Abstract: A method and apparatus for regulating rogue behavior in optical transmission devices. The apparatus, for example, may be implemented in one or more of the ONTs in a PON. The ONT includes an optical transmitter that may be disabled by a command generated by the ONT itself if rogue behavior is detected or suspected. To detect rogue behavior, at least one output indictor, such as LBC or MPC, is monitored during one or more monitoring windows. If monitoring indicates that the optical transmitter is transmitting more than a pre-determined threshold, a suspect rogue flag is set, for example in an I2C register. The register is read, preferably a number of successive times, and a determination is made whether to disable the optical transmitter. In some embodiments, the OLT is queried as part of this determination. The disability may be permanent, until a manual service operation, for example, or temporary.

Abstract: A visible light communication (VLC) device is provided for use in a VLC system. The VLC device detect a trigger condition indicating a failure of a VLC link associated with first allocated resources used to communicate with a second VLC device. In response to the detection, the VLC device terminates on the first allocated resources transmission of data to the second VLC device and transmits a fast link recovery (FLR) signal using the first allocated resources. The VLC device receives a fast link recovery response (FLR RSP) signal indicating the second VLC device received the FLR signal and, in response, the VLC device resumes transmission of data to the second VLC device.

Abstract: A protected passive optical communications system (10), having a plurality of terminals (14), a main head end (22) for communicating with the terminals (14) in an operational state, and, a back up head (37) end arranged to allow communication with the terminals (14) in the event of a fault, the main head end (22) and the back up head end (37) each being operable to transmit a sequence of frames for communicating with the terminals (14), each frame having a header portion and a payload portion, the main head end (22) being arranged, in normal operation, to transmit control instructions in the header portion and user data in the payload portion of each frame, the back up head end (37) being arranged, in response to a fault, to transmit control instructions in the payload portion of a frame, and a control message in the header portion of that frame, which control message is indicative of the presence of control instructions in the payload portion, the terminals being responsive to the control message, such that:

Abstract: A forward error correction (FEC) communication device that includes a transmitter photonic integrated circuit (TxPIC) or a receiver photonic integrated circuit (RxPIC) and a FEC device for FEC coding at least one channel with a first error rate and at least one additional channel with a second error rate, wherein the first error rate is greater than the second error rate. The TxPIC chip is a monolithic multi-channel chip having an array of modulated sources integrated on the chip, each operating at a different wavelength, wherein at least one of the modulated sources is modulated with a respective FEC encoded signal. The TxPIC also includes an integrated wavelength selective combiner for combining the channels for transport over an optical link.

Abstract: An optical communication system has two or more active interfaces, each controlling the transmission and reception of optical signals between a communication network and one or more subscriber terminals according to control information pertaining to the individual subscriber terminals. The control information used by all the active interfaces is stored in a memory. The optical communication system also has a standby interface that is functionally equivalent to the active interfaces, and an optical switching apparatus that switches data transmission paths among the network, the active and standby interfaces, and the subscriber terminals. If a fault is detected in an active interface, the standby interface extracts the control information of the faulty interface from the memory, and the optical switching apparatus switches the data transmission paths so that the standby interface replaces the faulty interface.

Abstract: Thermal chirp compensation in a chirp managed laser. In one example embodiment, a method for thermal chirp compensation in a chirp managed laser (CML) includes several acts. First, a first bias condition and temperature is selected. Next, a first thermal chirp compensation signal is generated. Then, the laser is driven by biasing a first input drive signal with the first thermal chirp compensation signal. Next, a second bias condition and temperature is selected. Then, a second thermal chirp compensation signal is generated. Finally, the laser is driven by biasing a second input drive signal with the second thermal chirp compensation signal.

Abstract: A WDM communication system that includes links traversing substantially inaccessible regions may tolerate multiple failures. In one implementation, a primary link spanning such a region is protected by a backup link. To provide further fault tolerance diverse paths may be provided to and from this backup link. The region may be, for example, an ocean.

Abstract: A technique for carrying out protection of data traffic in a multi-channel multi-section optical communications network by simultaneously using Optical Multiplexed Section (OMS) protection and Optical Channel (OCH) protection; the technique comprises selection of a working optical signal in an OCH protected optical channel, in case of a fault in a section of the network, by relying on an indication associated with OMS switching functionality that is required to overcome the mentioned fault.

Abstract: An optical termination system is provided. The optical termination system includes a working Optical Line Terminal (OLT) that communicates with a plurality of Optical Network Units (ONUs) near end users through an optical transmission line. The OLT includes a control information storage to store control information of the plurality of ONUs. In addition, the optical termination system includes a standby OLT, which includes a storage to store the control information to be transmitted from the working OLT. Furthermore, the optical termination system includes a controller that controls switching from the working OLT to the standby OLT.

Abstract: An interface between a network component and an optical fiber section communicates with a remote interface and a remote network component at an opposite end of the optical fiber section through over-modulation of optical signals sent between the interfaces and network components. The over-modulation is at frequencies much lower than the ordinary modulation of the optical signals. The over-modulation allows communication between the interfaces for performing management, testing and maintenance operations and permits superior detection of faults in the optical fiber section.

Abstract: An apparatus including a first circuit and a second circuit. The first circuit may be configured to generate an equalized signal in response to an input signal and an equalizer parameter signal. The equalizer parameter signal generally causes a cancellation of pre-cursor inter-symbol interference from a plurality of symbols in the input signal. The second circuit may be configured to generate (i) the equalizer parameter signal, (ii) a control signal and (iii) a data output signal in response to the equalized signal. The control signal generally causes an adjustment of the equalizer parameter signal. The adjustment of the equalizer parameter signal generally causes a decrease in the pre-cursor inter-symbol interference from the symbols.

Abstract: The present disclosure provides a passive optical network (PON) system and a method for protecting the service of the system for service recovery and fault locating in case of a failure of the network, wherein the PON system comprises an optical line terminal (OLT), an optical distribution network (ODN) and an optical network terminal (ONT) equipment protection group comprising a plurality of ONT equipment groups, each of which is connected to at least one of other ONT equipment groups within the ONT equipment protection group for the mutual protection relationship. The PON system of the present disclosure does not require equipment and link redundancy for backup, contributes to reduced cost and improved utilization of resources, and provides a means for diagnosing any faults of the links and equipment in the network.

Abstract: In a data transmission network, such as a passive optical network, the consecutive identical digit (CID) handling requirements may be reduced by providing a CID monitoring module at the transmitter end that monitors the number of CIDs in a transmission stream. Where the CID number exceeds a threshold, an error generation module induces an error in the transmission stream to reduce the CID below the threshold. The modified transmission stream may then be transmitted to a receiver, allowing clock recovery be performed with improved stability at the receiver. Once clock recovery is achieved, the receiver can then process the transmission stream to correct the errors induced at the transmitter end.

Abstract: The present invention provides an automatic power restoring method capable of reliably detecting continuity by the dissolution of a line fault, to restore the optical power, even in a structure including an optical amplification medium on an optical transmission path and an optical communication system using the method. To this end, in an optical communication system to which the automatic power restoring method of the invention is applied, a pilot signal having a low transmission rate, a wavelength of which is set based on loss wavelength characteristics obtained by combining loss wavelength characteristics of an optical fiber used for the optical transmission path and loss wavelength characteristics of the optical amplification medium on the optical transmission path, is transmitted and received between an optical transmitting station and an optical receiving station when a line fault occurs, and a detection of continuity is thus performed.

Abstract: A method includes detecting wavelength collision including identifying a pair or pairs of ONTs that transmit the colliding wavelengths and recovering from collision wherein wavelengths of the pair or pairs of ONTs that cause collisions are re-adjusted to eliminate the collision.

Abstract: A method for protecting a network comprising a first network element, which homes to a second network element and a third network element on the same ring network, the method comprising the steps of: establishing and transporting on the second network element a service from the first network element to the second network element, and detecting the status of the service; sending a switching message to the third network element upon detection of a failure of the service between the first network element and the second network element; and establishing and transporting a service as failed from the first network element to the second network element by the third network element, by using network resource of the ring network assigned on the second network element for the failed service prior to the failure of the service, in response to the switching message.

Abstract: A call setup and restoration schema is disclosed for providing an improved priority-based call setup and restoration schema in an optical communications system, allowing higher priority connections access to bandwidth by user-definable parameters. The call setup and restoration schema includes allowing higher priority connections to claim the resources of lower priority connections, and restoring them when bandwidth is available, or bumping another, even lower priority, connections. This aids in providing access to bandwidth by high priority services and in preventing outages or connection delays for high priority connections.

Abstract: A system, device, and method for managing alternate site switching in an optical communication system recovers from failures/degradations that are uncorrected by the core optical communication network. When an uncorrected failure/degradation is detected, communications for a protected end-system are switched from a primary end-system to a backup end-system. The backup end-system may be selected a priori, for example, during connection establishment, in order to reduce switching time once a decision has been made to switch communications from the primary end-system to the backup end-system. Provisions are made for completing the alternate site switching within a specified amount of time. Load balancing may be used to further reduce switching time from the primary end-system to the backup end-system. This alternate site switching augments the various protection mechanisms provided by the core optical communication network in order to provide end-to-end protection for the optical communication path.

Abstract: The synchronization of trunk failover between two FC-AL switches when a primary trunk failure occurs is disclosed. If primary trunk T1 should fail, S1 bypasses the cascade port and sends a MaRK (MRK) ordered set out over duplicate trunk T2 to switch S2. In response, S2 sends an acknowledgement MRK ordered set over T2 back to S1. S1 then reconfigures the switch to establish T2 as the primary trunk, and acts as a masters in the failover process and initiates LIP ordered sets which are communicated to all devices in the system to initialize them. Note that when S2 receives the MRK ordered set and acknowledges it by sending an acknowledgement MRK back to S1, it acts as a slave in the failover process and does not attempt to initiate LIPs, thereby eliminating the possibility of multiple Loop Initialization cycles and reducing the time in which data cannot be transmitted.

Abstract: One embodiment provides a system that performs protection switching in an Ethernet passive optical network (EPON), which includes an optical line terminal (OLT) and at least one optical network unit (ONU). The system is configured with at least one redundant component for the OLT and/or ONUs, wherein the redundant component can be optical or electrical, and can be a port, line card or link. The system provides protection by detecting a failure, and switching automatically to the redundant components to reduce service disruption time. The protection switching comprises: preserving the existing configuration over the loss of at least one of a multiple-point control protocol (MPCP) message; an operations, administration and maintenance (OAM) message; and a signal on the physical layer. The system recovers from the failure without performing ONU discovery.

Abstract: Systems and methods are described that provide a distributed restoration signaling protocol for shared mesh restoration with standbys for transparent optical networks.

Abstract: A transmissive active channel element is provided in each signal channel of a monolithic multi-channel TxPIC where each channel also includes a modulated source. The active channel element functions both as a power control element for both monitoring and regulating the output channel signal level of each signal channel and as a modulator for channel wavelength tagging or labeling to provide for wavelength locking the modulated sources. The power regulating function is also employed to control the channel signal power outputs of each channel to be uniform across the channel signal array. All of these functions are carried out by a feedback loop utilizing digital signal processing.

Abstract: The present disclosure describes methods and systems for the hierarchical mesh restoration of connections in an ASON or the like. These methods and systems provide a mesh restorable OTN server layer that carries an aggregate of mesh restorable SONET/SDH SNCs, without designating SONET/SDH/OTN hand-off ports or work/protect lines. Server layer SNCs are terminated on Virtual Trail Termination Points (VTTPs) on the NEs. These VTTPs maintain all of the attributes of physical Trail Termination Points (TTPs). The server routing protocol creates physical TTP interfaces at the server layer, and the server layer advertises bandwidth to the client layer routing protocol. A failure in the server layer results in the mesh restoration of an aggregate line, holding off the release of the individual client SNCs. Only when the server layer cannot restore are these individual client SNCs released.

Abstract: An optical network includes an optical ring that is capable of transmitting, between two or more nodes, a plurality of working traffic streams that include traffic transmitted in one of a plurality of wavelengths. A node is capable of transmitting, in a first wavelength, a first protection traffic stream associated with a first working traffic stream, in response to an interruption of the first working traffic stream. A node is also capable of transmitting, in a second wavelength, a second protection traffic stream associated with a second working traffic stream, in response to an interruption of the second working traffic stream. The optical network also includes a regeneration element capable of selectively regenerating the first protection traffic stream. The regeneration element is also capable of tuning the regeneration element to receive traffic in the second wavelength and of selectively regenerating the second protection traffic stream.

Abstract: The technique for improving the efficiency in use of power resources while maintaining the received signal quality that satisfies the requirement during a system operation is disclosed. The frame processing apparatus disclosed in the present case includes: a frame processing unit which performs frame processing on an input signal in the designated error processing mode; and a controlling unit which receives the monitoring result relating to the signal quality of the input signal and performs switching control of the designation of the above error processing mode in the frame processing unit based on the received monitoring result.

Abstract: A method of detecting transmission collisions in an optical data interconnect system including a transmitting node, a plurality of receiving nodes, and one or more remaining nodes connected through an optical data channel.

Abstract: A method for handling a fiber line fault is provided. The method includes: loading, by a station, an indication signal carrying alarm suppression information onto an optical signal to be output to a downstream fiber of the station upon detection of a fault on upstream fiber line; keeping in a normal working status, by a downstream station upon a receipt of the optical signal with the indication signal. The various embodiments of the present disclosure suppress alarms from downstream amplifier stations which are not adjacent to the failure link and further suppress APR or APSD procedures according to the alarms.

Abstract: The invention relates to a port marker which comprises lighting means and which is intended to be positioned next to the red and green light signals, lighting columns or lighthouses at the port entrance. The inventive marker comprises a self-supporting half-ring having a maximum height which is essentially greater than the height of the light source of the lighting column, lighthouse or light signal. The invention also comprises and electronic unit for powering and controlling the lighting means. The lighting means are distributed between the two ends of the self-supporting half-ring along the length thereof and are disposed such as to be visible to a boat or similar vessel approaching the port. The color of said lighting means corresponds to that of the respective obligatory lighting column, lighthouse or light signal at the entrance to the port, which, as mentioned above, can be red or green.

Abstract: The present invention provides Ethernet extension and demarcation functionality through a Multi-Source Agreement (MSA) pluggable transceiver in a customer or remote device. The pluggable transceiver is configured to frame an Ethernet client signal and to provide OAM&P functionality, such as with G.709 framing. The pluggable transceiver operates within existing multi-source agreement (MSA) specifications. Accordingly, the pluggable transceiver can operate in any customer device compliant to the MSA specifications. Additionally, the framing and OAM&P functionality are transparent to the customer device, but instead utilized by a service provider for demarcation functionality, eliminating the requirements for external demarcation equipment and for external transponders.

Abstract: The present invention provides multi-channel protection switching systems and methods for increased reliability and reduced cost. Advantageously, the multi-channel protection switching systems and methods of the present invention use pre-FEC rate measurements, and changes in pre-FEC rate measurements, to detect and avoid potential link failures before they occur. The multi-channel protection switching systems and methods of the present invention also use “wavelength-hopping” and other protection schemes to alleviate wavelength and time-dependent optical propagation impairments.

Abstract: The present invention discloses a passive optical network system based on wavelength protection and the protection backup method thereof. It comprises an optical line terminal (OLT) with two interface circuits, two trunk lines, an optical splitter network, multiple branch lines and optical network units (ONU), each of which is with two interface circuits. The two interface circuits of the OLT transmit downlink optical signals with different wavelengths, while the two interface circuits of the ONU transmit uplink optical signals with different wavelengths. Each trunk line is set with a wavelength division multiplexing (WDM) device and it can only pass the uplink and downlink signals of the connected interface circuits.

Abstract: A method for remotely restoring inoperative data communications is disclosed. The method includes generating a predetermined sequence of signals at a first Communications unit. The method further includes keying an output of a transmitter on and off at the first communications unit with the predetermined sequence of signals and conveying a transmission of the predetermined sequence of signals to a second communications unit. The method also includes recognizing the predetermined sequence of signals at the second communications unit as indicative of inoperability of the second communications unit and, responsive to the recognized predetermined sequence of signals, outputting a signal to attempt to restore inoperative data communications at said second communications unit.

Abstract: An optical protection switch and a method for optical protection switching are provided. The optical protection switch includes a loop mirror-based optical switch with two circulators and a direction-dependent phase shifter in the loop mirror. The direction-dependent phase shifter introduces phase shifts in counter-propagating optical signals in the loop mirror such that either one of a first optical signal and a second optical signal are switched as an output optical signal. The direction-dependent phase shifter is controlled by a controller which initiates switching from the first optical signal to the second optical signal if a drop in power level is detected in the first optical signal and a corresponding drop in power level is not detected in the second optical signal and vice versa.

Abstract: Provided are an optical output power control method that provides optical communication without harming the human body even when a transmission line is cut or no optical network unit (ONU) accesses a communication system, and a passive optical network (PON) system using the method. In the method, an optical signal having a predetermined period is transmitted when an optical communication system is in an abnormal state, wherein the predetermined period includes a laser-on time interval in which the optical signal has a normal power level and a laser-off time interval in which the optical signal is off or has a power level lower than the normal power level, so that the optical communication system performs optical communication without harming the human body even during the abnormal state.

Abstract: The objective of the present invention is to provide a solution for detecting timeslot conflict between Optical Network Units (ONUs) in an Optical Line Terminal (OLT). When all ONUs are deactivated and there is still any light signal in the upstream, each ONU is notified to switch off the power supply of its transmitter one by one to detect whether there is any failure ONU; when all ONUs are deactivated and there is no light signal in the upstream, the failed ONU can be located through ranging the ONUs separately. The solution can be implemented through software update, without affecting the intrinsic hardware design.

Abstract: A network connection device for performing a process of transmitting and receiving a SONET/SDH frame includes a first protocol processor which processes a first network management protocol; a second protocol processor which processes a second network management protocol; a frame header extractor which extracts a header from the received SONET/SDH frame; and a protocol identifier which identifies a network management protocol based on the extracted frame header. The network connection device selects one of a control signal outputted from the first protocol processor and a control signal outputted from the second protocol processor based on a result of the identification, and outputs the control signal.

Abstract: In order to prevent a spectrum of a wavelength channel from becoming narrower, a device according to the present invention includes a light dividing section capable of dividing a wavelength spectrum in an input light beam and outputting a plurality of divided light beams, which are spatially separated and have wavelength spectrum portions different from each other, and a wavelength-to-special-position-converter capable of spatially multiplexing the wavelength spectrum portions of the plurality of divided light beams from the light dividing section.

Abstract: A optical transmitter includes a light device provided a driving current and for emitting light on the bases of the driving current; a light measure for measuring an output light level of the light device; a controller for controlling the driving current to the light device and for changing the measured output light power level into a set output light power level of the light device; a current measure for measuring a level of the driving current to the light device; an abnormality detector for detecting an aged deterioration of the light device on the bases of the current level of the driving current by the current measure and the measured output light power level by the light measure; and an adjuster for reducing the set output light power level in the controller when the abnormality detector detects the aged deterioration of the light device.

Abstract: A synchronous optical signal generation device includes: an optical phase detector that compares the phase of a reference optical signal with a phase of an optical beat signal to generate a phase error signal; a shaping mechanism that shapes the phase error signal; and a voltage controlled optical signal generator that generates an optical beat signal based on the shaped phase error signal and that outputs the optical beat signal while feeding the optical beat signal back to the phase detector.

Abstract: A method for starting lasers in a network is provided. According to the method, a restart phase in automatic mode is initiated after the laser is disconnected. During the restart phase in automatic mode, which is accompanied by a delay time for the new laser connection, the automatic mode is switched to a different mode in which the new connection of the laser takes place faster than in automatic mode. This can be achieved by switching to manual mode as per ITU-T G.664. Furthermore, safety measures are defined before activating switching from the network management or a network element control so that safety is ensured during the fast, new connection of the laser.

Abstract: The present invention relates to a method for protecting an optical telecommunication network against possible failures and signal degrades, the network comprising at least two nodes; working resources and protecting resources connecting the nodes; a first protection mechanism operating at a first protection layer; and a second protection mechanism operating at a second protection layer, characterized by a) the step, carried out by a node detecting a failure that could be managed by said first mechanism, of sending downstream an in-band Forward Protection Indication signalling in order to instruct the second protection mechanism not to operate, or b) the step, carried out by a node detecting a Bit Error Rate between a certain range, of sending downstream an in-band Link Degradation Indication signalling in order to instruct the second protection mechanism to operate.

Abstract: The present invention provides an automatic power restoring method capable of reliably detecting continuity by the dissolution of a line fault, to restore the optical power, even in a structure including an optical amplification medium on an optical transmission path and an optical communication system using the method. To this end, in an optical communication system to which the automatic power restoring method of the invention is applied, a pilot signal having a low transmission rate, a wavelength of which is set based on loss wavelength characteristics obtained by combining loss wavelength characteristics of an optical fiber used for the optical transmission path and loss wavelength characteristics of the optical amplification medium on the optical transmission path, is transmitted and received between an optical transmitting station and an optical receiving station when a line fault occurs, and a detection of continuity is thus performed.

Abstract: A photonic integrated circuit (PIC) device is described. The PIC device includes a set of optical transceivers including optical transmitters and optical receivers, and an embedded optical interconnect mesh operatively associated with the set of optical transceivers and structured to enable at least one of the following network architectures: a star network architecture, a bus/broadcast network architecture, and a ring network architecture. Related apparatus and methods are also described.

Abstract: A method and a device of enhancing data integrity using hash function for data transmission over an optical network are disclosed. The device includes a hash circuit, a lookup circuit, a vector circuit, and a correcting circuit. The hash circuit is capable of performing a hash function and configured to generate a hash value in response to a value of header error correction (“HEC”) encapsulated in a gigabit passive optical network (“GPON”) frame. The lookup circuit is capable of obtaining a data entry from a lookup table in a memory in accordance with the hash value. While the vector circuit generates an error vector in response to the data entry, the correcting circuit corrects an error or errors in the header portion of a data structure encapsulated in the GPON frame.

Abstract: The present invention relates to the quality monitoring of optical signals, which have different symbol rates and generated by different modulation schemes, used for example in a wavelength division multiplexing network. The apparatus according to the invention includes an optical splitter for outputting the input optical signal to a first optical route and a second optical route, an optical coupler for coupling a optical signal from the first optical route with a optical signal from the second optical route, a delay unit provided on the first optical route, and a phase shift unit provided on the first optical route or the second optical route.

Abstract: A method and system for transmitting information in an optical communication system includes modulating a non-intensity characteristic of an optical carrier signal with a data signal to generate an optical information signal. The optical information signal is transmitted over an optical link. The optical information signal is amplified over a length of the optical link with a co-launched amplification signal traveling in the optical link in a same direction as the optical information signal.

Abstract: According to an aspect of an embodiment, in a light transmission device for switching a light transmission path for receiving an optical signal from a currently used system to a backup system when the light level of light input from a light transmission path of a currently used system becomes substantially equal to or less than a light input break detection threshold value which serves as a reference for detecting a light input break. The light transmission device includes light level measuring means for measuring the light level of the light input from the light transmission path of the currently used system, and light input break detection threshold value setting means for detecting only the light level of accumulated noise of the light level measured by the light level measuring means and setting the detected light level as the light input break detection threshold value.

Abstract: An operational optical transceiver configured to preserve a portion of volatile memory during a warm reboot process. The optical transceiver includes a persistent memory, a processor, and a system memory. The system memory includes a preserved memory space. The optical transceiver loads microcode from the persistent memory to the system memory without writing into the preserved memory space. The processor processes the microcode and writes certain information into the preserved memory space that will be preserved during a warm reboot. The optical transceiver may then initiate a warm reboot and load microcode from the persistent memory to the system memory that overwrites the existing microcode. However, the information written in the preserved memory space is not overwritten by the microcode loaded from the persistent memory. In this way, a portion of the information contained in the system memory prior to the warm reboot is preserved.

Abstract: An optical network terminal (ONT) battery backup unit (BBU) battery voltage alert system (BVAS). This alert is provided as a supplement to an audio alarm which sounds when the backup battery is discharging because that alarm may not be heard in sufficient time if the alarm is remotely located relative to its customer, e.g., in the customer's basement or outside. A battery monitoring facility, located in a telecommunication service provider's centralized serving office, or remotely located elsewhere, monitors the voltage of each of a plurality of batteries deployed in customers' premises within the geographical area served by that centralized serving office. That battery monitoring facility receives packets via a communication network from each such customer's premises, each of those packets representing a battery voltage measurement. That battery monitoring facility stores these on-going measurements thereby creating a trending database to point to any potential future battery failures.

Abstract: Between photonic cross connects (PXCs) 3 and 4, an in-use system path passing through transmission apparatus (WDMs) 5 and 7 are set, and a backup system passing through transmission apparatus (WDMs) 6 and 8 is set. The WDMs 3 and 4 link to the PXCs 5 and 7 and release masking therebetween when the in-use system path is set, and set masking therebetween when the in-use system path is deleted. The WDMs 5 and 7 included in the in-use system path transmit an alarm to a monitoring system when an input light is turned off, and the WDMs 6 and 8 included in the backup system path do not transmit an unintended alarm to the monitoring system even when the input light is turned off.