JPH0685367A - Optical communication system - Google Patents

Abstract

PURPOSE:To provide the coherent optical communication system causing no self phase modulation without decreasing the output of an optical amplifier. CONSTITUTION:After the modulation for specifying amplitude, a passive waveguide type optical limiter is arranged in the position immediately after an optical amplitude 1. Thus, the fluctuation in amplitude in high power can be avoided thereby enabling the self phase modulation to be avoided. Resultantly, the output of the optical amplifier 1 can be increased while avoiding the wave form distortion due to the self phase modulation so that the intervals between the optical amplifiers as well as between reproduction.relays may be lengthened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a coherent optical communication system, and more particularly to a technique for preventing a failure due to a non-linear phenomenon.

[0002]

2. Description of the Related Art In recent years, with the advent of practical optical fiber amplifiers, deterioration of receiving sensitivity due to a non-linear optical phenomenon occurring in a fiber has become a problem. Nonlinear optical phenomenon is a phenomenon that occurs when the optical power density is very large, and there is a phenomenon called self-phase modulation (stolen: ”
Nonlinearity in Fiber Transmission “, Proceedings o
f the IEEE, Vol.68 (1980), No.10, pp1232-1236). This is a phenomenon in which the refractive index of the optical fiber changes depending on the intensity of the propagating light, so that the propagation velocity changes and phase density (phase modulation) occurs. The spectrum is broadened by this phase modulation, just as the spectrum of transmitted light is broadened by ordinary data modulation. The width of spread depends on the output power and transmission distance of the optical fiber amplifier, but is sometimes on the order of nanometers. At this time, the receiving sensitivity is greatly deteriorated in a system such as coherent optical communication which has a severe line width requirement. This phenomenon occurs due to the strength of itself, and also occurs under the influence of other light, for example, when noise due to spontaneous emission light emitted from the optical fiber amplifier is superimposed on the signal light.

In coherent optical communication, a method of preventing deterioration of receiving sensitivity due to self-phase modulation has been a method of transmitting a pilot carrier (Yamazaki et al .: "Phase Noise Canceller").
The effect compensation method for Kerr for coherent optical amplification repeater transmission using "," 1992 Spring Meeting of the Institute of Electronics, Information and Communication Engineers, SB-9-7) has been reported. This is a method to compensate after self-phase modulation occurs. Therefore, compensation may not be possible depending on the degree of occurrence of self-phase modulation.

In order to prevent self-phase modulation, there is no choice but to shorten the propagation distance with large power or to reduce the output of the optical amplifier to such an extent that self-phase modulation does not occur. However, in that case, many optical amplifiers must be used at short intervals to transmit a long distance, and the advantage of using the optical amplifier is halved.

[0005]

As described above, in the conventional optical communication system, in order to prevent reception sensitivity deterioration due to self-phase modulation, compensation is performed after self-phase modulation occurs or the output of the optical amplifier is lowered to cause a non-linear phenomenon. There is no way to effectively prevent the reception sensitivity from being deteriorated.

The present invention has been made to solve such a conventional problem, and an object thereof is to provide a coherent optical communication system in which self-phase modulation does not occur without lowering the output of an optical amplifier. To do.

[0007]

To achieve the above object, the present invention is a coherent optical communication system using an optical amplifier, wherein the modulation system is a constant amplitude modulation system such as frequency modulation or phase modulation. The optical limiter is arranged on the output side of at least one of the optical transmitter, the optical postamplifier, and the optical repeater amplifier provided in the.

[0008]

The input / output characteristics of the optical limiter used in the present invention constructed as described above are as shown in FIG. That is, when light of large power is input to the optical limiter, when it is output, the portion of high power comes to a ceiling. Self-phase modulation is a phenomenon in which amplitude fluctuations in a strong power portion cause phase modulation. Therefore, in the constant amplitude modulation method such as PSK or FSK which is usually used in coherent optical communication,
After output from an optical amplifier or an optical transmitter with a large output power,
If the amplitude fluctuations such as residual AM and optical amplifier noise are eliminated by passing through the optical limiter, self-phase modulation does not occur. As the optical limiter, if a passive element made of a nonlinear optical crystal or the like is used instead of an active element capable of emitting light such as a semiconductor laser amplifier, the optical limiter itself does not generate noise, so it is effective in preventing self-phase modulation. is there. The passive optical limiter can be composed of a waveguide having both a linear portion and a non-linear portion, the principle of which is shown in FIG. This is because the electric field distribution of the propagating light in the non-linear portion depends on the light intensity of the propagating light.
Since it changes as shown in FIG. 2, it is based on the principle that the coupling efficiency when coupling from the nonlinear portion to the linear portion depends on the optical power. In the optical limiter having such a configuration, the input / output characteristics and the saturation output of the limiter can be freely determined by combining the refractive index of the material forming the waveguide, the nonlinear refractive index, the size and shape of the waveguide, the linear number, and the number of non-linear repetitions. Can be designed.

Furthermore, if the above passive optical waveguide is a fiber type, connection with an optical fiber amplifier or a transmission optical fiber is facilitated, coupling loss is reduced, and the relay interval can be expanded.

Further, in the optical limiter having such a principle, the power cut by the limiter operation becomes leaked light from the waveguide, and therefore the input power to the optical limiter can be known by detecting the power. If the output power of the optical transmitter or optical amplifier in front of the optical limiter is controlled so that the power of the leaked light is constant at an appropriate value, the output of the optical transmitter or optical amplifier is too small and the limiter operation is It is not performed, and the amplitude fluctuation does not remain. Further, it is possible to prevent the optical transmitter or the optical amplifier from outputting more power than necessary.

Further, the saturation output of the optical limiter can be reduced by externally irradiating the nonlinear portion with bias light or applying a bias voltage, as shown in the characteristic of FIG. Therefore, when designing the optical limiter, if the saturation output is set to the maximum value in the control range and the saturation output is controlled by applying a bias, amplitude fluctuations can be appropriately removed according to the output power of the optical amplifier. Allows flexibility in system design and operation.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an optical amplifier 1 and an optical limiter 2 which are main parts of an optical communication system to which the present invention is applied. As shown in the figure, an optical limiter 2 is arranged immediately after the optical amplifier 1. As the optical amplifier 1, a semiconductor laser amplifier may be used in addition to the rare earth-doped optical fiber amplifier which is often used at present. The optical limiter 2 is placed without passing through the optical fiber as much as possible after the output of the optical amplifier.

FIG. 2 shows an embodiment in which this is incorporated into a coherent optical communication system. The light output from the transmitter 4 is amplified several times by the optical amplifier 1 until it is received by the receiver or the regenerator 5, and the optical limiter 2 is arranged immediately after each optical amplifier 1. If the optical power of the transmitter output light is high, the optical limiter 2 is placed immediately after that.

FIG. 3 is a part of the embodiment of the present invention and shows an example of the structure of the optical limiter 2. A part of the linear channel type waveguide 7 is replaced by the nonlinear waveguide 8. The material forming the linear waveguide may be either inorganic or organic. Further, as shown in FIG. 4, a nonlinear thin film 9 may be provided as a clad on the linear channel waveguide 7. Also, if a substance such as diacetylene that is usually a linear substance and becomes a non-linear substance by some operation such as irradiation of ultraviolet rays is used, the structure shown in FIG. 5 can be obtained. In this way, the difference in mode distribution when the input power is small is small and the loss is small. Further, as shown in FIG. 6, when the linear part 7 and the non-linear part 8 are repeated several times, the limiter saturation output becomes small.

The above is the channel type waveguide 7, but a fiber type as shown in FIG. 7 may be used to obtain the same effect. That is, a part of the linear core 12 inserted in the linear clad 14 may be replaced with the nonlinear core 13. Further, such a fiber type is slightly difficult to fabricate as compared with the plate-shaped one as shown in FIG. 3, so that it is slightly difficult to manufacture, but the coupling loss with the optical fiber amplifier or the transmission optical fiber is smaller than the plate-shaped waveguide. Get smaller.

FIG. 8 shows a part of an embodiment of the present invention, in which leak light from the optical limiter 2 is detected by the photodetector 15, and the detected output is converted into an appropriate control signal by the optical amplifier controller 16. The optical amplifier 1 is fed back to the optical amplifier 1 to control the input power at which the optical limiter 2 operates most effectively. This allows the optical limiter 2 to be used under optimum conditions.

9 and 10 show a part of an embodiment of the present invention, in which the output of the optical amplifier 1 is divided by the linear optical distributor 17, one of which is received by the photodetector 15 and the input power to the optical limiter 2. To detect. This is converted into an appropriate control signal by the optical limiter control device 18, and the optical bias light source 19 is controlled as shown in FIG. Alternatively, it is used to control the bias voltage of the optical limiter 2 as shown in FIG. At this time, the control is performed so that the operation of the optical limiter 2 is performed most effectively or the output power of the optical limiter 2 satisfies the required value from the entire system. This allows the optical limiter 2 to operate under optimal conditions.

[0018]

As described above, according to the present invention,
In a coherent transmission system that uses an optical amplifier and has a large incident power to the optical fiber, the transmitted light is subjected to constant amplitude modulation, and an optical limiter is placed immediately after the optical amplifier or the optical transmitter to provide self-phase. Fluctuations in the optical amplitude that cause modulation can be eliminated. As a result, the output of the optical amplifier can be increased, and the optical amplifier interval in the transmission system can be lengthened. Further, since the waveform deterioration due to the self-phase modulation is eliminated, it is possible to obtain the effect that the regenerator interval can be lengthened.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an optical amplifier and an optical limiter which are main parts of an optical communication system to which the present invention is applied.

FIG. 2 is a block diagram showing the configuration of an embodiment of the optical communication system of the present invention.

FIG. 3 is a diagram showing a configuration example of an optical limiter in which a nonlinear waveguide is incorporated in a linear waveguide.

FIG. 4 is a diagram showing a configuration example of an optical limiter, in which a nonlinear thin film is a clad on a linear waveguide.

FIG. 5 is a diagram showing a configuration example of an optical limiter, in which a part of the cladding of the linear waveguide is a non-linear material.

FIG. 6 is a configuration example of an optical limiter, and is a diagram in which a nonlinear waveguide is repeatedly incorporated in a linear waveguide.

FIG. 7 is a diagram showing an example in which an optical limiter is configured as a fiber type.

FIG. 8 is an explanatory diagram showing an example in which leak light of an optical limiter is detected and feedback control is performed on an optical amplifier.

FIG. 9 is an explanatory diagram showing an example of detecting the input power of the optical limiter and controlling the saturated output of the optical limiter with bias light.

FIG. 10 is an explanatory diagram showing an example in which the input power of the optical limiter is detected and the saturation output of the optical limiter is controlled by a bias voltage.

FIG. 11 is a diagram showing the input / output characteristics of the optical limiter.

FIG. 12 is an explanatory diagram showing the principle of an optical limiter.

FIG. 13 is an explanatory diagram showing how the input / output characteristics of the optical limiter change with bias.

[Explanation of symbols]

 1 Optical Amplifier 2 Optical Limiter 3 Transmission Optical Fiber 4 Optical Transmitter or Regenerative Repeater 5 Regenerative Repeater or Optical Receiver 6 Substrate 7 Linear Waveguide 8 Nonlinear Waveguide 9 Nonlinear Clad 10 Linear Clad 11 Nonlinear Clad 12 Linear Core 13 Nonlinear core 14 Linear clad 15 Photodetector 16 Optical amplifier controller 17 Optical distributor 18 Optical limiter controller 19 Optical bias light source

Claims (5)

[Claims] 1. In a coherent optical communication system using an optical amplifier, the modulation method is a constant amplitude modulation method such as frequency modulation or phase modulation, and an optical transmitter, an optical post amplifier or an optical repeater amplifier provided in the system. An optical communication system characterized in that an optical limiter is arranged on at least one of the output sides. 2. The optical communication system according to claim 1, wherein the optical limiter is composed of a passive optical waveguide that performs a limiter operation by nonlinearity of coupling efficiency of a linear portion and a non-linear portion. 3. The optical communication system according to claim 2, wherein the passive optical waveguide is an optical fiber type. 4. A means for detecting leaked light from the optical limiter and feeding back a control signal corresponding to the power of the leaked light to a device immediately before the optical limiter; and an optical power input to the optical limiter according to the control signal. The optical communication system according to claim 2 or 3, further comprising a means for stabilizing. 5. The light according to claim 2, further comprising means for generating a bias signal according to an optical power input to the optical limiter and controlling a saturated output of the optical limiter according to the bias signal. Communications system.