Wavelength division multiplex bothway optical communication system

Bothway optical communication is carried out through a wavelength division multiplex system between two terminal stations (11a, 12a), each having a two-beam interference type filter (15) coupled with an optical cable (10) for multiplexing and/or de-multiplexing transmitted light and received light. The filter (15) has passbands and attenuation bands having periodic characteristics for the wavelength in the communication wavelength band. The wavelength of the transmitted light is essentially the same as the wavelength of the received light in each terminal station, with a small offset of the two wavelengths less than 5 nm. The oscillation wavelength of the laser (21) for transmission is adjusted so that it coincides with the passband of the filter, which doubles as a reference wavelength of the oscillation wavelength of the laser. Preferably, the two-beam interference type filter (15) is a Mach Zehnder type asymmetrical interferometer having a pair of directional couplers (F1, F2) connected to each other through a pair of optical fibers (f1, f2) so that the period of the passbands and/or the attenuation bands of the filter (15) is adjustable.

FIELD OF THE INVENTION 
The present invention relates to a bothway wavelength division multiplex 
optical communication system, in particular, relates to such a system in 
which wavelengths of light in two directions belong to a single wavelength 
band, although those wavelengths differ or offset from each other a little 
in the band, and the wavelengths are finely controlled based upon a single 
wavelength standard through a self tuning operation. The present invention 
is for instance used for subscriber lines in which large numbers of cables 
are necessary and therefore the use of a single optical cable for bothway 
communication is essential. 
BACKGROUND OF THE INVENTION 
Conventionally, a time compression multiplex (TCM) optical communication 
system has been used for a bothway communication system using a single 
optical line. In that system, an upward direction signal and a downward 
direction signal are forwarded to an optical cable alternatively, so that 
the system has been used for low rate communication up to 28 Mbits/second. 
When high rate communication is requested in a system with a single optical 
cable, for instance, the high rate communication is higher than 50 
Mbits/second, a wavelength division multiplex (WDM) optical communication 
system has been used. That system uses two wavelength bands, for instance 
1.3 .mu.m band and 1.5 .mu.m band for an upward direction and a downward 
direction, respectively. 
FIG. 1A shows a block diagram of a prior bothway wavelength division 
multiplex (WDM) optical communication system. In the figure, the numeral 
70 is a single optical fiber cable coupling a pair of terminal stations 71 
and 72. Each terminal station has an optical transmitter 73 which outputs 
a light signal through a laser, an optical receiver 74 which receives an 
optical signal from an opposite side, and a multiplexer/demultiplexer 75 
having an output port coupled with the single optical fiber cable 70 and a 
pair of input ports coupled with the transmitter 73 and the receiver 74 so 
that the light from the transmitter 73 is forwarded to the optical cable 
70 and the light from the line 70 is forwarded to the receiver 74. The 
wavelength of the output of the transmitter 73 differs from that on the 
other side, for instance, the wavelength in one direction is 1.3 .mu.m 
band, and the wavelength in the other direction is 1.5 .mu.m band. Those 
wavelengths of 1.3 .mu.m band and 1.5 .mu.m band are multiplexed in the 
optical cable 70. 
However, the system of FIG. 1A has the disadvantage that a laser in the 
transmitter 73 must provide an oscillation wavelength which coincides with 
a center of a passband of the multiplexer/demultiplexer. However, it 
should be noted that the oscillation wavelength of a laser depends upon 
ambient temperature, bias current and/or producing error, and therefore, 
it is rather difficult to obtain a laser with the requested accurate 
oscillation wavelength. Further, the difference between wavelengths in two 
directions must be large enough for suppressing cross talk between the two 
wavelengths, and therefore, two kinds of lasers through different 
producing processes must be used for a large oscillation wavelength 
difference, for instance, 1.3 .mu.m and 1.5 .mu.m. Therefore, producing 
yield rate of a laser is rather low, and so, the cost of the communication 
system of FIG. 1A is rather high. 
FIG. 1B shows another prior bothway wavelength division multiplex optical 
communication system, which uses the common single wavelength band in both 
directions. The system of FIG. 1B has not existed in the market, but we 
considered it in our research. In the figure, the numeral 80 is an optical 
cable having a pair of ends each coupled with terminal stations 81 and 82. 
Each of the terminal stations has an optical transmitter 83, an optical 
receiver 84 for receiving light from the other side, and a directional 
coupler 85 coupled with an end of the optical cable 80, an output of the 
optical transmitter 83, and an input of the optical receiver 84, so that 
the light from the transmitter 83 is applied to the optical cable 80, and 
the light from the optical cable 80 is applied to the optical receiver 84. 
However, the system is FIG. 1B has the disadvantage that the receiver 84 
receives not only the light from the optical cable 80 but also the light 
from the transmitter 83 in the same terminal station through leakage in 
the directional coupler 85, and therefore, the signal characteristics are 
deteriorated, although the lasers in the transmitters in each stations 
oscillate with the same wavelength as each other. 
SUMMARY OF THE INVENTION 
It is an object, therefore, of the present invention to overcome the 
disadvantages and limitations of the prior bothway wavelength division 
multiplex optical communication system by providing a new and improved 
bothway wavelength multiplex optical communication system. 
It is also an object of the present invention to provide a bothway 
wavelength division multiplex optical communication system which uses 
essentially a common wavelength band in both directions. 
It is also an object of the present invention to provide a bothway 
wavelength division multiplex optical communication system in which a 
filter, for separating receiving wavelengths from transmitting 
wavelengths, doubles as a wavelength standard for a laser in a 
transmitter. 
It is also an object of the present invention to provide a bothway 
wavelength division multiplex communication system in which two terminal 
stations have a master-slave relationship, and the filter for directional 
coupling in the master station provides 1) a wavelength standard for a 
laser in a transmitter in the master station, 2) a filter for directional 
coupling in the slave station, and 3) a laser for a transmitter in the 
slave station. 
The above and other objects are attained by a wavelength division multiplex 
bothway optical communication system comprising a first terminal station, 
a second terminal station coupled with the first terminal station through 
an optical communication cable. Each terminal station comprising a 
transmitter having a laser for converting a transmission electrical signal 
to an optical signal, an optical receiver for receiving the optical signal 
which is subject to conversion to an electrical signal, and a two-beam 
interference type first filter having a zero phase input port and a pi 
phase input port coupled with an output of the laser and an input of the 
optical receiver, respectively, and a zero phase output port and a pi 
phase output port with one port being coupled with the optical 
communication line and the other port being free standing. The filter has 
periodical characteristics of passband and attenuation band for 
wavelengths in the communication wavelength band. Each terminal station 
has a wavelength control means for controlling the oscillation wavelength 
of the laser so that the oscillation wavelength coincides with a center of 
the passband of the filter. The wavelength control means comprises a 
monitor means for monitoring the wavelength of an output of the laser, a 
comparator for comparing a strength of the output of the monitor means 
with a predetermined threshold level to provide an output of comparison, 
and a control means for adjusting the oscillation wavelength of the laser 
based upon the output of the comparator so that the oscillation wavelength 
coincides with the passband of the filter. The wavelength band of the 
laser in the first terminal station is essentially the same as the 
wavelength of the laser in the second terminal station in the 
communication wavelength band. The optical communication cable is coupled 
with the zero phase output port of one terminal station and the pi phase 
output port of the other terminal station.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Some of the important features of the present invention are the use of a 
two-beam interference type filter as a multiplexer/de-multiplexer in a 
wavelength division multiplex communication system, and the two-beam 
interference type filter doubles as a wavelength standard for determining 
an oscillation wavelength of a laser in a transmitter. 
First, a two-beam interference type filter is described in accordance with 
FIGS. 2A-2D. 
FIG. 2A shows the structure of a two-beam interference type directional 
coupler, having a pair of optical fibers C1 and C2 which are optically 
coupled with each other in the region F in a predetermined length so that 
the optical fibers C1 and C2 have a common clad layer in the region F. The 
coupler has a pair of input ports C1-1 and C2-1, and a pair of output 
ports C1-2 and C2-2. As the directional coupler is reversible, the input 
ports and the output ports may be reversed. Each of the input ports and 
each of the output ports are called a zero phase port and a pi phase port. 
In operation of the directional coupler, when input light is applied to one 
of the input ports, the input light is divided to the two output ports so 
that each output port outputs optical power having the level of -3B as 
compared with the input optical power. Therefore, the directional coupler 
of FIG. 2A is called a 3 dB coupler. 
When the coupling region F is designed properly, the ratio of the output 
optical power to each output port depends upon the wavelength of the input 
light. Therefore, it functions as a wavelength filter. 
When it operates as a wavelength filter, assuming that the input light is 
applied to the zero phase port C1-1, it is switched to the output port 
C1-2 or the output port C2-2, depending upon the wavelength of the input 
light. Similarly, input light to the pi phase port C2-1 is switched to the 
output port C1-2 and the output port C2-2, depending upon the wavelength 
of the input light. 
The transmission factor, and/or the attenuation factor of the filter has 
periodic characteristics as shown in FIG. 2B, where the horizontal axis 
shows the wavelength of the input light, and the vertical axis shows the 
strength of the output light when the input light is applied to the zero 
phase input port Cl-1. The solid curve (a) shows the output at the zero 
phase output port Cl-2, and the dotted curve (b) shows the output at the 
pi phase output port C2-2. 
Threshold level L.sub.l and/or L.sub.2 is defined for self tuning purposes 
of a laser as described later. The higher threshold level L.sub.1 is the 
same as the peak output of the filter, or a little lower than the peak 
output. The lower threshold L.sub.2 is almost zero or a little higher than 
zero. 
In practical use, a pair of directional couplers F1 and F2 of FIG. 2A are 
coupled through a pair of optical fibers f1 and f2 as shown in FIG. 2C. 
The symbol F1 and F2 shows a directional coupler in FIG. 2A. The zero 
phase port of the first directional coupler F1 is coupled with the zero 
phase port of the second directional coupler F2 through the optical fiber 
f1, and the pi phase port of the first directional coupler F1 is coupled 
with the pi phase port of the second directional coupler F2 through the 
optical fiber f2. The optical length of the fibers or the optical 
wave-guides f1 and f2 differs by .DELTA.L. We call the structure of FIG. 
2C an asymmetrical Mach Zehnder type filter, or an asymmetrical Mach 
Zehnder type interferometer. 
The advantage of the structure of FIG. 2C is that the period (=2.times.FSR; 
Free Spectrum range) between the wavelength for the peak of the passband 
and the peak of the attenuation band may be adjusted by adjusting the 
difference .DELTA.L of the length of the fibers f1 and f2. 
The frequency band .DELTA.F from the frequency for the peak output to the 
frequency for the minimum output of the filter in FIG. 2C is shown below. 
EQU .DELTA.F=c/(2.times.n.sub.eff .times..DELTA.L) 
where c is light velocity , n.sub.eff is the effective refractive index of 
a waveguide or a fiber, and .DELTA.L is the difference between the lengths 
of optical fibers f1 and f2 coupling directional couplers F1 and F2. A 
Mach Zehnder type filter having .DELTA.F up to 640 GHz is possible to 
produce. 
FIG. 2D shows the modification of an asymmetrical Mach Zehnder type filter, 
which has a heater H on one of the intermediate optical fibers f1 and f2. 
The heater H is heated by the power source P. The structure of FIG. 2D has 
the advantage that the period (FSR or .DELTA.F) or the tuning wavelength 
is controlled finely by controlling the temperature of the heater H. The 
value FSR is increased around 0.5-1.0 nm for each degree of temperature 
when the temperature is increased. That is described in The Institute of 
Electronics, Information and Communication Engineer in Japan, Trans. 
Commun., vol. E75-B, No.9, September 1992, pages 871-879. 
A Mach Zehnder type filter may be produced on a quartz waveguide. When the 
period FSR is wide, the size of the filter is small, the producing error 
is small, and the transmission characteristics of the filter may be 
uniform. 
A port of the filter may have a spherical tapered end so that it is well 
coupled with an optical fiber optically. 
The two-beam interference type directional coupler or a filter of FIGS. 2A 
through 2D is described in Journal of Lightwave Technology, vol 6, No. 6, 
June 1988, pages 1003-1010. 
FIG. 3 shows a block diagram of an embodiment of a wavelength division 
multiplex bothway optical communication system according to the present 
invention. In the figure, a pair of terminal stations 11a and 12a are 
coupled through an optical cable 10. Each of the terminal stations 11a and 
12a has an optical transmitter 13a, an optical receiver 14, and a filter 
15 which operates to multiplex/de-multiplex transmitted light and received 
light. 
The filter 15 is a two-beams interference type filter of FIG. 2A, or an 
asymmetrical Mach Zehnder type interferometer as shown in FIG. 2C and FIG. 
2D. In case of FIG. 2C or FIG. 2D, the period of the passbands of the 
filter is adjustable. 
The filter 15 has the transmission characteristics as shown in FIG. 2B, 
where the solid line shows the transmission characteristics when the input 
light is applied to the zero phase input port, and the output light is at 
the zero phase output port. The dotted line shows the transmission 
characteristics when the input light is applied to the pi phase port, and 
the output light is at the zero phase output port. The dotted line is the 
same as the transmission characteristics at the pi phase output port when 
the input light is applied to the zero phase input port. 
The filter 15 is reversible. Therefore, input ports and output ports may be 
reversed. However, we refer to input ports and output ports for the sake 
of simplicity of explanation. 
It is assumed that the input ports have zero phase port A.sub.0 and pi 
phase port A.sub.1, and the output ports have zero phase port B.sub.0 and 
pi phase port B.sub.1. 
The optical cable 10 is coupled with the outputs of the filters 15 so that 
the transmission characteristics of the output port of the first terminal 
station differs from that of the second terminal station. In the 
embodiment, the cable 10 is coupled with the port B.sub.0 (zero phase) in 
the station 11a, and the port B.sub.1 (pi phase) in the station 12a. 
The optical receiver 14 is coupled with the filter 15 in the port which has 
the same transmission characteristics as that of the output port at the 
opposite station. In the embodiment, the optical receiver 14 in the 
station 11a is coupled with the input port A.sub.1 (pi phase), (the output 
port of the opposite station 12a is B.sub.1 (pi phase), and the optical 
receiver 14 in the station 12a is coupled with the input port A.sub.0 
(zero phase), where the output port of the opposite station 11a is B.sub.0 
(zero phase). 
The optical transmitter 13a is coupled with the filter 15 at the input port 
having the same transmission characteristics as the output port which is 
coupled with the optical cable 10. In the embodiment, the optical 
transmitter 13a in the station 11a is coupled with the input port A.sub.0 
, (zero phase), where the optical cable 10 is coupled with the output port 
B.sub.0, (zero phase), and the optical transmitter 13a in the station 12a 
is coupled with the input port A.sub.1 (pi phase) where the optical cable 
10 is coupled with the output port B.sub.1 (pi phase). 
The light from the transmitter 13a in the station 11a is forwarded, through 
the port A.sub.0 (zero phase) of the filter 15 in the station 11a, the 
output port B.sub.0 , of the filter 15, the optical cable 10, the output 
port B.sub.1 (pi phase) of the filter 15 in the station 12a, and the input 
port A.sub.0 (zero phase) of the filter 15 in the station 12a, to the 
optical receiver 14 in the station 12a. Similarly, the light from the 
transmitter 13a in the station 12a is forwarded, through the port A.sub.1 
(pi phase) of the filter 15 of the station 12a, the port B.sub.1 (pi 
phase) of the filter 15 in the station 12a, the optical cable 10, the port 
B.sub.0 of the filter 15 in the station 11.sub.a, and the port A.sub.1 in 
the filter 15 in the station 11a, to the optical receiver 14 in the 
station 11a. 
Therefore, it should be noted that the ports of the filter are used in an 
opposite manner (opposite phase) in the corresponding means in each 
station. 
It should be appreciated that the wavelength band from the first station 
11a is in the same wavelength band as that from the second station 12a. It 
is for instance 1.3 .mu.m band, or 1.5 .mu.m band. The wavelength from the 
first station 11a differs a little from the wavelength from the second 
station 12a in the same wavelength band. The difference or offset between 
the two wavelengths is for instance less than 5 nm. Thus, two wavelengths 
are multiplexed in an optical cable 10, and bothway communication is 
carried out by using a single optical cable. 
Next, the self tuning of a laser so that the oscillation wavelength of a 
laser 21 coincides with a center of a passband of a filter 15 is 
described. In the embodiment, a second filter 22 for self tuning purposes 
is provided in each station 11a and 12a for defining a reference 
wavelength. The second filter 22 is also a two-beam interference type 
filter, or asymmetrical Mach Zehnder type interferometer, having the same 
passband characteristics as that of the first filter 15. The second filter 
22 has input ports A.sub.0 (zero phase) and A.sub.1 (pi phase), and output 
ports B.sub.0 (zero phase) and B.sub.1, (pi phase). 
A semiconductor laser provides front light and back light in opposite 
directions. Front light is used for transmitting a signal to another 
station, and back light is used for self tuning of the wavelength of the 
laser. 
Back light of the laser 21 is applied to one of the input ports of the 
tuning filter 22, which provides output light to an optical-electrical 
converter or sensor 23 through an output port having the same transmission 
characteristics as that of the output port of the filter 15. In the 
embodiment of FIG. 3, back light of the laser 21 in the station 11a is 
applied to the port A.sub.0 (zero phase) of the filter 22, and the output 
of the port B.sub.0 (zero phase) is applied to the converter 23. 
Similarly, back light in the station 12a is applied to the port A.sub.1 
(pi phase) of the filter 22, and the output of the port B.sub.1 (pi phase) 
is applied to the converter 23. 
The output of the converter 23 is applied to the comparator 24, which 
compares the output of the converter 23 with a predetermined threshold 
level L.sub.1 (FIG. 2B), which is close to the maximum level of the 
transmission power but a little lower than the same. The comparator 24 
provides positive output only when the output of the converter 23 exceeds 
the threshold level. The peak control 25 receives the output of the 
comparator 24, and controls the oscillation wavelength of the laser 21 so 
that output of the reference filter 22 is the maximum. The manner for 
adjusting the oscillation wavelength of a laser is conventional, and it is 
possible by adjusting the temperature of a laser, and/or bias current of a 
laser. 
Two terminal stations 11a and 12a have the above closed feedback loop for 
controlling oscillation wavelength. The station 11a receives back light of 
the laser 21 at the zero phase input port A.sub.0 of the filter 22 which 
outputs the light at the zero phase output port B.sub.0 and the peak 
control 25 controls so that the oscillation wavelength of the laser 21 
coincides with the passband of the filter 22, so that the output of the 
filter 22 is the maximum. Thus, the oscillation wavelength of the laser 21 
is controlled so that it coincides with the wavelength which provides the 
peak or the maximum transmission of the filter 22 for zero phase. 
Similarly, the oscillation wavelength of the laser 21 in the station 12a is 
controlled so that it coincides with the wavelength which provides the 
peak or the maximum transmission of the filter 22 for pi phase. 
Therefore, the wavelength band of two stations is in the same wavelength 
band, and the wavelength of a first station is offset or shifted by the pi 
phase from that of a second station. 
The combination of the reference filter 22, the sensor 23, the comparator 
24, and the control 25 constitute a monitor means for monitoring and 
controlling the oscillation wavelength of the laser 21. 
The two-beam interference type filter 15 has the features that the 
insertion loss is small, and the leakage of the transmitted light to an 
optical receiver is small. 
The filter 15 has the periodical characteristics as shown in FIG. 2B. 
Therefore, it is only necessary to coincide the oscillation wavelength of 
a laser with one of the transmission wavelengths of the filter. 
Further, it is easy and simple in the present invention to coincide the 
transmission wavelength of a filter in a received side with that of a 
transmitted side. 
Further, the requested value of the adjustment of the oscillation 
wavelength of a laser is small, and the use of a pair of lasers having the 
oscillation wavelength in the same band is possible in both the end 
terminals. It should be noted that a laser in a prior art of FIG. 1A must 
be adjusted in wide range so that the oscillation wavelength coincides 
with the passband of a filter, and further, lasers of different 
oscillation wavelengths are requested in two terminal stations. 
In one modification, a peak control 25 may operate so that the output of 
the optical-electrical converter 23 becomes the maximum so that 
transmission optical power into the cable 10 is the maximum, although the 
embodiment of FIG. 3 has the comparator 24 to compare the output of the 
converter 23 with the predetermined threshold. 
In another modification, a two-beams interference type filter may be 
substituted with another filter which has periodical and complementary 
characteristics shown in FIG. 2B, for instance, a ring resonator, and a 
Fabry-Perot Etalon filter, although the former has too high Q, which would 
cause the operation to be unstable, and is difficult to have a long 
period, and the latter is difficult to adjust an optical coupling axis, 
which cannot unify the periodical characteristics between each filter. 
FIGS. 4A, 4B and 4C show modifications of the embodiment of FIG. 3 of the 
present invention. The features of those modifications reside in the 
structure for controlling the oscillation wavelength of a laser to 
coincide with the passband of a filter 15. In those modifications, only 
one end terminal is shown for the sake of simplicity. 
In FIG. 4A, the terminal station 11b has an optical transmitter 13b, an 
optical receiver 14, and a filter 15 which is optically coupled with an 
optical cable 10. The optical transmitter 13b has a laser 21 for 
oscillating transmission light, a reference wavelength filter 22, an 
optical-electrical converter 23, a comparator 24, and a bottom control 26. 
The feature of FIG. 4A as compared with the embodiment of FIG. 3 is that 
the reference wavelength filter 22 receives back light of the laser 21 on 
the pi phase port A.sub.1 when front light of the laser is applied to the 
zero phase port A.sub.0 of the filter 15. That is to say, back light of 
the laser is received by the reference wavelength filter 22 on the port 
which is in a different phase from that of the receiving port of the 
filter 15. 
The comparator 24 in this case compares the output of the converter 23 with 
the threshold which is close to the bottom value L.sub.2 in the output of 
the filter (see FIG. 2B). The bottom control 26 controls the oscillation 
wavelength of the laser 21 so that the output of the reference filter 22 
is less than the threshold L.sub.2. Thus, the oscillation wavelength of 
the laser 21 coincides with the passband and of the filter 22. 
FIG. 4B shows a block diagram of another modification of the embodiment of 
FIG. 3. The feature of FIG. 4B is that the terminal station 11c has no 
reference wavelength filter, but an asymmetrical branch ratio type 
directional coupler 31 is provided between an output of a filter 15 and an 
optical cable 10, so that a small part of transmission optical power is 
branched to be applied to an optical-electrical converter or a sensor 23. 
A control 25 in this case is a peak control which controls a laser so that 
output power of the sensor 23 is higher than a predetermined threshold 
L.sub.1 . The filter 15 operates not only for multiplexing and/or 
de-multiplexing transmitted light and received light, but doubles as a 
wavelength reference filter. 
It should be noted that the filter 15 and the directional coupler 31 are 
integrated on the common substrate B, by using a quartz waveguide. 
FIG. 4C shows a block diagram of still another modification of the 
embodiment of FIG. 3. The feature of FIG. 4C is that the terminal station 
11d has no reference wavelength filter, but the optical-electrical 
converter 23 receives the light from the free standing output port B.sub.1 
of the filter 15 for the purpose of controlling the oscillation wavelength 
of the laser 21. The control 26 in this case is a bottom control which 
controls the output of the sensor 23 to be the bottom L.sub.2 or the 
minimum. 
FIGS. 5A-5B show a block diagram of wavelength division multiplex bothway 
optical communication system according to the present invention. The 
embodiment of FIGS. 5A-5B is the combination of the present wavelength 
division multiplex system of FIGS. 3 through 4C, and the prior art of FIG. 
1B, and may communicate by using four wavelengths. 
In FIGS. 5A-5B, the terminal stations 11e, 11f, 12e and 12f are a terminal 
station of FIG. 3, FIG. 4A, FIG. 4B or FIG. 4C. The terminal stations 11e 
in one terminal and the terminal station 12e in the other terminal operate 
for instance in 1.3 .mu.m band, and the terminal stations 11f and 12f 
operate for instance in 1.5 .mu.m. The wavelengths of 1.3 .mu.m band and 
1.5 .mu.m band are multiplexed or de-multiplexed in the 
multiplexer/demultiplexer 75. Therefore, four wavelengths (two wavelengths 
in 1.3 .mu.m band, and two wavelengths in 1.5 .mu.m band) are multiplexed 
in the optical cable 10. 
FIGS. 6A-6B show a block diagram of the optical communication system, 
according to the present invention, which functions to coincide the 
wavelength of a laser and a filter in both the end stations. In FIGS. 
6A-6B, one of the end stations operates as a master station, and the other 
end station operates as a slave station which follows the master station 
in wavelength of a laser and a filter. In FIGS. 6A-6B, the station (a) 
operates as a master station, and the station (b) operates as a slave 
station. 
The master station (a) in FIG. 6A has the structure of FIG. 4C. However,it 
should be appreciated of course that another structure, for instance, FIG. 
3, FIG. 4A or FIG. 4B may be possible. 
The basic idea for coinciding the wavelength of a laser and a filter in 
both the end stations is as follows: 
a) A filter for multiplexing/demultiplexing transmitted light and received 
light in a master station operates to provide a reference wavelength. 
b) The oscillation wavelength of a laser for providing transmitted light in 
a master station is controlled so that is coincides with the passband of 
the filter in the master station. 
c) The filter for multiplexing/demultiplexing transmitted light and 
received light in a slave station is then controlled so that the passband 
of the filter in the slave station receives the maximum level of light 
from the master station. 
d) The oscillation wavelength of a laser for providing transmitted light in 
the slave station is then controlled so that it coincides with the 
passband of the filter in the slave station. 
Therefore, it should be appreciated that the passband of the filter in both 
stations coincide with each other, and the oscillation wavelength of the 
lasers in both stations coincides with the passband. 
In FIGS. 6A-6B, the master station 11d has a filter 15M, which functions as 
the reference of the wavelength in communication system. The oscillation 
wavelength of a laser 21M for providing transmitted light in the master 
station 11d is then controlled so that it coincides with the passband of 
the filter 15M in the master station 11d, as described in accordance with 
FIG. 4C. 
In a slave station, optical light applied to the slave station (FIG. 6B) 
from the optical cable 10 is applied to an optical-electrical converter 
23b through the filter 15S. The electrical signal of the output of the 
converter 23b is then applied to an optical receiver 14 which has a linear 
amplifier 102 functioning as an AGC (automatic gain control). The output 
of the amplifier 102 provides the received signal of the optical 
communication system. The output of the amplifier 102 is also applied to a 
thermal control circuit 101 which supplies a heater H in the filter 15S 
power so that the passband of the filter 15S is shifted according to the 
power thus applied so that the output of the amplifier 102 has the maximum 
level. Thus, the center of the passband of the filter 15S, which has a 
pair of directional couplers F1 and F2 coupled with each other through a 
pair of asymmetrical optical fibers one of which has a heater H, is 
controlled so that it coincides with the wavelength of the received light. 
Then, the oscillation wavelength of the laser 21S, for providing 
transmitted light, is controlled so that it coincides with the passband of 
the filter 15S, through the feedback loop having the filter 15S, the 
optical-electrical converter 23a, the comparator 24 which receives the 
threshold level for comparison, and the bottom control circuit 26. The 
operation of the feedback loop has been described in accordance with FIG. 
4C. It should be appreciated of course that the feedback loop for 
controlling the oscillation wavelength of the laser 21S is not limited to 
that of FIG. 4C, but another circuit of FIG. 3, FIG. 4A or FIG. 4B is 
possible. 
In a practical embodiment, it is preferable that the time constant in the 
feedback loop for controlling the passband of the filter 15S, including 
the heater H, and the thermal control circuit 101, is shorter than the 
time constant in the feedback loop for controlling the oscillation 
wavelength of the laser 21, including the comparator 24 and the bottom 
control 26. 
Next, the period of the passband (FSR) of a filter, and the period of 
longitudinal modes of a laser are discussed. 
It is well known that there are two kinds of lasers, a multi-longitudinal 
mode oscillation laser (for instance a Fabry-Perot laser), and a 
single-longitudinal mode oscillation laser (distributed feedback laser, or 
distributed Bragg Reflection laser). 
A multi-longitudinal mode oscillation laser satisfies the following 
equation. 
EQU q.times.(.lambda.)/(2n)=L 
where (.lambda.) is the oscillation wavelength in a vacuum, 
L is the length of a resonator of a laser, 
n is the refractive index of the medium, 
q is the number of standing waves of half wavelength in the resonator. 
As L is much larger than wavelength (.lambda.), many waves having different 
wavelengths from each other are generated in the laser, and the 
oscillation wavelength of the laser is determined so that the gain is the 
maximum in the many waves. For instance, a laser of 1.3 .mu.m has the 
period of longitudinal modes of 0.8 nm (determined by n and L in the above 
equation). 
Therefore, it is preferable that the period (=2.times.FSR) of the passbands 
of a filter coincides with the period of the longitudinal modes of a laser 
so that the output power of the laser is transmitted to a receiver with 
high efficiency, and even when transmitted light of a laser is reflected 
by a connector at an end of an optical cable, it would be prevented by the 
filter with no affection of side modes of a signal light, so that the 
deterioration of an optical receiver due to reflected back light is 
solved. 
When a laser is a single-longitudinal mode oscillation laser, no 
consideration about the periods is necessary. 
FIG. 7 shows experimental curves of side modes in back light where the 
horizontal axis shows a difference of the center wavelength of the 
passband of a filter, and the center oscillation wavelength of a laser in 
nm, and the vertical axis shows attenuation (dB) in back light through the 
filter as compared with the front light. 
The curve (a) in FIG. 7 shows the case that the period of the passbands of 
a filter coincides with the period of the longitudinal modes of a laser, 
and FSR=0.8 nm. It should be appreciated in the curve (a) that the 
attenuation of back light is 30 dB when the center wavelength of the 
passband of the filter coincides with the center oscillation wavelength of 
the laser, and the attenuation is 16 dB when the difference of wavelengths 
is 0.6 nm. 
On the other hand, the curve (b) shows the case that the period of the 
filter does not coincide with the period of the laser, where FSR=5 nm. In 
that case, the attenuation of back light is around 10-12 dB for each 
difference of the wavelengths of the laser and the filter. 
The requested value of attenuation of back light as compared with the 
desired front light is generally 12 dB. Therefore, the curve (a) in which 
the period of the passband of a filter coincides with the longitudinal 
modes of a filter is satisfactory in practice. 
From the foregoing it will not be apparent that a new and improved 
wavelength division multiplex bothway optical communication system has 
been found. It should be understood of course that the embodiments 
discloses are merely illustrative and are not intended to limit the scope 
of the invention. Reference should be made therefore to the appended 
claims rather than the specification as indicating the scope of the 
invention.