JPH06265746A - Bi-directional multi-wavelength transmission equipment - Google Patents

Abstract

PURPOSE:To provide bi-directional multi-wavelength transmission equipment which performs communication at the same wavelength essentially in both directions on one optical cable. CONSTITUTION:Bi-directional optical communication between two terminal stations 11a, 11b (where, 11b is not shown in figure), is performed by wavelength division multiplexing, and each terminal station is coupled with the optical cable 1, and is provided with a two-beam interference type filter 15 which multiplexes/demultiplexes transmission light and reception light. The filter 15 is provided with a pass-band and an attenuation band periodically in a communication wavelength band. The wavelength of the transmission light is almost equal to that of the reception light in each terminal station, however, a small offset less than 5nm exists between them. The wavelength of a transmission laser 21 is controlled so as to conform to the pass-band of the filter also used as wavelength reference. It is desirable that the two-beam interference type filter 15 is a Mach-Zehnder type asymmetric interfer-meter and is provided with a pair of directional couplers and an optical fiber which couples them, and by which the period of the pass-band or the attenuation band can be adjusted.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-way wavelength division multiplexing optical communication system, and more particularly to a single wavelength band in which the wavelengths of light in both directions are slightly different, but the wavelengths comply with a single wavelength standard. The present invention relates to a system that can be precisely controlled by self-tuning operation. The present invention is used, for example, in a subscriber line in which a large number of cables are required and it is important to perform bidirectional communication with one cable.

[0002] Conventionally, as a technique for performing bidirectional communication through one optical communication path, time compression optical multiplex communication (Time C
compression Multiplex opti
cal communication) is known. In this technology, upstream and downstream signals are transmitted alternately,
Used for low speed communication up to 28 Mbits / s.

Wavelength multiplex communication (Wa) is a technique for performing high-speed communication of 50 Mbits / s or more with one optical cable.
velocity Division Multipl
optical communication; W
DM) is known. This technique uses light in different wavelength bands such as a 1.5 μm band and a 1.3 μm band in the up and down directions.

FIG. 1A shows a conventional WDM system, in which one optical cable 70 is coupled to a pair of terminal devices 71 and 72, and each terminal device outputs an optical signal from a laser 73. And an optical receiver 74 for receiving an optical signal from the partner station
And a multiplexer / demultiplexer 75. Optical multiplexer 70
Has an output port coupled to the transmitter 73 and an input port coupled to the receiver 74, the light from the transmitter 73 goes to the optical cable 70, and the light from the cable 70 receives the receiver 74.
Proceed to. The wavelength of the output light of the transmitter 73 is different from the wavelength of the transmitter on the other side, and for example, the wavelength in one direction is 1.3 μm band,
The wavelength in the opposite direction is the 1.5 μm band. Light in the 1.3 μm band and light in the 1.5 μm band are multiplexed in the cable 70.

However, the technique of FIG. 1A has the disadvantage that the oscillation wavelength of the laser of the transmitter 73 must match the center of the passband of the multiplexer / demultiplexer. However, it is difficult to accurately control the oscillation wavelength of the laser because the oscillation wavelength of the laser fluctuates due to temperature, bias current and manufacturing error. Furthermore, in order to reduce crosstalk between wavelengths, the transmission wavelengths in both directions must be widely separated, and therefore, two types of lasers having different manufacturing processes, for example, 1.3 μm band and 1.5 μm band, are used. Must be used. Therefore, the yield of the laser is low and the cost of the communication system is high.

FIG. 1B shows another conventional WDM system, which uses the same wavelength band in both directions. Figure 1B was considered in our study and was not in the market. In the figure, 80 is an optical cable, and 81 and 82 are terminal stations. Each terminal has an optical transmitter 83, an optical receiver 84, and a directional coupler 85. Light from the transmitter 83 goes to the cable 80, and light from the cable 80 goes to the optical receiver 84.

However, the system of FIG.
There is a disadvantage in that not only the light input to the cable 4 is input from the cable 80, but also the light from the transmitter 83 of the same terminal is input due to the leakage of the directional coupler 85. Therefore, although the lasers of the transmitters at both end stations have the same wavelength band, there is a drawback that the signal quality is deteriorated.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a bidirectional wavelength division multiplexing optical communication system which uses a substantially common wavelength in both directions.

Another object of the present invention is to provide a wavelength division multiplexing optical communication system in which a filter for separating a transmission wavelength and a reception wavelength also serves as a wavelength standard of a laser of a transmitter.

Another object of the present invention is that the two end stations are in a master / slave relationship, and the master end station filter is the master end station transmitter laser, the slave end station filter and the slave end station. Another object of the present invention is to provide a wavelength division multiplexing optical communication system which becomes the wavelength standard of the laser of the transmitter.

[0011]

A feature of the present invention is that a transmitter has first and second terminal stations connected by an optical cable, and each terminal station has a laser for converting a transmission electric signal into an optical signal. A receiver for receiving an optical signal and converting it into an electric signal, including a 0-phase port and a π-phase port, including an input port for coupling to the laser and the receiver, including a 0-phase port and a π-phase port, and coupling one to the optical cable A first two-beam interference type filter having an output port for transmitting the laser light, the filter having a pass band and an attenuation band which are periodic with respect to the wavelength within the communication wavelength band, and each terminal station is provided with a laser by a wavelength control means. Control is performed so that the oscillation wavelength of the laser beam coincides with the center of the pass band of the filter, and the wavelength control means monitors the output wavelength of the laser and a comparator that compares the output intensity of the monitoring means with a reference value. When, Control means for controlling the oscillation wavelength of the laser to match the pass band of the filter according to the output of the comparator, and the wavelength bands of the lasers of the first terminal station and the second terminal station are substantially the same, The optical cable is in a bidirectional wavelength division multiplex transmission device that is coupled between the 0-phase output of one terminal and the π-phase output of the other terminal.

[0012]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An important feature of the present invention is that a two-beam interference type filter is used as a multiplexer / demultiplexer of a wavelength division multiplexing communication system, and the two-beam interference type filter is used to control the oscillation wavelength of a laser of a transmitter. It also serves as a standard.

First, referring to FIG. 2, the two-beam interference type filter will be described.

FIG. 2A shows a two-beam interference type directional coupler,
In the region F, the pair of optical fibers C1 and C2 share a clad layer with a predetermined length and are optically coupled. The combiner has input ports C1-1 and C2-1 and output ports C1-2 and C2.
-2. Since the directional coupler is reversible, the input and output ports may be reversed. Each of the input port and the output port is called a 0-phase port or a π-phase port.

When input light is applied to one of the input ports, the input light is split into two output ports, and each output port outputs a power 3 dB lower than the input power.
Therefore, the directional coupler of FIG. 2A is referred to as a 3 dB coupler.

When the coupling region F is designed appropriately, the output light becomes dependent on the wavelength of the input light, and thus operates as a wavelength filter.

When operating as a wavelength filter, for example, when input light is applied to the 0-phase port C1-1, the input light is switched to either the output port C1-2 or C2-2 according to the wavelength. Similarly, the π-phase port C2-
The input light of No. 1 is output ports C1-2 and C2- according to the wavelength.
It is switched to 2.

The transmission characteristic or attenuation characteristic of the filter is shown in FIG. 2B.
The horizontal axis represents the wavelength, and the vertical axis represents the intensity of the output light when the input light is applied to the 0-phase input C1-1. Solid line (a)
Indicates the output of the 0-phase output port C1-2, and the dotted line (b) indicates the output of the π-phase output port C2-2.

Thresholds L ₁ and L ₂ are defined for automatic tuning of the laser described later. The upper threshold L ₁ is equal to or slightly lower than the peak output of the filter, and the lower threshold L ₂ is 0 or slightly higher than 0.

In actual use, a pair of directional couplers F
1 and F2 are coupled by optical fibers f1 and f2 as shown in FIG. 2C. The 0-phase port of the first directional coupler F1 is coupled to the 0-phase port of the second directional coupler F2 by the optical fiber f1, and the π-phase port of the first directional coupler F1 is an optical fiber. It is coupled to the π-phase port of the second directional coupler F2 by f2. The optical path lengths of the optical fibers (optical waveguides) f1 and f2 differ by ΔL. The structure of FIG. 2C is called a Mach-Zehnder type filter or an asymmetric Mach-Zehnder type interferometer.

The advantage of the structure of FIG. 2C is that the period of wavelengths (= 2 · FSR; free spectrum range) giving the peaks of the pass band and the attenuation band can be adjusted by the difference ΔL between the lengths of the fibers f1 and f2. .

The frequency band Δ from the frequency giving the peak output to the frequency giving the minimum output in the filter of FIG. 2C.
F is given by the following equation.

ΔF = C / (2 × n _eff ΔL) where C is the speed of light n _eff is the refractive index of the waveguide or optical fiber ΔL is the difference between the lengths of the optical fibers f1 and f2

640GH for Mach-Zehnder type filter
It is possible to make filters with ΔF up to z.

FIG. 2D shows a modification of the asymmetric Mach-Zehnder type filter, in which one of the intermediate fibers f1 and f2 has a heater H.
Can be made. The heater H is heated by the power source P.
The advantage of the structure of FIG. 2D is that the cycle (FSR or ΔF) can be finely controlled by adjusting the temperature of the heater H. FS
The value of R is 0.5 nm per degree when the temperature rises.
Increase by 1.0 nm. This applies to the Institute of Electronics, Information and Communication Engineers, Transactions, Communications, vol E75-B, N
o-9, September 1992, pp. 871-879.

The Mach-Zehnder type filter is manufactured on a quartz waveguide. If the FSR is wide, the filter size is small, the manufacturing error is small, and the transmission characteristics of the filter are uniform.

The port of the filter has a spherical taper end,
It is possible to improve the optical coupling with the optical fiber.

Two-beam interference filter or FIGS. 2A to 2
Filter D is Journal of Lightwave Technology, vol 6, No 6, June 1988, 1003
It is listed on page -1010.

FIG. 3 shows a block diagram of an embodiment of the present invention. In the figure, a pair of terminal stations 11 a and 12 a are connected by a single optical cable 10. Each terminal is an optical transmitter 1
3a, an optical receiver 14, and a filter 15 that combines and demultiplexes a transmitted wave and a received wave.

The filter 15 is the two-beam interference filter shown in FIG. 2A or the asymmetric Mach-Zehnder interferometer shown in FIGS. 2C and 2D. In the case of FIG. 2C or FIG. 2D, the pass band period of the filter can be adjusted.

The filter 15 has the transmission characteristic of FIG. 2B,
Here, the solid line shows the transmission characteristics to the 0-phase output port when input light is applied to the 0-phase input port, and the dotted line shows the transmission characteristics to the 0-phase output port when input light is input to the π-phase port. Show. The dotted line is the same as the transmission characteristic from the 0-phase input port to the π-phase output port.

The filter 15 is reversible, and the input port and the output port can be reversed, but the terms input port and output port are used here for the sake of simplicity of description.

The input port has a 0-phase port A ₀ and a π-phase port A ₁ , and the output port has a 0-phase port B ₀ and a π-phase port B ₁ .

The optical cable 10 is coupled to the output port of the filter 15 so that the transmission characteristic of the output port of the first terminal station is different from the transmission characteristic of the output port of the second terminal station. In the embodiment, the cable 10 is coupled to the port B ₀ (0 phase) of the terminal station 11a and the port B ₁ (π phase) of the terminal station 12a.

The optical receiver 14 is coupled to the filter 15 so that it has the same transmission characteristic as the output port of the opposite terminal station. In the embodiment, the optical receiver 14 of the terminal station 11a has an input port A ₁ (π phase) (the output port of the partner terminal station 12a is B _1).
(Π phase), and the optical receiver 14 of the terminal station 12a couples to the input port A ₀ (0 phase) (at this time, the partner terminal station 11).
The output port of a is B ₀ (0 phase)).

The coupling between the optical transmitter 13a and the filter 15 is an input port having the same transmission characteristic as that of the output port to the optical cable 10. In the embodiment, the optical transmitter 13a of the terminal station 11a is coupled to the input port A ₀ (0 phase), and the optical cable 10 is coupled to the output port B ₀ (0 phase) at this time. Similarly, the optical transmitter 13a of the terminal station 12a is coupled to the input port A ₁ (π phase), and the optical cable 10 is coupled to the output port B ₁ (π phase) at this time.

The light from the transmitter 13a of the terminal station 11a is transmitted to the terminal station 1
The port A ₀ (0 phase) of the filter 15 of 1a, the output port B ₀ (0 phase) of the filter, the optical cable 10, the output port B ₁ (π phase) of the filter 15 of the terminal station 12a,
The terminal station 1 passes through the input port A ₀ (0 phase) of the filter.
It is sent to the optical receiver 14 of 2a. Similarly, the light from the transmitter 13a of the terminal station 12a is transmitted to the port A ₁ (π phase) of the filter 15 of the terminal station 12a and the port B ₁ (π phase) of the filter.
Through the optical cable 10, the port B ₀ of the filter 15 of the terminal station 11a, and the port A ₁ of the filter, the terminal station 1
It is sent to the optical receiver 14 of 1a.

Therefore, the phase of the port of the filter is reversed by each corresponding means of each terminal station.

The transmission wavelength band from the first terminal station 11a is the second wavelength band.
This is the same as the transmission wavelength band from the terminal station 12a. The wavelength band is, for example, the 1.3 μm band or the 1.5 μm band. The transmission wavelength of the first terminal station 11a is slightly different from the transmission wavelength of the second terminal station 12a in the same wavelength band. The difference between the two wavelengths is, for example, 5 nm or less. In this way, the two wavelengths are multiplexed in the optical cable 10 and bidirectional communication is realized by one optical cable.

Next, the oscillation wavelength of the laser 21 is set to the filter 1
The self-tuning (self-tuning) matching the center of the pass band of No. 5 will be described. In the embodiment, the second filter 22 for self-tuning is provided in each of the terminal stations 11a and 12a to give a reference wavelength. The second filter 22 is also a two-beam interference type filter or an asymmetric Mach-Zehnder type interferometer, and has the same passband characteristics as the first filter 15. The second filter 22 receives the input port A ₀ (0
Phase), A ₁ (π phase) and output ports B ₀ (0 phase), B ₁
(Π phase).

The semiconductor laser outputs front light and rear light. The forward light is used for transmitting a signal to the other end station, and the backward light is used for self-tuning the wavelength of the laser.

The backward light of the laser 21 is applied to one of the input ports of the tuning filter 22 and its output light (filter 1
The output port of the filter 22 having the same transmission characteristic as that of the output port 5 is applied to the photoelectric converter (sensor) 23. Figure 3
In this embodiment, the backward light of the laser 21 of the terminal station 11a is applied to the port A ₀ (0 phase) of the filter 22, and the output light of its output port B ₀ (0 phase) is applied to the converter 23.
Similarly, the rear light of the terminal station 12 a is transmitted to the port A of the filter 22.
₁ (π phase), and the output of its output port B ₁ (π phase) is applied to the converter 23.

The output of the converter 23 is applied to the comparator 24 and compared with a predetermined threshold value. The threshold is approximately equal to the maximum level of transmitted power, but is set to a value slightly lower than this. Comparator 24 produces a positive output when the output of converter 23 is above the threshold. The peak control circuit 25 controls the oscillation wavelength of the laser 21 according to the output of the comparator 24 so that the output of the reference filter 22 becomes maximum. The method of controlling the oscillation wavelength of the laser is known, and is performed by controlling the temperature of the laser or the bias current of the laser.

The two terminal stations 11a and 12a have a closed foot-back loop for controlling the oscillation wavelength described above.
The filter 22 of the terminal station 11a receives the backward light of the laser 21 at the 0-phase input port A ₀ and outputs it from the 0-phase output port B ₀ . The peak control circuit 25 controls the output of the filter 22 to the maximum so that the oscillation wavelength of the laser 21 matches the pass band of the filter 22. In this way, the oscillation wavelength of the laser 21 is controlled to the wavelength that maximizes the zero phase transmission of the filter 22.

Similarly, the oscillation wavelength of the laser 21 of the terminal station 12a is controlled to the wavelength that maximizes the transmission of the π phase of the filter 22.

As described above, the transmission wavelength bands of the two terminal stations are the same, and the transmission wavelengths of the two terminal stations are slightly shifted by π phase.

The combination of the reference filter 22, the sensor 23, the comparator 24, and the control circuit 25 constitutes a monitor means for monitoring the oscillation wavelength of the laser 21.

The two-beam interference filter 15 is characterized by low insertion loss and small leakage of transmitted light to the optical receiver.

Since the filter 15 has the periodic characteristics shown in FIG. 2B, the oscillation wavelength of the laser only needs to match any one of the transmission wavelengths of the filter.

Further, according to the present invention, it is easy and simple to match the transmission wavelength of the filter of the receiving terminal with the transmission wavelength of the filter of the transmitting terminal.

Furthermore, since the adjustment range of the oscillation wavelength of the laser is small, it is possible to use the laser of the same oscillation wavelength band at both terminal stations. In the case of the conventional technique shown in FIG. 1A, since the wavelength adjustment range is wide, lasers operating in different wavelength bands must be provided in each terminal station.

In the embodiment of FIG. 3, the comparator 24 is the sensor 23.
However, as a modified example, the peak control circuit 25 may control so that the output of the sensor 23 is maximized so that the optical output to the cable 10 is maximized.

As another modification, the two-beam interference filter may be replaced with another filter having periodicity and complementarity shown in FIG. 2B, for example, a ring resonator or a Fabry-Perot etalon filter. However, the former has a high Q, which may cause unstable operation, and the latter makes it difficult to match the optical axes, so that the periodic characteristics of the filter tend to be uneven.

FIGS. 4A, 4B and 4C show modifications of FIG. The feature of these modifications is the control structure for matching the oscillation wavelength of the laser with the pass band of the filter 15. In each modification, only one terminal station is shown for simplicity.

In FIG. 4A, the terminal station 11b is the optical transmitter 13b.
And an optical receiver 14 and a filter 15 coupled to the optical cable 10. The optical transmitter 13b includes a laser 21 that oscillates transmitted light, a reference wavelength filter 22, and a photoelectric converter 23.
And a comparator 24 and a bottom control circuit 26. The feature of FIG. 4A compared with FIG. 3 is that the reference wavelength filter 22
However, the front light of the laser 21 is the 0 phase port A of the filter 15.
_When inputting ₀ , the backward light is received by the π-phase port A ₁ . That is, the backward light of the laser 21 is received by the reference wavelength filter 22 at the port having the opposite phase to the receiving port of the filter 15.

The comparator 24 outputs the output of the converter 23 as shown in FIG. 2B.
The threshold value of the output of the filter is compared with a threshold value close to the bottom value L ₂ . The bottom control circuit 26 uses the reference filter 2
The oscillation wavelength of the laser 21 is controlled so that the output of ₂ becomes smaller than the threshold L ₂ . In this way
The oscillation wavelength of the laser 21 matches the pass band of the filter 22.

FIG. 4B is another modification of FIG. 3, which is characterized in that the terminal station 11c does not include a reference wavelength filter, and instead, the asymmetric branch type directional coupler 31 is used as the filter 15.
Is provided between the optical output and the optical cable 10, and a part of the transmitted light output is branched and applied to the photoelectric converter (sensor) 23. The control circuit 25 is a peak control circuit in this example, and controls the laser so that the output of the sensor 23 becomes higher than the threshold L ₁ . The filter 15 operates not only as a wavelength division filter of the transmitted light and the received light, but also as a wavelength reference filter.

The filter 15 and the directional coupler 31 can be integrated on a common substrate B using a quartz waveguide.

FIG. 4C is a further modification of FIG. 3, which is characterized in that the terminal station 11d does not include a reference wavelength filter, and instead, the input light to the photoelectric converter 23 is the free end output of the filter 15. This is to control the oscillation wavelength of the laser 21 at the port B ₁ . In this example, the control circuit 26 controls the output of the sensor 23 to the bottom L _2, that is, to the minimum by the bottom control.

FIG. 5 (FIGS. 5A and 5B) shows FIGS.
Communication using four wavelengths can be performed in an embodiment in which the embodiments of the present invention up to C are combined with the conventional technique of FIG. 1B.

In FIG. 5, the terminal stations 11e, 11f, 12
e and 12f have the configuration of FIG. 3, FIG. 4A, FIG. 4B, or FIG. 4C. The one terminal station 11e and the other terminal station 12e operate in the 1.3 μm band, for example, and the terminal stations 11f and 12f operate in the 1.5 μm band, for example. 1.3 μm band and 1.5 μm
The wavelengths in the band are multiplexed / demultiplexed by the multiplexer / demultiplexer 75. Therefore, four waves (two waves in the 1.3 μm band and two waves in the 1.5 μm band) are multiplexed in the optical cable 10. At each terminal,
Reference numeral 13 is an optical transmitter, 14 is an optical receiver, and 15 is a multiplexing / demultiplexing filter.

FIG. 6 (FIGS. 6A and 6B) shows another embodiment of the present invention, which has a function of matching the wavelength of the laser with the filter of both end stations. In FIG. 6, one terminal station is the master station (master).
The other terminal station operates as a slave station (slave) and causes the wavelengths of the laser and the filter to follow the wavelength of the master station.
In FIG. 6, the station (a) is the master station and the station (b) is the slave station.

The main station (a) of FIG. 6 shows the configuration of FIG. 4C.
However, other configurations are possible, such as the configurations of FIGS. 3, 4A and 4B.

The basics of matching the wavelengths of the laser and the filter at both ends are as follows.

(A) The multiplexing / demultiplexing filter of the main station is used as the reference wavelength.

(B) The oscillation wavelength of the transmission laser of the master station is matched with the pass band of the filter of the master station.

(C) The sub-multiplexing / demultiplexing filter of the slave station is controlled so that the pass band receives the light from the master station at the maximum level.

(D) The oscillation wavelength of the transmitting laser of the slave station is matched with the pass band of the filter of the slave station.

Therefore, the passbands of the filters at both ends match, and the oscillation wavelength of the laser matches the passband.

In FIG. 6, the master station 11d has a filter 15M which functions as a reference wavelength of the communication system.

Laser 21 for providing transmission light at the main station 11d
The oscillation wavelength of M is controlled so as to match the pass band of the filter 15M of the master station 11d, as described with reference to FIG. 4C.

In the slave station, the light applied from the cable 10 to the slave station is applied to the photoelectric converter 23b through the filter 15S. The electric signal output from the converter 23b is AGC.
It is applied to the optical receiver 14 having the linear amplifier 102 having the function. The output of the linear amplifier 102 is the received signal of the communication system. The output of the amplifier 102 is also applied to the thermal control circuit 101, and by supplying power to the heater H of the filter 15S, the pass band of the filter 15S is shifted corresponding to the applied power, and the output of the amplifier 102 is maximized. So that Therefore, the heater H is asymmetrical on one side.
The pass band of the filter 15S having the directional couplers F1 and F2, which are coupled by the fiber having the same, matches the wavelength of the received light. Next, the oscillation wavelength of the transmission laser 21S is passed through a feedback loop having a filter 15S, a converter 23a, a comparator 24 having a reference value, and a bottom control circuit 26.
It is controlled so as to match the pass band of the filter 15S. The operation of the feedback loop has already been described in connection with Figure 4C. The feedback loop for controlling the oscillation wavelength of the laser 21S is not limited to that shown in FIG. 4C, and the circuits shown in FIGS. 3, 4A, and 4B are also possible.

Practically, the heater H and the heat control circuit 101
The time constant of the feedback loop for passband control with the laser 21 having the comparator 25 and the bottom control circuit 26 is
It is preferable that it is shorter than the time constant of the feedback loop for controlling the oscillation wavelength.

Next, the pass band period of the filter (FS
R) and the period of the vertical mode of the laser.

There are two types of lasers, a multi-axis vertical mode laser (for example, Fabry-Perot laser) and a single-axis vertical mode laser (distributed feedback laser or distributed Bragg reflection laser).
It has been known.

The multiaxial vertical mode laser satisfies the following equation.

G × λ / (2n) = L where λ is the wavelength in vacuum L is the length of the resonator of the laser n is the refractive index of the medium g is the number of half-wave standing waves in the resonator is there.

Since L is larger than the wavelength λ, many waves having different wavelengths are generated in the laser, and the oscillation wavelength of the laser has the maximum gain among them. For example, 1.3μ
The m-band laser has a vertical mode spacing of about 0.8 nm (determined by n and L in the above equation).

Therefore, the passband period of the filter (=
2 × FSR) preferably coincides with the cycle of the vertical mode of the laser, whereby the output of the laser is efficiently transmitted to the receiver, and the transmitted light from the laser is reflected at the connector etc. of the optical line section. Even in such a case, it can be blocked by the filter without the influence of the side mode of the signal light, and the characteristic deterioration of the light receiving section due to the reflected return light can be prevented.

When the laser is a single-axis vertical mode laser, the above consideration regarding the period is unnecessary.

FIG. 7 shows the experimental results on the side modes of the back light, the horizontal axis shows the difference (nm) between the center frequency of the pass band of the filter and the center oscillation wavelength of the laser, and the vertical axis shows the back surface after passing through the filter. Attenuation of light to front light (dB)
Indicates.

The curve (a) shows that when FSR = 0.8 nm,
The case where the period of the pass band of the filter matches the period of the vertical mode of the laser is shown. When the center of the pass band of the filter coincides with the center wavelength of the laser, there is an attenuation of 30 dB, and when the wavelength difference is 0.6 nm, there is an attenuation of 16 dB.

On the other hand, the curve (b) has FSR = 5 nm,
The case where the period of the filter does not match the period of the laser is shown. In this case, the amount of attenuation is 10 to 12 dB with respect to the wavelength difference between the laser and the filter.

The amount of attenuation required of the filter for the reflected return light is approximately 12 dB. Therefore, the curve (a) in which the filter cycle matches the laser cycle is practically satisfactory.

[0085]

As described above, according to the present invention, it is possible to perform bidirectional communication using substantially the same wavelength band with one optical fiber, and the filter for multiplexing and demultiplexing the light has the wavelength reference. Also, it is easy to control the oscillation wavelength of the laser. Therefore, it is possible to perform wideband bidirectional wavelength division multiplexing transmission with a simple configuration.

[Brief description of drawings]

FIG. 1A is a conventional wavelength division multiplexing optical communication system.

FIG. 1B is a conventional wavelength division multiplexing optical communication system.

FIG. 2 is an explanatory diagram of a two-beam interference filter.

FIG. 3 is an example of the present invention.

FIG. 4A is a modification of the present invention.

FIG. 4B is a modification of the present invention.

FIG. 4C is a modification of the present invention.

FIG. 5A is part of another embodiment of the invention.

FIG. 5B is part of another embodiment of the invention.

FIG. 6A is a portion of yet another embodiment of the present invention.

FIG. 6B is part of yet another embodiment of the present invention.

FIG. 7 is a diagram showing an experimental result regarding a cycle of a two-beam interference filter passband and a laser oscillation cycle.

[Explanation of symbols]

 10 Optical Cables 11a, 12a Terminal Station 13a Optical Transmitter 14 Optical Receiver 15 Filter 21 Laser 22 Wavelength Reference Filter 23 Photoelectric Converter 24 Comparator 25 Peak Control Circuit

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location H04B 10/24

Claims (10)

[Claims] 1. A first and a second connected by an optical cable.
Bidirectional wavelength division multiplex transmission apparatus having the following configuration; each terminal station having a laser for converting a transmission electric signal into an optical signal; a receiver for receiving an optical signal and converting it into an electric signal; And a first two-beam interference type having an input port that includes a 0-phase port and a π-phase port and is coupled to the laser and the receiver, and an output port that includes a 0-phase port and a π-phase port and one of which is coupled to the optical cable A filter having a pass band and an attenuation band that are periodic with respect to a wavelength within a communication wavelength band, and each terminal station has a wavelength control unit so that the oscillation wavelength of the laser coincides with the center of the pass band of the filter. The wavelength control means, the monitoring means for monitoring the output wavelength of the laser, a comparator for comparing the output intensity of the monitor means with a reference value, the oscillation wavelength of the laser according to the output of the comparator. And a control means for controlling so as to match the pass band of the filter, the wavelength bands of the lasers of the first terminal station and the second terminal station are substantially the same, and the optical cable is the 0 phase of one terminal station. Output and π of the other terminal
Combined during phase output. 2. The control means receives the back light of the laser at the same port as the input port of the first filter by the second two-beam interference type filter having the same characteristic as the first filter, and the sensor causes The transmission device according to claim 1, wherein the back light passing through the second filter is converted into an electric signal, and the control means controls the oscillation wavelength of the laser so that the output of the second filter becomes maximum. 3. The control means receives the back light of the laser at an input port having a phase opposite to that of the input port of the first filter by means of a second two-beam interference type filter having characteristics similar to those of the first filter, 2. The transmission device according to claim 1, wherein the back light that has passed through the second filter is converted into an electric signal by the control means, and the control means controls the oscillation wavelength of the laser so that the output of the second filter is minimized. 4. The two-beam interference type filter comprises a pair of directional couplers, a pair of optical fibers coupling the both and having different lengths, and a passband of the filter is controlled by temperature control of one of the optical fibers. The transmission device according to claim 1, wherein the transmission device is a Mach-Zehnder type asymmetric interferometer having a period control means for controlling a period. 5. The control means includes a directional coupler inserted between the first filter and the optical cable to take out a part of the laser output, a sensor for converting an optical signal of the output into an electric signal, and a sensor. 2. The transmission device according to claim 1, further comprising: a comparator for comparing the output of the laser with a reference value and a control means for controlling the oscillation wavelength of the laser so that the output of the sensor becomes maximum. 6. The control means comprises a sensor for receiving output light of an output port of the first filter which is not connected to the optical cable, and a comparator for comparing the output of the sensor with a reference value.
The transmission device according to claim 1, further comprising a control unit that controls the oscillation wavelength of the laser so that the output of the port of the filter is minimized. 7. The laser is a multi-axis vertical mode oscillation type,
The transmission device according to claim 1, wherein the cycle of the mode matches the cycle of the pass band of the filter. 8. The filter of the first terminal operates as a reference of the communication wavelength, the wavelength of the laser of the first terminal is controlled to match the pass band of the filter, and the filter of the second terminal is , The temperature of one of the optical fibers between the directional couplers is controlled to maximize the light received from the first terminal station, and the laser oscillation wavelength of the second terminal station is set to the second terminal station. The transmission device according to claim 4, wherein the transmission device is controlled so as to match the pass band of the filter. 9. The transmission device according to claim 1, wherein the communication wavelength band is 1.3 μm band. 10. The transmission device according to claim 1, wherein the communication wavelength band is 1.5 μm band.