Apparatus And Method For Transmitting In An Optical Communication Network

Abstract

An apparatus and method for transmitting in an optical communication network. Optical transmissions, and in particular burst mode transmissions, are subject to wavelength drift. Described herein is a manner of executing optical transmissions while mitigating wavelength drift, in some cases without significantly reducing transmit power. Emphasis is placed on timely and efficient feedback so that adjustments may be made. A network node such as an ONT in a PON is provided with a light source and means for modulating an upstream optical transmission. A tap provides a portion of the generated (and perhaps modulated) light beam to a wavelength control loop, which in a preferred embodiment includes a channel selection filter and a wavelength discrimination filter. The wavelength generated by the light source is adjusted, if necessary, according to, at least in part, the output of the wavelength control loop.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This provisional application is related to and claims the benefit of U.S. Provisional Application No. 62/316,034, filed on 31 Mar. 2016, and of U.S. Provisional Application No. 62/437,437, filed on 21 Dec. 2016, and of U.S. Provisional Application No. 62/437,363, filed on 21 Dec. 2016, the entire contents of each of which are incorporated herein by reference.

BACKGROUND

Field of the Disclosure

The present disclosure relates generally to network communication and, more particularly, to a manner of transmitting an optical signal while controlling for wavelength drift, such as might be advantageously applied to burst mode transmission in an optical communication network.

Description of the Related Art

The following abbreviations are herewith expanded, at least some of which are referred to within the following description.

APC Automatic Power Control

BM Burst Mode

CO Central Office

DFB Distributed FeedBack

EDFA Erbium-Doped Fiber Amplifier

EML Electro-absorption Modulation Laser

GPON Gigabit PON

IEEE Institute of Electrical and Electronics Engineers

ITU International Telecommunication Union

NG-PON2 Next-Generation PON2

OLT Optical Line Terminal

ONT Optical Network Terminal

ONU Optical Network Unit

PON Passive Optical Network

RS Reed-Solomon

RSSI Received Signal Strength Indication

TEC Thermo-Electric Control

WDM Wavelength Division Multiplexer/demultiplexer

WM Wavelength Multiplexing/demultiplexing module

A PON (passive optical network) uses modulated optical signals transmitted over a fiber optic cable to communicate between two or more network nodes. It is “passive” because it typically requires no power input along the communication path between the transmitting and receiving nodes. In a common implementation one node is an OLT located in a service provider's central office communicating with a number of ONTs, each located at a subscriber premises. A splitter/combiner located between distributes the downstream signal from the OLT and combines upstream ONT transmissions onto a single fiber for the OLT.

In such a scenario, upstream and downstream optical transmissions often use different wavelengths to avoid interfering with one another. In addition, upstream transmissions are typically done according to a schedule established by the OLT. Each ONT buffers its upstream transmissions until sending them in a burst when its allocated time slot opens.

The light signals transmitted in a PON are often produced by lasers or similar devices. Lasers are well-suited to this purpose but do have some drawbacks. One disadvantage is that a laser generates heat as it operates and this heating may cause the wavelength to drift from its original setting. If the wavelength drifts too far it may drift out of its assigned optical channel and interfere with other signals or become more difficult to detect. These problems may be especially expected when an ONT laser is operating in burst mode in a multi-wavelength system.

These and other problems are addressed by the system, apparatus, and method of the present invention. Although provided as background for describing the present invention, no implication or admission is made or intended that the information herein is known to others besides the inventors.

SUMMARY OF EMBODIMENTS

The following presents a summary in order to provide a basic understanding of some aspects of the disclosed subject matter. This summary is not an exhaustive overview of the disclosed subject matter. It is not intended to identify key or critical elements of the disclosed subject matter or to delineate the scope of the disclosed subject matter. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.

In one aspect, a method for mitigating the effects of wavelength drift in an optical communication network includes generating a light beam along an optical channel, tapping the optical channel to direct a portion of the light beam toward a wavelength control loop comprising a channel selection filter and a wavelength discrimination filter, wherein the wavelength discrimination filter is configured to output an optical signal at a power commensurate with the wavelength of the generated light, and determining a wavelength adjustment as a function of at least output from the channel selection filter and the wavelength discrimination filter.

In this aspect, the method may be executed by a network node, for example, an ONT in a PON. The method is expected to be of particular advantage when performed during burst-mode upstream transmissions, and in a preferred embodiment, an ONT is so configured.

The method may further include converting the output of the wavelength discrimination filter from an optical signal to an electrical signal, and then digitizing the electrical signal. The digitized electrical signal may then be provided to a microcontroller. The microcontroller may use this signal to perform the wavelength adjustment. The wavelength adjustment determination may include calculating the wavelength of the generated light, for example by comparing a wavelength/power table with the power commensurate with the generated light. The table may be stored on a memory device of the network node. The wavelength adjustment may then include comparing the calculated wavelength to the assigned wavelength.

In some embodiments, the method may also include dividing the portion of the generated light beam such that a sub-portion propagates toward the wavelength control loop and a sub-portion propagates toward at least one secondary control loop. The wavelength adjustment may then also be a function of output from the at least one secondary control loop.

In some embodiments, the method also includes executing the determined wavelength adjustment. This may be done, for example, by adjusting the light source output power, by regulating an embedded heating element associated with the light source, by regulating a thermo-electric cooler associated with the light source, or by a combination of these techniques.

In another aspect, a network node, includes a light source for generating a light beam, a transmission channel for propagating light generated by the light source toward an optical fiber port for transmission, a tap for redirecting a portion of the light beam away from the transmission channel, a wavelength control loop for receiving at least a sub-portion of the redirected light-beam portion, the wavelength control loop comprising a wavelength discrimination filter configured to output an optical signal at a power commensurate with the wavelength of the generated light. The network node also usually includes a microcontroller configured to determine a wavelength adjustment as a function of at least the output of the wavelength control loop and a memory device. The network node may be, for example, an ONT in a PON.

In some embodiments, the light source is a direct modulation light source that generates a modulated light beam. In others, the network node includes an external modulator for modulating the generated light beam. The light source may be, for example, a DFB laser.

In some embodiments, the network node wavelength control loop also includes an O/E (optical/electrical) converter configured to convert the output of the wavelength discrimination filter from an optical signal to an electrical signal and an A/D (analog/digital) convertor configured to digitize the output of the optical/electrical converter and provide the digitized electrical signal to the microcontroller. The wavelength control loop further comprises a channel selection filter configured at least to select an assigned optical channel and remove artifacts from adjacent channels before the at least a sub-portion enters the wavelength discrimination filter.

In some embodiments, the network node also includes at least one secondary wavelength control loop. In this case, the network node may also include a power divider for redirecting a sub-portion of the redirected light-beam portion to the at least one secondary wavelength control loop.

Additional aspects of the invention will be set forth, in part, in the detailed description, figures and any claims which follow, and in part will be derived from the detailed description, or can be learned by practice of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings. The use of the same reference symbols in different drawings indicates similar or identical items.

FIG. 1 is a schematic diagram illustrating selected components of an exemplary PON in which some embodiments may be advantageously implemented.

FIG. 2 is a block diagram illustrating selected components of an exemplary ONT according to some embodiments.

FIG. 3 is flow diagram illustrating a method of wavelength-drift mitigation according some embodiments.

DETAILED DESCRIPTION

Various exemplary embodiments will now be described, and in general they are directed to an advantageous manner of providing wavelength-drift mitigation in an optical communication network, for example a PON (passive optical network). Note that the term “PON” is herein intended to be inclusive of all such networks, including for example XG-PON and NGPON2. And again, the solutions presented herein may also be employed in other types of optical networks.

FIG. 1 is a simplified schematic diagram illustrating selected components of a typical PON 100 in which embodiments of the present invention may be implemented. Note that PON 100 may, and in many implementations will, include additional components, and the configuration shown in FIG. 1 is intended to be exemplary rather than limiting. Five ONTs, 110a through 110n, are shown, although in a typical PON there may be many more or, in some cases, fewer. In this illustration, each of the ONTs are presumed to be located at and serving a different subscriber, perhaps at their respective residences or other premises. The ONT at each location is connected or connectable to a device of the subscriber, or to a network of such devices (not shown). The term “ONT” is herein intended to include ONUs and similar devices as well.

PON 100 also includes an OLT 120, which communicates directly or indirectly with various sources of content and network-accessible services (not shown) that are or may be made available to the subscribers associated with PON 100. As should be apparent, OLT 120 handles the communications between these other entities and the ONTs. OLT 120 may also be involved in regulating the PON and individual ONTs. As mentioned above, the OLT 120 is typically located at a service provider location referred to as a central office. The central office may house multiple OLTs (not separately shown), each managing their own respective PON.

OLT 120 is in at least optical communication with each of the ONTs in the PON 100. In the embodiment of FIG. 1, OLT is connected with the ONTs 110a through 110n via a (feeder) fiber optic cable 125 and (access) fiber optic cables 115a through 115n. In this PON, a single splitter 105 is used to distribute a downstream transmission so that each ONT receives the same downstream signal. In this case, each ONT extracts and uses only its own portion of the downstream transmission.

In other optical networks, the splitter may also separate the signal into different wavelengths, if used, associated with each or various of the respective ONTs. The splitter in a PON is typically a passive element requiring no power. The splitter may be located, for example, in a street-side cabinet near the subscribers it serves (FIG. 1 is not necessarily to scale). This cabinet or similar structure may be referred to as the outside plant. Note, however, that no particular network configuration is a requirement of the present invention unless explicitly stated or apparent from the context.

In the example of FIG. 1, the splitter may also serve as a combiner for combining upstream traffic from the ONTs 110a through 110n to the OLT 120. Upstream transmissions are generally at a different wavelength (or wavelengths) than those of downstream transmissions to avoid interference. In addition, each ONT may be assigned a separate time slot, that is, a schedule for making upstream transmissions. This means that ONT upstream transmissions are often bursty in nature as the data is buffered for transmitting when the assigned time slot opens.

Unfortunately, as alluded to above, using burst-mode transmissions frequently introduces the problem of wavelength drift, especially where constraints imposed on the network tend to be intolerant of significant drift. In most implementations, there is a tradeoff between high (or sufficient) power output and “tight” wavelength control.

Wavelength drift may be mitigated by improvements in ONT, OLT, or both. Described herein is a novel ONT for use in wavelength-drift mitigation. ONT modifications may or may not obviate the need for OLT feedback although such feedback may also represent an improvement in the system.

FIG. 2 is a block diagram illustrating selected components of an exemplary ONT 200 according to some embodiments. In this embodiment, ONT 200 includes a light source 205 for generating light signals for upstream transmission. In a preferred embodiment, light source 205 includes a laser such as a DFB or Fabry-Perot laser for generating the light. In some embodiments, a monitor diode may also be present integrated or associated with light source 205 to monitor optical output. In that case a monitor diode port may be present to direct a portion of the optical output for this purpose.

In the embodiment of FIG. 2, light from light source 205 is provided to modulator 210, where it is modulated to carry data that may be buffered, for example, in memory 265. Note that propagating-light channels are in FIG. 2 provided with arrowheads to indicate a direction of propagation. The modulator 210 may be, for example, an electro-absorption modulator. The electro-absorption modulator may in some implementations be physically separated from the light source 205 while it may be integrated onto the same structure in others. In yet other embodiments (not shown), the light source 205 incorporates a directly modulated laser diode, and the modulator 210 may therefore be unnecessary.

In the embodiment of FIG. 2, modulated light then passes through diplexer 215 and is transmitted upstream toward an OLT or similar device. As illustrated in FIG. 2, diplexer 215 also receives downstream transmissions and provides them to a receive train (not shown) of the ONT 200. In this way diplexer 215 enables both upstream and downstream transmissions to share the same optical (feeder) fiber and feeder-fiber port.

In the embodiment of FIG. 2, an upstream wavelength control loop 270 is also provided. In this preferred embodiment, a power divider 230 directs some of the light from light source 205, for example via a monitor diode port, to another feedback loop or loops, such as a laser APC or dual-loop controller, or both (not shown). A portion of the light is also directed from power divider 230 to wavelength control loop 270. In this embodiment, wavelength control loop 270 includes an SOA 235 for amplifying the received signal before passing it to channel selection filter 240. Note that in other embodiments, the SOA may be omitted.

In the embodiment of FIG. 2, it is understood there may be more than one upstream channel in use and the channel selection filter 240 filters out unwanted frequency components from other channels. In embodiments where an SOA such as SOA 235 is used, any ASE noise of the SOA is band limited by channel selection filter 240 in an attempt to maximize the signal to noise ratio of the wavelength control loop 270. The resultant stream from the channel selection filter 240 is provided to wavelength discriminator filter 245. The wavelength discriminator filter 240 is configured to convert the wavelength of the incoming signal to a corresponding power level.

In this embodiment, the output of wavelength discriminator filter 240 is provided to optical-electrical converter 250, where it is converted into an electrical signal. The electrical signal is then digitized by analog to digital converter 255 and the result provided to micro controller 260.

In the embodiment of FIG. 2, the microcontroller 260 is configured to calculate the wavelength under measurement by the wavelength control loop 270, for example by comparing measured power with a data table calibrated to reflect the output power corresponding to the discriminator-filter wavelength. In this embodiment the microcontroller 260 is further configured to compare the measured wavelength to the desired operating wavelength and to direct any necessary adjustments. This operation is described in more detail below. Note that strictly speaking, the microcontroller 260 may also be considered simply a part of the wavelength control loop 270.

In the embodiment of FIG. 2, microcontroller 260 may be implemented in hardware or in software executing on a hardware device, or a combination of both. Memory device 265 in this embodiment is a physical storage device that may in some cases operate according to stored program instructions. In any case, memory 265 is non-transitory in the sense of not being merely a propagating signal, unless explicitly recited otherwise in a particular embodiment. Memory 265 is used for storing, among other things, data such as a table (not separately shown) of operating wavelengths as well as stored program instructions for execution by processor 260. In this embodiment, upstream traffic to be transmitted is buffered until an appropriate time slot is encountered and this may be done in memory 260 or a separate memory device (not shown).

Note that FIG. 2 illustrates selected components of an embodiment and some variations are described above. Other variations are possible without departing from the claims of the invention as there recited. In some of these embodiments, illustrated components may be integrated with each other or divided into subcomponents. There will often be additional components in the ONT and in some cases components shown in FIG. 2 will not be present. The illustrations components may also perform other functions in addition to those described above.

FIG. 3 is flow diagram illustrating a method 300 of wavelength-drift mitigation according some embodiments. At START, it is presumed that the components necessary for executing this process are available and operational at least according to this embodiment. The process is here described in terms of an ONT in a PON with the understanding that it may also be implemented in similar optical communication networks having analogous devices. And although the method 300 is expected to have the greatest advantage when addressing upstream burst mode transmissions, it may be used in other scenarios as well. For the purpose of this description, it is also presumed that the ONT or similar device has or will have data to be transmitted.

The process in this embodiment then begins with determining a time slot for upstream transmission (step 305). The time slot will ordinarily be received in a schedule from a management node, such as an OLT in a PON. An upstream transmission wavelength is also determined (step 310) and it may also be indicated in a received schedule. When the indicated time lot arrives, a light source begins to generate light at the indicated wavelength (step 315).

In this embodiment, the data to be transmitted is then modulated (step 320) onto the light wave, for example using an electro-absorption modulator. The modulated light stream then passes through a diplexer and transmitted (step 325) on the optical network. Note that while an EML or other external modulator is presently preferred, it may in some implementations be omitted and in that case data modulation may occur in the originating light source. The light source may be a laser, for example a DFB.

In the embodiment of FIG. 3, light from the laser diode is also tapped and provided to a power divider (step 330), where at least some of the optical power is directed (step 335) to a secondary wavelength control loop. The secondary wavelength control loop may embody, for example, an APC module or dual loop controller or both. Note that used of the term “secondary” is used here for convenience in description and is not intended to imply either necessity or a lack of importance.

In the embodiment of FIG. 3, light from the divider is also provided to an SOA and amplified (step 340). The amplified light is then filtered (step 345) by a channel selection filter that selects the desired channel and removes any frequency components that may be present in adjacent channels.

Note that SOA amplification is optional and may not be necessary or desirable in all implementations. It may be included, for example, if it is desirable to minimize the tap ratio of the monitor diode tap or power divider. The channel selection filter may also be omitted in some implementations, for example if the monitor diode tap ratio is large enough and optical discriminator filter is small enough. When this can be successfully done it has the advantage of cost savings.

In the embodiment of FIG. 3, the filtered laser diode signal is then converted to a specific optical power level (step 350) in a wavelength discriminator filter. The output of the filter is then converted to an electrical signal (step 355) and digitized (step 360) in an A/D converter.

In this embodiment, the actual wavelength of the laser diode output is then calculated (step 365), for example by comparing the measured power with a calibrated data table. The data table may be populated with the output power for each channel corresponding to the wavelength discriminator filter wavelength.

The measured wavelength is then compared (step 370) to the desired operating wavelength, for example in a microcontroller of the ONT. Any necessary wavelength adjustment is then determined (step 375). As implied in FIG. 3, this may also take into account any determinations made by a secondary wavelength control. If the light source is operating at the desired wavelength corresponding to its assigned channel, of course, no actual adjustment needs to be taken. If it is not, then a wavelength adjustment protocol is executed (step 380).

In one embodiment, for example, the wavelength adjustment protocol includes adjusting the laser output power. Wavelength drift caused by heating may be mitigated by reducing power levels as permitted by the PON operating requirements. In another embodiment, where an embedded resistor or heating element is present in the laser die, it may be adjusted to promote heating or cooling, as needed to mitigate wavelength drift. In yet another embodiment, where a TEC is present, it may be adjusted to promote heating or cooling, as needed to mitigate wavelength drift.

Finally, in yet another embodiment both the die-embedded resister or heating element and a TEC may be adjusted to promote heating or cooling, as needed to mitigate wavelength drift. As one example, The TEC may be set to maintain a temperature below the required temperature for the desired wavelength. The heating element may then be operated in a pulsed mode such that the laser temperature is on average maintained at the required temperature to maintain the desired wavelength. In this case, the short term drift may be set by the sum of the TEC and resistor or heating element, while the later may be adjusted when the burst begins as laser self-heating maintains the proper temperature.

Note that the sequence of operation illustrated in FIG. 3 represents an exemplary embodiment; some variation is possible within the spirit of the invention. For example, additional operations may be added to those shown in FIG. 3, and in some implementations one or more of the illustrated operations may be omitted. In addition, the operations of the method may be performed in any logically-consistent order unless a definite sequence is recited in a particular embodiment.

In some embodiments, certain aspects of the techniques described above may be implemented by one or more processors of a processing system executing software. The software comprises one or more sets of executable instructions stored or otherwise tangibly embodied on a non-transitory computer readable storage medium. The software can include the instructions and certain data that, when executed by the one or more processors, manipulate the one or more processors to perform one or more aspects of the techniques described above. The non-transitory computer readable storage medium can include, for example, a magnetic or optical disk storage device, solid state storage devices such as Flash memory, a cache, random access memory (RAM) or other non-volatile memory device or devices, and the like. The executable instructions stored on the non-transitory computer readable storage medium may be in source code, assembly language code, object code, or other instruction format that is interpreted or otherwise executable by one or more processors.

A computer readable storage medium may include any storage medium, or combination of storage media, accessible by a computer system during use to provide instructions and/or data to the computer system. Such storage media can include, but is not limited to, optical media (e.g., compact disc (CD), digital versatile disc (DVD), Blu-Ray disc), magnetic media (e.g., floppy disc, magnetic tape, or magnetic hard drive), volatile memory (e.g., random access memory (RAM) or cache), non-volatile memory (e.g., read-only memory (ROM) or Flash memory), or microelectromechanical systems (MEMS)-based storage media. The computer readable storage medium may be embedded in the computing system (e.g., system RAM or ROM), fixedly attached to the computing system (e.g., a magnetic hard drive), removably attached to the computing system (e.g., an optical disc or Universal Serial Bus (USB)-based Flash memory), or coupled to the computer system via a wired or wireless network (e.g., network accessible storage (NAS)).

Note that not all of the activities or elements described above in the general description are required, that a portion of a specific activity or device may not be required, and that one or more further activities may be performed, or elements included, in addition to those described. Still further, the sequence in which activities are listed are not necessarily the order in which they are performed. Also, the concepts have been described with reference to specific embodiments. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the present disclosure as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present disclosure.

Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any feature(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature of any or all the claims. Moreover, the particular embodiments disclosed above are illustrative only, as the disclosed subject matter may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. No limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope of the disclosed subject matter. Accordingly, the protection sought herein is as set forth in the claims below.

Although multiple embodiments of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it should be understood that the present invention is not limited to the disclosed embodiments, but is capable of numerous rearrangements, modifications and substitutions without departing from the invention as set forth and defined by the following claims.

Claims

1. A method for mitigating the effects of wavelength drift in an optical communication network, comprising:

generating a light beam along an optical channel;

tapping the optical channel to direct a portion of the light beam toward a wavelength control loop comprising a channel selection filter and a wavelength discrimination filter, wherein the wavelength discrimination filter is configured to output an optical signal at an amplitude proportional to the wavelength of the generated light; and

determining a wavelength adjustment as a function of at least output from the channel selection filter and the wavelength discrimination filter.

2. The method of claim 1, wherein the method is executed by an ONT.

3. The method of claim 2, wherein the ONT is configured to operate the wavelength control loop during upstream transmissions.

4. The method of claim 1, further comprising converting the output of the wavelength discrimination filter from and optical signal to an electrical signal and digitizing the electrical signal.

5. The method of claim 4, further comprising providing the digitized electrical signal to a microcontroller.

6. The method of claim 5, wherein determining the wavelength adjustment is performed by the microcontroller.

7. The method of claim 6, wherein determining the wavelength adjustment comprises calculating the wavelength of the generated light.

8. The method of claim 6, wherein calculating the wavelength of the generated light comprises comparing a wavelength/power table with the power commensurate with the generated light.

9. The method of claim 7, wherein determining the wavelength adjustment comprises comparing the calculated wavelength to the assigned wavelength.

10. The method of to claim 9, wherein the wavelength adjustment is a function of at least output from the at least one secondary control loop.

11. The method of claim 1, further comprising executing the determined wavelength adjustment.

12. The method of claim 11, wherein executing the determined wavelength adjustment comprises adjusting the light source output power.

13. The method of claim 11, wherein executing the determined wavelength adjustment comprises regulating an embedded heating element associated with the light source.

14. A network node, comprising:

a light source for generating a light beam;

a transmission channel for propagating light generated by the light source toward an optical fiber port for transmission;

a tap for redirecting a portion of the light beam away from the transmission channel;

a wavelength control loop for receiving at least a sub-portion of the redirected light-beam portion, the wavelength control loop comprising a wavelength discrimination filter configured to output an optical signal at a power commensurate with the wavelength of the generated light;

a microcontroller configured to determine a wavelength adjustment as a function of at least the output of the wavelength control loop; and

a memory device.

15. The network node of claim 14, wherein the light source is a direct modulation light source that generates a modulated light beam.

16. The network node of claim 14, further comprising a modulator external to the light source for modulating the generated light beam.

17. The network node of claim 14, wherein the network node is an ONT.

18. The network node of claim 14, further comprising at least one secondary control loop.

19. The network node of claim 14, wherein the wavelength control loop further comprises an optical/electrical converter configured to convert the output of the wavelength discrimination filter from an optical signal to an electrical signal.

20. The network node of claim 14, wherein the wavelength control loop further comprises and analog/digital convertor configured to digitize the output of the optical/electrical converter and provide the digitized electrical signal to the microcontroller.

21. The network node of claim 14, wherein the wavelength control loop further comprises a channel selection filter configured at least to select an assigned optical channel and remove artifacts from adjacent channels before the at least a sub-portion enters the wavelength discrimination filter.