COIL. This cloning technique therefore represents a drastic cost saving solution over the deployment of an individual high-quality laser 1202 at each home location (i.e., not utilizing this COIL technique). In this example, the total cost of providing a coherent laser source at each individual home location includes the individual cost of cloned FP laser 1210, together with a the respective shared portion of the costs of parent laser 1202 and optical circulator 1218. The cost-per-home utilizing this COIL technique is thus in the approximate range of conventional direct detection PON systems. Accordingly, through the present systems and methods, coherent technology that is presently available to P2P network business users, may be affordably provided to residential subscribers in a P2MP network. An exemplary merger of such P2P and P2MP services is illustrated further below with respect to
Different though, from P2MP CPON 1000, in an exemplary embodiment, P2MP CPON 1300 is further configured such that a plurality of P2P fibers 1312 are communicatively coupled with headend/hub 1302 which includes at least one parent laser source 1314 at an MTS (e.g., the OLT) thereof. Accordingly, each end user 1304 further includes at least one child laser source 1316 at a modem (e.g., the ONU) thereof, and configured for injection locking with parent laser source 1314 according to the COIL techniques described above. According to this exemplary P2MP configuration, 100G coherent optical technology services, which have been heretofore only financially within the reach of P2P business subscribers, may now be provided to residential homes at a greatly reduced price that is competitive with conventional direct detection PON schemes.
As illustrated in
Further implementations of the present injection locking systems and methods are described further below with respect to
In exemplary operation of system 1400, for the downlink, the received signal λD is split by first splitter 1410 (e.g., a 3-dB optical coupler) with one arm thereof fed directly to homodyne detection unit 1418 (i.e., the downlink coherent receiver), and the other arm filtered first through optical filter 1412 (e.g., a narrow optical filter) and then injected, through optical circulator 1414 (e.g., a 3-port optical circulator), into the low-cost laser source of slave laser 1408. In the exemplary embodiment, optical filter 1412 is configured such that master laser power is sufficiently balanced with the narrow width of striped signal bandwidth.
After injection locking with slave laser 1408, the newly injected signal of a resultant injection locked carrier signal 1422 is split by second splitter 1416 (e.g., a 3-dB splitter), with one arm thereof returned to homodyne detection unit/receiver 1418 as the LO with phase noise and carrier frequency offset removed. According to this configuration, homodyne detection unit 1418 is advantageously capable of achieving carrier recovery in the optical domain without any delay. The other arm from second splitter 1416 is sent to uplink modulator 1422 become the uplink optical source of upstream modulated wavelength λU, which may be communicated upstream by way of a full duplex coherent optical connection schemes using an optical circulator (e.g.,
In an embodiment, in the case of asymmetrical modulation, the upstream transmission may employ an intensity modulation scheme, such as a non-return to zero on-off keying (NRZ-OOK) format, instead of the coherent modulation scheme implemented for the downstream transmission. According to the advantageous configuration of system 1400, only one relatively low-cost slave laser (e.g., an FP laser source) need be employed at each ONU 1404 to realize the effective performance of the high-cost master laser (e.g., an ECL laser source). In the exemplary embodiment, no carrier recovery is then needed in the relevant DSP flow either.
In exemplary operation, system 1500 performs similarly to system 1400 with respect to the reception and processing of downlink signal 1502D by ONU 1504. Different though, from the embodiment depicted in
In an exemplary embodiment, both of hub optical circulator 1524 and end user optical circulator 1526 are 3-port optical circulators respectively disposed on either side of the uplink and the downlink. In further exemplary operation, at the hub location, downlink signal 1502D by is transmitted from port 1 to port 2 of hub optical circulator 1524, and an uplink signal 1502U is transmitted from port 2 to port 3 of hub optical circulator 1524. Similarly, at the end point location of ONU 1504, downlink signal 1502D by is transmitted from port 2 to port 3 of end point optical circulator 1526, and the modulated uplink signal 1502U is transmitted from port 1 to port 2 of end point optical circulator 1526.
In the exemplary embodiment of system 1500, to achieve the full benefit of the bidirectional full duplex scheme, coherent modulation is also implemented for the upstream transmission. The bidirectional coherent signals provide significantly higher optical signal to noise ratio (OSNR) sensitivity, and also higher tolerance to the impairments from spontaneous Rayleigh backscattering, in comparison with non-coherent, intensity-modulated systems. Additionally, according to the configuration of system 1500, the threshold of the stimulated Brillouin scattering (SBS) nonlinear effect is much higher because of the nature of phase-modulated signals on the reduction of optical carrier power and the increase of effective linewidth.
Different from system 1500 though, system 1600 further includes a semiconductor optical amplifier (SOA) 1628 disposed at the location of ONU 1604. In the exemplary embodiment, SOA 1628 is disposed along the path of modulated upstream wavelength λU between uplink modulator 1620 and end user optical circulator 1626. In an embodiment, SOA may have a similar structure to an FP laser diode (e.g., slave laser 1608), but will not function as a laser source. Although SOAs generally have higher noise than an EDFA, the low noise output achieved by cloning slave laser 1608 to perform as the master laser enables system 1600 to implement SOA 1628 in place of a traditional EDFA because noise is much less of a factor in this particular COIL-based full duplex bidirectional coherent system.
In exemplary operation, system 1700 performs similarly to system 1400 with respect to the reception and processing of downlink signal 1702D by ONU 1704, and downstream coherent wavelength λD again may serve as downlink signal 1702D for both the master source and the LO. In this example, different from the embodiment depicted in
The advantageous properties of these techniques are described above with respect to homodyne detection configurations. As described further below, these innovative systems and methods are also advantageously applicable to WDM-PON architectures, as well as heterodyne configurations of PON systems.
In an exemplary embodiment, the particular configuration of optical comb generator 1914 may be according to one or more of a mode-locked laser, an electro-optic modulation scheme, or a gain-switched laser, in accordance with the specific technical parameters of architecture 1900. In an embodiment, optical comb generator 1914 may further include, as an integral or separate component, an EDFA (not separately shown) for amplifying and enhancing the optical power of frequency tones (e.g., 32 tones, 64 tones) generated from optical comb generator 1914.
The generated wavelengths λ1-N may then be demultiplexed by a first demultiplexer/demultiplexer 1916 for individual transport to, and processing by, each of hub transceivers 1908. The demultiplexed wavelengths may represent multiple individual CW channels (e.g., 1-N), or may represent a plurality of individual CW source pairs of phase synchronized coherent tones (e.g., multiple separate individual phased synchronized coherent tone source pairs 166,
Further to the exemplary embodiment, and similar to the several architectures described above, each hub transceiver 1908 may include one or more of an injection locked hub slave laser 1922, a first hub optical circulator 1924, a hub splitter 1926, a hub modulator 1928, a second hub optical circulator 1930, and a hub homodyne detection unit 1932 (e.g., a homodyne receiver). First hub optical circulator 1924 is, for example, configured to receive the respective narrow CW or tone λ, and inject the linewidth properties thereof into hub slave laser 1922. Second hub optical circulator 1930 couples hub transceiver 1908 with fiber 1906 by way of multiplexer/demultiplexer 1918.
In a similar manner, each end user transceiver 1910 may include one or more of a first end user optical circulator 1934, a first end user splitter 1936, an optical filter 1938 (e.g., a bandpass filter, or BPF), a second end user optical circulator 1940, an injection locked end user slave laser 1942, a second end user splitter 1944, an end user homodyne detection unit 1946, and an end user modulator 1948. Operation of each end user transceiver 1910 may then be performed according to one or more of the principles and techniques described above for the respective transceivers depicted in
In an embodiment, ONU 2004 includes a slave laser 2008 and one or more of an optical filter 2010, a heterodyne detection unit 2012 (e.g., a downlink heterodyne coherent receiver), a splitter 2014, an optical circulator 2016, and an uplink modulator 2018 configured to output a modulated signal of wavelength λU. In the exemplary embodiment, system 2000 may be similarly implemented at the end user location of one or more of the respective systems described above (i.e., where heterodyne detection is desired). Downstream transmission 2002 may, for example, include both a master CW laser source and a plurality of downstream signals λ on two separate wavelengths generated at central office or hub location. Downstream transmission 2002 therefore serves to provide both remote LO and master source delivery to ONU 2004.
In exemplary operation of system 2000, for the downlink, the received transmission 2002 is separated by optical filter 2010 with the signal from one arm thereof (i.e., λD) fed directly to downlink heterodyne detection unit 2012, and the signal from the other arm (i.e., λU) split by splitter 2014 (e.g., a 3-dB splitter), with one split returned heterodyne detection unit/coherent receiver 2012 as the LO, and the other split injected into the low-cost laser source of slave laser 2008 by way of optical circulator 2016. After injection locking with slave laser 2008, the newly injected signal (i.e., modulated λU) is sent to uplink modulator 2018 to provide the uplink optical source, which may be communicated upstream by way of a full duplex coherent optical connection schemes using an optical circulator (e.g.,
Similar to system 1400,
According to the advantageous configuration of system 2000, a centralized comb light source at the OLT (e.g., laser source 1912 and optical home generator 1914,
In exemplary operation, system 2100 performs similarly to system 2000 with respect to the reception and processing of downstream transmission 2102D by ONU 2104. Different though, from the embodiment depicted in
System 2200 differs from system 2100 in regard to the structural configuration of ONU 2204. Specifically, and as described above with respect to
Different from system 2100 though, system 2300 further includes an SOA 2324 disposed along the path of uplink signal 2302U between uplink modulator 2318 and end user optical circulator 2320. Although not illustrated in
In exemplary operation, system 2400 performs similarly to system 2000 with respect to the reception and processing of downlink transmission 2402D by ONU 2404. In this example though, and different from the embodiment depicted in
According to the innovative systems and methods described herein, the problems experienced with conventional direct detection PONs (i.e., poor receiver sensitivity, power fading due to chromatic dispersion at high symbol rates and long transmission distances, bandwidth- and power-inefficient modulation, etc.) are overcome. The CPONs of the present systems and methods are capable of fully realizing the benefits of coherent optical technology (i.e., frequency selectivity, linear detection, superior receiver sensitivity, etc.) but in a significantly more cost-effective solution than has been previously available for coherent systems. That is, the present COIL-based architectural configurations for a P2MP network effectively extend the reach and split ratio to multiple end-users, and residential home subscribers in particular, at approximately the present cost of conventional direct detection systems, but also at performance levels comparable to present coherent P2P links.
Optical Injected Laser for Coherent CommunicationAt present, data demand is rapidly increasing at an exponential pace, driven by bandwidth-intensive applications such as “big data,” Cloud technology, the Internet of Things (IoT0, video, and augmented reality/virtual reality (AR/VR). For these reasons, access bandwidth requirements for delivering such high-speed data and video services is expected to grow as described above. PON access technologies have thus become dominant architectures for meeting end user high capacity demand. Future PONs seek to provide higher per-subscriber data rates and wider coverage of services and applications, and service providers seek minimization of CAPEX and OPEX while increasing reconfigurable capability for scalable solutions.
As described above, DSPs are often fabricated using CMOS technology and, with recent advancements to sub-10 nm CMOS technology, future DSPs are expected to support the higher data rates under growing demand, but with a reduced footprint and power dissipation. The overall cost of conventional systems is dominated by optical and electrical components such as low-linewidth tunable lasers source for transmitter and local oscillator, as well as high speed balanced detectors. For mass adoption of coherent access network technology in the P2MP paradigm, further innovations to coherent optical components are needed to significantly reduce the cost, and thus increase the affordability, to an end user's home.
The embodiments above describe COIL technology and applications that enable low-quality lasers/slave lasers to perform as high-quality lasers by injection locking from a high-quality laser/master laser. The injection locked slave laser thus performs like the high-quality laser in a coherent transmission systems. The embodiments above describe how COIL may be achieved by injecting a single frequency laser source from the master laser into the laser resonator of the slave laser, such as with multi-longitudinal modes. Through the implementation of COIL in a coherent fiber communication system (e.g., CPON), a single high-quality laser may be utilized to drive many slave lasers, and these slave lasers will behave like the single high-quality master laser upon locking to the master laser. By these COIL techniques, the spectral quality of the master laser will be effectively cloned into many low-quality, low-cost slave lasers, thereby dramatically reducing the overall system cost by avoiding the deployment of high-quality, high-cost master lasers in each coherent transceiver (e.g., P2P systems).
However, conventional injection locking techniques are known to experience degradation of the injection efficiency of the laser due to detuning, and thus also penalties to the side mode suppression ratio (SMSR). A high maximum SMSR (e.g., for fine tuning) may be significantly reduced by detuning. Conventional frequency locking ranges of a coherent light source are often less than half of the FP mode spacing, and therefore, the injected input wavelength of such conventional systems are only able to tolerate a small variation gap before locking is lost. One significant factor impacting the SMSR of an injection-locked FP laser is frequency misalignment about the master wavelength. The frequency misalignment is due to the asymmetry of the locking range, resulting in an injection locking detuning range that is not equal on both sides of the frequency spectrum about the master wavelength. Accordingly, improved techniques are desirable for controlling the injection locking capability with respect to the detuning range.
The present systems and methods therefore provide innovative structures and processes for COIL-based, low-cost transmitters that may be implemented with one or more of the embodiments described above. In an exemplary embodiment, the COIL-based transmitter described herein includes a master seed laser source with a single longitudinal lasing mode, an input data stream, and a laser injected modulator including at least one slave laser having a resonator frequency that is injection locked to the single longitudinal modal frequency of the master seed laser source. For example, as described above with respect to
The COIL transmitter embodiments described further herein thus further improve upon the innovative architectures described above by providing innovative structures and implementation methods that may be employed as, or in place of, either or both of downstream transmitter 126 and upstream transmitter 160. In an exemplary embodiment, both of transmitters 126 and 160 have similar internal architectures and operations. Accordingly, for ease of explanation and not in a limiting sense, the following description refers generally to a COIL-based transmitter, or COIL transmitter, that may be employed as either an upstream or downstream transmitter in one or more of the embodiments above. The architecture of a COIL transmitter for a fiber communication network is described further below with respect to
Slave laser 2504, optical circulator 2508, and modulator 2510, on the other hand, may be separately located from master laser source 2502 in a discrete, compact transmission device that is still otherwise disposed in or proximate to the OLT/hub/CO (e.g., downstream transmitters 126, 200, 300, 400, 500,
In exemplary operation of system 2500, the ONU thus is enabled to act as the device that functions as the service provider endpoint of a PON (e.g., end user 106). In an embodiment, polarization controller 2506 may be utilized to align the polarization of master laser source 2502 with slave laser 2504 for maximum internal injection coupling efficiency. In further operation of transmission system 2500, optical circulator 2508 is used to route master laser source 2502 into the resonance cavity of the slave laser 2504 (e.g., through Port 1). Thus, the same optical circulator 2508 may further serve to route the optical output power of the injection locked slave laser (e.g., Port 2) into the input port (e.g., Port 3), and thus to optical modulator 2510. The subsequent coherent locked laser light may then be modulated using modulator 2510 (e.g., which may be an external modulator) to convert a data stream 2514 from electrical signals to optical signals for fiber optical transmission as modulated data 2516.
In the exemplary embodiment, the architecture of COIL transmitter system 2500 may be fully integrated within the greater architecture of fiber communication system 100,
In the exemplary embodiment, an ideal master laser source 2502 has low spectral linewidth, low phase noise, well defined frequency spacing, high E/O quantum efficiency, high operation stability, and high output power. The channel spacing of master laser source 2502 may include, without limitation, one or more of 100 GHz, 50 GHz, 25 GHz, or 12.5 GHz, based on the signal bandwidth occupancy. The generated frequency tones/phase synchronized coherent tone pairs (e.g., 32 tones, 64 tones) may then be amplified by an EDFA (e.g., EDFA 122) to enhance optical power, and then further demultiplexed into multiple separate individual coherent sources used as LO signals, and/or for coherent injection locking.
In further operation of system 2500, placement of the optical frequency comb source (e.g., comb generator 114) at the OLT, optical hub, or CO, provides a remote source for generating multiple coherent tones as the LO and seed signal sources for both downstream and upstream directions. This centralized master laser source thus enables the overall system to employ relatively compact physical devices for the transmission subsystem arrangement of system 2500, to provide multi-wavelength emission capability for each individual COIL transmitter system 2500 in a significantly simplified, elegant, lower cost design that demonstrates significantly increased reliability. Furthermore, the implementation of a central comb source provides further advantages that are not readily provided by discrete lasers, such as simultaneous control of multiple sources.
As shown in
Conventional transmitters used in long-haul transport utilize high-quality lasers and optical modulators to convert electrical data into optical signals. In comparison with the present COIL transmitter architecture, the conventional long-haul transmitter may be considered to have a relatively simple design. Nevertheless, despite this relative simplicity, the conventional transmitters are considerably more expensive to implement, due to the high cost of the using a dedicated master laser source for each modulator. In the access network paradigm, to be economically viable, the cost of optical components should be considerably lower than components used in the long-haul network, and particularly within the realm of coherent WDM PON. The present COIL transmitter embodiments thus enable significant reductions to the transmitter cost, and without sacrificing quality of the modulator laser source. By sharing a high-quality master laser source among many transmitters using COIL, the present low-cost local slave lasers inherit phase characteristics from the master laser, including frequency and linewidth. According to the COIL transmission techniques described herein, a fiber communication network is able to operate a single (or very few) high-quality master laser sources to drive many times more low-cost coherent transmitters for each single master laser source with negligible performance penalty, where the multiple driven transmitters are respectively distributed throughout the network at end user's homes, and/or include multiple compact upstream transmitters at the OLT/hub/CO.
Many laser sources have been used as slave laser sources for injection locking across a wide spectral range, including multi-longitudinal mode FP lasers and reflective semiconductor optical amplifiers (RSOAs). However, DFB, DBR, and VCSEL lasers are also known to be capable of being injection locked with improved bandwidth and spectral width. For the WDM-PON based access network described herein, multimode FP lasers are described for use as the exemplary slave laser, since the gain spectrum occupied by the FP modes is considered wide enough to cover any WDM-PON channel on a fiber transmission window, and therefore the innovative concepts of the present embodiments may be illustrated for all of these known laser structures by the description of the FP laser as an exemplary embodiment. The person of ordinary skill in the art will understand that this example is thus used for illustrative purposes, and is not intended to be limiting. An injection locked FP laser additionally demonstrates good phase noise and linewidth properties. Therefore, by optimizing the FP laser resonance cavity length, a single (or a few) types of FP laser are adequate to illustrate the entire operational spectral range of a WDM-PON network, thereby further allowing for a significant reduction in the number of inventory parts and implementation costs.
Exemplary operation of smart controller 2512 within COIL transmission system 2500 is described further below with respect to the following figures. In an exemplary embodiment, smart controller 2512 further includes one or more of a master interface 2518 to master laser source 2502, a slave interface 2520 to slave laser 2504, and a modulation interface 2522 to modulator 2510.
Therefore, a simple comparison of plot 2800 with plot 2700 demonstrates how a slave FP laser is considered to be sufficiently coherently locked to the master laser only under certain conditions. If not properly controlled, the slave laser side modes might be unlocked (e.g., plot 2600), or only partially locked (e.g., plot 2700), rendering the particular slave laser unusable for the COIL techniques described herein. Accordingly, the present COIL transmitter systems and methods advantageously enable coherent optical fiber communication techniques to work more reliably in the field by locking the optical injection laser source with operational certainty and managing the ongoing operation with significantly greater stability.
In an exemplary embodiment, a smart controller (e.g., smart controller 2512,
In the embodiment illustrated in
As further illustrated in
Therefore, by considering the COIL information against the input power information, very valuable additional information is provided that enables the enhanced slave laser injection locking control described herein. For example, in the case where the master laser frequency is placed right in the middle of a corresponding FP side mode center, locking may be effectively realized, but the stability of this locking will not be optimized, since this locking frequency will not be centered/in the middle of detuning range 2902 (e.g., along optimized third subplot 2908). Therefore, as illustrated in plot 2900, when the master laser output power is increased (horizontal axis), detuning range 2902 of the slave laser locking also increases. Accordingly, plot 2900 demonstrates that the locking range and stability of the COIL slave laser may be improved by increasing the master laser power.
Referring back to
Similarly, smart controller 2512 may be further configured to also control slave laser 2504 by setting the bias current and the output power thereof. In an exemplary embodiment, smart controller 2512 is configured to control slave laser 2504 by managing the slave laser junction temperature, such as by setting the temperature controller (not shown) of slave laser 2504. In at least some embodiments, optical modulator 2510 may also be managed by smart controller 2512, such that modulator 2510 is provided with optimized power and bias for the incoming coherently locked FP slave laser output.
In the exemplary embodiment, process 3100 begins at step 3102, in which operation of smart controller 2512 begins. In step 3104, the master laser (e.g., master laser source 2502) is powered on. In an exemplary embodiment of step 3104, smart controller 2512 turns on master laser source 2502 by setting a predefined wavelength and optical power level. In step 3106, smart controller 2512 is configured to calculate the drive current and temperature needed for slave laser 2504 to be successfully injection locked to the known master laser source 2502. In an exemplary embodiment of step 3106, smart controller 2512 obtains the drive current by a lookup operation in memory, or may obtain the drive current from a precision current source and the temperature by direct or indirect sensing capability.
In step 3108, smart controller 2512 transmits operational commands to a driver of slave laser 2504, as well as a temperature controller thereof (not shown). In an exemplary embodiment of step 3108, temperature control may be provided using a thermal electrical ceramic (TEC) module, or using a simplified thermal resistive heater element may be implemented for temperature control. In at least one embodiment of step 3108, temperature control is provided by smart controller 2512 using a thermistor (not shown) co-packaged near the FP laser chip of slave laser 2504 to accurately control the FP chip junction temperature within one percent of a Celsius degree or better.
In the exemplary embodiment, smart controller 2512 is configured to maintain in memory a detailed knowledge of the FP laser detuning range, as well as optimized locking temperatures and currents for the particular FP slave laser sought to be controlled. In an exemplary embodiment, such detailed information and/or algorithms may be stored in a lookup table, in the smart controller memory, in an EEPROM, etc. In an exemplary embodiment, smart controller is, or includes, a server, a micro-processor, or an ASIC.
In step 3110, smart controller 2512 polls slave laser 2504 for the locking status of slave laser 2504. Step 3112 is a decision step. If, in step 3112, smart controller 2512 determines that slave laser 2504 has not achieved locking to master laser source 2502, process 3100 returns to step 3106. If, however, process 3100 determines in step 3112 that slave laser 2504 has achieved injection locking, process 3100 proceeds to step 3114. In step 3114, smart controller 2512 configured to verify the power and bias operation condition of modulator 2510 (e.g., using modulation interface 2522). In an exemplary embodiment of step 3114, smart controller 2512 is further configured to adjust, in real time, the modulator power and bias operation condition as desired or needed.
In step 3116, smart controller 2512 is configured to initialize the optical transmission, turn on modulator 2510, and enable communication of data stream 2514, which is then output from modulator 2510 as modulated data 2516. In step 3118, process 3100 is configured to wait, or hold, for a predetermined period of time (e.g., 0-n seconds), before returning to step 3104, thereby creating a continuous monitoring loop that is capable of adjusting and controlling in real-time the several laser parameters relevant to successfully initiating and maintaining injection locking of slave laser 2504 to master laser source 2502. In an exemplary embodiment, the continuous loop of process 3100 will run as long as smart controller 2512 is powered on.
According to the exemplary systems and methods described herein, an innovative optical injection locking-based coherent optical transmitter is provided for coherent fiber optical network communications. The present transmitter includes a master laser capable of providing single channel or multi-channel low-linewidth frequency channel(s) as the central laser source for the communication system. The transmitter may further include a slave laser having multi-longitudinal modes which may be coherent optical injection locked (COIL) to the master laser frequency of the master laser.
In some embodiments, a COIL-based transmitter system includes a polarization controller between the master laser and the slave laser to maximize the injection locking efficiency, and an optical circular may be employed to inject the master laser into the cavity of the slave laser. The output power from the locked slave laser may be transmitted along the same fiber from which the master laser is injected to the slave laser cavity, but alternatively routed to an optical modulator. In the exemplary embodiment, operation of the COIL-based transmitter system is managed by a smart controller having a memory, and/or programmed with smart algorithm, such that the master laser and the slave laser may be reliably placed, and then maintained, in a locking state.
In some embodiments, the smart controller represents an advanced control device having improved communication, sensing, and control capabilities. The smart controller may, for example, include or be a server, a microprocessor, or an ASIC. The smart controller may further include a memory or EEPROM capable of storing detailed tables of slave laser or FP locking conditions with respect to the injected master laser optical power, and which may further include detailed information regarding relevant slave laser drive currents and junction temperatures.
In exemplary embodiments of the systems and methods described herein, the master laser communicates with the smart controller such that optical frequency and optical power is known to the controller. The smart controller may further include one or more interfaces capable of communication with one or more respective components of the COIL transmitter system. The master laser may, for example, include or be a frequency comb, a tunable laser, a WDM array, and/or other types of lasers capable of providing low-linewidth, and low phase noise optical tone or tones. The master laser may thus advantageously communicate with the smart controller, and in some cases, may be controlled by the smart controller.
Similarly the slave laser may also communicate with the smart controller and be controlled by the smart controller. The slave laser may therefore include or be a multi-longitudinal FP laser, a VCSEL, a DFB, or another type of laser capable of being injection locked to provide a single tone COIL source. The slave laser may optimally be driven by high precision current source of the master laser to provide injection locking by aligning an FP side mode with the master laser frequency. Alternatively, or supplementally, the slave laser may be driven by high resolution temperature controller such that injection locking is provided by aligning the FP side mode with the master laser frequency. The temperature controller may be or include one or more of a TEC component close to the FP laser capable of heating or cooling the FP laser, a thermal resistor component close to the FP laser capable of heating the FP laser, and a thermistor or temperature sensor placed near the FP laser to monitor the FP laser junction temperature.
In the exemplary embodiments described herein, the COIL transmitter system includes optical modulator capable of converting an electrical data stream into a modulated optical signal using the COIL transmitter system source. In some embodiments, the bias and power of the optical modulator may also be advantageously controlled by the smart controller. The optical modulator may include or be an intensity modulator, a phase modulator, an intensity-phase modulator, and/or one or more of the modulator embodiments described above.
The present techniques further provide an advantageous processing algorithm for operating the smart controller manage the transmitter perform, and thereby advantageously ensure that COIL is not only achieved, but also maintained in real-time throughout the operational duration of the smart controller. The smart controller may thus be configured to seamlessly integrate with the operation or one or more of the hardware component and system structures described above.
The present smart controller may therefore further include one or more software algorithms, or sets of executable computer programming steps, including without limitation instructions to: (i) initiate the master and slave lasers; (ii) calculate or determine optimal conditions to ensure locking of the slave laser; (iii) send drive current and/or temperature control commands to the slave laser; (iv) poll the temperature and current of the slave laser; (v) set the optical modulator power and bias for data conversion; (vi) start modulator operation once injection locking is stabilized; and (vi) instruct or authorize the optical modulator to convert electrical data to modulated optical signals for communication over an optical fiber transport medium. The present systems and methods are therefore of particular utility in the paradigm of coherent communications systems, including without limitation P2P coherent transmission systems, P2MP coherent PON systems, and/or single wavelength or WDM systems.
Injection-Locking Full-Field TransmitterAs described above, data-driven applications, such as HD video streaming, mobile Internet, and cloud computing, have led to a significant growth in the recent overall traffic volume in optical communication networks. Furthermore, the demand for optical transmission at ultra-high data rates is also increasing for the optical transport, metro, and access network paradigms. These trends, along with a growing bit-per-Hz cost reduction requirement, have led to increasing needs in the industry for higher-speed optical transmission interfaces and higher spectrum efficiency technologies.
Recent developments in coherent optical technology have provided useful improvements for long-haul backbone transport systems. Advancements in digital signal processing (DSP) techniques, as well as signal formats (e.g., QPSK, 8QAM, 16QAM, etc.) utilizing polarization multiplexing, have enabled long-haul links, and metro networks more recently, to realize improve data rates and spectral efficiency. Coherent technologies are now being introduced to the access network, such as with the case of 100G coherent optics in short distance P2P links of the HFC access network. Other wired networks (e.g., PON) are also moving towards 100G coherent implementations for next generation (NG) access (e.g., NG-PON). However, the lower receiver sensitivity and limited power budget of PON systems present considerable challenges to high speed support using present PON direct detection schemes. The innovative coherent techniques presented herein though, address and solve these conventional challenges, rendering the implementation thereof significantly future-proof solutions for NG high-speed PON systems. The present systems and methods improve the link power budget, and with much greater sensitivity, while also enabling higher-order modulation formats to increase the data rate and network efficiency.
Cost is another significant obstacle to utilizing coherent optics in the access network. Conventionally, the overall system cost of the access network has been dominated by optical and optic-electronic components, such as low-linewidth tunable laser sources for the transmitter, as well as local oscillators and advanced modulators. Accordingly, there is a need in the industry for further innovations to the coherent optical components of the coherent access network to further reduce the cost thereof. Some such architectural innovations are described in greater detail above, and in U.S. Pat. No. 9,912,409, all of which are incorporated by reference herein in their entireties. These embodiments to the present inventors describe COIL techniques that enable the use of essentially “low-quality” and low-cost lasers (i.e., slave lasers) as high performing lasers by injection locking the low-cost laser by a high-quality laser (sometimes referred to herein as “master laser”). The injection locked slave laser thus behaves like a high-quality master laser which can be used in coherent transmission systems to greatly reduce the overall cost of the entire system.
Particular challenges with respect to injection locking up lasers are described in greater detail above. The present embodiments introduce new and advantageous implementation schemes for injection-locking full-field transmitters that incorporate newer technologies, such that these developing coherent technologies are able to meet the unique requirements of the access environment with cost-efficient approaches. For example, according to the present systems and methods, laser injection-locking and full-field signal modulation (e.g., as m-QAM or m-PSK) may both be achieved using the low-cost embodiments described herein. The present techniques are further enabled to achieve polarization multiplexing, such as for dual-polarization advanced signal modulations. The present embodiments thereby provide an injection-locked transmitter using a relatively low-cost configuration of optical components, but which achieves high spectrum efficiency for an optical coherent communication system. In comparison with conventional I/Q modulators that implement a parallel Mach-Zehnder Modulator (MZM) structure that suffers relatively high insertion and modulation losses, the present systems and methods represent significant reductions to not only the cost, but also to the modulation loss, while nevertheless increasing the output power.
The embodiments described further herein provide, for purposes of illustration, two exemplary system architectural scenarios for multiple injection-locked transmitters, namely: (i) single-wavelength source use cases; and (ii) multi-wavelength source use cases. The person of ordinary skill in the art though, when reading and comprehending the present disclosure, will understand that these exemplary embodiments are provided by way of example, and not in a limiting sense. Other architectural scenarios may be implemented with respect to the present embodiments without departing from the scope thereof. The present systems and methods are further of particular use in both P2P and P2MP networks, and in either network type, a single high-quality master laser may be cloned and modulated by one or more low-cost slave lasers which will exhibit the high-quality performance of the master laser, while also carrying the relevant advanced modulation formats. According to these innovative techniques, the overall network cost is greatly reduced by avoiding the deployment of high-quality, high-cost master lasers and modulators for each coherent transceiver.
In the embodiment depicted in
In first stage 3302, two direct-modulator COIL lasers (sometimes referred to herein as COILs) are directly modulated with data for I and Q signals. In the example illustrated in
According to this advantageous embodiment, the need or desire for an external modulator is eliminated, thus rendering optical system according to operational principle 3300 “external modulator-free.” As described above, in this exemplary embodiment, to direct-modulator COIL lasers are utilized along with two controlled phase shifters keep orthogonality (i.e., first and second stages 3302, 3304, respectively), with the at least one additional stage (i.e., third stage 3306) to cancel the DC offset resulting from the direct modulation of the COIL lasers.
In comparison with conventional modulators based on MZM or Mach-Zehnder Interferometers, which experience generally large insertion and modulation losses, the present injection-locking laser-based embodiments demonstrate still further advantages over these conventional systems, in that the present systems and methods are capable of providing a significantly higher output power due to the gain components present in the lasers. In comparison with operational principle 3300,
In the exemplary embodiment, COIL subsystem 3508 includes an optical circulator 3510 for routing master laser 3502 into the respective resonance cavities (not shown) of slave lasers 3504, 3506. COIL subsystem 3508 further includes a first coupler 3512 (Coupler-1) disposed opposite master laser 3502 with respect to optical circulator 3510, for splitting the power from the signal of master laser 3502 into a first path 3514 (Path-1), and a second 3516 (Path-2). In the embodiment depicted in
The injection-locked modulated optical (I) signal from first slave laser 3504 connects with first path 3514, and the injection-locked modulated optical (Q) signal from second slave laser 3506 connects with second math 3516 through a first phase shifter 3526. In this manner, first phase shifter 3526 functions to control the phase shift between first path 3514 and second path 3516. Accordingly, after modulation, the two injection-locked optical laser signals are combined at first coupler 3512. Through this advantageous configuration, implementation of first phase shifter 3526 enables control of the signals to achieve a round-trip of 90-degree phase difference between the two respective outputs of injection-locked slave lasers 3504, 3506, thereby achieving full I/Q modulation between the slave lasers. In this example, a phase shift of 90-degrees is provided by way of illustration, but not in a limiting sense. The person of ordinary skill in the art understand that different phase shift values may be employed with respect to the present embodiments to achieve a synthesized full-field optical signal with different phase and amplitude distributions.
According to the exemplary configuration of transmitter 3500, optical circulator 3510 advantageously provides additional functionality to route the modulated, synthesized full-field optical signal (i.e., the combination of first and second paths 3514, 3516) out of the transmitter. A final stage includes a second coupler 3528 (Coupler-2) disposed between master laser 3502 and first coupler 3512, and a third coupler 3530 (Coupler-3) disposed between optical circulator 3510 and a transmitter output 3532. In the exemplary embodiment, a second phase shifter 3534 (PS-2) is disposed along a third path 3530 (Path-3) between second coupler 3528 and a third coupler 3530, which effectively renders third path 3530 into a bypass route around optical circulator 3510 between master laser 3502 and transmitter output 3532. This advantageous configuration of second phase shifter 3534 thus serves as a shifted control, taken directly from master laser 3502 (i.e., at second coupler 3528), and then combined with the synthesized full-field optical signal from optical circulator 3510 (i.e., at third coupler 3530), to effectively cancel the direct-current (DC) component of the full-field signal. In an exemplary embodiment, second phase shifter 3534 is configured for a 180-degree phase shift, which enables (e.g., assuming an appropriate power level from second coupler 3528) the DC component of the synthesized full-field optical signal to be fully canceled at/after third coupler 3530.
In an exemplary embodiment, first and second phase shifters 3526, 3534 are further configured to operate with a fixed phase shift to achieve orthogonal I/Q modulation. For example, first phase shifter 3526 may be configured to have a 45-degree one-way phase shift (e.g., along second path 3516), and second phase shifter 3534 may be configured to have a 180-degree one-way phase shift (e.g., along third path 3536). In some embodiments, one or both of first and second phase shifters 3526, 3534 are tunable, and may utilize a bias voltage to achieve arbitrary or desired phase and amplitude modulations. Transmitter 3500 therefore represents a three-stage signal generation unit. In at least one embodiment, in a case where the injected power of master laser 3502 is considered relatively low, a gain component may be desirable to amplify the power along third path 3536, before or after second phase shifter 3534, for carrier cancellation (e.g., using a semiconductor optical amplifier (SOA), not shown in
Transmitter 3600 differs from transmitter 3500 though, in that COIL subsystem 3604 utilizes a single optical coupler 3626 between optical circulator 3624 and slave lasers 3606 and 3608 for combining the synthesized optical signals of first and second paths 3614, 3620. Transmitter 3600 further differs from transmitter 3500 in that transmitter 3600 includes an injection-locking third slave laser 3628 (S-LD C) in operable communication with optical coupler 3626 over a third path 3630. In an exemplary embodiment, a second phase shifter 3632 is disposed along third path 3630 between optical coupler 3626 and second phase shifter 3632. According to this alternative configuration, only one coupler (e.g., optical coupler 3626) is needed to split the input master laser power from master laser 3602 into the three separate lanes of first, second, and third paths 3614, 3620, 3630, respectively.
More particularly, in this example, transmitter 3600 applies first path 3614 for laser injection-locking and I-data modulation, and second path 3620 for laser injection locking and Q-data modulation, with a round-trip 90-degree phase shift provided by first phase shifter 3622. According to this configuration, transmitter 3600 is further enabled to apply third path 3630 for not only laser injection locking, but also for DC offset cancellation through utilization of second phase shifter 3622, which may achieve a 180-degree phase shift round-trip, in this example. Thus, a single optical circulator (e.g., optical circulator 3624) may still be utilized in this alternative configuration to both route the master laser signal into the respective resonance cavities of slave lasers 3606, 3608, 3628, and also to route the modulated, synthesized full-field, combined optical signal therefrom to a transmitter output 3634. In some embodiments, implementation of third slave laser 3628 may be optional according to the input power of master laser 3602, namely, in the case where the input power is above or below a predetermined threshold.
Transmitters 3500 and 3600 are described above with respect to single-polarization COIL configurations by way of illustration, and not in a limiting sense. For example, the principles described immediately above are fully applicable to dual-polarization transmitters through implementation of two sets of each of the single-polarization COIL full-field transmitters in combination with a polarization beam combiner (PBC) to combine the modulated signals on two polarizations to achieve polarization multiplexing, as described further below with respect to
The full-field COIL transmitter techniques described above for I/Q modulation are also applicable to amplitude and phase modulation by implementing an external phase modulator, as described further below with respect to
Different though, from the above embodiments, amplitude-modulated optical signal 3912 is first modulated by a phase modulator 3914 in communication with a phase a data source 3916 (e.g., polar coordinates), and then output as a full-field modulated optical signal 3918. Accordingly, through utilization of amplitude modulation from slave laser 3904 and phase modulation from phase modulator 3914, transmitter 3900 advantageously achieves full-field optical transmission. The configuration of transmitter 3900 thus represents a trade-off with respect to the full-field optical transmission principles described above with respect to
In the embodiments described above, networking of multiple injection-locked transmitters is an important consideration. The embodiments described above with respect to
Accordingly, at the transmitter-side of fiber 4210, each of full-field transmitters 4206 (i.e., including respective slave lasers injection-locked by single-wavelength master laser 4204) is multiplexed onto fiber 4210 as a single aggregate optical signal by a multiplexer 4212 for delivery to a receiver aggregation 4214, which includes a plurality (i.e. 1-N) of optical receivers 4216, through a demultiplexer 4218. In some embodiments, multiplexer 4212 is a mode multiplexer, in which case fiber 4210 may be a multi-mode fiber, and demultiplexer 4218 may be a mode demultiplexer. In other embodiments, multiplexer 4212 is a core multiplexer, in which case fiber 4210 may be a multi-core fiber, and demultiplexer 4218 may be a core demultiplexer.
Accordingly, at the transmitter-side of fiber 4310, each of full-field transmitters 4306 are injection-locked to at least one wavelength of the multi-wavelength comb source of master laser source 4304, and then multiplexed onto fiber 4310 as an aggregate optical signal by a WDM multiplexer 4312 for delivery to a receiver aggregation 4314 of 1-N optical receivers 4316 through a WDM demultiplexer 4318. That is, according to the embodiment depicted in
According to the preceding embodiments, innovative systems and methods are provided for injection-locking full-field transmitters that are fully enabled to incorporate new technologies that meet the unique requirements of the access network environment, but through cost-efficient approaches. For example, using significantly low-cost lasers, the present embodiments are able to simultaneously achieve both laser injection-locking and full-field signal modulation, such as m-QAM or m-PSK. Furthermore, as described above, full-field signal modulation may be successfully achieved through either I/Q modulation or amplitude/phase modulation. Additionally, the present systems and methods are fully adaptable from single-polarization configurations to dual-polarization configurations without complex structural modifications to the transmitters or the networks in which they are deployed.
The present systems and methods are still further fully scalable for system architectures that utilize multiple injection-locked transmitters, whether for single-wavelength source use cases or for multi-wavelength source use cases, and for both P2P and P2MP networks. According to the techniques described above, a single high-quality master laser may be efficiently cloned and modulated by many low-cost slave lasers, whereby these slave lasers will realize the same high-quality performance, and carry the same advanced modulation formats, of the high-quality source. Accordingly, in comparison with conventional networks, the overall cost of the present network is greatly reduced by avoiding the need to deploy high-quality, high-cost master lasers and modulators in each coherent transmitter/transceiver.
Phase Domain Signal Modulation and EqualizationAs described above, coherent technologies are being increasingly deployed for ultra-high-speed long-haul backbone transport systems/links and metro networks due to recent DSP advancements, advanced signal formats, polarization multiplexing, and data rate/spectrum efficiency improvements. There is a clear desire in the industry to further this trend in the access network environment, which is considered to be the most effective future-proof approach for optical access networks in brown and green field deployments. This need is felt particularly in the cable access environment, where coherent optics technologies enable operators to best leverage existing fiber infrastructures for the present exponential growth in capacity and services.
Introduction of digital coherent technologies into optical access networks has presented several engineering challenges, because the access network environment is a significantly different environment than the long-haul and metro environments, particularly with respect to the overall system cost that is dominated by optical and optic-electronic components, such as low-linewidth tunable lasers source and advanced modulators. The embodiments described above provide a number of innovative solutions for reducing the cost of lasers the access network, and also for simplifying the configuration of the various modulators. The embodiments described immediately above further provide novel phase modulation techniques using COIL technologies to realize both full-field optical transmitters, and which also further simplify the configuration and operation of the transmitters. These solutions are particularly advantageous over conventional MZM techniques for signal modulation, even with respect to some of the techniques described herein that implement I/Q signal modulation.
Such advantages include without limitation: (1) the total loss that is seen according to the present phase modulation techniques is significantly smaller than the loss experienced using a conventional MZM (i.e., >3 dB), or some I/Q modulation techniques (e.g., >6 dB), thereby greatly improving the present output power; (2) the cost of transmitter implementing the present phase modulator embodiments is expected to be significantly lower than the cost of implementing an MZM, since one MZM typically includes two phase modulators, and an also lower than the cost of implementing an I/Q modulator, since one I/Q modulator may include as many as phase modulators; (3) since the signal according to the present embodiments may be modulated in phase domain, the amplitude and intensity of the output signal, after phase modulation, will remain the same; and (4) the modulated signal, after passing through the present phase modulator, will achieve better performance under fiber nonlinearities.
Nevertheless, one particular challenge to implementation of phase domain signal modulation arises with respect to signal equalization. Conventional coherent DSP algorithms have not been able to successfully overcome phase domain inter-symbol-phase-interference in the phase-modulated signals. This interference problem though, is solved according to the following innovative systems, apparatuses, and methods for multi-level signal modulation and equalization in the phase domain for optical coherent communication systems. The present embodiments thus provide a complete end-to-end system configuration for signal modulation, coherent detection, and equalization for optically transmitted multi-level signals.
In an exemplary embodiment, a multi-level signal is mapped into the phase domain, and then modulated by a phase modulator on a particular polarization. Once modulated, the optical signal may then be coherently detected and processed by DSP to recovery. The following techniques for phase domain equalization to address inter-symbol-phase-interface may be advantageously implemented at either the transmitter-side or the receiver-side, whether as post-equalization and/or pre-equalization. More particularly, in some embodiments, post-equalization techniques are provided herein for receiver-side processing in the phase domain. In other embodiments, digital phase-domain pre-compensation techniques are provided for improving performance at the transmitter-side.
The following embodiments are therefore particularly useful to satisfy the unique requirements of the access environment, by providing architectural configurations of significantly lower complexity, and also by realizing a significantly reduced insertion loss and modulation loss for multi-level signals, in comparison with MZM- and I/Q modulator-based systems. As described further below, the sensitivity performance of multi-level signals modulated according to the present phase modulation embodiments is greatly enhanced in comparison with multi-level signals modulated in the amplitude or intensity domains.
In exemplary operation of phase domain mapping module 4500, input bit information (e.g., [1110000110 . . . ]) coded as PAM-4 symbols of multi-level signal 4502 by a symbol coding unit 4506. Once encoded, Then, the PAM-4 symbols of multi-level signal 4502 may be mapped to specific phase values of phase domain signal 4504 by a phase domain signal mapping unit 4508. In the exemplary embodiment depicted in
In a similar manner, the operational principle of module 4500 may be effectively applied for any N-level signal. For example, after PAM-N mapping or coding in the electrical domain of an N-level signal 4502N by symbol coding unit 4506, the optical carrier may be modulated as an N-PSK signal 4504N by phase domain signal mapping unit 4508. That is, the corresponding drive signal for phase modulation by phase domain signal mapping unit 4508 will have N levels, and therefore be mapped as N phases of N-PSK optical signal 4504N. Module 4500 may then be operable for a respective phase modulator on the transmitter-side (e.g., phase modulator 4410,
In exemplary operation of transmitter 4600, input data is first encoded into multi-level signals (e.g., PAM-N signals 4502N) by multi-level signal coding unit 4506, and then mapped to corresponding phase values of optical signals (e.g., signals 4504N) in the phase domain by phase domain signal mapping unit 4508. As described above, the drive signal for phase modulation will have N levels, which is mapped as N phases of the resulting N-PSK optical signal 4504N. In practical implementation, the drive voltage may be determined by the voltage Vπ of phase modulator 4606, where Vπ is the required voltage for a π phase change of phase modulator 4606. Accordingly, in some embodiments, transmitter 4600 may optionally further include at least one electrical amplifier 4610 to boost the drive voltage of N-level signal 4504N to meet the voltage requirement of Vπ.
In some use cases of amplifier 4600 or amplifier 4612, it may be additionally desirable to implement further DSP processing at the receiver-side (e.g., DSP 4420,
In the exemplary embodiment, phase recovery is performed primarily at the end of processing, namely, by phase domain noise estimation and equalization unit 4712, after preliminary CD compensation, clock recovery, channel equalization, and/or frequency-offset estimation processing may have been performed. More specifically, unit 4712 implements process 4700 as into steps: (1) a phase recovery step S4716; and (2) a phase domain equalization step S4718.
In step S4716, phase recovery is performed to demodulate the multi-level signal. More particularly, phase recovery step S4716 includes a substep S4720, in which the dynamic phase noise caused by laser linewidth is estimated as φn(t). In substep S4722 the estimated dynamic phase noise value is removed by multiplying (e.g., by a multiplication unit) by the inverse value as exp[−jφn(t)]. In some embodiments of process 4700, the phase noise estimation of substep S4720 may be realized according through use of conventional phase noise estimation algorithms, such as training-based phase estimation algorithms, Viterbi-viterbi algorithms, and blind phase search (BPS) algorithms.
After the dynamic phase noise value is removed, phase domain equalization step S4718 of process 4700 is enabled to execute a mapping substep S4724, in which the signal optical phase may be mapped back to multi-level signals, and in substep S4726, a time-domain adaptive equalization is applied to the obtained PAM-N signal (in this example), Which is then equalized by a K-tap finite impulse response (FIR) digital filter to output the modulated multi-level signals 4714. In step S4728, a tap value may be updated by an error function after equalization. In some embodiments of step S4728, updating may be performed using such common adaptive equalization algorithms as least-mean-square (LMS), and recursive least squares (RLS) for filter tap value updating. In an alternative embodiment of step S4726, other adaptive equalization configurations be implemented, such as a Decision-Feedback Equalizer (DFE) for PAM-N signal equalization. In at least one embodiment, in order to hasten the convergence and tracking speed of the adaptive equalizer, a training sequence may be used for error function calculation.
In other cases, it may be desirable to perform pre-equalization at the transmitter-side (e.g., transmitter-side 4402,
Exemplary operation of transmitter 4800, is therefore substantially performed in accordance with the operation of transmitter 4600,
Transmitter 4800 therefore represents an exemplary implementation of transmitter 4600,
Process 4900 begins at step 4902, in which a multi-level signal is generated without pre-equalization, and then is modulated onto an optical phase. In an exemplary embodiment of step 4902, the generated multi-level signal is further mapped into the phase domain without pre-equalization and modulated onto the phase of optical carrier. In step 4904, the relevant receiver coherently detects the signal and performs DSP, including application of phase domain post-equalization. In step 4906, a pre-equalization filter or frequency response may be derived based on the K-tap adaptive filter of the phase-domain post equalization (e.g., in the receiver DSP). In step 4908, the receiver communicates the derived pre-equalization filter or frequency response back to the transmitter to perform pre-equalization. In step 4910, the transmitter generates the multi-level signal with pre-equalization according to the techniques described above, and modulates the pre-equalized multi-level signal onto the optical phase.
Process 4900 may thus be equally implemented with respect to the general transmitter configuration depicted in
Through various real-world testing simulations proof of the above-described concepts was verified by the present inventors. In one test example, the performance of a 12.5 GBaud polarization-multiplexed PAM-4 signal was evaluated, since the PAM-4 signal may be deemed equivalent to a 12.5 GBaud polarization multiplexed QPSK signal after phase domain modulation. Verification of these principles, including phase domain multilevel signal modulation and post-equalization with coherent detection, is described below with respect to
Accordingly, despite the overt existence of ISPI in the case of large phase fluctuations in four QPSK phases, considerable improvements may nevertheless be readily seen through a simple comparison of the respective constellations of
According to the innovative systems and methods provided herein, unique systems, apparatuses, and methods are provided for multi-level signal modulation and equalization in the phase domain for optical coherent communication networks, and particularly useful in the access network environment. The present embodiments further provide end-to-end system configurations for signal modulation, coherent detection, and equalization for multi-level signal transmissions. In exemplary embodiments, the multi-level signals are mapped into phase domain, and then modulated by phase modulator on a single polarization. The resulting optical signals may then be coherently detected, and then further processed by a DSP to recovery.
The present systems and methods further provide two individual types of phase-domain equalization methods for resolving the inter-symbol-phase-interface that plagues conventional phase modulation proposals for coherent optical access networks. The present techniques provide for both post-equalization at the receiver-side, and also pre-equalization at the transmitter-side. As further described above, these unique solutions provide yet a third, hybrid solution that uniquely implements a synergistic combination of both of the post-equalization and pre-equalization techniques described herein. That is, a unique post-equalization process is implemented in the receiver-side DSP to resolve ISPI problems downstream, and a digital phase-domain pre-compensation process may also be implemented to realize significant performance improvements at the transmitter-side.
Exemplary embodiments of optical communication systems and methods are described above in detail. The systems and methods of this disclosure though, are not limited to only the specific embodiments described herein, but rather, the components and/or steps of their implementation may be utilized independently and separately from other components and/or steps described herein. Additionally, the exemplary embodiments can be implemented and utilized in connection with other access networks utilizing fiber and coaxial transmission at the end user stage.
As described above, the DOCSIS protocol may be substituted with, or further include protocols such as EPON, RFoG, GPON, Satellite Internet Protocol, without departing from the scope of the embodiments herein. The present embodiments are therefore particularly useful for communication systems implementing a DOCSIS protocol, and may be advantageously configured for use in existing 4G and 5G networks, and also for new radio and future generation network implementations.
Although specific features of various embodiments of the disclosure may be shown in some drawings and not in others, such illustrative techniques are for convenience only. In accordance with the principles of the disclosure, a particular feature shown in a drawing may be referenced and/or claimed in combination with features of the other drawings.
Some embodiments involve the use of one or more electronic or computing devices. Such devices typically include a processor or controller, such as a general purpose central processing unit (CPU), a graphics processing unit (GPU), a microcontroller, a reduced instruction set computer (RISC) processor, an application specific integrated circuit (ASIC), a programmable logic circuit (PLC), a field programmable gate array (FPGA), a DSP device, and/or any other circuit or processor capable of executing the functions described herein. The processes described herein may be encoded as executable instructions embodied in a computer readable medium, including, without limitation, a storage device and/or a memory device. Such instructions, when executed by a processor, cause the processor to perform at least a portion of the methods described herein. The above examples are exemplary only, and thus are not intended to limit in any way the definition and/or meaning of the term “processor.”
This written description uses examples to disclose the embodiments, including the best mode, and also enables a person skilled in the art to practice the embodiments, including the make and use of any devices or systems and the performance of any incorporated methods. The patentable scope of the disclosure is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.
Claims
1. An optical injection locking based coherent optical transmitter for a coherent optical communications network, the coherent optical transmitter comprising:
- a master laser source configured to provide a low linewidth frequency channel as a master laser signal;
- a first coherent optical injection locking (COIL) subsystem including (i) a first slave laser configured for COIL with a master frequency of the master laser signal, (ii) an optical circulator configured to inject the master laser signal into a cavity of the first slave laser, and (iii) a full-field modulator configured to output a first modulated optical signal based on an output of the first slave laser routed through the optical circulator.
2. The transmitter of claim 1, wherein the first slave laser is configured to receive electrical in-phase data, and wherein the first COIL subsystem further includes a second slave laser configured to receive electrical quadrature data.
3. The transmitter of claim 2, wherein the first COIL subsystem further includes a first optical coupler disposed between (i) the optical circulator and the first slave laser, and (ii) the optical circulator and the second slave laser.
4. The transmitter of claim 3, wherein the first COIL subsystem further includes a first phase shifter disposed between the first optical coupler and the second slave laser.
5. The transmitter of claim 4, wherein the first phase shifter is configured to implement a 45-degree one-way phase shift.
6. The transmitter of claim 4, wherein the first COIL subsystem further includes a second phase shifter.
7. The transmitter of claim 6, wherein the first COIL subsystem further includes a second optical coupler disposed between the master laser source and the optical circulator and a third optical coupler disposed between the optical circulator and the first modulated optical signal, and wherein the second phase shifter is disposed between the second and third optical couplers.
8. The transmitter of claim 6, wherein the first COIL subsystem further includes a third slave laser configured for DC offset cancelation, and wherein the second phase shifter is disposed between the first optical coupler and the third slave laser.
9. The transmitter of claim 1, wherein the full-field modulator comprises a phase modulator disposed between the optical circulator and the first modulated optical signal.
10. The transmitter of claim 9, wherein the first slave laser is configured to receive electrical amplitude data, and wherein the phase modulator is configured to receive electrical phase data.
11. The transmitter of claim 1, further comprising a second COIL subsystem configured to output a second modulated optical signal, and a polarization beam combiner configured to combine the first modulated optical signal with the second modulated optical signal.
12. The transmitter of claim 11, wherein the first COIL subsystem is configured to output the first modulated optical signal for X-polarization data, and wherein the second COIL subsystem is configured to output the second modulated optical signal for Y-polarization data.
13. An optical communication network, comprising:
- an optical hub including: a master laser source; and a plurality of full-field coherent optical transmitters disposed proximate the optical hub, wherein each full-field transmitter of the plurality of full-field coherent optical transmitters (i) is configured to transmit a downstream coherent optical signal using a center frequency of a master signal from the master laser source, and (ii) includes a first slave laser injection locked to the center frequency of the master signal;
- a receiver; and
- an optical transport medium operably connecting the plurality of full-field coherent optical transmitters to the receiver.
14. The network of claim 13, wherein the optical hub further includes a power splitter operably connecting each full-field transmitter of the plurality of full-field coherent optical transmitters to the master laser source.
15. The network of claim 14, wherein the receiver comprises a plurality of fiber nodes, and wherein the optical transport medium comprises a plurality of optical fibers connecting the plurality of fiber nodes to a respective full-field transmitter of the plurality of full-field coherent optical transmitters.
16. The network of claim 15, wherein the network is configured for point-to-multipoint operation.
17. The network of claim 14, wherein the receiver comprises a plurality of receiving units, wherein the optical transport medium comprises a multi-mode fiber, wherein the plurality of full-field coherent optical transmitters operably connect to the multi-mode fiber through a mode multiplexer, and wherein the plurality of receiving units operably connect to the multi-mode fiber through a mode demultiplexer.
18. The network of claim 14, wherein the receiver comprises a plurality of receiving units, wherein the optical transport medium comprises a multi-core fiber, wherein the plurality of full-field coherent optical transmitters operably connect to the multi-core fiber through a core multiplexer, and wherein the plurality of receiving units operably connect to the multi-core fiber through a core demultiplexer.
19. The network of claim 13, wherein the master laser source is a master comb source, and wherein the optical hub further includes a first wavelength-division-multiplexing (WDM) demultiplexer operably connecting each full-field transmitter of the plurality of full-field coherent optical transmitters to the master comb source.
20. The network of claim 19, wherein the receiver comprises a plurality of receiving units, wherein the optical transport medium comprises an optical fiber, wherein the plurality of full-field coherent optical transmitters operably connect to the optical fiber through a WDM multiplexer, and wherein the plurality of receiving units operably connect to the optical fiber through a second WDM demultiplexer.
Type: Application
Filed: Dec 11, 2019
Publication Date: Sep 17, 2020
Inventors: Junwen Zhang (Broomfield, CO), Zhensheng Jia (Superior, CO), Luis Alberto Campos (Superior, CO), Haipeng Zhang (Broomfield, CO), Mu Xu (Broomfield, CO), Jing Wang (Broomfield, CO), Curtis Dean Knittle (Superior, CO), Chuang Zhou (Greenwood Village, CO)
Application Number: 16/711,293
