JP4405976B2 - Optical signal processor - Google Patents

Description

  The present invention relates to an optical signal processor that suppresses polarization dependence.

  Wavelength division multiplexing has become an important technology due to further demands for long-distance and large-capacity communications due to the explosive increase in data communications represented by the Internet. As an indispensable device for this wavelength division multiplexing communication system, there is a wavelength multiplexing / demultiplexing filter for multiplexing / demultiplexing optical signals according to wavelengths. For example, as a simple wavelength multiplexing / demultiplexing filter, there is one using a Mach-Zehnder interferometer (MZI) using an optical waveguide (see Non-Patent Document 1).

  However, in such a Mach-Zehnder interferometer, the optical waveguide has a birefringence, and two orthogonal polarizations, that is, TE polarization and TM polarization, have different effective refractive indexes. As a result, there is a problem in that the optical path length difference differs depending on the polarization, and polarization dependence occurs in the extinction wavelength.

  As a technique for eliminating such polarization dependence, there is a technique of inserting a half-wave plate in the center of an arm waveguide of a Mach-Zehnder interferometer (see Non-Patent Document 2). In this method, an optical signal propagates by one polarization (for example, TM polarization) for a half distance of the optical waveguide. Thereafter, the optical signal is polarization-converted from one polarization to the other by the half-wave plate. And an optical signal propagates the other half distance by the other polarization (for example, TE polarization). As a result, the optical signal has a constant optical path length in any polarization, and the polarization dependence in the optical waveguide can be eliminated.

K. Inoue et al., “A four-Channel Optical Waveguide Multi / Demultiplexer for 5-GHz Spaced Optical FDM Transmission,” JLT, Vol.6, No.2, Feb. 1988, pp.339-345 K. Takiguchi et al., “Planar lightwave circuit dispersion equalizer module with polarisation insensitive properties,” Electronics Letters, 5th January 1995, vol.31, No.1, pp.57

  However, the above method is based on the premise that the TE polarization is completely converted to TM polarization (or TM polarization to TE polarization) using a half-wave plate. Since the half-wave plate generally has an in-plane film thickness distribution, its polarization conversion efficiency (here, the rate of conversion from TE polarization to TM polarization (or TM polarization to TE polarization)) It is not necessarily “100%”. Also, the polarization conversion efficiency varies depending on the location. When such a wave plate is inserted into the arm waveguide of a Mach-Zehnder interferometer, a difference in polarization conversion efficiency occurs between the two arm waveguides, thereby limiting the reduction of the polarization dependence of the Mach-Zehnder interferometer. Is done.

  For example, if the frequency interval (period) of the extinction wavelength is defined as FSR (Frequency Spectral Range) and the maximum shift amount of the extinction wavelength depending on the polarization is defined as PDf (Polarization Dependent Frequency), the FSR is 10 GHz. With the above method, PDf is generated as much as 3 GHz. In order to avoid the deterioration of signal quality, this PDf is required to be 1/100 or less of the FSR, and it has been difficult to satisfy this specification with the conventional technology.

  The present invention has been made in view of such problems, and an object of the present invention is to provide an optical signal processor having a smaller polarization dependency with a simple configuration.

  In order to achieve such an object, the present invention provides two couplers, two waveguides connecting the two couplers, and the two waveguides. In the optical signal processor provided with the optical filter, the two waveguides are close to each other at the portion where the optical filter is provided.

  According to a second aspect of the present invention, in the optical signal processor according to the first aspect, the two waveguides are close to each other at a portion where the optical filter is provided.

  The invention according to claim 3 is the optical signal processor according to claim 1 or 2, wherein the two waveguides are close to 2000 μm or less at a portion where the optical filter is provided. And

  According to a fourth aspect of the present invention, in the optical signal processor according to any one of the first to third aspects, the optical filter is a half-wave plate corresponding to a half wavelength of an optical signal, and the half wavelength The plate is provided such that the optical principal axis forms an angle of 45 degrees with the waveguide substrate surface.

  According to a fifth aspect of the present invention, in the optical signal processor according to any one of the first to fourth aspects, the coupler is a multimode interferometer.

  According to a sixth aspect of the present invention, in the optical signal processor according to any one of the first to fourth aspects, the coupler is a directional coupler.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  In the following description, a silica-based optical waveguide formed on a silicon substrate will be described as an example of the optical waveguide. This is because this combination is suitable for producing an optical waveguide device which is stable and excellent in reliability. However, the present invention is not limited to this combination. On a substrate such as silicon, quartz glass, or soda glass, an optical waveguide such as a quartz optical waveguide or a silicon oxynitride film (SiON), or an organic system such as PMMA. Of course, an optical waveguide or a silicon optical waveguide may be used.

(Comparative example)
FIG. 1 shows a schematic diagram of a basic Mach-Zehnder interferometer. The MZI 100 includes two couplers 110a and 110b and two arm waveguides 120a and 120b that connect them. Here, the operation principle of the Mach-Zehnder interferometer and its polarization dependence will be described.

When the input A1 → output B1 is defined as a through path, and the input A1 → output B2 is defined as a cross path, their optical outputs (I _through , I _cross ) are expressed by the equations (1) and (2) according to a known interference principle. Can be described as follows.

Here, I ₀ is the light intensity of the input light, n is the effective refractive index, ΔL is the difference in length between the two arm waveguides, and λ is the wavelength used. The coupling rate of the coupler was 50%.

  In other words, the optical signal is periodically extinguished at the signal wavelength satisfying nΔL = λm in the through path and nΔL = λ (m + 1/2) (m is an integer) in the cross path and output to the other path. It means to function as a wavelength multiplexing / demultiplexing filter.

Examples of the method for manufacturing an optical waveguide used in such a device include the following. A core layer in which GeO ₂ is added to the lower under cladding layer mainly composed of SiO ₂ is deposited on the silicon substrate by using a flame volume method. Then, the core layer is patterned using reactive ion etching, and an overcladding layer is deposited again using a flame deposition method to produce a buried optical waveguide.

Usually, such an optical waveguide has birefringence due to the difference in thermal expansion coefficient between the shape of the core and the silicon substrate and quartz glass. That is, the TE polarization and TM polarization, which are two orthogonal polarizations, have different effective refractive indexes (n _TE , n _TM ). As a result, the optical path length difference (n _TE ΔL, n _TM ΔL) differs depending on the polarization, and the polarization dependency occurs in the extinction wavelength. This polarization dependency becomes a cause of generation of polarization dependent loss, and greatly degrades signal quality. Therefore, a method for eliminating the polarization dependence of the optical output is desired.

FIG. 2 shows an example of a conventional technique for eliminating the polarization dependence of the Mach-Zehnder interferometer (Non-Patent Document 2). The Mach-Zehnder interferometer circuit 200 includes a half-wave plate 230 corresponding to ½ of the used wavelength at the center of the two arm waveguides 120a and 120b. Is inserted at an angle of 45 degrees. As a result, the optical signal propagates at half the distance (ΔL / 2) of the arm waveguide by one polarization (for example, TM polarization), is polarized by the half-wave plate, and the other half distance ( ΔL / 2) is propagated by the other polarization (for example, TE polarization). Thus, the propagated optical signal has an optical path length difference of nΔL = (n _TE + n _TM ) ΔL / 2 in any polarization, and the polarization dependency of the output light can be eliminated.

  However, the above technique is based on the premise that the TE polarization is completely converted to TM polarization (or TM polarization to TE polarization) using a half-wave plate. However, since the half-wave plate generally has an in-plane film thickness distribution, its polarization conversion efficiency (the rate of conversion from TE polarization to TM polarization or from TM polarization to TE polarization) is not necessarily “100%. "is not. Also, the polarization conversion efficiency varies depending on the location. For example, FIG. 4 shows the result of measuring the location dependence of the polarization conversion efficiency for the polyimide wave plate 230 shown in FIG.

  It can be seen from FIG. 4 that the polarization conversion efficiency of the polyimide wave plate 230 varies depending on the location. When such a wave plate is inserted into the arm waveguide of a Mach-Zehnder interferometer, the polarization conversion efficiency differs between the two arm waveguides, and this affects the polarization dependency of the Mach-Zehnder interferometer. It is.

  For example, when the frequency interval (period) of the extinction wavelength is defined as FSR (Frequency Spectral Range) and the maximum shift amount of the extinction wavelength depending on the polarization is defined as PDf (Polarization Dependent Frequency), the MSR of FSR is 10 GHz. In the interferometer circuit, PDf of 3 GHz is generated in the configuration of FIG. In order to avoid degradation of signal quality, PDf is required to be 1 / 100th or less of FSR, and it is difficult to satisfy this specification with the conventional method.

Example 1
FIG. 5 is a schematic diagram of a Mach-Zehnder interferometer according to a first embodiment of the present invention. The MZI 500 is formed so as to divide two multi-mode interferometers 510a and 510b, two arm waveguides 520a and 520b sandwiched between the multi-mode interferometers, and the two arm waveguides. And a polyimide wave plate 530 inserted in the groove. The inserted polyimide wavelength plate 530 has an optical principal axis inclined at 45 degrees from the horizontal direction of the substrate, and gives a phase difference equivalent to a half wavelength of the signal light wavelength to the polarization propagating through the slow axis and the fast axis. As a result, it functions as a polarization mode converter that performs polarization conversion of TM polarization to TE polarization (or TE polarization to TM polarization). Further, since this MZI has an FSR of 10 GHz, the difference in length (ΔL) between the two arm waveguides 520a and 520b was set to 20.7 mm.

  The feature of this embodiment is that the two arm waveguides 520a and 520b approach gradually toward the half-wave plate, and the distance between the arm waveguides is made close at the insertion portion of the half-wave plate. That is, by making the distance between the two arm waveguides close to each other, it is possible to suppress the influence due to the in-plane dependence of the polarization conversion efficiency of the half-wave plate.

  Actually, when the distance between the two arm waveguides was 700 μm, PDf could be suppressed to 30 MHz in the vicinity of the wavelength of 1.55 μm. In the MZI 200 in the comparative example, the interval between the two arm waveguides that divide the wave plate was 10 mm, and the PDf was 3 GHz. Thus, the polarization dependency of MZI can be greatly improved by the present invention.

  FIG. 6 shows an example of the measurement result of PDf with respect to the interval of the arm waveguide in one embodiment of the present invention. From this figure, in order to make PDf 100 MHz (1 / 100th of FSR) or less, it is preferable to set the waveguide interval to 2000 μm or less. In this measurement result, the interval between the arm waveguides in the half-wave plate insertion portion is set to 700 μm, but it can be easily estimated that the PDf can be further reduced by further reducing the interval between the arm waveguides.

(Example 2)
FIG. 7 shows a schematic diagram of a Mach-Zehnder interferometer according to a second embodiment of the present invention. Similar to the first embodiment, the MZI 700 includes two multimode interferometers 710a and 710b, two arm waveguides 720a and 720b sandwiched between the multimode interferometers, and two arm waveguides. And a polyimide wave plate 730 inserted in a groove formed so as to divide. The present embodiment is characterized in that the couplers 710a and 710b are opposed to each other as compared with the MZI of the first embodiment (FIG. 5). Also in this MZI700, the same effect as in Example 1 was obtained by making the distance between the two arm waveguides close to each other. Specifically, when the arm waveguide interval was 700 μm, PDf could be suppressed to 50 MHz near the wavelength of 1.55 μm.

  Incidentally, since the two multi-mode interferometers 510a and 510b are not opposed to each other, the light emitted from the front-stage coupler 510a is not recombined with the rear-stage coupler 510b. A large extinction ratio is obtained. On the other hand, the merit of the configuration of FIG. 7 is that an MZI having a small difference in length between the two arm waveguides 720a and 720b enables a more compact configuration and is suitable for arraying.

  While the present invention has been described with respect to several embodiments, the embodiments described herein are merely illustrative in view of many possible forms to which the principles of the present invention can be applied. It is not intended to limit the scope of the invention. For example, in the above embodiment, a multimode interferometer is used as a coupler, but it can be easily analogized that the same effect can be obtained even if a directional coupler, an X-type branch circuit, or the like is used. Therefore, the configuration and details of the embodiment illustrated here can be changed without departing from the spirit of the present invention. Further, the illustrative components and procedures may be changed, supplemented, or changed in order without departing from the spirit of the invention.

It is the schematic for demonstrating a Mach-Zehnder interferometer. It is the schematic for demonstrating the conventional Mach-Zehnder interferometer which improved the polarization dependence. It is a figure for demonstrating the measurement location in measuring the place dependence of the polarization conversion efficiency of a half-wave plate. It is a figure which shows an example of the result of having measured the place dependence of the polarization conversion efficiency of a half-wave plate. It is the schematic for demonstrating the Mach-Zehnder interferometer by the 1st Example of this invention. It is a figure which shows an example of the measurement result of PDf with respect to the space | interval of the arm waveguide in 1st Example of this invention. It is the schematic for demonstrating the Mach-Zehnder interferometer by the 2nd Example of this invention.

Explanation of symbols

100 Mach-Zehnder interferometer 110a, 100b Coupler 120a, 120b Arm waveguide 200 Mach-Zehnder interferometer 230 Half-wave plate 500 Mach-Zehnder interferometer 510a, 510b Coupler 520a, 520b Arm waveguide 530 Half-wave plate 700 Mach-Zehnder interferometer 710a, 710b 720a, 720b Arm waveguide 730 Half wave plate

Claims (5)

In a Mach-Zehnder interferometer comprising two couplers, two waveguides connecting the two couplers, and a half-wave plate provided in the two waveguides,
The half-wave plate is provided such that the optical principal axis forms an angle of 45 degrees with the waveguide substrate surface,
The Mach-Zehnder interferometer, wherein the two waveguides are close to each other at a portion where the half-wave plate is provided. The Mach-Zehnder interferometer according to claim 1,
The Mach-Zehnder interferometer, wherein the two waveguides are close to each other at a portion where the half-wave plate is provided. The Mach-Zehnder interferometer according to claim 1 or 2,
The Mach-Zehnder interferometer, wherein the two waveguides are close to 2000 μm or less at a portion where the half-wave plate is provided. In the Mach-Zehnder interferometer according to any one of claims 1 to 3 ,
The Mach-Zehnder interferometer , wherein the coupler is a multimode interferometer. In the Mach-Zehnder interferometer according to any one of claims 1 to 3 ,
The Mach-Zehnder interferometer , wherein the coupler is a directional coupler.

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