JP4405978B2 - Optical signal processor - Google Patents

Description

  The present invention relates to an optical signal processor that suppresses polarization dependency and temperature dependency on polarization.

  Wavelength multiplexing is an important technology because of further demands for long distance and large capacity optical communication. 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 circuit (hereinafter referred to as MZI) using an optical waveguide (for example, see Non-Patent Document 1).

  FIG. 1 shows a schematic diagram of the MZI. The MZI 100 includes two couplers (122, 124) and an arm waveguide (106, 108) connecting the two couplers. The coupler 122 includes two input waveguides (102, 104). 124 comprises two output waveguides (110, 112).

  In the following, the operating principle of the Mach-Zehnder interferometer and its polarization dependence will be described.

In the MZI 100, a path from the input I1 (input waveguide 102) to the output O1 (output waveguide 110) is a through path, and a path from the input I1 (input waveguide 104) to the output O2 (output waveguide 112) is a cross path. Define. At this time, the optical output (O ^through , O ^cross ) of each path can be described by the following equation using a known interference principle. (Here, the coupling rate of the coupler is 50%.)

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.

  Through path is nΔL = λm from equation (1), and cross path is from equation (2).

The optical signal is periodically extinguished and output to the other path at a signal wavelength satisfying the above condition, and functions as a wavelength multiplexing / demultiplexing filter.

  As a method for manufacturing an optical waveguide used in such a device, for example, the following method is available.

On a silicon substrate using flame hydrolysis deposition, depositing a core layer added to GeO ₂ lower under-cladding layer consisting mainly of SiO ₂ and the SiO ₂ in this order. The core layer is then patterned using reactive ion etching. Using the flame deposition method again, an overcladding layer is deposited to produce a buried optical waveguide.

Usually, such an optical waveguide has birefringence due to the stress generated by the shape of the core and the difference in thermal expansion coefficient between the substrate and the clad. That is, the effective refractive index (n _TE , n _TM ) of _TM polarized light having a polarization direction perpendicular to the substrate is different from that of TE polarized light having a polarization direction parallel to the substrate. Here, the optical path length difference Δ (BL) depending on the polarization is given by the following equation.

Here, l ₁ and l ₂ are line coordinates along the two arm waveguides, respectively. Also,

Is the line integral along the birefringent arm waveguide.

  Since the optical path length difference Δ (BL) depending on the polarization has a finite value, polarization dependence occurs in the extinction wavelength. This polarization dependence causes generation of polarization dependent loss (PDL) and polarization dependent frequency difference (PDf), and greatly degrades signal quality.

  The following methods are known as methods for eliminating the polarization dependence.

(First example of prior art)
A Mach-Zehnder interferometer in which a half-wave plate equivalent to ½ of the wavelength used is inserted on a straight line connecting the centers of the two arm waveguides so that the principal axis is inclined 45 degrees from the horizontal direction (or normal line) of the substrate surface. There is a circuit (see Non-Patent Document 2).

  FIG. 2 shows a schematic diagram of the MZI of the first example of the prior art. The MZI 200 includes two couplers (222, 224) and two arm waveguides (206, 208) connecting the two couplers. Further, the MZI 200 includes a half-wave plate 232 arranged to bisect the optical path length of the arm waveguides (206, 208). The coupler 222 includes two input waveguides (202, 204), and the coupler 224 includes two output waveguides (210, 212).

  In the MZI 200, an optical signal propagates half the distance of the arm waveguide (206, 208) by the TE polarization (or TM polarization) to the half-wave plate 232, and the half-wave plate 232 converts the TE polarization to the TM polarization ( Alternatively, polarization conversion is performed from TM polarization to TE polarization. Then, the other half distance of the arm waveguides (206, 208) is propagated by TM polarization (or TE polarization). All optical signals converted from TE polarization (or TM polarization) to TM polarization (or TE polarization) are optical path length differences.

Thus, the optical path length difference Δ (BL) depending on the polarization can be eliminated.

(Second example of prior art)
There is a Mach-Zehnder interferometer circuit in which an optical path length difference Δ (BL) depending on polarization is set to an integral multiple (including 0) of a used light wavelength (see Patent Document 1). This is because the Mach-Zehnder interferometer cannot discriminate a phase difference that is an integral multiple of the wavelength λ used. Therefore, it is apparent that the TM polarization interference condition and the TE polarization interference condition coincide with each other. The dependent optical path length difference Δ (BL) is an integral multiple (including 0).

Japanese Patent Publication No. 6-60982 Japanese Patent No. 3770313 International Publication No. 01/059495 Pamphlet K. Inoue et al., “A Four-Channel Optical Waveguide Multi / Demultiplexer for 5-GHz Spaced Optical FDM Transmission”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol.6, No.2, FEB. 1988, pp.339-345 Y. Inoue et al., “Elimination of Polarization Sensitivity in Silica-Based Wavelength Division Multiplexer Using a Polyimide Half Waveplate”, JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol.15, No.10, Oct. 1997, pp.1947-1957 B. L. Heffner, “Deterministic, Analyticall Complete Measurement of Polarization-Dependent Transmission Through Optical Devices”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL.4, NO.5, MAY 1992, pp.451-454 M. Okuno et al., “Birefringence Control of Silica Waveguides on Si and Its Application to a Polarization-Beam Splitter / Switch”, JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL.12, NO.4, APRIL, 1994, pp.625-633

  However, the aforementioned Mach-Zehnder interferometer circuit (hereinafter referred to as MZI) has the following problems.

The first example of the prior art is based on the premise that the TE polarization is completely converted to TM polarization (or TM polarization is changed to TE polarization) using a half-wave plate. However, since the film thickness of the half-wave plate deviates from a desired thickness due to manufacturing errors, it does not match the design wavelength. As a result, the polarization is not completely converted from the TE polarized wave to the TM polarized wave, and a part thereof remains as the TE polarized wave. When such a half-wave plate is used, in the first example of the prior art, the optical path length difference of TE polarized light that is incident as TE polarized light and propagated without polarization conversion is n _TE ΔL. In other words, the optical path length difference

The purpose is not achieved, and polarization dependence occurs in the extinction frequency.

  For example, when the frequency interval (period) of the extinction wavelength is defined as FSR (Frequency Spectral Range) and the maximum value of the difference in extinction wavelength depending on the polarization is defined as PDf (Polarization Dependent Frequency), the conventional technology with an FSR of 10 GHz In the first example of the Mach-Zehnder interferometer circuit, the PDf is generated at 0.4 GHz for both the cross path and the through path. In order to avoid deterioration of signal quality, this PDf is required to be 1/100 or less of FSR, and it has been difficult to satisfy the specifications with the conventional technology.

  In the second example of the prior art, the problem is that PDf has a large birefringence dependency. As a result, it is very difficult to satisfy the PDf that is 1 / 100th of the FSR, and there is a large PDf fluctuation with respect to the temperature dependence. This birefringence dependency will be briefly described.

  Quenching frequency of TE polarization and TM polarization in the through path

Satisfies the following equation.

Here, m is the order, FSR _TE and FSR _TM are the TE polarization and TM polarization FSR, and c is the speed of light. From the above two formulas, PDf can be transformed into the following formula.

  here,

It is. When the effective refractive index is 1.45 and the extinction frequency is 193 THz, the amount of change of the PDf with respect to birefringence is 133 × 10 ¹² from the above equation. This means that when birefringence changes by 0.1 × 10 ⁻⁴ , PDf also changes by 1.33 GHz, and it can be seen that PDf has a large birefringence dependency. Therefore, it is necessary to adjust birefringence with high accuracy, and it is very difficult to satisfy PDf which is 1/100 of FSR.

  On the other hand, the internal stress applied to the waveguide varies depending on the difference in the thermal expansion coefficient between the substrate and the clad and the difference in the thermal expansion coefficient between the heat dissipation substrate to which the circuit is bonded and the circuit board, which are caused by the environmental temperature. As a result, the value of birefringence changes via the photoelastic effect, so that PDf varies depending on the environmental temperature.

  For example, in a Mach-Zehnder interferometer with a FSR of 10 GHz, in which birefringence is adjusted to 0.33 GHz using the second example of the prior art, when the environmental temperature is changed from −10 ° C. to 80 ° C., PDf is 6 GHz The problem has also changed.

  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 that suppresses the polarization dependency of the transmission spectrum and the temperature dependency of the polarization. .

In order to achieve the above-mentioned object, the invention according to claim 1 is an optical signal processor for connecting two couplers and the two couplers each formed on a substrate. In a Mach-Zehnder interferometer circuit with two arm waveguides , the optical main axis is the same to convert horizontally polarized light into vertically polarized light and convert vertically polarized light into horizontally polarized light. The polarization rotator is provided in a groove that bisects the optical path length of each of the two arm waveguides, and birefringence is performed in the propagation direction of the optical signal for each of the two arm waveguides. A value obtained by dividing the difference between the line integral values by the used light wavelength is 2m−0.2 to 2m + 0.2 (m is an integer including zero).

The invention according to claim 2 is an optical signal processor, which is a Mach-Zehnder interferometer comprising two couplers and two arm waveguides connecting the two couplers, each fabricated on a substrate. In the circuit, two polarization rotators whose optical principal axes are orthogonal to each other, which convert horizontally polarized light into vertically polarized light and convert vertically polarized light into horizontally polarized light, and respectively provided in the groove bisects the optical path length of each of the arm waveguides of the present, using the difference between the values obtained by line integration of the birefringence toward the propagation direction of optical signals for each of the two arm waveguides The value divided by the light wavelength is (2m−1) −0.2 to (2m−1) +0.2 (m is an integer including zero).

  According to a third aspect of the present invention, the polarization rotator according to the first aspect is installed such that the optical principal axis is inclined 45 degrees from the normal of the surface of the substrate and is orthogonal to the propagation direction of the optical signal. It is characterized by being a half-wave plate.

According to a fourth aspect of the present invention, in the two polarization rotators according to the second aspect, the optical principal axis is inclined 45 degrees from the normal of the surface of the substrate and is orthogonal to the propagation direction of the optical signal. A half-wave plate installed on the polarization rotator, wherein one optical principal axis of the polarization rotator and the other optical principal axis of the polarization rotator are orthogonal to each other.

  The invention described in claim 5 is characterized in that the coupler according to any one of claims 1 to 4 is a directional coupler or a multimode interference coupler.

  According to a sixth aspect of the present invention, at least one of the two arm waveguides according to any one of the first to fifth aspects is a waveguide in which a waveguide width is partially changed. The birefringence is adjusted so that the difference between the values obtained by line-integrating the birefringence in the propagation direction of the optical signal in each of the arm waveguides becomes a desired value.

  According to a seventh aspect of the present invention, at least one of the two arm waveguides according to any one of the first to fifth aspects is a waveguide partially irradiated with a laser, and the two arm waveguides The birefringence is adjusted so that the difference between the values obtained by integrating the birefringence in the propagation direction of the optical signal in each of the waveguides becomes a desired value.

  The invention according to claim 8 is a waveguide in which at least one of the two arm waveguides according to any one of claims 1 to 5 is formed with a stress relief groove in a part of both sides, The birefringence is adjusted so that the difference between the values obtained by line-integrating the birefringence in the propagation direction of the optical signal in each of the two arm waveguides becomes a desired value.

  According to a ninth aspect of the present invention, at least one of the two arm waveguides according to any one of the first to fifth aspects is a waveguide having a stress applying film formed on an upper surface thereof. The birefringence is adjusted so that the difference between the values obtained by line-integrating the birefringence in the propagation direction of the optical signal in each of the arm waveguides becomes a desired value.

  According to a tenth aspect of the present invention, at least one of the two arm waveguides according to any one of the first to fifth aspects is a waveguide in which a thin film heater for changing stress is formed on an upper portion. The birefringence of each of the two arm waveguides is adjusted so that the difference between the values obtained by line integration of the birefringence toward the propagation direction of the optical signal becomes a desired value.

  As described above, according to the present invention, it is possible to provide an optical signal processor that suppresses the polarization dependence of the transmission spectrum and the temperature dependence of the polarization.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
First, with reference to FIG. 3, a description will be given of conditions under which the transmission spectrum does not depend on the polarization even when the operating wavelength of the half-wave plate is shifted from the design wavelength.

  When the operating wavelength of the half-wave plate deviates from the design wavelength, the problem is that the optical path length difference of the polarization that has not undergone polarization conversion does not match the optical path length difference of the polarization that has undergone polarization conversion. Therefore, it is considered to make the optical path length difference apparently the same.

Specifically, the optical path length difference n _TE ΔL of the TE polarized wave that has been incident on the TE polarized wave and propagated as it is without polarization conversion, and the TE polarized wave that has been incident on the TM polarized wave and polarized at the half-wave plate are converted. Difference in optical path length of polarized waves propagated by waves

And the difference between the optical path length n _TM ΔL of the TM polarized wave that is incident on the TM polarized wave and propagated as it is without polarization conversion, and the incident on the TE polarized wave and half Optical path length difference of polarized wave that was polarized wave converted by wave plate and propagated as TM polarized wave

The difference between is zero or an integer multiple of the wavelength used.
This will be described more specifically using mathematical expressions.

<When the main axis of the half-wave plate is in the same direction / through pass>
A Mach-Zehnder interferometer in which a half-wave plate is inserted in the center of two arm waveguides, and the direction of the optical principal axis of the half-wave plate is inclined by 45 degrees from a line (normal line) perpendicular to the surface of the substrate. In MZI, which is orthogonal to the signal propagation direction and the direction of the optical principal axis of the half-wave plate inserted in each arm waveguide is the same direction, the optical output propagated through I1, arm 1, and O1 and output. Becomes Equation (7).

Here, φ _TE and φ _TM are the phase differences between the TE polarization and the TM polarization between the two arm waveguides, respectively, and Ψ is the phase difference given by the half-wave plate. It is assumed that the coupling rate of the coupler is 50%, and Ψ is assumed to be other than π and an integral multiple thereof, since it is assumed that the wave plate deviates from the design wavelength. Further, the optical output that propagates and is output through I1, arm 2, and O1 is expressed by equation (8).

  From Expressions (7) and (8), the optical output of the through path is Expression (9).

  From equation (9), the condition for polarization independence at the extinction wavelength is

Since it is time to satisfy, the following equation should be satisfied.

Here, m, n, and α are integers. Therefore, the following equation is obtained.

  When formula (13) is rewritten, formula (15) is obtained.

  Further rewriting yields equation (16).

  From this, it can be seen that when the optical path length difference depending on the polarization is an even order of the extinction wavelength (use wavelength), the polarization becomes independent.

  In general, due to fabrication errors, the value of waveguide birefringence deviates. However, some variation in birefringence is allowed in order to satisfy PDf, which is 1 / 100th of the target FSR.

  Therefore, an allowable value of birefringence (or the order of the optical path length difference depending on the polarization) for achieving the target PDf is obtained.

The maximum value and minimum value of the optical output at a certain wavelength can be derived by obtaining the eigenvalue of the product of the complex conjugate transpose matrix (M ^* ) ^T and M of M, where M is the Jones matrix of MZI (Non-Patent Document). 3). From equation (9), the MZI Jones matrix can be written as:

The eigenvalues of ^(M *) ^T M calculated for a wavelength (or frequency) to derive the calculated PDf the spectrum around 1.55 .mu.m.

4 (a) and 4 (b) show the results of calculating the relationship between PDf and birefringence when the operating wavelength of the half-wave plate deviates from the working wavelength. In the calculation, it was assumed that the coupling rate of the coupler was 50%, and the operating wavelength of the half-wave plate was shifted by 4% from the operating wavelength. This is because the operating wavelength of the half-wave plate we are using has an error (standard deviation) of 4% from the design wavelength. 4 (a) and 4 (b), in order to make PDf 1 / 100th of FSR, the variation of birefringence may be ± 0.1 × 10 ⁻⁴ or less.

  5 (a) and 5 (b), with respect to FIGS. 4 (a) and 4 (b), the horizontal axis is the degree (the value obtained by dividing the polarization-dependent optical path length difference Δ (BL) by the wavelength used). A graph of the case is shown. 5 (a) and 5 (b), it can be seen that the allowable variation in the order is ± 0.2.

  Similarly, changes in PDf due to environmental temperature changes can be inferred using FIGS. 4 (a) and 4 (b).

  4 (a) and 4 (b) indicate the birefringence dependence of PDf in the present invention, and the thin line in the second example of the prior art. It can be seen that the birefringence dependence of the PDf of the present invention is relaxed compared to the second example of the prior art. That is, since birefringence changes with environmental temperature, it turns out that the environmental temperature dependence of PDf is eased.

<When the main axis of the half-wave plate is in the same direction / cross path>
Similarly, for the cross path, a condition that does not depend on the polarization at the extinction wavelength can be calculated.
The optical output that propagates through I1, arm 1, and O2 and is output is given by equation (18).

Here, φ _TE and φ _TM are the phase differences between the TE polarization and the TM polarization between the two arm waveguides, respectively, and Ψ is the phase difference given by the half-wave plate. The coupling rate of the coupler was 50%. Further, the optical output that is propagated and output through I1, arm 2, and O2 is expressed by equation (19).

  From the above two equations, the optical output of the cross path can be expressed as equation (20).

  From equation (20), the condition for polarization independence at the extinction wavelength is

Since it is time to satisfy, the following equation should be satisfied.

Here, m, n, and α are integers. Therefore, the following equation is obtained.

  Further rewriting yields equation (26).

  From this, it can be seen that when the optical path length difference depending on the polarization becomes even order at a certain extinction wavelength, the polarization becomes independent.

<When the main axis of the half-wave plate is orthogonal / through pass>
A Mach-Zehnder interferometer in which a half-wave plate is inserted in the center of two arm waveguides. The main axis of the half-wave plate provided in the two arm waveguides is inclined by 45 degrees from a line perpendicular to the surface of the substrate. In the MZI, which is orthogonal to the propagation direction of the optical signal and the directions of the principal axes of the wave plates inserted in the respective arm waveguides are orthogonal, the optical output that propagates through I1, arm 1, and O1 and is output. Becomes Equation (27).

  Further, the optical output that is propagated and output through I1, arm 2, and O1 is expressed by equation (28).

  From Expressions (27) and (28), the optical output of the through path is Expression (29).

  From equation (29), the condition for polarization independence at the extinction wavelength is

Since it is time to satisfy, the following equation should be satisfied.

Here, m, n, and α are integers. Therefore, the following equation is obtained.

  Further rewriting, Equation (35) is derived.

  From this, it can be seen that when the optical path length difference depending on the polarization becomes an odd order at a certain extinction wavelength, the polarization becomes independent.

<When the main axis of the half-wave plate is orthogonal / cross-pass>
Similarly, the polarization independent condition can be calculated for the cross path.
The optical output that propagates through I1, arm 1, and O1 and is output is given by equation (36).

Here, φ _TE and φ _TM are the phase differences between the TE polarization and the TM polarization between the two arm waveguides, respectively, and Ψ is the phase difference given by the half-wave plate. The coupling rate of the coupler was 50%. Also, the optical output that propagates through I1, arm 2, and O1 and is output is given by equation (37).

  From the above two equations, the optical output of the through path can be expressed as equation (38).

  From equation (38), the condition for polarization independence at the extinction wavelength is

Since it is time to satisfy, the following equation should be satisfied.

Therefore, the following equation is obtained.

  Further rewriting yields equation (44).

  From this, it can be seen that when the optical path length difference depending on the polarization becomes an odd order at a certain extinction wavelength, the polarization becomes independent.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
In the following description, a quartz optical waveguide formed on a silicon substrate as an optical waveguide will be described as an example. 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 silica-based optical waveguide or a silicon oxynitride film (SiON), PMMA (polymethyl methacrylate). Of course, an organic optical waveguide such as a resin or a silicon optical waveguide may be used.

  In the following description, a multimode interferometer type coupler is used as a coupler. However, a directional coupler can also be used.

With reference to FIG. 6, the waveguide manufacturing process of this embodiment will be briefly described. Flame hydrolysis deposition on the silicon substrate 602 under cladding glass particles 604 and the SiO ₂ mainly by (FHD), and depositing a core glass fine particles 606 with the addition of GeO ₂ in SiO ₂ in the order (FIG. 6 (1)). At this stage, the glass particles scatter light and appear as a white film.

  Then, glass transparency is performed at a high temperature of 1000 ° C. or higher. When the silicon substrate on which the glass fine particles are deposited is gradually heated, the glass fine particles are melted to form a transparent glass film. At this time, glass fine particles are deposited so that the lower clad glass layer 604 is 30 microns thick and the core glass layer 606 is 7 microns thick (FIG. 6 (2)).

  Subsequently, the core glass layer 606 is patterned by photolithography and reactive ion etching (RIE) (FIG. 6 (3)).

  Upper clad glass fine particles 608 are deposited on the upper portion of the core by flame deposition (FIG. 6 (4)). Finally, high-temperature transparency is performed to produce a buried waveguide (FIG. 6 (5)). Boron trioxide and phosphorus pentoxide are added as dopants to the upper cladding glass layer 608 to lower the glass transition temperature so that the core is not deformed in the final high-temperature transparency step.

  FIG. 7 shows a schematic diagram of MZI produced as the first embodiment of the present invention. The MZI 700 shown in FIG. 7 includes two multimode interferometer-type couplers (722, 724) and two arm waveguides (706, 708) connecting the two multimode interferometer-type couplers. Further, the MZI 700 has an arm waveguide 706 at the center so that the optical path length of the arm waveguide 706 is bisected, and the arm waveguide 706 is divided into an arm waveguide 706b at the center and the optical path length of the arm waveguide 708 is bisected. A half-wave plate 732 inserted into a groove formed so as to divide the waveguide 708 into an arm waveguide 708a and an arm waveguide 708b at the center thereof is provided. The multimode interferometer-type coupler 722 includes two input waveguides (702, 704), and the multimode interferometer-type coupler 724 includes two output waveguides (710, 712).

  The half-wave plate 732 as a polarization rotator is a polyimide wave plate, the optical principal axis is orthogonal to the propagation direction, and is inclined 45 degrees from the horizontal direction of the substrate surface (or the normal of the substrate surface), and the slow axis A phase difference equivalent to half the design wavelength is given to the polarized waves propagating along the fast axis. The half-wave plate 732 has a function of performing polarization conversion from TM polarization to TE polarization and from TE polarization to TM polarization.

  In the present MZI 700, the length difference (ΔL) of the two arm waveguides (706, 708) is set to 20.7 mm in order to set the FSR to 10 GHz.

  Further, the MZI 700 of this embodiment adjusts birefringence by irradiating a part of the arm waveguide with an ArF (argon fluoride) laser before and after the half-wave plate 732. Thereby, Δ (BL) in the present MZI 700 is set to zero, and polarization dependence is suppressed.

A method for setting Δ (BL) to zero will be specifically described below.
In the arm waveguide 1 (706), the length of the waveguide in the laser irradiation region is L ^ir ₁ , and the length of the waveguide in the ^non- laser irradiation region is L ^non ₁ . In the arm waveguide 2 (708), the length of the waveguide in the laser irradiation region is L ^ir ₂ , and the length of the waveguide in the ^non- laser irradiation region is L ^non ₂ . The birefringence of the laser irradiation region is B ^ir and the birefringence of the laser non-irradiation region is B ^non . Using these, Δ (BL) is expressed by Equation (45).

For simplicity, when L ^ir ₁ = 0, Expression (46) is obtained.

  Where the difference between the lengths of the two arm waveguides is

The difference in birefringence was ΔB = B ^ir -B ^non. The condition that Equation (46) becomes 0 is Equation (47).

Therefore, laser irradiation was performed so that L ^ir ₂ was 10 mm and ΔB was 2.1 × 10 ⁻⁴ so as to satisfy Equation (47). As the laser, an ArF excimer laser with a wavelength of 193 nm was used, the irradiation power was IJ / cm ² , the pulse repetition was 50 Hz, and the irradiation time was 530 sec. In addition, using a metal mask, laser irradiation was performed only on two laser irradiation regions sandwiching a half-wave plate not covered with the metal mask. The irradiation area is 50 μm × 5 mm. On the other hand, ΔL was 20.7 mm, B ^non was 1 × 10 ⁻⁴ , and Δ (BL) was almost zero.

  According to this embodiment, PDf can be suppressed to 0.04 GHz in the vicinity of the wavelength of 1.55 μm for both the cross path and the through path. In the first prior art MZI, the PDf was 0.35 GHz for both the cross path and the through path. Therefore, PDf could be suppressed by using the present invention.

  The variation of PDf when the environmental temperature was changed from −10 ° C. to 80 ° C. was 0.06 GHz for both the cross path and the through path. On the other hand, in the second prior art MZI, the fluctuation of the PDf was 6 GHz for both the cross path and the through path. Therefore, the temperature dependence of PDf could be reduced by using the present invention.

  Note that the portion of the arm waveguide (706, 708) that irradiates the ArF laser may be away from the half-wave plate 732, and need not be symmetrical with respect to the half-wave plate 732 if the irradiation amount is symmetric. .

In this example, an ArF excimer laser having a wavelength of 193 nm was used for adjusting the birefringence. This is because the change in the refractive index of the core is due to absorption related to GeO ₂ having a wavelength of 245 nm. By irradiating a laser having an oscillation wavelength near 245 nm, the refractive index can be changed efficiently or the birefringence can be changed. Because it becomes. Furthermore, a similar change can be induced by two-photon absorption even in a visible laser. Therefore, the laser is not limited to the ArF excimer laser, but various excimer lasers such as He-Cd laser, N ₂ laser, KrF excimer laser, F ₂ excimer laser, Ar ion laser, Nd ³⁺ : YAG laser, What is necessary is just to have a wavelength of ultraviolet and visible region, such as the second order, the third order, and the fourth harmonic of the alexandrite (Cr ³⁺ : BeAl ₂ O ₃ ) laser.

In this embodiment, ArF laser irradiation is used as the birefringence adjustment method, but the same effect can be obtained by using, for example, the following birefringence adjustment method.
(1) A method of controlling the birefringence by arranging a stress applying film on the waveguide and changing the stress induced in the waveguide can be used (see Non-Patent Document 4). That is, a stress applying film (not shown) is formed on the upper part of the arm waveguides (706, 708), and the birefringence values are integrated in the propagation direction of the optical signal in the two arm waveguides. The stress induced in the arm waveguide can be adjusted so that the difference becomes a desired value.
(2) A method of locally heating a thin film heater disposed near the upper portion of the waveguide to permanently control the effective refractive index or birefringence can be used (see Patent Document 2). That is, a thin film heater (not shown) is formed near the upper part of the arm waveguides (706, 708), and the birefringence values are integrated in the propagation direction of the optical signal in the two arm waveguides. The stress induced in the arm waveguide can be adjusted by controlling the difference to be a desired value.
(3) It is also possible to use a method of controlling the effective refractive index or birefringence by providing stress relief grooves on both sides of the waveguide and relaxing the stress applied to the waveguide (see Patent Document 1). That is, for each of the two arm waveguides (706, 708), the difference between the values obtained by line-integrating the birefringence toward the propagation direction of the optical signal becomes a desired value, so that both sides of the arm optical waveguide are provided. It is also possible to adjust the birefringence birefringence by forming a stress relief groove (not shown) in a part.

  FIG. 8 shows a schematic view of MZI produced as a second embodiment of the present invention. The MZI 800 shown in FIG. 8 includes two multimode interferometer couplers (822, 824) and two arm waveguides (806, 808) connecting the two multimode interferometer couplers. Further, the MZI 800 has an arm waveguide 806 that bisects the optical path length of the arm waveguide 808 at the center of the arm waveguide 806 so that the optical path length of the arm waveguide 806 is bisected. A half-wave plate 832 inserted in a groove formed so as to divide the waveguide 808 into an arm waveguide 808a and an arm waveguide 808b at the center thereof is provided. The multimode interferometer-type coupler 822 includes two input waveguides (802, 804), and the multimode interferometer-type coupler 824 includes two output waveguides (810, 812).

  The configuration, arrangement, and function of the half-wave plate 832 as a polarization rotator are the same as those of the half-wave plate 732 in the first embodiment.

  In the MZI 800 of the present embodiment, birefringence is adjusted by changing the width of the two-arm waveguide (806, 808). By changing the waveguide width, the optical path length difference Δ (BL) depending on the polarization can be made zero (see Patent Document 3).

  The width of the arm waveguide (806, 808) is 7 μm in the vicinity of the multimode interferometer coupler (822, 824), and the width of the arm waveguide is 12 μm in the vicinity of the half-wave plate 832.

  In this MZI 800, the difference in length (ΔL) between the two arm waveguides (806, 808) is set to 20.7 mm, similar to the two arm waveguides (706, 708) in the first embodiment. Yes.

The effect of using two waveguide widths will be specifically described using mathematical expressions. In the arm waveguide 1, the length of the wide waveguide is L ^w ₁ , and the length of the narrow waveguide is L ⁿ ₁ . In the arm waveguide 2, the length of the wide waveguide is L ^w ₂ , and the length of the narrow waveguide is L ⁿ ₂ . Also, ^let B ^{w be} the birefringence of the wide waveguide and B ^{n be} the birefringence of the narrow waveguide. Using these, the difference Δ (BL) between the integral values of the birefringence in the propagation direction of the two arm waveguides is expressed by Expression (48).

For simplicity, when L ^w ₁ = 0, Expression (49) is obtained.

  Where the difference between the lengths of the two arm waveguides is

The difference in birefringence was ΔB = B ^ir -B ^non. The condition that Equation (49) becomes 0 is Equation (50).

L ^w ₂ was determined in consideration of satisfying the above equation. Specifically, B ^w = 1.57 × 10 ⁻⁴ , B ⁿ = 0.87 × 10 ⁻⁴ , L ^w ₁ = 0.5 mm, and L ^w ₂ = 26.2 mm.

Here, if only Δ (BL) is set to zero, a thick waveguide is not necessary for the arm waveguide 1. However, it is desirable to dispose a thick waveguide for the arm waveguide 1 for the following reason.
• Eliminates the difference in loss between arm waveguides with and without tapered waveguides.
-By increasing the width of the waveguide that the half-wave plate divides, the diffraction loss when radiating with the half-wave plate having no confinement structure is suppressed.

  According to this embodiment, PDf can be suppressed to 0.05 GHz in the vicinity of a wavelength of 1.55 μm for both the cross path and the through path.

  Moreover, the fluctuation of PDf when the environmental temperature was changed from −10 ° C. to 80 ° C. could be suppressed to 0.06 GHz for both the cross path and the through path.

  FIG. 9 shows a schematic diagram of MZI produced as a third embodiment of the present invention. The MZI 900 shown in FIG. 9 includes two multimode interferometer-type couplers (922, 924) and two arm waveguides (906, 908) connecting the two multimode interferometer-type couplers. Further, the MZI 900 bisects the waveguide length of the arm waveguide 908 into the arm waveguide 906a and the arm waveguide 906b at the center of the arm waveguide 906 so that the waveguide length of the arm waveguide 906 is bisected. A half-wave plate 932 inserted in a groove formed so as to divide the arm waveguide 908 into an arm waveguide 908a and an arm waveguide 908b at the center thereof. The multimode interferometer-type coupler 922 includes two input waveguides (902, 904), and the multimode interferometer-type coupler 924 includes two output waveguides (910, 912).

  The configuration, arrangement, and function of the half-wave plate 932 as a polarization rotator are the same as those of the half-wave plate 732 in the first embodiment.

  In the present MZI900, the difference in length (ΔL) between the two arm waveguides (906, 908) is 20.7 mm, similar to the two arm waveguides (706, 708) in the first embodiment. Yes.

The feature of this configuration is that the birefringence value is 1.5 × 10 ⁻⁴ . Here, birefringence was adjusted by changing the thermal expansion coefficient and softening temperature of the upper clad glass. That is, the upper cladding glass was added with 10 mol% boron trioxide and 10 mol% phosphorus pentoxide with respect to quartz. As a result, the optical path length difference Δ (BL) depending on the polarization is 3.1 × 10 ⁻⁴ from the following equation.

  The optical path length difference Δ (BL) depending on the polarization is 2 when the wavelength used is 1.55 μm, and the expressions (16) and (26) are substantially satisfied.

  According to this example, PDf was able to be suppressed to 0.006 GHz for both the cross path and the through path in the vicinity of a wavelength of 1.55 μm.

  Moreover, the fluctuation of PDf when the environmental temperature was changed from −10 ° C. to 80 ° C. could be suppressed to 0.06 GHz for both the cross path and the through path.

10 and 11 are schematic views of MZI prepared as the fourth embodiment of the present invention. Also,
The MZI 1000 includes two multimode interferometer couplers (1022, 1024) and two arm waveguides (1006, 1008) connecting the two multimode interferometer couplers, which are formed on a silicon substrate 1102. Is provided. In addition, the MZI 1000 bisects the waveguide length of the arm waveguide 1008 into the arm waveguide 1006a and the arm waveguide 1006b at the center so that the waveguide length of the arm waveguide 1006 is bisected. A half-wave plate 1032 inserted in a groove 1120 formed so as to divide the arm waveguide 1008 into an arm waveguide 1008a and an arm waveguide 1008b at the center thereof. The multimode interferometer-type coupler 1022 includes two input waveguides (1002, 1004), and the multimode interferometer-type coupler 1024 includes two output waveguides (1010, 1012).

  As shown in FIG. 11, a half-wave plate 1032 as a polarization rotator includes a half-wave plate 1032a and a half-wave plate 1032b. In the half-wave plate 1032a and the half-wave plate 1032b, the optical principal axis is orthogonal to the propagation direction and is inclined 45 degrees from the horizontal direction (or normal line) of the substrate. The directions of the main axes of the wave plate 1032b are orthogonal to each other.

  The half-wave plate 1032a and the half-wave plate 1032b give a phase difference corresponding to the half wavelength of the design wavelength to the polarized waves propagating along the slow axis and the fast axis, respectively. The half-wave plate 1032 has a function of performing polarization conversion of TM polarization into TE polarization and TE polarization into TM polarization.

In this example, boron trioxide was added to the upper clad glass at a ratio of 20 mol% and phosphorus pentoxide at a ratio of 5 mol%, and the birefringence value was set to 0.775 × 10 ⁻⁴ . As a result, the optical path length difference depending on the polarization is 1.55 × 10 ⁻⁶ from the equation (52).

  The difference in optical path length depending on the polarization is 1 when the wavelength used is 1.55 μm, and can substantially satisfy the expressions (35) and (44).

  According to this example, PDf was able to be suppressed to 0.07 GHz for both the cross path and the through path in the vicinity of a wavelength of 1.55 μm.

  Moreover, the fluctuation of PDf when the environmental temperature was changed from −10 ° C. to 80 ° C. could be suppressed to 0.06 GHz for both the cross path and the through path.

  It goes without saying that the birefringence value can be adjusted by applying the first to third embodiments to the present embodiment.

  Furthermore, in the above embodiment, MZI having an FSR of 10 GHz was used. In addition, it is obvious that the same effect can be expected with other FSRs such as 20 GHz and 40 GHz.

It is the schematic of the Mach-Zehnder interferometer circuit using an optical waveguide. It is the schematic of the Mach-Zehnder interferometer circuit of the 1st example of a prior art. It is a figure for demonstrating the conditions from which a transmission spectrum becomes polarization independent. (A) is a figure which shows the result of having calculated the relationship between PDf and birefringence in case the operating wavelength of a half-wave plate has shifted | deviated by 4% from the use wavelength, (b) is an enlarged view of (a). (A) is the figure which made the horizontal axis the order regarding FIG. 4 (a), (b) is an enlarged view of (a). It is a figure for demonstrating the waveguide preparation process of the optical signal processor of embodiment of this invention. It is the schematic of the Mach-Zehnder interferometer circuit produced as a 1st Example of this invention. It is the schematic of the Mach-Zehnder interferometer circuit produced as a 2nd Example of this invention. It is the schematic of the Mach-Zehnder interferometer circuit produced as a 3rd Example of this invention. It is the schematic of the Mach-Zehnder interferometer circuit produced as the 4th Example of this invention. It is the schematic of the Mach-Zehnder interferometer circuit produced as the 4th Example of this invention.

Explanation of symbols

100, 200, 300, 700, 800, 900, 1000 Mach-Zehnder interferometer circuit 106, 108, 206, 208, 306, 308, 706, 708, 806, 808, 906, 908, 1006, 1008 Arm waveguide 122, 124 , 222, 224, 322, 324, 722, 724, 822, 824, 922, 924, 1022, 1024 couplers 232, 732, 832, 932, 1032 half-wave plates

Claims (10)

In a Mach-Zehnder interferometer circuit comprising two couplers and two arm waveguides connecting the two couplers, each fabricated on a substrate,
A polarization rotator with the same optical principal axis that converts horizontally polarized light into vertically polarized light and converts vertically polarized light into horizontally polarized light. It is provided in a groove that bisects each optical path length,
For each of the two arm waveguides, a value obtained by dividing the difference obtained by linear integration of birefringence in the propagation direction of the optical signal by the used light wavelength is 2m−0.2 to 2m + 0.2 (m is zero) An optical signal processor. In a Mach-Zehnder interferometer circuit comprising two couplers and two arm waveguides connecting the two couplers, each fabricated on a substrate,
Two polarization rotators that convert horizontally polarized light to vertically polarized light and convert vertically polarized light to horizontally polarized light and whose optical principal axes are orthogonal to each other are the two arms. It is provided in each groove that bisects the optical path length of each waveguide ,
For each of the two arm waveguides, a value obtained by dividing the difference between the values obtained by integrating the birefringence in the propagation direction of the optical signal by the used optical wavelength is (2m−1) −0.2 to (2m−). 1) An optical signal processor characterized in that it is +0.2 (m is an integer including zero). The optical signal processor according to claim 1,
The polarization rotator is an optical signal processing characterized in that the optical principal axis is a half-wave plate installed so that the optical principal axis is inclined 45 degrees from the normal of the surface of the substrate and is orthogonal to the propagation direction of the optical signal vessel. The optical signal processor according to claim 2, wherein
The two polarization rotators are half-wave plates installed so that an optical principal axis is inclined 45 degrees from a normal of the surface of the substrate and is orthogonal to a propagation direction of an optical signal, An optical signal processor characterized in that one optical principal axis of the detector and the other optical principal axis of the polarization rotator are orthogonal to each other. The optical signal processor according to any one of claims 1 to 4,
The optical signal processor according to claim 1, wherein the coupler is a directional coupler or a multimode interference coupler. The optical signal processor according to any one of claims 1 to 5,
At least one of the two arm waveguides is a waveguide whose waveguide width is partially changed, and birefringence is linearly integrated in the propagation direction of the optical signal for each of the two arm waveguides. An optical signal processor, wherein birefringence is adjusted so that a difference in values becomes a desired value. The optical signal processor according to any one of claims 1 to 5,
At least one of the two arm waveguides is a waveguide partially irradiated with a laser, and a value obtained by linearly integrating birefringence in the propagation direction of an optical signal for each of the two arm waveguides. The birefringence is adjusted so that the difference between the two becomes a desired value. The optical signal processor according to any one of claims 1 to 5,
At least one of the two arm waveguides is a waveguide in which a stress relief groove is formed on a part of both sides, and birefringence of each of the two arm waveguides toward the propagation direction of the optical signal A birefringence is adjusted so that a difference between values obtained by line integration of the two becomes a desired value. The optical signal processor according to any one of claims 1 to 5,
At least one of the two arm waveguides is a waveguide having a stress applying film formed on the upper surface, and birefringence is linearly integrated in the propagation direction of the optical signal for each of the two arm waveguides. An optical signal processor, wherein birefringence is adjusted so that a difference in values becomes a desired value. The optical signal processor according to any one of claims 1 to 5,
At least one of the two arm waveguides is a waveguide on which a thin film heater for changing a stress is formed, and each of the two arm waveguides is compounded in the optical signal propagation direction. An optical signal processor characterized in that birefringence is adjusted so that a difference between values obtained by line integration of refraction becomes a desired value.

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