JP3608983B2 - Polarization dispersion compensation circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a polarization dispersion compensation circuit that shapes optical signal distortion caused by polarization dispersion in an optical transmission line (optical fiber).
[0002]
Here, polarization mode dispersion (PMD) of the optical fiber is a phenomenon in which the propagation speed changes depending on the polarization state of the optical signal due to the birefringence of the optical fiber, and this is a factor of optical signal distortion. It has become one of
[0003]
[Prior art]
With the development of optical communication, there is an increasing demand for transmitting high-speed optical signals to existing optical fibers. However, since existing optical fibers have relatively high polarization dispersion, optical signal distortion when a high-speed optical signal is propagated becomes a serious problem.
[0004]
As a polarization dispersion compensation circuit for solving this problem, a circuit using two polarization prisms (polarization beam splitter) and a mechanical movable mirror is known (reference: F. Heismann et al., “AUTOMATIC”). COMPENSATION OF FIRST-ORSER POLARIZATION MODE DISPERSION IN A 10 Gb / s TRANSMISSION SYSTEM ", WdC11, ECOC '98, 1998).
[0005]
FIG. 6 shows a configuration example of a conventional polarization dispersion compensation circuit described in the above-mentioned document. In the figure, the optical signal input from the input port 101 is input to the first polarizing prism 102. Here, the TM light delayed in the optical transmission path is output to a port that travels straight, and the advanced TE light is output to a port that is orthogonal to perform polarization separation. The TE light is input to the second polarizing prism 104 via the mechanical movable mirror 103 including two reflecting mirrors, and is combined with the TM light that is directly input and is output to the output port 105 after being subjected to polarization synthesis. By moving the mechanical movable mirror 103, the optical path length in which TE light propagates spatially changes, and the amount of delay between the TE light and TM light is adjusted to perform polarization dispersion compensation.
[0006]
[Problems to be solved by the invention]
However, the conventional polarization dispersion compensation circuit has the following problems.
First, since the mechanical movable mirror 103 is provided, it is not reliable. Polarization dispersion has a characteristic that fluctuates greatly in time. However, in the conventional polarization dispersion compensation circuit, it is necessary to move the mechanical movable mirror 103 in order to follow the fluctuation of polarization dispersion, and long-term stability is improved. It becomes a problem.
[0007]
Secondly, since the optical system is spatially configured, 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.
[0008]
Third, since the conventional polarization dispersion compensation circuit has a configuration in which individual parts are combined and a configuration using a movable member including the mechanical movable mirror 103, there is a limit to downsizing.
[0009]
An object of the present invention is to provide a waveguide-type polarization dispersion compensation circuit that is small and excellent in reliability without using a movable member.
[0010]
[Means for Solving the Problems]
The polarization dispersion compensation circuit of the present invention includes a waveguide-type polarization separation unit that separates input light into two orthogonal polarization components and outputs them to two output ports on the substrate, and a waveguide-type polarization separation. A pair of optical delay lines connected to the two output ports of the means, and a waveguide-type polarization combining means for inputting two polarization components that have passed through the pair of optical delay lines, and combining and outputting the polarization components And a means for varying the delay amount in at least one of the pair of optical delay lines, and adjusting the delay amounts of the two separated polarization components and inputting them to the waveguide 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.
[0011]
The means for varying the delay amount of the optical delay line is such that when N is a positive integer, (N + 1) 2-input 2-output optical switches and N pairs of optical waveguides having different optical path lengths are alternately connected. One input port of the first-stage 2-input 2-output optical switch is used as an input port for the optical delay line, and one output port of the 2-stage 2-input optical switch at the last stage is used as an output port for the optical delay line. By setting the input / output ports of the two-output optical switch to the through state or the cross state, one of the optical waveguide pairs in each pair is selected to vary the delay amount.
[0012]
Further, the optical path length differences of the N optical waveguide pairs forming the variable delay line are configured to have a factorial ratio of 2 such as 1: 2: 4: 8:...: 2 ^N−1. Thus, a delay amount that is 1 to 2 ^N -1 times the unit delay amount can be freely selected in N stages.
[0013]
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 the loss of a pair of optical delay lines (for example, a fixed delay line and a variable delay line) to be the same.
[0014]
The waveguide-type polarization separating means and the waveguide-type polarization combining means comprise two optical couplers with two inputs and two outputs, and two waveguide arms that connect them. It is the structure which formed the means to adjust refraction. The means for adjusting the birefringence may be formed of a stress applying film.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an embodiment of a polarization dispersion compensation circuit of the present invention. Here, an example in which a silica-based optical waveguide formed on a silicon substrate is used will be described. This is because the silica-based optical waveguide can constitute a stable optical circuit excellent in matching with an optical fiber, but is not limited to this, for example, other optical waveguides such as a semiconductor optical waveguide and a photonic crystal. May be used.
[0016]
In FIG. 1, on a silicon substrate 10, an input channel optical waveguide 11, a waveguide-type polarization separation element 12, a fixed delay line 13 and a variable delay line 14 constituting a pair of optical delay lines, an optical attenuator 15, A waveguide-type polarization beam combiner 16 and an output channel optical waveguide 17 are sequentially arranged. The variable delay line 14 is configured to be able to set an arbitrary delay amount by combining several parts for selecting either a short path or a long path.
[0017]
The input port A of the waveguide type polarization separation element 12 is optically connected to the input channel optical waveguide 11. The input port C of the fixed delay line 13 is optically connected to the first output port B1 of the waveguide type polarization separation element 12, and the input port D of the variable delay line 14 is optically connected to the second output port B2. Connected. The first input port F 1 of the waveguide polarization synthesizer 16 is optically connected to the output port E of the fixed delay line 13 via the 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 beam combiner 16. An output channel optical waveguide 17 is optically connected to the output port H of the waveguide type polarization separation element 16.
[0018]
Here, the polarization dispersion compensation circuit of the present invention is divided into a variable delay line 14, a fixed delay line 13 and an optical attenuator 15, a waveguide polarization separation element 12, and a waveguide polarization synthesis element 16, and The configuration and function will be described. FIG. 2 shows a configuration example of the variable delay line 14. FIG. 3 shows a configuration example of the fixed delay line 13 and the optical attenuator 15. FIG. 4 shows a configuration example of the waveguide type polarization separation element 12. 2 to 4 each show a basic configuration, and each part of FIG. 1 is not extracted as it is.
[0019]
(Configuration of variable delay line 14)
The variable delay line 14 shown in FIG. 2 includes N (positive integer: N = 7 here) optical waveguide pairs 21a to 21g and (N + 1) two-input two-output optical switches 22a to 22h. . The 2-input 2-output optical switch 22a has a Mach-Zehnder interferometer configuration including two optical couplers 23a, 24a and two waveguide arms connecting the optical couplers 23a, 24a. A thin film heater 25a that operates as a phase shifter is formed. The same applies to the other 2-input 2-output optical switches 22b to 22h.
[0020]
One input port of the first 2-input 2-output optical switch 22a is the input port D of the variable delay line 14, and the optical waveguide pair 21a is connected to the two output ports. Two input ports of the second 2-input 2-output optical switch 22b are connected to the other end of the optical waveguide pair 21a, and the next optical waveguide pair 21b is connected to the two output ports. Similarly, two-input two-output optical switches 22a-22h are connected before and after each pair of optical waveguide pairs 21a-21g, and the short optical waveguide of each pair of optical waveguides 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 2-input 2-output optical switch 22 h is the output port G of the variable delay line 14.
[0021]
Each pair of optical waveguides 21a to 21g is a pair of a short optical waveguide and a long optical waveguide, and the optical path length difference between them is
ΔD, 2ΔD, 4ΔD, 8ΔD, ..., 2 ^N-1 ΔD
It is set to be either. The optical path length difference between each pair of optical waveguides 21a to 21g is not limited to this order, and the order may be changed. 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.
[0022]
The operation of the variable delay line 14 having such a configuration will be described. However, the input / output port of the 2-input 2-output optical switch 22 is “through state” when no power is applied to the thin film heater 25 and “cross state” when power is applied. Some 2-input 2-output optical switches are in a “cross state” when power is not applied to the thin film heater 25 and are in a “through state” when power is applied.
[0023]
When all the 2-input 2-output optical switches 22a to 22h are set to the "through state", the optical signal input to the input port D of the variable delay line 14 passes through the short optical waveguides of the optical waveguide pairs 21a to 21g of each set. To output port G. The delay amount of the entire variable delay line at this time is D _0. Next, when the 2-input 2-output optical switches 22c, 22d, 22f, and 22h are set to the “cross state”, the light passes through the long optical waveguides of the optical waveguide pairs 21c, 21f, and 21g, and the others pass through the short optical waveguides for output. To port G. The delay amount of the entire variable delay line at this time is
D ₀ + 4ΔD + 32ΔD + 64ΔD = D ₀ + 100ΔD
It becomes. In this way, by switching the respective two inputs and two output optical switch 22a~22h the "through state" or the "cross state", _D 0 ⁺ a delay amount of the variable delay line 14 from the generally _{D 0} ⁽² N -1) It can be realized in increments of ΔD up to ΔD.
[0024]
In the above description, a 2-input 2-output optical switch using the thermo-optic effect of a silica-based optical waveguide is shown as the optical switch. This is because this combination can realize a stable optical switch with an excellent extinction ratio, but the present invention is not limited to this, and other optical switches such as a semiconductor optical switch and a micromechanical switch are used. May be.
[0025]
(Configuration of Fixed Delay Line 13 and Optical Attenuator 15)
The fixed delay line 13 shown in FIG. 3 is disposed along the short optical waveguides of the two-input two-output optical switches 22a to 22h and the optical waveguide pairs 21a to 21g of the variable delay line 14 of FIG. In this case, the total optical delay amount of the fixed delay line 13 becomes the minimum delay amount D ₀ that the variable delay line 14 can provide. Therefore, the variable delay line 14 gives a larger delay with reference to the delay amount of the fixed delay line 13.
[0026]
Since the fixed delay line 13 in the configuration of FIG. 1 is 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 the minimum of the variable delay line 14. It becomes larger than the amount of delay _{D 0.} Therefore, the variable delay line 14 can give a larger or smaller delay based on the delay amount of the fixed delay line 13. 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 is equal to the minimum delay amount D ₀ of the variable delay line 14. Yes.
[0027]
The optical attenuator 15 gives the fixed delay line 13 the same loss as that caused by the optical switch of the variable delay line 14 and is connected to the output port E of the fixed delay line 13. Here, the optical attenuator 15 has a Mach-Zehnder type interferometer configuration including two optical couplers 23i and 24i and two waveguide arms connecting them, and TO (or both) TO arms are placed on one (or both) waveguide arms. A thin film heater 25i that operates as a phase shifter is formed to make the attenuation variable. However, the optical attenuator 15 is not limited to this, and a semiconductor optical gate or a mechanical attenuator may be used.
[0028]
(Configuration of Waveguide Type Polarization Separation Element 12 and Waveguide Type Polarization Combining Element 16)
Since the waveguide-type polarization separation element 12 and the waveguide-type polarization combination element 16 have reversible configurations, the waveguide-type polarization separation element 12 will be described below. The waveguide type polarization separation element 12 shown in FIG. 4 includes two optical couplers 31 and 32 having two inputs and two outputs, two arm waveguides 33a and 33b for connecting them, and both arm waveguides 33a and 33b. The thin film heaters 34a and 34b formed on the upper surface and the amorphous silicon thin film 35 as a stress applying film formed on the 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 the amorphous silicons is broken by a technique such as laser irradiation (so-called laser trimming) (reference: M. Okuno et al., “Birefrenceence control of silicon”). waveguides on Si and its application to a polarization-beam splitter / switch ", IEEE J. Lightwave Technology., Vol. 12, No. 4, pp. 625-633, 19).
[0029]
One input port of the optical coupler 31 becomes the input port A of the waveguide type polarization separation element 12, and two output ports of the optical coupler 32 become the output ports B 1 and B 2 of the waveguide type polarization separation element 12. The birefringence of the arm waveguide 33a is adjusted by stress induction by the amorphous silicon thin film 35, the thin film heaters 34a and 34b are adjusted, and the phase rotation difference of the TM light in the pair of arm waveguides 33a and 33b is (2m + 1) π, The phase rotation difference of TE light is set to 2 mπ (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 B1, and the TE light is output from the output port B2, and is polarized and separated from each other. The waveguide polarization combining element 16 is configured to combine the polarization through a path opposite to the waveguide polarization separating element 12 and output the result.
[0030]
In this example, a configuration using a Mach-Zehnder interferometer with an amorphous silicon thin film and a silica-based optical waveguide is shown as the waveguide-type polarization separation element. However, the configuration is not limited to this, and for example, stress release by a groove is performed. A Mach-Zehnder interferometer using a silica-based optical waveguide or a polarization separation element using a photonic crystal may be used.
[0031]
(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 ignored, if the polarization state of one optical pulse is V1, the polarization state of the other optical pulse is given by V2 orthogonal thereto.
[0032]
The separated optical pulse has a polarization that is not shown in the figure so that the delayed one of the intrinsic polarization state of the optical transmission line (here, V1) is TM light and the other (here, V2) is TE light. It is adjusted by the wave controller, and is input to the waveguide type polarization separation element 12 via the input channel optical waveguide 11 of FIG. Of the optical pulses input to the waveguide polarization separator 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. .
[0033]
As described above, the delay amount set in the variable delay line 14 can be changed by controlling the operation state of the two-input two-output optical switches (22a to 22h in FIG. 2) constituting the variable delay line 14. . Therefore, if the difference between the delay amount of the fixed delay line 13 and the delay amount of the variable delay line 14 is set to be approximately equal to the delay difference between the two separated optical pulses, the waveguide-type polarization beam combiner 16 can combine them. The delay difference between the two optical pulses to be waved is almost zero, and the original optical pulse waveform is restored.
[0034]
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 amount of the optical attenuator 15 connected to the fixed delay line 13 is adjusted so that the losses of the fixed delay line 13 and the variable delay line 14 are the same.
[0035]
(Production of polarization dispersion compensation circuit)
The polarization dispersion compensation circuit of the present invention was manufactured 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, followed by transparent vitrification in an electric furnace. Next, the core layer was fabricated by etching the core layer using the pattern as shown in FIG. Subsequently, a thin film heater, electrical wiring, and an amorphous silicon thin film were deposited on a predetermined optical waveguide. Finally, the residual stress was released by irradiating the optical waveguide with laser. The variable delay line 14 is designed so that the unit delay amount is 1 psec, the number of stages is 7, and the delay amount can be changed from 0 to 127 psec.
[0036]
FIG. 5 shows an example of characteristics when an optical pulse separated by polarization dispersion is input to the produced polarization dispersion compensation circuit. The original full width at half maximum of the optical pulse is 100 psec. However, before the input to the polarization dispersion compensation circuit, the two intrinsic polarization states of the optical transmission line are converted into TE light and TM light by adjusting with the polarization controller.
[0037]
FIG. 5A shows an optical pulse waveform at the time of input to the polarization dispersion compensation circuit, and the optical pulse is separated into two by the polarization dispersion of the optical transmission line. The amount of separation is about 120 psec. FIG. 5B shows an output optical pulse waveform when the delay difference between the fixed delay line and the variable delay line is set to 120 psec by the polarization dispersion compensation circuit of the present invention. For example, in the configuration of FIG. 2, the 2-input 2-output optical switches 22a, 22b, 22c, 22e, 22f, 22g are set to the “through state”, and the 2-input 2-output optical switches 22d, 22h are set to the “cross state”. Thereby, the waveform deterioration due to the separation of the optical pulse is compensated.
[0038]
【The invention's effect】
As described above, according to the present invention, since the main member can be constituted by the optical waveguide, it is possible to realize a polarization dispersion compensation circuit that is small in size and excellent in reliability. By combining such a polarization dispersion compensation circuit of the present invention and a conventional optical fiber transmission line, high-speed optical signal transmission is 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 illustrating a configuration example of a variable delay line 14;
FIG. 3 is a diagram illustrating a configuration example of a fixed delay line 13 and an optical attenuator 15;
FIG. 4 is a diagram illustrating a configuration example of a waveguide polarization separating element 12;
FIG. 5 is a diagram illustrating an example of characteristics of a polarization dispersion compensation circuit.
FIG. 6 is a diagram illustrating 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 separation element 13 Fixed delay line 14 Variable delay line 15 Optical attenuator 16 Waveguide-type polarization combiner 17 Output channel optical waveguide 21 Optical waveguide pair 22 2 inputs 2-output optical switches 23 and 24 Optical coupler 25 Thin film heaters 31 and 32 Optical coupler 33 Arm waveguide 34 Thin film heater 35 Amorphous silicon thin film 101 Input ports 102 and 104 Polarizing prism 103 Mechanical movable mirror 105 Output port

Claims (6)

On the board
Waveguide-type polarization separation means for separating input light into two orthogonal polarization components and outputting them to two output ports;
A pair of optical delay lines connected to the two output ports of the waveguide polarization separator;
Waveguide-type polarization beam combining means for inputting the two polarization components that have passed through the pair of optical delay lines, and combining and outputting the polarization components;
And means for varying the delay amount to at least one of the pair of optical delay lines, Ri configuration der to adjust the delay amounts of the two polarization components separated input into the waveguide-type polarization combining means,
The means for varying the delay amount of the optical delay line is such that when N is a positive integer, (N + 1) 2-input 2-output optical switches and N pairs of optical waveguides having different lengths are alternately arranged. One input port of the first-stage 2-input 2-output optical switch is used as an input port of the optical delay line, and one output port of the 2-input 2-output optical switch of the last stage is used as an output port of the optical delay line. wherein configuration der Rukoto which selects either one of the optical waveguides of each pair of optical waveguide pairs varying the delay amount by setting the input and output ports of the input 2-output optical switches into the through state or the cross state Polarization dispersion compensation circuit. 2. The optical path length difference of the N optical waveguide pairs is a factorial ratio of 2 such as 1: 2: 4: 8:...: 2 ^N−1. The polarization dispersion compensation circuit described. 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 the pair of optical delay lines. The waveguide-type polarization separation unit and the waveguide-type polarization combination unit each include two optical couplers with two inputs and two outputs, and two waveguide arms that connect the two optical couplers. The polarization dispersion compensation circuit according to claim 1, wherein means for adjusting birefringence is formed on the polarization dispersion compensation circuit. The polarization dispersion compensation circuit according to claim 4, wherein the means for adjusting the birefringence is a stress applying film. 6. The polarization dispersion compensating circuit according to claim 1, wherein the optical waveguide constituting the polarization dispersion compensating circuit is a silica-based waveguide.

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