JP2015219317A - Polarization beam splitter circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polarization beam splitter circuit which allows for compensating for a phase error between two arm waveguides caused by manufacturing variation or the like, and offers a high polarization extinction ratio.SOLUTION: A polarization beam splitter circuit comprises optical waveguides formed on a substrate including a Mach-Zehnder interference circuit. A first arm waveguide 102 has a first groove 105 cutting across the first arm waveguide 102 and a second arm waveguide 103 has a second groove 105 cutting across the second arm waveguide, where a wavelength plate 106 for providing retardation corresponding to one-half of a wavelength being used is inserted to the first groove 105 such that a birefringence axis thereof matches that of the optical waveguide and where an optical medium 107, having a length and refractive index in a travelling direction of light that would compensate for a phase error between the light travelling through the first arm waveguide 102 and the light travelling through the second arm waveguide 103, is provided in the second groove 105.

Description

  The present invention relates to a polarization separation circuit having a high polarization extinction ratio.

  For the purpose of expanding the transmission capacity of an optical communication system, a polarization multiplexed QPSK signal obtained by multiplexing different quadrature phase shift keying (QPSK) signals into orthogonal polarizations is widely used.

  For example, Non-Patent Document 1 shows a modulator for generating a polarization multiplexed QPSK signal. In the modulator shown in Non-Patent Document 1, two modulators that generate a QPSK signal are prepared, and a signal from one modulator is polarization-rotated using a half-wave plate, and a polarization beam combiner (PBC: Polarization synthesis is performed by Polarization Beam Combiner). In the modulator shown in Non-Patent Document 1, a width-modulated PBC in which the width of the arm waveguide is changed is used as the PBC. However, as shown in Non-Patent Document 2, a PBS / PBC using a wave plate is also reported. Has been. The wave plate insertion type PBS / PBC is useful because the waveguide design is simple. Hereinafter, PBC and a polarization beam splitter (PBS) are described as the same.

  A schematic diagram of a polarization separation circuit having a conventional structure shown in Non-Patent Document 2 is shown in FIG. FIG. 5 shows a polarization separation circuit in which input side directional couplers 1 and 2, first and second arm waveguides 2 and 3 in parallel, and output side directional coupler 4 are connected. A dimming groove 5 is formed in the first arm waveguide 2 and is inserted so that the birefringence axis of the half-wave plate 6 coincides with the birefringence axis of the first arm waveguide 2. .

  Next, the operation principle of the PBS using the wave plate will be briefly described with reference to FIG. The optical signals input to the Mach-Zehnder interference circuit are equally distributed by the input side directional coupler 1 and output to the first and second arm waveguides 2 and 3, respectively. The phase difference between the light propagating through the first and second arm waveguides 2 and 3 is shifted by 180 degrees between the TE polarized wave and the TM polarized wave by the wave plate 6. As a result, in the first arm waveguide 2 and the second arm waveguide 3, since the TE polarized wave is in the opposite phase and the TM polarized wave is in the same phase, polarization separation is performed.

H. Yamazaki, “PDM-QPSK Modulator With a Hybrid Configuration of Silica PLCs and LiNbO3 Phase Modulators,” JOURNAL OF LIGHTWAVE TECHNOLOGY, 2011, Vol.29, No.5, pp.721-727 K. Suzuki, “Polarization-Insensitive Operation of Lithium Niobate Mach-Zehnder Interferometer With Silica PLC-Based Polarization Diversity Circuit,” IEEE PHOTONICS TECHNOLOGY LETTERS, 2008, Vol. 20, No. 10, pp.773-775

  However, the wave plate insertion type PBS as shown in FIG. 5 should ideally have zero phase error between the first and second arm waveguides 2 and 3. However, when the waveguide core is manufactured, a phase error occurs in the arm waveguide due to variations in core width, uneven stress in the overcladding, and the like, and this phase error breaks the interference condition for operating as a PBS. For example, the TE polarized wave is not perfectly reversed between the first and second arm waveguides 2 and 3, but a part of the TE polarized wave is mixed with another output port that is not desired. . As a result, the polarization extinction ratio (in the decibel notation, the absolute value of the difference between the transmittances of the TE polarization and the TM polarization) deteriorates. Hereinafter, the deterioration of the polarization extinction ratio will be briefly described with reference to FIG.

An electric field E _TE1 of TE polarization from which light input from the first input port 9 shown in FIG. 5 is output to the first output port 7 is expressed as (Equation 1) below.

Where θ is the coupling angle of the directional coupler, n _f is the refractive index of the fast axis of the wave plate, L is the thickness of the wave plate, n _wg is the effective refractive index of the waveguide, and Δφ is the phase error [rad]. , Λ is the wavelength used. Since the coupling angle θ is ideally π / 4, when θ = π / 4 is substituted into (Equation 1) and the optical power | E _TE1 | ² is obtained, the following equation (Equation 2) is obtained. Is done.

Next, an electric field E _TM1 of TM polarization output to the first output port 7 is expressed as (Equation 3) below. Here, _ns is the slow axis refractive index of the wave plate.

Substituting θ = π / 4 into (Expression 2) to obtain the optical power | E _TM1 | ² , the following expression (Expression 4) is obtained.

Similarly, the TE polarized optical power | E _TE2 | ² output to the second output port 8 and the TM polarized optical power | E _TM2 | ² output to the second output port 8 are as follows: (Expression 5) and (Expression 6).

  Since it is necessary to output the TE polarized wave to the first output port 7 and the TM polarized wave to the second output port 8, the operating condition of the PBS is expressed by the following (formula 7).

  From (Equation 2), (Equation 4), (Equation 5), (Equation 6), and (Equation 7), the operating condition to be satisfied as PBS is (Equation 8) below.

  Here, M and N are diffraction orders and are integers. Assuming that the phase error is zero (Δφ = 0) and rearranging the above (Equation 8), the following (Equation 9) is obtained.

Therefore, for example, by setting M = 1, N = 1, n _f = 1.56, n _s = 1.6, n _wg = 1.52, and the wavelength plate is λ / 2, (Equation 9) Can be satisfied and PBS can be realized.

  However, when a phase error occurs, (Equation 9) does not hold in (Equation 9), and unnecessary polarization becomes crosstalk, which degrades the polarization extinction ratio.

  In order to solve the above problems, an object of the present invention is to compensate for a phase error that causes a deterioration in the polarization extinction ratio.

  In order to solve the above-described problem, the polarization separation circuit according to claim 1 includes: a demultiplexing circuit that bifurcates the optical waveguide; and a first arm waveguide that is connected to one branch of the demultiplexing circuit; A Mach-Zehnder interference circuit comprising: a second arm waveguide connected to the other branch of the branching circuit; and a multiplexing circuit connected to the first and second arm waveguides. The first arm waveguide is provided with a first groove so as to divide the first arm waveguide, and the second arm waveguide includes the second groove. The arm waveguide is provided with a second groove so as to divide the second arm waveguide, and in the first groove, a wave plate that gives a phase difference of ½ wavelength of the used wavelength is provided. It is inserted so as to be the same as the birefringence axis of the optical waveguide, and the second groove has the above-mentioned An optical medium having a length and a refractive index with respect to the optical waveguide direction is provided to compensate for a phase error between the light propagating through one arm waveguide and the light propagating through the second arm waveguide. It is characterized by being.

The polarization separation circuit according to claim 2 is a demultiplexing circuit for bifurcating an optical waveguide, a first arm waveguide connected to one branch of the demultiplexing circuit, and the other of the demultiplexing circuit A polarization composed of an optical waveguide on a substrate, comprising a Mach-Zehnder interference circuit including a second arm waveguide connected to the branch and a multiplexing circuit connected to the first and second arm waveguides. In the wave separation circuit, the first arm waveguide is provided with a first groove so as to divide the first arm waveguide, and the second arm waveguide includes the second arm waveguide. A second groove is provided so as to divide the arm waveguide, and in the first groove, a wave plate that gives a phase difference of ½ wavelength of the used wavelength is connected to the birefringence axis of the optical waveguide. It is inserted so as to be the same, and the second groove has a predetermined refraction in the optical waveguide direction. n is _com optical medium having a provided, the predetermined refractive index n _com is the refractive index n _s of the slow axis of the wavelength plate, the thickness of the wave plate L, the first arm guide When the phase error between the light propagating through the waveguide and the light propagating through the second arm waveguide is Δφ, the order is N, and the wavelength used is λ,

It is characterized by being represented by.

  The polarization separation circuit according to claim 3 is the polarization separation circuit according to claim 1 or 2, wherein the optical medium is a polyimide film inserted in the second groove, or It is an epoxy resin filled in the second groove.

  A polarization separation circuit according to a fourth aspect is the polarization separation circuit according to any one of the first to third aspects, wherein the substrate is a silicon substrate, and the optical waveguide is made of a quartz-based waveguide. It is characterized by becoming.

  A polarization separation circuit according to claim 5 is the polarization separation circuit according to any one of claims 1 to 4, wherein the branching circuit and the multiplexing circuit are directional couplers. Features.

  According to the present invention, it is possible to compensate for a phase error between the two arm waveguides caused by manufacturing variations and the like, so that a high polarization extinction ratio can be realized.

1 is a schematic diagram of a polarization separation circuit according to the present invention. It is the schematic of the other example of the polarization separation circuit which concerns on this invention. Is a diagram showing a polarization extinction ratio calculation results for the refractive index n _com in PBS. It is the schematic of the polarization splitting circuit which concerns on Example 2 of this invention. It is the schematic of the conventional polarization separation circuit.

  FIG. 1 is a schematic diagram of a polarization separation circuit according to the present invention. FIG. 1 shows an input-side directional coupler 101 that bifurcates an input optical waveguide, first and second arm waveguides 102 and 103 provided in parallel, and first and second arm waveguides 102. And a polarization separation circuit including an output-side directional coupler 104, a dicing groove 105, a half-wave plate 106, and an optical medium 107 connected to each other. The first arm waveguide 102 is connected to one branch of the input side directional coupler 101, and the second arm waveguide 103 is connected to the other branch of the input side directional coupler 101. The first and second arm waveguides 102 and 103 are connected to join at the output side directional coupler 104.

  A dicing groove 105 is formed in the first and second arm waveguides 102 and 103. In the dicing groove 105 formed in the first arm waveguide 102, the half-wave plate 106 is provided so that the birefringence axis of the half-wave plate 106 coincides with the birefringence axis of the first arm waveguide 102. The optical medium 107 is inserted in the dicing groove 105 formed in the second arm waveguide 103.

  In the present invention, not only the first arm waveguide 102 but also the second arm waveguide 103 is provided with an optical medium 107 for phase error adjustment having a refractive index comparable to that of the half-wave plate 106. As a result, the optical signal is equally distributed by the input side directional coupler 101 to compensate for the inter-arm phase difference caused by the phase error in the first arm waveguide 102.

<When the slow axis of the wave plate is perpendicular to the substrate and the TE output is at the first output port 108>
1, in the polarization separation circuit according to the present invention shown in FIG. A case where the half-wave plate 106 is arranged so as to be on the slow axis with respect to the electric field (TM polarization) will be described.

Let n _{com be} the refractive index of the optical medium 107 for phase error adjustment. Since the location of the second arm waveguide having the conventional structure is replaced with the newly provided optical medium 107, n _wg may be replaced with n _com in the equation. When n _wg in the above (Equation 8), which is the operating condition of the PBS, is replaced by n _com and arranged, the following (Equation 10) is obtained.

In other words, the wavelength plate provided in the first arm waveguide 102 with NM = 0 is a half-wave plate, and the refractive index n _com of the optical medium newly disposed thereon satisfies (Equation 10). If designed, the phase error can be compensated.

<When the slow axis of the wave plate is the vertical direction of the substrate and the TM output is output to the first output port 108>
In the above description, the operating conditions under which the TE polarized wave is output to the first output port 108 and the TM polarized wave is output to the second output port 109 are described. However, as will be described below, the same concept is effective even when TM polarization is output to the first output port 108 and TE polarization is output to the second output port 109.

The TE-polarized electric field E _TE1 output to the first output port 108 is expressed as (Equation 11) below.

Substituting an ideal coupling angle θ = π / 4 into (Equation 11) to obtain the optical power | E _TE1 | ² is expressed as (Equation 12) below.

Next, an electric field E _TM1 of TM polarization output to the first output port 108 is expressed as (Equation 13) below.

Substituting the ideal coupling angle θ = π / 4 into (Equation 13) to obtain the optical power | E _TM1 | ² , it is expressed as (Equation 14) below.

  The operating condition of the PBS is expressed by the following (Equation 15) because TM polarization is output to the first output port 108 and TE polarization is output to the second output port 109.

  From (Equation 12), (Equation 14), and (Equation 15), the operating condition to be satisfied as PBS is (Equation 16) below.

  Further, when (Equation 16) is arranged, the following (Equation 17) is obtained.

That is, if MN = 1 and a half-wave plate is used as the wavelength plate and the refractive index n _com of the optical medium 107 is designed to satisfy (Equation 17), the phase error between the arm waveguides can be reduced. Can be compensated.

<When the slow axis of the wave plate is the horizontal direction of the substrate and the TE output is the first output port 108>
Since the present invention is effective even when the slow axis of the wave plate is the horizontal direction of the substrate, and the TE polarized wave is output to the first output port 108 and the TM polarized wave is output to the second output port 109, They will be described with reference to FIG.

The TE-polarized electric field E _TE1 output to the first output port 108 is expressed as (Equation 18) below.

Substituting an ideal coupling angle θ = π / 4 into (Equation 18) to obtain the optical power | E _TE1 | ² is expressed as (Equation 19) below.

Next, an electric field E _TM1 of TM polarization output to the first output port 108 is expressed as (Equation 20) below.

Substituting the ideal coupling angle θ = π / 4 into (Equation 20) to obtain the optical power | E _TM1 | ² , it is expressed as (Equation 21) below.

  The operating condition of the PBS is expressed by the following (Equation 22) because TE polarization is output to the first output port 108 and TM polarization is output to the second output port 109.

  From (Equation 19), (Equation 21), and (Equation 22), the operating condition to be satisfied as PBS is (Equation 23) below.

  Furthermore, when (Equation 23) is arranged, the following (Equation 24) is obtained.

That is, if MN = 1 and a half-wave plate is used as the wavelength plate and the refractive index n _com of the optical medium 107 is designed to satisfy (Equation 24), the phase error between the arm waveguides can be reduced. Can be compensated.

<When the slow axis of the wave plate is the horizontal direction of the substrate and the TM output is output to the first output port 108>
Since the present invention is effective even when the slow axis of the wave plate is the horizontal direction of the substrate, and TM polarization is output to the first output port 108 and TE polarization is output to the second output port 109, Explain them.

  The operating condition of the PBS is expressed by the following (Equation 25) because TM polarization is output to the first output port 108 and TE polarization is output to the second output port 109.

  From (Equation 19), (Equation 21), and (Equation 25), the operating condition to be satisfied as PBS is (Equation 26) below.

  Further, when (Equation 26) is arranged, the following (Equation 27) is obtained.

That is, if N−M = 0, a half-wave plate is used as the wavelength plate, and the refractive index n _com of the optical medium 107 is designed to satisfy (Equation 27), the phase error between the arm waveguides can be reduced. Can be compensated.

Furthermore, the conditions to be satisfied by the refractive index n _com of the optical medium 107 can be summarized by the following (Expression 28) from (Expression 10) and (Expression 17), or (Expression 24) and (Expression 27). Here, N ′ is an integer.

Example 1
Hereinafter, the PBS according to Example 1 of the present invention will be described. In Example 1, in the PBS shown in FIG. 1, the half-wave plate 106 is made of polyimide, and a polyimide film made of the same kind of material as the half-wave plate 106 is inserted as the optical medium 107. However, the polyimide film used as the optical medium 107 does not have birefringence toward the propagation direction of the optical waveguide.

Next, a specific method for producing PBS according to Example 1 of the present invention will be described. A silicon substrate is prepared, and quartz glass is formed by thermally oxidizing silicon. Thereafter, tantalum pentoxide (Ta ₂ O ₅ ) -added quartz glass is deposited as a core layer by a sputter deposition method. At this time, the amount of Ta ₂ O ₅ added is adjusted so that the relative refractive index difference (Δ) is 5% with respect to quartz glass. Then, the core is patterned by a photolithographic technique and a reactive ion etching technique. The core size is 2.6 μm.

  Finally, the core is embedded by chemical vapor deposition (CVD) to produce a silica-based optical waveguide having an embedded structure. The dicing groove 105 for inserting the wave plate is formed by a dicing saw with a width of 30 μm and a depth of 100 μm. Thereafter, a half-wave plate 106 made of polyimide is inserted into the dicing groove 105 formed in the first arm waveguide 102 at a predetermined angle, and the dicing groove 105 formed in the second arm waveguide 103 is inserted into the dicing groove 105 formed in the second arm waveguide 103. Inserts a polyimide film having the same thickness as that of the half-wave plate 106 as the optical medium 107, and fills and fixes both the dicing grooves 105 with an adhesive. At that time, the polyimide film used as the optical medium 107 is selected to compensate for the phase error.

Next, the operation principle of the PBS according to the first embodiment of the present invention will be described. In the first embodiment, it is assumed that a phase error of +35 degrees has occurred as a phase error corresponding to Δφ. The slow-axis refractive index n _s of the half-wave plate 106 is 1.6, and the fast-axis refractive index n _f is 1.56. Therefore, when the used light wavelength λ is 1.55 μm, the film thickness L of the half-wave plate 106 is 19.375 μm from (Equation 10). On the other hand, at this time, the refractive index n _com of the polyimide film used as the optical medium 107 may be ˜1.608. In this case, (Equation 10) can be satisfied, which is an ideal PBS operating condition.

The refractive index of the polyimide film is ideally as described above, but in reality, a slight deviation occurs. As an adverse effect due to the deviation, for example, deterioration of the polarization extinction ratio, which is a characteristic required for PBS, can be mentioned. FIG. 3 shows the calculation result of the polarization extinction ratio with respect to the refractive index n _com at the first output port 108 of the PBS. As shown in FIG. 3A, a high polarization extinction ratio is obtained at a place where the order is periodically matched to the refractive index n _com .

FIG. 3B shows an enlarged polarization extinction ratio in the vicinity of the refractive index n _com = 1.608 in FIG. In general, the polarization extinction ratio of PBS is required to be about 20 dB. Accordingly, the allowable refractive index error is within about ± 0.2% (= 0.003 / 1.608 * 100) when the target refractive index is the denominator and the deviation is the numerator. Guidance. Therefore, the appropriate refractive index is not limited to the above-described value, and may be a value having another diffraction order satisfying (Equation 10).

  The present invention focuses on compensating for phase errors. However, even when there is no phase error (Δφ = 0), the PBS operates by satisfying (Equation 10). The merits of the present invention when compared with the conventional structure when there is no phase error are listed below.

  -Grooves are provided in both arm waveguides in the same manner, and excessive losses due to the grooves are generated to the same extent. As a result, loss imbalance is avoided, and the light is extinguished in an ideal state when interference is caused by the subsequent coupler, and the polarization extinction ratio is less deteriorated.

In the formula (9) used in the PBS having the conventional structure, if the effective refractive index n _{core of} the waveguide and the refractive indexes n _f and n _s of the wave plate are determined, only the orders M and N have degrees of freedom. And the combination of wave plates are restricted. On the other hand, according to the present invention, as shown in (Equation 10), the combination of the waveguide and the wave plate can be freely selected by appropriately setting the refractive index n _com of the additional optical medium.

  In the conventional structure, polyimide is inserted only in one arm waveguide, so light propagates through the quartz glass core in the other arm waveguide where light propagates through the polyimide in one arm waveguide. To do. On the other hand, since polyimide and quartz glass have different temperature dependences of the refractive index, the temperature dependence of the optical path length difference occurs between the two arm waveguides. However, in the present invention, it is possible to select the same kind of material as the additional optical medium, or a material having the same temperature dependency, so that the temperature can be made independent.

(Example 2)
FIG. 4 shows a schematic configuration of the PBS according to the second embodiment of the present invention. In FIG. 4, the input-side directional coupler 401, first and second arm waveguides 402 and 403 provided in parallel, the output-side directional coupler 404, and first and second grooves 405 are illustrated. And 407, a half-wave plate 406, and an epoxy resin 408 are shown. The first and second arm waveguides 402 and 403 are connected between two output ports of the input-side directional coupler 401 and two input ports of the output-side directional coupler 404, respectively. By the dry etching, a first groove 405 is formed in the first arm waveguide 402, and a second groove 407 is formed in the second arm waveguide 403. A half-wave plate 406 is inserted into the first groove 405 formed in the first arm waveguide 402 so that the slow axis is perpendicular to the substrate, and is formed in the second arm waveguide 403. The second groove 407 is filled with an epoxy resin 408 for adjusting the refractive index. However, the epoxy resin 408 is not birefringent in the propagation direction of the optical waveguide.

Next, a specific method for producing PBS according to Example 2 will be described. Since most of them are the same as the method for manufacturing PBS according to Example 1, only the differences are shown. In the second embodiment, the first and second grooves 405 and 407 that divide each arm waveguide have a width of 30 μm by applying a resist and patterning, and performing SiO ₂ etching and Si etching under different etching conditions. A groove having a depth of 100 μm was formed. Thereafter, a half-wave plate 406 is inserted into the first groove 405, and is filled with an adhesive and fixed. The second groove 407 is filled with an epoxy resin 408 and solidified.

Next, the operation principle of the PBS according to the second embodiment of the present invention will be described. Example 2 has the same characteristics as Example 1. The difference between the two is that, in Example 1, a polyimide film was selected as an optical medium having a refractive index n _com satisfying (Equation 10). In Example 2, an epoxy resin was selected.

  In Example 1, since there is a constraint condition of a film shape, the adjustment range of the refractive index is limited. On the other hand, since the second groove 407 is filled with the epoxy resin 408 as in the second embodiment, there is no restriction condition of the film shape. Therefore, the refractive index is adjusted by preparing the epoxy resin on the spot. be able to. Therefore, compared with the first embodiment, it is possible to adjust the refractive index more easily and precisely, and there is an advantage that the characteristics of the PBS can be brought close to an ideal state.

  In the above Examples 1 and 2, the core was deposited on the thermal oxide film substrate by the sputter deposition method, and the over clad was deposited by the CVD method. However, the combination of the manufacturing methods is not limited to this, and it is clear that the same effect can be obtained even if manufactured in various combinations with other manufacturing methods capable of depositing quartz glass, including other glass deposition methods such as flame deposition. is there.

  In Examples 1 and 2, both the waveguide and the clad are made of quartz glass. However, the present invention is not limited to this combination. For example, the waveguide is made of quartz glass, silicon, a compound semiconductor, or a polymer. Even when the clad is made of quartz glass, compound semiconductor, or polymer, it is obvious that the same effect can be obtained if an optical waveguide having a confinement effect can be formed.

  In the first and second embodiments, the branching circuit and the multiplexing circuit both use directional couplers. However, the same effect can be obtained by using another 2 × 2 branching circuit such as a 2 × 2 multimode interference waveguide (MMI). Even if a 1 × 2 circuit that branches in phase, such as a Y-branch circuit, is used as a branching circuit, the PBS can be realized with the same effect by slightly changing the design of the waveguide.

  This will be described in more detail. In a 2 × 2 demultiplexing circuit such as a directional coupler, the demultiplexed optical signal has a phase difference of 90 degrees. On the other hand, in the Y branch circuit, the signals are demultiplexed in the same phase. Therefore, by extending the length of one of the arm waveguides so that the same phase change as that of the 2 × 2 branching circuit can be obtained, the same effect can be obtained in the Y branch circuit and the PBS can be realized.

In the first and second embodiments, the main purpose is to adjust the refractive index n _com of the optical medium introduced for adjusting the refractive index. However, since the optical path length is the product of the refractive index and the length, the length of the optical medium (the film thickness of the polyimide film in Example 1 and the groove width in Example 2) is adjusted to adjust the first and second A similar effect can be obtained by equalizing the optical path lengths in the optical waveguide direction of the groove portions provided with the optical media in the arm waveguides.

In the second embodiment, the grooves 405 and 407 are separate grooves. However, if the refractive index of the adhesive used for the wave plate 406 is equal to the required refractive index n _com , the same groove in which the grooves 405 and 407 are connected is used, and the second adhesive is used instead of the epoxy resin 408. There is no problem even if it is used as an optical medium in the arm waveguide.

Input side directional coupler 1, 101, 401
Arm waveguide 2, 3, 102, 103, 402, 403
Output side directional coupler 4, 104, 404
Dicing groove 5, 105
1/2 wavelength plate 6, 106, 406
Optical medium 107
Groove 405, 407
Epoxy resin 408

Claims (5)

A branching circuit for bifurcating the optical waveguide;
A first arm waveguide connected to one branch of the branching circuit;
A second arm waveguide connected to the other branch of the branching circuit;
A polarization separation circuit comprising an optical waveguide on a substrate, comprising: a Mach-Zehnder interference circuit including a multiplexing circuit connected to the first and second arm waveguides;
The first arm waveguide is provided with a first groove so as to divide the first arm waveguide,
The second arm waveguide is provided with a second groove so as to divide the second arm waveguide,
In the first groove, a wave plate that gives a phase difference of ½ wavelength of the used wavelength is inserted so as to be the same as the birefringence axis of the optical waveguide,
The second groove has a length with respect to the optical waveguide direction so as to compensate for a phase error between light propagating through the first arm waveguide and light propagating through the second arm waveguide. A polarization separation circuit comprising an optical medium having a refractive index. A branching circuit for bifurcating the optical waveguide;
A first arm waveguide connected to one branch of the branching circuit;
A second arm waveguide connected to the other branch of the branching circuit;
A polarization separation circuit comprising an optical waveguide on a substrate, comprising: a Mach-Zehnder interference circuit including a multiplexing circuit connected to the first and second arm waveguides;
The first arm waveguide is provided with a first groove so as to divide the first arm waveguide,
The second arm waveguide is provided with a second groove so as to divide the second arm waveguide,
In the first groove, a wave plate that gives a phase difference of ½ wavelength of the used wavelength is inserted so as to be the same as the birefringence axis of the optical waveguide,
The second groove is provided with an optical medium having a predetermined refractive index n _com in the optical waveguide direction,
The predetermined refractive index n _com is the refractive index of the slow axis of the wave plate as n _s , the thickness of the wave plate as L, the light propagating through the first arm waveguide, and the second arm waveguide. When the phase error between the light propagating through the light is Δφ, the order is N, and the wavelength used is λ,

A polarization separation circuit represented by:   The polarization separation according to claim 1, wherein the optical medium is a polyimide film inserted into the second groove or an epoxy resin filled in the second groove. circuit. The substrate is a silicon substrate;
4. The polarization separation circuit according to claim 1, wherein the optical waveguide is made of a silica-based waveguide.   5. The polarization separation circuit according to claim 1, wherein the branching circuit and the multiplexing circuit are directional couplers.

Images