JP3706271B2 - Variable optical attenuation method and variable optical attenuator - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a variable optical attenuating technique used in the field of optical communications, and more specifically, a variable optical attenuating method and a variable optical attenuator as basic elements in an optical signal transmission apparatus for transmitting an optical signal in wavelength division multiplexing communication. It is about.
[0002]
[Prior art]
There is a wavelength division multiplexing system as an effective means for constructing a large-capacity optical communication network, and research and development have been actively conducted in recent years. FIG. 10 shows an optical communication apparatus using a general wavelength division multiplexing system. This optical communication apparatus includes an optical transmitter LT composed of N optical transmitters 301, Nx1 optical multiplexers 302, and optical amplifiers 307 that generate optical signals of different wavelengths, an optical amplifier 307, and a 1xN optical demultiplexer. The optical receiver LR includes 304 and N optical signal receivers 305, and the optical transmission path 306 is disposed between the optical transmitter LT and the optical receiver LR. Here, when the DFB distributed feedback (DFB) laser is directly modulated as the optical transmitter 301, the DFB laser and the lithium niobate light intensity modulator for encoding are used, and the DFB laser and the EA modulator are integrated. In some cases, the light source is used. An AWG (Arrayed Waveguide Grating) is used as the 1 × N optical multiplexer / demultiplexer. As the optical amplifier, an optical fiber type amplifier or a Raman amplifier to which a rare earth element is added is used. As the optical transmission line, a single mode optical fiber having a zero-dispersion wavelength of 1.3 μm or 1.55 μm is used, and an optical amplifier and a 3R repeater are arranged as necessary. Such a wavelength division multiplexing system has a feature that the capacity of a communication network can be increased because a signal multiplexed in a wavelength space on one optical fiber is transmitted. As a typical wavelength division multiplexing system, the number of wavelengths is 32 to 64, the bit rate is 10 Gbit / s, and the transmission distance is 1000 km. In addition, wavelength division multiplexing transmission with a throughput of 1 Tbit / s is in the research stage.
[0003]
When designing and manufacturing such an optical communication device, it is important to control the intensity of the optical signal to the transmission optical fiber line. This is because when the signal light intensity is higher than a desired value, the nonlinear optical effect of the optical fiber received by the wavelength multiplexed signal light causes the waveform deterioration of the signal light pulse and increases the code error rate. (For example, “M. Fukui, S. Aisawa, O. Ishida, K. Shimano, A. Umeda, T. Sakamoto, K. Oda, and N. Takachio,“ Allowable fiber input. power for installed dispersion shifted fiber in equal / unequal channel spaced WDM transmission systems, “Electron. Lett., vol. 33, pp693-694, 1997.”).
[0004]
When the signal light intensity is lower than a desired value, the S / N is reduced due to noise generated in the optical amplifier, and the code error rate is increased. Therefore, it is necessary to determine the optimum signal light intensity to the optical fiber so that the optical signal transmission characteristics are the best. Furthermore, the problem of the nonlinear optical effect in the optical fiber becomes more prominent with the recent widening of the signal light wavelength range. In other words, regarding the communication wavelength range, in addition to the conventional 1.55 μm band, the development of optical amplifiers in new wavelength bands such as gain-shifted optical amplifier (GS-EDFA), fiber Raman amplifier, thulium-doped fiber type optical amplifier (TDFA), etc. Accordingly, optical communication in the 1.58 μm band, 1.52 μm band, and 1.48 μm band is being developed.
[0005]
In a situation where optical communication technology using such a new wavelength band is advancing, a wideband wavelength division multiplexing system that extends beyond the conventional 1.55 μm band having a bandwidth of about 40 nm and extends to 200 nm in the wavelength range of 1400 to 1600 nm. The appearance of is essential. In optical communication using such a wideband wavelength division multiplexing system, the nonlinear optical effect in the optical fiber becomes more prominent, and it is important to control the signal light intensity more precisely than before (nonlinearity in the wideband wavelength division multiplexing system). Regarding optical effects, see, for example, `` J. Kani, M. Jinno, T. Sakamoto, K. Hattori, and K. Oguchi, “Bi-directional transmission to suppress inter-wavelength-band nonlinear interactions in ultra-wide band WDM transmissions systems. "In Tech. Digest of 3rd OECC, pp. 412-413, 1998.").
[0006]
Here, as means for controlling the signal light intensity to the optical fiber in the conventional wavelength multiplexing system, (1) a method of adjusting wavelength multiplexed light in a lump with a variable optical attenuator, and (2) output light of an optical transmitter A method of individually adjusting the intensity, and (3) a method of arranging a post-amplifier after the multiplexer and controlling the gain of the post-amplifier to collectively adjust the optical output of the wavelength multiplexed light are conceivable. Here, a practical realization method is a method using the variable optical attenuator of (1). In the method (2), since the optical signal transmitted from the optical transmitter usually requires strict optical frequency accuracy of about GHz, it is necessary to adjust the light intensity while keeping the optical frequency constant. As a result, the device configuration becomes complicated. Therefore, in reality, several variable optical attenuators are prepared for several wavelengths after the optical transmitter, which increases the scale of the apparatus.
[0007]
In the above method (3), the optical amplifier usually has a wavelength dependency of the gain. When the gain is changed, the gain on the short wavelength side and the long wavelength side are different (gain tilt), thereby suppressing the wavelength dependency of the gain. This is because there is a problem in that the device configuration becomes complicated. Therefore, the method using the variable optical attenuator (1) can simplify the apparatus configuration and realize high control accuracy.
[0008]
As described above, a variable optical attenuator with less wavelength dependency in the signal light wavelength band and good control accuracy is an elemental technology from the viewpoint of suppressing signal light waveform degradation and simple optical fiber input light intensity control in the wideband wavelength division multiplexing system. It is important.
[0009]
Conventionally, for variable optical attenuators, a method using the thermo-optic effect of a silica-based optical waveguide, a method of driving a bulk ND filter with a stepping motor, a method of controlling the Faraday rotation angle of an optical fiber type optical isolator, an acoustic There is a method using an optical element.
[0010]
Here, a method using a silica-based optical waveguide is excellent in optical characteristics such as miniaturization, mass productivity, and low loss by integration, and is superior to other technologies as an optical component used in optical communication. An optical circuit using a silica-based optical waveguide is manufactured based on circuit design, chip manufacture, and fiber mounting by Planar Lightwave Circuit (PLC) technology. Conventionally, a configuration using a Mach-Zehnder interferometer and a thermo-optic effect has been proposed as a method for realizing a variable optical attenuator with PLC.
[0011]
FIG. 9 shows the optical circuit configuration. It comprises a Mach-Zehnder interferometer 10 composed of two 3 dB couplers 11 and two arm waveguides 12, and a phase adjuster 13 provided in one arm waveguide 12. In FIG. 10, Lin is input signal light, and Lout is output signal light.
[0012]
The phase adjustment mechanism in the quartz optical waveguide is a method of changing the refractive index using the thermo-optic effect using a metal thin film heater formed on the waveguide as a heat source. For example, when the variable optical attenuator uses a symmetric Mach-Zehnder interferometer, the input signal light is output to the output port at a position diagonal to the input port without applying power to the heater, and applied to the heater. The amount of light attenuation increases with the power, and the light output decreases. Therefore, it is possible to control the light output intensity according to the power supplied to the heater.
[0013]
Furthermore, an automatic level control (ALC) that controls the optical output constant with respect to changes in the optical input intensity by providing a branching device at the output port and performing feedback control while monitoring the light intensity output from this branching device. It is also possible to add. For ALC using a PLC Mach-Zehnder interferometer, see, for example, “K. Hattori, M. Fukui, M. Jinno, M. Oguma, and K. Oguchi,“ All-PLC-based optical ADM with high isolation and polarization independent level. equalizer, "in Proc. ECOC '98, 1998, pp. 327-328."
[0014]
Currently, an optical attenuator having a maximum optical attenuation of about 20 dB is realized by a PLC type variable optical attenuator.
[0015]
[Problems to be solved by the invention]
However, the variable optical attenuator using the Mach-Zehnder interferometer that can be miniaturized and integrated as described above has a problem that the attenuation is wavelength-dependent. This is due to the fact that the phase difference between the two arm waveguides of the Mach-Zehnder interferometer has wavelength dependence, which is an essential characteristic for a circuit using optical interference. For example, the one-stage Mach-Zehnder interferometer shown in FIG. 9 has a wavelength dependence of about 1 dB for an attenuation of 3 dB, 2 dB for an attenuation of 6 dB, and about 4 dB for an attenuation of 10 dB in the wavelength region of 1400 to 1600 nm. As a method for eliminating the wavelength dependence of optical attenuation, an optical circuit in which two Mach-Zehnder interferometers are connected in series and heaters are arranged alternately has been proposed ("TV Clapp, S.Day, S.Ojha, and RGPeall, “Broadband variable optical attenuator in silica waveguide technology,” in Proc. ECOC '98, 1998, pp. 301-302.). In this optical circuit, it is reported that a flat optical attenuation characteristic within 1 dB can be obtained in a band of 1530 to 1560 nm.
[0016]
However, even if this optical circuit is used, it is impossible to suppress the wavelength dependence in a wide wavelength range extending from 1400 to 1600 nm. For example, according to the simulation, in the wavelength region of 1400 to 1600 nm, the wavelength dependence is about 1 dB at an attenuation of 3 dB, 2 dB at an attenuation of 6 dB, and 4 dB at an attenuation of 10 dB, and a conventionally proposed one-stage Mach-Zehnder interferometer It can be seen that wavelength dependency equivalent to the wavelength dependency possessed by is generated. In other words, the wavelength dependence of the phase difference of the interferometer that occurs in principle dominates the wavelength dependence of the optical attenuation, and as a result, a variable optical attenuator with flat characteristics in a wide wavelength range cannot be realized. It was. Therefore, it has been impossible to realize an optical communication apparatus that collectively controls the optical fiber input light intensity of the optical communication apparatus in FIG. 10 using a small PLC variable optical attenuator with excellent integration. .
[0017]
For the reasons described above, there has been a demand for the emergence of a technique for eliminating the wavelength dependence of the optical attenuation of the variable optical attenuator using the PLC.
[0018]
An object of the present invention is to provide a variable optical attenuator having a small wavelength dependency in a waveguide type optical component which is promising for integration and miniaturization.
The above and other objects and novel features of the present invention will become apparent from the description of this specification and attached drawings.
[0019]
[Means for Solving the Problems]
The following is a brief description of an outline of typical inventions among the inventions disclosed in the present application.
(1) The first optical interferometer and the second optical interferometer are connected in cascade, the phase difference of the signal light bifurcated by the optical interferometer is adjusted, and the intensity of the signal light is set to a desired value. A variable optical attenuation method to be set, wherein the signal light passes through the first optical interferometer. First Phase difference And the relationship of the second phase difference when passing through the second optical interferometer The sum of the differential wavelength of the transmitted light intensity of the first optical interferometer and the differential wavelength of the transmitted light intensity of the second optical interferometer at the center wavelength of the signal light is zero. Above After approximating the second phase difference as a quadratic function of the first phase difference and adjusting the first phase difference, the second phase difference was uniquely obtained from the quadratic function. Phase difference In It is a method to adjust.
[0020]
(2) In the variable optical attenuation method of the means (1), the optical interferometer is constituted by a cross type Mach-Zehnder interferometer using one input end of the Mach-Zehnder interferometer and an output end obliquely opposed to the input end. It is a method to do.
[0021]
(3) In the variable optical attenuation method of the means (1), the optical interferometer is constituted by a bar-type Mach-Zehnder interferometer using one input end of the Mach-Zehnder interferometer and an output end facing the input end in parallel. Is the method.
[0022]
(4) In the variable optical attenuation method of the means (1), one of the first and second Mach-Zehnder interferometers is a cross-type Mach-Zehnder interferometer and the other is a bar-type Mach-Zehnder interferometer.
[0024]
( 5 ) Said means (1 )of In the variable optical attenuation method, the phase difference is within 2π.
[0026]
( 6 ) The first optical interferometer and the second optical interferometer are connected in cascade, the phase difference of the signal light bifurcated by the optical interferometer is adjusted, and the intensity of the signal light is set to a desired value. A variable optical attenuator, first phase difference adjusting means for adjusting a phase difference when the signal light passes through the first optical interferometer; A relationship between a first phase difference when the signal light passes through the first optical interferometer and a second phase difference when the signal light passes through the second optical interferometer, The sum of the wavelength differential amount of the transmitted light intensity of the first optical interferometer and the wavelength differential amount of the transmitted light intensity of the second optical interferometer at the center wavelength of the signal light is zero. ,Previous Second phase difference The After approximating as a quadratic function of the first phase difference and adjusting the first phase difference, The second phase difference Uniquely obtained from the quadratic function Place A second phase adjusting means for adjusting to the phase difference is provided.
[0027]
( 7 ) Said means ( 6 ), The phase difference is within 2π.
[0028]
( 8 ) Said means ( 6) or (7 In the variable optical attenuator, the optical interferometer is composed of a planar optical waveguide formed of a core part having a high refractive index and a clad part surrounding the core part and having a refractive index lower than that of the core part. Two input optical waveguides, two optical multiplexers / demultiplexers with two inputs and two outputs, two output optical waveguides of the optical multiplexer / demultiplexer of the optical multiplexer / demultiplexers, and two input optical waveguides of the optical multiplexer The Mach-Zehnder interferometer is composed of two arm optical waveguides that connect the two, a phase adjuster provided in the arm optical waveguides, and two output optical waveguides.
[0029]
( 9 ) Said means ( 6) or (7 In the variable optical attenuator, the optical interferometer is composed of a planar optical waveguide formed of a core part having a high refractive index and a clad part surrounding the core part and having a refractive index lower than that of the core part. One input optical waveguide, one input and two output Y branch, two input and one output Y branch, and the connection between the two output optical waveguides of the Y branch and the two input optical waveguides The Mach-Zehnder interferometer is composed of two arm optical waveguides, a phase adjuster provided in the arm optical waveguide, and one output optical waveguide.
[0030]
( 10 ) Said means ( 8 ) Is configured with a cross-type Mach-Zehnder interferometer having one input optical waveguide of the Mach-Zehnder interferometer as an input end and an output optical waveguide diagonally opposed to the input end as an output end.
[0031]
( 11 ) Said means ( 8 ), A bar-type Mach-Zehnder interferometer having one input optical waveguide of the Mach-Zehnder interferometer as an input end and an output optical waveguide facing the input end parallel to the output end.
[0032]
( 12 ) Said means ( 8 1), one of the first and second Mach-Zehnder interferometers is a cross-type Mach-Zehnder interferometer, and the other is a bar-type Mach-Zehnder interferometer.
[0033]
The variable optical attenuating method and variable optical attenuator of the present invention are characterized in that the optical output intensity from the optical interferometer is set to a desired value by adjusting the phase of the optical interferometer and the variable optical attenuator. , Two optical interferometers are connected in series, and the differential amount of the transmitted light intensity at the center wavelength within the operating wavelength range of the first optical interferometer and the transmitted light intensity at the center wavelength of the second optical interferometer are It is to drive the phase adjusters of the first and second optical interferometers while maintaining the relationship that the sum of the wavelength differential amounts becomes zero.
[0034]
That is, the variable optical attenuation method and variable optical attenuator of the present invention satisfy the broadband optical attenuation characteristic necessary for broadband wavelength division multiplexing optical communication that will be realized in the future. The wavelength dependence of the optical attenuation amount of the optical interferometer generated in the above is essentially suppressed by the combination of the two-stage optical interferometer and its phase control method. That is, the differential value of the transmitted light intensity at the center wavelength of the second optical interferometer is equal to the wavelength differential amount of the transmitted light intensity at the center wavelength within the operating wavelength range of the first optical interferometer. By adjusting the phase of the second optical interferometer so that the optical signal transmitted through the two optical interferometers has a relationship that the sum of the wavelength differential amounts of the transmitted light intensity at the center wavelength becomes zero. be able to.
[0035]
In addition, by applying phase control that maintains the relationship that the sum of the wavelength differential amounts of the transmitted light intensity at the center wavelength is zero, the wavelength differential amount of the transmitted light intensity at the operation center wavelength is zero, that is, the transmitted light intensity at the operation center wavelength. The wavelength dependence of can be eliminated. As a result, the wavelength dependence of the transmitted light intensity in the operating wavelength range is minimized.
[0036]
Further, in realizing a phase control method that maintains a relationship in which the sum of wavelength differential amounts is zero, a method that approximates phase control to a quadratic function can also effectively reduce wavelength dependency.
[0037]
If this quadratic function approximation is used, the wavelength dependency can be effectively reduced by a simple analog control circuit.
[0038]
Further, a PLC type Mach-Zehnder interferometer is effective as the interferometer used in the variable optical attenuator of the present invention. This is because, as described above, the PLC is excellent in small size, integration, and mass productivity, and in particular, the Mach-Zehnder interferometer has its own characteristics compared to other interference circuits such as a ring resonator. This is because the wavelength dependency is small. In addition, the variable optical attenuator of the present invention has a configuration in which two Mach-Zehnder interferometers, which are simple basic circuits, are used to reduce wavelength dependence, and the complexity and size of the apparatus are minimal. That is, by using a phase control method approximated by a quadratic function and a two-stage PLC Mach-Zehnder interferometer, it is possible to realize a variable optical attenuator with a simple apparatus configuration and a small wavelength dependency. .
[0039]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail with embodiments (examples) according to the present invention with reference to the drawings.
Note that, in all drawings for explaining the present embodiment (examples), parts having the same functions are given the same reference numerals, and repeated explanation thereof is omitted.
[0040]
(Embodiment 1)
FIG. 1 is a schematic diagram showing a schematic configuration of a variable optical attenuator according to Embodiment 1 of the present invention. In the first embodiment, a Mach-Zehnder interferometer is used as an optical interferometer, and an approximation using a quadratic function is used as a phase control method. This is because the Mach-Zehnder interferometer has less wavelength dependency of the optical attenuation than other optical interferometers, and is effective in reducing the wavelength dependency in a broadband operation. Further, the approximation by the quadratic function is possible because the phase control method can be configured with a simple analog circuit, and even if this approximation is used, the effect of reducing the wavelength dependency is great.
[0041]
The variable optical attenuator of Embodiment 1 uses an optical waveguide. As shown in FIG. 1, two input optical waveguides 101, two optical multiplexers / demultiplexers 102 having two inputs and two outputs, and an optical multiplexer are used. Two arm optical waveguides 103A and 103B connecting the two output optical waveguides of the optical demultiplexer 102 and the two input optical waveguides of the optical multiplexer, and arm optical waveguides 103A and 103B, respectively. The Mach-Zehnder interferometers 100A and 100B, each including the phase adjusters 104A and 104B and the two output waveguides 105, are configured. The Mach-Zehnder interferometers 100A and 100B are basically constituted by cross-type Mach-Zehnder interferometers. Here, the cross-type Mach-Zehnder interferometer is one in which the input end and the output end of the two input / output optical waveguides of the Mach-Zehnder interferometer are at diagonal positions.
[0042]
In the first embodiment, it is assumed that the Mach-Zehnder interferometers 100A and 100B are symmetrical, that is, the lengths of the two arm waveguides are equal. That is, the respective phase amounts of the Mach-Zehnder interferometers 100A and 100B are set to 0 in advance. As shown in FIG. 1, an ALC function can be realized by providing an optical branching device 106 at the output end, monitoring the branched light with a photodetector 107, and performing feedback control with a feedback control circuit 108. Yes. The amount of phase adjustment by the phase adjuster 104B is controlled by the control circuit 109. The control circuit 109 performs control to give a phase adjustment amount by the phase adjuster 104B as a quadratic function to the phase adjustment amount by the phase adjuster 104A.
[0043]
FIG. 2 is a diagram for explaining a phase control method for realizing wavelength independence of optical attenuation. The horizontal axis represents the phase amount of one (first) Mach-Zehnder interferometer (for example, 100A) at the front stage or the rear stage, and the vertical axis represents the phase amount of the other (second) Mach-Zehnder interferometer (for example, 100B). The black circle marks indicate the coupling rate of the optical multiplexer / demultiplexer 102 constituting the Mach-Zehnder interferometer when the coupling rate of the optical multiplexer / demultiplexer 102 constituting the Mach-Zehnder interferometer (100A, 100B) is 50% at a wavelength of 1500 nm. When the wavelength is 1400 nm and 50%, the black square mark indicates that the coupling factor of the optical multiplexer / demultiplexer 102 constituting the Mach-Zehnder interferometer (100A, 100B) is 50% at the wavelength of 1600 nm. Each plot is an exact solution with a minimum wavelength dependency, and each curve is approximated by a quadratic function. The phase control method using the exact solution can minimize the wavelength dependence of the optical attenuation.
[0044]
In order to realize a control method using an exact solution, the respective phase amounts of the two-stage Mach-Zehnder interferometers 100A and 100B are stored in a memory, and controlled according to this phase information by software processing such as firmware.
[0045]
Further, the phase control method using the approximate solution by the quadratic function can provide the other phase amount as the quadratic function approximation for one phase amount by using a simple analog circuit. Next, a phase control method when the coupling rate of the optical multiplexer / demultiplexer 102 constituting the Mach-Zehnder interferometers 100A and 100B varies will be described.
[0046]
First, let us consider a case where the coupling ratio of the optical multiplexer / demultiplexer 102 is ideally manufactured and the coupling ratio of 50% is satisfied at 1500 nm which is the central wavelength in the assumed operating wavelength range of 1400 to 1600 nm. In this case, control for decreasing the phase of the other Mach-Zehnder interferometer (for example, 100B) to be approximated by a quadratic function as shown in FIG. By performing the above, wavelength dependency can be minimized.
[0047]
Next, it is considered that the coupling rate of the optical multiplexer / demultiplexer 102 constituting the Mach-Zehnder interferometers 100A and 100B is shifted due to a manufacturing error. For example, when the wavelength at which the coupling rate is 50% is shifted to 1400 nm or 1600 nm, control that is approximated by a quadratic function as indicated by the black triangle mark or black square mark shown in FIG. Thus, wavelength dependency can be minimized. This indicates that a production error of the variable optical attenuator can be allowed by adjusting the settings of the phase control circuits 104A and 104B, which has a great effect on the yield improvement in the manufacture of the variable optical attenuator.
[0048]
As described above, two stages of Mach-Zehnder interferometers 100A and 100B are connected in series, and the phase of one Mach-Zehnder interferometer (for example, 100B) is controlled by a quadratic function of the phase of the other Mach-Zehnder interferometer (for example, 100A). FIG. 3 shows a simulation of the spectrum of light attenuation in the case of the above. For reference, the wavelength dependence of the light attenuation when a conventional one-stage Mach-Zehnder interferometer (FIG. 9) is used is indicated by a thin line. From FIG. 3, the wavelength dependence of the variable optical attenuator of the first embodiment is suppressed to within 0.5 dB at an attenuation of 10 dB. In the case of the conventional one-stage Mach-Zehnder interferometer, it can be seen that the wavelength dependency is about 4 dB at the attenuation of 10 dB, so that it is suppressed to about 1/8. Accordingly, it has been clarified that the wavelength dependence of the optical attenuation can be effectively reduced despite the approximation of the phase control method by a quadratic function. Thus, it has been found that the variable optical attenuator of the first embodiment can realize optical attenuation characteristics with less wavelength dependence in a wide wavelength region, and is an optical attenuator effective for application to wideband wavelength division multiplexing communication. I can conclude.
[0049]
In the first embodiment, the phase of the phase adjuster 104B of the subsequent stage Mach-Zehnder interferometer 100B is given as a quadratic function with respect to the phase of the phase adjuster 104A of the previous stage Mach-Zehnder interferometer 100A. It is obvious that the same effect can be obtained even if the phase of the phase adjuster 104A of the preceding Mach-Zehnder interferometer 100A is given as a quadratic function with respect to the phase of the phase adjuster 104B of the latter-stage Mach-Zehnder interferometer 100B. . As the phase adjuster, a metal thin film heater is generally used in the case of a silica-based optical waveguide, and the heater may be provided in either of the two arm optical waveguides 103 of the symmetric Mach-Zehnder interferometer.
[0050]
(Embodiment 2)
FIG. 4 is a schematic diagram showing a schematic configuration of a variable optical attenuator according to Embodiment 2 of the present invention. As shown in FIG. 4, the variable optical attenuator of the second embodiment has one of the two symmetric Mach-Zehnder interferometers 100A and 100B of the first embodiment shown in FIG. 1 as an asymmetric Mach-Zehnder interferometer 100C. A phase amount of 2π is provided. The one (first) Mach-Zehnder interferometer 100A is symmetric, the lengths of the two arm optical waveguides 103 are equal, and the other (second) Mach-Zehnder interferometer 100C is asymmetric. The length of the arm waveguide 103 is different by 2π as the optical path length difference. When the phase 2π is converted into an actual waveguide length, for example, when quartz glass having a refractive index of about 1.45 is used as an optical waveguide material and an operating wavelength band of 1550 nm is assumed, the two arm optical waveguides 103A and 103B are used. The difference in length is about 1034 nm. Further, the phase adjuster 104C of the asymmetric Mach-Zehnder interferometer 100C is provided in the short arm optical waveguide 103A. This is because the phase adjuster generally increases the refractive index with respect to the drive electric signal intensity, and realizes the operation of reducing the phase difference provided in advance by 2π with an asymmetric configuration by the phase adjuster. Because. For example, in a silica-based optical waveguide, the phase of the asymmetric Mach-Zehnder interferometer 100C decreases as the drive power increases by using the heater as a phase adjuster and mounting the heater on the short side arm optical waveguide 103A.
[0051]
Further, as shown in FIG. 4, as the circuit configuration, an optical branching device 106 is provided in the main output optical waveguide 105, the branched light is monitored by the photodetector 107, and feedback control is performed by the feedback control circuit 108, whereby the ALC function is achieved. It can also be realized.
[0052]
FIG. 5 is a diagram for explaining a phase control method for realizing wavelength independence of optical attenuation. The horizontal axis represents the phase adjustment amount of the phase adjuster 104A of the symmetric Mach-Zehnder interferometer 100A, and the vertical axis represents the phase adjustment amount of the phase adjuster 104C of the asymmetric Mach-Zehnder interferometer 100C. Further, when the coupling ratio of the optical multiplexer / demultiplexer constituting the Mach-Zehnder interferometer is 50% at a wavelength of 1500 nm, the black circle mark indicates that the coupling ratio of the optical multiplexer / demultiplexer constituting the Mach-Zehnder interferometer is 50% at a wavelength of 1400 nm. In this case, a black square mark indicates a case where the coupling rate of the optical multiplexer / demultiplexer constituting the Mach-Zehnder interferometer is 50% at a wavelength of 1600 nm. Furthermore, each plot is an exact solution with a minimum wavelength dependence, and each curve represents an approximation by a quadratic function. The phase control method using the exact solution can minimize the wavelength dependence of the optical attenuation.
[0053]
In order to realize a control method using an exact solution, the respective phase amounts of the two-stage Mach-Zehnder interferometers 100A and 100C are stored in a memory and controlled according to this phase information by software processing such as firmware. Moreover, the phase control method using the approximate solution by the quadratic function can give the other phase amount as the quadratic function approximation to the one phase amount by using a simple analog circuit.
[0054]
A phase control method when the coupling rates of the multiplexers / demultiplexers 104A and 104B constituting the Mach-Zehnder interferometers 100A and 100C vary will be described below.
[0055]
First, let us consider a case where the coupling ratio of the optical multiplexer / demultiplexer 102 is ideally manufactured and satisfies a coupling ratio of 50% at 1500 nm, which is the center wavelength, in the assumed operating wavelength range of 1400 nm to 1600 nm. In this case, for the increase in the phase adjustment amount of the phase adjuster 104A of the symmetric Mach-Zehnder interferometer 100A, the phase adjustment amount of the phase adjuster 104C of the asymmetric Mach-Zehnder interferometer 100C is an exact solution as shown in FIG. Alternatively, the wavelength dependence can be minimized by performing control to increase so as to be approximated by a quadratic function. This is because the phase adjustment amount II of the phase adjuster 104C shown in FIG. 4 is a value obtained by subtracting the phase amount II shown in FIG. 2 from 2π, and the phase of 2π is previously asymmetrically configured on the variable optical attenuator. This is because the phase adjustment amount of the asymmetric Mach-Zehnder interferometer 100C becomes equivalent to the phase amount II shown in FIG. 2 by providing the amount and adding the phase adjustment amount II of FIG.
[0056]
Next, let us consider that the coupling rate of the optical multiplexer / demultiplexer 102 constituting the Mach-Zehnder interferometer is shifted due to a manufacturing error. For example, when the wavelength at which the coupling rate is 50% is shifted to 1400 nm or 1600 nm, control that is approximated by a quadratic function as indicated by the black triangle mark or black square mark shown in FIG. Thus, wavelength dependency can be minimized. This indicates that, as in the first embodiment, by adjusting the setting of the phase neutralizer 104C, a manufacturing error of the variable optical attenuator can be tolerated. In the manufacture of the variable optical attenuator, the yield is improved. Has a great effect.
[0057]
As described above, the wavelength dependence of the optical attenuation of the variable optical attenuator in which the symmetric Mach-Zehnder interferometer 100A and the asymmetric Mach-Zehnder interferometer 100C are combined is as shown in FIG. The attenuator is equivalent to the configuration of the variable optical attenuator shown in FIG. Furthermore, the configuration of the variable optical attenuator of the second embodiment is more effective in reducing power consumption than the configuration of the variable optical attenuator shown in FIG. This is clear when phase II in FIG. 5 and FIG. 2 are compared. That is, in the configuration of FIG. 1, the phase II is from π to 2π from FIG. 2, whereas in the configuration of FIG. 4, the phase adjustment amount II is smaller from 0 to π than from FIG. Therefore, the power consumption of the phase adjusters (104B, 100C) can be suppressed. That is, it is clear that the variable optical attenuator of the second embodiment can effectively suppress the wavelength dependence of the optical attenuation amount, even though the approximation by the quadratic function is used as in the first embodiment. became.
[0058]
As described above, it has been found that the variable optical attenuator of the second embodiment can realize optical attenuation characteristics with little wavelength dependency in a wide band, and it can be concluded that it is an effective optical attenuator in broadband wavelength division multiplexing communication.
[0059]
In the second embodiment, the Mach-Zehnder interferometer 100A at the front stage is a symmetric type and the Mach-Zehnder interferometer 100C at the rear stage is asymmetrical. It is obvious that the same effect can be obtained as a mold. In this case, the phase of the phase adjuster 104A of the preceding asymmetric Mach-Zehnder interferometer 100A is given as a quadratic function with respect to the phase of the phase adjuster 104C of the latter-stage symmetric Mach-Zehnder interferometer 100C. As the phase adjuster, a metal thin film heater is generally used in the case of a quartz optical waveguide, and the position of the heater of the symmetric Mach-Zehnder interferometer may be provided in either of the two arm waveguides 103.
[0060]
In the second embodiment described above, the configuration in which the cross-type Mach-Zehnder interferometers are connected in two stages has been specifically described. However, even if one or both of them is a bar-type Mach-Zehnder interferometer, the phase adjuster of one Mach-Zehnder interferometer is used. A similar effect can be obtained by driving the phase amount of the second approximation to a quadratic function of the phase amount of the phase adjuster of the other Mach-Zehnder interferometer. However, it is more effective to use a cross type Mach-Zehnder interferometer in which the optical attenuation is not affected by a deviation from 50% of the coupling ratio of the directional coupler, because a larger optical attenuation can be achieved.
[0061]
(Embodiment 3)
FIG. 6 is a schematic diagram showing a schematic configuration of a variable optical attenuator according to Embodiment 3 of the present invention.
[0062]
As shown in FIG. 6, in the variable optical attenuator of the third embodiment, the two Mach-Zehnder interferometers of the first embodiment shown in FIG. 1 are Mach-Zehnder interferometers 100A and 100B each including a Y optical splitter 102A. Is. The variable optical attenuator of Embodiment 3 uses an optical waveguide, and includes one input optical waveguide 101, a Y-branch 102A having one input and two outputs, and a Y-branch 102B having two inputs and one output. , Two arm optical waveguides 103 connecting the two output optical waveguides of the Y branching device 102A and the two input waveguides of the Y branching device 102B, and phase adjusters 104A provided in the arm optical waveguide 103, The Mach-Zehnder interferometers 100A and 100B are composed of 104B and one output waveguide 105, and the first and second Mach-Zehnder interferometers 100A and 100B are connected in series. Here, it is assumed that the two Mach-Zehnder interferometers 100A and 100B are symmetrical, and the lengths of the two arm optical waveguides 103 are equal. Further, as shown in FIG. 6, as the circuit configuration, an ALC function can be realized by providing a branch circuit 106 at the output end, monitoring the branched light with the photodetector 107, and performing feedback control with the feedback control circuit 108. It has become.
[0063]
As described in the first embodiment, the variable optical attenuator of the third embodiment is configured such that the phase of the other Mach-Zehnder interferometer 100B increases as the phase adjustment amount I of the phase adjuster 104A of one Mach-Zehnder interferometer 100A increases. By controlling the phase adjustment amount II of the adjuster 104B so as to be approximated by a quadratic function, the wavelength dependence of the optical attenuation can be minimized as shown in FIG. Further, it is obvious that wavelength dependency can be reduced even when a symmetric Mach-Zehnder interferometer and an asymmetric Mach-Zehnder interferometer as described in the second embodiment are combined.
[0064]
As described above, the variable optical attenuator of Embodiment 3 has been found to be able to realize optical attenuation characteristics with little wavelength dependence in a wide band, and is an effective optical attenuator in wideband wavelength multiplex communication.
[0065]
As described above, the variable optical attenuator described in the first to third embodiments has a circuit configuration applicable to a waveguide type optical component in general. Here, it is preferable to use the circuit configuration of the variable optical attenuator of the third embodiment for a silica-based optical waveguide excellent in low loss. Further, as the phase adjuster of the silica-based optical waveguide, it is preferable to use the thermo-optical effect by heating the heater as described in the above embodiment, but other means such as an electro-optical effect can also be used. . In addition to quartz glass, a multicomponent glass, lithium niobate, or a polymer can be used as the material of the optical waveguide, and an electro-optic effect or a thermo-optic effect can be used as the phase adjustment mechanism. In addition to the directional coupler, an MMI coupler can be used for the two combining spectrum circuits constituting the Mach-Zehnder interferometer.
[0066]
Furthermore, in order to realize a small ALC function, it is effective to hybridly integrate a photo detector on a waveguide substrate.
[0067]
In order to suppress the wavelength dependence as much as possible, it is desirable that the phase amount set in advance in the two-stage Mach-Zehnder interferometer is 2π or less. This is because by reducing the preset phase amount to 2π or less, the wavelength dependence of the optical attenuation when the one-stage Mach-Zehnder interferometer shown in FIG. 9 is used for the variable optical attenuator can be reduced. This is because the wavelength dependence of the light intensity that has passed through the two-stage Mach-Zehnder interferometer can be minimized.
[0068]
(Embodiment 4)
Embodiment 4 shows an example of control of an optical amplifier using a variable optical attenuator according to the present invention. A schematic configuration of the optical amplifier is shown in FIG. The optical amplifier 201 of the fourth embodiment is a configuration for making the output signal light intensity constant when the light intensity of the input signal varies in the constant gain control (AGC) optical amplifier. The automatic level control device (ALC) 200 using the variable optical attenuator of the present invention is arranged in the above. The operation of the optical amplifier 201 will be briefly described. The input signal light is amplified with a constant gain by the AGC operation of the AGC optical amplifier 201. Here, when the AGC optical amplifier 201 is adapted to a situation where the input signal light intensity is different, the output signal intensity changes according to the input signal light intensity. Here, the ALC 200 using the variable optical attenuator of the present invention is arranged at the subsequent stage of the AGC optical amplifier 201, and the output light intensity is controlled to be constant.
[0069]
In the fourth embodiment, the ALC 200 is disposed in the subsequent stage of the AGC optical amplifier 201. However, a configuration in which the ALC 200 is disposed in the preceding stage of the AGC optical amplifier 201 or a structure in which the ALC 200 is disposed between the two stages of the AGC optical amplifier is considered. It is done. As the AGC optical amplifier 201, an EDFA has been generally used. However, it is effective to apply the AGC to a wideband optical amplifier such as an optical amplifier combining a EDFA and a GSEDFA or a Raman amplifier. . This is because as the amplification band becomes wider, the light attenuation characteristic having a smaller wavelength dependency, which is a feature of the ALC 200 of the present invention, can be utilized.
[0070]
As described above, by combining the ALC using the variable optical attenuator of the present invention with the AGC optical amplifier 201, the output signal light intensity can be controlled to be constant even when the input signal light intensity fluctuates. This is important as an application to optical amplifiers.
[0071]
(Embodiment 5)
FIG. 8 is a schematic diagram showing a schematic configuration of an optical communication apparatus using the variable optical attenuator according to the fifth embodiment of the present invention. The optical communication apparatus according to the fifth embodiment includes an N level optical transmitter 301 that generates optical signals of different wavelengths, an Nx1 optical multiplexer 302, and an automatic level adjustment apparatus (ALC) 303 using the variable optical attenuator of the present invention. Between the optical signal transmitter LT, 1 × N optical demultiplexer 304 and N optical receivers 305, and between the optical signal transmitter LT and the optical signal receiver LR. The transmission optical fiber line 306 and the optical amplifier 307 are arranged. Here, if necessary, between the Nx1 optical multiplexer and the automatic level adjuster 303 in the optical signal transmitter LT or the subsequent stage of the automatic level adjuster 303, and the 1xN optical demultiplexer 304 of the optical signal receiver LR. An optical amplifier 307 is disposed in the previous stage. Since the variable optical attenuator of the present invention has a small wavelength dependency, it is possible to collectively control the light intensity of a wideband wavelength division signal input to the transmission fiber line 306. Thus, it is not necessary to use a plurality of variable optical attenuators.
[0072]
Further, the attenuation amount of the variable optical attenuator is controlled as necessary so that the code error rate of the signal received by the optical receiver 305 of the optical signal receiver LR is minimized. When such feedback control is performed, for example, when the optical communication apparatus according to the fifth embodiment is newly installed and communication is started, the receiver can optimize the signal light intensity to the optical fiber line 306. The error rate can be adjusted while measuring, and the reliability of communication at the start of communication can be ensured.
[0073]
Further, when the number of wavelengths is increased or removed during system operation, the optimum signal light intensity to the optical fiber line 306 can be determined according to the change in the number of wavelengths. Thereby, it is possible to provide a so-called self-supporting optical communication apparatus that can cope with an increase or decrease in the number of wavelengths without stopping the operation of the apparatus.
[0074]
Furthermore, it is also effective to integrate the variable optical attenuator of the present invention with an arrayed waveguide type diffraction grating using a 1 × N optical demultiplexer.
[0075]
As described above, since the variable optical attenuator of the present invention can achieve optical attenuation characteristics having no wavelength dependence in a wide wavelength range extending to 200 nm, in an optical communication system using a wideband wavelength division multiplexing system, This is effective in application areas where the intensity of the division multiplexed signal light is collectively controlled.
[0076]
【The invention's effect】
As described above, according to the present invention, it is possible to realize a light attenuation characteristic with a small wavelength dependency in a wideband region as long as 200 nm.
In addition, since an optical waveguide that can be miniaturized and integrated is used, the optical waveguide can be miniaturized with a simple circuit configuration that combines a two-stage Mach-Zehnder interferometer.
In addition, since the phase control of the interferometer uses a control circuit that gives one phase as a quadratic function of the other phase, the control accuracy can be increased.
Further, since it is possible to cope with the manufacturing error of the optical multiplexing / demultiplexing circuit constituting the Mach-Zehnder interferometer only by adjusting the setting of the control circuit, it can be manufactured with a high yield.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a schematic configuration of an automatic level control apparatus using a variable optical attenuator according to a first embodiment of the present invention.
FIG. 2 is a diagram for explaining a phase control method of a Mach-Zehnder interferometer in the variable optical attenuator of the first embodiment.
FIG. 3 is a diagram showing a simulation of a light attenuation amount spell according to the first embodiment;
FIG. 4 is a schematic diagram showing a schematic configuration of an automatic level control apparatus using a variable optical attenuator of Embodiment 2 according to the present invention.
FIG. 5 is a diagram for explaining a phase control method of a Mach-Zehnder interferometer in the variable optical attenuator according to the second embodiment.
FIG. 6 is a schematic diagram showing a schematic configuration of an automatic level control apparatus according to a third embodiment of the present invention.
FIG. 7 is a schematic diagram showing a schematic configuration of an optical amplifier using an automatic level control device according to a fourth embodiment of the present invention.
FIG. 8 is a schematic diagram showing a schematic configuration of an optical communication apparatus using an automatic level control apparatus according to a fifth embodiment of the present invention.
FIG. 9 is a schematic diagram showing a schematic configuration of a conventional variable optical attenuator.
FIG. 10 is a schematic diagram showing a schematic configuration of an optical communication apparatus using a conventional variable optical attenuator.
[Explanation of symbols]
Lin ... input signal light, Lout ... output signal light, 100A, 100B ... symmetric Mach-Zehnder interferometer, 100C ... asymmetric Mach-Zehnder interferometer, 101 ... input optical waveguide, 11,102 ... optical multiplexer / demultiplexer, 12, 103 ... arm Optical waveguide 13, 104A, 104B, 104C ... phase adjuster, 105 ... output optical waveguide, 106 ... branching device, 107 ... photo detector, 108 ... feedback control circuit, 109 ... phase adjustment amount II with respect to phase adjustment amount I Control circuit given by quadratic function, 200, 303 ... automatic level adjustment device (ALC), 201 ... AGC optical amplifier, 301 ... optical transmitter, 302 ... Nx1 optical multiplexer, 303 ... automatic level adjustment device (ALC), 304 1xN optical demultiplexer, 305 optical receiver, 306 optical fiber line for transmission, 307 optical amplifier.

Claims (12)

A variable in which the first optical interferometer and the second optical interferometer are connected in cascade, the phase difference of the signal light bifurcated by the optical interferometer is adjusted, and the intensity of the signal light is set to a desired value. A light attenuation method,
The relationship between the first phase difference when the signal light passes through the first optical interferometer and the second phase difference when the signal light passes through the second optical interferometer is expressed as a center wavelength of the signal light. the first as the sum of the wavelength derivative of the transmitted light intensity of the wavelength derivative amount of the transmitted light intensity and the second optical interferometer of the optical interferometer is zero, the prior SL second retardation It said first approximated as a quadratic function of the phase difference, the first After adjusting the phase difference, characterized by adjusting the second phase difference phase difference which uniquely determined from the quadratic function Variable optical attenuation method.   2. The variable optical attenuation method according to claim 1, wherein the optical interferometer includes a cross type Mach-Zehnder interferometer using one input end of the Mach-Zehnder interferometer and an output end obliquely opposed to the input end. A variable optical attenuation method characterized.   2. The variable optical attenuation method according to claim 1, wherein the optical interferometer includes a bar-type Mach-Zehnder interferometer using one input end of the Mach-Zehnder interferometer and an output end facing the input end in parallel. Variable optical attenuation method.   2. The variable optical attenuation method according to claim 1, wherein one of the first and second Mach-Zehnder interferometers is a cross-type Mach-Zehnder interferometer and the other is a bar-type Mach-Zehnder interferometer. Method. The variable optical attenuation method according to claim 1 , wherein the phase difference is within 2π. A variable in which the first optical interferometer and the second optical interferometer are connected in cascade, the phase difference of the signal light bifurcated by the optical interferometer is adjusted, and the intensity of the signal light is set to a desired value. An optical attenuator,
First phase difference adjusting means for adjusting a phase difference when the signal light passes through the first optical interferometer;
The relationship between the first phase difference when the signal light passes through the first optical interferometer and the second phase difference when the signal light passes through the second optical interferometer is expressed as a center wavelength of the signal light. the first such that the sum of the wavelength derivative of the transmitted light intensity of the wavelength derivative amount of the transmitted light intensity and the second optical interferometer of the optical interferometer is zero, pre-Symbol second phase difference at was approximated as a quadratic function of the first phase difference, after adjusting the first phase difference, the second for adjusting the second phase difference phase difference which uniquely determined from the quadratic function A variable optical attenuator comprising the phase adjusting means. The variable optical attenuator according to claim 6 , wherein the phase difference is within 2π. 8. The variable optical attenuator according to claim 6 , wherein the optical interferometer is formed of a core portion having a high refractive index and a clad portion surrounding the core portion and having a lower refractive index than the core portion. Two input optical waveguides, two input and two optical multiplexers / demultiplexers, and two output optical waveguides of the optical demultiplexer of the optical multiplexer / demultiplexer and optical multiplexing A Mach-Zehnder interferometer comprising two arm optical waveguides connecting two input optical waveguides of the detector, a phase adjuster provided in the arm optical waveguides, and two output optical waveguides A variable optical attenuator. 8. The variable optical attenuator according to claim 6 , wherein the optical interferometer is formed of a core portion having a high refractive index and a clad portion surrounding the core portion and having a lower refractive index than the core portion. One input optical waveguide composed of a type optical waveguide, a Y-branch having one input and two outputs, a Y-branch having two inputs and one output, and two output optical waveguides of the Y-branch It is a Mach-Zehnder interferometer comprising two arm optical waveguides connecting two input optical waveguides, a phase adjuster provided in the arm optical waveguides, and one output optical waveguide. Variable optical attenuator. 9. The variable optical attenuator according to claim 8 , comprising a cross type Mach-Zehnder interferometer having one input optical waveguide of the Mach-Zehnder interferometer as an input end and an output optical waveguide diagonally opposed to the input end as an output end. A variable optical attenuator characterized by that. 9. The variable optical attenuator according to claim 8 , comprising a bar-type Mach-Zehnder interferometer having one input optical waveguide of the Mach-Zehnder interferometer as an input end and an output optical waveguide facing in parallel with the input end as an output end. A variable optical attenuator. 9. The variable optical attenuator according to claim 8 , wherein one of the first and second Mach-Zehnder interferometers is a cross-type Mach-Zehnder interferometer and the other is a bar-type Mach-Zehnder interferometer. vessel.

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