Optical filter, method of manufacturing, design method, design device, and design program for optical filter

Abstract

To provide an optical filter which offers both suppressed temperature dependence and suppressed waveguide length, and to provide a manufacturing method, design method, design device, and design program for the optical filter.SOLUTION: An optical filter is provided, comprising three or more waveguides and a section provided in each of the three or more waveguides, where light propagates through the sections of the three or more waveguides in different modes.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present disclosure relates to an optical filter, a manufacturing method, a design method, a design device, and a design program for an optical filter. [Background technology]

[0002] Silicon photonics, which forms optical elements in silicon, has been attracting attention. For example, a Mach-Zehnder interferometer may be formed in the silicon layer of an SOI (Silicon on Insulator) substrate. The TE0 mode and TE1 mode are propagated separately through multiple waveguides in the Mach-Zehnder interferometer. The temperature dependence of the spectrum is reduced by adjusting the length and width of the waveguide according to the mode (Non-Patent Document 1). A polarization rotator is provided in the silicon waveguide to convert the light mode (Non-Patent Document 2). [Prior art documents] [Non-patent literature]

[0003] [Non-Patent Document 1] “Broadband CMOS-compatible SOI temperature insensitive Mach-Zehnder interferometer” Peng Xing and Jaime Viegas OPTICS EXPRESS Vol.23,No.19 pp.24098-24107(2015) [Non-Patent Document 2] “Polarizatio rotator-splitters in standard active silicon photonics platforms” Wesley D.Sacher et al. OPTICS EXPRESS Vol.22, No.4 pp.3777-3786(2014) Summary of the Invention [Problem to be solved by the invention]

[0004] According to formula (9) in Non-Patent Document 1, the length of the waveguide can be designed so that the temperature dependency is small. In order to miniaturize the optical filter, the shorter the waveguide, the better. The length of the waveguide depends on the width of the waveguide and the FSR (the interval between the spectral peaks). If the width is large, the length of the waveguide increases. The FSR (the interval between the spectral peaks) and the length of the waveguide are inversely proportional to each other. If the FSR is reduced, the length of the waveguide increases. That is, even if the temperature dependency is reduced, the length of the waveguide may increase. If the waveguide is long, the device may become large and the propagation loss in the waveguide may increase. Therefore, the object is to provide an optical filter, a manufacturing method, a design method, a design device, and a design program for an optical filter that can simultaneously suppress the temperature dependency and the waveguide length. [Means for solving the problem]

[0005] An optical filter according to the present disclosure comprises three or more waveguides and sections provided in each of the three or more waveguides, and modes of light propagating through the sections of the three or more waveguides are different from one another. Effect of the Invention

[0006] According to the present disclosure, it is possible to provide an optical filter that can achieve both suppression of temperature dependency and suppression of the waveguide length, as well as a manufacturing method, design method, design apparatus, and design program for an optical filter. [Brief description of the drawings]

[0007] [Figure 1] FIG. 1 is a plan view illustrating an optical filter according to an embodiment. [Figure 2A] FIG. 2A is a plan view illustrating an example of a waveguide. [Figure 2B] FIG. 2B is a plan view illustrating an example of a waveguide. [Figure 2C] FIG. 2C is a plan view illustrating an example of a waveguide. [Figure 2D] FIG. 2D is a plan view illustrating an example of a waveguide. [Figure 3A]FIG. 3A is a cross-sectional view illustrating an example of a waveguide. [Figure 3B] FIG. 3B is a cross-sectional view illustrating an example of a polarization rotator. [Figure 4A] FIG. 4A is a diagram illustrating the transmission spectrum of an optical filter. [Figure 4B] FIG. 4B is a diagram illustrating an example of the shift amount. [Figure 5A] FIG. 5A is a block diagram illustrating an optical filter design apparatus. [Figure 5B] FIG. 5B is a block diagram showing the hardware configuration of the control unit. [Figure 6A] FIG. 6A is a flow chart illustrating a method for manufacturing an optical filter. [Figure 6B] FIG. 6B is a flow chart illustrating a method for manufacturing an optical filter. [Figure 7A] FIG. 7A is a cross-sectional view illustrating a method for manufacturing an optical filter. [Figure 7B] FIG. 7B is a cross-sectional view illustrating a method for manufacturing an optical filter. [Figure 8] FIG. 8 is a plan view illustrating an optical filter according to a comparative example. [Figure 9] FIG. 9 is a diagram illustrating an example of the calculation result of the section length. [Figure 10] FIG. 10 is a diagram illustrating an example of the calculation result of the section length. [Figure 11] FIG. 11 is a diagram illustrating the results of calculation of the section length. [Figure 12] FIG. 12 is a diagram illustrating the results of calculation of the section length. [Figure 13A] FIG. 13A is an example of an approximation curve. [Figure 13B] FIG. 13B is an example of an approximation curve. [Figure 14A] FIG. 14A is a diagram illustrating an example of the difference in section length. [Figure 14B] FIG. 14B is a diagram illustrating the ratio of section lengths. [Figure 15] FIG. 15 is a diagram illustrating a wavelength locker. [Figure 16] FIG. 16 is a diagram illustrating the transmittance. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] [Description of the embodiments of the present disclosure] First, the contents of the embodiments of the present disclosure will be listed and described.

[0009] One embodiment of the present disclosure is an optical filter including (1) three or more waveguides and sections provided in each of the three or more waveguides, in which the modes of light propagating through the sections of the three or more waveguides are different from one another. By propagating three or more different modes, it is possible to simultaneously suppress temperature dependency and suppress the waveguide length. (2) In the above (1), at least one of the three or more waveguides may have a polarization rotator. The polarization rotator converts the mode of light. By propagating three or more different modes, it is possible to simultaneously suppress temperature dependency and suppress the waveguide length. (3) In the above (2), the waveguide may include a waveguide core, and the polarization rotator may include the waveguide core and a rib portion protruding from the waveguide core. The phase of light changes in the polarization rotator. The length of the section is designed taking into consideration the amount of phase change. It is possible to simultaneously suppress temperature dependency and suppress the waveguide length. (4) In the above (2) or (3), the three or more waveguides include a first waveguide, a second waveguide, a third waveguide, and a fourth waveguide, the sections include a first section, a second section, a third section, and a fourth section, the polarization rotator includes a first polarization rotator and a second polarization rotator, the first waveguide is optically coupled to the second waveguide at a first coupling section and optically coupled to the third waveguide at a second coupling section downstream of the first coupling section, the first section is between the first coupling section and the second coupling section, the second waveguide is optically coupled to the fourth waveguide at a third coupling section downstream of the first coupling section, the second section is between the first coupling section and the third coupling section, and the third waveguide is optically coupled to the fourth waveguide at a third coupling section downstream of the second coupling section. In the fourth coupling section, the fourth waveguide is optically coupled to a portion of the fourth waveguide that is upstream of the third coupling section, and the third section is between the second coupling section and the fourth coupling section, the third waveguide has the first polarization rotator and the second polarization rotator, the first polarization rotator is located between the second coupling section and the fourth coupling section, the second polarization rotator is located between the first polarization rotator and the fourth coupling section, the third section is located between the first polarization rotator and the second polarization rotator, the fourth waveguide has the fourth section between the third coupling section and the fourth coupling section, and the mode of light propagating through the first section and the fourth section, the mode of light propagating through the second section, and the mode of light propagating through the third section may be different from each other. Three modes propagate through the optical filter. It is possible to achieve both suppression of temperature dependency and suppression of the waveguide length. (5) In the above (4), a TE0 mode may propagate through the first section and the fourth section, a TE1 mode may propagate through the second section, and a TM0 mode may propagate through the third section. Three modes propagate through the optical filter. It is possible to simultaneously suppress temperature dependence and suppress the waveguide length. (6) A method for manufacturing an optical filter, the optical filter comprising three or more waveguides, each of the three or more waveguides having a section, the modes of light propagating through the sections of the three or more waveguides being different from one another, the sections having lengths according to the propagation constants in the sections and the amount of change in the phase of the light in the waveguide, the manufacturing method being a method for manufacturing an optical filter forming the three or more waveguides having the sections, it is possible to achieve both suppression of temperature dependency and suppression of the waveguide length. (7) In the above (6), the manufacturing method may include a step of designing the length of the section and a step of forming the three or more waveguides based on the length of the section, and the step of designing the length of the section may include a step of designing the length of the section based on Mathematical formula 1. It is possible to simultaneously suppress temperature dependency and suppress the waveguide length.

number

[0010] [Details of the embodiment of the present disclosure] Specific examples of an optical filter, a manufacturing method for an optical filter, a design method, a design device, and a design program according to an embodiment of the present disclosure will be described below with reference to the drawings. Note that the present disclosure is not limited to these examples, but is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0011] (Optical Filter) FIG. 1 is a plan view illustrating an optical filter 100 according to an embodiment. The optical filter 100 is a Mach-Zehnder interferometer filter, and is formed on a substrate 10. The substrate 10 is, for example, a silicon on insulator (SOI) substrate. Two sides of the substrate 10 are parallel to the X-axis direction. The other two sides are parallel to the Y-axis direction. The top surface of the substrate 10 is parallel to the XY plane. The Z-axis direction is a normal direction of the substrate 10. The X-axis direction, the Y-axis direction, and the Z-axis direction are perpendicular to each other.

[0012] Optical filter 100 has waveguide 20 (first waveguide), waveguide 22 (second waveguide), waveguide 24 (third waveguide), and waveguide 26 (fourth waveguide). Waveguide 20 has section 21 (first section). Waveguide 22 has section 23 (second section). Waveguide 24 has section 25 (third section). Waveguide 26 has section 27 (fourth section). In the figure, the sections are the parts surrounded by dotted lines.

[0013] Waveguide 20 has input port 1. Waveguide 26 has output port 2. Input port 1 is located at one end of substrate 10. Output port 2 is located at the other end of substrate 10. Arrows in FIG. 1 represent light propagating through optical filter 100. Light incident on input port 1 is split into multiple waveguides, propagates, and combined, and is output from output port 2. The front stage of the waveguide is the one closer to input port 1, and the rear stage is the one closer to output port 2 than the front stage.

[0014] The waveguides 22 and 24 are parallel to the X-axis direction. The waveguides 22 and 24 are aligned in the Y-axis direction. The waveguides 20 and 26 are located between the waveguides 22 and 24.

[0015] The waveguide 20 and the waveguide 22 form a directional coupler 28 (first coupling portion). In the directional coupler 28, the waveguide 20 and the waveguide 22 are optically coupled. A portion of the waveguide 20 subsequent to the directional coupler 28 and the waveguide 24 form a directional coupler 30 (second coupling portion). In the directional coupler 30, the waveguide 20 and the waveguide 24 are optically coupled.

[0016] A portion of waveguide 22 subsequent to directional coupler 28 and waveguide 26 form directional coupler 34 (third coupling portion). In directional coupler 34, waveguide 22 and waveguide 26 are optically coupled. A portion of waveguide 24 subsequent to directional coupler 30 and a portion of waveguide 26 prior to directional coupler 34 form directional coupler 32 (fourth coupling portion). In directional coupler 32, waveguide 24 and waveguide 26 are optically coupled. As long as the two waveguides are optically coupled, a coupler other than a directional coupler may be used.

[0017] The coupling length of directional coupler 30 and directional coupler 32 is greater than that of directional coupler 28 and directional coupler 34, for example, more than twice the coupling length of directional coupler 28 and directional coupler 34. The longer the coupling length, the higher the coupling coefficient, and the greater the intensity of the transferred light. The shorter the coupling length, the lower the intensity of the transferred light. The coupling coefficient of directional coupler 30 and directional coupler 32 is greater than that of directional coupler 28 and directional coupler 34.

[0018] The waveguide 24 has a polarization rotator 36 (first polarization rotator) and a polarization rotator 38 (second polarization rotator). The polarization rotator 36 is located between the directional coupler 30 and the directional coupler 32. The polarization rotator 38 is located between the polarization rotator 36 and the directional coupler 32. The section 25 is located between the polarization rotator 36 and the polarization rotator 38.

[0019] 2A to 2D are plan views illustrating waveguides. Fig. 2A is a plan view of the waveguide 20 and the waveguide 26. The waveguide 20 and the waveguide 26 are curved. The width W1 of the waveguide 20 and the waveguide 26 is constant, for example, 400 nm.

[0020] Section 21 of waveguide 20 is the portion between directional coupler 28 and directional coupler 30. Section 27 of waveguide 26 is the portion between directional coupler 32 and directional coupler 34. The length of section 21 is L1a. The length of section 27 is L1b. The combined length of sections 21 and 27 is L1.

[0021] FIG. 3A is a cross-sectional view illustrating the waveguide 26, and illustrates a cross section taken along line AA in FIG. 2A. The substrate 10 includes a substrate 12, a box layer 14, and a waveguide core 16. The substrate 12 and the waveguide core 16 are made of silicon (Si). The box layer 14 is made of an insulator such as silicon oxide (SiO2). The box layer 14 is laminated on one surface of the substrate 12. The waveguide core 16 is embedded in the box layer 14 and functions as a waveguide. The waveguide core 16 is a part of the silicon layer. The silicon layer includes a terrace (not shown). The waveguide core 16 is spaced apart from the terrace. The thickness T1 of the waveguide core 16 is, for example, 220 nm. The distance T0 from the substrate 12 to the waveguide core 16 is, for example, 2 μm.

[0022] 2B is a plan view of the waveguide 22. The waveguide 22 has a section 23, a section 40, a section 41, a section 42, and a section 43. The sections 40, 41, 23, 42, and 43 are arranged in this order along the X-axis direction. The cross section of the waveguide 22 is similar to that of FIG. 3A.

[0023] 1 is formed in the section 40. The directional coupler 34 is formed in the section 43. The width W2 of the sections 40 and 43 is larger than the width W1 of the waveguide 20 and 26 and is, for example, 831 nm. The widths of the sections 40 and 43 are constant.

[0024] Section 41 and section 42 have a tapered shape. The width of section 41 decreases from section 40 toward section 23. The width of section 42 increases from section 23 toward section 43. The widths of sections 41 and 42 vary from 831 nm to 660 nm, for example.

[0025] Section 23 is the portion surrounded by a dotted line in Fig. 2B, and is located between section 41 and section 42. The width W3 of section 23 is larger than the width W1 of waveguide 20 and waveguide 26, and smaller than the width W2 of section 40 and section 43, and is, for example, 660 nm. The width W3 of section 23 is constant. The length of section 23 is L2.

[0026] 2C is a plan view of the waveguide 24. The waveguide 24 has a section 44, a polarization rotator 36, a section 45, a section 25, a section 46, a polarization rotator 38, and a section 47. In the Y-axis direction, the section 44, the polarization rotator 36, the section 45, the section 25, the section 46, the polarization rotator 38, and the section 47 are arranged in this order.

[0027] The width W4 of the sections 44 and 47 is greater than the width W1 of the waveguides 20 and 26 and is, for example, 831 nm. The width of the section 44 is, for example, 880 nm near the polarization rotator 36. The width of the section 47 is, for example, 880 nm near the polarization rotator 38.

[0028] Sections 45 and 46 have a tapered shape. The width of section 45 increases from the polarization rotator 36 to section 25. The width of section 46 decreases from section 25 to the polarization rotator 38. The widths of sections 45 and 46 vary, for example, from 500 nm to 1200 nm.

[0029] Section 25 is the portion surrounded by a dotted line in Fig. 2C. The width W5 of section 25 is constant and is larger than the width W4 of sections 44 and 47 and the width W1 of waveguide 20 and waveguide 26, and is, for example, 1200 nm. The length of section 25 is L3. The cross sections of sections 44, 45, 25, 46 and 47 of waveguide 24 are similar to those in Fig. 3A.

[0030] Fig. 2D is a plan view illustrating the polarization rotator 36. Fig. 3B is a cross-sectional view illustrating the polarization rotator 36, taken along line BB in Fig. 2D. The polarization rotator 38 has the same configuration as the polarization rotator 36.

[0031] The polarization rotator 36 has a tapered planar shape. The polarization rotator 36 includes a waveguide core 16 and two rib portions 17. The waveguide core 16 is located in the center. The rib portions 17 protrude from the waveguide core 16 on both sides in the Y-axis direction.

[0032] The width W6 at one end of the waveguide core 16 is, for example, 880 nm. The width W7 at the center is, for example, 700 nm. The width W8 at the other end is, for example, 500 nm. The width W9 at the center of the rib portion 17 is, for example, 500 nm. The line BB in FIG. 2D is located at the center of the rib portion 17 in the X-axis direction. The length of the rib portion 17 in the X-axis direction is, for example, 100 μm.

[0033] 3B, the waveguide core 16 and the rib portion 17 are formed of a silicon layer. The thickness T2 of the rib portion 17 is smaller than the thickness T1 of the waveguide core 16, and is, for example, 60 nm.

[0034] As shown in FIG. 1, light is input to input port 1. The mode of the incident light is TE0 mode. The incident light propagates through waveguide 20. In directional coupler 28, a portion of the light is transferred to waveguide 22. For example, 50% of the light is transferred to waveguide 22, and the other 50% propagates through the portion of waveguide 20 subsequent to directional coupler 28. That is, the intensity ratio between the light that continues to propagate through waveguide 20 and the light that is transferred to waveguide 22 is, for example, 1:1.

[0035] In the directional coupler 28, mode conversion occurs as the light transitions. The light mode changes from the TE0 mode to the TE1 mode. The light in the TE1 mode propagates through the section 23 of the waveguide 22.

[0036] The light in the TE0 mode propagates through section 21 of the waveguide 20, and is transferred to the waveguide 24 in the directional coupler 30. For example, 95% or more of the light may be transferred to the waveguide 24, or 100% may be transferred to the waveguide 24. In the directional coupler 30, the mode of the light is converted from the TE0 mode to the TE1 mode.

[0037] Mode conversion occurs in the polarization rotator 36, and the mode is converted from the TE1 mode to the TM0 mode. The light in the TM0 mode propagates through the section 25 of the waveguide 24 and enters the polarization rotator 38. Mode conversion occurs in the polarization rotator 38, and the mode is converted from the TM0 mode to the TE1 mode.

[0038] The light in TE1 mode is transferred to the waveguide 26 in the directional coupler 32. For example, 95% or more of the light may be transferred to the waveguide 26, or 100% may be transferred to the waveguide 26. In the directional coupler 32, the mode of the light is converted from the TE1 mode to the TE0 mode. The light in the TE0 mode propagates through the section 27 of the waveguide 26.

[0039] The light propagated through section 23 of waveguide 22 is transferred to waveguide 26 in directional coupler 34. For example, 50% of the light is transferred to waveguide 26. In directional coupler 34, the mode of the light is converted from TE1 mode to TE0 mode. The light in TE0 mode propagates through waveguide 26 and is output from output port 2.

[0040] As described above, the optical filter 100 branches incident light in one mode (e.g., TE0 mode) into multiple waveguides, propagates the light in different modes (TE0 mode, TE1 mode, TM0 mode), and emits output light in one mode (e.g., TE0 mode).

[0041] FIG. 4A is a diagram illustrating the transmission spectrum of the optical filter 100. The horizontal axis represents the wavelength of light. The vertical axis represents the transmittance of the optical filter 100. The spectrum is calculated with the temperatures of the optical filter 100 set to 20° C., 40° C., and 60° C. The solid line represents the spectrum at 20° C. The dashed line represents the spectrum at 40° C. The dotted line represents the spectrum at 60° C. The FSR is set to 0.8 nm. The widths W1 to W9 are the values ​​mentioned above. The length of the section is optimized using the design method described below.

[0042] As shown in Figure 4A, the spectrum shows periodic peaks. As shown in the inset of Figure 4A, the peaks of the spectrum at each temperature overlap. In other words, the shift of the peaks is suppressed.

[0043] FIG. 4B is a diagram illustrating the shift amount. The horizontal axis represents wavelength. The vertical axis represents the shift amount of the resonance peak of the spectrum when the temperature is changed from 20° C. to 60° C. The shift amount is within ±1 pm. The change amount of the peak position with respect to the temperature change is small. According to the embodiment, the temperature dependency of the spectrum is suppressed to be low.

[0044] In order to suppress the temperature dependency as described above, the section length should be set to an appropriate value. The design of the optical filter 100 will now be described.

[0045] (design equipment) 5A is a block diagram illustrating a design device 110 for the optical filter 100. The design device 110 designs the length of the section of the optical filter 100.

[0046] The design device 110 has a control unit 50. The control unit 50 is a computer or the like, and functions as a first calculation unit 52, a second calculation unit 54, and a third calculation unit 56. The function of each unit of the control unit 50 will be described later.

[0047] Fig. 5B is a block diagram showing a hardware configuration of the control unit 50. As shown in Fig. 5B, the control unit 50 includes a CPU (Central Processing Unit) 60, a RAM (Random Access Memory) 62, a ROM (Read Only Memory) 63, a storage device 64, and an interface 66. The CPU 60, the RAM 62, the ROM 63, the storage device 64, and the interface 66 are connected to one another via a bus or the like. The RAM 62 is a volatile memory that temporarily stores programs, data, and the like. The storage device 64 is a solid state drive (SSD) such as a flash memory, a hard disk drive (HHD), or the like. The storage device 64 stores programs and the like.

[0048] The CPU 60 executes a program stored in the RAM 62, thereby realizing a first calculation unit 52, a second calculation unit 54, and a third calculation unit 56 in the control unit 50. Each unit of the control unit 50 may be hardware such as a circuit.

[0049] (Calculation of section length) The design device 110 designs the length of the section of the optical filter 100 based on the following equation 2. The matrix on the left side of equation 2 may be written as matrix A. The column vector on the left side may be written as L. The first term on the right side may be written as B, and the second term may be written as C.

[0050]

number

[0051] In equation 2, β1 to βn are the propagation constants of the sections. L1 to Ln are the lengths of the sections. The subscripts 1 to n in β and L are numbers corresponding to the sections. For example, β1 is the propagation constant in section 21 of waveguide 20 and section 27 of waveguide 26. L1 is the sum of the lengths of section 21 of waveguide 20 and section 27 of waveguide 26. T represents the temperature of optical filter 100. λ is the wavelength of light propagating through optical filter 100. k is the wave number. FSR is the distance between adjacent peaks in the transmission spectrum of optical filter 100. φ is the sum of the changes in the phase of light.

[0052] Let us explain the matrix A on the left side. Each element in the first row of matrix A is a partial derivative of the propagation constant with respect to temperature T. Each element in the second row is a partial derivative of the propagation constant with respect to temperature T and wavelength λ. Each element in the third row is a partial derivative of the propagation constant with respect to wave number k. The elements of the column vector L on the left side are the lengths of the intervals.

[0053] The first term on the right-hand side is column vector B. The numerator of the third row of column vector B is the square of the wavelength λ. The denominator is the FSR.

[0054] The second term on the right-hand side is a column vector C. The first row of the column vector C is the partial derivative of the phase change φ with respect to temperature T. The second row is the partial derivative of the phase change φ with respect to temperature T and wavelength λ. The third row is the partial derivative of the phase change φ with respect to wave number k.

[0055] The propagation constant is proportional to the refractive index of the section and depends on the width of the section. By determining the width of the section, the propagation constant is determined, and the matrix elements of matrix A are determined. The elements of column vector B are determined by the wavelength λ of the light propagating through optical filter 100 and the magnitude of FSR. The amount of phase change φ depends on the shape of the waveguide, in particular the shape of the polarization rotator and the shape of tapered portions such as section 41. By determining the shape of the waveguide, the amount of phase change φ is determined, and the components of column vector C are also determined.

[0056] As shown in FIG. 1, optical filter 100 has sections 21, 23, 25, and 27. TE0 mode propagates through sections 21 and 27. TE1 mode propagates through section 23. TM0 mode propagates through section 25. In optical filter 100, it is only necessary to calculate the lengths of the sections corresponding to the three modes. That is, the combined length L1 of sections 21 and 27, the length L2 of section 23, and the length L3 of section 25 are calculated. By setting n=3 in equation 2, equation 3 is obtained. In equation 3, matrix A is a 3×3 matrix. Vectors L, B, and C are each a column vector with three rows.

[0057]

number

[0058] An example of the calculation is shown below. The width W1 of the sections 21 and 27 is set to 400 nm. The width W3 of the section 23 is set to 660 nm. The width W5 of the section 25 is set to 1200 nm. In this case, the matrix A is given by the following equation 4.

number

[0059] In number 4, the elements in the first row are 7.95×10-4 rad / μm / K, 8.03×10 -4 rad / μm / K, 5.57×10 -4 rad / μm / K. The element in the second row is -7.95×10 -4 rad / μm 2 / K, -1.63 x 10 -3 rad / μm 2 / K, -1.41 x 10 -3 rad / μm 2 / K. The elements of the third row are 4.25, 4.51, and 3.92.

[0060] The wavelength λ is 1550 nm and the FSR is 0.8 nm. As shown in Equation 5, the element λ of the column vector B 2 / FSR is 3003μm.

number

[0061] In Expression 3, vector C (second term on the right side) is set to zero, and the length of the section (section length) is calculated using the values ​​of Expressions 4 and 5. The calculation result of L is shown in Expression 6.

number

[0062] The length L1 is 1.39 mm, L2 is -4.24 mm, and L3 is 4.12 mm. In the calculation, the sign depends on the waveguide geometry and can be positive or negative. In practical design and manufacturing, the sign is positive.

[0063] Equation 7 is an example of vector C, which is a correction term.

number

[0064] Starting from the first row, the elements are -7.3×10 -2 rad / K, 0.1×10 -1 rad / μm / K, -402μm.

[0065] A more accurate interval length can be obtained by solving Equation 3 including the column vector C of Equation 7. The calculation result of L is shown in Equation 8.

number

[0066] The length L1 is 1.33 mm, L2 is -4.26 mm, and L3 is 4.12 mm. There is a difference of about several tens of μm in the section length between Equation 6 and Equation 8. Due to the effect of the correction term C, the section length can be determined with high accuracy and the temperature dependency can be reduced.

[0067] (Manufacturing method) 6A and 6B are flow charts illustrating a method for manufacturing the optical filter 100. As shown in Fig. 6A, a design process is performed to design the length and width of the waveguide (step S1). A wafer is processed to form the waveguide based on the design (step S2).

[0068] 7A and 7B are cross-sectional views illustrating a method for manufacturing the optical filter 100. As shown in FIG. 7A, a box layer 14a and a silicon layer 16a are laminated in this order on one surface of the substrate 12. A resist pattern (not shown) is provided on the silicon layer 16a. As shown in FIG. 7B, the silicon layer 16a is etched to form a waveguide core 16 from the silicon layer 16a. A box layer is further laminated on the box layer 14a to form the box layer 14. The optical filter 100 is formed by cutting the wafer.

[0069] As described above, the waveguide core 16 is manufactured by etching the silicon layer 16a. The thinner the waveguide core 16, the more difficult the etching. The thicker the waveguide core 16, the easier the etching and the easier it is to manufacture the waveguide core 16. The design is carried out taking into consideration the size and temperature dependency as well as such productivity.

[0070] 6B is a flow chart illustrating the design process, which corresponds to step S1 in FIG. 6A. The first calculation unit 52 of the control unit 50 calculates the matrix A and the column vector B (step S10). The first calculation unit 52 sweeps the width of the interval and calculates the matrix elements of the matrix A corresponding to each width. The first calculation unit 52 calculates the λ of the column vector B based on the wavelength of the light and the FSR. 2 Calculate / FSR.

[0071] The third calculation unit 56 calculates L1, L2, and L3 in the column vector L using the matrix A and the column vector B obtained in step S10 (step S12). At the time of step S12, the calculation is performed without taking into account the correction term (column vector C), and the lengths L1, L2, and L3 of the sections corresponding to the swept width are calculated.

[0072] The second calculation unit 54 calculates the correction term (step S14). The third calculation unit 56 calculates the section length taking the correction term into consideration (step S16). In detail, an appropriate value is selected from the width and section length obtained in step S12. A larger width makes it easier to manufacture the waveguide. The shorter the section of the waveguide, the smaller the size of the optical filter 100 can be. In order to achieve both productivity and miniaturization, it is effective to have a large width and a short section. The third calculation unit 56 selects a width and section length suitable for these requirements and adds the column vector C to the selected section length. That is, the section length is calculated using Equation 2 (step S16). This ends the processing of FIG. 6B.

[0073] (Comparative Example) 8 is a plan view illustrating an optical filter 101 according to a comparative example. The optical filter 101 has a waveguide 70 and a waveguide 74. The waveguide 70 has a section 71 and a section 72. One of the two sections 71 has an input port 1. The other of the two sections 71 has an output port 2. The section 72 is provided between the two sections 71. The width of the section 72 is greater than that of the section 71. The portion of the waveguide 70 between the sections 71 and 72 has a tapered shape.

[0074] One section 71 of the waveguide 70 and the waveguide 74 form a directional coupler 75. The other section 71 and the waveguide 74 form a directional coupler 76.

[0075] Light in TE0 mode enters at input port 1 and propagates through waveguide 70. At directional coupler 75, a portion of the light transfers from waveguide 70 to waveguide 74. The mode of the transferred light changes from TE0 mode to TE1 mode. The light in TE1 mode propagates through waveguide 74. The light in TE0 mode propagates through section 72 of waveguide 70. At directional coupler 76, light transfers from waveguide 74 to waveguide 70. The mode of the light changes from TE1 mode to TE0 mode. The light in TE0 mode propagates through waveguide 70 and is output from output port 2.

[0076] The optical filter 101 according to the comparative example propagates two modes. The optical filter 100 according to the embodiment propagates three modes. When the temperature dependence of the characteristics of the optical filter 100 and the optical filter 101 is set to the same degree, the section length of the optical filter 100 can be made shorter than the section length of the optical filter 101. That is, according to the embodiment, it is possible to suppress the temperature dependence and to achieve a compact size at the same time. An example of the calculation result of the section length is shown below.

[0077] (section length) 9 to 12 are diagrams illustrating the calculation results of the section length. The smaller the width of the waveguide, the lower the effective refractive index. The larger the width, the higher the effective refractive index. In the examples of Figs. 9 to 11, the width is swept and the section length is calculated using Equation 3. Section lengths L1, L2, and L3 are calculated for each effective refractive index. The horizontal axis of Figs. 9 to 11 represents the minimum value of the effective refractive index of the multiple sections. The vertical axis represents the maximum value of the multiple section lengths L1, L2, and L3.

[0078] In the example of Figure 9, the width is swept from 150 nm to 1200 nm in 20 nm intervals, and the section length is calculated using Equation 3. The maximum values ​​of the section lengths L1, L2, and L3 calculated for each refractive index are plotted in Figure 9. The FSR is 0.8 nm, and the wavelength λ is 1550 nm.

[0079] The embodiment is shown by dotted lines and black circles. The comparative example is shown by solid lines and squares. The higher the refractive index, the longer the section length. The section length of the embodiment is shorter than that of the comparative example. For example, when the refractive index is about 1.85, the section length of the embodiment is about half that of the comparative example.

[0080] In the example of Fig. 10, the width is swept from 150 nm to 1600 nm at approximately 10 nm intervals, and the section length for each refractive index is calculated. Black circles represent the comparative example. White circles represent the embodiment. The section length of the embodiment is shorter than the section length of the comparative example.

[0081] In the example of FIG. 11, the set of waveguide widths used in the example of FIG. 10 is swept in the range of ±10 nm at 1 nm intervals. Black circles represent the comparative example. White circles represent the embodiment. Most of the section lengths in the embodiment are shorter than the section lengths in the comparative example. FIG. 12 plots only the minimum values ​​of the section lengths for each effective refractive index in FIG. 11. The section lengths in the embodiment are shorter than the section lengths in the comparative example.

[0082] Figures 13A and 13B are examples of approximation curves. Figure 13A illustrates an approximation curve of a comparative example in Figure 12. Figure 13B illustrates an approximation curve of an embodiment in Figure 12. In Figures 13A and 13B, the solid lines represent calculation results. The dashed lines represent approximation curves.

[0083] The approximation curve in FIG. 13A is expressed by the following formula (1). y=-2000.8x 5 +16725x 4 -55699x 3 +92423x 2 -76428x+25202 (1) The approximation curve in FIG. 13B is expressed by the following formula (2). y=-2192.6x 5 +18972x 4 -65282x 3 +111720x 2 -95112x+32235 (2)

[0084] 14A is a diagram illustrating the difference in section length. The horizontal axis represents the effective refractive index. The vertical axis represents the difference between the section length in the comparative example of FIG. 12 and the section length in the embodiment. Since the section length in the embodiment is shorter than the section length in the comparative example, the difference is a positive value. When the effective refractive index is in the range of 1.8 to 1.9, the difference becomes large, and is 2.5 mm or more.

[0085] FIG. 14B is a diagram illustrating the ratio of section lengths. The horizontal axis represents the effective refractive index. The vertical axis represents the ratio between the section length in the comparative example of FIG. 12 and the section length in the embodiment. When the effective refractive index is in the range of about 1.45 to 1.5, the ratio is 1. When the effective refractive index is in the range of more than 1.5, the section length in the embodiment is shorter than that in the comparative example, so the ratio is less than 1. When the effective refractive index is around 1.8, the ratio is the lowest, about 0.6. That is, the section length in the embodiment is reduced to about 60% of that in the comparative example.

[0086] (wavelength locker) A wavelength locker 120 will be described as an application example of the optical filter 100. Fig. 15 is a diagram illustrating the wavelength locker 120. The wavelength locker 120 includes the optical filter 100, a wavelength tunable laser element 80, a control unit 82, and monitor units 83 and 84.

[0087] The tunable laser element 80 is a light source and emits laser light. The control unit 82 controls the wavelength of the light emitted from the tunable laser element 80. The light emitted from the tunable laser element 80 is split by a beam splitter 81. One of the split lights is incident on a monitor unit 83. The monitor unit 83 detects the intensity of the incident light.

[0088] The other of the branched lights enters input port 1 of the optical filter 100. The light exiting output port 2 of the optical filter 100 enters the monitor unit 84. The monitor unit 84 detects the intensity of the light. The control unit 82 acquires the intensity of the light detected by the monitor unit 83 and the intensity of the light detected by the monitor unit 84. The control unit 82 calculates the transmittance of the optical filter 100 based on the intensities.

[0089] FIG. 16 is a diagram illustrating transmittance. The horizontal axis is the frequency of light. The vertical axis is the transmittance of the optical filter 100. When the intensity of the light emitted from the optical filter 100 changes, the transmittance also changes. The characteristics of the optical filter 100 are stable against temperature changes. On the other hand, the wavelength of the light emitted from the wavelength-tunable laser element 80 may change due to temperature changes, etc. When the wavelength of the light emitted from the wavelength-tunable laser element 80 changes, the intensity of the light emitted from the optical filter 100 changes, and the transmittance also changes.

[0090] For example, the target value of the transmittance is 0.5. The frequency corresponding to the transmittance of 0.5 is 193.4 THz. When the frequency is higher than 193.4 THz, the transmittance is higher than 0.5. When the frequency is lower than 193.4 THz, the transmittance is lower than 0.5. The control unit 82 changes the oscillation wavelength of the wavelength-tunable laser element 80 so that the transmittance becomes 0.5. The oscillation wavelength is maintained at a desired value.

[0091] According to this embodiment, the optical filter 100 propagates different modes in three or more sections. As shown in FIG. 4A and FIG. 4B, the temperature dependency of the spectrum can be reduced by setting the waveguide length to an appropriate size. As shown in FIG. 14A and FIG. 14B, the section length in this embodiment is shorter than the comparative example in which two modes are propagated. Since the section length is reduced, the waveguide length can also be reduced. It is possible to suppress both the temperature dependency and the waveguide length. The optical filter 100 can be miniaturized, and the change in the spectrum with respect to temperature can be reduced.

[0092] As shown in FIG. 1, the optical filter 100 has waveguides 20, 22, 24, and 26, and directional couplers 28, 30, 32, and 34. The waveguide 20 has a section 21. The waveguide 22 has a section 23. The waveguide 24 has a section 25. The waveguide 26 has a section 27. The directional coupler 28 and the directional coupler 30 cause light to transition and mode conversion. The TE0 mode propagates through the section 21 of the waveguide 20 and the section 27 of the waveguide 26. The TE1 mode propagates through the section 23 of the waveguide 22. The TM0 mode propagates through the section 25 of the waveguide 24. Three modes propagate through the optical filter 100. As shown in FIG. 14A and FIG. 14B, the section length can be shortened according to the embodiment compared to the comparative example. The suppression of temperature dependency and the suppression of the waveguide length can be achieved at the same time.

[0093] As shown in Figures 6A and 6B, in the manufacturing process of the optical filter 100, the length of the section is designed. The section length is designed based on the propagation constant in the section and the amount of change in the phase of the light. The optimal section length is designed, and a waveguide having that section length is manufactured. By propagating different modes in three or more sections, the temperature dependence of the spectrum can be reduced. Since the section length is shorter than in the comparative example, the waveguide length can be suppressed.

[0094] The section length is designed based on Equation 2. In Equation 2, matrix A on the left side contains the propagation constant. The column vector on the left side contains the section lengths L1 to Ln. The first term on the right side contains the wavelength λ and FSR. The second term on the right side contains the phase change amount φ. By using Equation 2, the section lengths L1 to Ln can be calculated. It is possible to suppress both the temperature dependence and the waveguide length.

[0095] By determining the width of the section, the propagation constant βn is determined and each term in the matrix A is calculated. Using equation 2, Ln is calculated from the section length L1 according to the width. As shown in Figures 7A and 7B, the waveguide is manufactured by etching the silicon layer 16a. If the width is small, it is difficult to manufacture the waveguide. If the width is large, it is easier to manufacture. For example, the width is swept and the section length for each width is calculated. A value of the width that is easy to manufacture is selected, and the section length corresponding to that width is adopted. A waveguide having the width and section length is manufactured by etching the silicon layer 16a. The optical filter is easy to manufacture, and the waveguide length and temperature dependency can be suppressed.

[0096] When sweeping the width and searching for an appropriate interval length, a correction term (the second term on the right side in Equation 2) may be calculated for the entire width to be swept. In order to reduce the amount of calculations, the correction term may not be calculated for the entire width to be swept, but may be calculated for only some of the widths to be swept. An interval length that does not take the correction term into consideration is calculated for the width to be swept. Of the calculated interval lengths, for example, a set of small values ​​is set as an interval length candidate. A correction term according to the interval length candidate is calculated, and the interval length is calculated using the correction term and the candidate (Figure 6B).

[0097] The matrix A corresponding to the expected width candidates may be calculated in advance and stored in the control unit 50. The wavelength of the light propagating through the optical filter 100 and the FSR of the spectrum may be determined, and the column vector B may be calculated in advance and stored in the control unit 50. The section length is proportional to the square of the wavelength λ and inversely proportional to the FSR. The FSR may be, for example, 1 nm or less. The spectrum has periodic peaks, as in the example of Figure 4A. By appropriately designing the section length, the section length and temperature dependence can be suppressed.

[0098] At least one of the multiple waveguides has a polarization rotator. The polarization rotator converts the optical mode and propagates the converted mode in the waveguide. As shown in FIG. 1 and FIG. 2C, the waveguide 24 has a polarization rotator 36 and a polarization rotator 38. In the polarization rotator 36, the optical mode is converted from the TE1 mode to the TM0 mode. In the polarization rotator 38, the optical mode is converted from the TM0 mode to the TE1 mode. Mode conversion can be performed within one waveguide 24, and the TM0 mode can be propagated to the section 25. Since three modes propagate through the optical filter 100, it is possible to suppress both the temperature dependency and the waveguide length.

[0099] As shown in Fig. 3A, the waveguide includes a waveguide core 16. As shown in Fig. 2D and Fig. 3B, the polarization rotators 36 and 38 have a tapered shape and include the waveguide core 16 and a rib portion 17. The waveguide and the polarization rotator can be manufactured by processing the Si layer by etching or the like.

[0100] As shown in Figures 2D and 3B, the polarization rotator includes a waveguide core 16 and a rib portion 17. The planar shape is tapered. As shown in Figure 2B, sections 41 and 42 of the waveguide 22 have tapered shapes. The phase is likely to change in these parts. As in the examples of equations 6 and 8, there is a difference of about several percent between the section length when the correction term C is included in the calculation and the section length when it is not included. By considering the correction term C including the phase change amount φ, the section length can be calculated with high accuracy.

[0101] The optical filter 100 propagates three modes. n in Equation 2 is determined according to the number of sections. In the optical filter 100, three section lengths L1, L2, and L3 are designed. In Equation 2, n=3, and the design can be performed based on Equation 3. The number of modes may be four or more, and the number of sections is changed according to the number of modes. When the number of section lengths to be calculated is four or more, n in Equation 2 is four or more. From the left of Equation 2, the general inverse matrix A of matrix A is -1 By multiplying this, the section length can be calculated.

[0102] The optical filter 100 may be formed in a SOI substrate or in components other than a SOI substrate. The waveguide may include a Si waveguide core 16 or may be formed of other materials. The polarization rotator may have a configuration other than that of FIG. 2D.

[0103] The above functions can be realized by a computer. In that case, a program is provided that describes the processing contents of the functions that the processing device should have. The above processing functions are realized on the computer by executing the program. The program describing the processing contents can be recorded on a computer-readable storage medium (excluding carrier waves).

[0104] When a program is distributed, it is sold in the form of a portable storage medium on which the program is recorded, such as a DVD (Digital Versatile Disc), a CD-ROM (Compact Disc Read Only Memory), etc. The program can also be stored in a storage device of a server computer and transferred from the server computer to other computers via a network.

[0105] A computer that executes a program stores, for example, a program recorded on a portable storage medium or a program transferred from a server computer in its own storage device. The computer then reads the program from its own storage device and executes processing in accordance with the program. The computer can also read the program directly from a portable storage medium and execute processing in accordance with that program. The computer can also execute processing in accordance with the received program each time a program is transferred from the server computer.

[0106] Although the embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to such specific embodiments, and various modifications and variations are possible within the scope of the gist of the present disclosure described in the claims.

[0107] In addition to the description of the embodiments, the following notes are further disclosed. (Appendix 1) Three or more waveguides; a section provided in each of the three or more waveguides; An optical filter in which the modes of light propagating through the sections of the three or more waveguides are different from one another. (Appendix 2) 2. The optical filter of claim 1, wherein at least one of the three or more waveguides has a polarization rotator. (Appendix 3) the waveguide includes a waveguide core; 3. The optical filter according to claim 2, wherein the polarization rotator includes the waveguide core and a rib portion protruding from the waveguide core. (Appendix 4) the three or more waveguides include a first waveguide, a second waveguide, a third waveguide, and a fourth waveguide; the sections include a first section, a second section, a third section and a fourth section; the polarization rotator includes a first polarization rotator and a second polarization rotator; the first waveguide is optically coupled to the second waveguide at a first coupling portion, and is optically coupled to the third waveguide at a second coupling portion downstream of the first coupling portion, and has the first section between the first coupling portion and the second coupling portion; the second waveguide is optically coupled to the fourth waveguide at a third coupling portion downstream of the first coupling portion, and has the second section between the first coupling portion and the third coupling portion; the third waveguide is optically coupled to a portion of the fourth waveguide preceding the third coupling portion at a fourth coupling portion subsequent to the second coupling portion, and has the third section between the second coupling portion and the fourth coupling portion; the third waveguide has the first polarization rotator and the second polarization rotator; the first polarization rotator is located between the second coupling portion and the fourth coupling portion, the second polarization rotator is located between the first polarization rotator and the fourth coupling portion, the third section is located between the first polarization rotator and the second polarization rotator, the fourth waveguide has the fourth section between the third coupling portion and the fourth coupling portion, 4. The optical filter according to claim 2, wherein a mode of light propagating through the first section and the fourth section, a mode of light propagating through the second section, and a mode of light propagating through the third section are different from each other. (Appendix 5) A TE0 mode propagates in the first section and the fourth section, A TE1 mode propagates in the second section, 5. The optical filter according to claim 4, wherein a TM0 mode propagates in the third section. (Appendix 6) A method for manufacturing an optical filter, comprising the steps of: the optical filter comprises three or more waveguides; each of the three or more waveguides has a section; the modes of light propagating through the sections of the three or more waveguides are different from each other; the section has a length according to a propagation constant in the section and an amount of change in the phase of light in the waveguide; The manufacturing method is a method for manufacturing an optical filter that forms the three or more waveguides having the sections. (Appendix 7) The manufacturing method includes the steps of: designing a length of the section; forming the three or more waveguides based on the lengths of the sections; The manufacturing method according to claim 6, wherein the step of designing the length of the section includes a step of designing the length of the section based on Equation 1. (Appendix 8) 8. A method for manufacturing an optical filter according to claim 7, wherein the step of designing the length of the section includes a step of calculating each term of the matrix of equation 1 by determining a width of the section. (Appendix 9) the three or more waveguides include a first waveguide, a second waveguide, a third waveguide, and a fourth waveguide; the sections include a first section, a second section, a third section and a fourth section; the first waveguide has the first section; the second waveguide has the second section; the third waveguide has the third section; the fourth waveguide has the fourth section; a mode of light propagating through the first section and the fourth section, a mode of light propagating through the second section, and a mode of light propagating through the third section are different from each other; 9. A method for manufacturing an optical filter according to claim 7 or 8, wherein the step of designing the lengths of the sections is a step of designing the combined length of the first section and the fourth section, the length of the second section, and the length of the third section. (Appendix 10) 1. A method for designing an optical filter, comprising the steps of: the optical filter comprises three or more waveguides; each of the three or more waveguides has a section; the modes of light propagating through the sections of the three or more waveguides are different from each other; The design method is a method for designing an optical filter, in which the length of the section is designed based on a propagation constant and an amount of change in the phase of light in the waveguide. (Appendix 11) 11. The design method according to claim 10, wherein the length of the section is designed based on equation 1. (Appendix 12) calculating each term of the matrix of Equation 1 by determining the width of the interval; and designing the length of the section based on the formula 1. (Appendix 13) the three or more waveguides include a first waveguide, a second waveguide, a third waveguide, and a fourth waveguide; the sections include a first section, a second section, a third section and a fourth section; the first waveguide has the first section; the second waveguide has the second section; the third waveguide has the third section; the fourth waveguide has the fourth section; a mode of light propagating through the first section and the fourth section, a mode of light propagating through the second section, and a mode of light propagating through the third section are different from each other; 13. A method for designing an optical filter according to any one of claims 10 to 12, further comprising designing a combined length of the first section and the fourth section, a length of the second section, and a length of the third section. (Appendix 14) An optical filter design apparatus, comprising: the optical filter comprises three or more waveguides; each of the three or more waveguides has a section; the modes of light propagating through the sections of the three or more waveguides are different from each other; The design device includes: A first calculation unit that calculates a propagation constant in the section; A second calculation unit that calculates an amount of change in the phase of light in the waveguide; and a third calculation unit that calculates the length of the section based on the propagation constant and the amount of phase change. (Appendix 15) 15. The design device according to claim 14, wherein the third calculation unit calculates the length of the section based on Equation 1. (Appendix 16) 16. The optical filter design apparatus according to claim 15, wherein the first calculation unit calculates each term of the matrix of the equation 1 based on a width of the interval. (Appendix 17) the three or more waveguides include a first waveguide, a second waveguide, a third waveguide, and a fourth waveguide; the sections include a first section, a second section, a third section and a fourth section; the first waveguide has the first section; the second waveguide has the second section; the third waveguide has the third section; the fourth waveguide has the fourth section; a mode of light propagating through the first section and the fourth section, a mode of light propagating through the second section, and a mode of light propagating through the third section are different from each other; 17. The optical filter design device according to claim 14, wherein the third calculation unit calculates the combined length of the first section and the fourth section, the length of the second section, and the length of the third section. (Appendix 18) An optical filter design program, the optical filter comprises three or more waveguides; each of the three or more waveguides has a section; the modes of light propagating through the sections of the three or more waveguides are different from each other; Computer, A first calculation unit that calculates a propagation constant in the section; A second calculation unit that calculates an amount of change in the phase of light in the waveguide; a third calculation unit that calculates the length of the section based on the propagation constant and the amount of change in phase, (Appendix 19) 19. The design device according to claim 18, wherein the third calculation unit calculates the length of the section based on Equation 1. (Appendix 20) 20. The optical filter design apparatus according to claim 19, wherein the first calculation unit calculates each term of the matrix of the formula 1 based on a width of the interval. (Appendix 21) the three or more waveguides include a first waveguide, a second waveguide, a third waveguide, and a fourth waveguide; the sections include a first section, a second section, a third section and a fourth section; the first waveguide has the first section; the second waveguide has the second section; the third waveguide has the third section; the fourth waveguide has the fourth section; a mode of light propagating through the first section and the fourth section, a mode of light propagating through the second section, and a mode of light propagating through the third section are different from each other; 21. The optical filter design device of claim 18, wherein the third calculation unit calculates the combined length of the first section and the fourth section, the length of the second section, and the length of the third section. [Explanation of symbols]

[0108] 1 Input port 2 Exit port 10, 12 Board 14, 14a Box layer 16 Waveguide Core 16a Silicon layer 17 Rib section 20, 22, 24, 26, 70, 74 Waveguide 21, 23, 25, 27, 40, 41, 42, 43, 44, 45, 46, 47, 71, 72 Sections 28, 30, 32, 34 directional coupler 36, 38 Polarization rotator 50, 80 Control section 52 1st calculation section 54 2nd calculation section 56 3rd calculation section 60 CPU 62 RAM 63 ROM 64 Storage device 66 Interface 80 Tunable laser element 81 Beam splitter 83, 84 Monitor section 100, 101 Optical filters 110 Design equipment 120 Wavelength Locker

Claims

1. Three or more waveguides; a section provided in each of the three or more waveguides; An optical filter in which the modes of light propagating through the sections of the three or more waveguides are different from one another.

2. 2. The optical filter of claim 1, wherein at least one of the three or more waveguides comprises a polarization rotator.

3. the waveguide includes a waveguide core; 3. The optical filter according to claim 2, wherein the polarization rotator includes the waveguide core and a rib portion protruding from the waveguide core.

4. the three or more waveguides include a first waveguide, a second waveguide, a third waveguide, and a fourth waveguide; the sections include a first section, a second section, a third section and a fourth section; the polarization rotator includes a first polarization rotator and a second polarization rotator; the first waveguide is optically coupled to the second waveguide at a first coupling portion, and is optically coupled to the third waveguide at a second coupling portion downstream of the first coupling portion, and has the first section between the first coupling portion and the second coupling portion; the second waveguide is optically coupled to the fourth waveguide at a third coupling portion downstream of the first coupling portion, and has the second section between the first coupling portion and the third coupling portion; the third waveguide is optically coupled to a portion of the fourth waveguide preceding the third coupling portion at a fourth coupling portion subsequent to the second coupling portion, and has the third section between the second coupling portion and the fourth coupling portion; the third waveguide has the first polarization rotator and the second polarization rotator; the first polarization rotator is located between the second coupling portion and the fourth coupling portion, the second polarization rotator is located between the first polarization rotator and the fourth coupling portion, the third section is located between the first polarization rotator and the second polarization rotator, the fourth waveguide has the fourth section between the third coupling portion and the fourth coupling portion, 4. The optical filter according to claim 2, wherein a mode of light propagating through the first section and the fourth section, a mode of light propagating through the second section, and a mode of light propagating through the third section are different from each other.

5. A TE0 mode propagates in the first section and the fourth section, The TE1 mode propagates in the second section, 5. The optical filter according to claim 4, wherein a TM0 mode propagates in the third section.

6. A method for manufacturing an optical filter, comprising the steps of: the optical filter comprises three or more waveguides; each of the three or more waveguides has a section; the modes of light propagating through the sections of the three or more waveguides are different from each other, the section has a length according to a propagation constant in the section and an amount of change in the phase of light in the waveguide; The manufacturing method is a method for manufacturing an optical filter in which the three or more waveguides having the sections are formed.

7. The manufacturing method includes the steps of: designing a length of the section; forming the three or more waveguides based on the lengths of the sections; The method for manufacturing an optical filter according to claim 6 , wherein the step of designing the length of the section includes the step of designing the length of the section based on Equation 1. [0010] L1 to Ln: Length of section β1 to βn: Propagation constants for each section T: temperature λ: wavelength k: wave number FSR: Spectral peak spacing φ: Phase change amount

8. The method for manufacturing an optical filter according to claim 7 , wherein the step of designing the length of the section includes the step of calculating each term of the matrix of Equation 1 by determining the width of the section.

9. the three or more waveguides include a first waveguide, a second waveguide, a third waveguide, and a fourth waveguide; the sections include a first section, a second section, a third section and a fourth section; the first waveguide has the first section; the second waveguide has the second section; the third waveguide has the third section; the fourth waveguide has the fourth section; a mode of light propagating through the first section and the fourth section, a mode of light propagating through the second section, and a mode of light propagating through the third section are different from each other; 9. The method for manufacturing an optical filter according to claim 7 or claim 8, wherein the step of designing the lengths of the sections is a step of designing the combined length of the first section and the fourth section, the length of the second section, and the length of the third section.

10. 1. A method for designing an optical filter, comprising the steps of: the optical filter comprises three or more waveguides; each of the three or more waveguides has a section; the modes of light propagating through the sections of the three or more waveguides are different from each other, A method for designing an optical filter, the method including: designing the length of the section based on a propagation constant in the section and an amount of change in the phase of light in the waveguide.

11. An optical filter design apparatus, comprising: the optical filter comprises three or more waveguides; each of the three or more waveguides has a section; the modes of light propagating through the sections of the three or more waveguides are different from each other, The design device includes: A first calculation unit that calculates a propagation constant in the section; A second calculation unit that calculates an amount of change in the phase of light in the waveguide; and a third calculation unit that calculates the length of the section based on the propagation constant and the amount of change in phase.

12. An optical filter design program, the optical filter comprises three or more waveguides; each of the three or more waveguides has a section; the modes of light propagating through the sections of the three or more waveguides are different from each other; Computer, A first calculation unit that calculates a propagation constant in the section; A second calculation unit that calculates an amount of change in the phase of light in the waveguide; a third calculation unit that calculates the length of the section based on the propagation constant and the amount of change in phase;

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