Abstract: In a PON system in which communication is performed at a plurality of types of transmission rate (L, M, and H) in an upstream direction from a plurality of terminals connected to a station apparatus through optical fibers, within a discovery period for allowing an unregistered terminal to be recognized by station apparatus, the terminal makes a discovery response at one type of transmission rate (L). With this configuration, station apparatus can wait for a discovery response with a receive function being allowed to support transmission rate (L).

Abstract: A method for providing access to a passive optical network for services to homes or businesses from two or more telecommunications service providers and a billing means is described. A first service provider connects to a point of presence at one side of passive optical network. The provider transmits the appropriate services through this network to an authorization receiver. The authorization receiver is used to receive a periodic authorization code from the network provider to enable the appropriate services from the service provider to be transmitted to a subscriber at a home or business. The authorization receiver enables an optical fiber path to be established for the services to flow to and from the home or business.

Abstract: A transmission speed of a protection (backup) line using 1G-OLT unit and 1G-ONU unit is set to be lower than a transmission speed of a working (primary) line using 10G-OLT unit and 10G-ONU unit. A priority and band of each communication service in ONU are preset individually and separately every LLID for a working (primary) line and a protection (backup) line. When bands are allocated to the protection (backup) line, the band allocation is performed so as to secure a minimum number of bands of each communication service. A monitoring controller refers to a predetermined band information table according to an operation state of the working (primary) line or the protection (backup) line with respect to a downstream signal packet.

Abstract: The present disclosure is directed to a passive optical avionics network system and method. A avionics network system may comprise: (a) a passive optical network, the passive optical network comprising an optical repeater; and (b) an avionics module operably coupled to the passive optical network. An integrated modular avionics (IMA) system may comprise: (a) a line-replaceable unit (LRU), the LRU comprising: (i) a processing unit; and (ii) an optical line terminal (OLT); (b) an optical repeater; (c) at least one optical network unit (ONU); and an avionics module operably coupled to the at least one ONU. A method for avionics network communication may comprise: (a) receiving optical avionics data signals; (b) monitoring the optical avionics data signals for compliance with a communications protocol; and (c) regulating transmission of the optical avionics data signals according to compliance with the communications protocol.

Abstract: In a method of allocating a bandwidth of a passive optical network, downward data are transmitted by varying a wavelength based on a wavelength division method and upward data are transmitted using a time division method. Thereby, by efficiently allocating a network bandwidth, data can be transmitted and by realizing statistical multiplexing, transmission efficiency can be improved.

Abstract: A system and computer readable medium for a burst mode optical receiver that enables an optical receiver to receive signals from a plurality of optical network units at different optical power levels. In an exemplary embodiment, the system may include a memory that stores a received signal strength indication, and a media access controller communicably coupled to the memory. The media access controller receives a received signal strength indication of an upcoming data stream to an optical network unit, and asserts a signal control voltage during a guard time to an optical receiver to optimize reception of incoming data streams of the optical network unit based upon the received signal strength indication received from the upcoming data stream.

Abstract: A method and an apparatus for implementing a storage function in a passive optical network (PON) system. On the basis of a LINK ID held by the data, data requiring storage, e.g. data making exclusive use of a wide bandwidth such as audiovisual data and large-volume data packages, are determined among all the data sent to an optical network unit. These data requiring storage are stored in a storage device installed in the optical network unit. In case the user requests data stored in the storage device, the stored data are transferred directly to the user, without any need to request and send these data for the second time via an IPTV server and the core network. It is possible to avoid duplicate transfers of data, and in particular to avoid duplicate transfers of data making exclusive use of a comparatively wide bandwidth such as audiovisual data and large-volume data packages, to improve the utilization factor of the downlink bandwidth, and to reduce flow congestion and data delays.

Abstract: An apparatus includes an electromagnetic signal device, a medium, and a sound wave generator. The sound wave generator includes a carbon nanotube structure. The electromagnetic signal device transmits an electromagnetic signal to the carbon nanotube structure. The carbon nanotube structure converts the electromagnetic signal into heat. The heat transfers to the medium and causes a thermoacoustic effect.

Abstract: The present invention is a fiber optic data network, such as a passive optical network (PON), which includes a primary sub-network and a secondary sub-network. The primary sub-network may include a primary optical line terminal communicatively coupled to a plurality of primary optical network units. The secondary sub-network may include a secondary optical line terminal communicatively coupled to a plurality of secondary optical network units and to an intermediate optical network unit. The secondary sub-network may be communicatively coupled to the primary sub-network via the intermediate optical network unit, thereby allowing the PON to be configured so that the primary optical line terminal and the secondary optical line terminal are cascaded.

Abstract: A passive optical network is provided, which includes an optical central office connected to a line termination device by a branch of the network including a passive amplification medium. The central office is adapted to send/receive a first data optical signal and has a first amplifier for sending a second amplification optical signal. The second signal exciting the amplification medium to amplify the optical power of an optical signal. The line termination device is adapted to receive the first optical signal, modulate the second amplification optical signal; and inject the modulated second signal into the network.

Abstract: In a PON system in which communication is performed at a plurality of types of transmission rate (L, M, and H) in an upstream direction from a plurality of terminals connected to a station apparatus through optical fibers, within a discovery period for allowing an unregistered terminal to be recognized by station apparatus, the terminal makes a discovery response at one type of transmission rate (L). With this configuration, station apparatus can wait for a discovery response with a receive function being allowed to support transmission rate (L).

Abstract: Examples of apparatus and methods are provided for controlling a bias in an optical modulator. An exemplary apparatus may comprise an optical modulator operable to modulate an optical signal. The optical modulation apparatus may comprise a photodetector disposed to receive at least a portion of the modulated optical signal. The optical modulation apparatus may comprise a bias controller coupled to both the optical modulator and the photodetector. The bias controller may be configured to receive a dither signal and to produce a bias feedback signal for the optical modulator. The bias feedback signal may be based on a ratio between an odd order harmonic signal of the modulated optical signal and an even order harmonic signal of the modulated optical signal.

Abstract: A removable optical interface module is used to allow an optical transmission apparatus to flexibly respond to a change from a type of service to another that uses a different light wavelength. To this end, the optical transmission apparatus is removably provided with an optical interface module storing attribute information and includes a plurality of service implementation units implementing services using signals transmitted and received through the optical interface module. An operation selecting unit is provided which selects and causes a service implementation unit among the plurality of service implementation units that implement a service corresponding to the attribute information to operate.

Abstract: An optical interconnect includes a reflective body having a plurality of faces, where the body is translatable in a plane; and an optical receiver configured to receive optical energy reflected by at least one of the faces.

Abstract: The present invention relates to an optical network terminal (ONT), a method for configuring rate limiting attributes of ports, and a method for processing packets. The ONT includes a passive optical network (PON) protocol processing module, and a user network interface (UNI) module, which are connected through an internal interface. The ONT also includes a port rate limiting module connected to a UNI. The port rate limiting module stores rate limiting attributes, and the ONT uses these attributes to control the traffic of the UNI. The port rate limiting attributes are configured for the ONT through an ONT management and control interface (OMCI) message of an optical line terminal (OLT). In this way, when the ONT receives data from the UNI, it can control the traffic of the UNI according to the port rate limiting attributes.

Abstract: This disclosure is directed to devices and methods for facilitating the upgrade of optical networks. An optical network terminal (ONT) that terminates an optical fiber link of an optical network comprises two or more transport engines that each converts data transmitted via different transports to data corresponding to a service. For example, the ONT may include a first transport engine and a second transport engine. The first transport engine converts data received over the optical network via a first transport, e.g., a legacy transport, into data corresponding to a service for one or more subscriber devices. The second transport engine converts the data received over the optical network via a second transport, e.g., a next generation transport, into the data corresponding to the service for the subscriber devices. The ONT is selectively configurable to select one of the first and second transport engines, thereby making the ONT upgrade-resilient.

Abstract: A passive optical network system (PON) has a plurality of OLTs and ONUs with different transmission rates. OLTs with different transmission rates share information of priority frames and destinations, and determine timing for frame transmission to ONUs so that the signal from each of the OLTs does not collide when multiplied in a splitter. OLTs transmit the data to the ONU as a burst signal to prevent the signals with different rates from colliding. ONU acquires the information of the following burst frames. ONU receives only the signal addressed to the own ONU or with the transmission rate of own ONU, therefore errors in ONUs can be avoided. OLT receives only the signal with the transmission rate of own OLT from ONUs based on the transmission timing from the ONUs shared by the line terminators, errors in OLTs can be avoided.

Abstract: An optical system and method includes a source-free optical network unit coupled to an optical fiber for receiving a centralized lightwave carrier signal with downstream data over the optical fiber. The optical network unit includes a coupler configured to split the original carrier signal into a first path and a second path. The first path includes an optical filter configured to reduce fading effects of the carrier signal. The second path includes a modulator configured to remodulate the centralized lightwave carrier signal with upstream data to produce an upstream data signal for upstream transmission.

Abstract: Methods and apparatuses for controlling transmission of converged data packets and for media access through a single next generation access (NGA) passive optical network (PON) which can coexist with EPON and GPON based systems and can interoperate with a 10 GEPON. A converged data packet is transmitted between a first node and a second node of the NGA network under NGA management control. The converged packet has a format which unifies a GEM header with the 10 GEPON preamble header and certain fields replaced in a EPON packet format to accommodate information corresponding to the preamble elements of a GEM packet. The converged data can be encoded in the line code of the 10 GEPON protocol, allowing use of a control protocol based either on MPCP or GTC for the NGA. Node apparatuses include NGA elements which enable preparation, encoding/decoding and transmission of converged packets.

Abstract: In a passive optical network system in which communication signals from a master station to a plurality of subsidiary stations are multiplexed by time division and transmitted, after converting (modulating) data with a high transmission speed to a low transmission speed, a base speed, in the master station, a header including an identifier destined for each subsidiary station is attached to the data with a low transmission speed and the converted data, respectively, the data and the header being multiplexed by time division inside a frame and transmitted to the subsidiary stations at the base speed. In the subsidiary station having received the concerned frame, only the data destined for the station itself is read, on the basis of the identifier included in the header, and by means of demodulation, the converted data are reconverted to the original high speed data and supplied to a user terminal.

Abstract: A method for data transfer in an optical network, including: amplitude modulating, by a central office, an optical carrier by a downstream data with a first rate to generate a downstream optical signal carrying the downstream data; sending the downstream optical signal to a user terminal; obtaining, by the user terminal, the downstream data from the downstream optical signal; amplitude modulating, by the user terminal, the downstream optical signal by an upstream data with a second rate to generate an upstream optical signal, wherein the ratio of the first rate to the second rate is greater than 1; sending the upstream optical signal to the central office; and receiving, by the central office, the upstream optical signal, and obtaining, by the central office, the upstream data from the upstream optical signal. Furthermore, the present invention discloses a system and device for data transfer in the optical network.

Abstract: A two-way optical amplifier system amplifies upstream and downstream optical signals in a passive optical network (PON). The downstream optical amplifier system includes an optical amplifier (EDFA), configured and arrange to amplify the downstream optical transport signal.

Abstract: In a passive optical network system, in order that by measuring a transmission distance and a transmission time between an OLT and an ONU in operation of the system, if the distance or the time is short, communication is conducted at a high transmission speed to increase a capacity of user; if the distance or the time is long, communication is conducted at a low transmission speed to increase the capacity of user communicating in a state in which a predetermined quality is secured, a main station includes a signal communication circuit to communicate with subsidiary stations at a first or second transmission speed and a controller to measure a transmission distance or a transmission time between the main station and each subsidiary station. Based on a result of the measurement, the controller selects a transmission speed for communication with the subsidiary station.

Abstract: Provided is a passive optical network (PON) based on a reflective semiconductor optical amplifier (RSOA). In the PON, seed-light-injection RSOAs are used in an optical line terminal (OLT) to achieve the color-less management of the wavelengths of OLT optic sources, and wavelength reuse RSOAs are used to achieve the color-less management of the wavelengths of ONTs. Therefore, problems related to ONT wavelength management can be eliminated by the wavelength reuse RSOAs, and problems related to OLT wavelength management can be eliminated by the seed-light-injection RSOAs.

Abstract: A method includes generating an optical millimeter wave signal for modulation of a first data signal, and deriving from the generated optical millimeter wave signal a subsequent light source for modulation of a second data signal. More specifically the generating includes modulating a light wave to a multiple of a frequency of an oscillating signal. Alternatively, the generating includes modulating a data signal mixed with an oscillating signal to a multiple of a frequency of the oscillating signal. The deriving includes modulating a frequency component filtered from a data modulation of the generated optical millimeter wave signal.

Abstract: A fiber optic network has alarmed fiber optic lines in the cables connecting a secured junction box to plural user lock boxes. An outgoing alarm line and return alarm line in each cable connect the junction box to each user box. The outgoing alarm line is looped to the return alarm line inside the user lock box. The return alarm line is looped to the outgoing alarm line of a different cable inside the junction box to interconnect a plurality of alarm lines passing through a plurality of user boxes. A detector detects an alarm signal in the connected alarm lines to trigger an intrusion alarm.

Abstract: A synchronous optical bus system for communication between computer system components is described. In one example, the optical bus system is used for communication between a memory controller and memory devices optically coupled to an optical interconnect. Optical bus interface units couple the components to the optical interconnect and are arranged on the optical interconnect in order that a sum of an optical path length from a controller component to each computer system component and from each computer system component to the controller component is the same for all the coupled computer system components. A synchronous protocol is used for communication between the components.

Abstract: A digital camera 800 receives from lighting units 120a-120l lights temporarily changing in luminance and produces a coordinates data list 8031 of the lighting units. Decoding unit 804 reads change in luminance at each luminance-point coordinates of the lighting unit to decode information. It is judged based on the decoded information of a data format 900, whether plural luminance-point coordinates having the same guidance point ID information 902 are found. When the plural luminance-point coordinates having the same guidance point ID information are found, a location of the plural luminance-point coordinates is analyzed to draw and display a circumscribed polygon having its vertices at these coordinates on a through image in an overlapping manner. A hatched area is set inside the circumscribed polygon and the decoded information is displayed within the hatched area.

Abstract: Systems and methods for encoding information in the topology of superpositions of helical modes of light, and retrieving information from each of the superposed modes individually or in parallel. These methods can be applied to beams of light that already carry information through other channels, such as amplitude modulation or wavelength dispersive multiplexing, enabling such beams to be multiplexed and subsequently demultiplexed. The systems and methods of the present invention increase the number of data channels carried by a factor of the number of superposed helical modes.

Abstract: Methods and apparatus are described for an n-Way, Serial-Channel interconnect. An apparatus includes a communications network interconnect including an input layer including a plurality of input channels; a multicast channel branching fabric coupled to the input layer; and a modular output layer coupled to the multicast channel branching fabric layer, the modular output layer including a plurality of individual serial data channels; and a plurality of sets of endpoints, each set of endpoints coupled to one of the plurality of individual serial data channels.

Abstract: A system for passively scrambling and unscrambling a, pulse optical signal transmitted through a multi-mode optical fiber is provided. The system includes a scrambling unit connected between a signal receiving end of said transmission fiber and an optical signal source that includes an optical fiber which creates a differential delay between two groups of optical modes of the signal that is at least one bit period long such that said optical signal is passively scrambled, and an unscrambling unit connected to a signal transmitting end of said transmission fiber having an optical fiber that counteracts said differential delay between said two groups of optical modes of the signal such that said optical signal is passively unscrambled.

Abstract: A system to convert upstream burst mode data into continuous mode data in a passive optical network (PON) is provided herein. The system includes a burst mode Serializer/Deserializer (SerDes) that recovers a clock and burst mode data from an Optical Network Unit (ONU). The burst mode unit recovers the burst mode data based on a start time of burst mode data transmission by the ONU and a round-trip time between the ONU and an Optical Line Terminal (OLT). The system further includes a continuous mode SerDes that is coupled to the burst mode SerDes. The continuous mode SerDes is configured to receive the recovered clock and recovered burst mode data from the burst mode SerDes and convert the burst mode data into continuous mode data by buffering and padding the burst mode data based on the recovered clock. The continuous mode Serdes is configured to transmit the continuous mode data to the OLT.

Abstract: A system for transporting signals through PON, including a device for transporting optical network signals and a device for transporting optical line signals, wherein: the device for transporting optical network signals is adapted to map upstream optical signals with a PON frame format to upstream optical channel data unit signals, to convert into upstream signals with an optical channel transport unit format, and to transport to the device for transporting optical line signals through OTN; and to perform corresponding downstream processing; the device for transporting optical line signals is adapted to convert the upstream signals into upstream optical channel data unit signals, to de-map to upstream optical signals with a PON frame format, and to transport to an optical line terminal; and to perform corresponding downstream processing. The invention further discloses relevant devices and methods.

Abstract: A system, method, and computer readable medium for passive optical network media access controller assisted clock recovery which enables an optical receiver to recover a clock phase of an incoming data stream from an optical network unit comprises collecting phase data from a previous data stream clock recovery for an optical network unit, and providing the collected phase data as a starting value phase clock for subsequent incoming data streams for the optical network unit.

Abstract: The present invention discloses a low-cost light source for optical transmission systems and optical networks based on wavelength-division multiplexing (WDM) technology. A light source in accordance with the present invention is implemented by externally injecting a narrow-band incoherent light into a Fabry-Perot laser diode (F-P LD). After injection of narrow-band incoherent light, the output of F-P LD becomes wavelength-selective rather than multi-mode and the output wavelength of F-P LD coincide with the peak wavelength of the injected incoherent light. Multi-channel WDM light sources according to the present invention can be implemented using a single broadband incoherent light source and plurality of F-P LDs. An optical transmission system for upstream signal transmission in an passive optical network using the light source according the present invention is also disclosed.

Abstract: A device may include a first network module to capture network data at a first location in a network and a second network module to capture network data at a second location in the network different from the first location in the network. The device may include a control module to receive control commands relating to the first network module and the second network module. The control module may forwarded the control commands to the first network module and the second network module. The control module may receive the captured network data from the first network module and the second network module.

Abstract: One embodiment of the disclosures made herein is a method for activating services in a Passive Optical Networking (PON) system. In accordance with such an embodiment, an Optical Line Terminal (OLT) within the PON system is pre-provisioned with PON services and with at least a portion of information required for facilitating automatic activation of the PON services. Pre-provisioning is defined herein to mean that such provisioning is performed prior to installing an ONT (i.e., a network interface device) that will serve such to services at the service subscriber's site and that is controlled by the OLT. After installation of the ONT is complete, the PON services served by the ONT are automatically activated using at least a portion of the pre-provisioned information and information discovered about the ONT during a post-installation discovery process.

Abstract: A system, method, and computer readable medium for burst mode optical receiver that enables an optical receiver to receive signals from a plurality of optical network units at different optical power levels comprising, collecting received signal strength indication information from a previous data stream of an optical network unit, and adjusting an optical receiver to optimize reception of subsequent incoming data streams of the optical network unit based upon received signal strength indication information received from the previous data stream. The signal strength indication may be collected from an upcoming data stream and a signal control voltage asserted during the guard time of the upcoming data stream to optimize reception of the incoming data stream.

Abstract: Provided are a hybrid optical transceiver module having an optical amplifier packaged thereto for outputting a high-power optical signal to remove problems regarding narrow emission angle and optical alignment, and a passive optical network (PON) system having an improved optical network terminal (ONT) accommodation capability using the hybrid optical transceiver module. The hybrid optical transceiver module includes a first package in which an LD (laser diode) is packaged, and a second package in which SOA (semiconductor optical amplifier) and a PD (photo diode) are packaged. The first and second packages are coupled to be one package so as to output a high-power optical signal.

Abstract: Provided is an upstream source light generator of a passive optical network (PON) system. The upstream source light generator includes an amplification part configured to amplify injection light, and a reflection part configured to receive the amplified injection light and generate reflection light by reflecting the amplified injection light with different optical delays according to wavelengths of the amplified injection light.

Abstract: Disclosed are an optical line terminal in a gigabit passive optical network and a method for transmitting a broadcast frame using the same. The optical line terminal allocates second port identifiers to optical network units such that a broadcast frame is prevented from being retransmitted to an optical network unit which has transmitted the broadcast frame to the optical line terminal.

Abstract: In an optical transmission device, when wavelength division multiplexed light formed from signals of a plurality of wavelengths are received, an optical signal of one wavelength comprised in this wavelength division multiplexed light is planarized and non-signal light of the same wavelength is reproduced. The reproduced light is modulated with a transmission data string, an optical signal is generated, and a wavelength division multiplexed light comprising the generated optical signal is transmitted.

Abstract: A system for monitoring ancillary elements of an electric power distribution network, includes an optical fiber path associated with the ancillary elements to be monitored, respective optical branches branching from the optical fiber path, wherein each optical branch includes at least one passive optical attenuator operatively coupled to, and having an attenuation adapted to change in response to a change in operating conditions of the respective ancillary element, and an optical reflector; an optical radiation source adapted to inject optical radiation into the optical fiber path; and an optical receiver adapted to detect back-reflected optical radiation reflected by the optical reflector; the monitoring system being further adapted to recognize a position of at least one of the ancillary elements based on a characteristic of the back-reflected optical radiation.

Abstract: System and techniques for transmitting information from a remote information source use a reflector for reflecting a laser beam or other light source back to its originating location. A reflector coupled to an information source receives a laser beam from a laser source. A movement of at least one surface of the reflector is controlled to modulate the laser beam, and the modulated laser beam is reflected toward the laser source. The modulated laser beam can be received by a receiver at or near the laser source to detect and decode data embedded in the modulated laser beam.

Abstract: A method is provided for managing connections in an optical access network. Also provided are a corresponding platform, exchange, network and computer software product. The method manages connections of a plurality of user modules to a plurality of transmission modules of an exchange in an optical access network. The management method includes a dynamic re-arrangement of the connection of at least one of the user modules to at least one of the transmission modules according to at least one predetermined optimization criterion.

Abstract: Various methods, systems, and apparatuses in which a wavelength-division-multiplexing passive-optical-network includes a first broadband light source and a second broadband light source. The first broadband light source supplies an optical signal containing a first band of wavelengths to a first plurality of optical transmitters. The second broadband light source supplies an optical signal containing a second band of wavelengths to a second plurality of optical transmitters. A fiber is used for bi-directional transmission of optical signals in at least two different wavelength bands.

Abstract: One embodiment of the present invention provides a system that facilitates transmission control in an Ethernet passive optical network, which includes a central node and at least one remote node, and wherein a remote node implements a data-link layer and a physical layer. During operation, the system starts by receiving, at the physical layer of a remote node, a word which is communicated from the data-link layer of the remote node, wherein the word may be a data word or an idle word. The system then delays the word for a pre-determined amount of time before allowing the word to be transmitted by a transmitter, thereby providing time for turning the transmitter on or off. The system also turns the transmitter on or off based on the content of the received words.

Abstract: A system, apparatus and method are described for controlling the gain across one or more amplifier nodes within an optical span. In one embodiment, a fast local amplifier constant gain control loop is provided that maintains a constant gain across an amplifier node for each of the channels within an optical signal. A slow link level gain setting control loop is provided to set and/or adjust the target gain on the amplifier node(s). A gain adjust sequence is performed by the slow link level gain setting control loop to adjust the target gain(s) in response to various events and mechanisms. A “time of flight” protection method is also provided to ensure consistency between the fast local amplifier gain control loop and the slow link level gain setting control loop.

Abstract: Disclosed herein is a shared Local Area Network (LAN) emulation method and apparatus. The method includes the following four steps. At the first step, a Logical Link Identifier (LLID) management table is set up to assign unique LLIDs to a plurality of Optical Network Units (ONUs) and manage the assigned LLIDs so as to identify the plurality of ONUs connected to a single Optical Line Terminal (OLT). Thereafter, a MAC address table is set up for the LLIDs to learn MAC addresses of the ONUs. Thereafter, the unique LLIDs are assigned to ONUs when the ONUs request registration from the OLT. Finally, data frames, which are received by a Shared LAN Emulation (SLE) layer of the OLT, are bridged using the LLIDs, VIDs of Virtual LANs to which the ONUs belong and destination MAC addresses of the data frames so as to provide a single matched port between a Logical Link Control (LLC) layer and a MAC layer of the OLT.

Abstract: The objective of the invention is to devise a method for establishing a subscriber connection and a system applying the method. According to the method, a digital subscriber connection consisting of a composition of an optical fiber and a metallic pair cable is implemented by transporting a digital information stream in two directions over an optical fiber (2) between a central site and equipment (103) which is connected to the metallic pair cable (4) of each subscriber. According to the invention, the information streams transmitted and received over the pair cables are multiplexed onto the optical fiber by using a multiplexing method of optical signals, whereby the information streams originating from different subscribers are combined onto the fiber by means of passive optical elements and whereby the information streams transmitted from the central site are distributed to each subscriber-specific optical-to-electric conversion element (105) by means of passive optical elements.