JP6119306B2 - Optical waveguide device - Google Patents

Description

  The present invention relates to an optical waveguide element that switches a light path based on a difference in wavelength.

  2. Description of the Related Art Recently, a subscriber optical access system is configured by connecting one station side device (OLT: Optical Line Terminal) and a plurality of subscriber side devices (ONU: Optical Network Unit) via an optical fiber and a star coupler. Passive optical subscriber networks (PON) have become mainstream. In this communication system, the downstream optical signal from the OLT to the ONU and the upstream optical signal from the ONU to the OLT are set to different wavelengths. As a result, the downstream optical signal and upstream optical signal do not interfere with each other.

  The OLT and ONU are configured to include optical elements such as photodiodes and laser diodes. These optical elements are spatially coupled using, for example, a lens after performing complicated optical axis alignment for adjusting the center position (light receiving position or light emitting position) of each optical element to the design position.

  Here, as means for coupling the optical elements in the OLT and the ONU, there is a technique that uses an optical waveguide element instead of a lens (for example, see Patent Document 1). When the optical waveguide element is used, light is confined in the optical waveguide and propagates, so that complicated optical axis alignment is not required unlike the case of using a lens. Therefore, the assembly process of the OLT and ONU is simplified, which is advantageous as a form suitable for mass production. The optical waveguide element is formed extremely small using, for example, silicon (Si) as a waveguide material. In addition, the manufacturing process of CMOS (Complementary Metal Oxide Semiconductor) is diverted for manufacturing, and cost reduction is realized.

  In OLT and ONU, an optical waveguide element provided with a function as a wavelength filter may be used to switch between a downstream optical signal path and an upstream optical signal path.

  The function as the wavelength filter is realized by, for example, a grating. The grating has a feature that a desired wavelength can be selected in a single stage as compared with, for example, a Mach-Zehnder interferometer. For example, Non-Patent Documents 1 and 2 and Patent Document 2 disclose a configuration in which a grating is formed in an optical waveguide mainly made of Si.

JP-A-8-163028 JP 2006-235380 A

IEICE Transactions of Electronics vol. E90-C, No1, p. 59, January 2007 Optical Fiber Communication Conference 2012 OTh3D. 3

  In a conventional optical waveguide device using a grating, light incident on the grating and reflected light that is Bragg-reflected by the grating are in the same order mode.

  Here, the Bragg reflection condition in the grating is expressed by the following equation (1).

(N _a + n _b ) Λ = λ (1)
Incidentally, n _a and n _b are coupled in the grating, an equivalent refractive index of the incident light and reflected light. _a and _b in na and nb indicate the orders of modes of incident light and reflected light, respectively. Λ indicates the period of the grating. In the grating, light having a wavelength λ satisfying the above equation (1), that is, light having a Bragg wavelength is Bragg reflected.

  However, when the optical waveguide element is made of Si, the equivalent refractive index of TE (Transverse Electric) polarization and the equivalent refractive index of TM (Transverse Magnetic) polarization of the same order mode at the same wavelength Are difficult to match and the structure is limited. Therefore, when combining incident light and reflected light of the same order mode (that is, when a = b), it is difficult to establish the Bragg reflection condition at the same wavelength independent of polarization. It is. Therefore, a conventional optical waveguide element using a grating has polarization dependency.

  In general, light used as an optical signal in a PON often includes TE polarized light and TM polarized light. Therefore, for example, when used as a path switching element in OLT and ONU, an optical waveguide element having small polarization dependence is required.

  An object of the present invention is to provide an optical waveguide element that can be used as a path switching element using a grating and that can be used independently of polarization.

  In order to solve the above-described problems, an optical waveguide device according to the present invention has the following features.

The optical waveguide device of the present invention includes a first optical waveguide core and a second optical waveguide core. A first bidirectional coupling region in which the first optical waveguide core and the second optical waveguide core are spaced apart from each other and arranged in parallel is set. A Bragg grating is formed in the first bidirectional coupling region. In the first bidirectional coupling region, the TE polarized wave and TM polarized wave having a specific wavelength propagating through the first optical waveguide core are reflected by the Bragg grating, and are transferred to the second optical waveguide core. The propagation mode of at least one of the polarized waves is converted. The Bragg grating is formed in a sub optical waveguide core provided in a region between the first optical waveguide core and the second optical waveguide core.

  In the optical waveguide device according to the present invention, the propagation mode of at least one of the TE polarized wave and the TM polarized wave is converted between the first optical waveguide core and the second optical waveguide core in the first bidirectional coupling region. Is done. Therefore, the Bragg reflection condition can be established at a common wavelength for the TE polarization and the TM polarization. Therefore, it is possible to reflect TE polarized waves and TM polarized waves having specific wavelengths in common with each other by reflecting them with the Bragg grating and coupling them between the first optical waveguide core and the second optical waveguide core. As a result, the optical waveguide element according to the present invention can be used as a polarization-independent path switching element.

(A) And (B) is the schematic which shows the optical waveguide element of this invention. It is a figure which shows the relationship of the equivalent refractive index with respect to the width | variety of an optical waveguide core. It is a figure which confirms that a Bragg reflection condition is materialized. (A) And (B) is the schematic which shows the modification of a Bragg grating. It is a figure which confirms that a Bragg reflection condition is materialized. It is a figure which shows the simulation result regarding the characteristic evaluation of the optical waveguide element 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.

(Constitution)
With reference to FIGS. 1A and 1B, an optical waveguide device according to the present invention will be described. FIG. 1A is a schematic plan view showing an optical waveguide element. In FIG. 1A, a clad layer to be described later is partially omitted. FIG. 1B is a schematic end view of the optical waveguide element shown in FIG.

  The optical waveguide element 100 includes a support substrate 10, a cladding layer 20, a first optical waveguide core 30, a second optical waveguide core 40, and a third optical waveguide core 50.

  In the optical waveguide device 100, a first bidirectional coupling region 60 and a second bidirectional coupling region 80 are set apart from each other. In the first bidirectional coupling region 60, the first optical waveguide core 30 and the second optical waveguide core 40 are spaced apart from each other and arranged in parallel. In the second bidirectional coupling region 80, the second optical waveguide core 40 and the third optical waveguide core 50 are spaced apart from each other and arranged in parallel. Further, a Bragg grating 70 is formed in the first bidirectional coupling region 60.

  The first optical waveguide core 30 includes a first coupling part 31, a first taper part 33, and a first input / output part 35. The first input / output part 35 is connected to the first coupling part 31 via the first taper part 33. The width of the first taper portion 33 is set so as to continuously change from the width of the one end 35a of the first input / output portion 35 to the width of the other end 31d of the first coupling portion 31 along the light propagation direction. Has been. By providing the first taper portion 33, reflection of light propagating between the first input / output portion 35 and the first coupling portion 31 can be reduced.

  The second optical waveguide core 40 has a second coupling part 41, a second taper part 43, and a second input / output part 45. The second input / output part 45 is connected to the second coupling part 41 via the second taper part 43. The width of the second tapered portion 43 is set so as to continuously change from the width of the one end 41a of the second coupling portion 41 to the width of the one end 45a of the second input / output portion 45 along the light propagation direction. ing. By providing the second tapered portion 43, reflection of light propagating between the second input / output portion 45 and the second coupling portion 41 can be reduced.

  The third optical waveguide core 50 has a third coupling part 51 and a third input / output part 57. The third coupling unit 51 is formed by sequentially connecting the TM polarization coupling unit 53 and the TE polarization coupling unit 55 along the light propagation direction.

  The first coupling portion 31 of the first optical waveguide core 30 and the second coupling portion 41 of the second optical waveguide core 40 are portions in the first bidirectional coupling region 60. The second input / output unit 45 of the second optical waveguide core 40 and the third coupling unit 51 of the third optical waveguide core 50 are portions in the second bidirectional coupling region 80.

  Further, the second bidirectional coupling region 80 includes a TM polarization coupling region 81 in which the second input / output unit 45 and the TM polarization coupling unit 53 are spaced apart from each other and arranged in parallel, and the second input / output unit 45. And the TE polarization coupling portion 55 include TE polarization coupling regions 83 that are spaced apart from each other and arranged in parallel.

  Hereinafter, each component of the optical waveguide device 100 will be described. In the following description, the direction along the thickness of the support substrate 10 is the thickness direction. In addition, regarding the first optical waveguide core 30, the second optical waveguide core 40, and the third optical waveguide core 50, the direction along the propagation direction of the light propagating through them is defined as the length direction. Moreover, let the direction orthogonal to a length direction and a thickness direction be a width direction.

  The optical waveguide device 100 according to this embodiment can be easily manufactured by using an SOI (Silicon On Insulator) substrate.

That is, first, an SOI substrate configured by sequentially stacking a support substrate layer, a silicon oxide (SiO ₂ ) layer, and an Si layer is prepared.

Next, the first optical waveguide core 30, the second optical waveguide core 40, and the third optical waveguide core 50 are formed by patterning the Si layer using, for example, an etching technique. As a result, the SiO ₂ layer is laminated on the support substrate layer as the support substrate 10, and the first optical waveguide core 30, the second optical waveguide core 40, and the third optical waveguide core 50 are formed on the SiO ₂ layer. Can be obtained.

Next, the first optical waveguide core 30, the second optical waveguide core 40, and the third optical waveguide core 50 are covered with a material layer made of SiO ₂ on the SiO ₂ layer by using, for example, a CVD method. Form. As a result, the cladding layer 20 including the first optical waveguide core 30, the second optical waveguide core 40, and the third optical waveguide core 50 is formed from the SiO ₂ layer and the material layer.

The first optical waveguide core 30, the second optical waveguide core 40, and a third optical waveguide core 50 is formed, for example, Si has a higher refractive index than the cladding layer 20 in which the SiO ₂ as the material as the material. As a result, each of these optical waveguide cores 30, 40, and 50 functions as a light transmission path, and the input light propagates in the propagation direction according to the planar shape of each optical waveguide core. Each of the optical waveguide cores 30, 40, and 50 is preferably formed at least 1 μm away from the support substrate 10 in order to prevent the propagating light from escaping to the support substrate 10.

  The optical waveguide element 100 is used as a path switching element between a downstream optical signal and an upstream optical signal in, for example, an OLT or an ONU. Here, a configuration example when the optical waveguide element 100 is used as a path switching element in the ONU will be described.

  When the optical waveguide element 100 is used as a path switching element in the ONU, the first input / output unit 35 of the first optical waveguide core 30 is connected to, for example, an optical fiber. This optical fiber is connected to the OLT.

  The first coupling portion 31 of the first optical waveguide core 30 is connected to a light emitting element such as a laser diode on the one end 31a side. Further, the third input / output unit 57 of the third optical waveguide core 50 is connected to a light receiving element such as a photodiode.

  The period of the Bragg grating 70 provided in the first bidirectional coupling region 60 is set so as to reflect the downstream optical signal.

  The downstream optical signal in the basic mode transmitted from the OLT through the optical fiber is input to the first input / output unit 35 of the first optical waveguide core 30 and is transmitted to the first coupling unit 31 through the first taper unit 33. Here, the first input / output unit 35 is set to a thickness and a width that achieve a single mode condition. Therefore, the light of the fundamental mode is propagated.

  The downstream optical signal sent to the first coupling unit 31 is reflected by the Bragg grating 70 in the first bidirectional coupling region 60 and moves to the second coupling unit 41 of the second optical waveguide core 40. In the first bidirectional coupling region 60, the first coupling unit 31 and the second coupling unit 41 are optimized so as to convert and couple the fundamental mode TE polarization and the secondary mode TE polarization. Yes. Further, the first coupling unit 31 and the second coupling unit 41 are optimized so as to convert and couple the fundamental mode TM polarization and the primary mode TM polarization. The downstream optical signal including the secondary mode TE polarization and the primary mode TM polarization is sent from the second coupling unit 41 to the second input / output unit 45 via the second taper unit 43.

  The downstream optical signal sent to the second input / output unit 45 moves to the third coupling unit 51 in the second bidirectional coupling region 80. In the second bidirectional coupling region 80, the second input / output unit 45 and the third coupling unit 51 are optimized to convert and couple the secondary mode TE polarized wave and the fundamental mode TE polarized wave. ing. Further, the second input / output unit 45 and the third coupling unit 51 are optimized so as to convert and combine the primary mode TM polarization and the fundamental mode TM polarization.

  The downstream optical signal obtained by converting the TE polarized wave and the TM polarized wave into the fundamental mode is sent from the third coupling unit 51 to the third input / output unit 57. The downstream optical signal output from the third input / output unit 57 is received by the light receiving element.

  The upstream optical signal in the basic mode generated by the light emitting element is input to the first coupling unit 31 from the one end 31a side. The upstream optical signal input to the first coupling unit 31 passes through the Bragg grating 70, passes through the first taper unit 33, and is sent to the first input / output unit 35. The upstream optical signal output from the first input / output unit 35 is sent to the OLT via the optical fiber.

  As already described, in the first bidirectional coupling region 60, the fundamental mode TE polarization and the secondary mode TE polarization are coupled between the first coupling unit 31 and the second coupling unit 41. In addition, between the first coupling unit 31 and the second coupling unit 41, the TM mode polarized wave and the primary mode TM polarized wave are coupled.

  In the first bidirectional coupling region 60, the width W1 of the first coupling portion 31 and the width W2 of the second coupling portion 41 are set so that the following expression (2) is established for a specific wavelength.

n _0TE −n _0TM = n _1TM −n _2TE (2)
Note that n _0TE indicates the equivalent refractive index of the TE-polarized light in the fundamental mode that propagates through the first coupling unit 31. N _0TM represents the equivalent refractive index of the TM polarization in the fundamental mode propagating through the first coupling unit 31. N _2TE indicates the equivalent refractive index of the TE polarized wave in the second-order mode that propagates through the second coupling unit 41. N _1TM represents the equivalent refractive index of the TM polarization of the first-order mode propagating through the second coupling unit 41.

  Here, from the above equation (1), the Bragg reflection condition of TE polarized light reflected by the Bragg grating 70 and coupled between the first coupling portion 31 and the second coupling portion 41 is expressed by the following equation (3). The

(N _0TE + n _2TE ) Λ = λ (3)
Similarly, the Bragg reflection condition of TM polarization reflected by the Bragg grating 70 and coupled between the first coupling unit 31 and the second coupling unit 41 is expressed by the following expression (4).

(N _0TM + n _1TM ) Λ = λ (4)
When the above equation (2) holds, the grating period Λ in the above equations (3) and (4) matches the common wavelength λ. Therefore, by setting the period of the Bragg grating 70 to the coincident period Λ, the first bidirectional coupling region 60 reflects both the TE polarized wave and the TM polarized wave of the common wavelength, and the first coupling. It can be coupled between the part 31 and the second coupling part 41.

  Here, with reference to FIG. 2, an example of the design that establishes the above equation (2) will be described. FIG. 2 is a diagram showing the relationship of the equivalent refractive index to the width of the optical waveguide core at a wavelength of 1577 nm, created using the finite element method. In FIG. 2, the vertical axis represents the equivalent refractive index on an arbitrary scale, and the horizontal axis represents the width of the optical waveguide core in nm units. In FIG. 2, the TE mode, the first mode, the second mode, the third mode, and the fourth mode TE polarization, and the basic mode, the first mode, the second mode, and the third mode TM polarization. In particular, the equivalent refractive index of the TE polarized wave in the fundamental mode is I, the equivalent refractive index of the TM polarized wave in the basic mode is II, and the equivalent refractive index of the TE polarized wave in the second mode is shown. III, the equivalent refractive index of the TM polarized light in the first-order mode is denoted by the symbol IV.

  As shown in FIG. 2, when the width of the optical waveguide core is 500 nm, a value obtained by subtracting the equivalent refractive index II of the fundamental mode TM polarization from the equivalent refractive index I of the fundamental mode TE polarization (arrow 201). When the width of the optical waveguide core is 900 nm, a value obtained by subtracting the equivalent refractive index III of the TE polarized wave in the first order mode from the equivalent refractive index IV of the TE polarized wave in the second order mode (indicated by an arrow 203) Match).

  Therefore, by setting the width of the first coupling portion 31 to 500 nm and the width of the second coupling portion to 900 nm, the above equation (2) is established for the TE polarized wave and TM polarized wave of 1577 nm.

  Next, referring to FIG. 3, when the above equation (2) is established, it is confirmed that the Bragg reflection conditions of the above equations (3) and (4) are established in common.

  The upper part of FIG. 3 is a diagram showing the relationship of the equivalent refractive index to the width of the optical waveguide core, which is the same as FIG. Further, the lower part of FIG. 3 is a diagram obtained by inverting the top and bottom of FIG. As in FIG. 2, I is the equivalent refractive index of the TE polarization in the fundamental mode, II is the equivalent refractive index of the TM polarization in the fundamental mode, and III is the equivalent refractive index of the TE polarization in the second mode. , IV show the equivalent refractive index of the TM polarization in the first mode.

The above formulas (3) and (4) are converted into the following formulas (5) and (6).
λ / Λ = n _0TE + n _2TE (5)
λ / Λ = n _0TM + n _1TM (6)
Can be transformed into

In FIG. 3, an arrow 301 indicates the equivalent refractive index I of the TE polarized wave in the fundamental mode when the width of the optical waveguide core is 500 nm, and the TE in the secondary mode when the width of the optical waveguide core is 900 nm. The sum of the polarization and the equivalent refractive index III, that is, n _0TE + n _2TE is shown. The arrow 303 indicates the equivalent refractive index II of the fundamental mode TM polarization when the width of the optical waveguide core is 500 nm, and the TM polarization of the primary mode when the width of the optical waveguide core is 900 nm. And the equivalent refractive index IV, that is, n _0TM + n _1TM . From FIG. 3, it can be confirmed that n _0TE + n _2TE is equal to n _0TM + n _1TM , that is, the above equation (2) is established. Therefore, it can be confirmed that both the TE polarized wave and the TM polarized wave having the common wavelength λ can be reflected by the Bragg grating 70 having the common period Λ.

  Next, the Bragg grating 70 will be described. The Bragg grating 70 is formed on at least one of the first coupling portion 31 and the second coupling portion 41, for example.

  In the configuration example shown in FIG. 1, the Bragg grating 70 is formed including a first Bragg grating 71 and a second Bragg grating 73. The first Bragg grating 71 and the second Bragg grating 73 are formed so that the grating periods Λ coincide with each other. The period Λ is set to a value calculated from the above equations (3) and (4) under the condition that the above equation (2) is satisfied.

  The first Bragg grating 71 is formed on the side surface 31 b of the first coupling portion 31 that faces the second coupling portion 41. The first Bragg grating 71 is configured by forming a concave engraving 71 a from the side surface 31 b of the first coupling portion 31. A plurality of concave engravings 71a are formed with a period Λ along the light propagation direction. Further, the second Bragg grating 73 is formed on the side surface 41 b of the second coupling portion 41 facing the first coupling portion 31. The second Bragg grating 73 is configured by forming a concave engraving 73 a from the side surface 41 b of the second coupling portion 41. A plurality of concave engravings 73a are formed with a period Λ along the light propagation direction. The engraving depths d1 and d2 of the engraving 71a constituting the first Bragg grating 71 and the engraving 73a constituting the second Bragg grating 73 are the same.

  In addition, the first Bragg grating 71 and the second Bragg grating 73 are preferably formed symmetrically so that the engraving 71a and the engraving 73a face each other. Diffraction efficiency can be improved by forming the first Bragg grating 71 and the second Bragg grating 73 symmetrically.

  Further, the first coupling portion 31 and the second coupling portion 41 are separated by a distance within a range in which an evanescent component of light propagating through one coupling portion reaches the other coupling portion. The separation distance between the first coupling portion 31 and the second coupling portion 41 is set to a value that allows light to be transferred according to the length L1 of the first bidirectional coupling region 60 along the light propagation direction. Has been. That is, when the length L1 of the first bidirectional coupling region 60 is expanded, the separation distance between the first coupling portion 31 and the second coupling portion 41 can be expanded. Further, when the distance between the first coupling portion 31 and the second coupling portion 41 is shortened, the length L1 of the first bidirectional coupling region 60 can be shortened. In FIG. 1A, the distance between the first coupling portion 31 and the second coupling portion 41 is the center distance D1 when the first coupling portion 31 and the second coupling portion 41 are viewed in plan. It is shown as

  In the Bragg grating 70, since the period Λ that matches the TE polarized wave and the TM polarized wave having the common wavelength is set, both the TE polarized wave and the TM polarized wave can be reflected.

  Here, FIGS. 4A and 4B are schematic plan views showing modifications of the Bragg grating, and an area corresponding to the first bidirectional coupling area 60 surrounded by a broken line in FIG. 1 is enlarged. FIG.

  As shown in FIG. 4A, as a first modification, the Bragg grating 170 may be formed on each side surface of the first coupling portion 31 and the second coupling portion 41 along the extending direction. it can.

  In the first modification, the Bragg grating 170 is formed including a first Bragg grating 171 and a second Bragg grating 173. The first Bragg grating 171 and the second Bragg grating 173 are formed so that the grating periods Λ coincide with each other. The period Λ is set to a value calculated from the above equations (3) and (4) under the condition that the above equation (2) is satisfied.

  The first Bragg grating 171 is formed on both side surfaces 31 b and 31 c along the extending direction of the first coupling portion 31. The first Bragg grating 171 is configured by forming concave engravings 171 a at opposite positions from both side surfaces 31 b and 31 c of the first coupling portion 31. A plurality of concave engravings 171a are formed with a period Λ along the light propagation direction. The second Bragg grating 173 is formed on both side surfaces 41 b and 41 c along the extending direction of the second coupling portion 41. The second Bragg grating 173 is configured by forming concave engravings 173 a at opposite positions from both side surfaces 41 b and 41 c of the second coupling portion 41. A plurality of concave engravings 173a are formed with a period Λ along the light propagation direction. The engraving 171a constituting the first Bragg grating 171 and the engraving 173a constituting the second Bragg grating 173 are formed so that the engraving depths coincide.

  In addition, the first Bragg grating 171 and the second Bragg grating 173 are preferably formed symmetrically so that the engraving 171a and the engraving 173a face each other. The diffraction efficiency can be improved by forming the first Bragg grating 171 and the second Bragg grating 173 symmetrically.

  4B, in the second modification, the Bragg grating 270 is formed in the sub optical waveguide core 90 provided in the region between the first coupling portion 31 and the second coupling portion 41. ing.

  In the second modification, the sub-waveguide core 90 is disposed in the region between the first coupling portion 31 and the second coupling portion 41 so as to be spaced apart from and parallel to the first coupling portion 31 and the second coupling portion 41. Is provided. The sub-waveguide core 90 is provided with a Bragg grating 270.

  The Bragg grating 270 is configured by forming a concave engraving 270a in the sub-waveguide core 90. A plurality of engravings 270a are formed with a period Λ along the light propagation direction. The period Λ is set to a value calculated from the above equations (3) and (4) under the condition that the above equation (2) is satisfied. 4B, the engraving 270a is 0% with respect to the width of the sub-waveguide core 90. Therefore, in this configuration example, the Bragg grating 270 has a so-called segment structure in which the sub-waveguide core 90 is periodically divided.

  The Bragg grating is not limited to the configuration examples of FIG. 1A and FIGS. 4A and 4B. If the period Λ of the grating is a value calculated from the above equations (3) and (4) under the condition that the above equation (2) is satisfied, the arrangement and shape can be arbitrarily changed according to the design. .

  Further, in the second bidirectional coupling region 80, in the TM polarization coupling region 81, the equivalent refractive index of the TM polarization of the primary mode propagating through the second input / output unit 45 and the TM polarization coupling unit 53 are propagated. The width W4 of the TM polarization coupling unit 53 with respect to the width W3 of the second input / output unit 45 is set so that the equivalent refractive index of the TM polarization in the basic mode is equal. Further, in the TE polarization coupling region 83, the equivalent refractive index of the TE polarization in the secondary mode propagating through the second input / output unit 45 and the equivalent refraction of the TE polarization in the fundamental mode propagating through the TE polarization coupling unit 55. The width W5 of the TE polarization coupling unit 55 with respect to the width W3 of the second input / output unit 45 is set so that the rates are equal. As a result, in the TM polarization coupling region 81, the primary mode TM polarization propagating through the second input / output unit 45 and the fundamental mode TM polarization propagating through the TM polarization coupling unit 53 are coupled. Further, in the TE polarization coupling region 83, the secondary mode TE polarization propagating through the second input / output unit 45 and the fundamental mode TE polarization propagating through the TE polarization coupling unit 55 are coupled.

  Here, an example of the width W3 of the second input / output unit 45, the width W4 of the TM polarization coupling unit 53, and the width W5 of the TE polarization coupling unit 55 will be described with reference to FIG.

  As shown in FIG. 2, when the width of the optical waveguide core is 1000 nm, the equivalent refractive index of TM polarization in the first-order mode (enclosed by a wavy line with a reference numeral 211), and the optical waveguide core When the width is 400 nm, the equivalent refractive index of the TM polarized wave in the fundamental mode (enclosed by a wavy line with a reference numeral 213) matches. Also, when the width of the optical waveguide core is 1000 nm, the equivalent refractive index of TE polarized light in the second-order mode (enclosed by a wavy line with reference numeral 215) and the width of the optical waveguide core is 320 nm. This corresponds to the equivalent refractive index of the TE polarized wave of the fundamental mode (enclosed by a wavy line with a reference numeral 217).

  Therefore, by setting the width W3 of the second input / output unit 45 to 1000 nm, the width W4 of the TM polarization coupling unit 53 to 400 nm, and the width W5 of the TE polarization coupling unit 55 to 320 nm, the second input / output unit 45 is set. In addition, the TM polarized wave of the primary mode and the TM polarized wave of the basic mode can be converted and coupled between the TM polarized wave coupling unit 53. Further, between the second input / output unit 45 and the TE polarization coupling unit 55, it is possible to convert and couple the secondary mode TE polarization and the fundamental mode TE polarization.

  Further, the second input / output unit 45 and the TM polarization coupling unit 53, and the second input / output unit 45 and the TE polarization coupling unit 55 are ranges in which the evanescent component of light propagating through one reaches the other. Are separated by an inner distance. In FIG. 1A, the separation distance between the second input / output unit 45 and the TM polarization coupling unit 53 is the same as that when the second input / output unit 45 and the TM polarization coupling unit 53 are viewed in plan. It is shown as the center distance D2. Further, the separation distance between the second input / output unit 45 and the TE polarization coupling unit 55 is shown as a center-to-center distance D3 when the second input / output unit 45 and the TE polarization coupling unit 55 are viewed in a plane. is there. The separation distance D2 between the second input / output unit 45 and the TM polarization coupling unit 53 is set to a value that allows light transition according to the length L2 of the TM polarization coupling region 81 along the light propagation direction. Has been. That is, when the length L2 of the TM polarization coupling region 81 is expanded, the separation distance D2 between the second input / output unit 45 and the TM polarization coupling unit 53 can be expanded. Further, when the distance D2 between the second input / output unit 45 and the TM polarization coupling unit 53 is shortened, the length L2 of the TM polarization coupling region 81 can be shortened. Similarly, the separation distance D3 between the second input / output unit 45 and the TE polarization coupling unit 55 can also shift light according to the length L3 of the TE polarization coupling region 83 along the light propagation direction. Is set to a value. As a specific design example, when the width W3 of the second input / output unit 45 is 1000 nm and the width W4 of the TM polarization coupling unit 53 is 400 nm, for example, the second input / output unit 45 and the TM polarization coupling unit 53 are used. By setting the separation distance D2 between them to 1000 nm and the length L2 of the TM polarization coupling region 81 to 28 μm, the TM polarization in the primary mode can be changed between the second input / output unit 45 and the TM polarization coupling unit 53. It is possible to convert and shift the fundamental mode TM polarization. Further, when the width W3 of the second input / output unit 45 is 1000 nm and the width W5 of the TE polarization coupling unit 55 is 320 nm, for example, the separation distance D3 between the second input / output unit 45 and the TE polarization coupling unit 55 Is 960 nm and the length L3 of the TE polarization coupling region 83 is 23 μm, so that between the second input / output unit 45 and the TE polarization coupling unit 55, the second mode TE polarization and the fundamental mode TE polarization Waves can be converted and shifted.

  The third input / output unit 57 is set to a thickness and a width that achieve a single mode condition. Therefore, the light of the fundamental mode is propagated. The third input / output unit 57 is connected to one end of the third coupling unit 51 (one end of the TE polarization coupling unit 55 in the configuration example of FIG. 1A) 51a.

  As described above, in the optical waveguide device 100 according to this embodiment, the first coupling portion 31 of the first optical waveguide core 30 and the second coupling of the second optical waveguide core 40 in the first bidirectional coupling region 60. The propagation modes of the TE polarization and the TM polarization are converted between the unit 41 and the unit 41. Therefore, the Bragg reflection condition can be established at a common wavelength for the TE polarization and the TM polarization. Accordingly, it is possible to reflect the TE polarized wave and the TM polarized wave having specific common wavelengths by the Bragg grating 70 and to couple between the first coupling unit 31 and the second coupling unit 41. As a result, the optical waveguide element 100 according to the present invention can be used as a polarization-independent path switching element.

In the configuration example described above, the example in which the TE polarizations and the TM polarizations are coupled in the first bidirectional coupling region 60 has been described. However, the optical waveguide device 100 according to the present invention is not limited to this configuration. In the 1st bidirectional | two-way coupling area | region 60, it can also be set as the structure which couple | bonds the light between different polarized waves. In that case, in the first bidirectional coupling region 60, for example, the TE polarization of the fundamental mode and the TM polarization of the primary mode are coupled between the first coupling unit 31 and the second coupling unit 41. In addition, for example, between the first coupling unit 31 and the second coupling unit 41, the TM mode polarized wave and the primary mode TE polarized wave are coupled. In the first bidirectional coupling region 60, the width W1 of the first coupling portion 31 and the width W2 of the second coupling portion 41 are set so that the following expression (7) is established for a specific wavelength.
n _0TE −n _0TM = n _1TE −n _1TM (7)
Note that n _1TE indicates the equivalent refractive index of the TE-polarized light in the first-order mode that propagates through the second coupling unit 41.

  The Bragg grating 70 converts the polarization and reflects it. The Bragg reflection condition for converting and reflecting the fundamental mode TE polarized wave propagating through the first coupling unit 31 and the primary mode TM polarized wave propagating through the second coupling unit 41 is expressed by the following equation (8). Is done.

(N _0TE + n _1TM ) Λ = λ (8)
Similarly, the Bragg reflection condition for converting and reflecting the fundamental mode TM polarization propagating through the first coupling unit 31 and the primary mode TE polarization propagating through the second coupling unit 41 is expressed by the following equation (9): ).

(N _0TM + n _1TE ) Λ = λ (9)
When the above equation (7) holds, the grating period Λ in the above equations (8) and (9) coincides with the common wavelength λ. Therefore, by setting the period of the Bragg grating 70 to Λ calculated from the above equations (8) and (9), in the first bidirectional coupling region 60, both the TE polarized wave and the TM polarized wave having the common wavelength are obtained. , And can be coupled between the first coupling portion 31 and the second coupling portion 41.

  Referring to FIG. 5, when the above equation (7) is established, it is confirmed that the Bragg reflection conditions of the above equations (8) and (9) are established in common.

  The upper part of FIG. 5 is a diagram showing the relationship of the equivalent refractive index to the width of the optical waveguide core, which is the same as FIG. Further, the lower part of FIG. 5 is a diagram obtained by inverting the top and bottom of FIG. As in FIG. 2, I is the equivalent refractive index of the fundamental mode TE polarization, II is the equivalent refractive index of the TM polarization of the fundamental mode, and IV is the equivalent refractive index of the TM polarization of the primary mode. V represents the equivalent refractive index of the TE polarization of the first-order mode.

The above formulas (8) and (9) are converted into the following formulas (10) and (11).
λ / Λ = n _0TE + n _1TM (10)
λ / Λ = n _0TM + n _1TE (11)
Can be transformed into

In FIG. 5, an arrow 501 indicates an equivalent refractive index I of TE polarization in the fundamental mode when the width of the optical waveguide core is 500 nm, and a TM in the first mode when the width of the optical waveguide core is 900 nm. The sum of the polarization and the equivalent refractive index IV, that is, n _0TE + n _1TM is shown. An arrow 503 indicates an equivalent refractive index II of TM polarization in the fundamental mode when the width of the optical waveguide core is 500 nm, and TE polarization in the first mode when the width of the optical waveguide core is 900 nm. And the equivalent refractive index V, that is, n _0TM + n _1TE . From FIG. 5, it can be confirmed that n _0TE + n _1TM and n _0TM + n _1TE are equal. Therefore, by setting the width W1 of the first coupling part 31 to 500 nm and the width W2 of the second coupling part 41 to 900 nm, the above expression (7) is established, and the Braggs of the above expressions (8) and (9) are established. The reflection condition is established in common. From this result, it can be confirmed that both the TE polarized wave and the TM polarized wave having the common wavelength λ can be reflected with the common period Λ even when the Bragg grating 70 is configured to reflect the polarized wave by converting the polarized wave. .

  In the configuration example described above, an example is described in which, in the first bidirectional coupling region 60, the TE mode polarization and TM polarization in the basic mode are converted to the TE polarization in the primary mode and the TM polarization in the primary mode. did. However, the optical waveguide device 100 according to the present invention is not limited to this configuration.

  For example, the TE polarization of the i-th mode (i is an integer of 0 or more) is converted to the TE polarization of the p-order mode (p is an integer of 0 or more), and It is also possible to adopt a configuration in which TM polarization is converted into TM polarization of q-order mode (q is an integer of 0 or more). In that case, the propagation mode of at least one polarization is converted. And about the specific wavelength, the width W1 of the 1st coupling | bond part 31 and the width W2 of the 2nd coupling | bond part 41 are set so that the following Formula (12) may be materialized.

n _iTE −n _jTM = n _qTM −n _pTE (12)
Note that n _iTE indicates the equivalent refractive index of the TE polarized wave in the i-th mode (i is an integer of 0 or more) propagating through the first coupling unit 31. N _jTM indicates the equivalent refractive index of the TM polarized wave of the j-th mode (j is an integer of 0 or more) propagating through the first coupling unit 31. N _pTE represents the equivalent refractive index of the TE polarized wave in the p-order mode (p is an integer of 0 or more) propagating through the second coupling unit 41. N _qTM represents the equivalent refractive index of the TM polarization of the q-order mode (q is an integer of 0 or more) propagating through the second coupling unit 41.

  Further, when the Bragg grating 70 that converts and reflects the polarization is used, for example, the TE polarization of the i-th mode (i is an integer of 0 or more) is changed to the q-order mode (q is an integer of 0 or more). It is also possible to adopt a configuration in which TM polarization is converted into TM polarization, and TM polarization in j-order mode (j is an integer of 0 or more) is converted into TE polarization in p-order mode. In that case, the propagation mode of at least one polarization is converted. And about the specific wavelength, the width W1 of the 1st coupling | bond part 31 and the width W2 of the 2nd coupling | bond part 41 are set so that the following Formula (13) may be materialized.

n _iTE −n _jTM = n _pTE −n _qTM (13)
Note that, in the first bidirectional coupling region 60, in the configuration example in which the TE polarization and j-order mode TM polarization in the i-th mode are converted to the TE polarization in the p-order mode and the TM polarization in the q-order mode, The first input / output unit 35 is preferably configured as a multimode waveguide according to the combination of modes.

  Further, the second bidirectional coupling region 80 is not limited to a configuration that converts the TE-polarized wave in the second-order mode and the TM-polarized light in the first-order mode into the TE-polarized light and the TM-polarized light in the basic mode. By appropriately setting the width W3 of the second input / output unit 45, the width W4 of the TM polarization coupling unit 53, and the width W5 of the TE polarization coupling unit 55, the TE polarization of the p-order mode is changed to the s-order mode ( s is an integer of 0 or more) and TM polarization of q-order mode can be converted to TM polarization of t-order mode (t is an integer of 0 or more). In the second bidirectional coupling region 80, in the configuration example in which the TE polarization in the p-order mode and the TM polarization in the q-order mode are converted into the TE polarization in the s-order mode and the TM polarization in the t-order mode, The third input / output unit 57 is preferably configured as a multimode waveguide according to the combination of propagation modes.

  Further, the optical waveguide element 100 according to the present invention may be configured such that the third optical waveguide core 50 is omitted. In this case, the second input / output unit 45 of the second optical waveguide core 40 is connected to the light receiving element on the other end 45b side. Then, the downstream optical signal output from the other end 45b of the second input / output unit 45 is received by the light receiving element.

(Characteristic evaluation)
The inventor conducted a simulation using a three-dimensional FDTD (Finite Difference Time Domain) method in order to evaluate the characteristics of the optical waveguide device 100.

  In this simulation, assuming that the fundamental mode TE polarized wave and TM polarized wave are input from the first input / output unit 35 of the first optical waveguide core 30, one end of the second coupling unit 41 of the second optical waveguide core 40 is assumed. The intensities of the TE polarization and TM polarization output from 41 a and the TE polarization and TM polarization intensities output from one end 31 a of the first coupling portion 31 of the first optical waveguide core 30 were calculated.

  In this simulation, the thickness of the first optical waveguide core 30 and the second optical waveguide core 40 is 220 nm. The Bragg wavelength in the Bragg grating 70 was set to 1577 nm. As a design for satisfying the above expression (2) at a wavelength of 1577 nm, the width W1 of the first coupling portion 31 is 500 nm, and the width W2 of the second coupling portion 41 is 900 nm. From the above equations (3) and (4), the period Λ of the Bragg grating 70 is 355 nm, and the engraving depths d1 and d2 of the Bragg grating 70 are both 50 nm. Further, the center distance D1 between the first coupling portion 31 and the second coupling portion 41 is set to 1000 nm. The length L1 of the first bidirectional coupling region 60 is 100 μm.

  The result of the simulation is shown in FIG. FIG. 6 is a diagram used for evaluating the polarization dependence of the optical waveguide device 100. The vertical axis indicates the light intensity in dB scale, and the horizontal axis indicates the wavelength in μm. A curve 601 shown in FIG. 6 indicates the intensity of the TE polarized wave output from the one end 41 a of the second coupling portion 41. A curve 603 indicates the intensity of the TM polarization output from the one end 41 a of the second coupling unit 41. A curve 605 indicates the intensity of the TE polarization output from the one end 31 a of the first coupling unit 31. A curve 607 indicates the intensity of the TM polarization output from the one end 31a of the first coupling unit 31.

  As shown in FIG. 6, a high peak can be confirmed in the vicinity of the wavelength 1577 nm set as the Bragg wavelength in the Bragg grating 70 for both the TE polarized wave and the TM polarized wave output from the one end 41 a of the second coupling portion 41. . From this result, it was confirmed that in the optical waveguide element 100, the Bragg grating 70 reflects the TE polarized wave and the TM polarized wave at a common wavelength. Therefore, it was confirmed that the optical waveguide element 100 can be used as a path switching element in which TE polarized wave and TM polarized wave can propagate through the same path, that is, polarization independent.

10: support substrate 20: clad layer 30: first optical waveguide core 31: first coupling portion 33: first taper portion 35: first input / output portion 40: second optical waveguide core 41: second coupling portion 43: first 2 taper portion 45: second input / output portion 50: third optical waveguide core 51: third coupling portion 53: TM polarization coupling portion 55: TE polarization coupling portion 57: third input / output portion 60: first bidirectional Coupling region 70, 170, 270: Bragg grating 71, 171: First Bragg grating 73, 173: Second Bragg grating 80: Second bidirectional coupling region 81: TM polarization coupling region 83: TE polarization coupling region 90: Sub-waveguide core 100: optical waveguide element

Claims (2)

A first optical waveguide core and
A second optical waveguide core and
With
A first bidirectional coupling region in which the first optical waveguide core and the second optical waveguide core are spaced apart and arranged in parallel is set;
A Bragg grating is formed in the first bidirectional coupling region,
In the first bidirectional coupling region, TE polarized waves and TM polarized waves having specific wavelengths propagating through the first optical waveguide core are reflected by the Bragg grating and transferred to the second optical waveguide core, The propagation mode of at least one of the TE polarized wave and the TM polarized wave is converted,
The Bragg grating, the first optical waveguide core and the second waveguide optical waveguide device you characterized in that it is formed in the sub-light waveguide core provided in a region between the cores. A third optical waveguide core;
A second bidirectional coupling region in which the second optical waveguide core and the third optical waveguide core are spaced apart from each other and arranged in parallel is set as a region separated from the first bidirectional coupling region;
In the second bidirectional coupling region, the TE polarized wave and the TM polarized wave having specific wavelengths propagating through the second optical waveguide core are transferred to the third optical waveguide core, and the TE polarized wave and the TM polarized wave are transferred. The optical waveguide device according to claim 1 , wherein at least one of the propagation modes is converted.

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