JP6397973B1 - Optical waveguide device and wavelength filter - Google Patents

Abstract

An optical waveguide device that uses a photonic crystal to convert input light into different modes and reflect the light is provided.
An optical waveguide core including a mode converter, a first compensator formed adjacent to one end of the mode converter, and a second compensator formed adjacent to the other end of the mode converter; And a clad including the optical waveguide core. In the mode converter, a photonic crystal including a first hole group including a plurality of holes formed periodically and a second hole group is formed. The photonic crystal is 2π / Λ with respect to the wavelength λ, where the period of the first hole group and the second hole group is Λ, the equivalent refractive index for the fundamental mode is n ₀ , and the equivalent refractive index n _q for the q-order mode. = 2π (n ₀ + n _q ) / λ is satisfied. Each of the first compensation unit and the second compensation unit is formed with one or a plurality of sub-holes, and the first compensation unit and the second compensation unit transmit the fundamental mode light having a specific wavelength λ to the fundamental mode. Reflect as it is.
[Selection] Figure 1

Description

  The present invention relates to an optical waveguide element that reflects light of a specific wavelength by a photonic crystal and a wavelength filter including the same.

  In recent years, Si waveguides using Si (silicon) as a waveguide material have attracted attention in developing optical devices advantageous for miniaturization and mass productivity.

  In the Si waveguide, an optical waveguide core that substantially becomes a light transmission path is formed using Si as a material. Then, the periphery of the optical waveguide core is covered with a clad made of, for example, silica having a refractive index lower than that of Si. With such a configuration, the refractive index difference between the optical waveguide core and the clad becomes extremely large, so that light can be strongly confined in the optical waveguide core. As a result, a small curved waveguide having a bending radius reduced to, for example, about 1 μm can be realized. Therefore, it is possible to create an optical circuit having a size comparable to that of an electronic circuit, which is advantageous for downsizing the entire optical device.

  Further, in the Si waveguide, it is possible to divert the manufacturing process of a semiconductor device such as a CMOS (Complementary Metal Oxide Semiconductor). Therefore, realization of photoelectric fusion (silicon photonics) in which electronic functional circuits and optical functional circuits are collectively formed on a chip is expected.

  By the way, in a passive optical network (PON) using a wavelength division multiplexing (WDM) system, different reception wavelengths are assigned to each subscriber side device (ONU: Optical Network Unit). . A station side device (OLT: Optical Line Terminal) generates a downstream optical signal to each ONU at a transmission wavelength corresponding to a reception wavelength of a destination, and multiplexes and transmits them. Each ONU selectively receives an optical signal having a reception wavelength allocated to itself from downstream optical signals multiplexed at a plurality of wavelengths. In the ONU, a wavelength filter is used to selectively receive a downstream optical signal of each reception wavelength. And the technique which comprises a wavelength filter with the Si waveguide mentioned above is implement | achieved.

  As a wavelength filter using a Si waveguide, for example, there are a filter using a Mach-Zehnder interferometer and a filter using an arrayed waveguide grating. Further, as a wavelength filter using a Si waveguide, a ring resonator (see, for example, Patent Documents 1 to 3), a grating type (for example, see Patent Document 4) or a directional coupler type (for example, see Patent Document 5) wavelength filter. There is. These wavelength filters have the advantage that the output wavelength can be made variable by providing electrodes and applying a voltage. Further, like a grating, there is a structure called a photonic crystal as an element that reflects a specific wavelength in the reverse direction by diffracting light.

JP 2003-215515 A JP 2013-093627 A JP 2006-278770 A JP 2006-330104 A JP 2002-353556 A

  Since the photonic crystal has a high diffraction efficiency, when used as a wavelength filter, it is expected that wavelength separation can be performed with high efficiency. However, the conventional photonic crystal has a structure that performs diffraction in the reverse direction between the fundamental modes and has a limited function. A photonic crystal that converts input light into a different mode and reflects it has not been known.

  Accordingly, an object of the present invention is to provide an optical waveguide element using a photonic crystal, which can convert input light into a different mode and reflect it, and a wavelength filter using the optical waveguide element. is there.

  In order to achieve the above-described object, an optical waveguide element according to the present invention is provided along a first imaginary line segment and a second imaginary line segment parallel to the first imaginary line segment, and the first imaginary line segment and the second imaginary line. An optical waveguide core provided at a position overlapping the line segment and a clad including the optical waveguide core are provided.

The optical waveguide core includes a mode conversion unit connected in series along the first virtual line segment and the second virtual line segment, a first compensation unit formed adjacent to one end of the mode conversion unit, and a mode conversion 2nd compensation part formed adjacent to the other end of a part. In the mode conversion unit, the first hole group including a plurality of holes periodically formed in a position overlapping with the first imaginary line segment, and the position overlapping with the second imaginary line segment, A photonic crystal including a second hole group including a plurality of holes formed at the same period as the first hole group and shifted from the holes included in the first hole group by a half cycle is formed. ing. For a specific wavelength λ, the photonic crystal has a period of holes included in the first hole group and a second hole group as Λ, and an equivalent refractive index for the fundamental mode as n ₀ , q ( q is an integer of q> 0) The equivalent refractive index for the next mode is n _q , and 2π / Λ = 2π (n ₀ + n _q ) / λ is satisfied. Each of the first compensation unit and the second compensation unit is formed with one or a plurality of sub-holes, and the first compensation unit and the second compensation unit transmit the fundamental mode light having a specific wavelength λ to the fundamental mode. Reflect as it is.

  A wavelength filter according to a first aspect of the present invention includes the above-described optical waveguide element and an output waveguide core including a coupling portion. The optical waveguide core further includes a multimode waveguide section that is connected in series with the mode conversion section and propagates light in the fundamental mode and the q-order mode. The first compensation unit is formed as a part of the multimode waveguide unit. The cladding includes an optical waveguide core and an output waveguide core. A coupling region is set in which the multimode waveguide section and the coupling section are spaced apart from each other and arranged side by side. In the coupling region, the q-order mode light propagating through the multimode waveguide section and the r (r is an integer of r ≧ 0) order mode light propagating through the coupling section are coupled.

  A wavelength filter according to a second aspect of the present invention includes the above-described optical waveguide element. The optical waveguide core includes n (n is an integer of 2 or more) mode conversion units, a first compensation unit, and a second compensation unit connected in series along the first virtual line segment and the second virtual line segment, n-1 cavity portions. The mode conversion part and the cavity part are connected alternately. The 1st compensation part and the 2nd compensation part which exist between mode conversion parts adjacent on both sides of a cavity part are formed as a part of cavity part pinched between adjacent mode conversion parts. The cavity portion matches the phase of light of a fundamental mode having a specific wavelength propagating through the cavity portion.

  In the optical waveguide device of the present invention, the mode conversion unit forms a photonic crystal including two rows of hole groups (the first hole group and the second hole group), so that the fundamental mode of a specific wavelength can be obtained. Light can be converted to the primary mode and reflected.

  Moreover, in the wavelength filter of this invention, a specific wavelength and another wavelength can be isolate | separated by using the optical waveguide element mentioned above, and it can take out by switching a path | route. And by using a photonic crystal, light of a specific wavelength can be reflected with high diffraction efficiency. Therefore, wavelength separation and path switching can be performed with high efficiency.

(A) is a schematic plan view showing the optical waveguide device of the present invention, and (B) is a schematic end view showing the optical waveguide device of the present invention. It is a schematic plan view which shows the modification of a photonic crystal. (A) And (B) is a figure which shows the result of the simulation regarding the characteristic evaluation of the optical waveguide element of this invention. It is a schematic plan view which shows the 1st wavelength filter of this invention. (A) is a schematic plan view showing a second wavelength filter of the present invention, and (B) is a schematic end view showing a second wavelength filter of the present invention. It is a schematic plan view which shows the modification of the 2nd wavelength filter of this invention. It is a figure which shows the result of the simulation regarding the characteristic evaluation of the 2nd wavelength filter of this invention. It is a schematic plan view which shows the 3rd wavelength filter of this invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and arrangement relationship of each component are merely schematically shown to the extent that the present invention can be understood. In the following, a preferred configuration example of the present invention will be described. However, the material and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many changes or modifications that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention.

(Optical waveguide element)
With reference to FIG. 1, the optical waveguide device of the present invention will be described. FIG. 1A is a schematic plan view showing an optical waveguide element. FIG. 1B is a schematic end view of the optical waveguide element shown in FIG. In FIG. 1A, only the optical waveguide core described later is shown, and the cladding, the support substrate, and the electrodes are omitted.

  In the following description, the direction along the light propagation direction is the length direction of each component. The direction along the thickness of the support substrate is the thickness direction. Moreover, let the direction orthogonal to a length direction and a thickness direction be a width direction.

  The optical waveguide device 100 includes a support substrate 10, a clad 20, an optical waveguide core 30, and an electrode 40.

  The support substrate 10 is composed of a flat plate made of, for example, single crystal Si.

The clad 20 is formed on the support substrate 10 so as to cover the upper surface 10 a of the support substrate 10 and to include the optical waveguide core 30. The clad 20 is formed using, for example, SiO ₂ as a material.

  The optical waveguide core 30 is made of, for example, Si having a refractive index higher than that of the clad 20. As a result, the optical waveguide core 30 functions as a substantial light transmission path, and the input light propagates in the propagation direction according to the planar shape of the optical waveguide core 30. The optical waveguide core 30 is preferably formed at least 1 μm or more away from the support substrate 10 in order to prevent the propagating light from escaping to the support substrate 10.

  Here, the optical waveguide core 30 preferably has a thickness in the range of 200 to 500 nm, for example, in order to achieve a single mode condition in the thickness direction. For example, when light having a wavelength of 1550 nm is propagated, it is preferable that the optical waveguide core 30 has a thickness in the range of 200 to 300 nm.

  The optical waveguide core 30 is along the first virtual line segment L1 connecting the first point P1 and the second point P2, and along the second virtual line segment L2 connecting the third point P3 and the fourth point P4, and these It is provided at a position overlapping the first virtual line segment L1 and the second virtual line segment L2. The second virtual line segment L2 is parallel to the first virtual line segment L1.

  Further, the optical waveguide core 30 includes a mode conversion unit 31 and a pair of compensation units (first compensation unit 131a and second compensation unit) connected in series along the first virtual line segment L1 and the second virtual line segment L2. 131b).

  A photonic crystal is formed in the mode converter 31. The photonic crystal is configured by forming a first hole group 61 and a second hole group 62 in the mode converter 31.

  The first hole group 61 includes a plurality of holes 51 that are periodically formed so as to be arranged at positions overlapping the first virtual line segment L1. The second hole group 62 includes a plurality of holes 52 that are arranged at positions overlapping the second imaginary line segment L <b> 2 and formed at the same period as the first hole group 61. The holes 51 included in the first hole group 61 and the holes 52 included in the second hole group 62 are formed at positions shifted from each other by a half cycle.

  The holes 51 and 52 are formed so as to penetrate the mode conversion part 31 in the thickness direction. Further, here, the holes 51 and 52 have a circular cross-sectional shape orthogonal to the thickness direction.

  The photonic crystal reflects TE (Transverse Electric) polarized light having a specific wavelength λ that is input from the fundamental mode into the primary mode. The photonic crystal transmits light of other wavelengths while remaining in the fundamental mode.

The phase matching conditions in the photonic crystal are as follows: the formation period of the holes 51 and 52 is Λ, the equivalent refractive index for the TE polarization of the fundamental mode is n ₀ , and the equivalent refractive index for the TE polarization of the first mode is n ₁ . It is represented by the following formula (1).

2π / Λ = 2π (n ₀ + n ₁ ) / λ (1)
In the photonic crystal, a TE wave having a wavelength λ that satisfies the above equation (1), that is, a TE polarized wave having a Bragg wavelength is Bragg reflected. Therefore, the periods of the holes 51 and 52 are designed so that the above equation (1) is established for a desired wavelength λ to be reflected. Further, other designs such as the width of the mode converter 31 and the diameters of the holes 51 and 52 are also designed according to the desired wavelength λ to be reflected.

  Here, as a modification of the photonic crystal, the holes 51 and 52 each have a unique diameter, and the diameter has a value of at least 2 or more. A modification of the photonic crystal will be described with reference to FIG. FIG. 2 is a schematic plan view for explaining a modification of the photonic crystal. In FIG. 2, only the mode conversion unit is shown, and other configurations are omitted.

  In the configuration example shown in FIG. 2, the diameter increases for each period with respect to the diameters of the holes 51 and 52 in the first period. The diameters of the holes 51 and 52 are the largest in the holes 51 and 52 near the center in the length direction of the mode conversion unit 31. Then, the diameters of the holes 51 and 52 decrease after the holes 51 and 52 which are the maximum.

  Thus, the scattering of light in the photonic crystal can be suppressed when the diameters of the holes 51 and 52 have different values. Note that the amount of change in the diameter of the holes 51 and 52 is designed according to the wavelength λ to be reflected and the diffraction efficiency.

  The first compensation unit 131a and the second compensation unit 131b are formed adjacent to both ends 31a and 31b of the mode conversion unit 31 with the mode conversion unit 31 in between. A first compensator 131 a is formed at one end 31 a of the mode converter 31, and a second compensator 131 b is formed at the other end 31 b of the mode converter 31.

  Sub-holes 90 are formed in the first compensation unit 131a and the second compensation unit 131b, respectively. The sub-hole 90 is formed at the center in the width direction of the first compensation unit 131a and the second compensation unit 131b. Further, the sub-hole 90 is formed so as to penetrate the first compensation unit 131a and the second compensation unit 131b in the thickness direction. Here, the sub-hole 90 has a circular cross section perpendicular to the thickness direction.

  By forming the sub-hole 90, the first compensator 131a and the second compensator 131b reflect the fundamental mode light having the specific wavelength λ as described above while maintaining the fundamental mode.

  A plurality of sub holes 90 may be formed in the first compensation unit 131a and the second compensation unit 131b. In this case, a plurality of sub-holes 90 are arranged along the length direction of the first compensator 131a and the second compensator 131b.

  The electrode 40 is formed at a position covering the mode converter 31 via the clad 20. Joule heat is generated by passing an electric current through the electrode 40, and the refractive index of the mode converter 31 is changed by the thermo-optic effect caused by the heat generation. As a result, the wavelength reflected by the mode converter 31 can be changed.

  As described above, in the optical waveguide device 100, the mode converter 31 is formed with a photonic crystal including two rows of hole groups (the first hole group 61 and the second hole group 62). The fundamental mode TE polarized wave of a specific wavelength can be converted to the primary mode TE polarized wave and reflected.

  Therefore, by using the optical waveguide element 100, it is possible to configure a wavelength filter that extracts light having a specific wavelength that passes through the photonic crystal.

  Here, at both ends of the photonic crystal (that is, both ends of the mode conversion unit 31), not only the reflection diffraction between the fundamental mode and the first order mode but also an unwanted reflection diffraction between the fundamental modes occurs. It has been found by simulation using FDTD (Finite Differential Time Domain) that the input light of the mode is reflected in the fundamental mode. However, in the optical waveguide device 100, the first compensation unit 131a and the second compensation unit 131b are provided at both ends of the mode conversion unit 31. As described above, the first compensation unit 131a and the second compensation unit 131b reflect the fundamental mode light while maintaining the fundamental mode. As a result, the reflection diffraction between the fundamental modes which is undesirably generated in the mode conversion unit 31 can be canceled by the reflection diffraction between the fundamental modes in the first compensation unit 131a and the second compensation unit 131b. Therefore, in the optical waveguide device 100, it is possible to prevent the reflected light in the fundamental mode from going backward from the mode conversion unit 31.

  Further, in the optical waveguide device 100, heat can be applied to the mode conversion unit 31 using the electrode 40. Therefore, the wavelength of light reflected and transmitted by the photonic crystal can be changed. Therefore, by using the optical waveguide element 100, a wavelength filter with a variable output wavelength can be configured.

In this embodiment, the configuration in which the light of a specific wavelength is converted from the fundamental mode to the primary mode and reflected in the photonic crystal of the mode conversion unit 31 has been described. However, the photonic crystal of the mode conversion unit 31 may be configured to reflect light of a specific wavelength in the fundamental mode by converting it to a q-order mode (q is q> 0). In that case, the phase matching condition in the photonic crystal of the mode converter 31 is that the formation period of the holes 51 and 52 is Λ, the equivalent refractive index for the fundamental mode light is n ₀ , and the equivalent refraction for the q-order mode light. The rate is expressed by the following formula (2), where n _q .

2π / Λ = 2π (n ₁ + n _q ) / λ (2)
In the photonic crystal, light having a wavelength λ that satisfies the above equation (2), that is, light having a Bragg wavelength is Bragg reflected. The photonic crystal is designed so that the above equation (2) holds for the wavelength λ to be reflected.

  When the q-order mode is an odd-order mode in which the amplitude distribution is antisymmetric, the holes 51 and the holes 52 are formed at positions shifted from each other by a half cycle. Further, when the q-order mode is an even-order mode in which the amplitude distribution is symmetric, the formation positions of the holes 51 and the holes 52 are matched without shifting the period. That is, the hole 51 and the hole 52 are formed at positions where they are symmetrical with each other.

  In this embodiment, the configuration in which the holes 51 and 52 are formed so as to penetrate the mode conversion portion 31 in the thickness direction has been described. However, the holes 51 and 52 are formed by digging the mode conversion section 31 from the upper surface of the mode conversion section 31 to the middle of the thickness of the mode conversion section 31 (that is, without penetrating the mode conversion section 31). You can also In this case, it has been found by simulation using FDTD that the splitting of the wavelength peak of the output light can be suppressed. Similarly, the sub-hole 90 can be formed by digging in the middle of the first compensation unit 131a and the second compensation unit 131b without penetrating the first compensation unit 131a and the second compensation unit 131b.

(Characteristic evaluation)
The inventor performed the simulation which evaluates the characteristic of the optical waveguide device 100 using FDTD.

  First, as a first simulation, a simulation regarding the formation position of the sub-hole 90 in the optical waveguide device 100 was performed.

  In the first simulation, for the optical waveguide device 100, the first mode TE polarization is input to the optical waveguide core 30 from the first compensation unit 131 a side, and the first mode generates reflection diffraction between the basic modes. The intensity of light in the fundamental mode reflected to the compensation unit 131a side (that is, the amount of diffraction between the fundamental modes) was measured. Here, the length of the sub hole 90 of the first compensation unit 131a and the hole 51 or 52 located closest to the one end 31a (here, the hole 52 existing at the leftmost end of the paper in FIG. 1A). While changing the center-to-center distance along the vertical direction (hereinafter also referred to as the center-to-center distance between the sub hole 90 and the hole 52), the amount of diffraction between the fundamental modes was measured.

  In the first simulation, the optical waveguide device 100 was designed as follows in the photonic crystal of the mode conversion unit 31 as a design for reflecting the fundamental mode TE polarized wave having a wavelength of 1550 nm by converting it to the primary mode. That is, the thickness of the optical waveguide core 30 was set to 200 nm. The width of the mode conversion unit 31 is 1000 nm. In addition, it is assumed that the formation cycle of the holes 51 and 52 is 326 μm, and 20 holes 51 and 20 holes 52 are formed. Moreover, the diameter of the hole 51 and the hole 52 was 100 nm. The center-to-center distance along the width direction of the mode conversion unit 31 between the holes 51 and 52 was set to 500 nm.

  In the first simulation, it is assumed that the mode conversion unit 31 does not have the other end 31b. Therefore, in the first simulation, diffraction between the fundamental modes on the other end 31b side of the mode conversion unit 31 does not occur, and the second compensation unit 131b is also omitted. However, in the actual optical waveguide device 100, the diffraction between the fundamental modes generated on the other end 31b side of the mode conversion unit 31 is the same as that on the one end 31a side. Therefore, the result of the first simulation can also be applied to the relationship between the formation position of the sub-hole 90 of the second compensation unit 131b and the amount of diffraction between the fundamental modes.

  The result of the first simulation is shown in FIG. In FIG. 3A, the vertical axis indicates the amount of diffraction between the fundamental modes in dB scale, and the horizontal axis indicates the distance between the centers of the sub-hole 90 and the hole 52 in units of μm.

  As shown in FIG. 3A, when the distance between the centers of the sub holes 90 and the holes 52 is 0.23 μm, the reflection diffraction between the fundamental modes is suppressed to −30 dB or less. In this way, by setting the optimum center-to-center distance between the sub-hole 90 and the hole 52 according to the design, it is possible to suppress reflection diffraction between undesired fundamental modes.

  Next, as a second simulation, a simulation regarding wavelength characteristics in the optical waveguide device 100 was performed.

  In the second simulation, for the optical waveguide element 100, the TE polarization of the fundamental mode is input to the optical waveguide core 30 from the first compensation unit 131a side, and the mode between the fundamental mode and the primary mode generated in the mode conversion unit 31 is obtained. The intensity of the first-order mode light reflected on the first compensation unit 131a side by reflection diffraction (that is, the amount of diffraction between the fundamental mode and the first-order mode) and the reflection diffraction between the fundamental modes generated in the mode conversion unit 31 Thus, the intensity of the fundamental mode light reflected to the first compensation unit 131a side (that is, the amount of diffraction between the fundamental modes) was measured.

  In the second simulation, the optimum value (0.23 μm) obtained by the first simulation described above was adopted as the center-to-center distance between the sub-hole 90 and the hole 52. Other design conditions are the same as those in the first simulation.

  The result of the second simulation is shown in FIG. In FIG. 3B, the vertical axis indicates the light intensity in dB scale, and the horizontal axis indicates the wavelength in nm. In FIG. 3B, a curve 101 indicates the intensity of reflected light in the first-order mode, and a curve 103 indicates the intensity of reflected light in the fundamental mode.

As shown in FIG. 3B, it was confirmed that the fundamental mode was reflected while being converted into the primary mode in the wide wavelength range of about 125 nm in the vicinity of the design wavelength of 1550 nm.
It was also confirmed that the reflection diffraction between the fundamental modes was suppressed to -30 dB or less in the vicinity of the design wavelength of 1550 nm.

(First wavelength filter)
With reference to FIG. 4, the 1st wavelength filter using the optical waveguide element 100 (refer FIG. 1) mentioned above is demonstrated. FIG. 4 is a schematic plan view showing the first wavelength filter. In FIG. 4, only the optical waveguide core and the output waveguide core described later are shown, and the cladding, the support substrate, and the electrodes are omitted. In addition, components common to the optical waveguide device 100 are denoted by the same reference numerals, and description thereof is omitted.

  The first wavelength filter 200 is configured by adding an output waveguide core 70 to the optical waveguide element 100 described above. In addition, the optical waveguide core 30 is added to the mode conversion unit 31 and the first compensation unit 131a and the second compensation unit 131b described above, and the input unit 32, the input side taper unit 33, the multimode waveguide unit 34, and the output side. A tapered portion 35 and an output portion 36 are included. The input unit 32, the input side taper unit 33, the multimode waveguide unit 34, the mode conversion unit 31, the second compensation unit 131b, the output side taper unit 35, and the output unit 36 are connected in series in this order.

  The first compensation unit 131a is formed as a part of the multimode waveguide unit 34. Therefore, a sub-hole 90 is formed near the end of the multimode waveguide section 34 on the mode conversion section 31 side. The region where the sub-hole 90 of the multimode waveguide portion 34 is formed functions as the first compensation portion 131a.

  The input unit 32 is set to a width that achieves a single mode condition for propagating light of TE polarization.

  The input-side taper portion 33 continuously increases in width from one end 33 a connected to the input portion 32 to the other end 33 b connected to the multimode waveguide portion 34. The width of the one end 33 a of the input side taper portion 33 is set equal to the width of the input portion 32. Therefore, the input side taper portion 33 is set so as to achieve the single mode condition for the propagation light of the TE polarized wave at the one end 33a.

  The multimode waveguide unit 34 propagates TE polarized waves in the fundamental mode and the primary mode.

  In the mode converter 31, a photonic crystal that satisfies the above equation (1) is formed with respect to a TE polarized wave having a specific wavelength. The photonic crystal reflects TE-polarized light having a specific wavelength that is input from the fundamental mode into the primary mode. The photonic crystal transmits light of other wavelengths while remaining in the fundamental mode.

  The output-side taper portion 35 continuously reduces in width from one end 35a connected to the second compensation portion 131b to the other end 35b connected to the output portion 36. The width of the other end 35b of the output side taper portion 35 is set so as to achieve a single mode condition for the propagation light of TE polarization.

  The output unit 36 is set to a width that achieves a single mode condition for propagating light of TE polarization.

  The output waveguide core 70 is made of, for example, Si having a refractive index higher than that of the clad 20, similarly to the optical waveguide core 30. The output waveguide core 70 is preferably formed at least 1 μm or more away from the support substrate 10 in order to prevent the propagating light from escaping to the support substrate 10.

  Here, the thickness of the output waveguide core 70 is preferably set to a thickness in the range of 200 to 500 nm, for example, in order to achieve a single mode condition in the thickness direction. For example, when light having a wavelength of 1550 nm is propagated, it is preferable that the output waveguide core 70 has a thickness in the range of 200 to 300 nm.

  The output waveguide core 70 includes a coupling portion 71 and a second output portion 72.

  The coupling portion 71 is disposed apart from and side by side with the multimode waveguide portion 34 of the optical waveguide core 30. The multimode waveguide section 34 of the optical waveguide core 30 and the coupling section 71 of the output waveguide core 70 are set as coupling areas 80 that are spaced apart from each other and arranged side by side. The second output unit 72 is connected to the coupling unit 71 on the opposite side of the mode conversion unit 31 with the coupling region 80 interposed therebetween.

  In the coupling region 80, the multimode waveguide section 34 and the coupling section 71 are arranged so that their central axes are parallel to each other.

  Further, the coupling portion 71 has a tapered shape in which the width continuously increases from the one end 71 a to the other end 71 b connected to the second output portion 72. The widths of the one end 71a and the other end 71b of the coupling portion 71 are set in correspondence with the equivalent refractive index capable of propagating the fundamental mode TE polarized wave. The coupling unit 71 includes an equivalent refractive index for the TE polarized wave of the first mode propagating through the multimode waveguide unit 34 and the TE of the fundamental mode propagating through the coupling unit 71 between the one end 71a and the other end 71b. It includes a width in which the equivalent refractive index to the polarization matches.

  As a result, in the coupling region 80, the primary mode TE polarized wave propagating through the multimode waveguide unit 34 and the fundamental mode TE polarized wave propagating through the coupling unit 71 can be coupled.

  The second output unit 72 is set to a width that achieves a single mode condition for TE-polarized propagation light.

  In the first wavelength filter 200, the fundamental mode optical signal is input to the input unit 32 of the optical waveguide core 30, and sent to the mode conversion unit 31 through the input side taper unit 33 and the multimode waveguide unit 34. The TE polarization of the fundamental mode having a specific wavelength propagating through the mode converter 31 is converted to the primary mode by the photonic crystal, reflected, and sent again to the multimode waveguide unit 34. Other light is output from the output part 36 through the output side taper part 35 in the fundamental mode. The primary mode TE polarized wave reflected by the photonic crystal and propagating through the multimode waveguide section 34 is converted into the fundamental mode in the coupling region 80 and is transferred to the coupling section 71 of the output waveguide core 70. The basic mode TE polarized wave transferred to the coupling unit 71 is output from the second output unit 72.

  As described above, the first wavelength filter 200 can be used as a wavelength filter that separates a specific wavelength from other wavelengths by using the above-described optical waveguide device 100 and switches and extracts the path. Then, by using a photonic crystal in the mode conversion unit 31, light having a specific wavelength can be reflected with high diffraction efficiency. Therefore, wavelength separation and path switching can be performed with high efficiency.

  Further, the first wavelength filter 200 includes the first compensator 131a and the second compensator 131b, so that reflection diffraction between the fundamental modes that is undesirably generated in the mode converter 31 is prevented from occurring in the first compensator 131a and the first compensator 131a. It is possible to cancel by the reflection diffraction between the fundamental modes in the two compensation units 131b. As a result, it is possible to prevent the reflected light in the basic mode from going backward from the mode conversion unit 31.

  In this embodiment, the configuration in which the light of a specific wavelength is converted from the fundamental mode to the primary mode and reflected in the photonic crystal of the mode conversion unit 31 has been described. However, by forming the photonic crystal so as to satisfy the above equation (2), the photonic crystal can also be configured to reflect light of a specific wavelength in the fundamental mode by converting it to the q-order mode. .

  Furthermore, in this embodiment, the light coupled between the multimode waveguide section 34 and the coupling section 71 in the coupling region 80 is not limited to the fundamental mode and the primary mode. The widths of the one end 71a and the other end 71b of the coupling portion 71 are set in correspondence with the equivalent refractive index capable of propagating light in the r (r is an integer of r ≧ 0) order mode. The width up to the other end 71b includes a width where the equivalent refractive index for the q-order mode light propagating through the multimode waveguide section 34 and the equivalent refractive index for the r-order mode light propagating through the coupling section 71 coincide. It can also be designed as follows. In that case, in the coupling region 80, the q-order mode light propagating through the multimode waveguide section 34 and the r-order mode light propagating through the coupling section 71 can be coupled.

(Second wavelength filter)
With reference to FIG. 5, the 2nd wavelength filter using the optical waveguide element 100 (refer FIG. 1) mentioned above is demonstrated. FIG. 5A is a schematic plan view showing the second wavelength filter. FIG. 5 (B) is a schematic end view of the second wavelength filter shown in FIG. 5 (A) taken along the line II-II. In FIG. 5A, only the optical waveguide core is shown, and the cladding, the support substrate, and the electrodes are omitted. In addition, components common to the optical waveguide device 100 are denoted by the same reference numerals, and description thereof is omitted.

  In the second wavelength filter 300, the optical waveguide core 30 is added to the mode conversion unit 31, the first compensation unit 131 a, and the second compensation unit 131 b, and the input unit 231, the input side taper unit 232, the cavity unit 234, and the output A side taper portion 236 and an output portion 237 are included.

  The optical waveguide core 30 includes two mode conversion units 31 (a first mode conversion unit 31-1 and a second mode conversion unit 31-2), and the first mode conversion unit 31-1 and the second mode conversion unit. 1st compensation part 131a and 2nd compensation part 131b which were formed in the both ends of 31-2 are included. The input part 231, the input side taper part 232, the first mode conversion part 31-1, the cavity part 234, the second mode conversion part 31-2, the output side taper part 236, and the output part 237 are connected in series in this order. Yes. In addition, between the input side taper part 232 and the 1st mode conversion part 31-1, the 1st compensation part 131a formed adjacent to the one end 31a of the 1st mode conversion part 31-1 is provided. Further, a second compensation unit 131b formed adjacent to the other end 31b of the second mode conversion unit 31-2 is provided between the second mode conversion unit 31-2 and the output side taper unit 236. .

  Note that the first compensation unit 131a and the second compensation unit 131b (here, the other end 31b of the first mode conversion unit 31-1) exist between the first mode conversion unit 31-1 and the second mode conversion unit 31-2. The second compensation unit 131b formed adjacent to the first compensation unit 131a and the first compensation unit 131a formed adjacent to one end 31a of the second mode conversion unit 31-2 are connected to the first mode conversion unit 31-1. And it forms as a part of cavity part 234 pinched | interposed between the 2nd mode conversion part 31-2. Accordingly, sub-holes 90 are formed in the cavity portion 234 near the end on the first mode conversion portion 31-1 side and near the end on the second mode conversion portion 31-2 side, respectively. The regions where the sub holes 90 of the cavity part 234 are formed function as the first compensation part 131a and the second compensation part 131b.

  The input unit 231 is set to a width that achieves a single mode condition for TE-polarized propagation light. Accordingly, the input unit 231 propagates light in the fundamental mode.

  The input-side taper portion 232 continuously increases in width from one end 232a connected to the input portion 231 to the other end 232b connected to the first compensation portion 131a. The width of the one end 232 a of the input side taper portion 232 is set to be equal to the width of the input portion 231. Therefore, the input side taper portion 232 is set so as to achieve the single mode condition for the propagation light of the TE polarized wave at the one end 232a.

  The cavity part 234 is formed with a constant width. The width of the cavity part 234 is set so that the fundamental mode and the primary mode TE polarized wave can propagate.

  The cavity 234 matches the phase of light having a specific wavelength among the light propagating through the cavity 234. The length of the cavity part 234 is designed according to the wavelength to be phase-matched.

  Further, as described above, the cavity portion 234 includes the first compensation portion 131a and the second compensation portion 131b as regions where the sub-holes 90 are formed.

  The output side taper portion 236 continuously reduces in width from one end 236a connected to the second compensation portion 131b to the other end 236b connected to the output portion 237. The width of the other end 236 b of the output side taper portion 236 is set to be equal to the width of the output portion 237. The output side taper portion 236 is set at the other end 236b so as to achieve the single mode condition for the propagation light of the TE polarized wave.

  The output unit 237 is set to a width that achieves the single mode condition for the TE polarized wave. Therefore, the output unit 237 propagates light in the fundamental mode.

  The electrode 40 is formed at a position covering one or both of the mode conversion portion 31 and the cavity portion 234 via the clad 20. When the electrode 40 is formed on the cavity part 234, the refractive index of the cavity part 234 can be changed by the thermo-optic effect due to the heat generated by the electrode 40. As a result, the wavelength to be phase-matched by the cavity part 234 can be changed. When the electrode 40 is provided on both the mode conversion unit 31 and the cavity part 234, the electrode 40 provided on the mode conversion part 31 and the electrode 40 provided on the cavity part 234 can be integrally formed.

  In the second wavelength filter 300, mode conversion is performed by the TE-polarized wave of the fundamental mode that is input from the input unit 231 and passes through the first mode conversion unit 31-1, and the photonic crystal of the second mode conversion unit 31-2. Of the fundamental mode TE polarized light that is reflected while being further mode-converted by the first mode converting unit 31-1, and having a wavelength whose phase is matched according to the length of the cavity 234, is output to the output unit. 237 is output.

  On the other hand, according to the length of the cavity part 234 among TE polarized waves of the first mode reflected while being mode-converted by the photonic crystals of the first mode conversion part 31-1 and the second mode conversion part 31-2. Light having a wavelength whose phase is matched is input to the input-side tapered portion 232. The reflected light propagates through the input side tapered portion 232 toward the input portion 231. However, as described above, the width of the one end 232a of the input side tapered portion 232 is set so as to satisfy the single mode condition for the TE polarized wave. Therefore, the reflected light is radiated without moving to the input unit 231.

  Therefore, the second wavelength filter 300 can be used as a wavelength filter for extracting light of a specific wavelength that is phase-matched by the cavity portion 234.

  Here, as already described, at both ends of the photonic crystal (that is, both ends of each of the first mode conversion unit 31-1 and the second mode conversion unit 31-2), between the fundamental mode and the primary mode. In addition to the reflection diffraction, an undesired reflection diffraction between the fundamental modes occurs, and the input light in the fundamental mode is reflected in the fundamental mode. As a result, in the cavity part 234, not only the fundamental mode TE polarized wave input from the first mode converter 31-1 to the cavity part 234 but also the second mode converter 31-2 is input to the cavity part 234. TE mode polarized wave propagates. Then, it was found by a simulation using FDTD that the TE mode polarized waves from the opposite directions interfere with each other to cause the wavelength peak from the output unit 237 to split.

  However, in the second wavelength filter 300, the first compensation unit 131a and the second compensation unit 131b are provided at both ends of the first mode conversion unit 31-1 and the second mode conversion unit 31-2, respectively. As described above, the first compensation unit 131a and the second compensation unit 131b reflect the fundamental mode light while maintaining the fundamental mode. As a result, the fundamental mode TE polarization input from the second mode conversion section 31-2 to the cavity section 234 can be canceled by reflection diffraction between the fundamental modes in the first compensation section 131a and the second compensation section 131b. Therefore, in the second wavelength filter 300, it is possible to prevent the TE mode polarized waves from interfering with each other in the cavity portion 234. As a result, it is possible to prevent the wavelength peak from the output unit 237 from being split.

  Further, the phase of the plurality of wavelengths is matched to the light of the fundamental mode propagating through the cavity portion 234 by making the cavity portion 234 have a length that generates a phase that is an integral multiple of π with respect to the light of the fundamental mode. be able to. Therefore, the second wavelength filter 300 can make the wavelength peak of the output light multimodal.

  In the second wavelength filter 300, heat can be applied to the first mode conversion unit 31-1, the second mode conversion unit 31-2, and the cavity unit 234 using the electrode 40. Therefore, it is possible to change the reflection wavelength by the first mode conversion unit 31-1 and the second mode conversion unit 31-2 and the wavelength to be phase-matched by the cavity unit 234. Therefore, the output wavelength of the second wavelength filter 300 is variable.

  The second wavelength filter 300 can be regarded as a wavelength filter equivalent to a ring resonator. In this case, the photonic crystals of the first mode converter 31-1 and the second mode converter 31-2 correspond to the directional coupler portion of the ring resonator. The cavity portion 234 corresponds to the ring waveguide portion of the ring resonator. Here, the ring resonator is easily affected by manufacturing errors in the directional coupler portion. On the other hand, the second wavelength filter 300 does not include a directional coupler as a configuration. Therefore, the second wavelength filter 300 has a function equivalent to that of the ring resonator, but is less susceptible to manufacturing errors than the ring resonator.

  In this embodiment, the configuration in which the optical waveguide core 30 includes the two mode conversion units 31 (the first mode conversion unit 31-1 and the second mode conversion unit 31-2) and the one cavity unit 234 has been described. However, the optical waveguide core 30 may include n (n is an integer of 2 or more) mode conversion units 31 and n−1 cavity units 234. With reference to FIG. 6, a configuration in the case of including n mode converters 31 and n−1 cavities 234 as a modification of the second wavelength filter will be described. FIG. 6 is a schematic plan view of a modified example (wavelength filter 350) of the second wavelength filter including n mode converters 31 and n−1 cavities 234. FIG. In FIG. 6, the cladding, the support substrate, and the electrodes are omitted.

  The n mode conversion units 31-1 to 31-n and the n−1 cavity portions 234-1 to n−1 are alternately connected in series between the input side taper portion 232 and the output side taper portion 236. . The first compensator 131a and the second compensator 131b existing between the mode converters 31 adjacent to each other with the cavity part 234 interposed therebetween are formed as a part of the cavity part 234 sandwiched between the adjacent mode converters 31. Yes.

  In each mode conversion section 31, the above-described photonic crystal is formed over the entire area. With this photonic crystal, each mode conversion unit 31 converts the input propagation light of a specific wavelength from the fundamental mode to the primary mode and reflects it. In addition, each mode conversion unit 31 transmits the propagation light having other wavelengths while maintaining the fundamental mode. The period of the photonic crystal holes 51 and 52 of each mode converter 31 is designed to satisfy the above formula (1) for the wavelength λ to be reflected under common conditions.

  Each cavity part 234 matches the phase of the light of a specific wavelength according to the length of the cavity part 234 among the TE polarized waves propagating through each cavity part 234.

  Thus, by connecting the mode conversion unit 31 and the cavity unit 234 in multiple stages, the flat top characteristic of the wavelength peak of the light output from the output unit 237 can be improved.

  In addition, the length (the number of holes 51 and 52 of the photonic crystal) of each mode conversion unit 31 can be set so that part or all of them differ. In this case, the flat top characteristic of the wavelength peak of transmitted light can be improved. The relationship between the optimal lengths of the mode converters 31 (the number of photonic crystal holes 51 and 52) for improving the flat-top characteristics is determined by, for example, simulation using FDTD and the mode converters 31 and the cavity. It can be determined as appropriate according to the number of parts 234. As an example, when the mode conversion unit 31 and the cavity unit 234 are multi-staged, the lengths of the mode conversion units 31-2 to n-1 sandwiched between the cavity units 234 are set to the mode conversion units 31 arranged at both ends. -1 and 31-n can be about twice as long.

(Characteristic evaluation)
The inventor performed the simulation which evaluates the characteristic of the 2nd wavelength filter 300 using FDTD.

  In this simulation, with respect to the second wavelength filter 300 having the configuration example shown in FIG. 5, the fundamental mode TE polarized wave is input to the first mode conversion unit 31-1, and the first mode conversion unit 31-1 and the second mode are input. Output light (transmitted light) that is transmitted through the conversion unit 31-2 and output light (reflected light) that is reflected and output by the first mode conversion unit 31-1 and the second mode conversion unit 31-2. The strength of was analyzed.

  In this simulation, the same analysis was performed for the comparative wavelength filter in which the first compensation unit 131a and the second compensation unit 131b are not provided (that is, the sub-hole 90 is not formed). The characteristics were compared between the second wavelength filter 300 and the comparative wavelength filter.

  In this simulation, the second wavelength filter was designed as follows. That is, the entire thickness of the optical waveguide core 30 is 200 nm. In addition, the widths of the first mode conversion unit 31-1, the cavity unit 234, and the second mode conversion unit 31-2 were set to a constant 1000 nm. Further, the number of holes 51 and 52 in the photonic crystal of the first mode conversion unit 31-1 and the second mode conversion unit 31-2 is 20, the formation period Λ of the holes 51 and 52 is 394 nm, the holes The distance between the centers along the width direction of the mode converter 31 between the 51 and the holes 52 is 500 nm, and the diameters of the holes 51 and 52 are 100 nm. The length of the cavity part 234 was 9850 nm. In this simulation, in order to clearly check the characteristics of the cavity part 234, the first compensation part 131a and the second mode conversion part formed adjacent to the one end 31a of the first mode conversion part 31-1. The second compensator 131b formed adjacent to the other end 31b of 31-2 and the sub-holes 90 in these regions are omitted.

  In addition, the sub hole 90 of the 2nd compensation part 131b formed adjacent to the other end 31b of the 1st mode conversion part 31-1 and the most other end 31b side of the 1st mode conversion part 31-1 are located. The distance between the centers along the length direction of the air holes 51 or 52 (here, in FIG. 5A, the air holes 51 present at the rightmost end of the paper surface in the first mode conversion unit 31-1). The length corresponding to 5/8 period of the formation period Λ of the holes 51 and 52 (here, 246.25 nm). Similarly, the sub holes 90 of the first compensation unit 131a formed adjacent to the one end 31a of the second mode conversion unit 31-2 are also connected to the holes 51 and 52 closest to the sub holes 90. The distance between the centers along the length direction was set to a length corresponding to 5/8 period of the formation period Λ of the holes 51 and 52.

  On the other hand, the comparative wavelength filter is designed in common with the second wavelength filter under the above-described conditions except that the configuration is different from the second wavelength filter in that the sub air holes 90 are not formed.

  The results of this simulation are shown in FIGS. 7 (A) and (B). FIG. 7A shows the intensity of the output light of the second wavelength filter. FIG. 7B shows the intensity of the output light of the comparative wavelength filter. In FIGS. 7A and 7B, the vertical axis indicates the light intensity in dB scale, and the horizontal axis indicates the wavelength in μm. A curve 201 in FIG. 7A and a curve 301 in FIG. 7B indicate the intensity of the transmitted light of the TE polarization in the basic mode. Also, a curve 203 in FIG. 7A and a curve 303 in FIG. 7B indicate the intensity of reflected light of the TE-polarized light in the basic mode. Further, a curve 205 in FIG. 7A and a curve 305 in FIG. 7B indicate the intensity of the reflected light of the TE polarization in the first mode.

  As shown in FIG. 7A, in the second wavelength filter 300, a plurality of peaks can be confirmed as the transmitted light of the TE-polarized light in the basic mode. Then, it is confirmed that the peaks of these transmitted lights are sharply suppressed with the splitting being remarkably suppressed particularly at the central peak. It can also be confirmed that the reflected light of the TE mode polarized light in the basic mode is sufficiently suppressed.

  On the other hand, as shown in FIG. 7B, in the comparative wavelength filter, splitting occurs in the peak of the transmitted light of the TE-polarized light in the fundamental mode. Also, a large peak is confirmed in the intensity of the reflected light of the TE polarized light in the fundamental mode.

  From this result, it was confirmed that the splitting of the wavelength peak of the transmitted light to be output can be suppressed by forming the sub-hole 90.

(Third wavelength filter)
With reference to FIG. 8, the 3rd wavelength filter using the optical waveguide element 100 (refer FIG. 1) mentioned above is demonstrated. FIG. 8 is a schematic plan view showing a third wavelength filter. In FIG. 8, only the optical waveguide core is shown, and the cladding, the support substrate, and the electrodes are omitted. In addition, the same reference numerals are given to components common to the optical waveguide device 100, the first wavelength filter 200, and the second wavelength filter 300, and description thereof is omitted.

  The third wavelength filter 400 is configured by adding an output waveguide core 70 to the second wavelength filter 300 described above. Further, the optical waveguide core 30 is added to the optical waveguide core 30 (see FIG. 5) of the second wavelength filter 300, and the input side taper portion 232 and the mode conversion portion 31 (here, the first wavelength conversion portion 31). A multimode waveguide section 34 connected in series with the input side taper section 232 and the first mode conversion section 31-1 is included between the mode conversion section 31-1). The output waveguide core 70 and the multimode waveguide section 34 are the same as the first wavelength filter 200 described above.

  The first compensation unit 131a formed adjacent to the one end 31a of the first mode conversion unit 31-1 on the multimode waveguide unit 34 side is formed as a part of the multimode waveguide unit 34. Accordingly, a sub-hole 90 is formed near the end of the multimode waveguide section 34 on the first mode conversion section 31-1 side. The region where the sub-hole 90 of the multimode waveguide portion 34 is formed functions as the first compensation portion 131a.

  In the third wavelength filter 400, the fundamental mode optical signal is input to the input unit 231 of the optical waveguide core 30, passes through the input side taper unit 232 and the multimode waveguide unit 34, and then enters the first mode conversion unit 31-1. Sent.

  The first mode conversion unit 31-1 reflects the fundamental mode TE polarized light that is transmitted through the first mode conversion unit 31-1, and the second mode conversion unit 31-2 while undergoing polarization conversion, and is further reflected by the first mode conversion unit 31-1. Of the fundamental mode TE polarized light that is reflected while being subjected to polarization conversion, light having a wavelength whose phase is matched according to the length of the cavity 234 is output from the output unit 237.

  On the other hand, according to the length of the cavity part 234 among TE polarized waves of the first mode reflected while being mode-converted by the photonic crystals of the first mode conversion part 31-1 and the second mode conversion part 31-2. Light having a wavelength whose phase is matched is input to the multimode waveguide section 34.

  The primary mode TE polarized wave propagating through the multimode waveguide section 34 is converted into the fundamental mode in the coupling region 80 and is transferred to the coupling section 71 of the output waveguide core 70. The basic mode TE polarized wave transferred to the coupling unit 71 is output from the second output unit 72.

  As described above, the third wavelength filter 400 can be used as a wavelength filter that separates a specific wavelength from other wavelengths and switches and extracts the path. And the light of a specific wavelength can be reflected with high diffraction efficiency by utilizing a photonic crystal in the 1st mode conversion part 31-1 and the 2nd mode conversion part 31-2. Therefore, wavelength separation and path switching can be performed with high efficiency.

  In this embodiment, the configuration in which the optical waveguide core 30 includes two mode conversion units (the first mode conversion unit 31-1 and the second mode conversion unit 31-2) and one cavity unit 234 has been described. . However, similarly to the second wavelength filter 350, the optical waveguide core 30 may include n (n is an integer of 2 or more) mode conversion units 31 and n−1 cavity units 234. Yes (see FIG. 6).

(Production method)
The optical waveguide device 100, the first wavelength filter 200, the second wavelength filter 300, and the third wavelength filter 400 described above can be easily manufactured by using, for example, an SOI (Silicon On Insulator) substrate. . Hereinafter, a method for manufacturing the optical waveguide device 100 will be described as an example.

That is, first, an SOI substrate is prepared in which a support substrate layer, a SiO ₂ layer, and a Si layer are sequentially stacked. Next, the optical waveguide core 30 is formed by patterning the Si layer using, for example, an etching technique. As a result, it is possible to obtain a structure in which the SiO ₂ layer is laminated on the support substrate layer as the support substrate 10 and the optical waveguide core 30 is formed on the SiO ₂ layer. Next, SiO ₂ is formed to cover the optical waveguide core 30 on the SiO ₂ layer by using, for example, a chemical vapor deposition (CVD) method. As a result, the optical waveguide core 30 is encompassed by the SiO ₂ cladding 20. Next, the electrode 40 can be formed on the clad 20 to manufacture the optical waveguide device 100. In the case where the first wavelength filter 200 and the third wavelength filter 400 are manufactured, the output waveguide core 70 can be formed at the same time in the step of forming the optical waveguide core 30 by patterning the Si layer. .

10: Support substrate 20: Cladding 30: Optical waveguide core 31: Mode conversion section 32, 231: Input section 33, 232: Input side taper section 34: Multimode waveguide section 35, 236: Output side taper section 36, 237: Output part 40: Electrode 51, 52: Hole 61: First hole group 62: Second hole group 70: Output waveguide core 71: Coupling part 72: Second output part 80: Coupling region 90: Sub-hole 100: Optical waveguide element 131a: First compensation unit 131b: Second compensation unit 200: First wavelength filter 234: Cavity unit 300: Second wavelength filter 350: Second wavelength filter 400 according to the modification: Third Wavelength filter

Claims (10)

An optical waveguide core provided along a first imaginary line segment and a second imaginary line segment parallel to the first imaginary line segment and overlapping the first imaginary line segment and the second imaginary line segment;
A clad including the optical waveguide core,
The optical waveguide core includes a mode converter connected in series along the first imaginary line segment and the second imaginary line segment;
A first compensation unit formed adjacent to one end of the mode conversion unit;
A second compensator formed adjacent to the other end of the mode converter,
In the mode conversion unit, a first hole group including a plurality of holes periodically formed in a position overlapping with the first virtual line segment, and a position overlapping with the second virtual line segment A photonic crystal including a second hole group including a plurality of holes formed at the same period as the first hole group is formed,
For the specific wavelength λ, the photonic crystal has a period of holes included in the first hole group and a hole included in the second hole group as Λ, and an equivalent refractive index for the fundamental mode as n _0. , Q (q is an integer of q> 0) where n _q is the equivalent refractive index for the mode, and 2π / Λ = 2π (n ₀ + n _q ) / λ is satisfied,
Each of the first compensation unit and the second compensation unit is formed with one or a plurality of sub-holes, and the first compensation unit and the second compensation unit have a fundamental mode of the specific wavelength λ. An optical waveguide device that reflects light in a fundamental mode. 2. The light guide according to claim 1, wherein each of the holes included in the first hole group and the second hole group has a unique diameter, and the diameter has a value of at least 2 or more. Waveguide element. The holes included in the first hole group and the holes included in the second hole group excavate the mode conversion unit from the upper surface of the mode conversion unit to the middle of the mode conversion unit in the thickness direction. The optical waveguide device according to claim 1, wherein the optical waveguide device is formed by being embedded. 4. The light guide according to claim 1, wherein an electrode for applying heat to the mode conversion unit is formed at a position covering the mode conversion unit via the clad. Waveguide element. The optical waveguide device according to any one of claims 1 to 4, and an output waveguide core including a coupling portion,
The optical waveguide core further includes a multimode waveguide unit connected in series with the mode conversion unit and propagating light of a fundamental mode and a q-order mode,
The first compensation unit is formed as a part of the multimode waveguide unit,
The cladding includes the optical waveguide core and the output waveguide core;
A coupling region is set in which the multimode waveguide part and the coupling part are spaced apart from each other and arranged side by side.
In the coupling region, q-order mode light propagating through the multimode waveguide section and r (r is an integer of r ≧ 0) order mode light propagating through the coupling section are coupled. Wavelength filter to be used. Comprising the optical waveguide device according to any one of claims 1 to 4,
The optical waveguide core includes n (n is an integer of 2 or more) mode conversion units, the first compensation units, and the series connected in series along the first virtual line segment and the second virtual line segment. A second compensation unit;
n-1 cavity portions,
The mode conversion section and the cavity section are connected alternately,
The first compensation unit and the second compensation unit that exist between the mode conversion units adjacent to each other with the cavity unit interposed therebetween are formed as part of the cavity unit sandwiched between the mode conversion units adjacent to each other. And
The wavelength filter, wherein the cavity portion matches the phase of light of a fundamental mode having a specific wavelength that propagates through the cavity portion. The optical waveguide core further includes an input side taper portion connected in series to the first compensation portion adjacent to one end of the mode conversion portion disposed at the outermost end,
The input side taper portion continuously increases in width from one end to the other end on the first compensation portion side,
The wavelength filter according to claim 6, wherein one end of the input side taper portion is set to a width corresponding to a fundamental mode. Further comprising an output waveguide core including a coupling portion;
The optical waveguide core further includes a multimode waveguide unit that is connected in series to the mode conversion unit disposed at the outermost end and propagates light of a fundamental mode and a q-order mode,
The first compensation unit formed adjacent to one end of the mode conversion unit disposed at the outermost end on the multimode waveguide unit side is formed as a part of the multimode waveguide unit,
The cladding includes the optical waveguide core and the output waveguide core;
A coupling region is set in which the multimode waveguide part and the coupling part are spaced apart from each other and arranged side by side.
In the coupling region, q-order mode light propagating through the multimode waveguide section and r (r is an integer of r ≧ 0) order mode light propagating through the coupling section are coupled. The wavelength filter according to claim 6. The wavelength filter according to any one of claims 6 to 8, wherein an electrode for applying heat to the cavity portion is formed at a position covering the cavity portion via the clad. The center-to-center distance along the length direction of the mode conversion portion closest to the sub-hole is included in the holes included in the first hole group and the second hole group. The wavelength filter according to any one of claims 6 to 9, wherein the wavelength filter has a length corresponding to 5/8 period of a hole period.

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