JP6194789B2 - Optical waveguide device - Google Patents

Description

  The present invention relates to an optical waveguide element that switches a path between a TE (Transverse Electric) polarization and a TM (Transverse Magnetic) polarization.

  As the amount of information transmitted increases, optical wiring technology has attracted attention. In the optical wiring technology, an optical device using an optical fiber or an optical waveguide as a transmission medium is used to transmit information between devices in information processing equipment, between boards, between chips, and the like using optical signals. As a result, it is possible to improve the band limitation of electrical wiring, which is a bottleneck in information processing equipment that requires high-speed signal processing.

  The optical device includes an optical element such as an optical transmitter or an optical receiver. These optical elements can be spatially coupled to each other 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, there is a technique that uses an optical waveguide element instead of a lens (see, for example, 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 optical device 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 (for example, see Non-Patent Document 1).

  By the way, when an optical device is used in a communication system using a wavelength multiplexing technique such as a passive optical network (PON), an element for switching the path of an optical signal for each wavelength is required. In order to realize this, there is a structure using an optical waveguide element provided with a function as a wavelength filter (see, for example, Patent Document 1).

  As a structure of the optical waveguide device, there are a rib-type waveguide and a Si fine wire waveguide. In the Si thin wire 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 several 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.

  Here, the characteristics of the Si wire waveguide differ depending on the polarization. Therefore, the wavelength filter using the Si fine wire waveguide has a defect that it has polarization dependency. Therefore, in order to eliminate the polarization dependence, there is a structure in which a polarization separation element and a polarization rotation element are provided in the previous stage of the region functioning as a wavelength filter (see, for example, Patent Document 2).

  In this structure, the input optical signal is first separated from the orthogonally polarized TE polarization and TM polarization by the polarization separation element. Next, one polarization is rotated by 90 ° by the polarization rotation element. As a result, the polarization state of the optical signal input to the wavelength filter is unified to either TE polarization or TM polarization. Therefore, the wavelength filter may be designed only for either the TE polarized wave or the TM polarized wave, and the polarization dependency is eliminated.

  The polarization separation element can be configured using, for example, a directional coupler having two Si wire waveguides arranged side by side (see, for example, Patent Document 2). In a polarization beam splitting element using a directional coupler, the core of a Si wire waveguide is formed with a flat cross-sectional shape. This causes a difference in the coupling action length of the directional coupler for the TE polarized wave and the TM polarized wave. As a result, the TE polarized wave and the TM polarized wave can be output through different paths.

JP 2011-77133 A JP 2009-244326 A

IEEE Journal of selected topics in quantum electronics, vol.12, No.6, November / December 2006 p.1371-1379

  However, a polarization separation element using a directional coupler has a drawback that it is easily affected by manufacturing errors (that is, manufacturing tolerance is small). For example, when an error occurs in the width of two Si fine waveguides constituting the directional coupler, the coupling efficiency between the respective Si fine waveguides deteriorates. As a result, the TE polarized wave and the TM polarized wave cannot be separated from each other, and the other polarized wave may be mixed in the path through which one polarized wave is to be output.

  An object of the present invention is to provide an optical waveguide element that can be used as a polarization separation element that switches a path between a TE polarization and a TM polarization and that has a high manufacturing tolerance.

  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 according to the present invention includes a first optical waveguide core and a second optical waveguide core. The first optical waveguide core is composed of an i-order mode (i is an integer of 0 or more), one of TE polarization and TM polarization, and an l-order mode (l is an integer of 0 or more different from i). ) Of TE polarization and TM polarization, and a Bragg reflector connected to the multimode waveguide. The second optical waveguide core has a coupling portion.

  The Bragg reflection portion is formed with a grating that converts the l-order mode and the i-order mode of one polarization to reflect Bragg and transmits the other polarization.

  In addition, a bidirectional coupling region is set in which the multimode waveguide portion and the coupling portion are arranged apart from each other and aligned. In the bidirectional coupling region, one polarization of the i-th mode propagating through the multimode waveguide section and one polarization of the m-th mode propagating through the coupling section (m is an integer of 0 or more different from i) Are combined.

  In the optical waveguide device according to the present invention, the TE polarization and the TM polarization can be separated by the grating of the Bragg reflection portion. Furthermore, one polarized wave reflected by the grating can be shifted between the first optical waveguide core and the second optical waveguide core in the bidirectional coupling region. Therefore, this one polarization can be output from an output port different from the input port. Therefore, the optical waveguide element according to the present invention can be used as a polarization separation element that switches the path between the TE polarization and the TM polarization.

  Further, in the optical waveguide device according to the present invention, each component including a grating for separating polarization can be created by a simple waveguide creation process. Therefore, manufacturing tolerance can be improved as compared with the polarization separation element using the directional coupler.

(A) And (B) is a schematic diagram for demonstrating the optical waveguide element by embodiment. It is a schematic diagram for demonstrating a grating. It is a schematic diagram for demonstrating a grating. It is a figure for demonstrating expansion of a Bragg reflection zone. It is a schematic diagram for demonstrating a grating. It is a schematic diagram for demonstrating a bidirectional | two-way coupling area | region. It is a figure for determining the design example of a grating. (A) And (B) is a figure for determining the design example of a grating. It is a figure for evaluating the characteristic of a grating. It is a figure for determining the design example of a bidirectional | two-way coupling area | region.

  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 this embodiment will be described. FIG. 1A is a schematic plan view showing an optical waveguide element. In FIG. 1, a clad layer described later is 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 port 35, a multimode waveguide part 31, a first taper part 39 a, a Bragg reflection part 33, a second taper part 39 b, and a second port 37. The first optical waveguide core 30 has a second optical waveguide core 50 having a coupling portion 51 and a third port 57. The Bragg reflector 33 is formed with a grating 40 that reflects light of a specific wavelength. In addition, a bidirectional coupling region 60 is set in which the multimode waveguide section 31 and the coupling section 51 are spaced apart from each other and arranged side by side.

  In the following description, the direction along the thickness of the support substrate 10 is the thickness direction. Moreover, about the 1st optical waveguide core 30 and the 2nd optical waveguide core 50, let the direction along the propagation direction of the light which propagates these be a length direction. Moreover, let the direction orthogonal to a length direction and a thickness direction be a width direction.

  The support substrate 10 is configured as a flat body made of, for example, single crystal Si.

The clad layer 20 is formed on the support substrate 10 so as to cover the upper surface 10 a of the support substrate 10 and include the first optical waveguide core 30 and the second optical waveguide core 50. The cladding layer 20 is formed using, for example, SiO ₂ as a material.

  The first optical waveguide core 30 is made of, for example, Si having a refractive index higher than that of the cladding layer 20. As a result, the first optical waveguide core 30 functions as a light transmission path, and the light incident on the first optical waveguide core 30 propagates in the propagation direction according to the planar shape of the first optical waveguide core 30. Further, the second optical waveguide core 50 is formed of, for example, Si having a refractive index higher than that of the cladding layer 20, similarly to the first optical waveguide core 30. As a result, the second optical waveguide core 50 functions as a light transmission path, and light incident on the second optical waveguide core 50 propagates in a propagation direction corresponding to the planar shape of the second optical waveguide core 50.

  The optical waveguide device 100 is used as, for example, a polarization separation device that switches a path between a TE polarization and a TM polarization of an input optical signal. Here, as an example, a fundamental mode (0th-order mode) optical signal is input from the first port 35, the fundamental mode TM polarization is output from the second port 37, and the fundamental mode TE polarization is third. A configuration example output from the port 57 will be described.

  In this example, the fundamental mode optical signal is input to the first port 35 of the first optical waveguide core 30 and sent to the black reflecting portion 33 via the multimode waveguide portion 31. The fundamental mode TE-polarized light included in the optical signal is converted to the primary mode by Bragg reflection in the grating 40 formed in the Bragg reflector 33, and sent to the multimode waveguide section 31 again. The TM polarized wave in the basic mode is output from the second port 37 through the grating 40 without undergoing mode conversion. The primary mode TE polarized wave reflected by the grating 40 and propagating through the multimode waveguide section 31 is converted into a fundamental mode in the bidirectional coupling region 60 and transmitted to the coupling section 51 of the second optical waveguide core 50. It is done. The TE polarized wave in the basic mode is output from the third port 57 via the coupling unit 51.

  The first port 35 is set to a thickness and a width that achieve a single mode condition. Therefore, the light of the fundamental mode is propagated. One end 35 a of the first port 35 is connected to the other end 31 b of the multimode waveguide section 31.

  The multimode waveguide unit 31 propagates the TE mode and TM polarization in the fundamental mode and the TE polarization in the primary mode. The geometric design of the multimode waveguide section 31 will be described later. The multimode waveguide section 31 is connected to the Bragg reflection section 33 via the first taper section 39a. The width of the first tapered portion 39a is set so as to continuously change from the width of the one end 31a of the multimode waveguide portion 31 to the width of the one end 33a of the Bragg reflecting portion 33 along the light propagation direction. Yes. By providing the first taper portion 39a, reflection of light propagating between the multimode waveguide portion 31 and the Bragg reflection portion 33 can be reduced.

  A grating 40 is formed on the Bragg reflector 33. The grating 40 will be described with reference to FIG. FIG. 2 is a schematic plan view for explaining the grating formed in the Bragg reflector. In FIG. 2, the support substrate and the clad layer are omitted.

  In this embodiment, the grating 40 Bragg-reflects TE polarized light having a specific wavelength by converting the fundamental mode and the primary mode. In addition, TM polarization in the basic mode is transmitted.

The Bragg reflection condition in the grating is expressed by the following formula (1). N _a and N _b indicate the equivalent refractive indexes of incident light and reflected light that are combined in the grating. A and _b in N _a and N _b are integers of 0 or more, and indicate the orders of incident light and reflected light, respectively. Λ indicates the period of the grating. In the grating, light having a wavelength λ satisfying the following expression (1), that is, light having a Bragg wavelength is Bragg reflected.

(N _a + N _b ) Λ = λ (1)

Since the equivalent refractive indexes N _a and N _b have wavelength dependence, the above equation (1) is established only for a specific wavelength λ. Further, since the equivalent refractive indexes N _a and N _b have polarization dependency, the equivalent refractive indexes N _a and N _b have different values for the TE polarized wave and the TM polarized wave.

Based on the above equation (1), the condition for Bragg reflection of the TE polarized wave of wavelength λ _TE by converting the fundamental mode and the primary mode is expressed by the following equation (2). _{Incidentally, N TE0} is the equivalent refractive index of the fundamental mode of the TE _{polarization, N TE1} is the equivalent refractive index of the first-order mode of the TE polarization, respectively.

(N _TE0 + N _TE1 ) Λ = λ _TE (2)

At a wavelength λ _TE , the fundamental mode TM polarization passes through the grating under the condition that the fundamental mode TM polarization is not coupled with the fundamental mode and other order mode TM polarizations. Therefore, the condition under which the fundamental mode TM polarized light passes through the grating is expressed by the following equation (3). N _TM0 represents the equivalent refractive index of the fundamental mode of TM polarization, and N _TMj represents the equivalent refractive index of the j-order mode (j is an integer of 0 or more) of TM polarization.

(N _TM0 + N _TMj ) Λ ≠ λ _TE (3)

  The grating 40 is configured to integrally include a base 41 and protrusions 43a and 43b. The base 41 has a constant width W1 and extends along the light propagation direction. A plurality of protrusions 43 a and 43 b are periodically formed on both side surfaces of the base 41. The width W1 of the base 41, the protrusion width D of the protrusions 3a and 43b, and the period Λ of the protrusions 43a and 43b are designed so that both the above equations (2) and (3) hold. Further, the protruding portion 43a formed on one side surface of the base 41 and the protruding portion 43b formed on the other side surface are arranged so as to be shifted by a half cycle (ie, Λ / 2).

As a result, the fundamental mode TE polarized wave of wavelength λ _TE sent from the multimode waveguide section 31 is converted into the primary mode and reflected by the grating 40. The Bragg-reflected primary mode TE polarization is sent to the multimode waveguide section 31 again. On the other hand, the TM-polarized light in the fundamental mode is transmitted through the grating 40 without being subjected to mode conversion and Bragg reflection. The TM polarization in the fundamental mode that has passed through the grating 40 is sent to the second port 37 via the second tapered portion 39b.

  Here, as a modification of the grating 40, the period Λ may be set constant, and the protrusion width D of the protrusions 43a and 43b may be changed for each period. A modification of the grating 40 will be described with reference to FIG. FIG. 3 is a schematic plan view for explaining a modified example of the grating. In FIG. 3, the support substrate and the clad layer are omitted.

In the configuration example shown in FIG. 3, with respect to protrusion width D = D ₀ of the first cycle of protrusions 43a and 43 b, protrusion width D in each cycle is increased by ΔD at a constant variation. Accordingly, in the k-th cycle, the protrusion width D of the protrusions 43a and 43b is D ₀ + ΔD (k−1).

By protrusion width D of the protruding portions 43a and 43b are changed, the Bragg wavelength lambda _TE is changed to satisfy the above equation (2) and (3), the equivalent refractive index is changed accordingly. Therefore, by changing the protrusion width D, the wavelength band (Bragg reflection band) in which Bragg reflection is performed in the grating 40 can be expanded.

The expansion of the Bragg reflection band will be described with reference to FIG. In FIG. 4, the horizontal axis represents wavelength and the vertical axis represents reflection intensity. Note that the Bragg wavelength in the _k−1 period of the grating 40 is λ _k−1 , the Bragg wavelength in the k period is λ _k , and the Bragg wavelength in the k + 1 period is λ _{k + 1} .

  As shown in FIG. 4, when the protrusion width D increases by ΔD for each period, the center wavelength of the Bragg wavelength in the adjacent period is shifted by Δλ toward the long wavelength side.

The shift amount Δλ of the Bragg wavelength for each period can be approximately expressed by the following expression (4) using the change amount ΔD of the protrusion width D. N ₀ represents the equivalent refractive index of the fundamental mode, and N ₁ represents the equivalent refractive index of the primary mode. In this embodiment, since the grating 40 is a configuration example in which the TE polarized wave is Bragg-reflected, N ₀ is the equivalent refractive index of the TE polarized fundamental mode, and N ₁ is the equivalent refractive index of the TE polarized primary mode. Each corresponds to a rate.

  Therefore, the Bragg reflection band can be expanded by adjusting ΔD and setting Δλ so that the Bragg wavelengths in each period overlap. For example, when the grating 40 having a k period is formed, the Bragg reflection band can be expanded by about Δλ × k compared to the case where the protrusion width D is constant. As a result, the wavelength dependence of Bragg reflection can be relaxed.

Further, as another modified example of expanding the Bragg reflection band in the grating 40, the protrusion widths 43 and the duty ratio of the protrusions 43a and 43b may be constant, and the period Λ may be changed for each period. This modification is shown in FIG. In the configuration example shown in FIG. 5, with respect to the period Λ = Λ ₀ in the first period, the period Λ increases by ΔΛ with a constant change amount for each period. Therefore, in the k-th period, the period Λ is Λ ₀ + ΔΛ (k−1).

By changing the period Λ, the Bragg wavelength λ _TE that satisfies the above equations (2) and (3) changes, and the equivalent refractive index changes accordingly. Accordingly, the Bragg reflection band can be expanded in the grating 40 by changing the period Λ. The shift amount Δλ of the Bragg wavelength for each period can be approximately expressed by the following equation (5) using the change amount ΔΛ of the period Λ.

  Therefore, by adjusting ΔΛ and setting Δλ so that the Bragg wavelengths in each period overlap, the Bragg reflection band can be expanded. Similar to the case where the protrusion width D is changed, for example, when the grating 40 having the k period is formed, the Bragg reflection band can be expanded by about Δλ × k compared to the case where the period Λ is constant. As a result, the wavelength dependence of Bragg reflection can be relaxed.

  The second port 37 is set to a thickness and a width that achieve a single mode condition. Therefore, the light of the fundamental mode is propagated. The second port 37 is connected to the Bragg reflector 33 via the second tapered portion 39b. The second taper portion 39 b is set so as to continuously change from the width of the other end 33 b of the Bragg reflector 33 to the width of the one end 37 a of the second port 37. By providing the second tapered portion 39b, reflection of light propagating between the Bragg reflecting portion 33 and the second port 37 can be relaxed.

  The coupling portion 51 is arranged apart from and side by side with the multimode waveguide portion 31 of the first optical waveguide core 30. The coupling portion 51 is set to a thickness and a width that achieve a single mode condition. Accordingly, the coupling unit 51 propagates the light in the fundamental mode. Further, the other end 51 b of the coupling portion 51 is connected to the third port 57.

  The third port 57 is set to a thickness and a width that achieve a single mode condition. Accordingly, the third port 57 propagates light in the fundamental mode.

  Further, in the optical waveguide device 100, the bidirectional coupling region 60 in which the multimode waveguide portion 31 of the first optical waveguide core 30 and the coupling portion 51 of the second optical waveguide core 50 are arranged apart from each other and arranged side by side. Is set. The bidirectional coupling region 60 will be described with reference to FIG. FIG. 6 is a schematic plan view for explaining the bidirectional coupling region 60. In FIG. 6, the support substrate and the clad layer are omitted.

  In the bidirectional coupling region 60, the primary mode TE polarized wave propagating through the multimode waveguide unit 31 and the fundamental mode TE polarized wave propagating through the coupling unit 51 are coupled.

  The central axes of the multimode waveguide section 31 and the coupling section 51 are parallel to each other. Further, the multimode waveguide section 31 and the coupling section 51 are arranged such that one end 31 a of the multimode waveguide section 31 and one end 51 a of the coupling section 51 are aligned. In addition, the other end 31 b of the multimode waveguide portion 31 and the other end 51 b of the coupling portion 51 are aligned.

  The multimode waveguide section 31 has a tapered shape whose width continuously decreases from one end 31a to the other end 31b. The width W2 of the one end 31a of the multimode waveguide section 31 is set so as to correspond to propagation constants capable of propagating the fundamental mode and the primary mode TE polarized waves and the fundamental mode TM polarized waves. Further, the width W3 of the other end 31b of the multimode waveguide section 31 is set corresponding to a propagation constant capable of propagating the fundamental mode TE polarized wave and TM polarized wave.

  Further, the coupling portion 51 has a tapered shape in which the width continuously increases from the one end 51a to the other end 51b. The width W4 of the one end 51a and the width W5 of the other end 51b of the coupling portion 51 are set corresponding to propagation constants capable of propagating the fundamental mode TE polarized wave.

  The propagation constant of the TE polarization of the primary mode at one end 31 a of the multimode waveguide section 31 is set to be larger than the propagation constant of the TE polarization of the fundamental mode at one end 51 a of the coupling section 51. The propagation constant of the fundamental mode TE polarization at the other end 31 b of the multimode waveguide section 31 is set to be larger than the propagation constant of the fundamental mode TE polarization at the other end 51 b of the coupling section 51.

  In the bidirectional coupling region 60 designed in this way, the propagation constant of the TE polarization of the primary mode of the multimode waveguide section 31 matches the propagation constant of the TE polarization of the fundamental mode of the coupling section 51. Exists. As a result, the primary mode TE polarized wave propagating through the multimode waveguide section 31 and the fundamental mode TE polarized wave propagating through the coupling section 51 can be coupled. Therefore, the TE polarization of the first-order mode transmitted from the Bragg reflector 33 and input to the multimode waveguide section 31 from the one end 31 a side is converted into the fundamental mode and transferred to the coupling section 51. As described above, the propagation constant of the TE polarized wave in the fundamental mode at the other end 31b of the multimode waveguide section 31 is set to be larger than the propagation constant of the TE polarized wave in the fundamental mode at the other end 51b of the coupling section 51. Has been. Therefore, there is no possibility that the fundamental mode TE polarized wave input from the other end 31 b side to the multimode waveguide section 31 is transferred to the coupling section 51.

  As described above, in the optical waveguide device 100 according to this embodiment, the TE polarized wave and the TM polarized wave can be separated by the grating 40 of the Bragg reflector 33. Furthermore, the TE polarized wave reflected by the grating 40 can be transferred from the first optical waveguide core 30 to the second optical waveguide core 50 in the bidirectional coupling region 60. Therefore, TE polarized wave can be output from an output port different from the input port. Therefore, the optical waveguide device 100 can be used as a polarization separation device that switches the path between the TE polarization and the TM polarization.

  In addition, each component of the optical waveguide device 100 including the grating 40 that substantially separates polarization can be created by a simple waveguide creation process. Therefore, manufacturing tolerance can be improved as compared with the polarization separation element using the directional coupler.

  Here, the configuration in which the grating 40 performs Bragg reflection by converting the fundamental mode and the first-order mode of TE polarization has been described. However, the optical waveguide device 100 according to the present invention is not limited to this configuration.

By designing so that the following equations (6) and (7) are both established, the TE-polarized l-order mode (l is an integer of 0 or more) and i-order mode (i is different from l) of wavelength λ _TE It is possible to form a grating 40 that converts Bragg reflection and transmits TM polarized light. N _TEl is the equivalent refractive index of the l-order mode of the TE polarized wave, N _TEi is the equivalent refractive index of the i-order mode of the TE polarized wave, N _TMl is the equivalent refractive index of the l-order mode of the TM polarized wave, N _TMj indicates the equivalent refractive index of the j-order mode of TM polarization (j is an integer of 0 or more).

(N _TEl + N _TEi ) Λ = λ _TE (6)

(N _TMl + N _TMj ) Λ ≠ λ _TE (7)

Alternatively, by designing so that both the following equations (8) and (9) are satisfied, the TM-polarized l-order mode and the i-order mode of wavelength λ _TM are converted to Bragg reflection, and the TE polarized wave A grating 40 that transmits light can also be formed.

(N _TMl + N _TMi ) Λ = λ _TM (8)

(N _TEl + N _TEj ) Λ ≠ λ _TM (9)

  Also, the light coupled in the bidirectional coupling region 60 is not limited to the fundamental mode TE polarization and the primary mode TE polarization. By appropriately setting the width W2 of the one end 31a and the width W3 of the other end 31b of the multimode waveguide section 31, and the width W4 of the one end 51a and the width W5 of the other end 51b of the coupling section 51, the multimode waveguide section One polarization of the i-th mode propagating through 31 can be combined with one polarization of the m-order mode (m is an integer of 0 or more different from i) propagating through the coupling unit 51.

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

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

Next, a material layer made of SiO ₂ is formed on the SiO ₂ layer so as to cover the first optical waveguide core 30 and the second optical waveguide core 50 by using, for example, a CVD method. As a result, the cladding layer 20 including the first optical waveguide core 30 and the second optical waveguide core 50 is formed from the SiO ₂ layer and the material layer.

(Usage form)
The inventor performed several simulations in order to determine a suitable design example as a usage form of the optical waveguide device 100. In the following simulations, it is assumed that the wavelength of the optical signal input to the optical waveguide device 100 is 1.5 μm. The grating 40 assumes a configuration example in which the fundamental mode and the primary mode of TE polarization are converted and Bragg reflected. In addition, the bidirectional coupling region 60 is assumed to be configured to couple the fundamental mode TE polarization and the primary mode TE polarization. The first optical waveguide core 30 and the second optical waveguide core 50 are made of Si having a thickness of 0.22 μm, and the cladding layer 20 is made of SiO ₂ .

  First, using a FEM (Finite Element Method), a preferred example of the design of the grating 40 in which both the above equations (2) and (3) are established was examined.

  In this simulation, the relationship between the width W1 of the base 41 of the grating 40 and the equivalent refractive indices of the fundamental mode and the first-order mode TE polarization, and the fundamental mode and the first-order mode TM polarization was confirmed. In this simulation, the protrusion width D of the protrusions 43a and 43b of the grating 40 is constant at 0.1 μm.

  The simulation results are shown in FIG. FIG. 7 is a diagram showing the relationship between the width W1 of the base 41 of the grating 40 and each equivalent refractive index. In FIG. 7, the vertical axis represents the equivalent refractive index on an arbitrary scale, and the horizontal axis represents the width W1 of the base 41 in units of μm. In FIG. 7, ◆ indicates the result of TE polarization in the basic mode, ▲ indicates the result of TE polarization in the primary mode, ■ indicates the result of TM polarization in the basic mode, and × indicates the result of the primary mode. The results of TM polarization are shown respectively.

  As shown in FIG. 7, in the configuration in which the width W1 of the base 41 is set to be smaller than 0.52 μm, the equivalent refractive index of the TM polarization in the first-order mode is cut off. Further, in the configuration in which the width W1 of the base portion 41 is set to be smaller than 0.3 μm, the equivalent refractive index of the TE polarization of the first mode is cut off. Accordingly, by setting the width W1 of the base 41 within the range of 0.3 <W1 <˜0.52, the grating 40 can propagate the TE polarized light in the first mode and the TM polarized light in the first mode. Can not be propagated.

Based on this result, the inventor determined the width W1 of the base 41 to 0.4 μm. As shown in FIG. 7, when the width W1 of the base 41 is 0.4 μm, the equivalent refractive index of the TE polarized wave in the basic mode is 2.487, and the equivalent refractive index of the TE polarized wave in the first mode is 1.602. At this time, assuming that the Bragg wavelength is λ _TE = 1.5 μm, the period of the grating 40 is determined to be Λ = 0.367 μm from the above equation (2). Therefore, by designing the width W1 of the base 41 to 0.4 μm, the protrusion width D of the protrusions 43a and 43b to 0.1 μm, and the period Λ to 0.367 μm, the grating 40 has a fundamental wavelength of 1.5 μm. The TE polarization of the mode can be converted to the primary mode and Bragg reflected.

As shown in FIG. 7, when the width W1 of the base 41 is 0.4 μm, the equivalent refractive index of the TM polarized light in the fundamental mode is 1.839. At this time, if the Bragg wavelength is λ _TE = 1.5 μm, the right side and the left side of the above equation (3) do not match. From this result, it was confirmed that the TM polarized wave of the fundamental mode having a wavelength of 1.5 μm is transmitted through the grating 40 in the above-described design.

  Next, the inventor examined a design for expanding the Bragg reflection band in the grating 40. Here, the amount of change ΔD of the protrusion width D is determined for a configuration in which the protrusion width 43 of the protrusions 43a and 43b changes for each period (see FIG. 3), with the period Λ being constant.

  By transforming the above equation (4), the following equation (10) is derived.

In this simulation, the chromatic dispersion and ΔD of the sum (N _TE0 + N _TE1 ) of the equivalent refractive index of the TE-polarized light in the fundamental mode and the equivalent refractive index of the TE-polarized light in the first-order mode are based on the above equation (10) The structural dispersion was confirmed.

The simulation results are shown in FIGS. 8 (A) and 8 (B). FIG. 8A is a diagram showing the relationship between the wavelength and the sum of the equivalent refractive indices (N _TE0 + N _TE1 ). In FIG. 8A, the vertical axis represents the sum of the equivalent refractive indexes (N _TE0 + N _TE1 ) on an arbitrary scale, and the horizontal axis represents the wavelength in μm units. FIG. 8B is a diagram showing the relationship between the change amount ΔD of the protrusion width D and the sum of the equivalent refractive indexes (N _TE0 + N _TE1 ). In FIG. 8B, the vertical axis represents the sum of the equivalent refractive indexes (N _TE0 + N _TE1 ) on an arbitrary scale, and the horizontal axis represents the variation ΔD of the protrusion width D in units of μm.

Assuming that the relationship between the wavelength and the sum of the equivalent refractive indexes (N _TE0 + N _TE1 ) in FIG. 8A is a linear approximation, a slope of −1.83 is obtained. This is substituted into {∂ (N ₀ + N ₁ )} / (∂λ) in the above equation (10) as the chromatic dispersion of the sum of equivalent refractive indices (N _TE0 + N _TE1 ). Further, assuming that the relationship between the change amount ΔD of the protrusion width D and the sum of the equivalent refractive indexes (N _TE0 + N _TE1 ) in FIG. 8B is a linear approximation, a slope of 2.63 is obtained. This is substituted into {∂ (N ₀ + N ₁ )} / (∂D ₀ ) in the above equation (10) as the structural dispersion of the sum of equivalent refractive indices (N _TE0 + N _TE1 ). Further, by substituting the period Λ = 0.367 μm determined in the simulation according to FIG. 7, Δλ / ΔD = 0.576 is obtained from the above equation (10).

  Here, for example, it is assumed that the grating 40 with 100 periods is formed and the Bragg wavelength is expanded by 0.1 μm (that is, Δλ × 100 = 0.1). In that case, ΔD can be determined to be 0.0018 μm using Δλ / ΔD = 0.576 obtained by simulation.

  Next, the inventor evaluated the characteristics of the grating 40 having a design determined from the simulations shown in FIGS. 7 and 8 by using a three-dimensional FDTD (Finite Difference Time Domain) method. In this simulation, light including the TE-polarized light and the TM-polarized light in the basic mode is input to the grating 40. Then, the intensity of each polarization included in the reflected light from the grating 40 and the intensity of each polarization included in the transmitted light from the grating 40 were observed. For the reflected light from the grating 40, the primary mode was observed. For the transmitted light from the grating 40, the fundamental mode was observed.

  The result of the simulation is shown in FIG. FIG. 9 is a diagram showing the results of characteristic evaluation. In FIG. 9, the vertical axis indicates the light intensity in dB scale, and the horizontal axis indicates the wavelength in μm.

  As shown in FIG. 9, the reflected light from the grating 40 includes a large amount of primary mode TE polarized light as compared to the primary mode TM polarized light. From FIG. 9, it was confirmed that the polarization extinction ratio between the TE polarization and TM polarization of the primary mode included in the reflected light can be secured at least about 35 dB. Also, as shown in FIG. 9, the transmitted light from the grating 40 contains a larger amount of fundamental mode TM polarization than the fundamental mode TE polarization. From FIG. 9, it was confirmed that the polarization extinction ratio of the TE mode and TM polarization in the fundamental mode included in the transmitted light can be secured at least about 50 dB. From the result of FIG. 9, it was confirmed that the TE polarization and the TM polarization can be separated with a sufficient polarization extinction ratio by using the grating 40.

  Next, the inventor examined a suitable design example of the bidirectional coupling region 60.

  As described above, the multimode waveguide section 31 included in the bidirectional coupling region 60 propagates the width W2 of the one end 31a through the TE mode and the primary mode TE polarized wave and the fundamental mode TM polarized wave. Set for possible propagation constants. Further, the width W3 of the other end 31b is set corresponding to the propagation constant capable of propagating the TE polarized wave and TM polarized wave in the basic mode. On the other hand, for the coupling portion 51 included in the bidirectional coupling region 60, the width W4 of the one end 51a and the width W5 of the other end 51b of the coupling portion 51 correspond to the propagation constants capable of propagating the TE polarized wave of the fundamental mode. To set. Then, the propagation constant of the primary mode TE polarized wave at the one end 31 a of the multimode waveguide section 31 is set to be larger than the propagation constant of the fundamental mode TE polarized wave at the one end 51 a of the coupling section 51. The propagation constant of the fundamental mode TE polarization at the other end 31 b of the multimode waveguide section 31 is set larger than the propagation constant of the fundamental mode TE polarization at the other end 51 b of the coupling section 51. The inventor has determined the multimode waveguide section 31 and the coupling section 51 that satisfy these conditions using FEM.

  FIG. 10 is a diagram showing the relationship between the width of the optical waveguide core and the equivalent refractive index calculated using FEM. In FIG. 10, the vertical axis indicates the equivalent refractive index on an arbitrary scale, and the horizontal axis indicates the width of the optical waveguide core in units of μm. In FIG. 10, ◆ indicates the result of TE polarization in the fundamental mode, ▲ indicates the result of TE polarization in the primary mode, and ■ indicates the result of TM polarization in the fundamental mode.

  First, the width W2 of the one end 31a of the multimode waveguide section 31 was determined to be W2 = 0.6 μm as a width capable of propagating the fundamental mode and the primary mode TE polarized wave and the fundamental mode TM polarized wave. When W2 = 0.6 μm, TM polarization can propagate only in the fundamental mode.

  Next, as the width W3 of the other end 31b of the multimode waveguide section 31, a condition is found in which TE polarized light and TM polarized light can propagate only in the fundamental mode. As shown in FIG. 10, under the condition that the width of the optical waveguide core is smaller than 0.4 μm, the equivalent refractive index of the TE polarization in the first mode is cut off. Accordingly, the width W3 of the other end 31b of the multimode waveguide section 31 is set to W3 <0.4 μm. Here, W3 = 0.35 μm was determined.

  Next, under the condition that the propagation constant of the TE polarization of the first mode at the one end 31a of the multimode waveguide section 31 is larger than the propagation constant of the TE polarization of the fundamental mode at the one end 51a of the coupling section 51. The width W4 of the one end 51a of 51 is determined. The width W2 of the one end 31a of the multimode waveguide section 31 is set to W2 = 0.6 μm. As shown in FIG. 10, the equivalent refractive index of TE polarized light in the first-order mode when the width of the optical waveguide core is 0.6 μm is that of the fundamental mode under the condition that the width of the optical waveguide core is smaller than 0.28. It becomes larger than the equivalent refractive index of TE polarized light. Accordingly, the width W4 of the one end 51a of the coupling portion 51 is set to W4 <0.28 μm. Here, W4 = 0.1 μm was determined.

  Next, under the condition that the propagation constant of the fundamental mode TE polarized wave at the other end 31 b of the multimode waveguide section 31 is larger than the propagation constant of the fundamental mode TE polarized wave at the other end 51 b of the coupling section 51. The width W5 of the other end 51b of the part 51 is determined. This condition is achieved by satisfying W3> W5> W4. Here, W5 = 0.25 μm was determined.

  The center-to-center distance W6 between the multimode waveguide section 31 and the coupling section 51 and the length (coupling length) L1 of the bidirectional coupling region 60 are TE deviations of the primary mode propagating through the multimode waveguide section 31. The dimensions are designed so that the coupling occurs between the wave and the TE mode polarized wave propagating through the coupling portion 51. Here, the center-to-center distance W6 = 0.78 μm and the coupling length L1 = 100 μm were determined (see FIG. 6).

  The inventor evaluated the characteristics of the bidirectional coupling region 60 having the above-described design using BPM (Beam Propagation Method). As a result, the coupling efficiency between the primary mode TE polarized wave propagating through the multimode waveguide unit 31 and the fundamental mode TE polarized wave propagating through the coupling unit 51 was about −0.15 dB. Further, it was confirmed that the TE mode and TM polarization of the fundamental mode propagating through the multimode waveguide unit 31 do not shift to the coupling unit 51. From this result, it was confirmed that the bidirectional coupling region 60 has a conversion and separation function for a specific mode of one polarization.

10: support substrate 20: clad layer 30: first optical waveguide core 31: multimode waveguide section 33: Bragg reflector 35: first port 37: second port 40: grating 50: second optical waveguide core 51: coupling Portion 57: Third port 60: Bidirectional coupling region 100: Optical waveguide device

Claims (4)

Polarization of one of TE polarization and TM polarization in the i-order mode (i is an integer of 0 or more), and TE polarization and TM of the l-order mode (l is an integer of 0 or more different from i) A first optical waveguide core having a multi-mode waveguide section for propagating polarized waves, and a Bragg reflection section connected to the multi-mode waveguide section;
A second optical waveguide core having a coupling portion;
The Bragg reflector is formed with a grating that converts the l-order mode and the i-order mode of the one polarized wave to reflect the Bragg and transmits the other polarized wave,
A bidirectional 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 bidirectional coupling region, the one polarization of the i-th mode propagating through the multimode waveguide section and the m-order mode (m is an integer of 0 or more different from i) propagating through the coupling section. An optical waveguide device, wherein one polarization is coupled. The grating is
Bragg wavelength lambda _TE, grating period lambda, TE polarization equivalent refractive index _{N TEL} of l-order mode, i order mode of the TE polarization equivalent refractive index _{N TEi,} l TM polarization equivalent refractive index of the order mode _{N TML} , And j-order mode (j is an integer greater than or equal to 0) TM polarization equivalent refractive index N _TMj satisfies both (N _TEl + N _TEi ) Λ = λ _TE and (N _TMl + N _TMj ) Λ ≠ λ _TE Design or
Bragg wavelength lambda _TM, grating period lambda, TM polarization equivalent refractive index _{N TML} of l-order mode, i order mode of the TM polarization equivalent refractive index _{N TMi,} l TE polarization equivalent refractive index of the order mode _{N TEL} , And the TE-polarized equivalent refractive index N _TEj of (N _TMl + N _TMi ) Λ = λ _TM and (N _TEl + N _TEj ) Λ ≠ λ _TM The optical waveguide device according to claim 1. The grating includes a base having a constant width and extending along a light propagation direction, and a plurality of protrusions periodically formed on both side surfaces of the base.
The protruding portion formed on one side surface of the base portion and the protruding portion formed on the other side surface of the base portion are arranged with a half-cycle shift,
The optical waveguide element according to claim 1, wherein the protrusion has a protrusion width that changes with a constant amount for each period. The grating includes a base having a constant width and extending along a light propagation direction, and a plurality of protrusions periodically formed on both side surfaces of the base.
The protruding portion formed on one side surface of the base portion and the protruding portion formed on the other side surface of the base portion are arranged with a half-cycle shift,
3. The optical waveguide device according to claim 1, wherein the period of the protruding portion changes with a constant change amount for each period.

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