JP2001148661A - Optical transmission system and its distortion compensation method - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To perform light intensity modulation capable of obtaining a sufficient S / N in an optical transmission line, and to compensate for a distortion component generated at that time. SOLUTION: In a main path from a main electric / optical converter 10 to a main amplifier 5, light intensity modulation is performed so as to obtain a sufficient S / N, a signal is transmitted, and an auxiliary electric / optical converter 11 supplies a signal. In the auxiliary path leading to the optical / electrical converter 15, a signal is transmitted by performing light intensity modulation to the extent that third-order distortion does not occur. The auxiliary amplifier 7 takes out the difference between the two paths to take out only the distortion component E8 (t) generated in the main path, and the combiner 8 cancels the distortion component in the signal passing through the main path by this distortion component, thereby obtaining the distortion. A signal E9 (t) having a small component and a large S / N is extracted.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical transmission system and a method for compensating for distortion thereof, and more particularly, to a method of converting a wideband signal (electric signal) into an optical signal and transmitting the signal, and amplifying the signal after the transmission. The present invention relates to an optical transmission system having an amplifying device and a distortion compensation method thereof.

[0002]

2. Description of the Related Art FIG. 4 is a block diagram showing a schematic configuration of a conventional optical transmission system. The optical transmission system shown in FIG. 4 includes an electrical / optical converter 1 for converting an input signal (analog electrical signal) into an optical signal, and optical transmission via an optical fiber connected to the electrical / optical converter 1 and transmitting an optical signal. A medium 2 is connected to the other end of the optical transmission medium 2 and includes an optical / electrical converter 3 for returning an optical signal to an electric signal.

The operation of the optical transmission system having the configuration shown in FIG. 4 will be described. An input electric signal is converted into an optical signal by an electric / optical converter 1. This conversion is performed by analogly modulating the intensity of output light from a laser or the like in accordance with the input electric signal. The optical signal obtained by the electrical / optical converter 1 is input to the optical transmission medium 2 and the optical transmission medium 2
Is transmitted to the optical / electrical converter 3. The optical / electrical converter 3 converts the received optical signal into an original electrical signal using a photoelectric conversion element, and sets it as an output signal. Such a transmission system transmits a wideband signal in which a large number of telephone signals and the like are multiplexed by a method such as frequency multiplexing, code multiplexing (spread spectrum method), and time division multiplexing, for example, relaying a radio signal such as mobile communication. And transmission of a plurality of video signals on a cable television (CATV).

As the optical transmission medium 2, an optical fiber having excellent characteristics such as low loss, light weight and small diameter is used. The light modulation method performed by the electrical / optical converter 1 includes a light intensity modulation method (direct modulation method) and a frequency modulation method, and the light intensity modulation method is generally used as described above. Yes,
It modulates the intensity of an optical signal with a high-frequency signal, and has the advantage that it is easy to convert a high-frequency signal into an optical signal. Specifically, intensity modulation of an optical signal, that is, direct modulation is performed by using a semiconductor laser as a light emitting device and changing a driving current of the light emitting device directly by a high-frequency signal.

[0005]

When transmitting an analog-modulated optical signal, transmission quality is limited by noise and distortion generated in the optical transmission system. Noise generated in an optical transmission system includes noise generated by a light emitting device (laser light emitting element) and noise generated by a light receiving device. Also,
Distortion is when the analog signal to be transmitted has a certain finite band and does not use other bands.
Odd-order distortion components occur in the frequency band of the transmission signal.
For example, in the field of mobile communication, when trying to transmit a mobile phone signal having a band of several tens of MHz at 800 MHz, secondary distortion occurs in the vicinity of DC and twice as high as 1600 MHz band. Is far away from the 800 MHz transmission target signal, so that it can be easily removed by a filter. However, since the third-order distortion component occurs in the 800 MHz band, it is difficult to remove it using a filter. This is a major problem, especially for transmission of wideband signals. As described above, since the odd-order distortion (particularly the third-order distortion) limits the transmission quality of the transmission system, a design that takes into account both the amount of noise generation and the magnitude of the distortion component in the optical transmission system is required.

More specifically, the absolute value of the amount of noise generated in the optical transmission system is almost irrelevant to the degree of light modulation, and is applied to devices such as a light emitting device (laser), a light receiving device, and an optical fiber. Dependent. That is, when the degree of light modulation is increased, the ratio of the signal to be transmitted to noise (CNR) increases. On the other hand, in the relationship between the distortion component and the degree of light modulation, increasing the degree of light modulation increases the number of generated distortion signals. If the optical modulation factor is increased in order to increase the ratio of noise to signal, that is, the S / N ratio, the amount of distortion increases. Therefore, in order to reduce this, it is necessary to increase the dynamic range of the optical transmission system. However, this leads to an increase in the price of the system, and there is a limit to the improvement effect.

It is an object of the present invention to reduce the influence of noise by performing light intensity modulation with a sufficient modulation factor, and to suppress odd-order distortion to a small value without particularly increasing the dynamic range of an optical transmission system. An object of the present invention is to provide an optical transmission system having the configuration and a distortion compensation method thereof.

[0008]

In order to achieve the above object, the present invention provides a first branching means for branching an analog electric signal as an input signal into two, and a first branching means for dividing the analog electric signal into two. First and second electric / optical conversion means for converting each of the two branch outputs into an optical signal by light intensity modulation;
First and second optical transmission paths for individually transmitting optical signals from the first and second electric / optical conversion means;
First and second optical / electrical conversion means for converting each of the optical signals transmitted by the first and second optical transmission lines into an electric signal, and a main amplifier for amplifying the output signal of the first optical / electrical conversion means. Amplifying means, second branching means for branching an output signal of the main amplifying means at a predetermined branching ratio, and a first phase shifter for shifting a phase of an output of the second optical / electrical converting means; One branch output of the second branching means and the first branch output;
Amplifying means for amplifying the difference from the output of the phase shifter, a second phase shifter for shifting the output phase of the auxiliary amplifying means, and an output of the phase shifter and the other of the second branching means And a first synthesizing means for synthesizing an output of the optical transmission system to obtain an output signal.

The present invention also provides a pilot signal generating means for generating a pilot signal, a second synthesizing means for synthesizing a pilot signal from the pilot signal generating means and an analog electric signal as an input signal, First branch means for branching the output of the means into two, and first and second electric / optical conversion means for converting each of the two branch outputs by the first branch means into an optical signal by light intensity modulation And first and second optical transmission lines for individually transmitting optical signals from the first and second electric / optical conversion means,
First and second optical / optical converters for converting each of the optical signals transmitted by the first and second optical transmission lines into an electric signal.
Electrical conversion means, main amplification means for amplifying the output signal of the first optical / electrical conversion means, second branching means for branching the output signal of the main amplification means at a predetermined branching ratio, A first variable phase shifter for shifting the phase of the output of the optical / electrical conversion means, a first variable attenuator for controlling the amplitude of the output of the variable phase shifter, and one of the second branch means A variable attenuator for controlling the amplitude of the branch output of the second variable attenuator, auxiliary amplification means for amplifying the difference between the output of the second variable attenuator and the output of the first variable attenuator, and the auxiliary amplification means A second phase shifter for shifting the output phase of the second phase shifter, and first combining means for combining the output of the phase shifter and the other output of the second branching means to produce an output signal. An optical transmission system is provided.

Further, according to the present invention, the light intensity is modulated by an input electric signal at a modulation factor such that the S / N ratio on the receiving side becomes a predetermined value, transmitted by the main optical transmission system, and the transmitted signal is transmitted. While being amplified by the main amplifier, an optical signal modulated at a modulation factor such that transmission distortion does not occur due to the input electric signal is transmitted by the auxiliary optical transmission system, and the signal transmitted by the auxiliary optical transmission system and the main amplifier The difference between the signal and the output signal is obtained by adjusting the phase and amplitude of the signal by the first adjusting means to extract the distortion component generated in the main optical transmission system and the main amplifier. The amplifier output is adjusted by adjusting the amplitude and phase of the signals by a second adjusting unit and combined to remove the distortion component from the main amplifier output. To provide a 償方 method.

Further, the present invention transmits the input electric signal and two pilot signals whose frequency bands are adjacent to the input electric signal via the main and auxiliary optical transmission systems.
A distortion component generated in the main optical transmission system and the main amplifier of two pilot signals, and controlling the first and second adjusting means so as to minimize the distortion component. A distortion compensation method is provided.

[0012]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing an example of the configuration of an optical transmission system according to the present invention, showing a relay station master station and a slave station of a mobile telephone system connected by an optical transmission line. In FIG. 1, the same reference numerals are used for the same components as those shown in FIG. 4, and the input signal 16 (analog electric signal) is converted into an optical signal and several hundreds After transmitting to a point several km away from m, the optical signal is converted back to an electrical signal at that point, and the desired power is amplified and output as an output signal 17. In this case, the input signal 16 is a multiplexed mobile phone radio signal.

In FIG. 1, a main electric / optical converter 10 and an auxiliary electric / optical converter 11 are connected to two branch output terminals of the branch unit 4. The main electric / optical converter 10 includes:
Optical fiber 12 as optical transmission line, main optical / electrical converter 1
3, the main amplifier 5, the splitter 6, and the combiner 8 are connected in series. The auxiliary electrical / optical converter 11 includes an optical fiber 14 as an optical transmission line and the auxiliary optical / electrical converter 1.
5 are connected in series. An input terminal 71 of the auxiliary amplifier 7 is connected to a branch end of the splitter 6, and an input terminal 72 is connected to an output end of the auxiliary optical / electrical converter 15. The output terminal of the auxiliary amplifier 7 is connected to the other input terminal of the synthesizer 8. Further, phase shifters 21 and 22 for adjusting the phase of each signal are inserted at the positions shown in the figure. In this transmission system, the system of the optical fiber 12 is called a main optical transmission system, and the system of the optical fiber 14 is called an auxiliary optical transmission system.

Prior to the description of the operation of this system, a method of compensating distortion of a wideband amplifier used in a relay device of a mobile communication system will be described. In a slave station of a relay device using an optical transmission line, when a broadband signal multiplexed by frequency division multiplexing or the like is transmitted from the master station via an optical transmission line, the signal is amplified by a wideband amplifier for amplifying the signal. The output is transmitted from the antenna to the mobile device. In the case of frequency division multiplexing, it is also possible to individually amplify individual channels in a slave station and then combine and transmit them. There are many. However, in this case,
Since a wideband signal is amplified, unnecessary signals due to intermodulation distortion are likely to occur. The reduction of the unnecessary signal can be realized by using an amplifier having a large dynamic range. However, since a high-output amplifier is used for that purpose, power consumption, heat generation, and the like are increased, and practicality is poor. Therefore, a method of reducing a distortion signal (distortion compensation method) has been proposed so that amplification with low distortion can be performed without using an amplifier having such a large output. Examples of the distortion compensation method include a negative feedback method, an envelope feedback method, a heterodyne feedback method, a predistortion method, a Cuba predistortion method, and a feedforward method.

Among the above-mentioned distortion compensation methods, the feedforward method has a feature that the amount of distortion is remarkably improved. For this reason, a relay station of a mobile phone system currently uses a wideband amplifier that performs distortion compensation using the feedforward method. FIG.
Is a block diagram showing an amplifying device configured by this feedforward system. The input signal is branched by the branching unit 4 to become E (t), and the main amplifier 5 and the auxiliary amplifier 7
Are input respectively. Assuming that the amplification degree of the main amplifier 5 is A, the output is the fundamental wave component AE (t) and the distortion component IM.
(T). This main amplifier output signal
The branching is performed according to the branching ratio D1: D2 (D11D2), D1 (AE (t) + IM (t)) is output to the combiner 8, and D2 (AE (T) is output to the auxiliary amplifier 7. t) +
IM (t)) is output.

Since the branching ratio of the branching unit 6 is D 1 ≫D 2, the signal level of the input unit 71 of the auxiliary amplifier 7 is much smaller than the output of the main amplifier 5. The input signal E (t) from the splitter 4 is input to the input unit 72 of the auxiliary amplifier 7, and the signal applied to the input unit 72 and the input
Amplify the difference from the signal applied to 1. Therefore, if the amplification of the auxiliary amplifier 7 is A2, the output is

(Equation 1) Becomes Here, so that (1−D2 · A) = 0,
If the branching ratio of the branching device 6 is adjusted or the level is adjusted by an attenuator, an amplifier, etc., the output of the auxiliary amplifier 7 is proportional to the distortion component of the main amplifier 6 -A2.D2.IM (t).
And this is output to the synthesizer 8. Since the level of the input signal to the auxiliary amplifier 7 is sufficiently small, the level of the distortion signal generated in the auxiliary amplifier 7 is negligibly small and can be ignored. Therefore, the output signal of the auxiliary amplifier 7 and the signal D1 ·
When (AE (t) + IM (t)) is synthesized by the synthesizer 8 at a synthesis ratio C1: C2, the output O (t) of the synthesizer 8 becomes
It is expressed by the following equation.

(Equation 2) Here, the branching ratio (D1: D2) of the branching unit 6, the combining ratio (C1: C2) of the combiner 8, and the auxiliary amplifier 7 are set so that (C1.D1 -C2.A2.D2) = 0. By selecting the amplification degree A2, the distortion signal component of the output signal of the amplifier can be reduced. The present invention utilizes the above-described amplifier distortion compensation method, and extends this method to distortion compensation that includes both an optical transmission system and a wideband amplifier. The details will be described below.

Returning to FIG. 1, both the main electric / optical converter 10 and the auxiliary electric / optical converter 11 are formed using semiconductor laser elements, and generate distortion components corresponding to the dynamic range of the input signal. I do. Therefore, in the main electric / optical converter 10, the modulation degree of the light intensity is increased with emphasis on the noise characteristics, while in the auxiliary electric / optical converter 11, the modulation degree of the light intensity is reduced with emphasis on the distortion characteristics. ing. Therefore, the input signal to the main optical transmission system and the auxiliary transmission system is E1 (t), the transmission coefficient including the degree of light modulation of the main optical transmission system is A1, and the distortion component generated in this optical transmission system is IM1 (t). Then, the output signal E2 (t) becomes

(Equation 3) Becomes Further, assuming that the amplification degree of the main amplifier 5 is A2 and the generated distortion component is IM2 (t), the output signal E3
(T) is as follows.

(Equation 4) On the other hand, the transmission coefficient including the optical modulation degree of the auxiliary light transmission system is A3,
Assuming that the generated distortion component is IM3 (t), the output signal E6 (t) is

(Equation 5) However, since the degree of light modulation is small in the auxiliary transmission system, the distortion component IM3 (t) is assumed to be negligible.

The output signal E3 (t) of the main amplifier is supplied to the branching device 6
To C1: C2, and one of the signals E5 (t) =
C2 · E3 (t) is input to the + input terminal 71 of the auxiliary amplifier 7. The output signal E6 (t) of the auxiliary light transmission system undergoes a phase shift in the phase shifter 21. If a constant representing the phase shift is α1, the minus input terminal 7 of the auxiliary amplifier 7 is used.
2, E7 (t) = α1 · E6 (t) is input. Therefore, when the amplification degree of the auxiliary amplifier is A4, the output signal E8 (t) of the auxiliary amplifier 7 is obtained from (Equation 4) and (Equation 5).

(Equation 6) Here, the transmission coefficients A1 and A3 are complex numbers representing the attenuation rate and the amount of phase rotation in the optical transmission path,

(Equation 7) If the branching ratio of the branching unit 6, the phase amount of the phase shifter, the amplification factor of the amplifier, and the like are adjusted so that the output of the auxiliary amplifier 7 becomes only a distortion component;

(Equation 8)

The combiner 8 outputs a signal E9 obtained by phase-shifting the signal E4 (t) = C1 · E3 (t) branched from the brancher 6 and the output signal E8 (t) of the auxiliary amplifier 7 by the phase shifter 22.
Since (t) = α2 · E8 (t) is synthesized at the synthesis ratio C4: C3 (α2 is a constant indicating the amount of phase shift of the phase shifter 22), the output signal E10 (t) becomes (Equation 4) From Equation 8)

(Equation 9) Becomes Therefore, by adjusting the synthesis ratio of the synthesizer 8 and the phase shift amount of the phase shifter 22,

(Equation 10) If the output signal E1 of the synthesizer 8 is adjusted so that
0 (t) is

[Equation 11] And a signal from which the distortion component has been removed. Thus, even if the S / N ratio is set to be large by increasing the light intensity modulation degree in the main optical transmission system, it is possible to substantially eliminate the distortion signal generated from the optical transmission system and the main amplifier. Therefore, transmission quality can be improved, and therefore, there is no particular increase in cost.

FIG. 2 is a block diagram showing another embodiment of the optical transmission system of the present invention. 2, the same components as those shown in FIG. 1 are denoted by the same reference numerals. In this embodiment, a pilot signal generator 32 and a combiner 31 for determining a distortion compensation constant are provided on the master station side with respect to the configuration of FIG. 1, and two phase shifters are provided on the slave station side. The variable phase shifters 23 and 24, the variable attenuators 33 and 34 for varying the magnitude of the compensation parameter, the filter 36 for separating the distortion component of the pilot signal, and the variable attenuator 3 according to the distortion signal
3, and a detection controller 37 for controlling the variable phase shifters 23 and 24 is added.

In an actual optical transmission system, the length of the optical fiber is about several hundreds to several kilometers, and the transmission loss and the phase shift amount in the optical transmission line differ for each system. These amounts also change due to aging and temperature changes of the system. Therefore, in the configuration of FIG. 1, since each constant is fixed after the initial adjustment, it may be necessary to readjust the constant with time. In addition, initial adjustment is required when installing the system.
Therefore, in the present embodiment, by adding a pilot signal, the gain branching ratio is automatically adjusted using a variable attenuator.
The amount of phase shift can be automatically adjusted by a variable phase shifter.

For this purpose, a pilot signal is generated from its generator 32 and is combined with an input signal by a combiner 31. For example, in mobile communication, the frequency of the pilot signal is a signal having a band of several MHz to several tens of MHz, so that it is set to be a frequency in the immediate vicinity of this signal band. And this pilot signal is
As shown in FIG. 3, two frequencies f1 and f2 (f1 <f
2) signals P1 and P2,

(Equation 12) Is sufficiently smaller than the frequencies f1 and f2. The input signal to which such a pilot signal is added is shown in FIG.
Is transmitted to the slave station via the main optical transmission system and the auxiliary optical transmission system in the same manner as in the case of (1). The auxiliary amplifier 7 takes out the distortion components generated in the main optical transmission system and the main amplifier, and the combiner 8 cancels the distortion component and the distortion component in the signal from the main amplifier. Only the signal component is output. The process of this operation is similar to that of FIG.
In the case of FIG. 2, the amplitude of the input signal to the auxiliary amplifier 7 is controlled by the variable attenuator in the following manner.
The phase shift amounts of 3, 24 are also variably controlled.

The signals output from the splitter 8 include, in addition to the signal components, pilot signals P1 and P2, and distortion components Q1 and Q1 generated by passing through the main transmission system and the main amplifier.
Q2 is included. Since this distortion component is mainly due to the third-order distortion of the transmission system and the amplifier, as shown in FIG.
The frequencies of the distortion components Q1 and Q2 are

(Equation 13) It is. Since Δf is sufficiently small, the distortion components Q1, Q
2 is also within the band of the pilot signal, and is separated in frequency from the signal to be transmitted. Therefore, the distortion components Q1 and Q2 generated from the pilot signal are extracted by the filter 36, and the detection controller 37 controls the attenuation of the variable attenuators 33 and 34 and the variable phase shifter 23 so that the two distortion components are both reduced. , 24 are controlled. Since the frequency of the pilot signal is close to that of the signal, such a control method can automatically suppress the distortion component of the signal even if the constants of the transmission system change, so that the workability during system installation and maintenance can be improved. Can be improved.

Note that the phase shifter or variable phase shifter and the variable attenuator in FIGS. 1 and 2 are inserted so that the signal component can be removed in the auxiliary amplifier 7.
Further, since the distortion component can be removed in the synthesizer 8, it is needless to say that these components may be inserted at other positions, and the position is not limited to the positions shown in FIGS.

[0025]

As described above, according to the optical transmission system of the present invention, an analog electric signal as an input signal
The optical signal is converted into a system optical signal, and each is transmitted through a separate optical transmission line. The main amplifier is connected to the main system optical-electric conversion output, and the auxiliary amplifier is connected to the auxiliary system optical-electric conversion output. Means, the difference between the output of the main amplifying means by the auxiliary amplifying means and the output of the branching means is obtained, and the output of the auxiliary amplifying means and the output of the branching means are combined to obtain an output signal. Therefore, the dynamic range of the transmission signal can be expanded without individually improving the noise characteristics and distortion characteristics of the components of the system. In other words, if the same device is used, the transmission characteristics can be further improved. On the other hand, if the required dynamic range is the same, a device with lower characteristics can be used. .

[Brief description of the drawings]

FIG. 1 is a block diagram showing a first embodiment of an optical transmission system according to the present invention.

FIG. 2 is a block diagram illustrating a second embodiment of the optical transmission system according to the present invention.

FIG. 3 is an explanatory diagram of a pilot signal of FIG. 2;

FIG. 4 is a block diagram illustrating a schematic configuration of a conventional optical transmission system.

FIG. 5 is a block diagram showing a configuration of a feedforward type amplifying device used in an optical transmission system.

[Explanation of symbols]

 4, 6, 35 Splitter 5 Main amplifier 7 Auxiliary amplifier 8, 31 Combiner 10 Main electric / optical converter 11 Auxiliary electric / optical converter 12, 14 Optical fiber 13 Main optical / electrical converter 15 Auxiliary optical / electrical conversion Devices 21, 22 phase shifters 23, 24 variable phase shifters 32 pilot signal generators 33, 34 variable attenuators 36 filters 37 detection controllers

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5J090 AA01 AA56 CA21 CA32 CA41 FA09 GN03 GN07 HA44 HN08 HN16 KA00 KA02 KA16 KA23 KA41 KA55 MA14 TA01 TA03 5J092 AA01 AA56 CA21 CA32 CA41 FA09 HA44 KA00 KA23 KA16 KA01 UL01 UL07 5K002 CA01 CA13 CA15 DA08

Claims (4)

[Claims] 1. An analog electric signal as an input signal
First branching means for branching into two, first and second electrical / optical converting means for converting each of the two branch outputs by the first branching means into an optical signal by light intensity modulation, And first and second optical transmission lines for individually transmitting optical signals from the second electric / optical conversion means, and an electric signal transmitted by the first and second optical transmission lines, respectively. First and second optical / electrical converting means for converting the output signal from the first optical / electrical converting means, main amplifying means for amplifying the output signal of the first optical / electrical converting means, and branching the output signal of the main amplifying means at a predetermined branching ratio Second branching means, a first phase shifter for shifting the phase of the output of the second optical / electrical conversion means, one branch output of the second branching means, and the first phase shift. Auxiliary amplification means for amplifying the difference from the output of the amplifier, and the output of the auxiliary amplification means. A second phase shifter for shifting the phase,
An optical transmission system comprising: a first combining unit that combines an output of the phase shifter and the other output of the second branching unit to generate an output signal. 2. A pilot signal generating means for generating a pilot signal, a second synthesizing means for synthesizing a pilot signal from the pilot signal generating means and an analog electric signal as an input signal, and an output of the synthesizing means. First branching means for branching into two, first and second electrical / optical converting means for converting each of the two branch outputs by the first branching means into an optical signal by light intensity modulation, First and second optical transmission lines for individually transmitting optical signals from the first and second electric / optical conversion means, and an optical signal transmitted by the first and second optical transmission lines, respectively. First and second optical / electrical converting means for converting the signal into a signal, main amplifying means for amplifying an output signal of the first optical / electrical converting means, and an output signal of the main amplifying means at a predetermined branching ratio. No. to branch 2 branching means and the second light /
A first variable phase shifter for shifting the phase of the output of the electric conversion means, a first variable attenuator for controlling the amplitude of the output of the variable phase shifter, and one branch output of the second branch means Variable attenuator for controlling the amplitude of the signal, auxiliary amplification means for amplifying the difference between the output of the second variable attenuator and the output of the first variable attenuator, and the output phase of the auxiliary amplification means And a second phase shifter for shifting the output of the second phase shifter and the second phase shifter.
An optical transmission system comprising: a first combining unit that combines the other output of the branching unit with the other output to generate an output signal. 3. An S / S signal on the receiving side based on a force signal.
The signal is modulated by a modulation factor such that the N ratio becomes a predetermined value, transmitted by the main optical transmission system, the transmitted signal is amplified by the main amplifier, and the modulation factor is such that transmission distortion is not caused by the input electric signal. The auxiliary signal transmission system transmits the optical signal modulated by the step (a), and the difference between the signal transmitted by the auxiliary optical transmission system and the signal output from the main amplifier is determined by a first adjusting unit. The distortion component generated in the main optical transmission system and the main amplifier is taken out by adjusting and obtaining, and the distortion component and the output of the main amplifier are combined by adjusting the amplitude and phase of the signals by the second adjustment means. A distortion compensating method for the optical transmission system, wherein the distortion component is removed from the output of the main amplifier. 4. The input electric signal and two pilot signals whose frequency bands are adjacent to the input electric signal are transmitted through the main and auxiliary optical transmission systems, and the main optical transmission system and the main amplifier of the two pilot signals are transmitted. 4. The distortion compensation method for an optical transmission system according to claim 3, wherein the distortion component generated in step (1) is extracted, and the first and second adjusting units are controlled so that the distortion component is minimized.