JP2004147201A - Loss tilt variable filter, light amplifier and optical communication system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a loss tilt variable filter that is used suitably as a gain equalizer whose loss is linear to a wavelength and whose loss tilt is variable. <P>SOLUTION: Light inputted to an input port 101 of the loss tilt variable filter 100 suffers loss in accordance with wavelength when the light respectively passes through a variable filter 110 and a fixed filter 120 to be outputted from an output port 102. When connection efficiency of the variable filter 110 is at any level within a variable range, the entire loss spectrums of the loss tilt variable filter 100 are almost flat because the respective loss spectrums of the variable filter 110 and the fixed filter 120 have characteristics reverse from each other. A corresponding loss tilt changes in the entire loss spectrums of the loss tilt variable filter 100 while the loss remains linear with respect to a wavelength because temperature adjustment by a Peltier element 130 changes the connection efficiency of the variable filter 110. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention provides a filter having a variable loss slope that gives a loss to light within a predetermined wavelength band input to an input end and outputs the light from an output end, an optical amplifier including the loss slope variable filter, and The present invention relates to an optical communication system including the variable loss tilt filter or the optical amplifier.
[0002]
[Prior art]
An optical communication system can transmit and receive a large amount of information at high speed by transmitting signal light through an optical fiber transmission line. In particular, a WDM (Wavelength Division Multiplexing) optical communication system multiplexes and transmits multi-wavelength signal light included in a signal light wavelength band, and can further increase the capacity. . Further, in the optical communication system, since the signal light suffers a loss while being transmitted through the optical fiber transmission line, an optical amplifier for optically amplifying the signal light is provided to compensate for the loss.
[0003]
As an optical amplification device, an optical fiber amplifier (EDFA: Erbium Doped Fiber Amplifier) using an amplification optical fiber (EDF: Erbium Doped Fiber) doped with an Er element as an optical amplification medium is known. The EDFA can collectively amplify the multi-wavelength signal light of the 1.55 μm band in the EDF by supplying the EDF with excitation light (wavelength 0.98 μm band or 1.48 μm band).
[0004]
In addition, the optical amplifying device further includes a gain equalizer that equalizes the gain spectrum because the gain spectrum in the optical amplifying medium may not be flat in the signal light wavelength band (for example, Non-Patent Document 1 and Non-Patent Document 1). See Patent Document 2). That is, the loss spectrum of the gain equalizer has the same shape as the gain spectrum of the optical amplification medium, and the entire gain spectrum of the optical amplifier is flattened in the signal light wavelength band.
[0005]
[Non-patent document 1]
H. Nakaji, et al. , "Ultra-Wide Dynamic Range Erbium Doped Fiber Amplifiers Employing Variable Attenuation Slope Compensator", OAA2000, OWA2 (2000)
[Non-patent document 1]
H. J. Patrick, et al. , "Analysis of the Resonance of Long Period FiberGratings to External Index of Refraction", Journal of Lightwave Technology, Journal of Lightwave Technology. 16, No. 9, pp. 1606-1612 (1998)
[0006]
[Problems to be solved by the invention]
By the way, it is known that the gain spectrum of the above EDF fluctuates due to various factors (for example, input signal light intensity, input signal light wave number, temperature, etc.). For example, the EDF has a substantially linear gain with respect to wavelength in a wavelength range of 1540 nm or more, and its gain gradient changes depending on the input signal light intensity. Therefore, as a gain equalizer that can always equalize the gain of an optical amplifying medium whose gain slope changes, a gain equalizer whose loss is substantially linear with respect to wavelength and whose loss slope is variable is required. It becomes.
[0007]
SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and a loss slope variable filter that can be suitably used as a gain equalizer in which a loss is substantially linear with respect to wavelength and the loss slope is variable. It is another object of the present invention to provide an optical amplifying device including the variable loss inclination filter and an optical communication system including the variable loss inclination filter or the optical amplification device.
[0008]
[Means for Solving the Problems]
The variable loss slope filter according to the present invention is a filter having a variable loss slope for giving a loss to the input light in the predetermined wavelength band and outputting the light, and (1) the coupling efficiency depends on the wavelength; It is characterized by comprising: a variable filter having variable efficiency; and (2) a fixed filter having a predetermined loss spectrum for light in a predetermined wavelength band. Further, when the coupling efficiency of the variable filter is in any state within the variable range, the spectrums of the variable filter and the fixed filter in the predetermined wavelength band have characteristics opposite to each other.
[0009]
The entire loss spectrum of the loss slope variable filter is a sum of the loss spectrums of the variable filter and the fixed filter. When the coupling efficiency of the variable filter is in any state within the variable range, the predetermined wavelength band Is substantially flat. Then, by adjusting the coupling efficiency of the variable filter, the loss slope of the entire loss spectrum of the loss slope variable filter changes while maintaining a state in which the loss is substantially linear with respect to the wavelength.
[0010]
In the variable loss tilt filter according to the present invention, it is preferable that the variable filter includes a first single mode optical fiber, a graded index optical fiber, and a second single mode optical fiber which are sequentially fused. In this case, by controlling the temperature or the tension of the graded index optical fiber, the coupling efficiency of the variable filter is adjusted, and the loss slope in the entire loss spectrum of the loss slope variable filter is adjusted.
[0011]
In the variable loss inclination filter according to the present invention, the length of the graded index optical fiber is preferably 25 mm or more, and more preferably 60 mm or more. In this case, the wavelength dependence of the coupling efficiency of the variable filter appears effectively.
[0012]
In the loss inclination variable filter according to the present invention, it is preferable that the numerical aperture of one or both of the first single mode optical fiber and the second single mode optical fiber is 0.2 or more. In this case, since the wavelength selectivity of the coupling of the loss inclination variable filter is increased, the rate of change of the inclination with respect to temperature or humidity is increased.
[0013]
The optical amplifier according to the present invention includes: (1) an optical amplifier for optically amplifying a signal light within a predetermined wavelength band; and (2) an optical amplifier connected to the optical amplifier for providing a loss to the signal light. (3) adjusting the transmission spectrum of the loss slope variable filter according to the present invention to control the overall gain including the optical amplifier and the loss slope variable filter so as to be constant within a predetermined wavelength band. And a control unit. It is also preferable to further include a gain equalizer.
[0014]
In the optical amplifying device according to the present invention, the signal light is optically amplified by the optical amplifying unit and suffers a loss by the loss inclination variable filter. The overall gain spectrum of the optical amplifying device is obtained by integrating the gain spectrum of the optical amplifying section and the loss spectrum of the loss slope variable filter. By adjusting the coupling efficiency of the variable filter included in the loss slope variable filter, the entire gain spectrum of the optical amplifier can be made flat in a predetermined wavelength band.
[0015]
An optical communication system according to the present invention includes the above-described optical amplifying device according to the present invention, and optically amplifies and transmits signal light using the optical amplifying device. In this optical communication system, the signal light is optically amplified and transmitted by the optical amplifying device according to the present invention, so that high-quality signal light transmission is possible.
[0016]
An optical communication system according to the present invention is connected to (1) a plurality of optical amplifiers provided on a signal light transmission path for optically amplifying signal light within a predetermined wavelength band, and (2) a plurality of optical amplifiers, (3) adjusting the transmission spectrum of the loss slope variable filter according to the present invention to impart a loss to the signal light, and adjusting the transmission spectrum of the loss slope variable filter to include a plurality of optical amplifiers and the loss slope variable filter. A control unit that controls the gain so as to be constant within a predetermined wavelength band. In this optical communication system, the signal light is optically amplified by each of the plurality of optical amplifiers, and the loss according to the wavelength is given to the signal light by the loss inclination variable filter according to the present invention, so that the overall gain is reduced. Since it is constant within a predetermined wavelength band, high-quality signal light transmission is possible.
[0017]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements will be denoted by the same reference symbols, without redundant description.
[0018]
First, an embodiment of a loss slope variable filter according to the present invention will be described. FIG. 1 is a configuration diagram of a loss slope variable filter 100 according to the present embodiment. The variable loss slope filter 100 shown in this figure has a variable loss slope that gives a loss to light within a predetermined wavelength band input to an input terminal 101 and outputs the light from an output terminal 102. The variable loss slope filter 100 includes a variable filter 110 and a fixed filter 120 connected in series between an input terminal 101 and an output terminal 102. Further, the loss slope variable filter 100 also includes a Peltier element 130 for adjusting the temperature of the variable filter 110.
[0019]
In the variable filter 110, the light coupling efficiency between the first end 110a and the second end 110b depends on the wavelength. The coupling efficiency is variable and can be controlled by adjusting the temperature by the Peltier element 130. The fixed filter 120 is for inputting light within a predetermined wavelength band, giving a loss and outputting the light, and has a fixed loss spectrum in the predetermined wavelength band. When the coupling efficiency of the variable filter 110 is in any state within the variable range, the spectra of the variable filter 110 and the fixed filter 120 have characteristics opposite to each other in a predetermined wavelength band.
[0020]
In the loss slope variable filter 100, light within a predetermined wavelength band input to the input terminal 101 undergoes a loss according to the wavelength when passing from the first end 110a to the second end 110b of the variable filter 110. Even when the light passes through the fixed filter 120, it is output from the output terminal 102 while suffering a loss according to the wavelength. In the loss slope variable filter 100, a loss spectrum for light passing from the input terminal 101 to the output terminal 102 is a sum of the loss spectra of the variable filter 110 and the fixed filter 120.
[0021]
When the coupling efficiency of the variable filter 110 is in any state within the variable range, the loss spectrums of the variable filter 110 and the fixed filter 120 have characteristics opposite to each other in a predetermined wavelength band. Accordingly, the entire loss spectrum of the loss slope variable filter 100 is substantially flat. Further, since the coupling efficiency of the tunable filter 110 is changed by the temperature adjustment by the Peltier element 130, the loss spectrum of the entire loss slope variable filter 100 is such that the loss slope is substantially linear with respect to the wavelength and the loss slope is not linear. Change.
[0022]
FIG. 2 is a configuration diagram of a loss slope variable filter 100A according to another embodiment. The variable loss tilt filter 100A shown in this figure further includes an optical coupler 140, a monitor unit 150, and a control unit 160 in addition to the configuration of the loss tilt variable filter 100 shown in FIG.
[0023]
The optical coupler 140 is provided between the fixed filter 120 and the output terminal 102, branches a part of light traveling from the fixed filter 120 to the output terminal 102, and outputs the branched light to the monitor 150. The monitor 150 receives the light that has arrived from the optical coupler 140 and detects the spectrum of the light. Then, the control unit 160 controls the temperature adjustment of the variable filter 110 by the Peltier element 130 based on the spectrum of the light detected by the monitor unit 150, so that the entire loss slope of the loss slope variable filter 100A is reduced. The spectrum of the light output from the output end 102 of the loss inclination variable filter 100A is adjusted to be substantially flat.
[0024]
FIG. 3 is a configuration diagram of the variable filter 110 included in the loss slope variable filters 100 and 100A according to the present embodiment. As shown in this figure, the tunable filter 110 includes a single mode optical fiber 111, a graded index optical fiber 112, and a single mode optical fiber 113 that are sequentially fused and connected between a first end 110a and a second end 110b. Including. FIG. 3 shows a cross section of the tunable filter 110 when cut along a plane including the optical axis, and also schematically shows an optical path of light guided through the graded index optical fiber 112.
[0025]
The single mode optical fiber 111 has a core region 111a having a high refractive index and a cladding region 111b having a low refractive index surrounding the core region 111a, and confine the light in the core region 111a to guide the light. Can be done. Similarly, the single mode optical fiber 113 has a high refractive index core region 113a and a low refractive index cladding region 113b surrounding the core region 113a. Can be guided.
[0026]
The graded index optical fiber 112 has a high refractive index core region 112a and a low refractive index cladding region 112b surrounding the core region 112a. The refractive index distribution of the core region 112a in the radial direction in the radial direction increases as the distance from the optical axis increases, and gradually decreases as the distance from the optical axis increases. The graded index optical fiber 112 receives the light output from the core region 111a of one single mode optical fiber 111, and guides the light toward the other single mode optical fiber 113. Here, since the refractive index of the core region 112a depends on the wavelength (that is, has a wavelength dispersion), the coupling efficiency of light to the single mode optical fiber 113 differs depending on the wavelength. Further, since the refractive index of the core region 112a also depends on the temperature, the coupling efficiency can be controlled by adjusting the temperature by the Peltier element 130.
[0027]
FIG. 4 is a diagram showing an optical system equivalent to the variable filter 110. As an optical element equivalent to the graded index optical fiber 112, a lens 112c having chromatic aberration is shown. The light output from the core region 111a of the single mode optical fiber 111 enters the lens 112c while diverging, and is collected by the lens 112c. Light of a certain wavelength lambda ₂ is converged on the end face position of the core region 113a of the single-mode optical fiber 113, a high coupling efficiency to the core region 113a. However, since the lens 112c has chromatic aberration, light of wavelengths λ ₁ and λ ₃ different from the wavelength λ ₂ is condensed at a position different from the end surface position of the core region 113a of the single mode optical fiber 113. Therefore, the coupling efficiency to the core region 113a is low. The same applies to the variable filter 110 shown in FIG. 3, and the light coupling efficiency between the first end 110a and the second end 110b depends on the wavelength. The numerical aperture of both or any one of the single mode optical fiber 110 and the single mode optical fiber 113 included in the variable filter 110 is preferably larger than the numerical aperture of the single mode optical fiber generally used for optical communication. Preferably, the value is 0.2 or more. In this case, the wavelength selectivity of the coupling of the loss inclination variable filter 100 is increased, so that the rate of change of the inclination with respect to temperature or humidity is increased.
[0028]
FIG. 5 is a diagram showing a transmission spectrum of the variable filter 110. Here, the length of the graded index optical fiber 112 was set to 60 mm, and the temperature of the graded index optical fiber 112 was set to a value in the range of −20 ° C. to + 80 ° C. in increments of 20 ° C. by temperature adjustment by the Peltier element 130. As can be seen from this figure, the light coupling efficiency between the first end 110a and the second end 110b of the tunable filter 110 depends on the wavelength, and the wavelength dependency depends on the temperature. In order for the wavelength dependence of the coupling efficiency of the variable filter 110 to appear effectively, the length of the graded index optical fiber 112 is preferably 25 mm or more, and the length is 60 mm or more. Is more preferable.
[0029]
FIG. 6 is a diagram illustrating a transmission spectrum of the fixed filter 120. Here, the transmission spectrum of the fixed filter 120 has characteristics opposite to those of the variable filter 110 at the temperature of 20 ° C. shown in FIG. The fixed filter 120 having a transmission spectrum as shown in this figure can be realized by, for example, a dielectric multilayer filter, a fiber grating, or the like.
[0030]
FIG. 7 is a diagram illustrating a transmission spectrum of the loss slope variable filter 100 according to the present embodiment. The transmission spectrum of the variable loss tilt filter 100 is a sum of the transmission spectrum of the variable filter 110 shown in FIG. 5 and the transmission spectrum of the fixed filter 120 shown in FIG. The temperature of the graded index optical fiber 112 included in the variable filter 110 was set to each value in the range of −20 ° C. to + 80 ° C. in steps of 20 ° C. by the temperature adjustment by the Peltier element 130.
[0031]
As can be seen from this figure, when the temperature of the graded index optical fiber 112 is 20 ° C., the transmittance of the loss slope variable filter 100 is a constant value of −3 dB in a predetermined wavelength band regardless of the wavelength. As the temperature of the graded index optical fiber 112 becomes higher than 20 ° C., the transmission spectrum of the loss slope variable filter 100 shows that the loss is substantially linear with respect to wavelength in a predetermined wavelength band, and the loss slope is positive. It is getting bigger. On the other hand, as the temperature of the graded index optical fiber 112 becomes lower than 20 ° C., the transmission spectrum of the loss slope variable filter 100 shows that the loss is substantially linear with respect to wavelength in a predetermined wavelength band, and the loss slope is negative. In the direction of.
[0032]
As described above, in the transmission spectrum of the loss slope variable filter 100, in a predetermined wavelength band, the loss is substantially linear with respect to the wavelength, the loss slope is variable, and the loss slope can be controlled by adjusting the temperature. Therefore, the loss slope variable filter 100 can be suitably used as a gain equalizer for compensating fluctuations in the gain slope of the EDF. The same applies to the variable loss slope filter 100A shown in FIG.
[0033]
Next, an embodiment of the optical amplifying device according to the present invention will be described. FIG. 8 is a configuration diagram of the optical amplifying device 10 according to the present embodiment. The optical amplifying device 10 shown in FIG. 1 includes an optical coupler 21, an EDF 22 as an optical amplifier, a gain equalizer 23, and a loss slope variable filter 100 on a signal light transmission path from an input terminal 11 to an output terminal 12. The pump light source 24 is provided in order, and further includes an excitation light source 24 connected to the optical coupler 21.
[0034]
The optical coupler 21 receives the signal light input to the input terminal 11 and the pump light output from the pump light source 24, multiplexes them, and outputs them to the EDF 22. The EDF 22 is an optical fiber to which an Er element is added, and can optically amplify the signal light by supplying the excitation light. The gain equalizer 23 equalizes the gain of the EDF 22 to make the gain gradient substantially linear. Further, the loss slope variable filter 100 according to the above-described present embodiment, and flattens the gain slope approximately linearized by the gain equalizer 23.
[0035]
In the optical amplifying device 10, the pump light output from the pump light source 24 is supplied to the EDF 22 via the optical coupler 21. The signal light input to the input terminal 11 is input to the EDF 22 via the optical coupler 21 and is optically amplified by the EDF 22. The optically amplified signal light is given a loss according to the wavelength by each of the gain equalizer 23 and the loss inclination variable filter 100, and is output from the output terminal 12.
[0036]
The overall gain spectrum of the optical amplifier 10 is a sum of the gain spectrum of the EDF 22, the loss spectrum of the gain equalizer 23, and the loss spectrum of the loss tilt variable filter 100. Under certain conditions, the loss spectrum of the gain equalizer 23 and the loss spectrum of the loss slope variable filter 100 are set so as to equalize the gain spectrum of the EDF 22, and the entire gain spectrum of the optical amplifying device 10 is set. Are flat in a predetermined wavelength band.
[0037]
However, when the input signal light intensity fluctuates, the gain spectrum obtained by integrating the gain spectrum of the EDF 22 and the loss spectrum of the gain equalizer 23 has a gain gradient while maintaining substantially linearity. At this time, by adjusting the coupling efficiency of the variable filter 110 included in the loss slope variable filter 100 (specifically, by adjusting the temperature of the graded index optical fiber 112 by the Peltier element 130), the loss slope variable filter is adjusted. By adjusting the loss slope of 100, the gain spectrum of the entire optical amplifier 10 can be flattened in a predetermined wavelength band.
[0038]
Next, an embodiment of the optical communication system according to the present invention will be described. FIG. 9 is a configuration diagram of the optical communication system 1 according to the present embodiment. The optical communication system 1 shown in this figure includes N stages (N is an integer of 2 or more) of optical amplifiers 30 ₁ to 30 _N and a variable loss inclination filter 100 provided on a signal light transmission path. Each optical amplifier 30 ₁ to 30 _N, those components except the loss slope variable filter 100 from the configuration shown in FIG. The variable loss inclination filter 100 in the optical communication system 1 is according to the above-described embodiment. In the optical communication system 1, signal light, after being optically amplified by each optical amplifier 30 ₁ to 30 _N, loss with wavelength is given by the loss slope variable filter 100.
[0039]
The gain spectrum of the whole optical communication system 1 can have comprehensively the loss spectrum of the optical amplifier 30 ₁ to 30 _N respectively of the gain spectrum with the loss slope variable filter 100 of N stages. Under certain conditions, each of the gain spectrum optical amplifier 30 ₁ to 30 _N of the N stage is flat in the predetermined wavelength band.
[0040]
However, when the input signal light intensity varies, the action of the gain equalization by the gain equalizer included in each optical amplifier 30 ₁ to 30 _N of N stages becomes incomplete, gain tilt occurs. At this time, by adjusting the coupling efficiency of the variable filter 110 included in the loss slope variable filter 100 (specifically, by adjusting the temperature of the graded index optical fiber 112 by the Peltier element 130), the loss slope variable filter is adjusted. By adjusting the loss slope of the optical communication system 100, the gain spectrum of the entire optical communication system 1 can be made flat in a predetermined wavelength band.
[0041]
The present invention is not limited to the above embodiment, and various modifications are possible. For example, the order of connection between the variable filter 110 and the fixed filter 120 included in the loss slope variable filters 100 and 100A is arbitrary. As a means for adjusting the coupling efficiency of the variable filter 110, a heater may be used instead of the Peltier element. In addition to the means for adjusting the temperature of the graded index optical fiber 112, the graded index optical fiber Means for adjusting the tension of 112 may be used. The optical communication system includes one or a plurality of optical amplifiers 10 (FIG. 8), and it is also preferable that the optical amplifiers optically amplify the signal light and transmit the signal light.
[0042]
【The invention's effect】
As described above in detail, according to the present invention, the entire loss spectrum of the loss slope variable filter is the sum of the loss spectra of the variable filter and the fixed filter, and the coupling efficiency of the variable filter is within the variable range. In any one of the above states, it is substantially flat in a predetermined wavelength band. Then, by adjusting the coupling efficiency of the variable filter, the loss slope of the entire loss spectrum of the loss slope variable filter changes while maintaining a state in which the loss is substantially linear with respect to the wavelength. Therefore, this loss slope variable filter can be suitably used as a gain equalizer whose loss is substantially linear with respect to wavelength and whose loss slope is variable.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of a loss slope variable filter 100 according to the present embodiment.
FIG. 2 is a configuration diagram of a loss slope variable filter 100A according to another embodiment.
FIG. 3 is a configuration diagram of a variable filter 110 included in the loss slope variable filters 100 and 100A according to the present embodiment.
FIG. 4 is a diagram showing an optical system equivalent to the variable filter 110.
FIG. 5 is a diagram showing a transmission spectrum of the variable filter 110.
FIG. 6 is a diagram showing a transmission spectrum of a fixed filter 120.
FIG. 7 is a diagram illustrating a transmission spectrum of the loss slope variable filter 100 according to the present embodiment.
FIG. 8 is a configuration diagram of an optical amplifying device 10 according to the present embodiment.
FIG. 9 is a configuration diagram of an optical communication system 1 according to the present embodiment.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Optical communication system, 10 ... Optical amplifier, 21 ... Optical coupler, 22 ... EDF, 23 ... Gain equalizer, 24 ... Excitation light source, 30 ... Optical amplifier, 100, 100A ... Loss inclination variable filter, 101 ... Input end, 102 output end, 110 variable filter, 111 single mode optical fiber, 112 graded index optical fiber, 113 single mode optical fiber, 120 fixed filter, 130 peltier element, 140 optical coupler, 150 monitor unit, 160 control unit.

Claims (8)

A filter having a variable loss slope for giving a loss to light within a predetermined wavelength band and outputting the light,
A variable filter whose coupling efficiency depends on the wavelength and the coupling efficiency is variable;
A fixed filter having a predetermined loss spectrum for light in the predetermined wavelength band,
Wherein the variable filter and the fixed filter have mutually opposite spectra in the predetermined wavelength band when the coupling efficiency of the variable filter is in any state within the variable range. filter. The variable loss filter according to claim 1, wherein the variable filter includes a first single mode optical fiber, a graded index optical fiber, and a second single mode optical fiber that are fusion-spliced in order. 3. The variable loss tilt filter according to claim 2, wherein the length of the graded index optical fiber is 25 mm or more. 3. The variable loss tilt filter according to claim 2, wherein the length of the graded index optical fiber is 60 mm or more. 3. The variable loss slope filter according to claim 2, wherein the numerical aperture of one or both of the first single mode optical fiber and the second single mode optical fiber is 0.2 or more. An optical amplifier for optically amplifying signal light within a predetermined wavelength band,
The loss inclination variable filter according to claim 1, which is connected to the optical amplifier, and applies a loss to the signal light.
A control unit that adjusts a transmission spectrum of the loss slope variable filter and controls the overall gain including the optical amplification unit and the loss slope variable filter to be constant within the predetermined wavelength band. Optical amplification device. An optical communication system including the optical amplifying device according to claim 6, wherein the optical amplifying device optically amplifies and transmits the signal light. A plurality of optical amplifiers provided on the signal light transmission path for optically amplifying the signal light within a predetermined wavelength band,
The loss inclination variable filter according to claim 1, wherein the loss inclination variable filter is connected to the plurality of optical amplifiers, and applies a loss to the signal light.
A control section that adjusts a transmission spectrum of the loss slope variable filter and controls the overall gain including the plurality of optical amplifiers and the loss slope variable filter to be constant within the predetermined wavelength band. An optical communication system, comprising:

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