JP2005269246A - Light transmission path monitor switching device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To simultaneously monitor a main transmission path and an auxiliary transmission path independently of a state of communication wavelength light. <P>SOLUTION: The light transmission path monitor switching device is composed of a monitor light transmitting part for transmitting monitor wavelength light, a first light multiplexing/demultiplexing part for multiplexing the monitor wavelength light and the communication wavelength light transmitted by an external transmitting device, a light distributing part for transmitting the light wave multiplexed by the first light multiplexing/demultiplexing part after branching the light wave into two to the main transmission path and the auxiliary transmission path, second/third light multiplexing/demultiplexing parts for demultiplexing the respective propagated light waves into the communication wavelength light and the monitor wavelength light, first/second photoelectric conversion parts for respectively executing the photoelectric conversion to the demultiplexed monitor wavelength light, a level determining part for determining the disconnection/non-disconnection of the main transmission path and the auxiliary transmission path by comparing the level of an electric signal from the photoelectric conversion part with a preset disconnection level, and a light switch part which makes either of the communication wavelength light demultiplexed by the second/third light multiplexing/demultiplexing parts to be conducted to the external transmitting device by a control signal from the level determining part. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an optical transmission line monitoring and switching device for monitoring loss values of both transmission lines in a redundant configuration having a main transmission line and a standby transmission line and switching the optical transmission line when a failure such as a disconnection occurs. is there.

  FIG. 1 shows a conventional optical transmission line switching device disclosed in Japanese Patent Laid-Open No. 2003-229816. This conventional example is a device that bi-directionally transmits optical signals of different wavelengths, upstream and downstream, via a single-core transmission line. The optical transmission / reception device 1 and the optical transmission / reception device 2 are connected to the main transmission line 12 and the standby transmission line 11. It is a device that is connected by two cores and switches to the standby transmission line 11 when a failure such as disconnection occurs in the main transmission line 12. The optical transceiver 1 includes an optical transmitter 3 that converts an input electrical signal into light of a specific wavelength, a photoelectric converter 4 that converts an input optical signal into an electrical signal, and lights of two different wavelengths. It comprises a mixer 5 for multiplexing / demultiplexing, and an optical switch 6 for switching the main transmission path 12 and the standby transmission path 11. The opposite optical transmitter / receiver 2 has the same configuration. However, the transmission wavelengths of the optical transmitters 3 and 9 are different from each other.

The operation of this Japanese Patent Application Laid-Open No. 2003-229816 shown in FIG. 1 is as follows.
In the optical transceiver 1, the electrical signal input to the optical transmitter 3 is converted into light having a specific wavelength λ1. The converted optical signal is transmitted to the main transmission path 12 through the mixer 5 and the optical switch 6. Similarly, in the opposite optical receiver 2, the electrical signal input to the optical transmitter 9 is converted into light having a specific wavelength λ 2 and transmitted to the main transmission path 12 via the mixer 8 and the optical switch 7. Is done.
Optical signals λ1 (rightward) and λ2 (leftward) transmitted bi-directionally to the main transmission path 12 are received by the optical transceivers 2 and 1, respectively. The received optical signals λ1 and λ2 are photoelectrically converted by the optical / electrical converters 10 and 4 via the optical switches 7 and 6 and the mixers 8 and 5, respectively.
As described above, single-core bidirectional communication is performed through the main transmission path 12. At this time, if an excessive loss in the transmission line due to disconnection or the like occurs in the main transmission line 12, the reception level of the optical / electrical converters 10 and 4 decreases, and the optical switchers 7 and 6 are reached when the level reaches a preset level. Then, a control signal for switching to the protection line 11 is transmitted, and the optical switches 7 and 6 are switched from the main transmission line 12 to the protection transmission line 11, and normal communication is resumed.
JP 2003-229816 A

  However, in the above conventional example, the communication wavelength light is used for monitoring the loss of the transmission line, so that the communication line light is switched to the protection line when the communication wavelength light is interrupted due to the failure of the communication port even though the transmission line is normal. The problem is that the communication wavelength light is set to a high output to improve the transmission distance, resulting in a short MTTF (Mean Time To Fairue), with any media converter or WDM (Wavelength Division Multiplexing) transmission device There was a problem with lack of interconnectivity.

In addition, since only the transmission line that is performing communication is monitored, there is no way to know the state of the standby transmission line that is not performing communication.For example, after the main transmission line is disconnected and switched to the standby transmission line, Even when the main transmission line is restored, the main transmission line is not monitored, so that it cannot be automatically switched back.
The present invention has been made in view of the above problems, and is equipped with a dedicated LD (Laser Diode) for monitoring the transmission line, and monitors the main transmission line and the standby transmission line with a wavelength light different from the communication wavelength light. This is to solve the above problem.

  That is, the present invention includes a monitoring light transmitter that transmits monitoring wavelength light, and a first optical multiplexing / demultiplexing unit that combines the monitoring wavelength light transmitted by the monitoring light transmitter and the communication wavelength light transmitted by the external transmission device. An optical distribution unit (hereinafter referred to as a monitoring wavelength optical transmission side device) that splits the optical wave combined by the first optical multiplexing / demultiplexing unit into two and transmits it to the main transmission line and the standby transmission line, the main transmission line and the standby Each light wave propagated through the transmission line is demultiplexed by the second and third optical multiplexing / demultiplexing units that demultiplex the communication wavelength light and the monitoring wavelength light, respectively, and the second and third optical multiplexing / demultiplexing units. The first and second optical / electrical converters that photoelectrically convert the monitored wavelength light, respectively, and the level of the electrical signal from the first and second optical / electrical converters is compared with a preset disconnection level. A level determination unit that determines disconnection / non-disconnection of the transmission line and the backup transmission line; An optical switch unit (hereinafter referred to as a monitoring wavelength light receiving side) that conducts either one of the communication wavelength light demultiplexed by the second and third optical multiplexing / demultiplexing units to an external transmission device by a control signal from the level determining unit An optical transmission line monitoring / switching device.

  In the present invention, the monitoring light transmission unit modulates the monitoring wavelength light at a constant frequency, and the first and second optical / electrical conversion units have an electrical narrow band centered on the frequency. A band-pass filter is provided, and the monitor current output from the photodiode built in the laser diode is fed back to the modulation current in order to keep the extinction ratio of the monitoring wavelength light transmitted by the monitoring light transmitter constant. Preferably, the monitoring light transmitter uses a wavelength band of 1370 nm to 1450 nm that is not used for normal communication as a monitoring wavelength, or a wavelength band of 1630 nm or more, and the optical switch The unit is an optical transmission line monitoring switching device that preferably has a latch function for holding the state when the power is turned off.

Since the optical transmission line monitoring switching device of the present invention uses a dedicated wavelength for monitoring, it is possible to monitor the transmission line independently of the state of the communication wavelength light, and the transmission device connected oppositely The communication direction is also unquestioned, and the output of the monitoring LD can be lowered, so that the MTTF as a device can be increased. Further, since the main transmission line and the standby transmission line are simultaneously monitored, it is possible to detect disconnection of the standby transmission line and to switch back when the disconnected transmission line is restored.
In addition, the optical transmission line monitoring switching device of the present invention modulates the monitoring wavelength light at a constant frequency and uses a narrow bandpass filter of the frequency on the light receiving side, so that a photodiode or a preamplifier is generated. By removing noise, monitoring wavelength light can be detected with high sensitivity, so it becomes possible to use wavelengths in the wavelength range where absorption of OH groups in fibers that are not used for communications is large, as monitoring wavelengths. It is possible to make the best use of the number of wavelengths.
Since the optical transmission line monitoring switching device of the present invention feeds back the monitoring current from the PD with a built-in LD that transmits the modulated monitoring wavelength light to the modulation current, the extinction ratio of the modulated monitoring wavelength light is constant, and the accuracy It is possible to detect a high transmission line loss value.
Since the optical transmission line monitoring switching device of the present invention normally uses a wavelength that is not used for communication as a monitoring wavelength, it can be interconnected with a transmission device that does not use the monitoring wavelength as a communication wavelength. There is.

Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 2 shows a functional block diagram of the optical transmission line monitoring switching device of the present invention, and FIGS. 3 to 10 show details of individual functional blocks.
In FIG. 2, the present optical transmission line monitoring switching device is composed of a monitoring wavelength light transmitting side device 14 and a monitoring wavelength light receiving side device 13. The monitoring wavelength light transmitter 14 includes a monitoring light transmitter 15, an optical multiplexer / demultiplexer 16, and an optical distributor 17. The monitoring wavelength light receiving side device 13 includes optical multiplexing / demultiplexing units 18 and 19, optical / electrical conversion units 20 and 21, a level determination unit 22, and an optical switch unit 23.

Hereinafter, the function and operation of each component will be described for each functional block. The monitoring light transmitter 15 of the monitoring wavelength light transmitter 14 is composed of an LD unit 42 and an LD driver unit 43 as shown in FIG. The LD unit 42 is driven by the LD driver unit 43 with a sine modulation current having a constant frequency. The LD unit 42 keeps the average light output constant by feeding back to the LD driver unit 43 a monitor current that is an output of a built-in light output monitoring PD (Photo Diode) (not shown).

FIG. 9 is a diagram for explaining the relationship between the LD drive current and the optical output.
As shown in FIG. 9, the drive current of the LD is a sinusoidal modulation current having a constant frequency, and its amplitude is output from the LD because the lower side is modulated so as to be lower than the threshold current of the LD. The extinction ratio of the optical waveform is almost infinite. Also, since the monitor current of the PD built in the LD is fed back to the LD driver and APC (Auto Power Control) is performed, the change in the LD ambient current, the change in the LD threshold current due to the deterioration over time, and the change in the external differential quantum efficiency The average output power of the LD does not change.
Therefore, the extinction ratio does not change due to changes in the ambient environment temperature or deterioration of the LD over time, so that the transmission line loss can be monitored by detecting this amplitude intensity on the light receiving side. In order to make the extinction ratio constant at a finite value, the bias current and modulation current of the LD must be controlled separately depending on the ambient temperature of the LD. According to the above method, the extinction ratio is controlled only by controlling the modulation current by APC. Can be kept almost constant and infinite.

  Since the extinction ratio is almost infinite, the side mode suppression ratio at the rising edge of the waveform is small. When the side mode suppression ratio is small, the side mode component becomes large, and there is a concern that the waveform may become dull due to the influence of chromatic dispersion of the fiber. Therefore, the modulation frequency was set low. In the trial production, the modulation frequency of the monitoring wavelength light was 100 kHz. The optical output of the monitoring LD is preferably low so that the MTTF of the LD can be secured sufficiently large.

Next, selection of the monitoring wavelength will be described.
For example, as a wavelength for CWDM (Coarse WDM), G.264 recommended by ITU-T (International Telecommunication Union) in April 2002. In 694.2, 18 wavelengths from 1270 nm to 1610 nm at a 20 nm pitch are standardized. Of these, the wavelength range with low fiber loss that is suitable for communication is intended to be assigned to communication as much as possible. Therefore, the 1.4 μm band, which is normally absorbed by OH groups in fibers not used for communication, is used for monitoring. It is desirable to assign as In the fabricated prototype, a 1430 nm DFBLD (Distributed Feedback Laser Diode) was used, but the monitoring wavelength applicable to the present invention is not limited to this.

The optical multiplexing / demultiplexing unit 16 of the monitoring wavelength light transmitting side device 14 of FIG. 2 and the optical multiplexing / demultiplexing units 18 and 19 of the monitoring wavelength light receiving side device 13 of FIG. 2 are realized by WDM couplers as shown in FIG.
The WDM coupler 26 is provided with a reflection port 29 and a transmission port 27 with respect to the common port 28. For example, as shown in FIG. It reflects wavelength light. 4 shows a spectral profile of insertion loss between the common port 28 and the transmission port 27 of the WDM coupler 26, and FIG. 5 shows a spectral profile of insertion loss between the common port 28 and the reflection port 29. In FIG. 4 and 5, B1 is the wavelength range of the transmission port, B2 is the insertion loss of the transmission port, B3 is the flatness of the insertion loss of the transmission port, B4 is the crosstalk of the transmission port, C1 is the wavelength range of the reflection port, C2 represents the insertion loss of the reflection port, C3 represents the flatness of the insertion loss of the reflection port, and C4 represents the crosstalk of the reflection port.

  The wavelength range B1 of the transmission port needs to take into account individual variations in the peak wavelength of the LD for transmission line monitoring and wavelength fluctuations due to changes in the environmental temperature. In the manufactured prototype, since the monitoring wavelength was 1430 nm, it was set to 1424.5 to 1437.5 nm. Further, it is desirable that the insertion loss B2 and flatness B3 of the transmission port are small. The crosstalk B4 of the transmission port is designed so that the communication wavelength light used in the transmission apparatus connected to the optical transmission line monitoring switching apparatus of the present invention is input to the optical / electrical converters 20 and 21 and no malfunction occurs. There is a need. The wavelength range C1 of the communication wavelength light reflection port needs to be designed to cover the communication wavelength used in the transmission apparatus connected to the optical transmission line monitoring switching apparatus of the present invention. In the trial production, it was set to 1264 to 1384 nm and 1464 to 1618 nm so as to cover a wavelength band widely used as a communication wavelength. It is desirable that the reflection port insertion loss C2 and the flatness C3 of the reflection port insertion loss be smaller. It is necessary to design the crosstalk C4 of the reflection port so that the monitoring wavelength light is not mixed into the transmission apparatus connected to the optical transmission line monitoring switching apparatus of the present invention and affects communication.

The reason why the monitoring wavelength light is used for transmission by the WDM coupler will be described below.
In general, a dielectric multilayer filter has a reflection port crosstalk of about 10 dB and a transmission port crosstalk of about 50 dB, and the transmission port crosstalk is larger. When a wavelength with a large fiber loss is used as the monitoring wavelength, the monitoring wavelength receives a loss that is greater than the allowable transmission loss of the transmission device connected to the optical transmission line monitoring switching device of the present invention. For example, ITU-T G.I. The loss at 1430 nm is about 1.7 times in dB / km with respect to the wavelength 1550 nm at which the fiber loss is the smallest among the CWDM wavelengths defined in 694.2. If the communication wavelength light receives a loss of 25 dB through the transmission line, the monitoring wavelength light receives a loss of 42.5 dB, which is 1.7 times that loss. If the transmission level to the transmission line is the same between the communication wavelength and the monitoring wavelength, the reception level difference on the receiving side is calculated to be 17.5 dB higher at the communication wavelength than at the monitoring wavelength light.

  Therefore, when the reception level of the monitoring wavelength is detected on the receiving side, it is necessary to take sufficient crosstalk of the communication wavelength light. Therefore, the transmission port of the WDM coupler is assigned to the monitoring wavelength. When the transmission apparatus connected to the optical transmission line monitoring switching apparatus of the present invention is WDM, the number of communication wavelengths increases and the amount of crosstalk increases, so that the optical / electrical converters 20 and 21 that receive the monitoring wavelength light described later are received. It is necessary to take measures to further suppress the crosstalk of the communication wavelength light by incorporating the dielectric multilayer filter used in the WDM coupler into the photodiode.

Hereinafter, a connection configuration of each port of the optical multiplexing / demultiplexing units 16, 18, and 19 will be described.
In the monitoring wavelength light transmission side device 14 (FIG. 2), the transmission port 27 of the WDM coupler of the optical multiplexing / demultiplexing unit 16 is connected to the monitoring LD of the monitoring light transmission unit 15, and the reflection port 29 is the optical transmission of the present invention. The common port 28 is connected to an input port 32 of the optical distribution unit 17 described later, which is connected to the optical interface of the external transmission device connected to the path monitoring switching device.
By connecting in this way, the monitoring wavelength light from the monitoring light transmission unit 15 and the communication wavelength light output from the transmission device connected to the optical transmission line monitoring switching device of the present invention are multiplexed by the optical multiplexing / demultiplexing unit 16. And output to the optical distributor 17.
In the monitoring wavelength light receiving device 13 (FIG. 2), the common ports 28 of the WDM couplers of the optical multiplexing / demultiplexing units 18 and 19 are connected to the main transmission line 24 and the standby transmission line 25, respectively. The transmission ports 27 of the optical multiplexing / demultiplexing units 18 and 19 are connected to optical / electrical conversion units 20 and 21 to be described later, respectively, and the reflection ports 29 of the optical multiplexing / demultiplexing units 18 and 19 are respectively first optical switch units to be described later. The output port 36 and the second output port 37 are connected.

2 has a first output port 33 and a second output port 34 with respect to the input port 32 as shown in FIG. The optical power input to the port 32 is divided by half and output to the first and second output ports 33 and 34, and conversely, the optical power input from the first or second output port 33 and 34 Has a function of being halved and output to the input port 32. As the optical distributor having such a function, a well-known fiber fusion type 3 dB coupler or a 3 dB coupler that divides a light wave by a half mirror in free space can be used.
The first output port 33 and the second output port of the optical distribution unit 17 are connected to the main transmission line 24 and the backup transmission line 25, respectively. By connecting in this way, the monitoring wavelength light multiplexed by the optical multiplexing / demultiplexing unit 16 and the communication wavelength light of the external transmission device connected to the optical transmission line monitoring switching device of the present invention are split into two, respectively. It is transmitted to the transmission line 24 and the backup transmission line 25.

Further, as shown in FIG. 10, the optical / electrical converters 20 and 21 include a photodiode 44, a preamplifier 45, a band pass filter 46, a log amplifier 47, and an A / D converter 48. The modulated light signal input to the photodiode 44 is photoelectrically converted, the electric signal is amplified by a high-impedance preamplifier 45, and transmitted through a band-pass filter 46 whose center frequency is the modulation frequency of the monitoring wavelength light. The dark current generated by the diode and the noise generated by the preamplifier are removed. The transmission band of the bandpass filter 46 is desirably a narrow band for the purpose of noise removal.
The electrical signal that has passed through the band-pass filter 46 is subjected to linear logarithmic conversion by the log amplifier 47 in the subsequent stage, and the received power level of the photodiode is digitized by the A / D converter 48. In the fabricated prototype, the received light power level is digitized by the A / D converter 48 so that it can be confirmed via SNMP (Simple Network Management Protocol). However, it may be processed by the level determination unit 22 described later as an analog signal. As it is possible, the A / D converter 48 is not a requirement of the present invention.

  Further, as shown in FIG. 7, the optical switch unit 23 includes a first output port 37 and a second output port 38 with respect to the input port 36, and the input port 36 and the second output port 38 according to an external electrical control signal. The optical continuity with the first output port 37 or the second output port 38 is switched. Although the mirror is switched by an electrical control signal, it is necessary to use an optical switch that maintains the previous state when the power is turned off. Moreover, since an optical switch is for switching a transmission line, the switching speed is required to be on the order of several tens of msec. Although the mirror type optical switch is described in FIG. 7, the present invention is not limited to this as long as the optical switch satisfies the above functions.

  The level determination unit 22 determines the normal / disconnection of the main transmission line 24 and the backup transmission line 25 by comparing the light reception power level of the optical / electrical conversion units 20 and 21 with a preset disconnection determination level. If it is determined, the control signal for switching to the normal side is sent to the optical switch unit 23.

As described above in detail, the optical transmission line monitoring switching device of the present invention is equipped with a monitoring wavelength light different from the communication wavelength, and the monitoring wavelength light is an optical multiplexing / demultiplexing unit of the monitoring wavelength light transmitting side device 14. 16, the light is transmitted to the main transmission line 24 and the standby transmission line 25 with the same power through the optical distribution unit 17. In the monitoring wavelength light receiving side device 13, the monitoring wavelength light from the main transmission line 24 and the standby transmission line 25 is The received light power is converted into an electrical signal by the optical / electrical converters 20 and 21 through the optical multiplexer / demultiplexers 18 and 19, respectively, and the monitoring wavelength light from the main transmission line 24 and the standby transmission line 25 is converted by the level determination part 22. It is possible to determine whether or not the received light level has reached a preset disconnection level, determine disconnection / non-disconnection, and if it is determined that the disconnection has occurred, the optical switch unit can be switched to the transmission line on the non-disconnection side. ing. Further, since the light reception level of the monitoring wavelength light is always monitored for the disconnected transmission line, it can be determined that the transmission line has been restored, and for example, the transmission line can be switched back. On the other hand, the communication wavelength of the transmission apparatus connected to the optical transmission line monitoring and switching apparatus of the present invention can be transmitted in one direction or in both directions through the transmission line selected by the switch unit 23.
If the communication wavelength of the transmission device connected to the optical transmission line monitoring switching device of the present invention is within the reflection band of the optical multiplexing / demultiplexing units 16, 18, and 19, the number of wavelengths of one-way communication, bidirectional communication, and WDM Regardless of the connection is possible.

It is drawing which shows the functional block diagram of the conventional optical transmission path switching apparatus. It is drawing which shows the functional block diagram of the optical transmission line monitoring switching apparatus of this invention. It is drawing which shows the structure of the optical multiplexing / demultiplexing part of the optical transmission line monitoring switching apparatus of this invention. It is drawing which shows the insertion loss spectrum of the transmission port of the optical multiplexing / demultiplexing part of the optical transmission line monitoring switching apparatus of this invention. It is drawing which shows the insertion loss spectrum of the reflection port of the optical multiplexing / demultiplexing part of the optical transmission line monitoring switching apparatus of this invention. It is drawing which shows the structure of the optical distribution part of the optical transmission line monitoring switching apparatus of this invention. It is drawing which shows the structure of the optical switch part of the optical transmission line monitoring switching apparatus of this invention. It is drawing which shows the functional block of the monitoring light transmission part of the optical transmission line monitoring switching apparatus of this invention. It is drawing which shows the effect | action of the monitoring light transmission part of the optical transmission line monitoring switching apparatus of this invention. It is drawing which shows the functional block of the optical / electrical conversion part of the optical transmission line monitoring switching apparatus of this invention.

Explanation of symbols

1: Optical transceiver, 2: Optical transceiver, 3: Optical transmitter, 4: Optical / electrical converter, 5: Mixer, 6: Optical switch, 7: Optical switch, 8: Mixer, 9: 10: optical / electrical converter, 11: standby transmission line, 12: main transmission line, 13: monitoring wavelength light receiving side device, 14: monitoring wavelength light transmitting side device, 15: monitoring light transmitting unit, 16 : Optical multiplexing / demultiplexing unit, 17: Optical distributing unit, 18: Optical multiplexing / demultiplexing unit, 19: Optical multiplexing / demultiplexing unit, 20: Optical / electrical converter, 21: Optical / electrical converter, 22: Level determining unit, 23: Optical switch unit, 24: main transmission line, 25: backup transmission line,
26: WDM coupler, 27: transmission port, 28: common port, 29: reflection port, 30: dielectric multilayer filter,
31: 3 dB coupler, 32: input port, 33: first output port, 34: second output port,
35: optical switch, 36: input port, 37: first output port, 38: second output port, 39: mirror, 40: mirror, 41: mirror switching operation unit,
42: LD part, 43: LD driver part,
44: Photodiode, 45: Preamplifier, 46: Band pass filter 47: Log amplifier, 48: A / D converter,
B1: Wavelength range of transmission port, B2: Insertion loss of transmission port, B3: Flatness of insertion loss of transmission port, B4: Crosstalk of transmission port,
C1: Reflection port wavelength range C2: Reflection port insertion loss, C3: Flatness of reflection port insertion loss, C4: Reflection port crosstalk

Claims (5)

A monitoring light transmitter for transmitting the monitoring wavelength light, a first optical multiplexing / demultiplexing unit for combining the monitoring wavelength light transmitted by the monitoring light transmitter and the communication wavelength light transmitted by the external transmission device, and the first An optical distribution unit that splits the optical wave combined by the optical multiplexing / demultiplexing unit into two branches and transmits it to the main transmission line and the backup transmission line, and the respective light waves propagated through the main transmission line and the backup transmission line are monitored with the communication wavelength light, respectively. Second and third optical multiplexing / demultiplexing units that are demultiplexed into wavelength light, and first and second light / second light that photoelectrically convert the monitoring wavelength light demultiplexed by the second and third optical multiplexing / demultiplexing units, respectively. An electric conversion unit and a level determination unit that compares the levels of electric signals from the first and second optical / electrical conversion units with a preset disconnection level to determine disconnection / non-disconnection of the main transmission line and the backup transmission line And demultiplexed by the second and third optical multiplexing / demultiplexing units Re or the other of the optical transmission path monitoring switching device characterized by consisting of an optical switch portion for conducting the external transmitting device communication wavelength light by the control signal from the level determination unit. The monitoring light transmitting unit modulates monitoring wavelength light at a constant frequency, and the first and second optical / electrical conversion units are provided with an electrically narrow bandpass filter centered on the frequency. Item 4. The optical transmission line monitoring switching device according to Item 1. 3. The optical transmission line monitoring switching device according to claim 2, wherein a monitoring current output from a photodiode having a built-in laser diode is fed back to a modulation current in order to maintain a constant extinction ratio of the monitoring wavelength light transmitted by the monitoring light transmitter. 2. The optical transmission line monitoring switching device according to claim 1, wherein the monitoring light transmission unit uses a wavelength band of 1370 nm to 1450 nm that is not used for normal communication as a monitoring wavelength, or a wavelength band of 1630 nm or more. The optical transmission line monitoring switching device according to claim 1, wherein the optical switch unit has a latch function for holding a state when the power is turned off.

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