JP2001318354A - Gain equalizer, optical amplifier using the same, and wdm optical transmission system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a gain equalizer which can dynamically compensate not only a linear slope, but also a ripple of high order as to wavelength characteristics of the power of WDM signal light with high precision and has high reliability. SOLUTION: This gain equalizer is equipped with variable optical filters 101 and 102 which are able to vary wavelength characteristics of transmissivity by using magnetooptic effect and have their wavelength characteristics of transmissivity set differently from each other and a control part 12 which detect the wavelength characteristics of the power of WDM signal lights transmitted through the various optical filters 101 and 102 and performs feedback control over the transmission wavelength characteristics of the variable optical filters 101 and 102 so that the light power deviation between the wavelengths is suppressed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a compensation technique for equalizing the wavelength characteristic of the optical power of a WDM signal light using a variable optical filter utilizing a magneto-optical effect, and more particularly, to a linear (linear) wavelength characteristic. A) a gain equalizer that can dynamically compensate not only for a gradient but also for higher-order ripples, and
The present invention relates to an optical amplifier and a WDM optical transmission system using the same.

[0002]

2. Description of the Related Art In recent years, as one method for realizing a large capacity of an optical transmission system, wavelength multiplexing (WD) in which two or more optical signals having different wavelengths are multiplexed and transmitted on one transmission line.
M) An optical transmission system is receiving attention. In a WDM optical amplifying repeater transmission system combining a WDM optical transmission system and an optical amplifying repeater transmission system, an optical amplifier (optical repeater) can be used to amplify a wavelength-multiplexed optical signal collectively. It is possible, and with a simple configuration (economical), large-capacity and long-distance transmission can be realized.

In the WDM optical amplifying repeater transmission system described above, the power of the WDM signal light has wavelength dependence due to the loss wavelength characteristic of the transmission line and the gain wavelength characteristic of the optical amplifier, and the transmission characteristic is degraded. It is known to end up.
In order to cope with such wavelength dependence of WDM signal light,
For example, a technique has been proposed in which a fixed gain equalizer for individually compensating the gain wavelength characteristics of an optical amplifier is provided, and a so-called pre-emphasis is performed on the optical transmitter side so that the optical signal power of each wavelength has a deviation. Have been. By applying such a technique, the deviation of the optical signal power of each wavelength on the optical receiver side becomes relatively small, and the optical SNR (signal-to-noise ratio) between the wavelengths becomes almost the same. Deterioration of characteristics can be suppressed.

By the way, in the technology for compensating for the wavelength dependency of the optical power as described above, since various setting conditions are optimized at the design stage, when the optical transmission service is started, the optimum condition is obtained. However, there is a case where sufficient compensation cannot be performed as time passes. For example, the gain wavelength characteristic of an optical amplifier changes due to a change in an operating point or temperature. For this reason,
Under the initially designed compensation conditions, deviations occur between the optical signal powers of the respective wavelengths. Therefore, the deviation of the optical SNR between each wavelength light after transmission cannot be remedied by the pre-emphasis on the optical transmitter side. There is a problem of breaking.

Here, the change in the gain wavelength characteristic due to the change in the operating point of the optical amplifier will be specifically described. In general, the section loss of a transmission line in an optical transmission system increases due to interruption or deterioration over time, so that the power of WDM signal light input to an optical amplifier tends to decrease. For this reason, in an optical amplifier operating under constant optical output control (ALC) such that the total output optical power to the transmission line is constant, the operating point is initially designed with a decrease in input optical power. It will be shifted from what you have.

For example, in a conventional signal light band of 1.55 μm (hereinafter referred to as C band), as shown in FIG.
When the gain of an erbium-doped optical fiber amplifier (EDFA) operating in ALC increases by about 1 dB with an increase in section loss, the gain tilt (gain tilt) with respect to wavelength changes by about -0.012 to -0.015 dB / nm. Become like In other words, when the input optical power of the EDFA is reduced by about 1 dB,
In the signal light band of 12 nm, the gain on the short wavelength side is relatively increased by about 0.2 dB as compared with the gain on the long wavelength side. Hereinafter, such a phenomenon is referred to as “input dependency of gain tilt”.

[0007] The above-mentioned influence of the input dependence of the gain tilt is caused by a submarine WDM that requires a product life of 25 years or more.
This is especially important for optical amplification repeater transmission systems,
This must be taken into account when designing. This is because there is a high possibility that the section loss will increase in 25 years due to the interruption or the aging deterioration.

Further, recently, with the increase in the transmission capacity of the WDM optical amplifying repeater transmission system, the expansion of the signal light band is required, and the signal light band of the 1.58 μm band (hereinafter, referred to as L band) is required. It is receiving attention as the next signal light band. For example, in the case of a known optical amplifier such as an EDFA having an amplification band in the L band, the input dependency of the gain slope is as follows.
Instead of changing in a state close to a straight line as in the case of the C band shown in FIG. 18 described above, for example, it changes in a quadratic curve as shown in FIG. In the case of long-distance optical transmission with a large number of relay sections such as a submarine WDM optical amplification relay transmission system, in order to make the deviation of the optical SNR accumulated for the optical signal of each wavelength after transmission uniform, It is necessary to provide a technique for compensating for the input dependency of the gain gradient changing like a quadratic curve in the middle of the transmission path.

As a conventional technique for dynamically compensating up to a quadratic curve change in the gain wavelength characteristic caused by a change in the operating point or temperature as described above, for example, John W. Arkwrigh
t etal., "Custom designed gain-flattening filters
with highly reproduciblespectral characteristics ",
OAA'99, pp.207-210, 1999, and SP Parry et al.,
"Dynamic gain equalization of EDFAs with Fourier
Filters ", OAA'99, pp.221-224, 1999.
There has been proposed a technique for performing compensation by combining Mach-Zehnder (MZ) type optical filters having different periods.

The basic concept in this prior art is to combine loss filters with desired gain wavelengths by combining optical filters having different periods with respect to wavelength (in other words, free spectral range (FSR)). This is a so-called Fourier expansion compensation technique that compensates up to higher-order ripples by matching the characteristics. As an optical filter used in such a conventional technique, a bulk-type or waveguide-type optical filter is used.

[0011]

However, in the conventional compensation technique as described above, when a bulk-type optical filter is used, the transmission (loss) wavelength characteristic is made variable by a mechanical operation. There was a problem that it was weak and the reliability was low. Further, when a waveguide type optical filter is used, insertion loss is large, cost is high,
It has drawbacks such as relatively high power consumption, and at present it is difficult to mount it as a gain equalizer on a transmission line.

Further, by combining a plurality of optical filters, not only linear compensation (hereinafter, referred to as first-order compensation) but also quadratic-curve compensation (hereinafter, referred to as second-order compensation) is dynamically performed. For this purpose, it is necessary to control the transmission (loss) wavelength characteristics of each optical filter with high accuracy in accordance with changes in gain wavelength characteristics caused by operating point fluctuations and the like. Such control increases the number of combined optical filters,
As higher-order compensation is required, the complexity becomes more complicated. Therefore, an effective and accurate control technique is required.

In addition, when a lower-noise and wider-band Raman amplifier is put to practical use in the future, a technique for compensating for the above-mentioned change in gain wavelength characteristic will be important. That is, the gain wavelength characteristic of a Raman amplifier used in place of the EDFA or in combination with the EDFA is considered to change due to a change in the operating point or the temperature. It is considered necessary to prevent deterioration.

The present invention has been made by paying attention to the above points, and can dynamically and highly accurately compensate not only a linear slope but also a high-order ripple in the wavelength characteristic of the optical power of WDM signal light. It is another object of the present invention to provide a gain equalizer having high reliability and an optical amplifier and a WDM optical transmission system using the same.

[0015]

In order to achieve the above object, a gain equalizer according to the present invention utilizes a magneto-optical effect in a gain equalizer for equalizing the wavelength characteristic of the optical power of a WDM signal light. A plurality of variable optical filters that are capable of changing the wavelength characteristics of transmittance, and are set so that the wavelength characteristics of each transmittance are different from each other; and a WDM that transmits the plurality of variable optical filters. Control means for detecting the wavelength characteristic of the optical power of the signal light and controlling the wavelength characteristic of the transmittance of each of the variable optical filters so that the optical power deviation between the wavelengths is suppressed based on the detection result; , Are provided.

In the gain equalizer having such a configuration, when a WDM signal light having a deviation in optical power between respective wavelengths is input,
The WDM signal light is sequentially sent to a plurality of variable optical filters. The wavelength characteristics of the transmittances of the variable optical filters are set to be different from each other, and the optical power deviation of the WDM signal light is compensated by transmitting the WDM signal light according to the respective transmission wavelength characteristics. Further, the wavelength characteristic of the optical power of the WDM signal light transmitted through the plurality of variable optical filters is detected by the control means, and the optical power deviation between the remaining wavelengths is obtained. Each variable optical filter can change the transmission wavelength characteristic using the magneto-optical effect, so that each transmission wavelength characteristic is feedback-controlled by the control means so that the optical power deviation between the remaining wavelengths is suppressed. You. As a result, dynamic compensation for the wavelength characteristic of the optical power generated in the WDM signal light can be performed with high accuracy. In addition, since each variable optical filter can variably control the transmission wavelength characteristic without mechanical operation, a highly reliable gain equalizer can be realized.

As a specific configuration of the plurality of variable optical filters used in the above-mentioned gain equalizer, first and second polarizers for cutting linearly polarized light from transmitted light, and the first and second polarizers are described.
A birefringent element provided between the first and second polarizers and providing a phase difference between the transmitted orthogonal two polarization components, and a Faraday variable provided to the transmitted polarized light between the first and second polarizers. And a Faraday rotator that provides a rotation angle.

In each tunable optical filter having such a configuration, the wavelength characteristic of the transmittance is obtained by appropriately setting the thickness of the birefringent element in the light transmission direction and the refractive index difference between the ordinary ray and the extraordinary ray. It is possible to arbitrarily set the shape of the curve in the direction of the wavelength axis, and by appropriately setting the angle and the Faraday rotation angle between the transmission axis of each polarizer and the optical axis of the birefringent element, It is possible to arbitrarily change the shape of the characteristic curve giving the wavelength characteristic of the transmittance in the direction of the axis of the transmittance.

In the above gain equalizer, when n variable optical filters (n is an integer of 2 or more) are provided, the transmittance of the k-th (k is an integer from 1 to n) variable optical filter is used. Is preferably set in advance such that (k-1) extreme points exist within the band of the WDM signal light. With this setting, the first variable optical filter has a linear (primary) transmission wavelength characteristic, and the second variable optical filter has
The transmission wavelength characteristic has a second-order curve, and the n-th variable optical filter has a transmission wavelength characteristic having an n-th curve. This allows
Compensation from the first order to the nth order is performed by the n variable optical filters.

Further, as a specific configuration relating to the control means of the gain equalizer, the WDM signal light transmitted through the plurality of variable optical filters may have at least two preset WDM signal lights corresponding to the respective variable optical filters. An optical filter unit for extracting an optical signal in a monitor wavelength band, a power detection unit for detecting an optical signal power in each monitor wavelength band extracted by the optical filter unit, and a detection result of the power detection unit. An arithmetic unit that calculates a light power deviation corresponding to the variable optical filter, and generates a control signal for controlling the wavelength characteristic of the transmittance of the corresponding variable optical filter so that the optical power deviation is suppressed. The wavelength characteristic of each variable optical filter may be feedback-controlled in accordance with the control signal generated by the arithmetic unit.

In the control means having such a configuration, the optical signal power corresponding to the monitor wavelength band corresponding to each variable optical filter is detected, and the corresponding optical power is determined in accordance with the optical power deviation obtained based on the detection result. Feedback control of the transmission wavelength characteristic of the variable optical filter is performed.

In addition, as a specific setting of the above-mentioned optical filter unit, each center wavelength of at least two monitor wavelength bands set in advance corresponding to the odd-numbered variable optical filters is set to be equal to the bandwidth of the WDM signal light. Among the two wavelengths at which the variable amount of transmittance of the variable optical filter is maximum, each center wavelength of at least two monitor wavelength bands set in advance corresponding to the even-numbered variable optical filters,
It is preferable to include a wavelength at which the variable amount of the transmittance of the variable optical filter is maximum and a wavelength at which the variable amount of the transmittance of the variable optical filter is within the band of the WDM signal light. As a result, the optical power deviation corresponding to each variable optical filter can be obtained with high accuracy, and effective feedback control can be realized.

[0023] Further, with respect to the calculation section of the control means,
It is preferable that each time constant of the feedback control corresponding to each variable optical filter is set to be different from each other. With such a setting, oscillation of each feedback control can be prevented.

The above-mentioned gain equalizer may be provided with an optical amplifier for compensating for the insertion loss of the plurality of variable optical filters. Thus, a gain equalizer without insertion loss is realized.

The gain equalizer described above may be provided with output light power control means for controlling the total power of the WDM signal light transmitted through the plurality of variable optical filters to be constant. Thereby, the WDM signal light whose total power is controlled to be constant is output from the gain equalizer.

Further, regarding the above-mentioned gain equalizer, W
When the DM signal light includes signal lights of a plurality of wavelength bands,
A demultiplexing means for demultiplexing the WDM signal light into signal light of each wavelength band, and a plurality of variable optical filters and control means corresponding to the signal light of each wavelength band demultiplexed by the demultiplexing means. May be provided, and multiplexing means for multiplexing the WDM signal light of each wavelength band transmitted through the plurality of variable optical filters may be provided. This makes it possible to apply the gain equalizer according to the present invention even when, for example, the C-band and L-band signal lights are transmitted collectively.

An optical amplifier according to the present invention comprises: a gain equalizer as described above; an optical amplifying means for amplifying WDM signal light;
And a change in gain wavelength characteristic of the optical amplifying means is compensated for by a gain equalizer. In the optical amplifier having such a configuration, a change in the gain wavelength characteristic caused by a change in the operating point, temperature, or the like is automatically compensated.

The WDM optical transmission system according to the present invention
An optical transmitter that generates a DM signal light and transmits it to a transmission line; an optical receiver that receives the WDM signal light transmitted through the transmission line; and an optical receiver that is arranged at a predetermined relay interval in the middle of the transmission line. In a WDM optical transmission system including a plurality of optical amplifiers, the above-described gain equalizer according to the present invention includes:
The gain equalizer is arranged for each predetermined compensation section including a plurality of relay sections, and changes in gain wavelength characteristics of a plurality of optical amplifiers in the compensation section are compensated by the gain equalizer.

In the WDM optical transmission system having such a configuration,
Since the change in the gain wavelength characteristic of the optical amplifier in the compensation section is dynamically compensated for each compensation section, signal light having a small optical power deviation between wavelengths can be received by the optical receiver. As a result, a wideband WDM signal light can be stably transmitted over a long distance, so that the transmission capacity and transmission distance of the WDM optical transmission system can be increased.

In the WDM optical transmission system, a plurality of variable optical filters of the gain equalizer may be collectively arranged at the end of any one of the relay sections in the compensation section. They may be distributed and arranged at the respective ends of different relay sections in the section.

[0031]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of the first embodiment of the gain equalizer according to the present invention.

[0032] In FIG. 1, the gain equalizer 1 includes an input terminal IN and the output terminal 2 of the variable optical filter 10 ₁ connected in series between OUT, 10 _2, the variable optical filter 10 ₂ of the rear stage Optical coupler 11 for branching a part of output light of
And a control unit 12 as control means for controlling the transmission (loss) wavelength characteristics of the variable optical filters 10 ₁ and 10 ₂ based on the branched light from the optical coupler 11.

[0033] Each variable optical filter 10 _1, 10 ₂ is a known optical filter utilizing magneto-optical effect, the transmission wavelength characteristics of each, as will be described later is set to be different from each other. As a variable optical filter utilizing the magneto-optical effect, for example, Japanese Patent Application Laid-Open No.
It is preferable to use the technology described in Japanese Patent Application Laid-Open No. 1-212044 and Japanese Patent Application Laid-Open No. 2000-66137.

FIG. 2 is a diagram showing a preferred configuration example of the variable optical filter. This configuration example is common to the variable optical filters 10 ₁ and 10 ₂ . The configuration of the variable optical filter used in the present invention is not limited to this, and various configurations described in the above-mentioned publications and the like can be applied.

In the configuration example of FIG. 2, the input side lens 10A,
Input side wedge plate 10B, birefringent plate (birefringent element) 10
C, variable Faraday rotator 10D, output side wedge plate 10
E and the output side lens 10F are arranged in this order on the optical path OP of the transmitted light.

The input side lens 10A is a lens for collimating the light emitted from the input side optical fiber or the like and supplying the collimated light to the input side wedge plate 10B. The output-side lens 10F is a lens for condensing a beam from the output-side wedge plate 10E and coupling it to an output-side optical fiber or the like.

The input-side wedge plate 10B is made of a birefringent material, has an optical axis Pi, and functions as a first polarizer. The output-side wedge plate 10E is also made of a birefringent material, has an optical axis Po, and functions as a second polarizer.
Here, the top and bottom of the input-side wedge plate 10B face the bottom and top of the output-side wedge plate 10E, respectively, and the corresponding surfaces are arranged so as to be parallel to each other. In addition, each wedge plate 10 as a polarizer
The transmission axis of B and 10E (determining the polarization axis of the transmitted polarized light) is defined as the polarization direction of an extraordinary ray whose polarization plane is parallel to the optical axis or the polarization direction of an ordinary ray whose polarization plane is perpendicular to the optical axis. You.

The birefringent plate 10C is for giving a phase difference between the two orthogonally polarized light components to be transmitted, and has an optical axis C1 for determining the phase difference. Here, the birefringent plate 10
The thickness of C in the direction of the optical path OP is d, and the difference in refractive index between the ordinary ray and the extraordinary ray in the birefringent plate 10C is μ. Further, the optical axis C1 of the birefringent plate 10C and the input-side wedge plate 10
B is the angle formed by the optical axis Pi of B, and the birefringent plate 10C
Is the angle formed between the optical axis C1 of FIG. 1 and the optical axis Po of the output side wedge plate 10E.

The variable Faraday rotator 10D imparts variable Faraday rotation to transmitted polarized light. Generally, Faraday rotation is a magnetic field (magnetic field) in a magneto-optical crystal.
For example, when linearly polarized light passes through the magneto-optical crystal while the magneto-optical crystal is in a certain magnetic field, the polarization direction always rotates in a fixed direction regardless of the propagation direction of the linearly polarized light. It is a phenomenon that does. The magnitude of the rotation angle of the polarization direction (Faraday rotation angle θ _F ) depends on the direction and intensity of the magnetization of the magneto-optical crystal generated by the applied magnetic field. The specific configuration and operation of such a variable Faraday rotator 10D are described in the above-mentioned Japanese Patent Application Laid-Open No. 11-21204.
Since this is described in detail in Japanese Patent Publication No. 4 and the like, only the outline thereof will be briefly described here.

FIG. 3 is a perspective view showing an example of a specific configuration of the variable Faraday rotator 10D. The configuration of the variable Faraday rotator used in the present invention is not limited to this.

In the configuration example of FIG. 3, the variable Faraday rotator 10D includes a magneto-optical crystal 41 and a magneto-optical crystal 41.
A permanent magnet 42 and an electromagnet 43 for applying a magnetic field in a direction orthogonal to each other, and a variable current source 44 for supplying a drive current to the electromagnet 43. In such a configuration, by adjusting the drive current applied to the electromagnet 43, the permanent magnet 4
2 and the electromagnet 43 change the direction and strength of the synthetic magnetic field applied to the magneto-optical crystal 41, and the Faraday rotation angle θ _F is variably controlled.

The control unit 12 (see FIG. 1) includes, for example, optical couplers 20A and 20B, optical filters 21A, 21B and 21C as optical filter units, and light receiving elements (PD) 22A and 22B as power detection units. It has a 22C, an arithmetic circuit 23 _1, 23 ₂ as an arithmetic unit. Optical coupler 20A
Splits the light split by the optical coupler 11 into two, sends one split light to the optical filter 21A, and sends the other split light to the optical coupler 20B. The optical coupler 20B is an optical coupler 20A.
Is further split into two, and one of the split lights is sent to the optical filter 21B, and the other split light is sent to the optical filter 21C.
Send to Each of the optical filters 21A to 21C is an optical filter that passes only light in a required band (monitor wavelength band). Here, the center wavelengths of the pass bands of the optical filters 21A to 21C are λ _CA , λ _CB and λ _CC . In addition,
The center wavelengths λ _CA , λ _CB , λ of the optical filters 21A to 21C.
The setting of the _CC will be described later. Each light receiving element 22A, 22
B and 22C convert the light passing through the corresponding optical filters 21A to 21C into electric signals and output the electric signals. Arithmetic circuit 2
3 ₁ is inputted an output signal from the light receiving element 22A at one input terminal, to the other input terminal is input the output signal from the light receiving element 22C, a control signal corresponding to the level difference between the output signal The control signal is generated and the control signal is
It is fed back to the 0 _1. Arithmetic circuit 23 ₂ is supplied with the output signal from the light receiving element 22A at one input terminal, to the other input terminal is input the output signal from the light receiving element 22B, control corresponding to the level difference between the output signal generates a signal, feeds back the control signal to the variable optical filter 10 _2. Note that constant T _F2 when the feedback loop including the constant T _F1 and arithmetic circuit 23 ₂ when the feedback loop including the operational circuit 23 _1, have been set in accordance with the capacitance of the capacitor provided in each arithmetic circuit Here, the time constant T _F1 is set to be shorter than the time constant T _F2 (T _F1 <T _F2 ).

Next, the specific setting of each of the variable optical filters 10 ₁ and 10 ₂ will be described. First, consider the basic characteristics of the variable optical filter as shown in FIG.
Here, in order to make the explanation easy to understand, a quantitative analysis in the case where the variable Faraday rotator 10D is omitted is performed on the configuration of FIG. The characteristics will be described.

When linearly polarized light sin (ωt) is incident parallel to the optical axis Pi of the input side wedge plate 10B, the birefringent plate 10
The component E1 parallel to the optical axis C1 and the component E2 perpendicular to the optical axis C1 of the light passing through C are expressed by the following equations (1), respectively, assuming that the phase delays of both components are ε1 and ε2, respectively. Can be.

E1 = sinφsin (ωt + ε1) E2 = cosφsin (ωt + ε2) (1) After these two components E1 and E2 are combined at the output of the birefringent plate 10C, they pass through the output side wedge plate 10E. The optical axis P of the light passing through the output side wedge plate 10E
The amplitude (E1 sin θ + E2 cos θ) of the component parallel to o can be expressed by the following equation (2) using the above equation (1).

E1 sinθ + E2cosθ = sinφsinφsin (ωt + ε1) + cosφcosθsin (ωt + ε2) = (sinφsinθcosε1 + cosφcosθcosε2) sin ωt + (sinφsin = sinθsin cosθ2) I = cos ² (φ + θ) + sin (2φ) sin (2θ) cos ² ((ε1−ε2) / 2) (3) where the second term of the above equation (3) (Ε1−ε2) at
/ 2 is the thickness d of the birefringent plate 10C, where λ is the wavelength of light.
When the refractive index difference μ between the ordinary ray and the extraordinary ray in the birefringent plate 10C is used, the following equation (4) holds.

(Ε1−ε2) / 2 = πμd / λ (4) Therefore, from the above equations (3) and (4), the intensity I of the transmitted light is represented by the following function I (λ) of the wavelength λ. It can be expressed by equation (5).

I (λ) = cos ² (φ + θ) + sin (2φ) sin (2θ) cos ² (μd / λ) (5) According to the above equation (5), the intensity (transmittance) of the transmitted light is represented by the wavelength It can be seen that it has a dependency and changes periodically with the wavelength. Here, if the value of the wavelength is larger than the width of the wavelength optical band actually used, 1 / λ becomes a linear function 1 / λ =
It can be approximated by aλ + b (a and b are constants). Further, when only the relative wavelength is considered ignoring b, the intensity I (λ) of the transmitted light can be expressed as the following equation (6).

I (I) = cos ² (φ + θ) + sin (2φ) sin (2θ) cos ² (πλ / FSR) (6) where FSR represents a wavelength period in a wavelength characteristic of transmittance. It is given by the following equation (7).

FSR = 1 / aμd (7) Accordingly, in order to obtain a required FSR, for example, if the refractive index difference μ determined by the material of the birefringent plate 10C is constant, the birefringent plate 10C It can be understood that the thickness d of the substrate may be adjusted.

From equation (5), it can be seen that changing at least one of the angle φ and the angle θ also changes the transmitted light intensity. The angle θ is 10E of the output side wedge plate.
Is the angle between the optical axis Po of the birefringent plate 10C and the optical axis C1 of the birefringent plate 10C, and the angle between the polarization axis of the light incident on the output side wedge plate 10E and the optical axis Po of the output side wedge plate 10E. You can also. Therefore, if the azimuth of the polarized light incident on the output side wedge plate 10E is rotated by the variable Faraday rotator 10D, the same state as the change in the angle θ can be realized, and the transmitted light intensity is changed according to the rotation. Can be changed.

Here, the variable Faraday rotator 10D is disposed between the birefringent plate 10C and the output side wedge plate 10E to change the angle θ. The birefringent plate 1C is disposed between the input side wedge plate 10B and the birefringent plate 10C.
By rotating the azimuth of the polarized light incident on 0C, the same state as the change of the angle φ can be realized, and the transmitted light intensity can be changed according to the rotation.

As described above, the variable optical filter having the configuration shown in FIG. 2 gives the wavelength characteristic of the transmittance by appropriately setting the thickness d and the refractive index difference μ of the birefringent plate 10C. It is possible to arbitrarily set the shape of the characteristic curve in the direction of the wavelength axis. Specifically, FIG.
As shown in FIG. 4, the peak wavelength at which the transmittance becomes minimum can be changed in the direction of the wavelength axis, or the FSR can be changed as shown in FIG. 4B. FIG.
In the example of (B), the refractive index difference μ
, The transmittance also changes in the axial direction. However, this can be made constant by appropriately changing the thickness d of the birefringent plate 10C.

Further, by appropriately setting the arrangement of the optical axes Pi and Po of the wedge plates 10B and 10E with respect to the optical axis C1 of the birefringent plate 10C and the drive current of the variable Faraday rotator 10D, the transmittance can be reduced. The shape of the characteristic curve giving the wavelength characteristic can be arbitrarily changed in the direction of the axis of the transmittance.

Next, the individual setting of the transmission wavelength characteristic of each of the variable optical filters 10 ₁ and 10 ₂ will be specifically described.
Figure 5 is a diagram showing a setting example of a transmission wavelength characteristic of the variable optical filter 10 _1. Each curve in the figure represents each transmittance when the Faraday rotation angle θ _F is changed in a range of 10 ° to 80 °.

As shown in FIG. 5, the variable optical filter 10
The transmission wavelength characteristic of ₁ has a required signal light band Δλ (for example,
The thickness d of the birefringent plate 10C, the refractive index difference μ, and the like are set so that the transmittance changes linearly in the C band and the L band. The configuration example shown in FIG. 5, with respect to a change in the Faraday rotation angle theta _F, the change in transmittance at both ends of the signal light band Δλ becomes the maximum, respectively, the minimum change in the transmittance at the central portion of the signal light band Δλ Becomes More specifically, when the Faraday rotation angle is small, the transmittance decreases monotonically, when the Faraday rotation angle theta _F is large, the transmittance has a wavelength characteristic as to increase monotonically. As an example of design values for realizing such transmission wavelength characteristics, for the L band, the thickness d of the birefringent plate 10C is 0.001666 m, the refractive index difference μ is 0.015, and the angle φ is 45 °. And can be. In the variable optical filter 10 ₁ having such wavelength characteristics, by the Faraday rotation angle theta _F is feedback controlled in accordance with a control signal sent from the arithmetic circuit 23 ₁ of the controller 12, linearly varying transmission wavelength It is automatically adjusted so that the characteristic slope becomes a value suitable for the first-order compensation.

[0057] Figure 6 is a diagram showing a setting example of a transmission wavelength characteristic of the variable optical filter 10 _2. Note that each curve in the figure has a Faraday rotation angle θ _F of 10 as in the case of FIG.
The respective transmittances when changed in the range of ° to 80 ° are shown.

As shown in FIG. 6, the variable optical filter 10
The transmission wavelength characteristic of No. ₂ has a required signal light band Δλ (for example,
The thickness d of the birefringent plate 10C and the refractive index difference μ are changed so that the transmittance changes in a quadratic curve in the C band and the L band.
Etc. are set. Here, the quadratic change in the transmittance is a change such that there is one extremum point in the required signal light band Δλ. Note that the linear (primary) change in the transmittance described above corresponds to a case where no extreme point exists. In the setting example shown in FIG. 6, the Faraday rotation angle θ _F
, The change in transmittance becomes the maximum (maximum value or minimum value) at the center of the signal light band Δλ, and the signal light band Δ
The change in transmittance at both ends of λ is minimum. Specifically, when the Faraday rotation angle is small, the transmittance changes like a quadratic curve convex upward, and when the Faraday rotation angle θ _F is large, the transmittance changes like a quadratic curve convex downward. Wavelength characteristic. As an example of design values for realizing such transmission wavelength characteristics, for the L band, the thickness d of the birefringent plate 10C is 0.001664 m, the refractive index difference μ is 0.015, and the angle φ is 45 °. And can be. In the variable optical filter 10 ₂ having such wavelength characteristics, by the Faraday rotation angle theta _F is feedback controlled in accordance with a control signal sent from the arithmetic circuit 23 _{and second} controller 12, an extreme point of the transmission wavelength characteristics Is automatically adjusted so that the depth becomes a value suitable for the secondary compensation.

Here, the first order with respect to the change of the gain wavelength characteristic
The compensation and the second-order compensation are specifically described with reference to FIGS.
Will be explained. For example, as shown in FIG.
The change in the gain wavelength characteristic decreases downward as shown by the solid line I.
When a convex curve is drawn (for example, ED
Changes in gain wavelength characteristics of FA correspond to this)
Is a variable optical filter 10 that performs first-order compensation₁Transmission wavelength
The characteristic is monotonically increasing as indicated by the broken line I '(only
And the transmittance is set in the vertical axis direction).
As a result, the change in the gain wavelength characteristic after the primary compensation is represented by the solid line I
A substantially downward quadratic curve as shown by I is drawn.
Further, it is possible to perform second-order compensation corresponding to the solid line II.
Variable optical filter 10 _TwoIs represented by a broken line II '.
A quadratic curve that is convex upward as shown (however, the vertical axis
Rate). Specifically, the solid line I
Wavelength λ at which I is minimum_IIVariable optical filter
TA10_TwoTransmission wavelength characteristics so that the maximum
Set. As a result, the change of the gain wavelength characteristic after the secondary compensation
Ideally has a flat characteristic as shown by the solid line III.
You.

For example, as shown in FIG. 8, when the change in the gain wavelength characteristic of the optical amplifier draws a curve that rises to the right and rises upward as shown by a solid line I (for example, ED due to temperature fluctuation).
The such as a change in the gain wavelength characteristic equivalent) of FA, the transmission wavelength characteristic of the variable optical filter 10 ₁ to perform the primary compensation is set to be a monotonically decreasing characteristic as shown by a broken line I ' As a result, the change of the gain wavelength characteristic after the first-order compensation draws a substantially quadratic curve convex upward as shown by a solid line II. Further, in correspondence with the solid line II, the transmission wavelength characteristic of the variable optical filter 10 ₂ for the second compensation, by setting so that quadratic curve is convex downward as shown by the broken line II ', 2 The change of the gain wavelength characteristic after the next compensation ideally becomes a flat characteristic as shown by a solid line III.

For example, as shown in FIG. 9, when the change in the gain wavelength characteristic of the optical amplifier draws a downwardly convex curve as shown by the solid line I (for example, the gain of the Raman amplifier due to the variation of the operating point). the change in the wavelength characteristic like equivalent), by the transmission wavelength characteristic of the variable optical filter 10 ₁ to perform the primary compensation is set to be a monotonically increasing characteristic as shown by the broken line I ', The change in the gain wavelength characteristic after the first-order compensation draws a substantially quadratic curve convex downward as shown by a solid line II. Further, corresponding to the solid line II,
The transmission wavelength characteristic of the variable optical filter 10 ₂ for the second compensation, by setting such that the quadratic curve upwardly convex as indicated by the broken line II ', the change in the gain wavelength characteristic after the secondary compensation The characteristics are substantially flat as shown by the solid line III.

In the design stage of the present gain equalizer,
And the gain wave of the optical amplifier connected via a transmission line
Each variable optical filter responds to the changing tendency of the long characteristic.
10 ₁, 10_TwoSet the design parameters of
In the use stage, each variable is adjusted according to a control signal from the control unit 12.
Optical filter 10₁, 10_TwoFaraday rotation angle
Gain control due to operating point fluctuation
Primary compensation and secondary compensation that correspond to changes in characteristics
Will be applied.

Here, the control operation of the control unit 12 will be described in detail. First, the signal light monitored by the control unit 12 will be specifically described. In the present embodiment, the control unit 1
Of three light filters 21A~21C provided in 2, the optical filter 21A, with the light extracted by the 21C, the feedback control of the variable optical filter 10 ₁ for primary compensation is performed, the optical filter 21A , using the light extracted in 21B, the variable optical filter 10 ₂ for the secondary compensation
Is performed.

In the first-order compensation in the variable optical filter 10 ₁ , the slope of the linear change is accurately monitored for the change in the gain wavelength characteristic of the optical amplifier, and the Faraday rotation angle θ _F
It is necessary to feedback control of the slope of the transmission wavelength characteristic of the variable optical filter 10 ₁ which changes according to the. For this purpose, for example, the signal light band Δ
The optical power at both ends of λ is monitored, and the variable optical filter 10 is adjusted so that the difference between the optical powers is compensated.
_It is effective to adjust the Faraday rotation angle θ _F of ₁ .
In the present embodiment, as shown in FIG. 5, the center wavelength λ _CA of the pass band of the optical filter 21A is equal to the signal light band Δλ.
, And the center wavelength λ _CC of the pass band of the optical filter 21C is set at the long wavelength end of the signal light band Δλ.

Further, ₂ in the variable optical filter 10 2
In the next compensation, the change in the gain wavelength characteristic of the optical amplifier
Depth at extreme points of the quadratic curve changes (the magnitude of the transmittance) accurately monitors the feedback depth of the extreme point of the variable optical filter 10 ₂ that varies depending on the Faraday rotation angle theta _F You need to control. To this end, the compensated signal light has the wavelength λ _II shown in FIGS.
An optical power in, and an optical power monitor each at one end of the signal light band [Delta] [lambda], may be adjusted variable optical filter 10 ₂ of the Faraday rotation angle theta _F so that the difference of each of the optical power is compensated. In the present embodiment, as shown in FIG. 6 described above, the center wavelength λ _CB of the pass band of the optical filter 21B is changed to the wavelength (wavelength λ _{II) at} which the amount of change in transmittance with respect to the change in the Faraday rotation angle θ _F becomes maximum. Is set). Here, the optical filter 21A is shared for monitoring the optical power at one end of the signal light band Δλ. However, this may be provided separately.

Each optical filter 21 set as described above
The monitor lights extracted by A to 21C are respectively
The light receiving elements 22A to 22C correspond to each optical power.
Is converted into an electric signal,₁Or arithmetic circuit
23_TwoSent to Arithmetic circuit 23₁Then, the light receiving element 22
A, using each output signal from 22C, the signal light band Δλ
The deviation of the optical power at both ends of is obtained. This deviation
Is a variable optical filter 10 ₁To the first-order compensation error at
Variable optical filter so that the error is reduced.
10₁Control signal to adjust the slope of the transmission wavelength characteristic of
Constant T_F1Arithmetic circuit 23 according to₁Variable optical filter from
10₁Sent to The arithmetic circuit 23_TwoThen, the light receiving element
A quadratic curve using each output signal from 22A and 22B
The depth at the extreme point of the change is determined. This extreme point
Is the variable optical filter 10_TwoSecond complement in
Variable, so that the error is reduced.
Optical filter 10_TwoControl signal to adjust the depth of the extreme point of
Is the time constant T_F2Arithmetic circuit 23 according to_TwoVariable optics
Filter 10_TwoSent to

Here, by setting the time constant T _F1 of the feedback loop for the primary compensation to be shorter than the time constant T _F2 of the feedback loop for the secondary compensation, each feedback control oscillates. Is preventing. That is, when the feedback control of each order is performed with the same time constant, when the control direction is inconsistent with each order, each feedback control may not converge. In order to avoid such a situation, for example, the time constant T of each order
_It suffices if _F1 and _TF2 are made different, and by setting the time constant T _F1 of the lower order to be shorter than the time constant T _{F2 of the} higher order, the feedback control is more easily converged.

[0068] As described above, according to the first embodiment of the gain equalizer according to the present invention, the primary variable optical filter 10 ₁ capable of compensating the secondary compensation possible variable optical filter 1
0 ₂ and the Faraday rotation angles of the variable optical filters 10 ₁ and 10 ₂ are adjusted based on the monitoring result of the signal light after the compensation to feedback-control each transmission wavelength characteristic. Dynamic compensation for changes in gain wavelength characteristics caused by operating point fluctuations and temperature fluctuations can be performed with high accuracy. In addition, the variable optical filters 10 ₁ and 10 ₂ used in the present gain equalizer can variably control the transmission wavelength characteristics without mechanical operation, and thus have an advantage that they have high reliability against vibrations and the like. is there. Further, by setting the time constant T _F1 of the primary side feedback loop to be shorter than the time constant T _{F2 of the} secondary side, the feedback control is easily converged, so that the primary compensation and the secondary compensation become more stable. It is possible to do.

Next, a description will be given of a second embodiment of the gain equalizer according to the present invention. FIG. 10 is a block diagram illustrating a configuration of a gain equalizer according to the second embodiment. 10, that the configuration of the gain equalizer 1 is different from the configuration of the first embodiment, the feedback provided with a variable optical filter 10 ₃ for 3-order compensation, the control unit 12, a variable optical filter 10 ₃ The point is that a configuration for controlling is added. Specifically, an optical coupler 20C, the optical filter 21D, 21E, the light receiving element (PD) 22D, the 22E and the operational circuit 23 ₃ are attached to the control unit 12.

The variable optical filter 10 ₃ is a known optical filter utilizing the magneto-optical effect, and has a transmission wavelength characteristic of other variable optical filters 10 ₁ and 10 ₂ as described later.
Is set differently. The specific configuration of the variable optical filter 10 ₃ is the variable optical filters 10 ₁ , 10
₂ (see FIGS. 2 and 3), and a description thereof will be omitted.

The optical coupler 20C of the control unit 12 further splits the light split by the optical coupler 20A into two, sends one split light to the optical filter 21D, and sends the other split light to the optical filter 21E. Here, it is assumed that the branched light from the optical filter 11 is branched into three by the optical filter 20A. Each of the optical filters 21D and 21E is an optical filter that allows only light in a required band to pass therethrough. Here, the center wavelengths of the respective pass bands are λ _CD and λ _CE . Note that the center wavelengths λ _CD and λ _{CE of the} optical filters 21D and 21E, respectively _.
The setting will be described later. Each light receiving element 22D, 22E
Converts the light passing through the corresponding optical filters 21D and 21E into an electric signal and outputs the electric signal. Arithmetic circuit 23 ₃ is input the output signal from the light receiving element 22D to one input terminal, to the other input terminal is input the output signal from the light receiving element 22E, control corresponding to the level difference between the output signal generates a signal, feeds back the control signal to the variable optical filter 10 _3. Constant T _F3 when the feedback loop including the operational circuit 23 _3, have been set in accordance with the capacitance of the capacitor provided in the arithmetic circuit, wherein the time constant T _F3 is longer than the time constant T _F2 (T _F1 <T _F2
<T _F3 ) is set.

Next, specifically described setting of the transmission wavelength characteristics of the variable optical filter 10 _3. Figure 11 is a diagram showing a setting example of a transmission wavelength characteristic of the variable optical filter 10 _3. Each curve in the figure represents each transmittance when the Faraday rotation angle θ _F is changed in a range of 10 ° to 80 °.

As shown in FIG. 11, the variable optical filter 1
The transmission wavelength characteristic of O ₃ is such that the birefringent plate 10C has a transmittance that changes in a cubic curve in the required signal light band Δλ.
, The refractive index difference μ, etc. are set. The cubic change of the transmittance referred to here is a required signal light band Δλ.
Is a change such that two extreme points exist. FIG.
In the setting example shown in FIG. 1, the change in the transmittance near the both ends of the signal light band Δλ (exactly, the wavelengths λ _CD and λ _CE in the figure) becomes maximum with respect to the change in the Faraday rotation angle θ _F. , The change in transmittance at the center of the signal light band Δλ is minimized. Specifically, when the Faraday rotation angle is small, the transmittance is minimum at the wavelength λ _CD and maximum at the wavelength λ _CE ,
When the Faraday rotation angle θ _F is large, the transmittance becomes maximum at the wavelength λ _CD and becomes minimum at the wavelength λ _CE . As an example of design values for realizing such transmission wavelength characteristics, for the L band, the thickness d of the birefringent plate 10C is 0.001662 m,
Refractive index difference μ = 0.0279, angle φ = 45 °, and the like. In the variable optical filter 10 ₃ having such wavelength characteristics, by the Faraday rotation angle theta _F is feedback controlled in accordance with a control signal sent from the arithmetic circuit 23 ₃ of the control section 12, the transmission wavelength characteristic cubic compensation Is automatically adjusted to a value suitable for.

The third-order compensation for the change in the gain wavelength characteristic is as follows:
This is performed in order to compensate for higher-order ripples and the like that remain only with the primary compensation and the secondary compensation as described with reference to FIGS. Such higher-order compensation may be appropriately performed depending on how much change in gain wavelength characteristics of the optical amplifier due to operating point fluctuation or the like is allowed in the system.

In the third order compensation in the variable optical filter 10 ₃ , the change in the gain wavelength characteristic of the optical amplifier is accurately monitored in the form of a third-order curve, and the variable optical filter 10 3 changes according to the Faraday rotation angle θ _{F. It} is necessary to perform feedback control on the transmission wavelength characteristics of ( ₃ ). To do this,
For example, the wavelength lambda _CD near both ends of the signal light band Δλ signal light after compensation, respectively monitors the optical power at lambda _CE, Faraday rotation of the variable optical filter 10 ₃ so that the difference of each of the optical power is compensated it is preferable to adjust the angle θ _F. In the present embodiment, the center wavelength λ _CD of the pass band of the optical filter 21D is set to be a wavelength corresponding to the extreme point on the short wavelength side, and the center wavelength λ _CE of the pass band of the optical filter 21E is , The wavelength corresponding to the extreme point on the long wavelength side.

Each optical filter 21 set as described above
D, the monitor light extracted respectively by 21E, each light-receiving element 22D, is converted into an electrical signal corresponding to each of the optical power by 22E, it is sent to the arithmetic circuit 23 _3. The arithmetic circuit 23 _3, the light-receiving element 22D, by using each output signal from 22E, the deviation of the optical power at each extreme point of the signal light band Δλ ends is obtained. This deviation corresponds to an error of the third-order compensation in the variable optical filter 10 _3,
Control signal said error to adjust the transmission wavelength characteristic of the variable optical filter 10 ₃ so as to decrease is sent from the arithmetic circuit 23 ₃ in accordance with a time constant T _F3 to the variable optical filter 10 _3.

Here, the time constant T for the third order compensation is
_By setting _F3 to be longer than the time constant T _F2 for the second-order compensation, the feedback control of each order is prevented from oscillating, as in the first embodiment.

[0078] As described above, according to the second embodiment of the gain equalizer according to the present invention, with the addition of variable optical filter 10 ₃ possible cubic compensation, based on the monitoring result of the signal light after compensation the function of feedback control of the adjustment to the transmission wavelength characteristic of Faraday rotation angle of the variable optical filter 10 ₃ by the addition to the control unit 12 Te, to become as compensation from the primary to tertiary is achieved, the gain Dynamic compensation for changes in wavelength characteristics can be performed with higher accuracy.

In each of the above-described embodiments, the configuration for realizing the second-order or third-order compensation has been described. However, the present invention is not limited to this, and it is also possible to realize fourth-order or higher-order compensation. is there. When this is generalized and written, the gain equalizer of the present invention that realizes the n-order compensation has a basic configuration as shown in the block diagram of FIG. That is, the input terminal IN
N variable optical filters 10 ₁ to 10 _n corresponding to the first-order compensation to the n-th compensation, respectively, between the output terminals OUT and OUT.
Are connected in order, and a control unit 12 that performs feedback control of the transmission wavelength characteristic of each of the variable optical filters 10 ₁ to 10 _n using the monitor light branched by the optical coupler 11 is provided.

In this case, the transmission wavelength characteristic of the k-th (k = 1 to n) variable optical filter 10 _k is determined by the required signal light band Δ
The characteristic is such that (k-1) extreme points of the transmittance exist for λ. Further, the monitor wavelength of the feedback control for the compensation of each order may be set to two wavelengths at which the amount of change in transmittance with respect to the change in the Faraday rotation angle is maximized when the order is odd. In the case of an even number, the wavelength may be set to a wavelength at which a change in transmittance with respect to a change in the Faraday rotation angle is maximized and a wavelength at which a change in transmittance is minimized. Furthermore, if the time constant of the feedback loop of each order is set to be shorter as the order is lower, each feedback control is more likely to converge.

Next, a third embodiment of the gain equalizer according to the present invention will be described. FIG. 13 is a block diagram illustrating a configuration of a gain equalizer according to the third embodiment. In FIG. 13, the present gain equalizer has, for example, the generalized configuration for performing the nth-order compensation shown in FIG. 12 as an optical amplifying means for compensating for a loss occurring in the gain equalizer. The optical amplifier 13 is additionally provided. When many variable optical filters are provided to realize higher-order compensation, the influence of the insertion loss of each variable optical filter may become a problem. To cope with such a case, in the third embodiment, for example, the optical amplification section 1 between the variable optical filter 10 of _the input terminal IN and the first stage
3 is provided to compensate for the insertion loss generated in each of the variable optical filters 10 ₁ to 10 _n .

As the optical amplifying unit 13, for example, an EDFA
It is possible to use a known optical amplifier such as a Raman amplifier. The gain in the optical amplifier unit 13 is preset to exceed it equal to or insertion loss of total generated at the variable optical filter 10 ₁ to 10 _n.

By providing the optical amplifier 13 in this manner, a gain equalizer substantially free from insertion loss can be realized. This makes it possible to apply the present gain equalizer to various optical transmission systems.

[0084] In the third embodiment, is provided with the optical amplifying section 13 between the input terminal IN and the first stage of the variable optical filter 10 _1, the arrangement of the optical amplifying section 13 is not limited to this, and an input terminal IN The optical amplifying unit 13 is located at an arbitrary position between the output terminals OUT.
Can be provided. Although the optical amplifier 13 has a single-stage configuration, an optical amplifier having a multi-stage configuration may be provided in the gain equalizer.

Next, a fourth embodiment of the gain equalizer according to the present invention will be described. FIG. 14 is a block diagram illustrating a configuration of a gain equalizer according to the fourth embodiment. In FIG. 14, the present gain equalizer includes, for example, the output light power for controlling the total power of the signal light output to the outside via the output terminal OUT to be substantially constant in the configuration of the above-described third embodiment. Output light power control unit 1 as control means
4 is provided between the optical coupler 11 and the output terminal OUT.

The output light power controller 14 has, for example, a variable optical attenuator (VATT) 14A, an optical coupler 14B, a light receiving element (PD) 14C, and an arithmetic circuit 14D. The variable optical attenuator (VATT) 14A is a known optical device capable of changing an optical attenuation amount according to a control signal from the outside. Here, the signal light that has passed through the optical coupler 11 enters. The optical coupler 14B includes a variable optical attenuator 14
A part of the signal light sent from A to the output terminal OUT is branched and sent to the light receiving element 14C. The light receiving element 14C converts the branched light from the optical coupler 14B into an electric signal,
Output to D. The arithmetic circuit 14D compares the output signal level from the light receiving element 14C with a preset reference level, and adjusts the optical attenuation of the variable optical attenuator 14A so that the output optical power becomes constant at a required value. And outputs the control signal to the variable optical attenuator 14A.

In the gain equalizer having such a configuration, the output light power controller 14 monitors the total power of the signal light having been subjected to the first to n-th compensations, and according to the monitoring result, the variable optical attenuator. The feedback control of the optical attenuation of 14A makes the total power of the signal light output to the outside via the output terminal OUT constant at a required value. As a result, even if the power of the signal light input to the gain equalizer changes, the power of the output signal light can be kept constant, so that more stable transmission of the WDM signal light can be realized. Become.

Next, a fifth embodiment of the gain equalizer according to the present invention will be described. In the fourth embodiment, for example, C
A gain equalizer compatible with a WDM optical transmission system in which band signal light and L band signal light are transmitted collectively will be described.

FIG. 15 shows gain equalization according to the fifth embodiment.
FIG. 3 is a block diagram showing a configuration of a container. In FIG.
The gain equalizer 1 receives a WDM signal input to an input terminal IN.
WD that splits light into C-band and L-band signal light
M coupler 15A, C-band and L-band signal light
Coupler 15B that multiplexes the signals and sends them to the output terminal OUT
And the C band output terminal of the WDM coupler 15A and the WDM coupler.
It is located between the C-band input terminal of the plastic 15B and
Variable optical filter 10_1C, 10_2C,… 10_nC, Optical coupler 11
_CAnd control unit 12_CAnd L band of WDM coupler 15A
Between the output terminal and the L-band input terminal of the WDM coupler 15B.
Variable optical filter 10 interposed _1L, 10_2L,… 1
0_nL, Optical coupler 11_LAnd control unit 12_LAnd

The variable optical filter 10 for C band
Variable optical filter 10 _1L for _1C to 10 _nC and L band
10 to 10 _nL are the same as those of the variable optical filters 10 ₁ to 10 _n described above. Also, an optical coupler 1 for the C band
1 _C , control unit 12 _C and optical coupler 11 _L for L band,
The control unit 12 _L is the same as the optical coupler 11 and the control unit 12 described above.

In the gain equalizer 1 having such a configuration, when the WDM signal light including the C-band and L-band signal lights is input to the input terminal IN, the WDM signal light is
The signal is demultiplexed into the C-band signal light and the L-band signal light by the DM coupler 15A, and the n-order compensation for the change in the gain wavelength characteristic is performed for each band in the same manner as in the above-described embodiments. Will be Then, each signal light that has been n-order compensated for each band is multiplexed by the WDM coupler 15B, and then output to the outside via the output terminal OUT. This makes it possible to apply the compensation technique according to the present invention to a system in which two signal lights having different bands are transmitted collectively.

In the fifth embodiment, the case where the signal lights of the C band and the L band are transmitted collectively is considered. However, the present invention is not limited to this, and the signal lights of three or more different bands are considered. Can be dealt with by performing compensation for each signal light band in the same manner as in the above case.

The gain equalizers of the first to fifth embodiments as described above can be provided inside a known optical amplifier such as a rare earth element-doped optical fiber amplifier or a Raman amplifier. Such an optical amplifier having a built-in gain equalizer according to the present invention can automatically compensate for a change in gain wavelength characteristic caused by a change in operating point, temperature, or the like. Further, the gain equalizers of the first to fifth embodiments can be provided for each required compensation section in a general WDM optical transmission system. In such a WDM optical transmission system in which the gain equalizer according to the present invention is arranged, a change in the gain wavelength characteristic generated by a plurality of optical amplifiers (optical repeaters) provided in a required compensation section is different for each compensation section. Will be compensated.

Hereinafter, a WDM optical transmission system using the gain equalizer according to the present invention will be specifically described. FIG. 16 is a block diagram showing a configuration of a WDM optical transmission system using a gain equalizer according to the present invention.

The WDM optical transmission system shown in FIG. 16 comprises, for example, a WDM optical transmitter 2, a WDM optical receiver 3, a transmission line 4 connecting these optical transceivers, and a required transmission line A plurality of optical amplifiers (optical repeaters) 5 arranged at relay intervals and a gain equalizer provided for each compensation section L including a plurality of relay sections (in the figure, for example, sections having three optical amplifiers 5) And 1.

The WDM optical transmitter 2 has, for example, wavelengths λ ₁ to λ ₁ .
_N optical transmitters (O) each outputting an optical signal of λ _N
S) 2A, a multiplexer 2B that wavelength-multiplexes optical signals of wavelengths λ _{1 to} λ _N , and a post-amplifier that amplifies the WDM signal light from the multiplexer 2B to a required level and outputs it to the transmission line 4 2C
And

The WDM optical receiver 3 includes, for example, a preamplifier 3A that amplifies a WDM signal light transmitted via the transmission path 4 to a required level, and outputs an output light from the preamplifier 3A having a wavelength of λ ₁ to λ _N. It has a demultiplexer 3B for demultiplexing each optical signal, and N optical receivers (OR) 3C for receiving and processing optical signals of wavelengths λ _{1 to} λ _N , respectively.

The transmission path 4 connects the WDM optical transmitter 2, each optical amplifier 5, the gain equalizer 1, and the WDM optical receiver 3 respectively. Each optical amplifier 5 amplifies the WDM signal light transmitted via the transmission line 4 to a required level and outputs the amplified signal. These optical amplifiers 5 are general well-known optical amplifiers whose gain wavelength characteristics change due to operating point fluctuations and temperature fluctuations as described above. Specifically, an optical fiber amplifier (for example, EDFA or the like) doped with a rare earth element, a Raman amplifier using Raman amplification, or the like can be used. Further, the optical amplifier 5 may be a combination of an EDFA and a Raman amplifier and employ a configuration in which the input light power of the EDFA is controlled to be substantially constant by Raman amplification.

As the gain equalizer 1, any one of the above-described first to fifth embodiments can be appropriately selected and used according to the compensation accuracy required for the system. For example, when the system requires compensation up to the second order, the gain equalizer 1 as in the first embodiment described above may be applied. Further, when compensation of up to the third order is required for a specific compensation section L, the gain equalizer 1 as in the above-described second embodiment may be applied to the compensation section L. In such a case, gain equalizers having different compensation orders are mixed on the same system. Further, when it is necessary to compensate for the insertion loss in the gain equalizer 1, the gain equalizer 1 as in the third embodiment described above is applied. In addition, when it is necessary to make the signal light power output from the gain equalizer 1 to the transmission line constant, the gain equalizer 1 as in the above-described fourth embodiment may be applied. Further, for example, in a system in which C-band and L-band signal lights are transmitted collectively, the gain equalizer 1 as in the fifth embodiment described above may be applied.

In the WDM optical transmission system having such a configuration, each of the optical amplifiers 5 caused by fluctuations in the operating point, temperature, and the like.
Of the gain wavelength characteristic is dynamically compensated for each predetermined compensation section L, so that the WDM optical receiver 3 can receive signal light having a small optical power deviation between wavelengths. As a result, a wideband WDM signal light can be stably transmitted over a long distance, so that the transmission capacity and transmission distance of the WDM optical transmission system can be increased.

Next, an application example of the above WDM optical transmission system will be described. FIG. 17 is a block diagram illustrating a configuration example of a WDM optical transmission system to which the compensation technology according to the present invention is applied.

In the WDM optical transmission system shown in FIG. 17, for example, when performing first-order and second-order compensation, the gain equalizer 1
Of a variable optical filter 10 ₁ and the variable optical filter 10 ₂ is obtained so as to place the end of the different repeating section. Here, a variable optical filter 10 ₁ is provided at the end of the repeater section is located in the center portion of the compensation interval L, and the variable optical filter 10 ₂ is provided at the end of the compensation interval L, the output from the variable optical filter 10 ₂ The light is monitored by the control unit 12, and the transmission wavelength characteristics of the variable optical filters 10 ₁ and 10 ₂ are feedback-controlled.

As described above, according to the compensation technique of the present invention, it is possible to perform compensation of each order in different relay sections within one compensation section L in a distributed manner. Here, an example in which the first-order and second-order compensations are performed has been described. However, the above description is the same when the third-order or higher-order compensation is performed. The setting of each variable optical filter corresponding to each order is performed in consideration of changes in the gain wavelength characteristics of all the optical amplifiers within the compensation section L, and compensation of each order is performed in units of one compensation section L. To be

[0104]

As described above, according to the gain equalizer of the present invention, the wavelength characteristic of the transmittance is variable by utilizing the magneto-optical effect, and the wavelength characteristics of the transmittance are mutually different. By combining a plurality of variable optical filters set differently, and by performing feedback control of the transmission wavelength characteristic of each variable optical filter based on the monitoring result of the WDM signal light transmitted through the plurality of variable optical filters, Dynamic compensation for the wavelength characteristic of the optical power generated in the WDM signal light can be performed with high accuracy.
In addition, since each variable optical filter can variably control the transmission wavelength characteristic without mechanical operation, a highly reliable gain equalizer can be realized. Further, by setting the time constants of the feedback control corresponding to the respective variable optical filters to be different from each other, it is possible to prevent the feedback control from oscillating. In addition, if an optical amplifier for compensating for insertion loss is provided, a gain equalizer without insertion loss can be realized. Further, by providing the output light power control means, it becomes possible to output WDM signal light whose total power is controlled to be constant.

In the optical amplifier according to the present invention, the gain equalizer having the above-mentioned effects is provided inside, so that the change of the gain wavelength characteristic caused by the fluctuation of the operating point and the temperature is automatically compensated. It becomes possible.

In the WDM optical transmission system according to the present invention, the gain equalizer having the above-described effect is arranged for each predetermined compensation section, so that the change in the gain wavelength characteristic of the optical amplifier in the compensation section is compensated. Since the compensation is dynamically performed for each section, the optical receiver can receive the signal light having the small optical power deviation between the wavelengths.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of a first embodiment of a gain equalizer according to the present invention.

FIG. 2 is a diagram showing a preferred configuration example of a variable optical filter used in a gain equalizer according to the present invention.

FIG. 3 is a perspective view showing an example of a specific configuration of the variable Faraday rotator of FIG. 2;

4A and 4B are diagrams illustrating basic transmission wavelength characteristics of the variable Faraday rotator in FIG. 2; FIG. 4A illustrates a case where a peak wavelength at which the transmittance is minimum is changed in the direction of the wavelength axis; (B) is a diagram showing a case where the FSR is changed.

FIG. 5 is a diagram showing a setting example of a transmission wavelength characteristic of a variable optical filter for primary compensation used in a gain equalizer according to the present invention.

FIG. 6 is a diagram showing an example of setting transmission wavelength characteristics of a variable optical filter for secondary compensation used in a gain equalizer according to the present invention.

FIG. 7 is a first exemplary diagram illustrating first-order compensation and second-order compensation in the present invention.

FIG. 8 is a second exemplary diagram illustrating first-order compensation and second-order compensation in the present invention.

FIG. 9 is a third exemplary diagram illustrating first-order compensation and second-order compensation in the present invention.

FIG. 10 is a block diagram showing a configuration of a second embodiment of the gain equalizer according to the present invention.

FIG. 11 is a diagram showing an example of setting transmission wavelength characteristics of a variable optical filter for tertiary compensation used in a gain equalizer according to the present invention.

FIG. 12 is a block diagram showing a basic configuration of a gain equalizer that realizes n-order compensation according to the present invention.

FIG. 13 is a block diagram showing a configuration of a third embodiment of the gain equalizer according to the present invention.

FIG. 14 is a block diagram showing a configuration of a fourth embodiment of the gain equalizer according to the present invention.

FIG. 15 is a block diagram showing a configuration of a fifth embodiment of the gain equalizer according to the present invention.

FIG. 16 is a block diagram showing a configuration of a WDM optical transmission system using a gain equalizer according to the present invention.

17 is a block diagram illustrating an application example of the WDM optical transmission system in FIG.

FIG. 18 is a diagram illustrating a change (C band) in the gain wavelength characteristic due to a change in the operating point of the optical amplifier.

FIG. 19 is a diagram illustrating a change (L band) in the gain wavelength characteristic due to a change in the operating point of the optical amplifier.

[Explanation of symbols]

_{REFERENCE SIGNS LIST} 1 gain equalizer 2 WDM optical transmitter 3 WDM optical receiver 4 transmission line 5 optical amplifier 10 ₁ to 10 _n , 10 _1C to 10 _nC , 10 _1L to 10 _nL variable optical filter 10A, 10F ... lens 10B ... input wedge plate (polarizer) 10C ... birefringent plate 10D ... variable Faraday rotator 10E ... output side wedge plate _{(polarizer) 11,11 C, 11 L, 20A~20C} ... optical coupler 12, 12 _C , 12 _L ... control unit 14 ... output light power control unit 15 A, 15 B ... WDM couplers 21 A to 20 E ... optical filters 22 A to 22 E ... light receiving elements (PD) 23 ₁ , 23 ₂ , 23 ₃ ... arithmetic circuit

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) H04B 10/02 H04B 9/00 M 10/18 F term (Reference) 2H049 BA02 BA08 BB03 BB66 BC25 2H079 AA03 BA01 CA04 EA11 EB18 FA01 FA04 HA11 KA01 KA05 KA11 KA19 KA20 2K002 AA02 AB30 BA01 CA15 DA06 HA23 5F072 AB07 AK06 JJ09 JJ20 KK30 MM20 QQ07 YY17 5K002 BA02 CA01 CA10 CA13 DA02

Claims (15)

[Claims] 1. A gain equalizer for equalizing wavelength characteristics of optical power of WDM signal light, wherein a wavelength characteristic of transmittance can be changed by utilizing a magneto-optical effect. A plurality of variable optical filters set so that wavelength characteristics of transmittance are different from each other; a wavelength characteristic of optical power of the WDM signal light transmitted through the plurality of variable optical filters is detected, and each wavelength is determined based on the detection result. Control means for controlling the wavelength characteristic of the transmittance of each of the variable optical filters so that the optical power deviation between them is suppressed. 2. The gain equalizer according to claim 1, wherein the plurality of tunable optical filters include first and second polarizers for extracting linearly polarized light from transmitted light, and the first and second polarizers. A birefringent element provided between two polarizers and providing a phase difference between two orthogonally polarized light components to be transmitted;
And a Faraday rotator provided between the polarizers and providing a variable Faraday rotation angle to the transmitted polarized light. 3. The gain equalizer according to claim 1, wherein when the number of the variable optical filters is n (n is an integer of 2 or more), a k-th variable (k is an integer from 1 to n). Wherein the wavelength characteristic of the transmittance of the variable optical filter is preset so that (k-1) extreme points exist within the band of the WDM signal light. 4. The gain equalizer according to claim 1, wherein said control means presets a WDM signal light transmitted through said plurality of variable optical filters in correspondence with each of said variable optical filters. An optical filter unit for extracting the extracted optical signals of at least two monitor wavelength bands, a power detector for detecting the optical signal power of each monitor wavelength band extracted by the optical filter unit, and a detection result of the power detector And a control signal for controlling the wavelength characteristic of the transmittance of the corresponding variable optical filter is generated such that the optical power deviation is suppressed based on the obtained optical power deviation. A gain unit, wherein the wavelength characteristic of each of the variable optical filters is feedback-controlled according to a control signal generated by the calculation unit. 5. The gain equalizer according to claim 4, wherein said optical filter unit has a center wavelength of at least two monitor wavelength bands set in advance corresponding to odd-numbered variable optical filters. , Including at least two wavelengths at which the variable amount of the transmittance of the variable optical filter is maximum within the band of the WDM signal light, and at least two monitor wavelength bands set in advance corresponding to the even-numbered variable optical filters. A gain equalizer, wherein each center wavelength includes a wavelength at which a variable amount of transmittance of the variable optical filter is maximum and a minimum within a band of the WDM signal light. 6. The gain equalizer according to claim 4, wherein said arithmetic unit is configured to set each time constant of feedback control corresponding to each of said variable optical filters to be different from each other. The gain equalizer characterized. 7. The gain equalizer according to claim 1, further comprising: an optical amplifier for compensating insertion loss of said plurality of variable optical filters. 8. The gain equalizer according to claim 1, further comprising output light power control means for controlling the total power of the WDM signal light transmitted through the plurality of variable optical filters to be constant. And a gain equalizer. 9. The gain equalizer according to claim 1, wherein when the WDM signal light includes signal lights of a plurality of wavelength bands, the WDM signal light is demultiplexed into signal lights of each wavelength band. Demultiplexing means is provided, and the plurality of variable optical filters and the control means are respectively provided in correspondence with the signal light of each wavelength band demultiplexed by the demultiplexing means, and further, the plurality of variable optical filters are provided. A gain equalizer characterized by comprising a multiplexing means for multiplexing the transmitted WDM signal light of each wavelength band. 10. A gain equalizer according to claim 1, further comprising:
An optical amplifier for amplifying signal light, wherein a change in gain wavelength characteristic of the optical amplifier is compensated for by the gain equalizer. 11. The optical amplifier according to claim 10, wherein said optical amplification means includes a rare earth element doped optical fiber amplifier. 12. The optical amplifier according to claim 10, wherein said optical amplifier includes a Raman amplifier. 13. An optical transmitter for generating a WDM signal light and transmitting the generated signal to a transmission line, an optical receiver for receiving the WDM signal light transmitted through the transmission line, A plurality of optical amplifiers arranged at a relay interval
In the M optical transmission system, the gain equalizer according to claim 1 is arranged for each predetermined compensation section including a plurality of relay sections, and changes in gain wavelength characteristics of a plurality of optical amplifiers in the compensation section. Is compensated by the gain equalizer. 14. The WDM optical transmission system according to claim 13, wherein the plurality of variable optical filters of the gain equalizer are collectively arranged at an end of one of the relay sections in the compensation section. WDM optical transmission system. 15. The WDM optical transmission system according to claim 13, wherein the plurality of variable optical filters of the gain equalizer are dispersedly arranged at respective ends of different relay sections in the compensation section. A WDM optical transmission system, comprising: