JP2001021848A - Polarized light dispersion compensating circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a waveguide type polarized light dispersion compensating circuit which does not use a movable member, and is small in size and excellent in reliability, by providing at least one of a pair of optical delay lines with a means for varying a delay amount, and adjusting delay amounts of the two separated polarized light components and inputting them to a waveguide type polarized light synthesis means. SOLUTION: Among optical pulses inputted to a waveguide type polarized light separating element 12 through an input channel optical waveguide 11, an optical pulse delayed due to a polarized light dispersion characteristic of the optical transmission line is guided to a fixed delay line 13, and a leading optical pulse is guided to a variable delay line 14. By controlling a double-in double-out optical switch comprising the variable delay line 14, a difference between the delay amounts of the fixed delay line 13 and the variable delay line 14 is set to almost the same delay amount difference between the two separated optical pulses. The difference between the delays in the two optical pulses to be synthesized by a waveguide type polarized light synthesis means 16 becomes almost zero, and the optical pulses are restored to the original optical pulse waveform. Attenuation of an optical attenutor 15 is adjusted so that the fixed delay line 13 and the variable delay line 14 have the same loss of light.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a polarization dispersion compensation circuit for shaping optical signal distortion caused by polarization dispersion of an optical transmission line (optical fiber).

Here, the polarization dispersion (PMD:
Polarization Mode Dispersion is a phenomenon in which the propagation speed changes depending on the polarization state of an optical signal due to the birefringence of an optical fiber, and this is one of the causes of optical signal distortion.

[0003]

2. Description of the Related Art With the development of optical communication, there is an increasing demand for transmitting a high-speed optical signal to an existing optical fiber. However, existing optical fibers have a relatively high polarization dispersion, so that optical signal distortion when a high-speed optical signal is propagated becomes a serious problem.

As a polarization dispersion compensating circuit for solving this problem, two polarizing prisms (polarizing beam splitters) are used.
And those using a mechanical movable mirror (reference: F. Heismann et al., "AUTOMATIC COMPENSATION OF
FIRST-ORSER POLARIZATIONMODE DISPERSION IN A 10 G
b / s TRANSMISSION SYSTEM ", WdC11, ECOC'98, 1998).

FIG. 6 shows a configuration example of a conventional polarization dispersion compensating circuit described in the above document. In the figure, an optical signal input from an input port 101 is input to a first polarizing prism 102. Here, the TM delayed in the optical transmission path
The light is output to a straight port, and the advanced TE light is output to an orthogonal port to perform polarization separation. TE light is
The light is input to the second polarizing prism 104 via a mechanical movable mirror 103 including two reflecting mirrors, is polarization-synthesized with the directly input TM light, and is output to an output port 105. By moving the mechanical movable mirror 103, the optical path length in which the TE light propagates spatially changes, and the amount of delay between the TE light and the TM light is adjusted to perform polarization dispersion compensation.

[0006]

However, the conventional polarization dispersion compensating circuit has the following problems. First, since it has the mechanical movable mirror 103, it lacks reliability. Although the polarization dispersion has a characteristic that greatly fluctuates with time, the conventional polarization dispersion compensating circuit needs to move the mechanical movable mirror 103 in order to follow the fluctuation of the polarization dispersion. It becomes a problem.

Second, since the optical system is spatially constructed, strict accuracy is required for the position of each component (102, 103, 104). In particular, in the conventional polarization dispersion compensation circuit, extremely strict positional accuracy is required so that the optical coupling amount does not change even when the mechanical movable mirror 103 is moved.

Third, the conventional polarization dispersion compensating circuit has a configuration in which individual components are combined, and the mechanical movable mirror 103
However, there is a limit to miniaturization because of the configuration using the movable member including.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a small-sized, highly reliable waveguide-type polarization dispersion compensation circuit that does not use a movable member.

[0010]

SUMMARY OF THE INVENTION A polarization dispersion compensating circuit according to the present invention comprises a waveguide type polarization separation circuit for separating an input light into two orthogonal polarization components on a substrate and outputting the resultant to two output ports. Means, a pair of optical delay lines connected to two output ports of the waveguide type polarization separation means, and a pair of optical delay lines passing through the pair of optical delay lines.
And a waveguide-type polarization combining means for inputting the two polarization components, combining the two polarization components, and outputting the combined signals. At least one of the pair of optical delay lines has means for varying the amount of delay. In this configuration, the delay amount of the wave component is adjusted and input to the waveguide type polarization combining means. That is, the pair of optical delay lines may be a combination of a fixed delay line and a variable delay line, or a combination of a variable delay line and a variable delay line.

The means for varying the delay amount of the optical delay line includes (N + 1) 2-input 2-output optical switches and N sets of optical waveguide pairs having different optical path lengths, where N is a positive integer. Alternately connected, one input port of the first-stage two-input two-output optical switch is used as the input port of the optical delay line, and the last-stage two-input two-output optical switch is used.
One output port of the output optical switch is set as the output port of the optical delay line, and one of each pair of optical waveguides is set by setting a through state or a cross state between the input and output ports of each two-input two-output optical switch. And the amount of delay can be varied by selecting the optical waveguide.

The difference in the optical path lengths of the N sets of optical waveguide pairs forming the variable delay line is a factor of 2 such as 1: 2: 4: 8:...: 2 ^N-1. , N
At the stage, a delay amount of 1-2 ^N -1 times the unit delay amount can be freely selected.

Further, an optical attenuator for adjusting the intensity of the optical signal may be connected to at least one of the pair of optical delay lines. The optical attenuator is for adjusting a pair of optical delay lines (for example, a fixed delay line and a variable delay line) to have the same loss.

The waveguide type polarization separating means and the waveguide type polarization combining means comprise two optical couplers having two inputs and two outputs, and two waveguide arms connecting them, and one of the waveguide arms is provided. This is a configuration in which means for adjusting birefringence is formed above. The means for adjusting birefringence may be formed by a stress applying film.

[0015]

FIG. 1 shows an embodiment of a polarization dispersion compensating circuit according to the present invention. Here, an example in which a quartz optical waveguide formed on a silicon substrate is used will be described. This silica-based optical waveguide can constitute a stable optical circuit having excellent matching with an optical fiber, but is not limited to this. For example, another optical waveguide such as a semiconductor optical waveguide or a photonic crystal can be used. May be used.

In FIG. 1, on a silicon substrate 10,
Input channel optical waveguide 11, waveguide-type polarization splitter 1
2. A fixed delay line 13 and a variable delay line 14, which constitute a pair of optical delay lines, an optical attenuator 15, a waveguide-type polarization combining element 1.
6. The output channel optical waveguides 17 are sequentially arranged. The variable delay line 14 has a configuration in which an arbitrary delay amount can be set by combining some parts for selecting one of a short path and a long path.

The input port A of the waveguide type polarization beam splitter 12 is optically connected to the input channel optical waveguide 11. First output port B1 of waveguide type polarization splitting element 12
Is optically connected to the input port C of the fixed delay line 13, and the input port D of the variable delay line 14 is optically connected to the second output port B2. An output port E of the fixed delay line 13 is optically connected to a first input port F1 of a waveguide type polarization combining device 16 via an optical attenuator 15.
The output port G of the variable delay line 14 is optically connected to the second input port F2 of the waveguide type polarization combining element 16. An output channel optical waveguide 17 is optically connected to the output port H of the waveguide type polarization separation element 16.

Here, the polarization dispersion compensating circuit of the present invention comprises a variable delay line 14, a fixed delay line 13, an optical attenuator 15, a waveguide type polarization splitting element 12, and a waveguide type polarization combining element 16.
And the respective configurations and functions will be described. FIG. 2 shows a configuration example of the variable delay line 14. FIG.
2 shows a configuration example of a fixed delay line 13 and an optical attenuator 15. FIG. 4 shows a configuration example of the waveguide type polarization separation element 12. FIGS. 2 to 4 each show a principle configuration, and do not directly extract each part of FIG.

(Configuration of Variable Delay Line 14) The variable delay line 14 shown in FIG. 2 has N (positive integers: N = 7 in this case) optical waveguide pairs 21a to 21g, and (N + 1) 2-inputs. It is composed of two output optical switches 22a to 22h. 2 input 2
The output optical switch 22a includes two optical couplers 23a and 24.
a and a Mach-Zehnder interferometer composed of two waveguide arms connecting them, and one (or both)
A thin-film heater 25a that operates as a TO phase shifter is formed on the waveguide arm. The same applies to the other two-input two-output optical switches 22b to 22h.

One input port of the first two-input two-output optical switch 22a is an input port D of the variable delay line 14, and an optical waveguide pair 21a is connected to two output ports. The other end of the pair of optical waveguides 21a is connected to two input ports of a second two-input / two-output optical switch 22b, and the two output ports are connected to the next pair of optical waveguides 21b. Similarly, two-input two-output optical switches 22a to 22h are connected before and after each pair of optical waveguide pairs 21a to 21g, and the short optical waveguide of each pair of optical waveguide pairs is set according to the setting of each two-input two-output optical switch. Alternatively, one of the long optical waveguides is selected. One output port of the eighth two-input two-output optical switch 22h is an output port G of the variable delay line 14.

Further, each set of optical waveguide pairs 21a~21g has become a pairs short optical waveguide and long optical waveguide, the optical path length difference therebetween, ΔD, 2ΔD, 4ΔD, 8ΔD , ..., 2 N-1 ΔD. The optical path length difference between the pairs of optical waveguides 21a to 21g is not limited to this order, and may be arranged in a different order. The actual variable delay line 14 shown in FIG. 1 has an arrangement in which the order of the optical path length differences is appropriately changed in order to reduce the size of the entire circuit.

The operation of the variable delay line 14 having such a configuration will be described. However, the two-input two-output optical switch 22
Indicates that the input / output port is in a “through state” when no power is applied to the thin-film heater 25 and is in a “cross state” when power is applied. The two-input two-output optical switch enters a “cross state” when power is not applied to the thin-film heater 25 and a “through state” when power is applied.
Some are.

All the two-input two-output optical switches 22a-
22h is set to the “through state”, the optical signal input to the input port D of the variable delay line 14
The light passes through the short optical waveguide of a to 21 g and reaches the output port G. The amount of delay in the entire variable delay line at this time is D ₀ . Next, the two-input two-output optical switches 22c, 22d, and 2
Assuming that 2f and 22h are in the “cross state”, the optical waveguide pair 2
The light passes through the long optical waveguides 1c, 21f, and 21g, and passes through the short optical waveguide to reach the output port G. At this time, the delay amount of the entire variable delay line is D ₀ + 4ΔD + 32ΔD + 64ΔD = D ₀ + 100ΔD. Thus, each two-input two-output optical switch 22a
To 22h in the “through state” or the “cross state”, the delay amount of the variable delay line
It can be realized from ₀ to D ₀ + (2 ^N −1) ΔD in ΔD steps.

In the above description, a two-input two-output optical switch using the thermo-optic effect of a silica-based optical waveguide is shown as an optical switch. This is because this combination can realize a stable optical switch having an excellent extinction ratio, but the present invention is not limited to this, and uses another optical switch such as a semiconductor optical switch or a micromechanical switch. You may.

(Configuration of Fixed Delay Line 13 and Optical Attenuator 15) The fixed delay line 13 shown in FIG.
4 are arranged along the short optical waveguide of each of the two-input two-output optical switches 22a to 22h and the respective optical waveguide pairs 21a to 21g. In this case, the entire optical delay amount of the fixed delay line 13 is the minimum delay amount D ₀ that the variable delay line 14 can provide. Therefore, in the variable delay line 14, the fixed delay line 13
Will be given a larger delay based on the amount of delay.

The fixed delay line 13 in the configuration of FIG.
Are arranged along either the short optical waveguide or the long optical waveguide of the variable delay line 14, the delay amount of the fixed delay line 13 is larger than the minimum delay amount D ₀ of the variable delay line 14. Therefore, in the variable delay line 14, the fixed delay line 13
, And a delay larger or smaller than that can be given. However, in the configuration of FIG. 1, the adjustment portion I is provided immediately before the output port G of the variable delay line 14 so that the delay amount of the fixed delay line 13 becomes equal to the minimum delay amount D ₀ of the variable delay line 14.
Is provided.

The optical attenuator 15 generates the same amount of loss as that of the variable delay line 14 due to an optical switch or the like.
And is connected to the output port E of the fixed delay line 13. Here, the optical attenuator 15 has a Mach-Zehnder interferometer configuration including two optical couplers 23i and 24i and two waveguide arms connecting them.
A thin-film heater 25i operating as a TO phase shifter is formed on one (or both) waveguide arms to make the attenuation variable. However, the optical attenuator 15 is not limited to this, and a semiconductor optical gate, a mechanical attenuator, or the like may be used.

(Configuration of Waveguide Polarization Separation Element 12 and Waveguide Polarization Combining Element 16) Waveguide Polarization Separation Element 12
And the waveguide-type polarization combining element 16 have a reversible configuration,
Hereinafter, the waveguide type polarization separation element 12 will be described. FIG.
The waveguide type polarization separation element 12 shown in FIG.
Optical couplers 31, 32, two arm waveguides 33a, 33b connecting them, and both arm waveguides 33a, 33b.
It is composed of thin film heaters 34a and 34b formed on 33b and an amorphous silicon thin film 35 as a stress applying film formed on one arm waveguide 33a. The amorphous silicon thin film 35 has an advantage that the amount of stress can be adjusted because the bond between amorphous silicon is broken by a method such as laser irradiation (so-called laser trimming) (reference: M. Okuno et al., "Birefringence cont."
rol of silica waveguideson Si and its application
to a polarization-beam splitter / switch ", IEEEJ.Lig
htwave Technol., Vol. 12, No. 4, pp. 625-633, 1994).

One input port of the optical coupler 31 is an input port A of the waveguide type polarization splitting element 12, and
2 are output ports B1 and B2 of the waveguide type polarization beam splitter 12. Amorphous silicon thin film 3
5, the birefringence of the arm waveguide 33a is adjusted, and the thin-film heaters 34a and 34b are adjusted. The phase rotation difference of the TM light in the pair of arm waveguides 33a and 33b is (2m + 1) π, 2mπ phase rotation difference
(M is an integer). At this time, among the optical signals input from the input port A, the TM light is output from the output port B.
1 and the TE light is output from the output port B2 and is polarization-separated from each other. Waveguide type polarization combining element 16
Has a configuration in which polarization is combined through a path opposite to that of the waveguide type polarization separation element 12 and output.

Here, a configuration using a Mach-Zehnder interferometer using an amorphous silicon thin film and a silica-based optical waveguide as the waveguide type polarization separation element has been described. However, the present invention is not limited to this. A Mach-Zehnder interferometer using a silica-based optical waveguide utilizing stress release, or a polarization separation element using a photonic crystal may be used.

(Operation of Polarization Dispersion Compensation Circuit) The operation of the polarization dispersion compensation circuit based on the above configuration will be described. The optical signal pulse propagated through the optical transmission line is separated into two by the polarization dispersion characteristic of the optical transmission line. Here, when the polarization dependent loss of the optical transmission line can be neglected, if the polarization state of one optical pulse is V1, the polarization state of the other optical pulse is given by V2 orthogonal to this.

The separated optical pulses are arranged such that the delayed one (here, V1) of the intrinsic polarization state of the optical transmission line becomes TM light and the other (here, V2) becomes TE light. The waveguide-type polarization splitting device 1 is adjusted by an external polarization controller and passes through the input channel optical waveguide 11 of FIG.
2 is input. Of the optical pulses input to the waveguide type polarization splitting element 12, the delayed optical pulse (TM light) is guided to the fixed delay line 13, and the advanced optical pulse (TE light) is guided to the variable delay line 14. .

As described above, by controlling the operation states of the two-input two-output optical switches (22a to 22h in FIG. 2) constituting the variable delay line 14, the delay amount set in the variable delay line 14 is changed. be able to. Therefore, the difference between the delay amount of the fixed delay line 13 and the delay amount of the variable delay line 14 is
If it is set so as to be substantially equal to the delay difference between the two separated optical pulses, the delay difference between the two optical pulses multiplexed by the waveguide-type polarization combining element 16 becomes almost 0, and the original optical pulse waveform becomes Will be restored.

Further, since the variable delay line 14 has a plurality of two-input two-output optical switches (22a to 22h in FIG. 2), the loss is generally larger than that of the fixed delay line 13. Therefore, the attenuation of the optical attenuator 15 connected to the fixed delay line 13 is adjusted to make the fixed delay line 13 and the variable delay line 14 equal in loss.

(Preparation of Polarization Dispersion Compensation Circuit) The polarization dispersion compensation circuit of the present invention was prepared using a silica-based optical waveguide. First, a SiO ₂ lower cladding layer was deposited on a silicon substrate by a flame deposition method, and then a SiO ₂ glass core layer to which GeO ₂ was added as a dopant was deposited. Next, the core layer was formed by etching the core layer using the pattern as shown in FIG. Subsequently, a thin film heater, electric wiring, and an amorphous silicon thin film were deposited on a predetermined optical waveguide. Finally, the optical waveguide was irradiated with a laser to release the residual stress. The variable delay line 14 is designed so that the unit delay amount is 1 psec, the number of stages is seven, and the delay amount can be changed from 0 to 127 psec.

FIG. 5 shows an example of characteristics when an optical pulse separated by polarization dispersion is input to the manufactured polarization dispersion compensation circuit. The original full width at half maximum of the light pulse is 100 psec. However, before input to the polarization dispersion compensating circuit, the two intrinsic polarization states of the optical transmission line are adjusted by the polarization controller to T
Convert to E light and TM light.

FIG. 5A shows an optical pulse waveform at the time of input to the polarization dispersion compensation circuit. The optical pulse is separated into two by the polarization dispersion of the optical transmission line. This separation amount is about 120 psec. FIG. 5B shows the polarization dispersion compensation circuit of the present invention.
It is an output light pulse waveform when the delay difference between the fixed delay line and the variable delay line is set to be 120 psec. For example, in the configuration of FIG.
The 2b, 22c, 22e, 22f, and 22g are set to the “through state”, and the two-input and two-output optical switches 22d and 22h are set to the “cross state”. As a result, waveform deterioration due to separation of optical pulses is compensated.

[0038]

As described above, according to the present invention, the main member can be constituted by the optical waveguide, so that a small and highly reliable polarization dispersion compensating circuit can be realized. By combining such a polarization dispersion compensating circuit of the present invention with a conventional optical fiber transmission line, high-speed optical signal transmission becomes possible, and communication quality can be improved and economy can be achieved.

[Brief description of the drawings]

FIG. 1 is a diagram showing an embodiment of a polarization dispersion compensation circuit of the present invention.

FIG. 2 is a diagram showing a configuration example of a variable delay line 14.

FIG. 3 is a diagram showing a configuration example of a fixed delay line 13 and an optical attenuator 15;

FIG. 4 is a diagram showing a configuration example of a waveguide type polarization separation element 12.

FIG. 5 is a diagram illustrating an example of characteristics of a polarization dispersion compensation circuit.

FIG. 6 is a diagram showing a configuration example of a conventional polarization dispersion compensation circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Silicon substrate 11 Input channel optical waveguide 12 Waveguide-type polarization splitter 13 Fixed delay line 14 Variable delay line 15 Optical attenuator 16 Waveguide-type polarization synthesizer 17 Output channel optical waveguide 21 Optical waveguide pair 22 2-input 2 output optical switch 23, 24 Optical coupler 25 Thin film heater 31, 32 Optical coupler 33 Arm waveguide 34 Thin film heater 35 Amorphous silicon thin film 101 Input port 102, 104 Polarizing prism 103 Mechanical movable mirror 105 Output port

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Hiroaki Yamada 3-19-2 Nishishinjuku, Shinjuku-ku, Tokyo Japan Telegraph and Telephone Corporation (72) Inventor Katsutoshi Okamoto 3-19 Nishishinjuku, Shinjuku-ku, Tokyo No. 2 Nippon Telegraph and Telephone Corporation F term (reference) 2H047 KB04 LA21 NA01 QA04 RA08 TA21 TA22 2H079 AA06 AA07 AA12 BA02 BA03 CA04 CA05 CA08 CA24 DA05 DA22 EA04 EB24 EB27

Claims (7)

[Claims] 1. A waveguide type polarization separation means for separating input light into two orthogonal polarization components and outputting the same to two output ports on a substrate, and a waveguide type polarization separation means. A pair of optical delay lines connected to an output port; and a waveguide-type polarization combining unit that receives two polarization components that have passed through the pair of optical delay lines, combines the polarization components, and outputs the resultant. At least one of the pair of optical delay lines has a means for varying the amount of delay, and adjusts the amount of delay of the two separated polarization components and inputs the adjusted amount to the waveguide type polarization combining means. Polarization dispersion compensation circuit. 2. The means for varying the delay amount of the optical delay line includes: (N + 1) 2-input 2-output optical switches and N sets of optical waveguides having different lengths, where N is a positive integer. Pairs are alternately connected, one input port of the first-stage two-input two-output optical switch is used as an input port of the optical delay line,
One output port of the two-input two-output optical switch is set as the output port of the optical delay line, and the input / output port of each two-input two-output optical switch is set to a through state or a cross state, whereby any one of the pairs of optical waveguides is set. 2. The polarization dispersion compensating circuit according to claim 1, wherein said one optical waveguide is selected to vary a delay amount. 3. The optical path length difference between the N sets of optical waveguide pairs is:
3. The polarization dispersion compensating circuit according to claim 2, wherein the ratio is a factor of 2 such as 1: 2: 4: 8:...: 2 ^N-1 . 4. The polarization dispersion compensation circuit according to claim 1, wherein an optical attenuator for adjusting the intensity of an optical signal is connected to at least one of said pair of optical delay lines. 5. The waveguide type polarization separating means and the waveguide type polarization combining means include two optical couplers having two inputs and two outputs, and two waveguide arms connecting them. 5. The polarization dispersion compensation circuit according to claim 1, wherein a means for adjusting birefringence is formed on said waveguide arm. 6. The polarization dispersion compensation circuit according to claim 5, wherein said means for adjusting birefringence is a stress applying film. 7. The polarization dispersion compensation circuit according to claim 1, wherein the optical waveguide constituting the polarization dispersion compensation circuit is a silica-based waveguide.