JP6300437B2 - 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 the upstream optical signal are prevented from interfering 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 (see, for example, Non-Patent Documents 1 and 2). 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 addition, in the OLT and ONU, there is a structure using an optical waveguide element provided with a function as a wavelength filter in order to switch the path of the downstream optical signal and the path of the upstream optical signal (see, for example, Patent Document 1).

  The function as the wavelength filter is realized by, for example, a grating. If the reflection intensity of the grating is sufficiently increased, the transmittance within the transmission band can be made constant. 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. A configuration in which a grating is formed in an optical waveguide mainly made of Si is disclosed in Non-Patent Document 3, for example.

JP 2011-77133 A

IEEE Journal of selected topics quantum electronics, vol.11, p232-240, 2005 IEEE Journal of selected topics quantum electronics, vol.12, No.6, November / December 2006 p.1371-1379 IEICE Transactions of Electrons vol. E-90-C, No.1, p.59, January 2007

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

Here, the Bragg reflection condition in the grating is expressed by the following equation (1). N _ei and N _ej indicate equivalent refractive indexes of incident light and reflected light that are combined in the grating. I and j in N _ei and N _ej 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 _ei + N _ej ) Λ = λ (1)
Since the equivalent refractive indexes N _ei and N _ej are wavelength dependent, the above equation (1) is established only for a specific wavelength λ.

  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 i = j), 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 according to the present invention includes a first optical waveguide core and a second optical waveguide core. The first optical waveguide core includes a multimode waveguide section that propagates light of i-th mode (i is an integer of 0 or more) and light of j-th mode (j is an integer of 0 or more different from i), and multimode It has a Bragg reflector connected to the waveguide. The second optical waveguide core has a coupling portion.

  The Bragg reflector is formed with a grating that Bragg-reflects by converting the i-order mode and the j-order mode for a TE polarized wave and a TM polarized wave having the same wavelength.

  A bidirectional coupling region is set in which the multimode waveguide portion and the coupling portion are spaced apart from each other and arranged in parallel.

In the bidirectional coupling region, the TE-polarized wave of the j-th mode propagating through the multi-mode waveguide section and the TE-polarized wave of the k-th mode (k is an integer of 0 or more) propagating through the coupling section are coupled. The TM polarization of the j-th mode propagating through the mode waveguide section and the TM polarization of the k-order mode propagating through the coupling section are combined.
Grating, the equivalent refractive index _{N EiTE} of i-th order mode TE polarization equivalent refractive index _{N EjTE} the j-th order mode of the TE polarization, the equivalent refractive index _{N EITM} the i-order mode of the TM polarization, and the TM polarization The equivalent refractive index N _ejTM of the j-th mode is formed so that N _eiTE + N _ejTE = N _eiTM + N _ejTM is established at a specific same wavelength .
The coupling portion has a tapered shape whose width continuously changes along the light propagation direction.

  In the optical waveguide device according to the present invention, the incident light and the reflected light that is Bragg-reflected are coupled in different orders in the grating formed in the Bragg reflector. Therefore, by forming the grating so that the sum of the equivalent refractive index of the i-th mode and the equivalent refractive index of the j-order mode is equal between the TE polarization and the TM polarization at a specific same wavelength, The Bragg reflection condition at the same wavelength can be established for both TM polarized waves.

  Further, as described above, in the bidirectional coupling region, the j-order mode TE polarization propagating through the multimode waveguide section and the k-order mode TE polarization propagating through the coupling section are also converted into multimode guides. The TM polarization of the j-th mode propagating through the waveguide part and the TM polarization of the k-order mode propagating through the coupling part are respectively coupled.

  Therefore, in the optical waveguide device according to the present invention, the TE polarized wave and the TM polarized wave can be propagated through the same path according to the wavelength in the first optical waveguide core and the second optical waveguide core. Therefore, the optical waveguide element according to the present invention can be used as a polarization-independent path switching element.

It is a schematic plan view which shows an optical waveguide element. 1 is a schematic end view showing an optical waveguide element. It is a schematic plan view which shows the modification of a bidirectional | two-way coupling area | region. It is a figure which shows the relationship between the design of a grating, and the sum of the equivalent refractive index of a fundamental mode, and the equivalent refractive index of a primary mode. It is a figure which shows the relationship between the equivalent refractive index of the fundamental mode of TE polarization in a 1st coupling part, the equivalent refractive index of the fundamental mode of TM polarization in a 2nd coupling part, and each width | variety. It is a figure where it uses for evaluation of the polarization dependence of an optical waveguide element.

  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)
An optical waveguide device according to the present invention will be described with reference to FIGS. FIG. 1 is a schematic plan view showing an optical waveguide device. In FIG. 1, a clad layer described later is omitted. FIG. 2 is a schematic end view of the optical waveguide element shown in FIG. 1 taken along line II.

  The optical waveguide device 100 includes a support substrate 10, a cladding layer 20, a first optical waveguide core 30 having a first port 35, a multimode waveguide section 31, a Bragg reflection section 33, and a second port 37, and a coupling section. 51 and a second optical waveguide core 50 having a third port 57. The Bragg reflector 33 is formed with a grating 40 that reflects light of a specific wavelength. Further, a bidirectional coupling region 60 in which the multimode waveguide section 31 and the coupling section 51 are arranged in parallel to each other is set.

  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. Accordingly, a configuration example in the case where the optical waveguide element 100 is used as a path switching element in the ONU will be described below.

  When the optical waveguide element 100 is used as a path switching element in the ONU, the first port 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 second port 37 of the first optical waveguide core 30 is connected to a light receiving element such as a photodiode. The third port 57 of the second optical waveguide core 50 is connected to a light emitting element such as a laser diode.

  In this example, the fundamental mode upstream optical signal generated by the light emitting element is input to the third port 57 of the second optical waveguide core 50, and in the bidirectional coupling region 60, the coupling portion 51 of the second optical waveguide core 50. To the multimode waveguide section 31. The bidirectional coupling region 60 is optimized so as to couple primary mode light and fundamental mode light between the multimode waveguide section 31 and the coupling section 51. Accordingly, the upstream optical signal propagates through the multimode waveguide section 31 in the primary mode. Further, the upstream optical signal is sent from the multimode waveguide unit 31 to the Bragg reflector 33. Then, in the grating 40 formed in the Bragg reflector 33, the upstream optical signal is converted from the primary mode to the fundamental mode and reflected. The reflected upstream optical signal is sent to the multimode waveguide section 31 again. In the bidirectional coupling region 60, since the fundamental modes of the multimode waveguide unit 31 and the coupling unit 51 are set so as not to be coupled, the reflected upstream optical signal propagates through the multimode waveguide unit 31, Sent to the first port 35. The upstream optical signal output from the first port is sent to the OLT via the optical fiber.

  The fundamental mode downstream optical signal sent from the OLT through the optical fiber is input to the first port 35 of the first optical waveguide core 30 and sent to the multimode waveguide section 31. Further, the downstream optical signal propagating through the multimode waveguide unit 31 is transmitted to the second port 37 through the Bragg reflector 33. The downstream optical signal output from the second port 37 is received by the light receiving element.

  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.

  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. The first port 35 is connected to the multimode waveguide section 31 through the first tapered section 39a. The width of the first taper portion 39a is set so as to continuously change from the width of the one end 35a of the first port 35 to the width of the one end 31a of the multimode waveguide portion 31 along the light propagation direction. Yes. By providing the first tapered portion 39a, reflection of light propagating between the first port 35 and the multimode waveguide portion 31 can be reduced.

  The multimode waveguide section 31 is formed with a width and thickness for propagating fundamental mode light and primary mode light. The multimode waveguide section 31 is connected to the Bragg reflection section 33 via the second taper section 39b. The width of the second tapered portion 39b is set so as to continuously change from the width of the other end 31b 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. ing. By providing the second taper part 39b, reflection of light propagating between the multimode waveguide part 31 and the Bragg reflection part 33 can be reduced.

  A grating 40 is formed on the Bragg reflector 33.

  In the present invention, the grating 40 performs Bragg reflection by converting the fundamental mode and the primary mode with respect to the TE polarized wave and the TM polarized wave having the same wavelength.

  The grating 40 is configured such that first regions 41 and second regions 43 are alternately and periodically formed in the Bragg reflector 33 along the light propagation direction. The first region 41 is a region where a concave engraving is formed from one side surface 33c of the Bragg reflector 33 along the propagation direction. The second region 43 is a region in which a concave engraving is formed from the other side surface 33d of the Bragg reflector 33 along the propagation direction. The concave engravings formed in the first region 41 and the second region 43 are set to the same engraving depth d.

Furthermore, the width W2 of the Bragg reflector 33 and the engraving depth d of the grating 40 are set so that N _ei + N _ej in the above equation (1) matches between the TE polarized wave and the TM polarized wave at a specific same wavelength. Has been. Here, the grating 40 includes the TE-polarized fundamental mode equivalent refractive index N _e0TE , the TE-polarized primary mode equivalent refractive index N _e1TE , the TM-polarized fundamental mode equivalent refractive index N _e0TM , and the TM polarized _light . The equivalent refractive index N _e1TM of the primary mode of the wave is formed so that N _e0TE + N _e1TE = N _e0TM + N _e1TM is established at a specific same wavelength. As a result, the above equation (1) can be established at the same wavelength λ for both the TE polarization and the TM polarization. That is, the polarization can be made independent.

Therefore, the primary mode light having a wavelength matching the Bragg wavelength transmitted from the multimode waveguide section 31 to the Bragg reflector 33 is converted into fundamental mode light in the grating 40 by both TE polarization and TM polarization. Bragg is reflected. Then, the Bragg-reflected fundamental mode light is sent to the multimode waveguide section 31. In addition, light having a wavelength deviating from the Bragg wavelength transmitted to the Bragg reflector 33 is transmitted to the second port 37 through the grating 40 for both the TE polarized wave and the TM polarized wave. A specific example of a specific design of the grating 40 for establishing N _e0TE + N _e1TE = N _e0TM + N _e1TM will be described later.

  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 third tapered portion 39c. The third taper portion 39 c 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 third tapered portion 39c, reflection of light propagating between the Bragg reflecting portion 33 and the second port 37 can be relaxed.

  The second optical waveguide core 50 is made of, for example, Si having a refractive index higher than that of the cladding layer 20. 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 coupling portion 51 is spaced apart from and parallel to the multimode waveguide portion 31 of the first optical waveguide core 30.

  In the configuration example illustrated in FIG. 1, the coupling unit 51 includes a first coupling unit 53 and a second coupling unit 55 connected to the first coupling unit 53. The first coupling portion 53 and the second coupling portion 55 have different widths, and are set to thicknesses and widths that achieve single mode conditions, respectively. Accordingly, the first coupling unit 53 and the second coupling unit 55 propagate the fundamental mode light.

  Further, one end 51 a of the coupling portion 51 is connected to the third port 57. In the configuration example shown in FIG. 1, one end on the first coupling portion 53 side 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 spaced apart from each other and arranged in parallel. Is set.

  In the bidirectional coupling region 60, the primary mode light propagating through the multimode waveguide section 31 and the fundamental mode light propagating through the coupling section 51 are coupled.

  In the configuration example illustrated in FIG. 1, the bidirectional coupling region 60 includes a first bidirectional coupling region 61 in which the multimode waveguide unit 31 and the first coupling unit 53 are spaced apart from each other and arranged in parallel. The multimode waveguide unit 31 and the second coupling unit 55 include a second bidirectional coupling region 63 that is spaced apart from and parallel to each other. In the first bidirectional coupling region 61, the primary mode TE polarized wave propagating through the multimode waveguide unit 31 and the fundamental mode TE polarized wave propagating through the first coupling unit 53 are coupled. In the second bidirectional coupling region 63, the primary mode TM polarization propagating through the multimode waveguide section 31 and the fundamental mode TM polarization propagating through the second coupling section 55 are coupled.

  Here, the mode coupling between the two optical waveguide cores will be described. For example, the following expressions (2) and (3) are established for inter-mode coupling between two optical waveguide cores A and B that are spaced apart from each other and arranged in parallel. Note that κ represents a mode coupling constant between the optical waveguide cores A and B.

  In the above equation (2), A (0) and A (z) are functions indicating the amplitude of light propagating through one optical waveguide core A in position coordinates. The above equation (2) represents the ratio of the energy at the position coordinate 0 and the energy at the position coordinate z for the light propagating from the position coordinate 0 to the position coordinate z of the optical waveguide core A. B (z) is a function indicating the amplitude of light propagating through the other optical waveguide core B in position coordinates. The above expression (3) is obtained by using the energy at the position coordinate 0 of the optical waveguide core A and the position coordinate z of the optical waveguide core B for light propagating from the position coordinate 0 of the optical waveguide core A to the optical waveguide core B. It represents the ratio with energy. Therefore, if the coupling length of the optical waveguide core A and the optical waveguide core B is z, the right side of the above equation (2) is 0, and the right side of the above equation (3) is 1 from the one optical waveguide core A. The light is completely transferred to the other optical waveguide core B.

Further, δβ in the above equations (2) and (3) can be expressed by the following equation (4). Β _a represents the propagation constant of the optical waveguide core A, and β _b represents the propagation constant of the optical waveguide core B.

Further, β _a and β _b can be expressed by the following formulas (5) and (6). N _ea represents the equivalent refractive index of the optical waveguide core A, N _eb represents the equivalent refractive index of the optical waveguide core B, and λ represents the wavelength.

From the above formulas (2) to (6), when N _ea = N _eb , light is completely transferred from the optical waveguide core A to the optical waveguide core B.

  Therefore, in the optical waveguide device 100, the equivalent refractive index of the primary mode of the TE polarized wave propagating through the multimode waveguide unit 31 and the equivalent refractive index of the fundamental mode of the TE polarized wave propagating through the first coupling unit 53 are The equivalent refractive index of the primary mode of TM polarization propagating through the multimode waveguide section 31 is equal to the equivalent refractive index of the fundamental mode of TM polarization propagating through the second coupling section 55. The width W1 of the multimode waveguide section 31, the width W3 of the first coupling section 53, and the width W4 of the second coupling section 55 are set. As a result, in the first bidirectional coupling region 61, the primary mode TE polarized wave propagating through the multimode waveguide unit 31 and the fundamental mode TE polarized wave propagating through the first coupling unit 53 are reliably coupled. be able to. Further, in the second bidirectional coupling region 63, the primary mode TM polarization propagating through the multimode waveguide section 31 and the fundamental mode TM polarization propagating through the second coupling section 55 are reliably coupled. Can do. Therefore, in the bidirectional coupling region 60, both the TE polarized wave and the TM polarized wave can be coupled between the multimode waveguide unit 31 and the coupling unit 51.

  Note that the equivalent refractive index of the fundamental mode in the multimode waveguide section 31 is different from the equivalent refractive index of the primary mode. Therefore, the difference between the equivalent refractive index of the TE polarized fundamental mode in the multimode waveguide section 31 and the equivalent refractive index of the TE polarized fundamental mode in the first coupling section 53 is set large. Further, the difference between the equivalent refractive index of the fundamental mode of TM polarization in the multimode waveguide section 31 and the equivalent refractive index of the fundamental mode of TM polarization in the second coupling section 55 is set large. As a result, the coupling between the fundamental modes between the multimode waveguide section 31 and the coupling section 51 can be suppressed. Specific preferred examples of the width W1 of the multimode waveguide section 31, the width W3 of the first coupling section 53, and the width W4 of the second coupling section 55 will be described later.

  Here, FIG. 3 is a schematic plan view showing a modification of the bidirectional coupling region 60, and is an enlarged view of a region corresponding to the bidirectional coupling region 60 surrounded by a broken line in FIG.

  As shown in FIG. 3, in this modification, the coupling portion 151 may have a tapered shape whose width continuously changes along the light propagation direction. In that case, a width W3 in which the equivalent refractive index of the fundamental mode of the TE polarized wave propagating through the coupling unit 151 is equal to the equivalent refractive index of the first mode of the TE polarized wave propagating through the multimode waveguide unit 31, A width W4 at which the equivalent refractive index of the fundamental mode of TM polarization propagating through the coupling portion 151 is equal to the equivalent refractive index of the first mode of TM polarization propagating through the multimode waveguide portion 31 is defined as the coupling portion 151. Design to include. As a result, both the TE polarized wave and the TM polarized wave can be coupled between the multimode waveguide unit 31 and the coupling unit 151.

  As described above, in the optical waveguide device 100 according to the present invention, in the grating 40 formed in the Bragg reflector 33, the incident light and the reflected light reflected by the Bragg are coupled between modes of different orders. Then, by forming the grating 40 so that the sum of the equivalent refractive index of the fundamental mode and the equivalent refractive index of the first-order mode is equal between the TE polarized wave and the TM polarized wave at a specific same wavelength, the TE polarized wave In addition, the Bragg reflection condition at the same wavelength can be established for both the TM polarization and the TM polarization.

  Further, 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, and the multimode waveguide unit 31. Can be coupled to the TM polarization of the primary mode propagating through the coupling unit 51 and the TM polarization of the fundamental mode propagating through the coupling unit 51.

  Therefore, in the optical waveguide device 100 according to the present invention, the TE polarized wave and the TM polarized wave can be propagated through the same path according to the wavelength in the first optical waveguide core 30 and the second optical waveguide core 50. Therefore, the optical waveguide element 100 according to the present invention can be used as a polarization-independent path switching element.

  Here, the configuration in which the grating 40 converts the fundamental mode and the primary mode and performs Bragg reflection has been described. However, the optical waveguide device 100 according to the present invention is not limited to this configuration. By appropriately setting the width W2 of the Bragg reflector 33 and the engraving depth d of the grating 40, the i-order mode (i is an integer of 0 or more) and j for a specific TE polarized wave and TM polarized wave. It is also possible to adopt a configuration in which the next mode (j is an integer of 0 or more different from i) is converted and Bragg reflected. In that case, the multimode waveguide section 31 is formed so as to propagate at least i-order mode light and j-order mode light. Further, the light coupled in the bidirectional coupling region 60 is not limited to the fundamental mode and the primary mode. By appropriately setting the width W1 of the multimode waveguide section 31 and the widths W3 and W4 of the coupling section 51, the TE-polarized wave of the j-th mode that propagates through the multimode waveguide section 31 in the bidirectional coupling region 60 And the k-order mode TE polarization propagating through the coupling unit 51 (k is an integer of 0 or more) and the j-order mode TM polarization propagating through the multimode waveguide unit 31 and the coupling unit 51 Can be combined with TM polarization of the kth order mode propagating through the.

(Production method)
The optical waveguide device 100 according to the present invention can be easily manufactured by using 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 a simulation to search for a suitable design example as a utilization form of the optical waveguide device 100.

First, using an FEM (Finite Element Method), the TE-polarized fundamental mode equivalent refractive index N _e0TE , the TE-polarized primary mode equivalent refractive index N _e1TE , and the TM-polarized fundamental mode equivalent refractive index N _e0TM As for the equivalent refractive index N _e1TM of the first-order mode of TM polarization, a suitable example of the design of the grating 40 in which N _e0TE + N _e1TE = N _e0TM + N _e1TM is found.

In this simulation, the first optical waveguide core 30 is made of Si and has a thickness of 300 nm. Also, to form the cladding layer 20 in the SiO _2. Further, the Bragg wavelength λ in the above formula (1) was set to 1277 nm. For each of the TE polarized wave and the TM polarized wave, the width W2 of the Bragg reflector 33 and the engraving depth d of the grating 40, the equivalent refractive index _Ne0 of the fundamental mode, and the equivalent refractive index of the primary mode of the TM polarized wave. The relationship with the sum of N _e1 (N _e0 + N _e1 ) was confirmed.

The result of the simulation is shown in FIG. FIG. 4 is a diagram showing the relationship between the design of the grating 40 and N _e0 + N _e1 . In FIG. 4, the vertical axis represents N _e0 + N _e1 on an arbitrary scale, and the horizontal axis represents the width W2 of the Bragg reflector 33 in nm units.

From the result of FIG. 4, when the width W2 of the Bragg reflector 33 is in the range of 330 to 340 nm and the engraving depth d is 50 nm, N _e0 + N _e1 of the TE polarization and the TM polarization match. That is, it was confirmed that N _e0TE + N _e1TE = N _e0TM + N _e1TM . For example, when the width W2 of the Bragg reflector 33 is 340 nm and the engraving depth d is 50 nm, N _e0TE + N _e1TE and N _e0TM + N _e1TM are about 4 from FIG. Therefore, in this case, it is preferable to set the period Λ of the grating 40 to 319 nm from the above equation (1).

  Next, using the FEM method, the width W1 of the multimode waveguide portion 31 of the first optical waveguide core 30, the width W3 of the first coupling portion 53 of the second optical waveguide core 50, and the second A suitable combination of the width W4 of the two connecting portions 55 was found.

In this simulation, the first optical waveguide core 30 and the second optical waveguide core 50 are made of Si and have a thickness of 300 nm. Also, to form the cladding layer 20 in the SiO _2. Further, the wavelength of light coupled in the bidirectional coupling region 60 was set to 1277 nm. Further, the width W1 of the multimode waveguide section 31 is set to 450 nm. In the multimode waveguide section 31 with the width W1, the equivalent refractive index N _em1TE of the primary mode of TE polarization is 1.890, and the equivalent refractive index N _em1TM of the primary mode of TM polarization is 2.063. Met. Therefore, the width W3 at which the equivalent refractive index N _es0TE of the fundamental mode of the TE polarization in the first coupling portion 53 _matches the above-described equivalent refractive index N _em1TE was confirmed. In addition, the width W4 in which the equivalent refractive index N _es0TM of the fundamental mode of TM polarization in the second coupling unit 55 _matches the above-described equivalent refractive index N _em1TM was confirmed.

The result of the simulation is shown in FIG. Figure 5 is an equivalent refractive index _{N Es0TM} the equivalent refractive index _{N Es0TE} and second coupling portion 55 of the first coupling portion 53 is a diagram showing the relationship between the widths W3 and W4. In FIG. 5, the vertical axis represents the equivalent refractive index with an arbitrary scale, and the horizontal axis represents the widths W3 and W4 of the first coupling portion 53 and the second coupling portion 55 in nm units.

From the results of FIG. 5, when the width W3 is 210 nm, that the equivalent refractive index _{N Es0TE} of the fundamental mode of the TE polarization in the first coupling part 53 coincides with the equivalent refractive index _{N Em1TE} multimode waveguide section 31 confirmed. Further, when the width W4 is 165 nm, the equivalent refractive index _{N Es0TM} of the fundamental mode of the TM polarization in the second coupling portion 55, it was confirmed to match the equivalent refractive index _{N Em1TM} multimode waveguide section 31.

Accordingly, by setting the width W1 of the multimode waveguide section 31 to 450 nm, the width W3 of the first coupling section 53 to 210 nm, and the width W4 of the second coupling section 55 to 165 nm, the TE coupling in the bidirectional coupling region 60 is achieved. Both waves and TM polarized waves can be coupled between the multimode waveguide section 31 and the coupling section 51. In addition, in the combination of these widths W1, W3, and W4, the difference between the equivalent refractive index of the fundamental mode of the TE polarized wave in the multimode waveguide portion 31 and the equivalent refractive index N _es0TE of the first coupling portion 53 becomes large. . _Further , the difference between the equivalent refractive index of the fundamental mode of TM polarization in the multimode waveguide section 31 and the equivalent refractive index _Nes0TM of the second coupling section 55 becomes large. Therefore, coupling between fundamental modes between the multimode waveguide section 31 and the coupling section 51 can be suppressed.

  Further, using BPM (Beam Propagation Method), an optimal combination of the TE polarization of the primary mode propagating through the multimode waveguide section 31 and the TE polarization of the fundamental mode propagating through the first coupling section 53 is achieved. The bond length L1 was determined. In addition, the optimal coupling length L2 for coupling the primary mode TM polarization propagating through the multimode waveguide section 31 and the fundamental mode TM polarization propagating through the second coupling section 55 was determined. It is assumed that the center distance D1 between the multimode waveguide section 31 and the first coupling section 53 and the center distance D2 between the multimode waveguide section 31 and the second coupling section 55 are both 0.7 μm. (See FIG. 1).

  As a result, in each dimension of the widths W1, W3, and W4 found from FIG. 5, the coupling length L1 of the multimode waveguide section 31 and the first coupling section 53, that is, the length L1 of the first bidirectional coupling area 61 is , 13 μm. Further, the coupling length L2 of the multimode waveguide unit 31 and the second coupling unit 55, that is, the length L2 of the second bidirectional coupling region 63 is 25 μm.

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

  In this simulation, TE polarization and TM polarization were input from the first port 35, respectively. Then, the intensity of each polarized wave reflected by the grating 40 and output from the third port 57 through the bidirectional coupling region 60 was calculated. In addition, the intensity of each polarization output from the second port 37 through the grating 40 was calculated.

In this simulation, the first optical waveguide core 30 and the second optical waveguide core 50 are made of Si, and the thickness thereof is 300 nm. Also, to form the cladding layer 20 in the SiO _2. The Bragg wavelength in the grating 40 was set to 1277 nm. Based on the result of the simulation shown in FIG. 4 described above, the width W2 of the Bragg reflector 33 is 340 nm, the engraving depth d of the grating 40 is 50 nm, and the period Λ of the grating 40 is 319 nm. Further, based on the result of the simulation shown in FIG. 5 described above, the width W1 of the multimode waveguide section 31 is 450 nm, the width W3 of the first coupling section 53 is 210 nm, and the width W4 of the second coupling section 55 is 165 nm. . Further, the length L1 of the first bidirectional coupling region 61 is 13 μm, the length L2 of the second bidirectional coupling region 63 is 25 μm, and the center distance D1 between the multimode waveguide portion 31 and the first coupling portion 53, The center-to-center distance D2 between the multimode waveguide section 31 and the second coupling section 55 is 0.7 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, where the vertical axis indicates the light intensity in dB scale, and the horizontal axis indicates the wavelength in nm. A curve I shown in FIG. 6 indicates the intensity of the TE polarized wave output from the third port 57. A curve II indicates the intensity of TE polarized light output from the second port 37. A curve III indicates the intensity of the TM polarization output from the third port 57. A curve IV indicates the intensity of the TM polarized wave output from the second port 37.

  From the results of FIG. 6, there is a peak in the light intensity output from the third port 57 in both the TE polarized wave and the TM polarized wave in the wavelength band near 1277 nm. Further, in both the TE polarized wave and the TM polarized wave, the light intensity output from the second port 37 is defective in the wavelength band near 1277 nm. From this result, in the optical waveguide device 100, the multi-mode waveguide portion 31 and the coupling portion 51 are reflected in the grating 40 in which both the TE polarization and the TM polarization are reflected at the same Bragg wavelength, and in the bidirectional coupling region 60. It was confirmed that the TE polarized wave and the TM polarized wave are coupled with each other. Therefore, in the optical waveguide device 100, it was confirmed that the TE polarized wave and the TM polarized wave can be propagated through the same path according to the wavelength in the first optical waveguide core 30 and the second optical waveguide core 50.

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 cores 51, 151 : Coupling portion 53: first coupling portion 55: second coupling portion 57: third port 60: bidirectional coupling region 61: first bidirectional coupling region 63: second bidirectional coupling region 100: optical waveguide element

Claims (2)

a multimode waveguide section for propagating light of i-order mode (i is an integer of 0 or more) and light of j-order mode (j is an integer of 0 or more different from i), and is connected to the multimode waveguide section A first optical waveguide core having a Bragg reflector,
A second optical waveguide core having a coupling portion;
With
The Bragg reflector is formed with a grating that Bragg-reflects by converting the i-order mode and the j-order mode for the TE polarized wave and the TM polarized wave having the same wavelength.
A bidirectional coupling region in which the multimode waveguide part and the coupling part are spaced apart from each other and arranged in parallel is set.
In the bidirectional coupling region, a TE polarized wave of the j-th mode propagating through the multimode waveguide section and a TE polarized wave of the k-order mode (k is an integer of 0 or more) propagating through the coupling section are coupled. And the TM polarization of the j-order mode propagating through the multimode waveguide section and the TM polarization of the k-order mode propagating through the coupling section are combined,
The grating is
The TE-polarized i-order mode equivalent refractive index N _eiTE , the TE-polarized j-order mode equivalent refractive index N _ejTE , the TM-polarized i-order mode equivalent refractive index N _eiTM , and the TM-polarized j-order mode _Is equivalent so that N _eiTE + N _ejTE = N _eiTM + N _ejTM is established at a specific same wavelength .
The coupling portion includes an optical waveguide element you characterized in that the width continuously along the propagation direction of light is tapered varying. The coupling unit is configured such that the equivalent refractive index of the TE-polarized wave propagating through the coupling unit is equal to the equivalent refractive index of the TE-polarized wave propagating through the multimode waveguide unit. A width and a second width at which an equivalent refractive index of a TM polarized wave propagating through the coupling portion is equal to an equivalent refractive index of a TM polarized wave propagating through the multimode waveguide portion. The optical waveguide device according to claim 1 , comprising:

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