Wavelength division multiplexing transmitter receiver, optical transmission system, and redundant system switching method

For wavelength division multiplexing transmission, a standby-system processor capable of handling all wavelengths is included in relation to a plurality of working system processors associated with different wavelengths. If any of the plurality of working system processors fails, the wavelength of a light wave to be processed by the standby-system processor is controlled according to the wavelength being handled by a current- system processor that has failed. A switching unit switches the working system processor that has failed to the standby-system processor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a wavelength division multiplexing 
transmission technology for transmitting and receiving a plurality of 
light waves having different wavelengths over a single optical 
transmission line. More particularly, this invention is concerned with a 
wavelength- division multiplexing transmitter receiver having a redundant 
configuration, an optical transmission system, and a redundant system 
switching method. 
2. Description of the Related Art 
Technologies for effectively utilizing a wide frequency band of 
approximately 20 THz permitted by an optical fiber serving as an optical 
transmission line have been studied in years. In recent years, development 
of a technology for handling the wavelength of light including development 
of optical devices or the like has made progress, an increase in 
processing speed of an electronic circuit has come to a standstill, and an 
optical amplifier repeater has made its debut. For these reasons, there is 
some prospect of realizing a high-speed and large-capacity economic 
repeater transmission line through combination of a technology of 
wavelength division multiplexing (WDM) transmission, in which a plurality 
of light waves having different wavelengths are multiplexed and 
transmitted over one optical fiber, and a technology of optical 
amplification repeating transmission, in which the light waves to be 
multiplexed in wavelength are amplified simultaneously. 
Various kinds of apparatuses designed for optical communication are 
expected to offer high reliability. In particular, apparatuses connected 
over a trunk are requested to guarantee higher reliability. In a WDM 
transmission approach, since a large amount of information can be 
transmitted by performing multiplexing relative to a plurality of 
wavelengths, it has a significant meaning whether or not high-reliability 
hardware can be realized. 
For a known optical transmitter receiver for transferring a light wave 
having a single wavelength which has been adopted before the WDM 
transmission approach is established, a configuration in which, for 
example, an auxiliary system having the same configuration as a current 
system is included so that if the current system fails, it can be switched 
to the auxiliary system has been adopted as a redundant configuration for 
keeping line reliability high. Like the known redundant configuration, a 
WDM transmitter receiver may presumably include current systems and 
auxiliary systems, and thus guarantee high reliability. This redundant 
configuration includes current systems and auxiliary systems associated 
with wavelengths of light waves to be multiplexed. 
According to the above redundant configuration, if for example, part of 
current systems for processing a light wave with a certain wavelength 
fails, the current system is switched to an auxiliary system. At this 
time, the other parts of the current systems for processing light waves 
with the other wavelengths (current systems that do not fail) are also 
switched to auxiliary systems. This poses a problem that use efficiency is 
poor and a transmitter receiver costs high. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide an 
inexpensive WDM transmitter receiver having a simple redundant 
configuration capable of ensuring high reliability and an optical 
transmission system, and to also provide a redundant system switching 
method for the WDM transmitter receiver. 
A WDM transmitter receiver in accordance with the present invention for 
accomplishing the above object is a WDM transmitter receiver for 
transmitting and receiving a plurality of light waves having different 
wavelengths by combining and separating them. The WDM transmitter receiver 
comprises: a plurality of working system processing means, associated with 
the different wavelengths, for processing the plurality of light waves; a 
standby-system processing means capable of handling all the wavelengths of 
the plurality of light waves; a standby-system control means that if any 
of the plurality of working system processing means fails, produces a 
wavelength control signal which changes the wavelength of a light wave to 
be processed by the standby-system processing means according to the 
wavelength of a light wave being processed by a working system processing 
means that has failed; and a switching means for switching the working 
system processing means that has failed to the standby-system processing 
means. 
Owing to the above configuration, a standby-system processing means capable 
of handling wavelengths being handled by a plurality of working system 
processing means is included in relation to the plurality of working 
system processing means associated with different wavelengths. If any of 
the plurality of working system processing means fails, the wavelength of 
a light wave to be processed by the standby-system processing means is 
controlled according to the wavelength being handled by a working system 
processing means that has failed. A switching means then switches the 
working system processing means that has failed to the standby-system 
processing means. 
Compared with a redundant configuration in which current systems and 
auxiliary systems are included in one- to-one correspondence, the 
configuration of a standby-system processing means can be simplified. 
Consequently, a WDM transmitter receiver in which minimizing the scale and 
cost of the transmitter receiver has been achieved, and use efficiency and 
line reliability are retained high can be provided. The redundant 
configuration is independent of the number of concurrent wavelengths to be 
handled by a WDM system. Therefore, the larger the number of multiplex 
wavelengths is, the simpler the configuration of an auxiliary system gets 
relative to that of a current system. The effect of minimizing cost and 
improving reliability increases. In addition, upgrading the WDM system, 
such as, modifying the number of multiplex wavelengths can be coped with 
flexibly. 
Moreover, the standby-system processing means may include a light-emitting 
unit capable of emitting a plurality of light waves having different 
wavelengths, and an emission wavelength control means for changing the 
wavelength of a light wave to be emitted by the light- emitting unit 
according to a wavelength control signal. 
Specifically, if part of a working system processing means for transmitting 
a light wave fails, the wavelength of an optical output of the 
light-emitting unit in the standby-system processing means is controlled 
by the emission wavelength control unit according to the wavelength of a 
light wave being transmitted by the part of the working system processing 
means. The light-transmitting part of the working system processing means 
that has failed is then switched to the standby-system processing means. 
Furthermore, the standby-system processing means includes a light-receiving 
unit capable of receiving a plurality of light waves having different 
wavelengths. In addition, the standby-system processing means may include 
a variation optical filter capable of varying the passband of wavelengths 
according to a wave control signal so that a light wave passing through 
the variation optical filter can be received by the light-receiving unit. 
Owing to the above configuration, if part of a current- system processing 
means for receiving a light wave fails, the light-receiving part of the 
working system processing means is switched to a standby-system processing 
means including a light-receiving unit capable of receiving the light wave 
to be received by the light-receiving part. In addition, when the 
variation optical filter is included, only a light wave having the 
wavelength being handled by the working system processing means that has 
failed is received by the light-receiving unit. Accordingly, compared with 
a configuration in which a plurality of light waves with different 
wavelengths, which have propagated over an optical transmission line, are 
received directly by the light- receiving unit, the influence of noises is 
minimized, and a signal-to-noise ratio improves. Consequently, reception 
sensitivity can be raised. This is especially effective in coping with 
accumulated spontaneous emission light (ASE) noises stemming from 
long-distance systematization or multi-stage repeating realized with 
optical amplifiers, or with deterioration of reception sensitivity caused 
by various phenomena including the nonlinear effect of an optical fiber. 
The WDM transmitter receiver may be configured to include an 
occurrence-of-failure conveying means for conveying occurrence of a 
failure in any of the plurality of working system processing means to a 
WDM transmitter receiver in a remote station connected over an optical 
transmission line. The occurrence-of-failure conveying means may include 
an occurrence-of-failure signal production unit for producing an 
occurrence-of-failure signal indicating that any of the plurality of 
working system processing means has failed, and a signal superposition 
unit for superposing the occurrence-of-failure signal on a multiplexed 
light wave to be transmitted over the optical transmission line. 
Alternatively, the occurrence-of-failure conveying means may be configured 
so that if any of the plurality of working system processing means fails, 
the occurrence of the failure is conveyed to the WDM transmitter receiver 
in the remote station on the basis of the intermittent discontinuity of 
the multiplexed light wave, which stems from the occurrence of the 
failure, being transmitted over the optical transmission line . 
Specifically, since the occurrence-of-failure conveying means is included, 
switching performed in a local station with occurrence of a failure can be 
used to trigger inspection at a remote station. This is effective when the 
cause of a failure lies in a remote station or a whole WDM system 
employed. Moreover, when occurrence of a failure in a working system 
processing means is conveyed to a remote station with intermittent 
discontinuity of a multiplexed light wave, occurrence of a failure can be 
conveyed by a simpler configuration. Furthermore, the cost of a 
transmitter receiver can be minimized and the reliability thereof can be 
improved. 
Moreover, a redundant system switching method for a WDM transmitter 
receiver including current systems and an auxiliary system in accordance 
with the present invention comprises: a local station switching step at 
which if a current system for processing one of a plurality of light waves 
fails, a wavelength to be handled by the auxiliary system in a local 
station is controlled according to the wavelength of the light wave, and 
the current system that has failed is switched to the auxiliary system in 
the local station; an occurrence-of-failure conveying step of conveying 
occurrence of a failure in the local station to a WDM transmitter receiver 
in a remote station connected over an optical transmission line; and a 
remote station switching step of controlling a wavelength to be handled by 
an auxiliary system in the remote station according to the occurrence of a 
failure conveyed from the local station, and switching a corresponding 
current system in the remote station to the auxiliary system in the remote 
station. 
Herein, the occurrence-of-failure conveying step may include a step of 
producing an occurrence-of-failure signal indicating that a failure has 
occurred in the local station, and a step of superposing the 
occurrence-of-failure signal on a multiplexed light wave to be transmitted 
over the optical transmission line. Alternatively, at the 
occurrence-of-failure conveying step, occurrence of a failure in the local 
station may be conveyed to the remote station on the basis of the 
intermittent discontinuity of the multiplexed light wave, which stems from 
the occurrence of the failure, to be transmitted over the optical 
transmission line. 
Concurrent switching in a local station and a remote station with 
occurrence of a failure can be achieved by the method described above, and 
this method is effective in a case that a source of failure is in the 
remote station or in the whole system currently used, therefore, can cope 
with various applications. 
An optical transmission system in accordance with the present invention is 
an optical transmission system including at least first and second 
terminal stations each having a WDM transmitter receiver for transmitting 
and receiving a plurality of light waves with different wavelengths by 
combining and separating them. In the optical transmission system, the 
first and second terminal stations each comprise a plurality of working 
system processing means, associated with the wavelengths of the plurality 
of light waves, for processing the light waves, a standby-system 
processing means capable of handling all the wavelengths of the plurality 
of light waves, and a standby-system control means that if any of the 
plurality of working system processing means fails, produces a wavelength 
control signal which changes the wavelength of a light wave to be 
processed by the standby-system processing means according to the 
wavelength of a light wave being processed by a working system processing 
means that has failed, and a switching means for switching the working 
system processing means that has failed to the standby-system processing 
means. 
According to the above configuration, in each of the first and second 
terminal stations, a standby-system processing means capable of handling 
wavelengths to be handled by a plurality of working system processing 
means is included in relation to the plurality of working system 
processing means associated with different wavelengths. If any of the 
plurality of working system processing means fails, the wavelength of a 
light wave to be handled by the standby-system processing means is 
controlled according to the wavelength being handled by a working system 
processing means that has failed. A switching means switches the working 
system processing means that has failed to the standby-system processing 
means. 
Compared with a redundant configuration in which current systems and 
auxiliary systems are included in one- to-one correspondence in each 
terminal station, the above configuration makes it possible to simplify 
the configuration of the standby-system processing means. Consequently, 
the scale and cost of a WDM system can be minimized, and an optical 
transmission system enjoying high use efficiency and high circuit 
reliability can be provided. This redundant configuration is independent 
of the number of concurrent wavelengths to be handled by the WDM system. 
Therefore, the larger the number of multiplex wavelengths is, the simpler 
the configuration of an auxiliary system becomes relative to the 
configuration of a current system. The effect of minimizing cost and 
improving reliability increases. In addition, upgrading the WDM system, 
such as, modifying the number of multiplex wavelengths can be coped with 
flexibly. 
Herein, the optical transmission system may include an 
occurrence-of-failure conveying means for conveying the fact that a 
failing working system processing means in the first terminal station will 
be switched to the standby-system processing means to the second terminal 
station. Furthermore, the second terminal station may be designed to 
detect failure information sent from the first terminal station and to 
switch the working system processing means to the standby-system 
processing means. 
In other words, occurrence of a failure in the first terminal station is 
conveyed to the second terminal station by the occurrence-of-failure 
conveying means. Switching a corresponding working system processing means 
to the standby-system processing means in the second terminal station is 
carried out according to the failure information sent from the first 
terminal station. 
As mentioned above, since the occurrence-of-failure conveying means is 
included, switching in a local station deriving from occurrence of a 
failure can be used to trigger inspection in a remote station. This is 
effective when the cause of a failure is present in a remote station or in 
a whole WDM system employed. 
Other objects, features, and advantages of the present invention will be 
apparent from the description below of embodiments relevant to the 
appended drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To begin with, a WDM transmission approach will be described briefly. 
FIG. 1 shows a typical example of the system configuration conformable to a 
general WDM transmission approach (for multiplexing two waves). 
In FIG. 1, low-order group signals Trib.1 to Trib.6 that are six groups of 
electric waves are input in threes to multiplexers 11M and 12M (MUX1 and 
MUX2) in a transmission side, and multiplexed. Thereafter, the resultant 
signals are input to optical transmission units 21S and 22S (OS(.lambda.1) 
and OS(.lambda.2)), and converted electro-optically into light waves with 
wavelengths .lambda.1 and .lambda.2. These light waves are combined by an 
optical coupler 31 (CPL1) and then transmitted over an optical fiber F 
serving as an optical transmission line. 
In a reception side, the light wave propagating over the transmission line 
is bifurcated by an optical coupler 32 (CPL2). From one of the light 
waves, only a light wave component with a wavelength .lambda.1 is 
extracted by an optical filter 21F (Fil(.lambda.1)). Extracted light wave 
components are converted into electric waves by an optical reception unit 
21R (OR(.lambda.1)), and separated into low-order group signals Trib.1 to 
Trib.3 by a demultiplexer 11D (DMUX1). The same applies to the other light 
wave. That is to say, the other light wave is recomposed into low-order 
group signals Trib.4 to Trib. 6 by means of an optical filter 22F 
(Fil(.lambda.2)), an optical reception unit 22R (OR(.lambda.2)), and a 
demultiplexer 12D (DMUX2). 
FIG. 2 is a block diagram of a WDM transmitter receiver employed in the 
aforesaid WDM transmission system. 
A WDM transmitter receiver shown in FIG. 2 has a configuration in which the 
transmission side and reception side shown in FIG. 1 are united. To be 
more specific, a multiplexer demultiplexer 11 (MULDEX1) that is a 
combination of the multiplexer 11M and demultiplexer 11D, and an optical 
transmission/reception unit 21 (OS/OR1) that is a combination of the 
optical transmission unit 21S, optical reception unit 21R, and optical 
filter 21F are included as a component (system 1) for processing a light 
wave with a wavelength .lambda.1. Moreover, a multiplexer demultiplexer 12 
(MULDEX2) that is a combination of the multiplexer 12M and demultiplexer 
12D, and an optical transmission/reception unit 22 (OS/OR2) that is a 
combination of the optical transmission unit 22S, optical reception unit 
22R, and optical filter 22F are included as a component (system 2) for 
processing a light wave with a wavelength .lambda.2. Furthermore, an 
optical coupler unit 30 (CUP) that is a combination of the optical coupler 
31 for combining and transmitting light waves with wavelengths .lambda.1 
and .lambda.2 over an optical fiber Fs, and the optical coupler 32 for 
bifurcating a light wave propagating over the optical fiber Fr is also 
included. 
Now, a redundant configuration including a current system and an auxiliary 
system will be discussed in relation to the foregoing WDM transmitter 
receiver. 
FIG. 3 shows an example of a block diagram of a WDM transmitter receiver 
having a redundant configuration. 
In a WDM transmitter receiver shown in FIG. 3, two systems each having the 
configuration shown in FIG. 2 are included. One of the systems is used as 
a current system, and the other system is used as an auxiliary system. In 
both the systems, high-order groups of signals (light waves) are combined 
by optical couplers 71 and 72 (CPLs and CPLr), and low-order groups of 
signals (electric waves) are combined by switching units 81 and 82 (SEL1 
and SEL2). 
In a normal operation mode, the optical transmission/reception units 21 and 
22 in the auxiliary system are retained in a cold standby state. No light 
wave is output from the auxiliary system. If the current system fails, 
optical transmission units in the optical transmission/reception units 21 
and 22 in the current system stop outputting a light wave. Instead, the 
optical transmission/reception units 21 and 22 in the auxiliary system 
retained in the cold standby state become active, and transmit and receive 
a light wave. A transmitted light wave is sent over the optical fiber Fs 
via the optical coupler 71, while a received light wave propagating over 
the optical fiber Fr is processed by the auxiliary system via the optical 
coupler 72. Responsively, the switching units 81 and 82 that have 
connected to low-order signal input/output ports of the current system 
before the occurrence of the failure select low-order group signal 
input/output ports in the auxiliary system. Thus, switching the current 
system to the auxiliary system is completed. 
However, using the foregoing redundant configuration, for example, if the 
multiplexer demultiplexer 11 or optical transmission/reception unit 21 in 
the current system fails, the current system is switched to the auxiliary 
system. In this case, the multiplexer demultiplexer 12 and optical 
transmission/reception unit 22 in the current system which do not fail are 
also switched to those in the auxiliary system. Thus, use efficiency is 
poor and the transmitter receiver costs high. FIG. 3 shows the 
configuration in which two wavelengths are handled for multiplexing. As 
the number of concurrent wavelengths increases to be 4, 8, 16, etc., and 
n, the transmitter receiver gets larger in scale. Besides, since the 
number of components increases, this invites an increase in cost. 
Embodiments of the present invention, which are realized in consideration 
of the above problems, will be described. 
FIG. 4 is a block diagram showing the configuration of a WDM transmitter 
receiver of a first embodiment in accordance with the present invention. 
In the first embodiment, a WDM transmitter receiver to be used when the 
number of concurrent wavelengths is two, that is, to be used for 
transmitting light waves with two different wavelengths .lambda.1 and 
.lambda.2 by performing wavelength multiplexing will be described. 
In FIG. 4, the WDM transmitter receiver comprises, like the configuration 
of the current system shown in FIG. 3, a multiplexer demultiplexer 11 
(MULDEX1) and optical transmission/reception unit 21 (OS/OR1) constituting 
a component (system 1) for processing a light wave with a wavelength 
.lambda.1, a multiplexer demultiplexer 12 (MULDEX2) and optical 
transmission/reception unit 22 (OS/OR2) constituting a component (system 
2) for processing a light wave with a wavelength .lambda.2, and an optical 
coupler unit 30 (CUP) for combining or bifurcating light waves to be 
transmitted or received by the optical transmission/reception units 21 and 
22. Thus, the system 1 and system 2 function as working system processing 
means. 
An standby-system processing means includes a multiplexer demultiplexer 40 
(MULDEX1-2), an optical transmission/reception unit 50 (OS/OR1-2) for 
selecting and processing one of the light waves with the wavelengths 
.lambda.1 and .lambda.2, and a wavelength control unit 60 serving as a 
standby-system control means for controlling changing of the wavelength of 
a light wave to be processed by the optical transmission/reception unit 
50. 
Furthermore, switching units 81 and 82 (SEL1 and SEL2) for switching 
low-order group signals Trib.1 to Trib.3 and Trib.4 to Trib.6 over to the 
multiplexer demultiplexers 11 and 12 in the current system or the 
multiplexer demultiplexer 40 in the auxiliary system, an optical coupler 
71 (CPLS) for outputting a light wave transmitted from the optical coupler 
unit 30 in the current system or a light wave transmitted from the optical 
transmission/reception unit 50 in the auxiliary system over an optical 
fiber Fs serving as an optical transmission line used for transmission, 
and an optical coupler 72 (CPLr) for bifurcating a light wave propagating 
over an optical fiber Fr serving as an optical transmission line used for 
reception, and then transmitting resultant light waves to the optical 
coupler 30 in the current system and the optical transmission/reception 
unit 50 in the auxiliary system are included as a means for switching the 
current system to the auxiliary system. 
The multiplexer demultiplexer 11 (12) is, like the one shown in FIG. 2, a 
combination of the capability of a multiplexer and the capability of a 
demultiplexer. The multiplexer demultiplexer 11 (12) multiplexes low-order 
group signals Trib.1 to Trib.3 (Trib.4 to Trib.6) input via the switching 
unit 81 (82), and outputs a resultant signal to the optical 
transmission/reception unit 21 (22). Moreover, the multiplexer 
demultiplexer 11 (12) derives the low-order group signals Trib.1 to Trib.3 
(Trib.4 to Trib.6) from a light wave output from the optical 
transmission/reception unit 21 (22), and outputs the low-order group 
signals. 
The optical transmission/reception unit 21 includes, like the one shown in 
FIG. 2, an optical transmission unit 21S, an optical reception unit 21R, 
and an optical filter 21F. The optical transmission unit 21S allows an 
external modulator or the like to modulate continuous-wave light with the 
wavelength .lambda.1, which is generated by, for example, a laser diode, 
according to a signal output from the multiplexer demultiplexer 11, and 
transmits resultant light to the optical coupler unit 30. The optical 
reception unit 21R uses a light-receiving device to receive a light wave 
passing through the optical filter 21F, converts it into an electric wave, 
and sends the electric wave to the multiplexer demultiplexer 11. The 
light-receiving device shall have sufficient sensitivity relative at least 
to light with the wavelength .lambda.1. Moreover, the optical transmission 
unit 21S and optical reception unit 21R include an optical output 
monitoring unit and optical input monitoring unit respectively which are 
not shown. The optical output monitoring unit and optical input monitoring 
unit output an optical output interception alarm signal OS1 and optical 
input interception alarm signal OR1 respectively to the wavelength control 
unit 60 and switching unit 81 respectively. The optical output 
interception alarm signal OS1 and optical input interception alarm signal 
OR1 make, for example, a low-to-high transition when no optical output or 
input is supplied because of occurrence of a failure. The optical filter 
21F inputs one of light waves resulting from bifurcation performed on a 
light wave propagating over the optical fiber Fr by the optical coupler 
unit 30, extracts a component with the wavelength .lambda.1 from the input 
light, and sends the component to the optical reception unit 21R. 
The optical transmission/reception unit 22 has the same configuration as 
the optical transmission/reception unit 21, but handles the wavelength 
.lambda.2 instead of the wavelength .lambda.1. The description of the 
optical transmission/reception unit 22 will therefore be omitted. 
The optical coupler unit 30 is, like the one shown in FIG. 2, a combination 
of an optical coupler 31 for combining light waves with the wavelengths 
.lambda.1 and .lambda.2, which are output from the optical 
transmission/reception units 21 and 22, and outputting a resultant light 
wave over the optical fiber Fs, and an optical coupler 32 for bifurcating 
a light wave propagating over the optical fiber Fr and outputting 
resultant light waves to the optical transmission/reception units 21 and 
22. 
The multiplexer demultiplexer 40 multiplexes low-order group signals Trib.1 
to Trib.3 or low-order group signals Trib.4 to Trib.6, which are input 
responsively to switching of the switching units 81 and 82, outputs a 
resultant signal to the optical transmission/reception unit 50. Moreover, 
the multiplexer demultiplexer 40 derives the low-order group signals 
Trib.1 to Trib.3 or low-order group signals Trib.4 to Trib.6 from a light 
wave output from the optical transmission/reception unit 50, and outputs 
the low-order group signals Trib.1 to Trib.3 or low-order group signals 
Trib.4 to Trib.6. 
The optical transmission/reception unit 50 includes, as shown in FIG. 5, an 
optical transmission unit 50S (OS(.lambda.1-2)) connected to the optical 
coupler 71, a variable passband-of- wavelengths optical filter 50F 
(Fil(.lambda.1-2)) serving as a variable optical filter connected to the 
optical coupler 72, and an optical reception unit 50R (OR(.lambda.1-2)) 
serving as a light reception unit connected to the variable passband-of- 
wavelengths optical filter 50F. 
The optical transmission unit 50S includes, as shown in FIG. 6(a), a laser 
diode LD serving as a light-emitting unit, and an external modulator LN. 
The laser diode LD is driven according to an LD wavelength control signal, 
which will be described later, sent from the wavelength control unit 60, 
and the wavelength of an optical output of the laser diode LD is 
controlled according thereto. As a means for controlling the wavelength of 
an optical output, for example, a Peltier device is used to control the 
temperature of the laser diode LD in order to control the wavelength of an 
optical output so that the wavelength thereof is variable between the 
wavelengths .lambda.1 and .lambda.2. FIG. 6(b) shows an example of the 
characteristic of the wavelength of an optical output relative to a 
control voltage (proportional to the temperature of the laser diode LD) to 
be applied to the Peltier device. FIG. 6(b) shows an example in which the 
wavelength of an optical output changes substantially in proportion to the 
control voltage to be applied to the Peltier device, and the control 
voltage must be changed by a value .DELTA.V (=V2-V1) in order to vary the 
wavelength from the value .lambda.1 to .lambda.2. An optical output of the 
laser diode LD is modulated by the external modulator LN according to a 
signal sent from the multiplexer demultiplexer 40, and then transmitted to 
the optical coupler 71. The technology for controlling temperature so as 
to control and vary the wavelength of an optical output of the laser diode 
LD has already been known. The means for controlling the wavelength of an 
optical output of the laser diode LD is not limited to the foregoing one. 
Alternatively, for example, a current or the like may be controlled so 
that the wavelength of an optical output will be variable. 
The variable passband-of-wavelengths optical filter 50F is, for example, a 
known mechanical or electronic optical filter capable of varying the 
passband of wavelengths according to a passband-of-wavelengths control 
signal, which will be described later, output from the wavelength control 
unit 60. Herein, when a voltage v1 or v2 is applied to the variable 
passband-of-wavelengths optical filter 50F, the passband of wavelengths is 
changed to the wavelength .lambda.1 or .lambda.2. When a voltage v0 is 
applied, light is intercepted. 
The optical reception unit 50R uses a light-receiving device, which is not 
shown, to receive a light wave passing through the variable 
passband-of-wavelengths optical filter 50F, converts it into an electric 
wave, and outputs the electric wave to the multiplexer demultiplexer 40. 
The light-receiving device shall have sufficient sensitivity relative at 
least to light with the wavelengths .lambda.1 and .lambda.2. 
The wavelength control unit 60 inputs, as shown in FIG. 5, optical output 
interception alarm signals OS1 and OS2, and optical input interception 
alarm signals OR1 and OR2 which are output from the optical 
transmission/reception units 21 and 22. Based on these input signals, a 
control processing unit 61 produces LD wavelength control data and 
passband-of-wavelengths control data. The LD wavelength control data and 
passband-of-wavelength control data are converted into analog signals; the 
LD wavelength control signal and passband-of-wavelengths control signal by 
D/A converters 62 and 63, and then sent to the optical 
transmission/reception unit 50. 
When a high-level optical output interception alarm signal OS1 or a 
high-level optical input interception alarm signal OR1 is input, the LD 
wavelength control signal is used to drive the optical transmission unit 
50S and to apply a voltage V1 to the Peltier device in order to control 
the wavelength of an optical output so that the wavelength thereof will be 
set to the wavelength .lambda.1. By contrast, when a high-level optical 
output interception alarm signal OS2 or a high-level optical input 
interception alarm signal OR2 is input, the optical transmission unit 50S 
is driven, and a voltage V2 is applied to the Peltier device in order to 
control the wavelength of an optical output so that the wavelength thereof 
will be set to the wavelength .lambda.2. In any other case, that is, in a 
normal state in which the optical output interception alarm signals OS1 
and OS2 and the optical input interception alarm signals OR1 and OR2 are 
low, the optical transmission unit 50S is brought to a cold standby state. 
When a high-level optical output interception alarm signal OS1 or 
high-level optical input alarm signal OR1 is input, the 
passband-of-wavelengths control signal is used to apply a voltage v1 to 
the variable passband-of-wavelengths optical filter 50F and thus control 
the passband of wavelengths so that the passband of wavelengths will be 
set to the wavelength .lambda.1. By contrast, when a high-level optical 
output interception alarm signal OS2 or high-level optical input 
interception alarm signal OR2 is input, a voltage v2 is applied to the 
variable passband-of-wavelengths optical filter 50F in order to control 
the passband of wavelengths so that the passband of wavelengths will be 
set to the wavelength .lambda.2. In the normal state, as mentioned above, 
a voltage v0 is applied to the variable passband-of-wavelengths optical 
filter 50F in order to intercept light. 
When the optical output interception alarm signal OS1 (OS2) and optical 
input interception alarm signal OR1 (OR2) are low, the switching unit 81 
(82) transmits low-order group signals Trib.1 to Trib.3 (Trib.4 to Trib.6) 
to the multiplexer demultiplexer 11 (12). When the optical output 
interception alarm signal OS1 (OS2) or optical input interception alarm 
signal OR1 (OR2) is driven high, switching is carried out so that the 
low-order group signals Trib.1 to Trib.3 (Trib.4 to Trib.6) will be 
transmitted to the multiplexer demultiplexer 40. 
Next, operations carried out in accordance with the first embodiment will 
be described. 
To begin with, a state in which the components of the WDM transmitter 
receiver operate normally will be discussed. In this case, low-order group 
signals Trib.1 to Trib.3 and Trib.4 to Trib.6 pass through the switching 
units 81 and 82, are input to the working system multiplexer 
demultiplexers 11 and 12, multiplexed, and then sent to the optical 
transmission/reception units 21 and 22. The optical transmission/reception 
units 21 and 22 output light waves with the wavelengths .lambda.1 and 
.lambda.2, which have been modulated according to the signals sent from 
the multiplexer demultiplexer 11 and 12, to the optical coupler unit 30. 
The light waves with the wavelengths .lambda.1 and .lambda.2 are combined 
by the optical coupler unit 30, and then sent over the optical fiber Fs 
via the optical coupler 71. At this time, since the standby-system optical 
transmission unit 50S is retained in a cold standby state, only the light 
wave sent from the current system is input to the optical coupler 71. 
A light wave propagating over the optical fiber Fr and containing 
components with the wavelengths .lambda.1 and .lambda.2 is bifurcated by 
the optical coupler 72, and sent to the current system and auxiliary 
system. At this time, since the variable passband-of-wavelengths optical 
filter 50F is in a state for intercepting light, the auxiliary system does 
not receive a light wave from the optical coupler 72. In the current 
system, the light wave output from the optical coupler 72 is bifurcated by 
the optical coupler unit 30 and then sent to the optical 
transmission/reception units 21 and 22. In the optical 
transmission/reception units 21 and 22, the optical filters 21F and 22F 
extract the components with the wavelengths .lambda.1 and .lambda.2 from 
in the light waves sent from the optical coupler unit 30. Thereafter, the 
optical reception units 21R and 22R convert the components opti 
electrically, and then send resultant electric waves to the multiplexer 
demultiplexers 11 and 12. The multiplexer demultiplexers 11 and 12 derives 
low-order group signals Trib.1 to Trib.3 and low-order group signals 
Trib.4 to Trib.6 respectively from the electric waves sent from the 
optical transmission/reception units 21 and 22, and provide the low-order 
group signals as output signals to an external unit via the switching 
units 81 and 82. 
As mentioned above, in a normal operation mode, since the working system 
optical transmission/reception units 21 and 22 each generate an optical 
output and optical input, and allow the optical output monitoring unit and 
optical input monitoring unit to sense the optical output and input, the 
optical output interception alarm signals OS1 and OS2, and the optical 
input interception alarm signals OR1 and OR2 are driven low. These states 
are listed in the columns concerning the normal operation mode in Table 1. 
In Table 1, the alarm signals and control signals, and the wavelength at 
which the auxiliary system operates are classified and written down in the 
transmission-related columns and reception-related columns. Numerical 
characters in the column "Item" correspond to the numerical characters in 
FIG. 5. 
TABLE 1 
______________________________________ 
Both 
Signal & Operating 
Normal 
systems 
System 
System 
Unit Item wavelength state fail 2 fails 
2 fails 
______________________________________ 
Optical output 
Low High High Low 
interception 
alarm signal OS1 
Optical output 
Low High Low High 
interception 
alarm signal OS2 
LD wavelength 
V0 V0 V1 V2 
control signal 
Wavelength of 
Not Not .lambda.1 
.lambda.2 
output of optical 
emitted 
emitted 
transmission 
unit OS (.lambda.1-2) 
Optical input 
Low High High Low 
interception 
alarm signal OR1 
Optical input 
Low High Low High 
nterception 
alarm signal OR2 
Passband-of- v0 v0 v1 v2 
wavelengths 
control signal 
Wavelength passed 
Inter- 
Inter- 
.lambda.1 
.lambda.2 
by variable cepted 
cepted 
optical filter 
Fil (.lambda.1-2) 
______________________________________ 
Next, a situation in which a failure occurs in the current system will be 
discussed. The situation in which a failure occurs includes, as listed in 
Table 1, a situation in which both systems 1 and 2 fail and a situation in 
which either system 1 or 2 fails. This embodiment adopts the redundant 
configuration in which if one of the systems fails, the current system 
that has failed is switched to the auxiliary system. Accordingly, if both 
the systems should fail, the optical output interception alarm signals OS1 
and OS2, and the optical input interception alarm signals OR1 and OR2 
would be driven high. In this case, the transmitter receiver would stop 
operating. A practical example of the situation in which both the systems 
fail is presumably a situation in which the optical transmission/reception 
units 21 and 22 break down simultaneously or a situation in which no 
optical input is supplied to both the systems because of disconnection of 
the optical fiber Fr. 
To begin with, if system 1 fails, since at least one of an optical output 
and optical input is not supplied from or to the optical 
transmission/reception unit 21, the optical output interception alarm 
signal OS1 and optical input interception alarm signal OR1 are driven high 
(See the columns concerning "System 1 fails" in Table 1) and sent to the 
wavelength control unit 60 and switching unit 81. 
In the wavelength control unit 60, the control processing unit 61 produces 
LD wavelength control data and passband-of-wavelengths control data on the 
basis of the high-level optical output interception alarm signal OS1 and 
high-level optical input interception alarm signal OR1. The LD wavelength 
control data is converted into an analog LD wavelength control signal by 
the D/A converter 62. The LD wavelength control signal is used to drive 
the auxiliary- system optical transmission unit 50S and to apply a voltage 
V1 to the Peltier device in order to control the wavelength of an optical 
output so that the wavelength thereof will be set to the wavelength 
.lambda.1. The passband-of-wavelengths control data is converted into an 
analog passband-of-wavelengths control signal by the D/A converter 63. The 
passband-of-wavelengths control signal is used to apply a voltage v1 to 
the variable passband-of-wavelengths optical filter 50F in order to 
control the passband of wavelengths so that the passband of wavelengths 
will be set to the wavelength .lambda.1. 
The switching unit 81 switches the transmission route of low-order group 
signals Trib.1 to Trib.3 from the multiplexer demultiplexer 11 to the 
multiplexer demultiplexer 40 in response to a high-level optical output 
interception alarm signal OS1 and optical input interception alarm signal 
OR1. 
Owing to the foregoing operations, the auxiliary system operates on behalf 
of current system 1 that has failed. The standby-system optical 
transmission unit 50S outputs a light wave with the wavelength .lambda.1, 
and the optical coupler 71 combines the light wave with a light wave with 
the wavelength .lambda.2 sent from the current system 2, and transmits a 
resultant light wave. A light wave propagating over the optical fiber Fr 
is bifurcated by the optical coupler 72, and sent to the current system 2 
and auxiliary system. In the auxiliary system, the variable 
passband-of-wavelengths optical filter 50F extracts a light wave component 
with the wavelength .lambda.1, and the optical reception unit 50R and 
multiplexer demultiplexer 40 separate low-order group signals Trib.1 to 
Trib.3. In the current system, low-order group signals Trib.4 to Trib.6 
are separated in the same manner as they are in a normal state. The 
low-order group signals Trb.1 to Trib.3 and Trib.4 to Trib.6 are output to 
outside of the transmitter receiver via the switching unit 82. 
Next, if system 2 fails, since at least one of an optical output and 
optical input of the optical transmission/reception unit 22 is not 
supplied, the optical output interception alarm signal OS2 and optical 
input interception alarm signal OR2 are driven high (See the columns 
concerning "System 2 fails" in Table 1), and then sent to the wavelength 
control unit 60 and switching unit 82. 
The wavelength control unit 60 allows the control processing unit 61 and 
D/A converters 62 and 63 to produce an LD wavelength control signal and 
passband-of-wavelengths control signal in response to the high-level 
optical output interception alarm signal OS2 and optical input 
interception alarm signal OR2. The LD wavelength control signal is used to 
drive the optical transmission unit 50S and to apply a voltage V2 to the 
Peltier device in order to control the wavelength of an optical output so 
that the wavelength thereof will be set to the wavelength .lambda.2.The 
passband-of-wavelengths control signal is used to apply a voltage v2 to 
the standby-system variable passband-of-wavelengths optical filter 50F in 
order to control the passband of wavelengths so that the passband of 
wavelengths will be set to the wavelength .lambda.2. 
The switching unit 82 switches the transmission route of low-order group 
signals Trib.4 to Trib.6 from the multiplexer demultiplexer 12 to the 
multiplexer demultiplexer 40 in response to a high-level optical output 
interception alarm signal OS2 and optical input interception alarm signal 
OR2. 
Owing to the foregoing operations, the auxiliary system operates on behalf 
of current system 2 that has failed. The standby-system optical 
transmission unit 50S outputs a light wave with the wavelength .lambda.2. 
The optical coupler 71 combines the light wave with a light wave with the 
wavelength .lambda.1 output from current system 1, and transmits a 
resultant light wave over the optical fiber Fs. A light wave propagating 
over the optical fiber Fr is bifurcated by the optical coupler 72 and sent 
to the current system 1 and auxiliary system. In the auxiliary system, the 
variable passband-of-wavelengths optical filter 50F extracts a light wave 
component with the wavelength .lambda.2, and the optical reception unit 
50R and multiplexer demultiplexer 40 separate low-order group signals 
Trib.4 to Trib.6. In the current system 1, low-order group signals Trib.1 
to Trib.3 are separated in the same manner as they are in a normal state. 
These low-order group signals Trib.1 to Trib.3 and Trib.4 to Trib.6 are 
output to outside of the transmitter receiver via the switching unit 81. 
As mentioned above, according to the first embodiment, an auxiliary system 
for processing a light wave with a certain wavelength is included in 
relation to a current system for processing light waves with two different 
wavelengths .lambda.1 and .lambda.2. If a component in the current system 
for handling one of the wavelengths fails, a wavelength to be handled by 
the auxiliary system is controlled according to the one wavelength. The 
current system that has failed is then switched to the auxiliary system. 
This obviates the necessity of including working system components and 
standby-system components in one-to-one correspondence. The auxiliary 
system can therefore be configured simply. Consequently, a WDM transmitter 
receiver whose scale and cost has been minimized and which enjoys high use 
efficiency and high circuit reliability can be provided. Moreover, since 
the variable passband-of-wavelengths optical filter 50F is included in the 
reception stage in the auxiliary system, only light waves with the 
wavelength .lambda.1 or .lambda.2 are received by the optical reception 
unit 50R. Compared with a configuration in which only a light-receiving 
device, of which quantum efficiency is plotted substantially flat relative 
to a wavelength employed, is used to make the optical reception unit 50R 
compatible with certain wavelengths, this configuration makes it possible 
to minimize the influence of noises, improve a signal-to-noise ratio, and 
raise reception sensitivity. This is especially effective in coping with 
deterioration of reception sensitivity caused by various phenomena such as 
accumulated spontaneous emission light (ASE) noises stemming from 
long-distance systematization or multi-stage repeating realized with 
optical amplifiers, or by a nonlinear effect exerted by an optical fiber. 
Next, a second embodiment of the present invention will be described. 
In the second embodiment, a WDM transmitter receiver in which the number of 
concurrent wavelengths is n will be discussed. 
FIG. 7 is a block diagram showing the configuration of a WDM transmitter 
receiver of the second embodiment. Components identical to those in the 
first embodiment shown in FIG. 4 are assigned the same reference numerals. 
The description of the components will be omitted. 
In FIG. 7, the WDM transmitter receiver has a current system composed of 
systems 1 to n for processing light waves with wavelengths .lambda.1 to 
.lambda.n, and an optical coupler unit 30 for combining or separating 
light waves transmitted and received by the systems. The configuration of 
systems 1 to n is identical to that of systems 1 and 2 in the first 
embodiment. Numerical characters corresponding to system numbers are 
appended to the symbols denoting components of the systems. 
Moreover, a multiplexer demultiplexer 40' (MULDEX1-n), an optical 
transmission/reception unit 50' (OS/OR1-n) for selecting one of the light 
waves with the wavelengths .lambda.1 to .lambda.n, and processing the 
selected one, and a wavelength control unit 60 for controlling changing of 
the wavelength of a light wave to be processed by the optical 
transmission/reception unit 50' are included as an auxiliary system. The 
multiplexer demultiplexer 40' multiplexes or separates three groups of 
signals out of low-order group signals Trib.1 to Trib.3n responsively to 
switching by switching units 81 to 8n (SEL1 to SELn). The optical 
transmission/reception unit 50' has components that are similar to those 
of the optical transmission/reception unit 50 in the first embodiment, and 
that are designed to be able to handle the wavelengths .lambda.1 to 
.lambda.n. 
Operations carried out in accordance with the second embodiment having the 
above components are basically identical to those carried out in 
accordance with the first embodiment except that the number of concurrent 
waves has increased from two to n. States of signals and the like 
indicating the operations in the second embodiment are listed in Table 2. 
The description of the operations will be omitted. Noted is that V1 to Vn 
in Table 2 denote voltages to be applied to the Peltier device in order to 
control the wavelength of an optical output so that the wavelength thereof 
will be set to the wavelengths .lambda.1 to .lambda.n, and v1 to vn denote 
voltages to be applied to the variable passband-of-wavelengths optical 
filter (Fil(.lambda.1-n)) in order to give control the passband of 
wavelengths so that the passband of wavelengths will be set to the 
wavelengths .lambda.1 to .lambda.n. 
TABLE 2 
______________________________________ 
Sys- Sys- Sys- . . . 
Sys- 
Signal & Ope- 
Nor- tem tem tem . . . 
tem 
rating wave- 
mal 1 2 3 . . . 
n 
Unit Item length state 
fails 
fails 
fails 
. . . 
fails 
______________________________________ 
Optical out- 
Low High Low Low . . . 
Low 
put inter- 
ception alarm 
signal OS1 
Optical input 
Low Low High Low . . . 
Low 
interception 
alarm signal 
OS2 
Optical out- 
Low Low Low High . . . 
Low 
put inter- 
ception alarm 
signal OS3 
Optical out- 
Low Low Low Low . . . 
High 
put inter- 
ception alarm 
signal OSn 
LD wavelength 
V0 V1 V2 V3 . . . 
Vn 
control signal 
Wavelength of 
Not .lambda.1 
.lambda.2 
.lambda.3 
. . . 
.lambda.n 
output of op- 
emit- 
tical trans- 
ted 
mission unit 
OS (.lambda.1-n) 
Optical input 
Low High Low Low . . . 
Low 
interception 
alarm signal 
OR1 
Optical input 
Low Low High Low . . . 
Low 
interception 
alarm signal 
OR2 
Optical input 
Low Low Low High . . . 
Low 
interception 
alarm signal 
OR3 
Optical input 
Low Low Low Low . . . 
High 
interception 
alarm signal 
ORn 
Passband-of- 
v0 v1 v2 v3 . . . 
vn 
wavelengths 
control 
signal 
Wavelength Inter- 
.lambda.1 
.lambda.2 
.lambda.3 
. . . 
.lambda.n 
passed by cept- 
variable ed 
filter Fil (.lambda.1-n) 
______________________________________ 
As mentioned above, according to the second embodiment, when the number of 
multiplex wavelengths is n, an auxiliary system for processing a light 
wave with a certain wavelength is included in relation to current systems 
1 to n for processing light waves with wavelengths .lambda.1 to .lambda.n. 
If any of the current systems fails, a wavelength to be handled by the 
auxiliary system is controlled according to the wavelength being handled 
by a current system that has failed, and the current system that has 
failed is switched to the auxiliary system. The same advantages as those 
provided by the first embodiment can therefore be exerted. Thus, the 
redundant configuration of the WDM transmitter receiver is independent of 
the number of multiplex wavelengths to be handled by a wavelength 
multiplexing system. Therefore, the larger the number of multiplex 
wavelengths is, the simpler the configuration of the auxiliary system 
becomes relative to the configuration of the current systems. The effect 
of minimizing cost and improving reliability increases. Moreover, 
upgrading the wavelength multiplexing system, such as, modifying the 
number of multiplex wavelengths can be coped with flexibly. 
In the second embodiment, an auxiliary system for handling one wavelength 
is included in relation to current systems 1 to n. Alternatively, a 
plurality of configurations each having the same configuration as the 
auxiliary system (however, the number of configurations is smaller than 
the number of wavelengths, n) may be included so that a plurality of 
failing systems can be coped with. 
Next, the third embodiment of the present invention will be described. 
The third embodiment is concerned with a WDM transmitter receiver in which 
the number of multiplex wavelengths is, similarly to that in the first 
embodiment, two. If a failure occurs, a signal indicating a state in which 
a failing system in a local station is switched to an auxiliary system 
therein is transmitted to a remote station, and a corresponding system in 
the remote station is also switched to an auxiliary system therein. This 
configuration can be regarded as an optical transmission system having the 
local station as a first terminal station and the remote station as a 
second terminal station. 
FIG. 8 is a block diagram showing the configuration of a WDM transmitter 
receiver of the third embodiment. Components identical to those in the 
first embodiment shown in FIG. 4 are assigned the same reference numerals. 
The description of the components will be omitted. 
In FIG. 8, the configuration of the WDM transmitter receiver is different 
from that in the first embodiment in points that an optical coupler 72' 
for trisecting a light wave propagating over the optical fiber Fr is 
substituted for the optical coupler 72, and that a command detection unit 
90 for inputting one of light waves output from the optical coupler 72' 
and detecting a switching control command, which will be described later, 
serving as an occurrence-of-failure signal, and a switching control unit 
100, which serves as an occurrence-of-failure signal production unit, for 
sending a wavelength control command to the wavelength control unit 60 on 
the basis of the detected switching control command, and sending a 
switching control command to optical transmission/reception units 21' and 
22' (OS/OR1' and OS/OR2') in the current system according to a switching 
control instruction sent from the wavelength control unit 60 are included. 
In the optical transmission/reception units 21' and 22', the configuration 
of optical transmission units 21S' and 22S' is, as described later, 
different from that of the optical transmission units 21S and 22S in the 
first embodiment. The other components are identical to those in the first 
embodiment. 
The command detection unit 90 includes, as shown in FIG. 9, an optical 
filter 90F (Fil(.lambda.1, 2)) for extracting light wave components with 
wavelengths .lambda.1 and .lambda.2 from a light wave output from the 
optical coupler 72', a light- receiving unit 90R (O/E) for receiving a 
light wave having passed through the optical filter 90F and converting it 
into an electric wave, and a detector 90D (DET) for detecting a switching 
control command sent from a remote station on the basis of the 
photo-electrically converted wave, and outputting the command to the 
switching control unit 100. The light-receiving unit 90R is a 
light-receiving unit having a bandwidth characteristic that covers the 
frequency of a carrier, which will be described later, modulated according 
to the switching control command. 
The switching control unit 100 produces a wavelength control command on the 
basis of the switching control command sent from the remote station and 
detected by the command detection unit 90, and transmits it to the 
wavelength control unit 60. The wavelength control command is a command 
used to control the wavelength to be handled by an auxiliary system in a 
local station according to the switched state in a remote station which is 
expressed by the switching control command. A switching control 
instruction indicating that a current system in a local station has failed 
and been switched to an auxiliary system is sent from the wavelength 
control unit 60. Based on the switching control instruction, the switching 
control unit 100 produces a switching control command to be transmitted to 
the remote station and transmits it to the optical transmission/reception 
units 21' and 22'. 
The optical transmission unit 21S' includes, as shown in FIG. 10, an 
internal oscillator OSC, a mixing modulator MOD, an LD drive DRV, and an 
optical amplifier AMP in addition to a laser diode LD and external 
modulator LN which are identical to those of the optical transmission unit 
21S (FIG. 3) in the first embodiment. A switching control command sent 
from the switching control unit 100 is mixed with a carrier, of which 
frequency is, for example, 10 MHz and which is supplied from the internal 
oscillator OSC, by the mixing modulator MOD. A resultant signal is 
superposed on an LD driving signal produced by the LD drive DRV. The LD 
driving signal on which the switching control command is superposed is 
used to drive the laser diode LD, whereby a light wave directly modulated 
according to the switching control command is generated. The light wave is 
then modulated by the external modulator LN according to primary signal 
data, amplified by the optical amplifier AMP, and then transmitted to the 
optical coupler unit 30. The optical transmission unit 21S' thus serves as 
a signal superposition unit. Incidentally, by monitoring an optical output 
of the optical amplifier AMP, an optical output interception alarm signal 
OS1 is produced in the same manner as that in the first embodiment. 
Moreover, the configuration of the optical transmission unit 22S' is 
identical to that of the optical transmission unit 21S'. The description 
of the configuration will therefore be omitted. 
A switching control command, and light waves generated by the optical 
transmission units 21S' and 22S' will be described more particularly. 
To begin with, the switching control command is, as shown in FIG. 11(a), 
for example, a four-bit digital signal indicating a switched state in the 
auxiliary system. The first and last bits of the four bits are start and 
stop bits indicating the start and end of a switching control command. Two 
intermediate bits indicate a switched state. A state in which no failure 
occurs and no current system is switched to the auxiliary system is 
indicated with a bit stream of, for example, (1,0,0,1). A state in which 
current system 1 fails and a wavelength to be handled by the auxiliary 
system is set to the wavelength .lambda.1 is indicated with a bit stream 
of, for example, (1,0,1,1). A state in which current system 2 fails and 
the wavelength to be handled by the auxiliary system is set to the 
wavelength .lambda.2 is indicated with a bit stream of, for example, 
(1,1,0,1). 
When the foregoing switching control command is sent from the switching 
control unit 100 to the optical transmission units 21S' and 22S', the 
mixing modulator MOD mixes the command with a carrier so as to modulate 
the carrier. The mixing is illustrated in FIG. 11(b). The switching 
control command component is expressed with the envelope of the carrier. 
Furthermore, the waveform of an optical output produced by the LD drive, 
which is driven with an LD driving signal on which the carrier modulated 
with the switching control command is superposed, and modulated externally 
by the external modulator LN is shown schematically in FIG. 11(c). 
Apparently, the light intensity (amplitude) of a primary signal data 
component modulated at a high speed varies depending on the carrier. 
Next, the operations in the third embodiment in accordance with the present 
invention will be described. 
Similarly to the operations carried out in accordance with the first 
embodiment, if one of current systems in a local station fails, a 
high-level optical output interception alarm signal and high-level optical 
input interception alarm signal indicating the failing system are sent to 
the wavelength control unit 60. A wavelength to be handled by the 
auxiliary system is then controlled, and the failing system is switched to 
the auxiliary system. Concurrently, the wavelength control unit 60 sends a 
switching control instruction, of which contents depend on a state in 
which switching to the auxiliary system is carried out, to the switching 
control unit 100. Based on the switching control instruction, the 
switching control unit 100 produces a switching control instruction shown 
in FIG. 11(a). This control command is input to the optical 
transmission/reception units 21' and 22'. The optical 
transmission/reception units 21' and 22' each send an optical output shown 
in FIG. 11(c) to the optical coupler unit 30. The sent optical outputs are 
combined by the optical coupler unit 30, and sent over the optical fiber 
Fs via the optical coupler 71. Information concerning a failure occurring 
in the local station is thus conveyed to a remote station by a light wave 
propagating over the optical fiber Fs. 
In a remote station having the same configuration as the local station, the 
light wave containing the switching control command and propagating over 
the optical fiber Fr is trisected by the optical coupler 72'. One of the 
three portions of the trisected light wave is sent to the command 
detection unit 90, and the other two portions are sent to the current 
system and auxiliary system in the same manner as those in the first 
embodiment. In the command detection unit 90, an optical filter 90F inputs 
a light wave output from the optical coupler 72', and extracts light wave 
components with wavelengths .lambda.1 and .lambda.2 from the light wave. 
The light wave passing through the optical filter 90F is converted into an 
electric wave by a light-receiving unit 90R. The light-receiving unit 90R 
has a bandwidth characteristic that covers the frequency (10 MHz) of a 
carrier. Only a carrier component contained in a received light wave is 
therefore converted photo-electrically. An output of the light-receiving 
unit 90R is like the one shown in FIG. 11(b). The output of the 
light-receiving unit 90R is input by the detector 90D, whereby the 
switching control command is demodulated and sent to the switching control 
unit 100. An output of the detector 90D is like the one shown in FIG. 
11(a). 
The switching control unit 100 recognizes a state, in which a current 
system in a local station is switched to the auxiliary system, on the 
basis of the switching control command detected by the command detection 
unit 90, produces a wavelength control command used to control the 
wavelength to be handled by the auxiliary system according to the result 
of the recognition, and sends the command to the wavelength control unit 
60. 
The wavelength control unit 60 drives the auxiliary system and controls the 
wavelength to be handled by the auxiliary system in the same manner as 
that in the first embodiment according to the wavelength control command 
sent from the switching control unit 100. The current system associated 
with the wavelength is then switched to the auxiliary system. However, 
unlike that in the first embodiment, a current system to be switched 
operates normally. 
As mentioned above, according to the third embodiment, when a current 
system in a local station which has failed is switched to an auxiliary 
system, the switched state is conveyed to a remote station. A 
corresponding current system in the remote station is then switched to an 
auxiliary system in line with the local station. Consequently, if a 
failure occurs, switching can be made concurrently in both a local station 
and a remote station, that is, one optical transmission line can be 
switched to another. A range of variation of system construction can 
therefore be expanded. In the WDM transmission approach, physically, one 
optical transmission line is used for each of transmission and reception 
(one pair of optical fibers). Optically, this means that there are the 
same number of transmission lines as the number of multiplex wavelengths. 
If a WDM transmission system is configured so that one transmission line 
is switched to another in case of a failure, a flexible redundant 
configuration can be constructed. For example, an application in which if 
one transmission channel fails, the channel is switched to another channel 
can be coped with by the WDM transmission system. Specifically, in the 
aforesaid embodiments, a wavelength to be handled by an auxiliary system 
can be set to either a wavelength .lambda.1 or .lambda.2. If the 
wavelength can also be set to another wavelength .lambda.3, another 
transmission line can be constructed. A difference between the wavelengths 
.lambda.1 and .lambda.2 gets smaller because of time-passing changes of 
components concerned. In this case, it may become hard for a reception 
unit to identify a wavelength. In this case, if a current system 
associated with one of the wavelengths is switched to an auxiliary system 
associated with the wavelength .lambda.3 for each transmission line 
(remote station), the WDM transmission system can be rescued. 
Next, a fourth embodiment in accordance with the present invention will be 
described. 
In the fourth embodiment, unlike the third embodiment, it is not carried 
out that a switching control command is superposed on an optical output in 
order to convey a state, in which a current system is switched to an 
auxiliary system, to a remote station. Instead, a remote station judges 
the state, in which a current system is switched to an auxiliary system, 
on the basis of intermittent discontinuity of an optical output. 
FIG. 12 is a block diagram showing the configuration of a WDM transmitter 
receiver of the fourth embodiment. Components identical to those in the 
first embodiment shown in FIG. 4 are assigned the same reference numerals. 
The description of the components will be omitted. 
In FIG. 12, the configuration of the WDM transmitter receiver is different 
from that in the first embodiment in a point that optical 
transmission/reception units 21" and 22" (OS/OR1" and OS/OR2") are 
substituted for the optical transmission/reception units 21 and 22 in the 
first embodiment. The other components are identical to those in the first 
embodiment. 
The optical transmission/reception unit 21" includes the same facilities as 
the optical transmission/reception unit 21 in the first embodiment. In 
addition, the optical reception unit 21R has the ability to sense 
intermittent discontinuity of an optical input. Specifically, when a light 
wave with a wavelength .lambda.1 having propagated the optical fiber Fr, 
been bifurcated by the optical coupler 72 and optical coupler unit 30, and 
then passed through the optical filter 21F is received by the optical 
reception unit 21R, if intermittent discontinuity of the optical input is 
sensed, an intermittent discontinuity alarm signal OR1' makes a 
low-to-high transition. Intermittent discontinuity of an optical input 
occurs at intervals of a period proportional to a wavelength 
associated-with a system in a remote station when the system is switched 
to an auxiliary system. When the intermittent discontinuity alarm signal 
OR1' is driven high, it is judged that a current system in the remote 
station associated with the wavelength .lambda.1 is switched to the 
auxiliary system. The intermittent discontinuity alarm signal OR1' is sent 
to the wavelength control unit 60. 
The optical transmission/reception unit 22" has the same configuration as 
the optical transmission/reception unit 21". The optical 
transmission/reception unit 22" senses intermittent discontinuity of an 
optical input with a wavelength .lambda.2 and outputs a high-level 
intermittent discontinuity alarm signal OR2' to the wavelength control 
unit 60. 
With generation of the high-level intermittent discontinuity alarm signal 
OR1' or intermittent discontinuity alarm signal OR2', the wavelength 
control unit 60 drives the auxiliary system, controls a wavelength to be 
handled by the auxiliary system, and then switches the current system 
associated with the wavelength to the auxiliary system in the same manner 
as that in the first embodiment. However, unlike the first embodiment, the 
current system to be switched operates normally. 
Table 3 below lists states of signals and the like indicating operations 
performed when a failure occurs in a local station and a corresponding 
system in a remote station is switched. 
TABLE 3 
______________________________________ 
Failure in local 
Normal System System 
station state 1 fails 2 fails 
______________________________________ 
Intermittent dis- 
Low High Low 
continuity signal 
OR1' in remote 
station 
Intermittent dis- 
Low Low High 
continuity signal 
OR2' in remote 
station 
LD wavelength control 
V0 V1 V2 
signal in remote 
station 
Passband-of- v0 v1 v2 
wavelengths control 
signal in remote 
station 
Selected wavelength - 
-- .lambda.1 
.lambda.2 
to be handled by 
auxiliary system in 
remote station 
______________________________________ 
When a local station operates normally, the intermittent discontinuity 
alarm signals OR1' and OR2' in a remote station remain low. No current 
system is switched to the auxiliary system. If system 1 in the local 
station fails, the intermittent discontinuity alarm signal OR1' in the 
remote station is driven high. This causes the LD wavelength control 
signal and passband-of-wavelengths control signal to assume voltages V1 
and v1 respectively. A selected wavelength to be handled by the auxiliary 
system is set to the wavelength .lambda.1. By contrast, if system 2 in the 
local station fails, the intermittent discontinuity alarm signal OR2' in 
the remote station is driven high. This causes the LD wavelength control 
signal and passband-of-wavelengths control signal to assume voltages V2 
and v2 respectively. The selected wavelength to be handled by the 
auxiliary system is then set to the wavelength .lambda.2. 
As mentioned above, according to the fourth embodiment, the optical 
transmission/reception units 21" and 22" sense intermittent discontinuity 
of an optical input. A current system is switched to the auxiliary system 
with generation of the high-level intermittent discontinuity alarm signals 
OR1' and OR2'. Owing to this configuration, intermittent discontinuity of 
an optical output occurring in a local station when a failing current 
system is switched to the auxiliary system is sensed as intermittent 
discontinuity of an optical input in a remote station. The state in the 
local station in which the current system is switched to the auxiliary 
system is thus conveyed. Despite the simpler configuration, switching 
(switching one optical transmission line to another) can be achieved 
concurrently in both a local station and remote station. 
In the aforesaid first to fourth embodiments, if one system associated with 
a wavelength fails, a transmitting/receiving part of the system is 
switched to an auxiliary system. The present invention is not limited to 
this mode. Alternatively, only a transmitting part or receiving part that 
has failed may be switched to an auxiliary system. 
The third and fourth embodiments have been described on the assumption that 
the number of multiplex wavelengths is two. Similarly to the second 
embodiment, the third and fourth embodiments are apparently applicable to 
a configuration in which the number of multiplex wavelengths is n. 
Furthermore, occurrence of a failure is conveyed to a remote station with 
switching of a current system in a local station to an auxiliary system. 
Alternatively, since occurrence of a failure to the remote station should 
merely be conveyed with sensing of a failure, a WDM transmitter receiver 
may be configured so that, for example, after a failure is sensed, before 
a failing system in a local station is switched to an auxiliary system, 
the occurrence of the failure is conveyed to a remote station. 
The present invention is not limited to the aforesaid embodiments. From the 
above description, it will be apparent to people with ordinary skill in 
the art that variants can be formed.