JPH09331293A - Optical transmitter and optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the waveform distortion of signals on the reception side of an optical transmission system after light is transmitted through an optical fiber by performing wavelength dispersion on the optical fiber. SOLUTION: A light transmitter is constituted of an attenuator 1, a semiconductor laser 2, a fiber-optic amplifier 3, an optical degree-of-modulation detector 4, an optical amplification gain detecting circuit 5, and a processor 8 which calculates an optical degree of modulation (m) from a required carrier-to-noise ratio (CNR) and an optical amplification gain G. The optical transmitter suppresses the occurrence of wavelength chirping in the semiconductor laser 2 by lowering the degree of modulation of input signals without lowering the CNR by controlling the degree of modulation and optical amplification gain of the input signals to calculated or inputted values and amplifying the output light 2a of the laser 2 by means of the amplifier 3.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmitter used in an optical communication system for transmitting light modulated by a laser device through an optical fiber transmission line, and an optical transmission system using this optical transmitter. More specifically, the present invention reduces the distortion of the signal waveform due to the optical fiber wavelength dispersion caused by the wavelength chirping in the semiconductor laser, and reduces the distortion of the signal waveform caused by the non-linearity of the current-optical output characteristic in the semiconductor laser. Regarding
In order to reduce the signal waveform distortion after optical fiber transmission,
The present invention relates to an optical transmitter in which a modulation degree of an input signal to a semiconductor laser and an optical amplification gain in an optical amplifier are controlled.

[0002]

2. Description of the Related Art In an optical communication system using an optical fiber having wavelength dispersion, when optical transmission is performed using an amplitude modulation method, the optical signal incident on the optical fiber is the amplitude of an input signal, that is, a semiconductor laser. Due to the change in the magnitude of the input current, the oscillation wavelength undergoes wavelength chirping. In this way, when the wavelengths of light propagating in the optical fiber are different, the wavelength dispersion of the optical fiber causes wavelength dependence of the propagation speed, and the signal waveform at the output end of the optical fiber is distorted. For example, when an optical pulse signal is transmitted, there is a problem that the pulse waveform spreads at the exit end of the optical fiber and the transmission band is limited. Further, when an analog signal such as a video is transmitted, in order to reduce the waveform distortion of the received signal after the transmission due to the optical fiber wavelength dispersion, there arises a problem that the modulation degree and the transmission distance are limited.

The amount of distortion of the signal waveform is proportional to the amount of wavelength chirping of the light source and is a factor that limits the transmission speed and the transmission distance in optical fiber communication. The wavelength chirping occurs when the intensity of output light is changed by changing the injection current as in the direct modulation method of the semiconductor laser, and is caused by the change in the refractive index due to the change of the charge carriers inside the semiconductor laser element. Therefore, in the amplitude modulation, the wavelength chirping amount cannot be made zero, and it is required to suppress it to the minimum.

A 500-channel optical transmission method using quadrature amplitude modulation (QAM) is proposed ("Development of QAM 500ch optical transmission method", IEICE Technical Report OCS94-98, pp7-1.
3), 1995-03), but when transmitting an optical signal in the 1.55 μm band that enables optical amplification with extremely low loss in a 1.3 μm zero-dispersion single-mode fiber (transmission distance: 7 km). The degree of modulation to obtain the second-order distortion of −40 dB is 1.5
It has been reported that it must be less than%, and an increase in the amount of wavelength chirping, that is, the amount of distortion due to chromatic dispersion proportional to the degree of modulation is a problem. Therefore, 1.3μ
In order to realize longer-distance transmission in 1.55 μm band optical transmission using m zero-dispersion single-mode fiber, it is necessary to reduce the amount of distortion due to chromatic dispersion.

In order to reduce the amount of distortion due to wavelength dispersion,
Research and development of semiconductor lasers exhibiting low distortion and low wavelength chirping characteristics have been carried out, and the strained quantum well structure has been adopted for the active layer, which is the light emitting layer of the semiconductor laser, and distributed feedback as a reflection mirror necessary for optical resonance. A type (DFB) reflecting mirror has been introduced and the relaxation frequency has been increased to reduce nonlinear distortion and wavelength chirping in a semiconductor laser. However, a semiconductor laser exhibiting these low distortion and low wavelength chirping characteristics may have a complicated structure and a high cost as compared with a conventional Fabry-Perot cavity laser, and the nonlinear distortion and wavelength in the semiconductor laser may be increased. Improving device performance such as reducing the amount of chirping may not be enough to reduce distortion of the transmission signal.

Therefore, in order to reduce the amount of distortion due to dispersion, a dispersion compensating fiber having an inverse dispersion characteristic is used for the chromatic dispersion generated in the 1.3 μm zero-dispersion single mode fiber.
Report on optical transmission system that compensates for dispersion effects (“AM / QAM /
FM 102 carrier multi-stage amplification optical transmission experiment ", IEICE Fall Conference B-840, 4-81, 1993). However, since the amount of distortion due to optical fiber dispersion is proportional to the transmission distance, each user's house In addition, when the above transmission system is applied as a subscriber transmission line having a different fiber length, there is a possibility that the compensation value is different for each user's house and the design becomes complicated and difficult. Values from 1.3 μm band to 1.5 in optical fiber structure design
A system that reduces dispersion distortion using a dispersion-shifted fiber that has been shifted to the 5 μm band has also been proposed, but as with a system that uses low distortion, low-wavelength chirping semiconductor laser and dispersion compensation fiber, the cost may increase. There is.

That is, in an optical communication system using such an optical fiber having wavelength dispersion, a received signal is distorted due to wavelength chirping (fluctuation) in a semiconductor laser which converts an input signal into an optical signal. There is a problem that the band and the transmission distance are limited. In order to deal with this problem, until now, an optical transmission system has been configured using a semiconductor laser having a low wavelength chirping characteristic, or a chromatic dispersion amount (-) that is the reverse of the optical chromatic dispersion amount (D) of an optical fiber.
Although the above distortion is compensated by inserting a dispersion compensating fiber having D) into the transmission line, there is a problem that the cost becomes high.

[0008]

As described above, the amount of distortion due to optical fiber wavelength dispersion depends on the modulation degree, the wavelength chirping amount of the light source, the optical fiber dispersion value, and the transmission distance.
When the fiber dispersion value and the transmission distance are constant, the required signal-to-noise power ratio (CNR) can be increased by increasing the modulation degree of amplitude modulation. However, the wavelength chirping in a semiconductor laser is the input current change rate. Since it is proportional to, the degree of modulation can be increased only to some extent in consideration of the distortion of the signal waveform due to the wavelength dispersion of the optical fiber. Further, in order to prevent an increase in non-linear distortion in the semiconductor laser, it is necessary to limit the modulation degree within a region where the linearity of the current-optical output (IL) characteristic is maintained.

On the other hand, when a signal with a small modulation degree is input to the semiconductor laser, the wavelength chirping can be made smaller than that with a large modulation degree, and the distortion of the signal waveform due to wavelength dispersion and the nonlinearity in the semiconductor laser are caused. Although the distortion can be reduced, the CNR after signal transmission is lowered due to the low signal power, which causes a problem that noise characteristics are deteriorated. Therefore, it is necessary to determine the modulation factor in consideration of nonlinear distortion in the semiconductor laser, wavelength chirping, and CNR after signal transmission.

[0010]

SUMMARY OF THE INVENTION In order to suppress wavelength chirping in a semiconductor laser, the present invention inputs an electric signal having a reduced degree of modulation of an input signal to the semiconductor laser and outputs the semiconductor laser output light to an optical fiber amplifier. Therefore, we propose an optical transmitter that compensates for the decrease in CNR due to the reduction of the modulation degree by directly performing optical amplification. By suppressing wavelength chirping, it is possible to reduce signal waveform distortion due to optical fiber wavelength dispersion. Further, by reducing the modulation degree, it is possible to perform modulation within a range in which a linear optical output is obtained with respect to an input current in the semiconductor laser, and it is possible to reduce nonlinear distortion in the semiconductor laser.

Signal-to-noise power ratio (CNR) in communications
Is used as a reference to evaluate the transmission characteristics, and the CN of the received signal after signal transmission when a sine wave is used as the modulation signal
R is given by equation (1). Note that the noise characteristic in the optical fiber amplifier is taken into consideration in the equation (1).

[0012]

[Equation 1]

In equations (1) and (2), m is the modulation factor, ξ is the receiver conversion efficiency, Ps is the received light power, Pn is the optical amplifier noise power, e is the electron charge, and Be is the electrical signal band,
Br is an optical signal band, RIN is relative noise intensity, Id is dark current, In is equivalent input noise current density, G is optical amplifier gain, hν is photon energy, and nsp is population inversion index.

In order to suppress the wavelength chirping which causes the signal waveform distortion due to the optical fiber wavelength dispersion, it is necessary to reduce the modulation degree m. However, when the modulation degree m is lowered, the above equation (1) is used. CNR becomes lower. Therefore CN
In order to prevent the decrease of R, it is required to increase the received light power on the receiving side. In the present invention, the semiconductor laser output light is directly amplified by the optical fiber amplifier to compensate for the decrease in CNR due to the decrease in the modulation degree m. By reducing the modulation degree m, wavelength chirping of the semiconductor laser proportional to the modulation degree m can be suppressed, and the signal waveform distortion after transmission due to the optical fiber wavelength dispersion, which is the object of the present invention, can be reduced.

By setting the optical amplification gain G of the optical fiber amplifier to a certain value, a certain required C can be obtained by using the above equation (1).
The modulation degree m for obtaining the NR can be calculated.

FIG. 5 shows the calculation result of the CNR in the receiver after the optical fiber transmission with respect to the modulation degree when the sine wave is used as an input signal with the optical amplification gain G as a parameter. Here, the oscillation wavelength of the semiconductor laser was 1.55 μm, the output power of the semiconductor laser was 6 dBm, the optical fiber length was 10 km, the optical fiber loss was 0.2 dB / km, and the optical fiber amplifier gain G was 2 dB, 4 dB, and 6 dB, respectively. If the required signal-to-noise power ratio (CNR) is 50 dB, the optical amplification gain G of the optical fiber amplifier is set to 6 dB, so that the modulation degree m is reduced from 5.3% to 2.7% when the optical amplifier is not used. It can be reduced.

It has been reported that the complex second-order distortion (CSO) is proportional to the square of the modulation factor ("Dispersion-Induced Composi").
te Second-Order Distortion at 1.5μm ”, IEEE PHOT
ONICS TECHNOLOGY LETTERS, Vol. 3, pp59-61, 1991)
When the optical amplifier with the optical amplification gain of 6 dB is used, the CSO can be reduced to about 1/4 as compared with the case where the optical amplifier is not used.

The operation contents of the optical transmitter of the present invention will be described below. If the modulation degree of the input signal is unknown in the initial state, the output light of the semiconductor laser with the modulation signal input is converted into an electric signal by a photodiode, and the obtained electric signal is converted into a DC voltage and a signal voltage. Separate and calculate the modulation factor in the present state from the ratio. When there is a difference between the optical amplification gain and the modulation degree calculated by the equation (1) and the modulation degree in the current state, the modulation degree of the input signal is changed by the attenuator installed in the preceding stage of the semiconductor laser to calculate the calculated modulation. Control to match the degree.

The modulation degree can be set to the minimum value when the maximum value in the flat state where the optical amplification gain of the optical fiber amplifier has no wavelength dependence is used. If the gain in the current state is unknown, the semiconductor laser output light that is the input light to the optical fiber amplifier and the optical fiber amplifier output light are converted into an electric signal by the photodiode, and the DC voltage of the obtained electric signal is converted. The optical amplification gain in the current state is calculated from the ratio. When there is a difference between the maximum gain that can minimize the modulation factor or the set optical amplification gain, the pumping light power of the optical amplifier is changed to change the optical amplification gain so as to match the set value.

In the present invention, an electric signal having a reduced modulation degree of an input signal is input to a semiconductor laser, and the optical signal after the output of the semiconductor laser is directly amplified by an optical fiber amplifier to reduce the modulation degree. It is possible to compensate for the decrease in CNR due to this. By suppressing the wavelength chirping by lowering the modulation degree, it is possible to reduce the signal waveform distortion after optical fiber transmission, and according to the required transmission quality, CN
R and optical amplification gain can be set. Further, by lowering the modulation degree, it is possible to perform modulation within a range in which a linear optical output is obtained with respect to the input current in the current-optical output characteristics of the semiconductor laser, and it is possible to reduce nonlinear distortion in the semiconductor laser. it can.

[0021]

1 is a block diagram of a first embodiment of the present invention.
This will be described with reference to FIG. In the first embodiment of FIG. 1, a modulation degree is calculated by using a processor from the signal-to-noise power ratio (CNR) on the receiving side and the optical amplification gain of the optical amplifier, which is defined by the user, for the input signal, and the detection is performed. If there is a difference between the modulation factor of the input signal in the current state and the optical amplification gain, the control is performed so that the modulation factor of the input signal in the detected current state matches the calculated modulation factor, and In this embodiment, control is performed so that the optical amplification gain of the optical amplifier matches the set optical amplification gain value. The configuration and operation contents will be described below according to the signal processing.

Referring to FIG. 1, the optical transmitter 100 of the present invention.
Is an attenuator 1 for attenuating an input signal, a semiconductor laser 2 optically modulated by the input signal, an optical fiber amplifier 3 for optically amplifying the output of the semiconductor laser 2, and a degree of optical modulation of the semiconductor laser 2 in its present state. Optical modulation degree detection circuit 4 for detecting
An optical amplification gain detection circuit 5 for detecting the optical amplification gain of the optical fiber amplifier 3 in the current state, and a modulation degree for controlling the modulation amount of the input signal of the semiconductor laser by controlling the attenuation amount of the attenuator 1. It comprises a control circuit 6, an optical amplification gain control circuit 7 for controlling the optical amplification gain of the optical fiber amplifier 3, and a processor 8 for calculating the modulation degree.

The optical modulation degree detecting circuit 4 includes a photodiode 41 for converting a part of the optical output of the semiconductor laser into an electric signal, and a low-pass filter 42 for extracting a DC component from the electric signal output of the photodiode 41. A bandpass filter 43 for extracting a signal component from an electric signal output of the photodiode 41, a peak voltage detector 44 for detecting a peak voltage of an output signal of the bandpass filter 43, an output of the low-pass filter 42 and the peak voltage detection. And a divider circuit 45 to which the output of the device 44 is input.

The optical amplification gain detection circuit 5 converts a light output of the semiconductor laser, that is, a part of the input of the optical fiber amplifier 3 into an electric signal, and a DC component from the electric signal output of the photodiode 41. A low-pass filter 42 for extracting, a photodiode 51 for converting a part of the optical output of the optical fiber amplifier 3 into an electric signal, a low-pass filter 52 for extracting a DC component from the electric signal output of the photodiode 51, and a low-pass filter. It is composed of a divider circuit 53 to which the output of 42 and the output of the low pass filter 52 are input.

The modulation degree control circuit 6 receives the output of the division circuit 45 and the output of the processor 8 as a differential amplifier 60.
The optical amplification gain control circuit 7 includes a differential amplifier 7 to which the output of the division circuit 53 and the optical amplification gain signal are input.
It consists of 0.

The user applies the voltage (Vcnr) corresponding to the CNR required on the receiving side after the optical fiber transmission to the CN.
A voltage (Vg) corresponding to the optical amplifier gain is input from the R voltage input terminal 9 from the optical amplification gain voltage input terminal 10.
The processor 8 calculates the modulation factor m from the input voltage (Vcnr) corresponding to the CNR required on the receiving side after transmission through the optical fiber and the voltage (Vg) corresponding to the optical amplifier gain using the equation (1). , And outputs a voltage 8a corresponding to the calculated modulation factor m. The light modulation degree detection circuit 4 detects the voltage 4a corresponding to the light modulation degree M in the current state. The photodiode 41 receives a part of the semiconductor laser output light 2a and converts the optical signal into an electric signal. The photodiode output signal 41a is separated into a signal voltage 43a and a DC voltage 42a by using a low pass filter 42 and a band pass filter 43. Further, input the signal voltage 43a to the peak voltage detector 44,
The amplitude voltage 44a of the signal component is obtained. The obtained amplitude voltage 44a and DC voltage 42a are input to the division circuit 45, and the signal voltage 4a corresponding to the modulation degree M in the current state is detected.

The optical amplification gain detection circuit 5 detects the voltage 5a corresponding to the optical amplification gain Go in the present state. A part of the output light 2a of the semiconductor laser 2 is received by the photodiode 41, an optical signal is converted into an electric signal 41a, and this output signal 41a is converted into a DC voltage 42a by using a low pass filter 42. Further, a part of the output light 3a of the optical fiber amplifier 3 is received by the photodiode 52, the optical signal is converted into an electric signal 51a, and the output signal 51a is converted into the low pass filter 5a.
2 is used to convert to a DC voltage 52a. DC voltage 42a
Then, the DC voltage 52a is input to the division circuit 53, and the voltage 5a corresponding to the optical amplification gain Go in the present state is detected.

The modulation degree control circuit 6 inputs the voltage 8a corresponding to the modulation degree m calculated by the processor 8 and the signal voltage 4a corresponding to the detected modulation degree M in the present state to the differential amplifier 60, and the deviation thereof. The amount of attenuation of the attenuator 1 is changed by the differential amplifier output voltage 6a according to the above, and the modulation factor of the input signal is changed so that the modulation factor M in the current state matches the modulation factor m calculated by the processor 8.

The optical amplification gain control circuit 7 outputs a voltage (Vg) 10 corresponding to the input optical amplification gain G and a signal voltage 5a corresponding to the detected optical amplification gain Go in the present state to the differential amplifier 70.
Is input to the optical fiber amplifier 3 and the pumping light power is changed to the optical amplification gain value G input by the user.

With the above configuration, the optical transmitter according to this embodiment of the present invention can compensate the decrease in signal power due to the decrease in the modulation degree by the optical amplification.
The degree of modulation can be reduced without lowering the NR,
It is possible to reduce signal waveform distortion on the receiving side due to optical fiber wavelength dispersion due to suppression of wavelength chirping. Further, by reducing the modulation degree, the semiconductor laser can obtain a modulated optical output within a range in which a linear output is obtained with respect to the input current, and the amount of nonlinear distortion in the semiconductor laser itself can be reduced. A low-cost semiconductor laser with slightly inferior distortion characteristics can be used, and an optical transmission system can be economically constructed by using such a transmission system for a subscriber line having a relatively short transmission distance.

A second embodiment of the present invention will be described with reference to FIG. Compared with the first embodiment, this embodiment has an optical amplification gain G in advance so that the optical fiber amplifier 3 can minimize the modulation degree calculated in the processor 8.
The signal voltage (Vgmax) corresponding to the maximum value is input, and the optical amplification gain detection circuit 5 and the optical amplification gain control circuit 7 are omitted. That is, the optical transmitter 100 ′ of this embodiment shown in FIG. 2 has the attenuator 1 that attenuates the input signal.
A semiconductor laser 2, an optical fiber amplifier 3, and an optical modulation degree detection circuit 4 for detecting the optical modulation degree of the semiconductor laser 2.
A modulation degree control circuit 6 for controlling the modulation degree of the input signal,
It is composed of a processor 8 for calculating the modulation degree.
To the optical fiber amplifier 3, a signal voltage (Vgmax) corresponding to the maximum value of the optical amplification gain is input in advance so that the modulation degree calculated by the processor 8 can be minimized. In this embodiment, the modulation degree m is calculated, the modulation degree M in the current state is detected,
The operation content for controlling the modulation degree is the same as that of the first embodiment.

[0032]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT An embodiment in which the present invention is applied to an optical transmission system is shown in FIGS. The optical transmission system of FIG. 3 transmits the optical output signal 3a from the optical transmitters 100 and 100 ′ shown in FIG. 1 or 2 through the optical fiber 110, and the transmitted optical signal is received by the optical receiver 120. And is compatible with an optical transmission system in which the information transmitting side and the information receiving side are one-to-one. The optical transmitter 10 shown in FIG. 1 or 2.
0,100 'can reduce the modulation degree required for obtaining the required CNR on the receiving side, so that the wavelength chirping in the semiconductor laser can be suppressed and the signal waveform after the optical fiber transmission due to the optical fiber wavelength dispersion. Distortion can be reduced.

The optical transmission system of FIG. 4 is the same as that of FIG.
Optical output signal 3 from the optical transmitters 100 and 100 'shown in FIG.
This is a system for optically distributing a to a plurality of receivers 120-1 to 120-n, and an optical transmitter 100 according to the present invention, an optical distributor 130, and a plurality of optical devices for transmitting the optically distributed optical signals. It is composed of fibers 110-1 to 110-n and a plurality of receivers 120-1 to 120-n for receiving the transmitted optical signals. In addition to the configuration of FIG. 3, this optical transmission system has a configuration in which an optical distributor 32 is inserted and an optical signal transmitted by one optical fiber is branched into n optical fibers for optical transmission. , A system in which the information transmitting side and the information receiving side are set to 1 to n. Here, FIG. 1 or FIG.
The optical fiber amplifier 3 described in the above embodiment has two functions of compensating for the decrease in the power of the optical signal due to the light distribution and for compensating for the decrease in the power of the optical signal due to the decrease in the modulation degree. The amplification gain G in this case is a total of the gains required by the plurality of receivers.

[0034]

As described above, according to the present invention, in the amplitude modulation type optical transmission system, the modulation degree of the input signal can be lowered without lowering the signal to noise power ratio (CNR). Furthermore, by reducing the modulation factor, wavelength chirping in the semiconductor laser can be suppressed, and signal waveform distortion due to optical fiber wavelength dispersion can be reduced. Further, since the degree of modulation can be lowered, the modulation can be performed within the linear region of the current-light output characteristic of the semiconductor laser, and the nonlinear distortion in the semiconductor laser can be reduced.

Further, since the distortion of the signal waveform on the receiving side can be reduced, it can be expected to improve the optical transmission characteristics such that the optical fiber transmission distance can be increased. Further, it is possible to construct an optical transmission system by using a semiconductor laser having a little inferior linearity in place of the semiconductor laser having excellent linearity in current-optical output characteristics of the semiconductor laser, which contributes to cost reduction. Can be expected.

[Brief description of drawings]

FIG. 1 is a conceptual diagram showing a first embodiment of an optical transmitter according to the present invention.

FIG. 2 is a conceptual diagram showing a second embodiment of an optical transmitter according to the present invention.

FIG. 3 is a conceptual diagram showing a configuration of an optical transmission system using an optical transmitter according to the present invention.

FIG. 4 is a conceptual diagram showing the configuration of another optical transmission system using the optical transmitter according to the present invention.

FIG. 5 is a curve diagram showing a signal-to-noise power ratio (CNR) calculation result with respect to modulation degree in the amplitude modulation method.

[Explanation of symbols]

1 attenuator 1a attenuator output 2 semiconductor laser 2a semiconductor laser output 3 optical fiber amplifier 3a optical output signal 4 optical modulation degree detection circuit 4a optical modulation degree detection circuit (division circuit) output M 5 optical amplification gain detection circuit 5a optical amplification gain Detection circuit (division circuit) output Go 6 Modulation degree control circuit 6a Modulation degree control circuit output 7 Optical amplification gain control circuit 7a Optical amplification gain control circuit output 8 Processor 8a Processor output m 9 CNR voltage (Vcnr) input terminal 10 Optical amplification gain Voltage (Vg) or maximum optical amplification gain voltage (Vgmax) input terminal 41,51 Photo diode 41a, 51a Photo diode output 42,52 Low pass filter 42a, 52a Low pass filter output 43 Band filter 43a Band filter output 44 Peak voltage detection 44a Peak voltage detector output 45 Division circuit 45a Division circuit output 60,7 Differential amplifier 100 optical transmitter 110 optical fiber transmission line 120 optical receiver 130 optical distributor

Claims (11)

[Claims] 1. A variable attenuator capable of controlling the amount of attenuation of an input signal, a laser device in which an output optical signal is modulated by an output signal from the attenuator, and an optical amplification of the output optical signal from the laser device. An optical amplifier, which detects an optical modulation degree M of the laser device and an optical amplification gain Go of the optical amplifier, and an optical amplification of the optical amplifier so that the detected optical amplification gain Go matches a predetermined optical amplification gain G. The gain is controlled, the required optical modulation degree m is calculated from the signal-to-noise power ratio CNR required on the receiving side, the optical amplifier input light Pin, and the predetermined optical amplification gain G, and the detected optical modulation degree M is calculated. The optical transmitter is characterized in that the attenuator is controlled so as to match the required optical modulation degree m. 2. An attenuator for attenuating an input signal, a semiconductor laser for converting an output signal from the attenuator into an optical signal,
In an optical transmitter including an optical fiber amplifier for optically amplifying output light from the semiconductor laser, an optical modulation degree detection circuit for branching and receiving a part of the semiconductor laser output light and detecting a modulation degree M, An optical amplification gain detection circuit for detecting the optical amplification gain Go by branching and receiving a part of the input light Pin and the output light of the optical fiber amplifier is provided, and the detected optical amplification gain Go matches the predetermined optical amplification gain G. From the optical amplification gain control circuit for controlling the optical amplification gain Go of the optical fiber amplifier, the signal-to-noise power ratio (CNR) required on the receiving side, the optical fiber amplifier input light Pin, and the predetermined optical amplification gain G. A processor for calculating a required optical modulation degree m, and a converter for controlling the attenuator so that the modulation degree M obtained by the optical modulation degree detection circuit matches the required optical modulation degree m calculated by the processor. Optical transmitter characterized in that it is constituted from degrees control circuit. 3. A photodiode in which an optical modulation degree detection circuit receives a part of output light of a semiconductor laser and converts it into an electric signal, a bandpass filter for obtaining a signal voltage from the electric signal, and a peak of the signal voltage. It comprises a peak voltage detector for detecting a voltage, a low-pass filter for obtaining a DC voltage from the output electric signal of the photodiode, and a division circuit for obtaining a required modulation factor m from the signal peak voltage and the DC voltage. The optical transmitter according to claim 2. 4. An optical amplification gain detection circuit, a photodiode for receiving a part of input light of an optical fiber amplifier and converting it into an electric signal, a low-pass filter for obtaining an input DC voltage from the received electric signal, and the light. A photodiode for receiving a part of the output light of the fiber amplifier and converting it into an electric signal, and a low-pass filter for obtaining an output DC voltage from the received electric signal,
4. A division circuit for obtaining an optical amplification gain from the obtained input DC voltage and output DC voltage.
The optical transmitter described. 5. A differential amplifier in which a modulation degree control circuit obtains an output voltage according to a difference between a voltage corresponding to an optical modulation degree obtained by an optical modulation degree detection circuit and a voltage corresponding to an optical modulation degree calculated by a processor. 5. The optical transmitter according to claim 2, further comprising: an output voltage of the differential amplifier, which is fed back to an attenuator to change a modulation degree of an input signal. 6. A differential amplifier in which an optical amplification gain control circuit obtains an output voltage corresponding to a difference between a voltage corresponding to an optical amplification gain obtained by an optical amplification gain detection circuit and a voltage Vg corresponding to a set optical amplification gain. 6. The optical transmitter according to claim 2, further comprising: an output voltage of the differential amplifier being fed back to the optical fiber amplifier to change an optical amplification gain of the optical fiber amplifier. 7. An attenuator for attenuating an input signal, a semiconductor laser for converting an output signal from the attenuator into an optical signal,
An optical transmitter including an optical fiber amplifier for optically amplifying output light from the semiconductor laser, wherein an optical modulation degree detection circuit for branching and receiving a part of the semiconductor laser output light and detecting a modulation degree M is provided. A processor for calculating a required optical modulation degree m from a signal-to-noise power ratio (CNR) required on the receiving side, the semiconductor laser output light, and a maximum optical amplification gain Gmax set in the optical fiber amplifier, An optical transmission comprising a modulation degree control circuit for controlling the attenuator so that the modulation degree M calculated by the optical modulation degree detection circuit matches the required optical modulation degree m calculated by the processor. vessel. 8. An optical modulation degree detection circuit receives a part of output light of a semiconductor laser and converts it into an electric signal, a bandpass filter for obtaining a signal voltage from the received electric signal, and a signal voltage of the signal voltage. 8. A peak voltage detector for detecting a peak voltage, a low-pass filter for obtaining a DC voltage from an output electric signal of the photodiode, and a division circuit for obtaining a modulation factor from the signal peak voltage and the DC voltage. The optical transmitter described. 9. A difference in which the modulation degree control circuit obtains an output voltage according to a difference between a voltage corresponding to the optical modulation degree M obtained by the optical modulation degree detection circuit and a voltage corresponding to the optical modulation degree m calculated by the processor. The optical transmitter according to claim 7, further comprising a dynamic amplifier, wherein an output voltage of the differential amplifier is fed back to an attenuator to change a modulation degree of an input signal. 10. The optical transmitter according to claim 1, an optical fiber for transmitting the output light from the optical transmitter, and an optical receiver for receiving an optical signal transmitted through the optical fiber. Optical transmission system consisting of a device. 11. The optical transmitter according to claim 1, an optical distributor for distributing output light from the optical transmitter, and a light for transmitting an optical signal distributed from the optical distributor. An optical transmission system comprising a fiber and an optical receiver for receiving an optical signal transmitted through the optical fiber.