JP2000004062A - Optical amplifier - Google Patents

Abstract

(57) [Summary] [PROBLEMS] In a WDM transmission, a time variation of a signal light level and a level deviation between signal lights, which cause a limitation of a transmission distance and a deterioration of transmission quality, are absorbed. SOLUTION: A wavelength-division multiplexed signal light optically amplified by constant gain control is demultiplexed, and constant output level control is performed on the signal light of each wavelength by an optical variable attenuator. Thereby, the signal light levels of the respective wavelengths can be individually controlled or adjusted, and the signal light levels can be made uniform.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical amplifying apparatus for amplifying signal light of each wavelength multiplexed at once and then demultiplexing the signal light of each wavelength.

Optical amplifiers for amplifying wavelength-multiplexed signal light collectively include a post-optical amplifier of an optical transmitter, a relay optical amplifier of an optical repeater, and a pre-optical amplifier of an optical receiver.
The optical amplifying device of the present invention relates to a pre-amplifier therein. The pre-amplifier has a function of amplifying a signal light level that has been reduced by propagating through an optical fiber immediately before an optical demultiplexer to an electrically reproducible level.

[0003]

2. Description of the Related Art Wavelength-multiplexed signal light of each wavelength can be collectively amplified by using an optical fiber amplifier or a semiconductor optical amplifier. As a method of controlling the signal light level, there are gain constant control and output level constant control.

FIG. 5 shows an example of the configuration of a conventional optical amplifying apparatus that performs constant gain control. This optical amplifier comprises a two-stage optical fiber amplifier 1a for amplifying a wavelength multiplexed signal light in the 1550 nm band.
1b and an optical demultiplexer 2 for demultiplexing the amplified wavelength multiplexed signal light into signal lights of each wavelength. The signal light of each wavelength demultiplexed by the optical demultiplexer 2 is received by the light receiving devices 3-1 to 3-n
Are respectively converted into electric signals, and a reproduction process is performed.

The optical fiber amplifiers 1a and 1b include an erbium-doped optical fiber 11, an excitation light source 12, and an optical multiplexer 1 for inputting excitation light to the erbium-doped optical fiber 11.
3. An optical demultiplexer 14-that demultiplexes a part of the input signal light and a part of the output signal light of the erbium-doped optical fiber 11-
1, 14-2, and a control circuit 15 that controls the pump light source 12 so that the gain is constant by comparing the input / output signal light levels. The optical fiber amplifier 1a is configured to pump with 0.98 μm band pump light in order to improve noise characteristics, and the optical fiber amplifier 1b is configured to pump 1.48 μm in order to obtain high output.
In this configuration, excitation is performed with excitation light in the μm band. The optical isolators 16-1 and 16-16 are disposed downstream of the optical demultiplexer 14-1 of the optical fiber amplifier 1 a, between the optical fiber amplifiers 1 a and 1 b, and between the optical fiber amplifier 1 b and the optical demultiplexer 2.
-2 and 16-3.

In this optical amplifying device, each optical fiber amplifier 1
Since the gains are controlled to be constant at a and 1b, when the input signal light level changes, the same change occurs at the inputs of the light receiving devices 3-1 to 3-n. In addition, the signal light level of each wavelength that has been demultiplexed is generally different. Therefore, in the light receiving device, the automatic gain control (AG) capable of absorbing the time variation of the input signal light level and the level deviation between the signal lights.
C) Function is required.

An ordinary light receiving device has an AGC function capable of absorbing an input level difference of 10 dB or more. SDH system
Taking a 2.5 Gbit / s transmission device as an example, a light receiving device using a pin photodiode alone can be -20 dBm to -5 dB.
It has an input dynamic range of about 15 dB of Bm. This light receiving device is connected to the above-mentioned optical amplifier (pre-optical amplifier)
When used in combination with, the lower limit is about -15 dBm. This is because SN design is performed in a region where beat noise between amplified spontaneous emission light (ASE) emitted from an optical fiber amplifier and signal light is dominant. Therefore,
The effective input dynamic range is (-5dBm)-
(−15 dBm) = about 10 dB.

[0008]

The time variation of the input signal light level and the level deviation between signal lights appear due to the following factors.

(1) Loss fluctuation due to contact with an optical fiber,
(2) Polarization-dependent loss due to time change of polarization state, (3) Optical amplifier gain fluctuation due to insertion / extraction of multiplexed signal light, (4) Gain deviation (gain of wavelength) of optical amplifier with wavelength (5) Wavelength dependence), (5) Difference in section loss due to difference in relay section distance.

The time variation of the input signal light level is caused by the following factors (1)
(3), and the level deviation between signal lights is a factor (4).
caused by. Note that factor (4) also occurs as a result of factor (3). The level deviation between signal lights increases as the number of relays of the relay optical amplifier increases, due to the accumulation of gain deviations. Also, the level of the input signal light to each of the repeater optical amplifiers and the pre-optical amplifier has a steady difference of 10 dB or more due to the factor (5). Although a certain degree of adjustment is possible by using a fixed optical variable attenuator at the time of initial introduction of the system, it is necessary to allow for several dB for the light receiving device.

In an actual optical transmission system, an allowable range is allocated to each of the above factors. For example, the factor
In contrast to (3), it is necessary to expect about 4 dB with the current gain control technology.
dB may be expected. There is already an acceptable range for both
Since it exceeds 10 dB, the input dynamic range 10
With dB, other factors (1), (2) and (5) cannot be absorbed.

By performing constant output level control in the optical amplifier, the factors (1), (3) and (5) can be absorbed. FIG. 6 shows a configuration example of a conventional optical amplifying device that performs this output level constant control. This is based on the maximum value detection control method, the details of which are described in Japanese Patent Application No. 8-309504 (optical amplifying device for wavelength multiplex transmission).

In FIG. 6, optical fiber amplifiers 1a and 1b for performing constant gain control, optical demultiplexer 2, and light receiving devices 3-1 to 3-1 are provided.
3-n is the same as that shown in FIG. An optical variable attenuator 4 capable of adjusting an optical attenuation is inserted between the optical fiber amplifiers 1a and 1b. Optical demultiplexer 2 and each light receiving device 3-
The light intensity detection circuit 5 inserted between 1 and 3 -n individually detects the light intensity of the signal light of each wavelength split by the optical splitter 2. The maximum light intensity detection circuit 6 detects the maximum value of the light intensity of the signal light of each wavelength, and controls the variable optical attenuator 4 so that the maximum value of the light intensity becomes a predetermined level.

By controlling the maximum level of the signal light of each wavelength multiplexed in the above-described configuration to be constant, it is possible to absorb a component whose signal light level fluctuates with time to some extent. For example, although depending on the response speed of the optical variable attenuator 4, the factors (1) and (3) can be absorbed up to about 1 dB (Reference: Suzuki et al., "Transient in the maximum optical power selection control type optical amplifier"). Response propagation characteristics ", 1998 IEICE General Conference, B-10-157).

However, even when this output level constant control method is used, it is difficult to absorb the main factor (4) that causes a level deviation between signal lights. It is also difficult to absorb factor (2).

It is an object of the present invention to provide an optical amplifying apparatus capable of absorbing a time variation of a signal light level and a level deviation between signal lights, which are factors that limit transmission distance and deteriorate transmission quality in WDM transmission. Aim.

[0017]

An optical amplifying apparatus according to the present invention comprises:
The wavelength-division multiplexed signal light optically amplified by the gain constant control is demultiplexed, and the output level constant control is performed on the signal light of each wavelength by a variable optical attenuator. Thereby, the signal light levels of the respective wavelengths can be individually controlled or adjusted, and the signal light levels can be made uniform. Therefore, not only the above factors (1), (3), (5), but also factors (2),
(4) can also be absorbed.

It should be noted that the AGC circuit of the light receiving device only needs to function for high-speed fluctuation components that cannot be followed by the optical variable attenuator, and as a whole, the same level of performance as a single-channel optical transmission system can be realized. it can.

The variable optical attenuator 4 between the two optical fiber amplifiers 1a and 1b required in the conventional configuration shown in FIG.
Becomes unnecessary. The optical variable attenuator 4 shown in FIG. 6 is required to have strict specifications for wavelength dependency and polarization dependency, but this is eased by the configuration of the present invention that controls the optical level for each signal light of each wavelength.

[0020]

FIG. 1 shows an embodiment of an optical amplifying device according to the present invention. In FIG. 1, optical fiber amplifiers 1a and 1b for performing constant gain control, optical demultiplexer 2, and light receiving device 3-1.
３−3-n are the same as those shown in FIG. Here, a two-stage configuration of a 0.98 μm pumping optical fiber amplifier and a 1.48 μm pumping optical fiber amplifier for amplifying the wavelength multiplexed signal light in the 1550 nm band is shown, but a one-stage configuration may be used. Further, an optical fiber amplifier having another amplification band may be used.

The feature of this embodiment is that the level of the output light of the demultiplexed signal light of each wavelength becomes constant between the optical splitter 2 and each of the light receiving devices 3-1 to 3-n. The optical variable attenuators 7-1 to 7-n which are automatically controlled are provided. Examples of the variable optical attenuator include a configuration using a Faraday rotator capable of high-speed operation, a configuration using a Mach-Zehnder interferometer suitable for integration, and a configuration using a Y-branch waveguide formed of a polymer material. Can be used.

FIG. 1 shows how the level deviation between signal lights can be suppressed by controlling the output level of the signal light of each wavelength using the variable optical attenuators 7-1 to 7-n.
See (2) and (3). FIG. 1 (2) shows an optical fiber amplifier 1a,
1b shows the spectrum of the wavelength multiplexed signal light amplified in 1b.
The level deviation between signal lights reaches 7 dB as described above.
The ASE level is also proportional to each signal light level.
By splitting the wavelength multiplexed signal light having a relative level deviation between the signal lights and controlling each signal light level by each of the variable optical attenuators 7-1 to 7-n, FIG. As shown, the output light level can be made uniform. By setting the equalization level at an intermediate level between the input dynamic ranges of the light receiving devices 3-1 to 3-n, the time variation of the signal light level can be absorbed by the light receiving devices. Thereby, the bit error rate can be further reduced.

FIG. 2 shows a configuration example of an optical variable attenuator using a Faraday rotator (reference: N. Fukushima et a).
l., "Non-mechanical variable attenuator module usin
g faraday effect ", International Conference on opt
ical amplifiers and thier applications (OAA'96), FD
9-1, 1996).

The present variable optical attenuator includes an input optical fiber 21,
Two lenses 22-1, 22-2, two birefringent wedge plates 23-1, 23-2, a Faraday rotator 24 whose rotation angle is variable, an output optical fiber 25, an optical coupler 26, and a photodiode 27 , A Faraday rotator drive circuit 28.

The signal light split by the optical splitter 2 is input to an input optical fiber 21, a lens 22-1, a birefringent wedge plate 23-.
The signal light input to the Faraday rotator 24 via the Faraday rotator 1 and output from the Faraday rotator 24
-2, coupled to the output optical fiber 25 via the lens 22-2. A part of the output light of the output optical fiber 25 is branched by the coupler 26, received by the photodiode 27, and converted into a photocurrent corresponding to the light level. The Faraday rotator drive circuit 28 drives the Faraday rotator 24 so that the photocurrent becomes constant. That is, by rotating the magnetization direction with respect to the optical path while the magneto-optical crystal is saturated, the Faraday rotation angle is continuously changed, and the output optical fiber 25 is changed by the transmittance change according to the rotation angle.
Is controlled so that the light level coupled to the light source becomes constant.

FIG. 3 shows a configuration example of an optical variable attenuator using a Mach-Zehnder interferometer (reference: T. Kawai et a).
l., "PLC type compact variable optical attenuator f
or photonic transport network ", Electron. Lett., vo
l.34, no.3, pp.264-265, 1998).

This optical variable attenuator comprises a Mach-Zehnder interferometer and its control system. The Mach-Zehnder interferometer has two input waveguides 31-1, 31-2 and two sets of 3d
B couplers 32-1 and 32-2, two arm waveguides 33
-1, 33-2, thermo-optic phase shifters (heater elements) 34-1, 34-2 provided on the arm waveguides, and two output waveguides 35-1, 35-2. The control system includes an optical coupler 36, a photodiode 37, and a heater drive circuit (constant current circuit) 38.

The signal light split by the optical splitter 2 is input to one input waveguide 31-1 of the Mach-Zehnder interferometer and output from one output waveguide 35-1. A part of the output light from the output waveguide 35-1 is branched by the coupler 36, received by the photodiode 37, and converted into a photocurrent corresponding to the light level. The heater drive circuit 38 controls the thermo-optical phase shifters 34-1 and 34- so that the photocurrent becomes constant.
2 is driven (electrically heated). That is, the thermo-optic phase shifter 3
Due to the thermo-optic effect of 4-1 and 34-2, the arm waveguide 3
The transmittance is changed by controlling the refractive indices (waveguide lengths) of 3-1 and 33-2 so that the level of the signal light coupled to the output waveguide 35-1 is controlled to be constant.

FIG. 4 shows a configuration example of a variable optical attenuator using a Y-branch waveguide (reference: Japanese Patent Laid-Open No. 10-20348).
No.). This variable optical attenuator includes a Y-branch waveguide formed of a polymer material and a control system therefor. The Y branch waveguide includes an input waveguide 41, two output waveguides 42-
1, 42-2, and thermo-optic phase shifters (heater elements) 43-1 and 43-2 provided on the output waveguide. The control system includes an optical coupler 44, a photodiode 45,
It comprises a heater drive circuit (constant current circuit) 46.

The signal light split by the optical splitter 2 is input to the input waveguide 41, and is output to one of the output waveguides 4 of the Y-branch waveguide.
Output from 2-1. A part of the output light from the Y-branch waveguide 42-1 is branched by the coupler 44 and
5, and is converted into a photocurrent corresponding to the light level. The heater drive circuit 46 drives (heats and energizes) the thermo-optical phase shifters 43-1 and 43-2 so that the photocurrent becomes constant. That is, the thermo-optic phase shifters 43-1 and 43-2
The asymmetric Y-branch waveguide is formed by controlling the refractive index of the output waveguides 42-1 and 42-2 by the thermo-optic effect described above so that the signal light level coupled to the output waveguide 42-1 becomes constant. Control.

[0031]

As described above, the optical amplifying device of the present invention can individually control or adjust each signal light level by the optical variable attenuator corresponding to the signal light of each wavelength.
As a result, it is possible to limit the transmission distance in WDM transmission and to reduce the time variation of the signal light level and the level deviation between signal lights, which are factors that degrade the transmission quality.

Further, since the signal light levels of the respective wavelengths can be made uniform, the transmission capacity of the lightwave network for performing routing in units of wavelengths can be increased, and the transmission quality can be made uniform.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of an optical amplifying device of the present invention.

FIG. 2 is a diagram illustrating a configuration example of an optical variable attenuator using a Faraday rotator.

FIG. 3 is a diagram showing a configuration example of an optical variable attenuator using a Mach-Zehnder interferometer.

FIG. 4 is a diagram showing a configuration example of an optical variable attenuator using a Y-branch waveguide.

FIG. 5 is a diagram showing a configuration example of a conventional optical amplifier that performs gain constant control.

FIG. 6 is a diagram showing a configuration example of a conventional optical amplifier that performs output level constant control.

[Explanation of symbols]

 1a, 1b Optical fiber amplifier 2 Optical demultiplexer 3 Light receiving device 4 Optical variable attenuator 5 Optical intensity detection circuit 6 Maximum optical intensity detection circuit 7 Optical variable attenuator 11 Erbium-doped optical fiber 12 Excitation light source 13 Optical multiplexer 14 Optical demultiplexer Waver 15 Control circuit 16 Optical isolator 21 Input optical fiber 22 Lens 23 Birefringent wedge plate 24 Faraday rotator 25 Output optical fiber 26 Optical coupler 27 Photodiode 28 Faraday rotator drive circuit 31 Input waveguide 32 3 dB coupler 33 Arm waveguide 34 thermo-optic phase shifter (heater element) 35 output waveguide 36 optical coupler 37 photodiode 38 heater drive circuit (constant current circuit) 41 input waveguide 42 output waveguide 43 thermo-optic phase shifter (heater element) 44 optical coupler 45 Photodiode 46 Heater drive times (Constant current circuit)

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) H04B 10/17 H04B 9/00 J 10/16 F term (reference) 2H079 AA06 AA08 AA12 BA01 CA06 CA09 CA12 CA13 DA17 EA09 EB27 FA03 FA04 HA07 KA18 5F072 AB09 AK06 FF09 GG09 HH02 HH03 JJ09 KK30 MM01 PP07 TT22 TT27 YY17 5K002 AA06 BA02 BA07 CA08 CA13 DA02

Claims (5)

[Claims] 1. A wavelength multiplexed n by a constant gain control.
An optical amplifier that collectively amplifies signal light of a wavelength (n is an integer of 2 or more); an optical demultiplexer that demultiplexes the signal light of each wavelength amplified by the optical amplifier; An optical amplifying device comprising: n variable optical attenuators for controlling the output light level of the demultiplexed signal light to be constant. 2. An optical amplifier whose gain is controlled to be constant is 0.98 μm.
Optical fiber amplifier pumped by m-band pump light, 1.48μ
2. The optical amplifying device according to claim 1, wherein the optical amplifying device has a configuration in which optical fiber amplifiers excited by m-band excitation light are connected in cascade. 3. The variable optical attenuator uses a Faraday rotator whose Faraday rotation angle changes according to the magnitude of a current magnetic field, and controls the output light level by changing the transmittance according to the rotation angle. The optical amplifier according to claim 1, wherein: 4. An optical variable attenuator uses a Mach-Zehnder interferometer in which a refractive index of a waveguide changes due to a thermo-optic effect, and controls an output light level coupled to an output waveguide by changing a refractive index of the waveguide. The optical amplifying device according to claim 1, wherein the optical amplifying device has a configuration. 5. The variable optical attenuator uses a Y-branch waveguide formed of a polymer material whose refractive index changes by a thermo-optic effect, and changes the refractive index of the branch waveguide to an output waveguide. The optical amplifying device according to claim 1, wherein the optical amplifying device is configured to control an output light level to be coupled.