JPH05313109A - Waveguide type polarization controller - Google Patents

Abstract

PURPOSE:To reduce work for realizing polarization independence for every individual waveguide circuit. CONSTITUTION:The waveguide type polarization controller consists of an optical waveguide type polarization beam splitter 11 which converts light in an optional polarized wave state into linearly polarized waves having mutually orthogonal planes of polarization, a polarization mode converter 12 which has an amorphous silicon stress imparting film 24 for polarization mode conversion to one of two optical waveguides 22 and 23 coupled with two output optical waveguides of the polarization beam splitter 11, i.e., optical waveguide 22 and converts the linear polarized wave propagated in the optical waveguide 22 into the other orthogonal linear polarized wave, an optical phase shifter 13 which is provided on the optical waveguide 23 and controls its optical path length, and a tunable coupler 14 which multiplexes the output light beams of the two optical waveguides 22 and 23 with each other at an optional coupling rate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a waveguide type polarization controller used in the field of optical communication or optical signal processing.

[0002]

2. Description of the Related Art In order to further popularize optical fiber communication, in addition to high performance and low cost of receiving and emitting elements, various optical integration such as optical branching / coupling devices, optical multiplexer / demultiplexers, and optical switches. Development of circuit parts is essential. And silicon (S
i) The glass optical waveguide circuit formed on the substrate has small optical loss and is expected as a practical optical integrated circuit component.

However, this glass optical waveguide circuit
Since it has birefringence due to thermal stress generated in the manufacturing process, it has a problem of polarization dependency. That is, in the glass optical waveguide circuit on the Si substrate, thermal stress is generated due to the difference in thermal expansion coefficient between the Si substrate and the glass in the cooling process after the glass is heat-treated, so that stress birefringence occurs in the optical waveguide. .. This stress birefringence is 3 × 10 ^-4
It is about the same value as that of the polarization maintaining fiber on the market. Here, the birefringence is defined as the difference between the effective refractive index n _{TM of TM} light and the effective refractive index n _TE of TE light (TE light is light having an electric field component parallel to the substrate surface, TM light is light having an electric field component perpendicular to the substrate surface). That is, the TE component and the TM component of the incident light of the glass optical waveguide circuit propagate individually without interfering with each other in the circuit. Moreover, in the asymmetric Mach-Zehnder interferometer, since the arm waveguide has birefringence, the optical path length difference between the TE light and the TM light seems to be slightly different. Therefore,
In the same optical circuit, a difference occurs between the TE light transmission characteristic and the TM light transmission characteristic, and as a result, there is a problem that the transmission characteristic of the optical circuit changes depending on the polarization state of the incident light.

As a method of eliminating the polarization dependence of the glass optical waveguide circuit, Japanese Patent Laid-Open No. 01-77002 proposes a method of laser trimming the stress imparting film. This is a method of controlling the stress birefringence of the optical waveguide by trimming the stress imparting film and eliminating the polarization dependence of the optical circuit.

[0005]

However, as the optical loss of the waveguide is greatly reduced and the scale of the optical circuit is increased,
Laser trimming for polarization independence for each individual circuit has become a major part of the time required to fabricate an optical waveguide circuit. For example, FIG. 5 shows a 16-wave optical frequency multiplexer / demultiplexer in which 15 asymmetric Mach-Zehnder interferometers are integrated. As shown in the figure, the optical frequency multiplexer / demultiplexer 1
00 is 15 asymmetric Mach-Zehnder interferometers 101 to 101.
It has 115. In principle, it is possible to eliminate the polarization dependence of these 15 asymmetric Mach-Zehnder interferometers 101 to 115 individually by laser trimming of the stress applying film 116 as described above. However, the labor required therefor is enormous, which is a major factor that hinders the manufacture of such optical integrated circuit components.

In view of such circumstances, the present invention provides a waveguide type polarization controller capable of reducing the laser trimming work steps for making the polarization independent of each individual circuit of the optical waveguide circuit. The purpose is to do.

[0007]

[Means for Solving the Problems] The present invention for achieving the above object
The waveguide-type polarization controller according to Ming
Waveguide and optical phase shifter for controlling optical path length of the optical waveguide
And a stress applying film that controls the birefringence of the optical waveguide.
The two output optical waveguides have polarization planes orthogonal to each other.
Optical waveguide type polarization beam splitter that outputs
And two outputs of this optical waveguide type polarization beam splitter.
Has two optical waveguides that are coupled to the optical waveguide.
Other linear polarizations orthogonal to the linearly polarized wave propagating in one optical waveguide
Polarization mode converter for converting into a wave, and the two optical waveguides
The optical path length of the optical waveguide is provided on at least one of
It controls the optical phase shifter and the light of the two optical waveguides.
It consists of a tunable coupler that couples at the desired coupling rate.
It is characterized by

[0008]

When light enters the waveguide type polarization controller having the above structure, it is split into two linearly polarized waves (TE light and TM light) orthogonal to each other by the optical waveguide type polarization beam splitter, and polarization mode conversion is performed. Is guided to the other side and one of the linearly polarized waves is converted into another linearly polarized wave which is orthogonal to each other. Then, the two linearly polarized waves (TE light or TM light having the same plane of polarization) are combined by a tunable coupler. And output. That is, in the waveguide type polarization controller, the output can always be controlled to the linear polarization of either TE light or TM light.

By integrating such a waveguide type polarization controller with another optical waveguide circuit, either the TE light or the TM light is provided in the optical waveguide circuit even if the optical waveguide circuit has polarization dependence. Since only the linearly polarized wave of is incident, there is virtually no problem. In other words, conventionally, it was necessary to individually perform laser trimming on multiple optical waveguide circuits integrated on the same substrate in order to make them polarization independent. It becomes unnecessary by integrating.

[0010]

EXAMPLES The present invention will be described below based on examples.

FIG. 1 is a plan view of a waveguide type polarization controller according to an embodiment, and FIGS. 2 to 4 show its II-II line and III-III.
The line and the IV-IV line are enlarged cross-sectional views.

As shown in these drawings, this waveguide type polarization controller includes a waveguide type polarization beam splitter 11 and
It includes a polarization mode converter 12, a thermo-optical phase shifter 13, and a tunable coupler 14. The waveguide type polarization beam splitter 11 is a waveguide in which directional couplers 18 and 19 having a coupling rate of 50% are arranged at two positions of two single mode glass optical waveguides 16 and 17 formed on a silicon substrate 15. Type Mach-Zehnder interferometer, and an amorphous silicon stress imparting film 20 for controlling waveguide birefringence and a thermo-optic phase shifter 21 are formed on the arms of the optical waveguides 16 and 17, respectively. The optical waveguides 16 and 1
One end of 7 serves as light input / output ports 16a and 17a.

Optical waveguides 22 and 23 forming the polarization mode converter 12 are continuously formed at the other ends of the optical waveguides 16 and 17, respectively. An amorphous silicon stress imparting film 24 for polarization mode conversion is provided above one optical waveguide 22 asymmetrically in the width direction. An optical waveguide 26 is coupled to the optical waveguide 21 via a directional coupler 25 having a coupling rate of 1%. Both ends of the optical waveguide 26 are light input / output ports 26a and 26b. A thermo-optical phase shifter 13 is provided above one of the optical waveguides 23, which is used to operate the tunable coupler 13 normally.

The tunable coupler 14 is composed of two optical waveguides 2 formed continuously with the optical waveguides 16 and 17.
Directional couplers 29 with a coupling rate of 50% at two locations 7, 28,
It consists of a waveguide type Mach-Zehnder interferometer provided with 30,
A thermo-optic phase shifter 31 is provided above the optical waveguide 30, which is one of the arms. Then, the optical waveguide 27,
One end of 28 serves as light input / output ports 27a and 28a.

Next, each structure will be described in more detail while showing a manufacturing method. The method of manufacturing the waveguide type polarization beam splitter 11 is described in detail in Japanese Patent Laid-Open No. 64-77002, and the characteristics thereof are described in detail in the Institute of Electronics, Information and Communication Engineers Autumn 1990 National University C-215. When manufacturing the waveguide type polarization beam splitter 11, the embedded single mode glass optical waveguides 16 and 17 are formed on the silicon substrate 15 by the fire deposition method, the photolithography and the reactive ion etching method. The core glass is doped with Ge so that the refractive index is 0.75% higher than that of the surrounding clad glass. Further, the amorphous silicon stress imparting film 20 and the thermo-optical phase shifter 21 are partially deposited on the surface of the clad glass on the optical waveguides 16 and 17. Here, the amorphous silicon stress imparting film 20 is formed by a sputtering method and a reactive ion etching method, and the thermo-optical phase shifter 21 is formed by a vapor deposition method and a lift-off method.
A membrane was used. The phase shift by the thermo-optical phase shifter 21 is performed by supplying appropriate power to the thermo-optical phase shifter 21.

The adjustment of the stress applying film 20 of the waveguide type polarization beam splitter 11 by trimming is performed as follows. That is, the optical path length difference between the two arms of the Mach-Zehnder interferometer is, for example, N · λ (N is an integer, λ is an optical wavelength) for TE light and (N + 1/2) · for TM light.
The value is set to λ using the stress applying film 20 and the thermo-optical phase shifter 21. At this time, the TE light component of the light incident from the optical waveguide 17 is output to the waveguide 22, and the TM light component is output to the waveguide 23. Then, the stress applying film 20 of the polarization beam splitter 11 is trimmed by allowing the light incident from the input / output port 26b of the optical waveguide 26 for monitoring to pass through the directional coupler 25 and the optical waveguide 22. This is performed by making the light incident on 11 and detecting the transmitted light through the optical waveguide 16 or 17.

In the polarization mode converter 12, the amorphous silicon stress imparting film 23 is provided asymmetrically in the width direction of the optical waveguide 21, so that the stress principal axis of the optical waveguide 22 is tilted at 45 degrees with respect to the substrate surface, and TE , TM polarization mode conversion is performed. This polarization mode converter 12 is disclosed in Japanese Patent Laid-Open No. 63-
This is an application of the waveguide type optical phase plate described in Japanese Patent No. 147114. In the waveguide type optical phase plate disclosed in Japanese Patent Laid-Open No. 63-147114, the stress releasing groove is used to incline the stress principal axis of the optical waveguide, but the stress releasing groove can be used in the present invention. However, in terms of easy fine adjustment, a polarization mode converter using a stress applying film can realize a higher performance.

Fine adjustment in the polarization mode converter 12 is performed as follows. First, with the coupling rate of the tunable coupler 14 set to 0%, the TE or TM linearly polarized wave is incident on the optical waveguide 26 for monitoring from the optical input / output port 26b.
This linearly polarized light enters the optical waveguide 22 of the polarization mode converter 12, passes under the stress applying film 24, and the polarization-mode-converted light passes through the directional coupler 25 to the optical input / output port 26 a. Is output from. By observing the polarization state of this output light, it can be seen whether the polarization has been completely converted. Actually, the stress applying film 24 is additionally provided, and laser trimming is gradually performed to set the state in which the polarization mode is completely converted. In this embodiment, when TE light is incident on the polarization mode converter, this light is converted to TM mode and output from the optical input / output port 26a of the optical waveguide 26.

Next, the control of the coupling rate of the tunable coupler 14 will be described. The thermo-optical phase shifter 13,
The configuration of 31 is similar to that of the thermo-optical phase shifter 20.

Mach-Zehnder interferometer tunable coupler 14 using two 3 dB directional couplers 29, 30.
The transfer matrix of is shown in Equation 1. However, α is the amount of phase shift by the thermo-optical phase shifter 31.

[0021]

[Equation 1] The light incident from one input port is output to the two output ports in a state where the phase change and the amplitude change shown in Formula 2 are received. However, ω is the optical angular frequency of the signal light.

[0022]

[Equation 2]

It can be seen from the following expression 3 that the relative phase difference between the two output lights is zero.

[0024]

[Equation 3]

Further, it can be seen from the following formula 4 that the optical power ratio of two outputs changes in the range from infinity to 0 with the phase shift amount α from 0 to π by the thermo-optical phase shifter 31. Further, FIG. 5 shows the phase shift amount α by the thermo-optic phase shifter 31 and the ratio of two optical output powers.

[0026]

[Equation 4]

Since the tunable coupler 14 is a reciprocal element, it is possible to split the light incident from one port into two or conversely combine the light incident from two ports into one. Is. However, when one light is split into two, the relative phase difference between the two lights is 0. Therefore, when the two lights are combined, the phases of the two incident lights are opposite to each other. Need to be equal. In order to realize this, the thermo-optical phase shifter 13 gives a phase shift amount β so that the relative phase difference between the two lights becomes zero.
The phase shift amount β needs to be controlled so as to compensate for the phase difference between the TE light and the TM light incident on the optical input / output port 16a. Further, the phase shifter amount α by the thermo-optical phase shifter 31 needs to be determined by the power ratio of the TE component and the TM component incident on the light input / output port 16a.
For this, for example, the output light from the optical input / output port 28a may be detected by the photodetector, and the phase shift amount α may be determined so that the output is minimized.

In the embodiment described above, the waveguide type polarization beam splitter 11, the polarization mode converter 12, the thermo-optical phase shifter 13, and the tunable coupler 14 are formed on the same substrate, so that the loss is low. A polarization controller is realized.

As an example of the polarization controller of the above embodiment, T
Supply power and output light to the thermo-optical phase shifters 21, 13, 31 when linearly polarized light is inclined by 45 degrees and is incident from the light input / output port 16a so as to excite E-mode light and TM-mode light in equal amounts. Table 1 shows the relationship. From the results shown in Table 1, it is understood that TE or TM linearly polarized waves can be output from the optical input / output port 27a or 28a by applying appropriate power to the thermo-optic phase shifters 21, 13, 31.

[0030]

[Table 1]

FIG. 6 shows the configuration of a polarization controller according to another embodiment, and FIG. 7 shows an enlarged sectional view taken along line VII-VII thereof. It should be noted that the members having the same functions as those in FIGS.

In the polarization mode converter 12A of this embodiment, the polarization mode conversion is not performed by stress control but by the quartz thin film 32 which is a birefringent medium as a wave plate. More specifically, a groove 33 that crosses the waveguide 22 is formed in the middle of the optical waveguide 22, and the crystal thin film 32 is inserted into the groove 33. Here, the groove 33 is formed by the lithography and the reactive etching method, but the groove 33 is not limited to this, and the groove may be cut by, for example, a cutter coated with diamond abrasive grains. Although the method using a cutter can make a groove in a short time, it cannot make a groove that traverses only the optical waveguide 22, and a groove that also traverses the optical waveguide 26 and the optical waveguide 23 is formed. .. However, forming such a groove is advantageous in that light loss occurs in the optical waveguide 22 and the optical waveguide 23 in the same manner.

The characteristics of the waveguide type polarization controller shown in FIGS. 6 and 7 were almost the same as those of the above-mentioned embodiment, but about 1 dB loss was large only with respect to the optical insertion loss.

An example of using the waveguide type polarization controller shown in FIGS. 1 to 4 is shown in FIG. As shown in the figure, in this embodiment, a waveguide type polarization controller 34 and a 16-wave optical frequency multiplexer / demultiplexer 35 are integrated on the same substrate. In this case, light of an arbitrary polarization state that is incident from the optical input / output port 16a of the waveguide type polarization controller 34 is converted into TE or TM linearly polarized light by the waveguide type polarization controller 34, and the The 16-wave optical frequency multiplexer / demultiplexer 35 enters the waveguide 28. Therefore, since only TE or TM linear polarization exists in the optical frequency multiplexer / demultiplexer 35, it is not necessary to eliminate the polarization dependence of each asymmetric Mach-Zehnder interferometer as in the conventional case.
For example, the trimming for controlling the birefringence becomes unnecessary.

[0035]

As described above, the waveguide type polarization controller of the present invention can convert light in an arbitrary polarization state into a constant linearly polarized wave, and other optical integrated circuits. In addition, since they can be integrated on the same substrate, there is an effect that the conventional birefringence control for eliminating the polarization dependence of individual optical integrated circuits becomes unnecessary. Therefore, the manufacturing time and cost of the waveguide type optical component can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a plan view showing a configuration of a waveguide type polarization controller according to an embodiment.

FIG. 2 is an enlarged sectional view taken along line II-II of FIG.

FIG. 3 is an enlarged sectional view taken along line III-III of FIG.

FIG. 4 is an enlarged sectional view taken along line IV-IV of FIG.

FIG. 5 is a graph showing the relationship between the phase shift amount α by the thermo-optic phase shifter 31 in the tunable coupler 14 and the cross port / through port light output ratio.

FIG. 6 is a plan view showing the configuration of a waveguide type polarization controller according to another embodiment.

FIG. 7 is an enlarged sectional view taken along line VII-VII of FIG.

FIG. 8 is an explanatory diagram showing a usage example of the waveguide type polarization controller according to the embodiment.

FIG. 9 is an explanatory diagram showing a 16-wave optical frequency multiplexer / demultiplexer according to a conventional technique.

[Explanation of symbols]

 11 Waveguide Polarization Beam Splitter 12 Polarization Mode Converter 13, 21, 31 Thermo-Optical Phase Shifter 14 Tunable Coupler 15 Silicon Substrate 16, 17, 22, 23, 26, 27, 28 Optical Waveguide 18, 19, 29 , 30 Directional coupler with 50% coupling rate 20 Amorphous silicon stress imparting film for controlling waveguide birefringence 24 Amorphous silicon stress imparting film for polarization mode conversion 25 Directional coupler with 1% coupling rate 32 crystal thin film 33 groove 34 waveguide type polarization controller 35 16-wave optical frequency multiplexer / demultiplexer

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Noboru Kochio 1-1-6 Uchisaiwaicho, Chiyoda-ku, Tokyo Nihon Telegraph and Telephone Corporation

Claims (4)

[Claims] 1. An optical waveguide having a light propagating action, an optical phase shifter for controlling an optical path length of the optical waveguide, and a stress applying film for controlling birefringence of the optical waveguide. An optical waveguide type polarization beam splitter that outputs linearly polarized waves having mutually orthogonal polarization planes, and two optical waveguides that are coupled to the two output optical waveguides of this optical waveguide type polarization beam splitter are provided. Polarization mode converter for converting a linearly polarized wave propagating through the optical waveguide into another orthogonal linearly polarized wave; and an optical phase for controlling the optical path length of the optical waveguide provided in at least one of the two optical waveguides. A waveguide type polarization controller, comprising: a shifter; and a tunable coupler that couples the lights of the two optical waveguides at an arbitrary coupling ratio. 2. The stress imparting device according to claim 1, wherein the optical waveguide is a glass optical waveguide formed on a substrate, and the optical phase shifter is a thermo-optical phase shifter including a thin film heater arranged on the glass optical waveguide. In the Mach-Zehnder interferometer, the film is an amorphous silicon film arranged on a glass waveguide, while the optical waveguide type polarization beam splitter has the thermo-optical phase shifter and the stress applying film on the arm waveguide. And the tunable coupler is a Mach-Zehnder interferometer having a thermo-optic phase shifter on its arm waveguide.
A waveguide type polarization controller characterized by the above. 3. The polarization mode converter according to claim 1, wherein the polarization mode converter performs mode conversion by a stress imparting film or a stress release groove which is arranged in the vicinity of the optical waveguide in the width direction asymmetrically. A characteristic waveguide-type polarization controller. 4. The waveguide type polarization controller according to claim 1, wherein the polarization mode converter performs mode conversion by a wave plate inserted in a groove that traverses the optical waveguide.