JPS6316058B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to optical communication systems, and particularly to optical multiplex transmission systems.

In recent years, optical communication systems have made remarkable progress and are being used in a variety of fields.One of the ways in which they are used is the simultaneous bidirectional transmission of multiple optical signals using a single optical transmission path. There is an optical multiplex transmission system.

Conventional methods for performing this type of multiplex transmission include:
Wavelength division multiplexing transmission method combines light sources with different emission wavelengths and optical demultiplexers to perform bidirectional multiplex transmission by wavelength division, and optical directional couplers are used to demultiplex transmission and reception to utilize the directionality. A bidirectional multiplex transmission system is being considered in which the optical transmission signal and the optical reception signal are separated based only on the difference in the transmission direction.

However, even with the wavelength division multiplexing transmission method, which has the best characteristics, using an optical demultiplexer with a simple configuration,
It is not possible to sufficiently suppress the reflected light of the optical transmission signal caused by backscattering on the optical transmission path or end face reflection at the connection point from going around to the optical receiver of the local station. Therefore, the optical reception sensitivity deteriorates due to the reflected interference light, making it difficult to transmit over long distances.

Therefore, in the case of long-distance transmission, methods such as using an optical demultiplexer composed of a combination of multiple optical filters have been considered in order to increase the amount of attenuation against interference light, but this method does not work. The drawback is that the optical demultiplexer has a plurality of configurations and is expensive.

Another possible improvement method is to supply an electrical signal proportional to the optical transmission signal from the optical transmitter of the local station to the optical receiver, thereby electrically removing the wraparound interference signal component. However, since the delay time of the reflected light usually changes depending on the distance to the point where the reflected light is generated, rather than just one fixed point, this method is not suitable for special systems with very slow transmission speeds. However, there are drawbacks that make it generally inapplicable.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate such drawbacks and to provide an optical multiplex transmission system that is capable of bidirectional multiplex transmission over relatively long distances even when using an optical demultiplexer with a simple configuration.

According to the present invention, a plurality of optical transmitters and a plurality of optical receivers are optically connected to both ends of a single optical transmission line via an optical demultiplexer, and optical multiplexing is performed to perform bidirectional multiplex transmission. In the transmission system, an electrical signal proportional to an optical transmission signal of the optical transmitter is obtained from at least one optical transmitter among the plurality of optical transmitters, and the electrical signal is passed through a low-pass filter. and at least one of the plurality of optical receivers receives a difference signal between the received input signal and the signal obtained by the means. An optical multiplex transmission system is obtained. Note that the optical demultiplexer also includes an optical directional coupler without wavelength selectivity.

In this optical multiplex transmission system, the largest amount of light that goes around from the optical transmitter to the local optical receiver is the end face reflected light that occurs at the input end face from the optical demultiplexer to the optical transmission line. This is backscattered light that occurs in the optical transmission line when the optical transmission line is relatively long.

Note that this backscattered light is the sum of the backscattered light at each point from the beginning to the end of the optical transmission line, so the response is calculated by taking into account the transmission line loss and delay time to and from the scattering point. It can be obtained by integrating, and when calculated for a long optical transmission path, the output waveform of backscattered light when an optical pulse is input is the pulse response of a first-order low-pass filter in a predetermined band. It can be approximated by a waveform.

Therefore, the present invention focuses on the above-mentioned properties, and by subtracting an electrical signal of an appropriate level proportional to the optical transmission signal from the reception input signal in the optical receiver, it is possible to eliminate the effects of the above-mentioned end face reflection component, etc. The influence of backscattered light contained in the received input signal can also be removed by using the signal obtained by passing the electric signal proportional to the optical transmission signal through a low-pass filter. As a result, it is possible to almost eliminate the influence of the optical transmission signal from the local station, so even if an optical demultiplexer with a simple configuration is used, relatively long-distance bidirectional multiplex transmission is possible.

Next, the present invention will be explained in detail with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of a first embodiment of the present invention, which relates to an optical multiplex transmission system that performs one-to-one bidirectional transmission by wavelength division.

The two optical transmitters 11 and 12 convert transmission signals 111 and 112 into optical signals 12 with different wavelengths, respectively.
The optical demultiplexers 31 and 32 are connected to the optical transmitters 11 and 1, respectively.
2 output optical signals 121 and 122 are connected to the optical transmission line 1 with low loss, and the optical signals transmitted through the optical transmission line 1 are connected to the optical receivers 21 and 22 with low loss. On the other hand, erasure signal synthesis circuits 61 and 62
receive electrical signals 131, 132 obtained from optical transmission signals 121, 122 using optical directional couplers 41, 42 and photodetectors 51, 52, respectively, and transmit the electrical signals 131, 132 to remove the loop interference components. signal 13
1,132 and a signal obtained by passing the electric signals 131, 132 through a low-pass filter, and outputs erasing signals 141, 142, which are obtained by combining them at an appropriate ratio. In this system, due to backscattering in the optical transmission line 1 and end face reflection at connection points, etc., part of the power of the optical signals sent from the optical transmitters 11 and 12 returns, and furthermore, some of the power is returned. The optical receivers 21 and 22 pass through optical demultiplexers 31 and 32, respectively.
Go around. However, in the optical receivers 21 and 22, the cancellation signals 141 and 1 are synthesized by the cancellation signal synthesis circuits 61 and 62 from the received input signals to which interference light from the optical transmitters 11 and 12 of the own station is added, respectively.
By electrically subtracting the wrap-around interference components using 42, most of the interference components can be eliminated, so that deterioration due to wrap-around interference can be almost eliminated.

FIG. 2 is a circuit diagram showing the configuration of a low-pass filter used in the erasure signal synthesis circuit of the first embodiment, and is an RC filter circuit in which a series resistor R and a parallel capacitor C are connected in cascade. In this circuit, by matching the time constant determined by the values of resistance R, capacitance C, and input/output impedance to the response time constant of backscattered light, it is possible to create an electrical signal with the same waveform as the response waveform of backscattered light. It can be used to remove interference parts.

The second embodiment of the present invention has the same configuration as the first embodiment shown in FIG. 1, except for the low-pass filter, which uses a different circuit from the first embodiment.

FIG. 3 is a circuit diagram showing the configuration of a low-pass filter used in the erasure signal synthesis circuit of this embodiment, and is an LR filter circuit in which a series inductor L and a parallel resistor R are connected in cascade. Similar to the first embodiment, this circuit can also generate an electrical signal with the same waveform as the response waveform of the backscattered light, and by using that signal, it is possible to almost eliminate deterioration due to wraparound interference.

FIG. 4 is a block diagram showing the configuration of a third embodiment of the present invention, which relates to an optical multiplex transmission system that performs two-to-one bidirectional transmission. Similarly to the first embodiment described above, optical transmitters 11, 12, 13 are provided, respectively.
Bidirectional multiplex transmission is performed between the optical receivers 21, 22, and 23 by wavelength division. Further, as in the first embodiment, output optical signals 121, 122, 123 of optical transmitters 11, 12, 13 are transmitted to optical directional couplers 41, 42, 43 and photodetectors 51, 5, respectively.
2, 53 and erase signal synthesis circuits 61, 62, 6
3 to erase the necessary erase signals 141, 142, 14
3 are synthesized. In this system, the optical receiver 21 of the left station which transmits two waves electrically removes the wraparound interference component from the received input signal using the two cancellation signals 141 and 143. Furthermore, the two optical receivers 22 and 23 of the right station that transmit one wave each use one cancellation signal 142 to electrically eliminate the loop interference portion. This makes it possible to almost eliminate deterioration due to wraparound interference.

The fourth embodiment has the same configuration as the above-mentioned FIG. This is an optical multiplex transmission system that couples only the wavelength range components to the optical transmission line 1 and essentially performs bidirectional transmission by wavelength division. In this system as well, similar to the first embodiment, the optical transmission signal is detected, and then the cancellation signals 141 and 142 are combined to eliminate the loop interference component, thereby almost eliminating the deterioration due to the loop interference. Can be done.

The fifth embodiment has the same configuration as in FIG.
This is an optical multiplex transmission system using optical directional couplers for 1 and 32. In this system as well, interference components can be removed in the same way.

In addition, in each of the above embodiments, the optical directional couplers 41 to 43 and the photodetectors 51 to 53 are used,
Although a method of detecting the optical transmission signals 121 to 123 is used, the optical directional couplers 41 to 4
It is also possible to use a method of detecting the monitor light of the optical transmitter or the light leaking to the outside from the light source without using 3.

In addition, in a system using an optical transmitter with stabilized optical transmission output power, the transmission signals 111 to 111 are simply
It is also possible to synthesize the cancellation signal directly from 113.

Although each of the above embodiments performs one-to-one or two-to-one bidirectional transmission, it is equally applicable to a system that performs bidirectional multiplex transmission of any number of signals greater than that. It is.

In addition, in the optical multiplex transmission systems of the above embodiments, if there is a combination in which the optical demultiplexer has sufficient attenuation against decoupling, the circuit for cancellation may be omitted for that combination. good.

As described in detail above, according to the present invention, even when using an optical demultiplexer with a simple configuration, it is possible to eliminate the wraparound interference component from the local optical transmitter, almost eliminate deterioration due to interference, and achieve a relatively long An optical multiplex transmission system capable of bidirectional multiplex transmission over long distances can be provided.

[Brief explanation of the drawing]

FIG. 1 shows the first, second, fourth and fifth embodiments of the present invention.
FIG. 2 is a block diagram showing the configuration of an embodiment of the invention.
FIG. 3 is a circuit diagram of a low-pass filter used in the cancellation signal synthesis circuit of the second embodiment, and FIG. 4 is a circuit diagram of a low-pass filter used in the second embodiment. FIG. 2 is a block diagram showing an example configuration. 1... Optical transmission line, 11-13... Optical transmitter, 2
1-23... Optical receiver, 31-33... Optical demultiplexer, 41-43... Optical directional coupler, 51-53
...Photodetector, 61-63...Elimination signal synthesis circuit.

Claims (1)

[Claims] 1. Optical multiplexing in which a plurality of optical transmitters and a plurality of optical receivers are optically connected to both ends of a single optical transmission line via an optical demultiplexer to perform bidirectional multiplex transmission. In the transmission system, an electrical signal proportional to an optical transmission signal of the optical transmitter is obtained from at least one optical transmitter among the plurality of optical transmitters, and the electrical signal is passed through a low-pass filter. and at least one of the plurality of optical receivers receives a difference signal between the received input signal and the signal obtained by the means. An optical multiplex transmission system characterized by: 2. The optical multiplex transmission system according to claim 1, wherein the low-pass filter comprises an RC filter circuit.