JP2000261384A - Variable light gain equalizer and optical amplifier for multi-wavelength transmission using same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To ensure the gain deviation of an optical amplifier in a wavelength multiplexed light transmitting system, to suppress the deterioration of NF at the time of operation and to reduce a burden on an energizing light source. SOLUTION: This system is formed of an input-side lens fiber unit 101 for image-converting inputted light to parallel light, an output-side lens fiber unit 102 for guiding the parallel light to a fiber, mirrors 103, 104 arranged in parallel with each other between the unit 101 and the unit 102 and forming a Fabry- Perot etalon filter, and a driving element 105 varying a central wavelength by varying an interval between the mirrors 103 and 104. The FSR of the etalon filter is larger than four-fold of a transmitting band.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable optical gain equalizer and an optical amplifier for wavelength division multiplexing transmission using the same, and more particularly to a variable optical gain equalizer for compensating a gain deviation of an optical amplifier at a plurality of wavelengths. And an optical amplifier for wavelength division multiplexing transmission using the same.

[0002]

2. Description of the Related Art Conventionally, as a means for realizing a large-capacity optical communication system, a wavelength division multiplexing optical transmission system for transmitting a plurality of optical signals of different wavelengths in one optical transmission line to increase the amount of transmitted information has been developed. Is being promoted.

In such a wavelength division multiplexing optical transmission system, it is necessary to arrange linear repeaters (optical amplifiers) at predetermined intervals and collectively amplify optical signals of each wavelength. Therefore, a gain deviation occurs between the long wavelength optical signal and the short wavelength optical signal. Since the gain deviation in the wavelength division multiplexing optical transmission system increases as the number of wavelengths and the spacing between repeaters change from the design values, the problem that optical amplifiers must be individually designed individually according to the number of wavelengths and the spacing between repeaters. There is a point. In addition, since the gain deviation is accumulated and increased for each optical amplifier, there is a problem that the transmission quality (S / N ratio) is deteriorated or a transmission path in which transmission is impossible occurs.

In order to solve such a problem, an optical amplifier having a built-in variable optical attenuator for loss compensation has been proposed (Takeda et al., 1998 IEICE Communications Society Conference, B-10-). 155).

FIG. 7 is a diagram showing an example of a conventional optical amplifier incorporating a variable optical attenuator.

In this conventional example, as shown in FIG.
(Erbium-doped fiber) 711, a pre-gain control circuit 701, which comprises a GEQ (gain equalizer) 712 and a pump LD 713 and controls the input-side gain to be constant.
EDF721, 724, GEQ722, excitation LD723
And a post-stage gain control circuit 702 for controlling the output side gain to be constant, a VAT (variable optical attenuator) 731, a DCF (dispersion compensation fiber) 73.
2, an output constant control circuit 703 connected to an input terminal and an output terminal, the output constant control circuit 703 being provided with a constant total gain amplified by the preceding-stage gain control circuit 701 and the subsequent-stage gain control circuit 702. A monitoring circuit 704 monitors the state.

In this conventional example, the pre-stage gain control circuit 7
When the input level of the optical signal incident on 01 is the design value level, the optical signal is amplified by the former-stage gain control circuit 701 and the latter-stage gain control circuit 702, and the output level is controlled to be constant.

On the other hand, when the input level of the optical signal changes, the former-stage gain control circuit 701 and the latter-stage gain control circuit 7
02, the output level of the amplified optical signal fluctuates, so that the output constant control circuit 703 operates.
The output level is controlled to be constant. Note that a VAT (variable optical attenuator) 731 provided in the output constant control circuit 703 is provided.
Has a characteristic that the wavelength dependency is small.

[0009]

However, in the conventional optical amplifier as described above, when the input level rises, it is necessary to increase the attenuation of the variable optical attenuator and reduce the total gain of the two amplifiers. At this time, there is a problem that NF (Noise Figure) is deteriorated.

When the input level is reduced, it is necessary to increase the total gain of the two amplifiers from a design value. At this time, even in an operation state at the design value level, signal attenuation by the variable optical attenuator occurs. Would. Because of the signal attenuation, the gain is lost by several dB. Therefore, it is necessary to correct the loss by increasing the level of the design value, and there is a problem that the load on the pump light source increases.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and guarantees a gain deviation of an optical amplifier in a wavelength division multiplexing optical transmission system and suppresses NF degradation during operation. It is another object of the present invention to provide a variable optical gain equalizer capable of reducing a load on a pump light source and an optical amplifier for wavelength division multiplexing transmission using the same.

[0012]

According to the present invention, there is provided an optical transmission line having a predetermined transmission band, wherein a center wavelength is varied based on a control signal input from the outside. In a variable optical gain equalizer that compensates for a signal level deviation that fluctuates due to temporal and artificial operations at each wavelength of an optical signal transmitted in the optical transmission path, a Fabry-Perot type etalon filter is formed, An FSR (Free Spectral range) of the Fabry-Perot type etalon filter is set to be four times or more the transmission band.

Further, each of the optical signals transmitted in the optical transmission line, which is provided in an optical transmission line having a predetermined transmission band and whose center wavelength is varied based on a control signal inputted from the outside, is changed. In a variable optical gain equalizer for compensating for a signal level deviation that fluctuates due to a temporal or artificial operation at a wavelength, a Maha-Zehnder optical circuit is formed.
The SR is set at least four times the transmission band.

In a wavelength division multiplexing transmission optical amplifier for transmitting a plurality of optical signals of different wavelengths in one optical transmission line having a predetermined transmission band and amplifying the optical signals of each wavelength collectively, A first optical amplifier that amplifies and outputs an optical signal with a constant gain, and a constant signal level deviation at each wavelength with respect to the optical signal output from the first optical amplifier. A fixed optical gain equalizer for compensating, and a center wavelength variable, and a signal level deviation that fluctuates with respect to the optical signal output from the first optical amplifier by temporal and artificial operations at each wavelength. A variable optical gain equalizer that compensates for
An optical signal which is provided on the output side, is output from the pre-stage optical amplifier, and has a signal level deviation compensated by the fixed optical gain equalizer and the variable optical gain equalizer, is amplified with a constant gain and output. A second optical amplifier, a first optical signal splitter that splits light output from the second optical amplifier into two optical signals, and the optical signal splitter according to the optical signal output from the optical signal splitter. An output monitoring circuit for controlling characteristics of the variable optical gain equalizer, wherein the variable optical gain unit forms a Fabry-Perot type etalon filter, and the FSR of the Fabry-Perot type etalon filter is in the transmission band. It is characterized by being set to four times or more.

In a wavelength division multiplexing transmission optical amplifier for transmitting a plurality of optical signals of different wavelengths in one optical transmission line having a predetermined transmission band and amplifying the optical signals of each wavelength collectively, A first optical amplifier that amplifies and outputs an optical signal with a constant gain, and a constant signal level deviation at each wavelength with respect to the optical signal output from the first optical amplifier. A fixed optical gain equalizer for compensating, and a center wavelength variable, and a signal level deviation that fluctuates with respect to the optical signal output from the first optical amplifier by temporal and artificial operations at each wavelength. A variable optical gain equalizer that compensates for
An optical signal which is provided on the output side, is output from the pre-stage optical amplifier, and has a signal level deviation compensated by the fixed optical gain equalizer and the variable optical gain equalizer, is amplified with a constant gain and output. A second optical amplifier, a first optical signal splitter that splits light output from the second optical amplifier into two optical signals, and the optical signal splitter according to the optical signal output from the optical signal splitter. An output monitoring circuit for controlling the characteristics of the variable optical gain equalizer, wherein the variable optical gain device forms a Maha-Zehnder optical circuit, and the FSR of the Maha-Zehnder optical circuit is four times the transmission band. It is characterized by being set as described above.

The output monitoring circuit includes a second optical signal splitter for splitting the optical signal output from the first optical signal splitter into two signals, a long wavelength signal and a short wavelength signal.
Two filters for transmitting the two optical signals split by the second optical signal splitter with different transmission center wavelengths, and converting the two optical signals respectively transmitted through the two filters into electric signals. Two output optical / electrical signal converters, and the variable optical gain such that the short-wavelength electric signal level and the long-wavelength electric signal output from the two optical / electrical signal converters are equal. A control circuit for controlling characteristics of the equalizer.

Further, the second optical signal splitter is a wavelength selecting coupler.

(Function) In the present invention configured as described above, the FSR of the variable optical gain equalizer used in the wavelength division multiplexing transmission optical amplifier is set to be at least four times as large as the wavelength division multiplexing transmission band. This makes it possible to obtain a loss wavelength characteristic that is linear with respect to the wavelength within a certain band. Further, the band in which the above-described loss wavelength characteristic can be obtained can be wider than the wavelength multiplex transmission band.

In addition, since the operation is performed to change the center wavelength according to the level of the output signal detected by the output monitoring circuit and to correct the slope of the loss wavelength characteristic, the gain deviation of the optical amplifier for wavelength division multiplexing transmission is reduced. It is possible to compensate.

Further, since the loss of the variable optical gain equalizer when the center wavelength is changed is small, it is possible to reduce the NF deterioration and the load on the pump light source due to the operation of the variable optical gain equalizer.

[0021]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a diagram showing one embodiment of the variable optical gain equalizer of the present invention.

In this embodiment, as shown in FIG. 1, an input-side lens fiber unit 101 for converting light guided through a fiber (not shown) into parallel light, and an output-side lens for guiding parallel light to the fiber. A fiber unit 102 and mirrors 103 arranged parallel to each other between the input-side lens fiber unit 101 and the output-side lens fiber unit 102 to form a Fabry-Perot etalon filter;
It comprises a driving element 105 for changing the interval between the mirrors 103 and 104.

The input-side lens fiber unit 10
The first and output side lens fiber units 102 are composed of a single mode fiber and an aspheric lens, and the respective optical elements are fixed by YAG laser welding.

The reflectance and the gap between the mirrors 103 and 104 have an FSR of 300 nm, an amplitude of 5 dB,
The transmission center wavelength is set to 1551 nm, and the FSR of the etalon filter is wide.

The driving element 105 is a piezoelectric element.
When a voltage of 20 V is applied, the gap between the mirrors 103 and 104 can be displaced by about 0.5 μm. Therefore, by driving the driving element 105, the mirror 1
The wavelength of the etalon filter formed between 03 and 104 can be selectively varied.

In the variable optical gain equalizer configured as described above, the light incident on the input-side lens fiber unit 101 is wavelength-selected by the mirrors 103 and 104 due to Fabry-Perot interference. The light is emitted through the fiber unit 102.

FIG. 2 is a diagram showing an embodiment of an optical amplifier for wavelength division multiplexing transmission using the variable optical gain equalizer shown in FIG.

In this embodiment, as shown in FIG. 2, a first-stage optical amplifier 1 which is provided on the input side and amplifies an optical signal with a constant gain and outputs the amplified signal, and an output from the first-stage optical amplifier 1 A fixed optical gain equalizer 3 for compensating for a steady signal level deviation at each wavelength with respect to the obtained optical signal, and an optical signal output from the pre-stage optical amplifier 1 whose characteristics are changed by an externally input signal. A variable optical gain equalizer 4 for compensating a signal level deviation that fluctuates due to temporal and artificial operations at each wavelength with respect to the signal; 3 and variable optical gain equalizer 4
The second-stage optical amplifier 2 which is a second optical amplifier that amplifies the optical signal, the signal level deviation of which has been compensated for, with a constant gain and outputs the amplified signal.
A coupler 5 serving as a first optical signal splitter for splitting the light output from the post-stage optical amplifier 2 into two optical signals, and a variable optical gain equalizer 4 according to the optical signal output from the coupler 5 And an output monitoring circuit 100 for controlling the characteristics of. The output monitoring circuit 100 includes a coupler 6 serving as a second optical signal splitter for splitting the optical signal output from the coupler 5 into two optical signals of a long wavelength signal and a short wavelength signal.
And optical bandpass filters 7 and 8 for transmitting the two optical signals branched by the coupler 6 at different transmission center wavelengths, and converting the optical signals transmitted through the optical bandpass filters 7 and 8 into electric signals. Monitor PDs 9 and 10 which are optical / electrical signal converters for outputting and outputting
The level of each electric signal output from the monitor PD 9 is detected, and the level of the electric signal output from the monitor PD 9 and the level of the electric signal output from the monitor PD 10 are equalized. And a control circuit 11 for controlling characteristics.

The pre-stage optical amplifier 1 and the post-stage optical amplifier 2 use erbium-doped fibers doped with Al at a high concentration, and have a wavelength multiplex transmission band of 1538 nm.
From 1564 nm to 26 nm. The erbium-doped fiber can directly amplify the input light as an optical signal, eliminating the need for electronic circuits, high gain,
It has advantages such as high output.

The fixed optical gain equalizer 3 is designed to operate optimally when the input level of the optical signal input to the pre-stage optical amplifier is a design value.

The variable optical gain equalizer 4 uses the variable optical gain equalizer shown in FIG. 1 and has an FSR of 300 nm, an amplitude of 5 dB, and a transmission center wavelength of 1551 nm. The FSR of No. 4 is 12 times the wavelength multiplex transmission band. At this time, the loss deviation of each wavelength in the wavelength division multiplexing transmission band is 0.15 dB.

The transmission center wavelength of the optical bandpass filter 7 is adjusted to 1538 nm, which is the shortest wavelength of the band signals used in the wavelength division multiplexing transmission system of the present embodiment. The transmission center wavelength of the filter 8 is adjusted to 1564 nm, which is the longest signal wavelength.

In the optical amplifier for wavelength division multiplex transmission configured as described above, the optical signal incident on the pre-stage optical amplifier 1 is amplified by the pre-stage optical amplifier 1, and the fixed optical gain equalizer 3 and the variable optical gain The signal level deviation is compensated for by the equalizer 4, and thereafter, the signal is amplified again by the post-stage optical amplifier 2. The optical signal re-amplified by the latter-stage optical amplifier 2 is split into two optical signals by the coupler 5, and one of the two optical signals split by the coupler 5 is input to the output monitoring circuit 100 and the other is output. The optical signal is output via the output terminal.

The optical signal incident on the output monitoring circuit 100 is again split into a long wavelength signal and a short wavelength signal in the coupler 6 and transmitted through the optical bandpass filters 7 and 8 having different transmission center wavelengths, respectively. The data is input to the monitors PD9 and PD10. The optical signals input to the monitor PDs 9 and 10 are converted into electric signals by the monitor PDs 9 and 10, and then output to the control circuit 11. In the control circuit 11, the levels of the electric signals output from the monitors PD9 and PD10 are detected, and the transmission center wavelength of the variable optical gain equalizer 4 is controlled to the optimum wavelength so that the electric signal levels become equal. ing.

FIG. 3 shows the variable optical gain equalizer 4 shown in FIG.
FIG. 4 is a diagram illustrating the loss wavelength characteristics of the present invention.

As shown in FIG. 3, the variable optical gain equalizer 4
Can selectively change the wavelength of the etalon filter, so that in a certain band, it is possible to obtain a loss wavelength characteristic such that the loss becomes linear with respect to the wavelength. Further, by setting the FSR of the etalon filter wider than the wavelength multiplex transmission band, the band in which the above-described loss wavelength characteristic can be obtained can be made larger than the wavelength multiplex transmission band.

Accordingly, the FSR having a linear loss wavelength characteristic with respect to the wavelength and capable of changing the amount of inclination thereof.
The etalon filter having a wide width can operate as a variable optical gain equalizer for correcting the slope of the gain loss of the optical amplifier. Note that the amount of increase in excess loss when the wavelength is variably operated is as small as about 0.5 dB.

Hereinafter, the operation of the wavelength division multiplexing transmission optical amplifier of the present embodiment configured as described above will be described in further detail.

When the level of the optical signal input from the input terminal is within the range of the design value, the output level of each wavelength of the optical signal in the wavelength division multiplexing transmission band is made flat by the fixed optical gain equalizer 3. Because they are compensated, they are almost equal. Therefore, the monitor currents of the monitor PD9 and the monitor PD10 of the output monitoring circuit 100 become equal. At this time, the gain deviation of the output level of the post-stage optical amplifier 2 is 0.3 dB.

On the other hand, when the level of the optical signal incident from the input terminal decreases and deviates from the design value, the gain of the post-stage optical amplifier 2 that performs the constant output control increases. However, since the gain change in the wavelength multiplexing transmission band is not constant, the gain on the short wavelength side is larger than that on the long wavelength side, and the monitor current I1 of the monitor PD10 is larger than the monitor current I2 of the monitor PD9. Become. Therefore, the monitor current I
By shifting the transmission center wavelength of the variable optical gain equalizer 4 to the shorter wavelength side by the control circuit 11 so that 1 and I2 become equal, it is possible to compensate for the gain deviation of each wavelength.

As a specific example, when the level of the optical signal input from the input terminal is reduced by 3 dB, the gain deviation of the output level of each wavelength is 1.3 dB, and the control circuit 11
In, it is detected that the monitor current I1 of the monitor PD10 is larger than the monitor current I2 of the monitor PD9 by 1.3 dB. Therefore, after adjusting the transmission center wavelength of the variable optical gain equalizer 4 to 1512 nm so that the monitor current I1 and the monitor current I2 detected by the control circuit 11 are equal, the gain of the output level of the post-stage optical amplifier 2 is adjusted. When the deviation was confirmed, the gain deviation was 0.4 dB.

Therefore, even when the input level of the optical signal deviates from the design value, the transmission center wavelength of the variable optical gain equalizer 4 is controlled to the optimum wavelength, thereby comparing the input level with the design value. As a result, the gain deviation was only about 0.1 dB, and it was confirmed by the output monitoring circuit 100 that the gain compensation was more effective than the gain deviation of 1.3 dB before being controlled. . In addition, the loss increase of the variable optical gain equalizer due to the wavelength variable operation was about 0.5 dB, and no NF deterioration was observed unlike the conventional example using the variable optical attenuator.

In this embodiment, the FSR of the etalon filter
Was set to 12 times the wavelength multiplexing transmission band, and the effect was confirmed. It was also confirmed that the same effect was obtained in an experiment in which the wavelength was set to 4 times the wavelength multiplexing transmission band.

FIG. 4 is a diagram showing wavelength characteristics when the wavelength division multiplexing transmission optical amplifier shown in FIG. 2 is driven.

As shown in FIG. 4, when the input level of the optical signal input to the pre-amplifier 1 increases and the gain of the post-amplifier 2 decreases, the gain on the short wavelength side decreases. The transmission center wavelength of the optical gain equalizer 4 shifts to the longer wavelength side, whereby it is possible to compensate for the gain deviation of each wavelength.

FIG. 5 is a diagram showing another embodiment of the wavelength division multiplexing transmission optical amplifier using the variable optical gain equalizer shown in FIG.

As shown in FIG. 5, in this embodiment, a wavelength selecting coupler 60 is provided in the output monitoring circuit 100a.

The wavelength selecting coupler 60 has a function of again splitting the optical signal output from the coupler 5 into a long wavelength signal and a short wavelength signal, similarly to the coupler 6 shown in FIG.

When the wavelength division multiplexing transmission optical amplifier shown in FIG. 5 was operated and the gain deviation of the output level of the post-stage optical amplifier 2 was confirmed, the same effect as that shown in FIG. .

FIG. 6 is a diagram showing another embodiment of the variable optical gain equalizer of the present invention.

In this embodiment, as shown in FIG. 6, a coupler 601 for splitting incident light into two optical signals,
And a optical path length varying mechanism 602 for changing the optical path length of one optical signal of the two optical signals branched by the coupler 601. Is formed.

In the variable optical gain equalizer configured as described above, the FSR is set to 300 nm, and the center wavelength is changed by the optical path length changing mechanism 602.

The variable optical gain equalizer shown in FIG. 6 is incorporated in the variable optical gain equalizer 4 shown in FIG. 2 to operate the wavelength division multiplexing transmission optical amplifier. When the gain deviation was confirmed, the same effect as that shown in FIG. 2 was confirmed for the gain compensation.

[0055]

As described above, according to the present invention, a plurality of optical signals of different wavelengths are transmitted in one optical transmission line having a predetermined transmission band, and the optical signals of each wavelength are collectively amplified. Since the FSR of the variable optical gain equalizer used for the wavelength division multiplexing transmission optical amplifier is set to four times or more with respect to the transmission band, the center wavelength of the variable optical gain equalizer is changed to correct the loss wavelength characteristic. Thus, the gain deviation of the wavelength division multiplexing transmission amplifier can be compensated.

Further, since the loss of the variable optical equalizer when the wavelength is varied can be reduced, the NF deterioration due to the operation of the variable optical equalizer and the load on the pump light source can be reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of a variable optical gain equalizer of the present invention.

FIG. 2 is a diagram showing an embodiment of an optical amplifier for wavelength division multiplexing transmission using the variable optical gain equalizer shown in FIG.

FIG. 3 is a diagram illustrating a loss wavelength characteristic during operation of the variable optical gain equalizer illustrated in FIG. 1;

FIG. 4 is a diagram illustrating wavelength characteristics when the wavelength division multiplexing transmission optical amplifier shown in FIG. 2 is driven.

FIG. 5 is a diagram showing another embodiment of the optical amplifier for wavelength division multiplexing transmission of the present invention.

FIG. 6 is a diagram showing another embodiment of the variable optical gain equalizer of the present invention.

FIG. 7 is a diagram illustrating a conventional optical amplifier for wavelength division multiplexing transmission.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Front-stage optical amplifier 2 Rear-stage optical amplifier 3 Fixed optical gain equalizer 4 Variable optical gain equalizer 5, 6 Coupler 7, 8 Optical bandpass filter 9, 10 Monitor PD 11 Control circuit 100 Output monitoring circuit 101 Input side lens fiber Unit 102 Output-side lens fiber unit 103, 104 Mirror 105 Driving element 60 Wavelength selecting coupler 70 Optical bandpass filter 100a Output monitoring circuit 601 603 Coupler 602 Optical path length variable mechanism

[Procedure amendment]

[Submission date] March 9, 1999 (1999.3.9)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Claim 3

[Correction method] Change

[Correction contents]

Claims (6)

[Claims] 1. An optical signal which is provided in an optical transmission line having a predetermined transmission band and whose center wavelength is variable based on a control signal input from the outside, and which is transmitted through the optical transmission line. In a variable optical gain equalizer that compensates for a signal level deviation that fluctuates due to temporal and artificial operations at wavelengths, a Fabry-Perot type etalon filter is formed, and an FSR of the Fabry-Perot type etalon filter corresponds to the transmission band. A variable optical gain equalizer characterized by being set to four times or more. 2. An optical signal, which is provided in an optical transmission line having a predetermined transmission band and whose center wavelength is variable based on a control signal input from the outside, and which is transmitted through the optical transmission line, In a variable optical gain equalizer that compensates for a signal level deviation that fluctuates due to temporal or artificial operation at a wavelength, a maha-zehnder optical circuit is formed, and the FSR of the maha-zehnder optical circuit is four times or more with respect to the transmission band. A variable optical gain equalizer characterized by being set. 3. A wavelength division multiplexing transmission optical amplifier for transmitting a plurality of optical signals of different wavelengths in one optical transmission line having a predetermined transmission band and amplifying the optical signals of each wavelength collectively. A first optical amplifier that amplifies the optical signal with a constant gain and outputs the same, and the optical signal output from the first optical amplifier is temporally and artificially operated at each wavelength. A fixed optical gain equalizer for compensating for a varying signal level deviation; and a central wavelength variable, the optical signal output from the first optical amplifier being varied by temporal and artificial operations at each wavelength. A variable optical gain equalizer that compensates for the signal level deviation that occurs, and a signal level deviation that is provided on the output side and is output from the pre-stage optical amplifier, and is fixed by the fixed optical gain equalizer and the variable optical gain equalizer. Amplify the compensated optical signal with a constant gain A second optical amplifier for outputting, a first optical signal splitter for splitting the light output from the second optical amplifier into two optical signals, and an optical signal output from the optical signal splitter. An output monitoring circuit for controlling the characteristics of the variable optical gain equalizer, wherein the variable optical gain device forms a Fabry-Perot type etalon filter, and an F of the Fabry-Perot type etalon filter.
An optical amplifier for wavelength division multiplexing transmission, wherein SR is set to four times or more of the transmission band. 4. An optical amplifier for wavelength-division multiplexing transmission in which a plurality of optical signals of different wavelengths are transmitted in one optical transmission line having a predetermined transmission band and the optical signals of each wavelength are collectively amplified. A first optical amplifier for amplifying an optical signal with a constant gain and outputting the same, and a constant signal level deviation at each wavelength with respect to the optical signal output from the first optical amplifier. A fixed optical gain equalizer for compensating, and a center level variable, and a signal level deviation that fluctuates with respect to the optical signal output from the first optical amplifier by temporal and artificial operations at each wavelength. A variable optical gain equalizer for compensating for the following factors: light provided on the output side, output from the pre-stage optical amplifier, and compensated for signal level deviation by the fixed optical gain equalizer and the variable optical gain equalizer. A second optical amplifier for amplifying and outputting a signal with a constant gain A first optical signal splitter for splitting light output from the second optical amplifier into two optical signals; and a variable optical gain or the like according to the optical signal output from the optical signal splitter. An output monitoring circuit for controlling the characteristics of the modulator, wherein the variable optical gain device forms a Mach-Zehnder optical circuit, and the FSR of the Mach-Zehnder optical circuit is 4 to the transmission band.
An optical amplifier for wavelength-division multiplex transmission, wherein the optical amplifier is set to be twice or more. 5. The optical amplifier for wavelength division multiplexing transmission according to claim 3, wherein the output monitoring circuit converts the optical signal output from the first optical signal splitter into a long wavelength signal and a short wavelength signal. A second optical signal splitter for splitting into two signals, a signal and two signals; two filters for transmitting the two optical signals split by the second optical signal splitter with transmission center wavelengths different from each other; Two optical / electrical signal converters that convert two optical signals transmitted through the two filters into electric signals and output the electric signals, and a short-wavelength electric signal level output from each of the two optical / electrical signal converters And a control circuit for controlling the characteristics of the variable optical gain equalizer so that the level of the electric signal of the long wavelength becomes equal to the level of the electric signal of the long wavelength. 6. The optical amplifier for wavelength division multiplexing transmission according to claim 5, wherein said second optical signal splitter is a coupler for wavelength selection.