Optical waveguide element

Abstract

To provide an optical waveguide element that can achieve wavelength selection not depending on polarization with as few elements as possible, and that does not require strict manufacture accuracy.SOLUTION: An input side polarization conversion part 100 is formed to have a larger width along an input light propagation direction. A ridge part includes a wide part and a narrow part in this order. The wide part is formed to have a larger width or a uniform width along the input light propagation direction. The narrow part is formed to have a narrowing width along the input light propagation direction. The end part of the narrow part on the narrow side has the width that is more than or equal to the minimum width that can be manufactured, and has the width that is more than or equal to the minimum width that can be manufactured by the terrace part at the end part where the light is input. The ridge part is shorter than a rib waveguide. A grating part 200 includes antisymmetric grating on both sides of an optical waveguide core. An output side polarization conversion part 300 has a structure similar to an input side polarization conversion part and is provided facing the input side polarization conversion part.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an optical waveguide element that multiplexes and demultiplexes light and can be used, for example, when transmitting light of a plurality of different wavelengths through a single optical fiber. [Background technology]

[0002] Passive Optical Networks (PONs) are the mainstream subscriber optical communication systems. In a PON, one central office device (OLT: Optical Line Terminal) and multiple subscriber devices (ONUs: Optical Network Units) are connected via optical fiber and a star coupler, with multiple ONUs sharing one OLT. In a PON, the optical signal wavelengths used for downstream communication from the OLT to the ONUs and upstream communication from the ONUs to the OLT are different from each other to prevent mutual interference between the downstream communication from the OLT to the ONUs and the upstream communication from the ONUs to the OLT.

[0003] Therefore, multiplexing / demultiplexing elements are required to demultiplex and multiplex the optical signals with different wavelengths used for downstream and upstream communications. In general, OLTs and ONUs are configured by spatially coupling optical wavelength filters, photodiodes (PDs), and laser diodes (LDs) as multiplexing / demultiplexing elements to realize the function of transmitting and receiving optical signals with different wavelengths.

[0004] Spatial coupling requires alignment work to align the optical axes between the optical wavelength filter, PD, and LD. Optical wavelength filters that use waveguides have been developed to eliminate the need for this optical axis alignment. Furthermore, silicon (Si) waveguides, which use silicon-based materials as the waveguide material, are attracting attention for their compactness and ease of mass production when forming these optical wavelength filters.

[0005] In a Si waveguide, the optical waveguide core, which essentially serves as the optical transmission path, is made of Si. The optical waveguide core is then surrounded by a cladding made of a material with a lower refractive index than Si, such as silica. This configuration significantly increases the difference in refractive index between the optical waveguide core and the cladding, allowing for strong confinement of light within the optical waveguide core. As a result, it is possible to realize compact curved waveguides with bending radii as small as 1 μm. This makes it possible to create optical circuits of similar size to electronic circuits, which is advantageous for miniaturizing the entire optical device.

[0006] Furthermore, with Si waveguides, it is possible to reuse the manufacturing processes of semiconductor devices such as CMOS (Complementary Metal Oxide Semiconductor), which is why it is expected to realize photonics-electronics convergence (silicon photonics), in which electronic and optical functional circuits are formed simultaneously on a chip.

[0007] Recently, a system has been proposed in which a large number of wavelengths are prepared and distributed to each subscriber. In this system, a wavelength filter is required to select an arbitrary wavelength from the large number of wavelengths.

[0008] Optical wavelength filters using Si waveguides for this purpose include, for example, those using a Mach-Zehnder interferometer and those using an arrayed waveguide grating (AWG).Also, optical wavelength filters using Si waveguides include ring resonator, grating, and directional coupler type tunable wavelength filters that can tune the output wavelength and have the advantage of being easy to use due to their simple element structure (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Patent Publication 2013-093627 Summary of the Invention [Problem to be solved by the invention]

[0010] However, the conventional tunable wavelength filters mentioned above only work with specific polarized waves. To solve this problem, a method is often used in which the polarized waves are separated and aligned before being introduced into the wavelength selection element.

[0011] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an optical waveguide element that can realize polarization-independent wavelength selection with as few elements as possible and that does not require strict manufacturing precision. [Means for solving the problem]

[0012] In order to achieve the above-mentioned object, the optical waveguide element of the present invention is an optical waveguide element in which a waveguide core is formed on a lower clad, and the waveguide core is embedded in a clad composed of the lower clad and an upper clad, and is configured to include an input side polarization conversion section, a grating section, and an output side polarization conversion section arranged in series in this order.

[0013] The input polarization conversion section and the output polarization conversion section are provided opposite to each other and each comprises a ridge portion and a terrace portion that is thinner than the ridge portion in a direction perpendicular to the upper surface of the lower cladding.

[0014] On the input side polarization conversion section, the ends of the ridge portion and the terrace portion on the side where light is input are arranged in the same plane perpendicular to the direction of light propagation, and the length of the ridge portion in the direction along the direction of light propagation is shorter than the length of the terrace portion in the direction along the direction of light propagation.

[0015] The ridge portion has a wide portion that widens or has a uniform width along the propagation direction of the input light, and a tapered portion that narrows along the propagation direction of the input light, arranged in that order along the propagation direction of the input light, and at the end where the wide portion and the tapered portion are connected, the widths of the wide portion and the tapered portion are equal to each other, and the end where the tapered portion is narrower is configured with a width equal to or greater than the minimum width that can be produced. The terrace portion is configured to widen along the propagation direction of the input light, and is formed with a width equal to or greater than the minimum width that can be produced at the end where the light is input.

[0016] On the output polarization conversion section side, the ends of the ridge portion and the terrace portion from which light is output are arranged in the same plane perpendicular to the light propagation direction, and the length of the ridge portion in the direction along the light propagation direction is shorter than the length of the terrace portion in the direction along the light propagation direction.

[0017] The ridge portion has, in order along the propagation direction of the input light, a tapered portion configured to widen in width along the propagation direction of the input light, and a wide portion formed to narrow in width or with a uniform width along the propagation direction of the input light, and at the end where the wide portion and the tapered portion are connected, the widths of the wide portion and the tapered portion are equal to each other, and the end where the tapered portion is narrower is configured with a width equal to or greater than the minimum width that can be fabricated. The terrace portion is configured to narrow in width along the propagation direction of the input light, and is formed with a width equal to or greater than the minimum width that can be fabricated at the end where the light is output. The grating portion has antisymmetric gratings on both sides of the optical waveguide core.

[0018] According to a preferred embodiment of the optical waveguide element described above, the grating section is formed to have the same thickness as the terrace section.

[0019] Furthermore, a tapered waveguide formed to the same thickness as the terrace portion may be provided between the grating and each of the input polarization conversion portion and the output polarization conversion portion. In this case, the tapered waveguide and the input polarization conversion portion may have equal widths at the end where the tapered waveguide and the input polarization conversion portion are connected, the tapered waveguide and the output polarization conversion portion may have equal widths at the end where the tapered waveguide and the output polarization conversion portion are connected, and the tapered waveguide and the grating may have equal widths at the end where the tapered waveguide and the grating are connected.

[0020] Furthermore, according to another preferred embodiment of the optical waveguide element described above, the grating section is formed to the same thickness as the ridge section, and a thickness conversion structure is provided between the grating section and each of the input-side polarization conversion section and the output-side polarization conversion section.

[0021] The thickness conversion structure may be configured to include a tapered section with a thin waveguide thickness, which extends from the terrace section toward the grating section and has the same thickness as the terrace section, narrows toward the grating section, and has a width at its tip that is equal to or greater than the minimum width that can be created; a tapered section with a thick waveguide thickness, which extends from the grating section toward the terrace section and narrows toward the terrace section, and has a width at its tip that is equal to or greater than the minimum width that can be created; and a pair of sub-waveguides, each having the same thickness as the grating section, located between the tapered section with a thin waveguide and the tapered section with a thick waveguide.

[0022] According to another preferred embodiment of the optical waveguide element described above, the grating section is made of a silicon nitride film, the input-side polarization conversion section and the output-side polarization conversion section are made of silicon optical waveguide cores, the silicon nitride film optical waveguide cores are arranged at a distance from the silicon optical waveguide cores in the thickness direction, transition terrace sections are provided on the grating section sides of the input-side polarization conversion section and the output-side polarization conversion section by extending the terrace sections and formed so that their widths become narrower in the direction toward the grating section, the width of the tip of the transition terrace section on the grating section side is the smallest width that can be created, and in the region where the transition terrace sections are provided, transition SiN sections by extending the grating sections are provided on the input-side polarization conversion section and output-side polarization conversion section sides of the grating section.

[0023] Furthermore, the grating section may have one or more resonance regions in which no grating is formed, and the regions in which a grating is formed and the resonance regions may be arranged alternately along the longitudinal direction, with one resonance region or each of the multiple resonance regions being sandwiched between regions in which a grating is formed. [Effects of the Invention]

[0024] The optical waveguide element of the present invention is easy to fabricate and can realize polarization-independent wavelength selection with as few elements as possible. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a schematic diagram (1) illustrating a first optical waveguide element. [Figure 2] FIG. 2 is a schematic diagram (2) illustrating the first optical waveguide element. [Figure 3] FIG. 10 is a diagram for explaining the operation of the input-side polarization conversion unit, and is a diagram for explaining the relationship between the propagation distance in the polarization conversion unit and the equivalent refractive index for an area where a ridge portion is provided. [Figure 4] FIG. 4 is a schematic diagram illustrating a second optical waveguide element. [Figure 5] FIG. 10 is a diagram illustrating the characteristics of a second optical waveguide element. [Figure 6] FIG. 10 is a schematic diagram illustrating a third optical waveguide element. [Figure 7] FIG. 10 is a diagram illustrating the characteristics of a fourth optical waveguide element. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the shape, size, and positional relationship of each component are merely shown in a schematic manner to enable understanding of the present invention. Furthermore, preferred configuration examples of the present invention will be described below. However, the materials and numerical conditions of each component are merely preferred examples. Therefore, the present invention is not limited to the following embodiments, and many modifications and variations that can achieve the effects of the present invention can be made without departing from the scope of the configuration of the present invention. Note that, although some hatching is applied in the plan view, it should be understood that this does not represent a cross section, but is added to facilitate understanding of the invention.

[0027] (First optical waveguide element) An optical waveguide element according to a first embodiment of the present invention (hereinafter also referred to as a first optical waveguide element) will be described with reference to Figures 1 and 2. Figures 1 and 2 are schematic diagrams for explaining the first optical waveguide element. Figure 1 is a schematic plan view of the optical waveguide element, omitting a support substrate and a lower cladding, which will be described later, and showing only the waveguide core. Figures 2(A), (B), and (C) are diagrams showing cut end faces of the optical waveguide element taken along lines AA, BB, and CC, respectively.

[0028] The first optical waveguide element includes a lower clad 22 on a support substrate 10 and a waveguide core 30 on the lower clad 22. The first optical waveguide element can be easily manufactured by using, for example, a commercially available SOI (Silicon On Insulator) substrate. The SOI substrate is configured by sequentially stacking a support substrate layer, an SiO2 layer, and an Si layer, and the support substrate layer serves as the support substrate 10. The SiO2 layer serves as the lower clad 22.

[0029] The Si layer is dry-etched or the like to pattern the Si layer, thereby forming the waveguide core 30. As will be described later, the waveguide core 30 has thick and thin portions. For this reason, dry etching is performed, for example, in two stages. In this case, the Si layer is first dry-etched (first time) to pattern it until it reaches the lower clad 22 so as to obtain the planar shape of the waveguide core 30, and then the patterned Si layer is dry-etched (second time) to form the thin portion. Alternatively, the waveguide core 30 may be formed by thinning the Si layer by dry-etching (first time) in areas other than the areas that will become the thick portions of the waveguide core 30, and then dry-etching (second time) the thinned portion of the Si layer until it reaches the lower clad 22 so as to obtain the desired planar shape of the waveguide core 30.

[0030] Thereafter, SiO2 is deposited on the lower clad 22 by a chemical vapor deposition (CVD) method or the like to form the upper clad 24 that covers the waveguide core 30. As a result, a configuration is obtained in which the waveguide core 30 is embedded in the clad 20 consisting of the lower clad 22 and the upper clad 24.

[0031] To prevent light propagating through the waveguide core 30 from escaping to the support substrate 10, the distance between the support substrate 10 and the waveguide core 30, i.e., the thickness of the lower cladding 22, is preferably 1 μm or more. Furthermore, the thickness of the waveguide core 30 is preferably 200 to 400 nm, a value that can achieve single-mode conditions in the thickness direction. Typically, a waveguide core 30 with a thickness of 220 nm is used. In the example described here, the thickness of the thick portion is, for example, 220 nm. Furthermore, the difference in height between the top surfaces of the thick and thin portions is typically, for example, 70 nm in a rib waveguide. Therefore, in the example described here, the thickness of the thin portion is 150 nm.

[0032] Light propagates according to the planar shape of the waveguide core 30, achieving the desired function of the optical waveguide element. Light input to the first optical waveguide element propagates in the longitudinal direction of the waveguide core 30. In the following description, the longitudinal direction may also be referred to as the propagation direction. The direction perpendicular to the upper surface of the lower cladding 22 may also be referred to as the thickness direction, and the direction perpendicular to both the propagation direction and the thickness direction may also be referred to as the width direction. The dimension in the width direction may also be referred to as the width, and the dimension in the longitudinal direction may also be referred to as the length.

[0033] The optical waveguide element is configured to include an input section 400 , an input-side polarization conversion section 100 , a grating section 200 , an output-side polarization conversion section 300 , and an output section 500 , which are obtained by the planar shape of the waveguide core 30 .

[0034] The input-side polarization conversion section 100 has a so-called rib waveguide structure that includes a ridge portion 110 and a terrace portion 120. The ridge portion 110 extends along the longitudinal direction (the light propagation direction) and is thick in the direction perpendicular to the upper surface of the lower cladding 22. The terrace portion 120 is thin in the direction perpendicular to the upper surface of the lower cladding 22. The ends of the ridge portion 110 and the terrace portion 120 on the light input side are provided in the same plane perpendicular to the light propagation direction.

[0035] The rib waveguide, particularly the terrace portion 120, is configured so that its width increases along the longitudinal direction from the input waveguide 300 side toward the grating section 200.

[0036] The ridge portion 110 is configured to include, in order from the input section 400 side along the longitudinal direction, a wide portion 112 and a tapered portion 114. The length of the ridge portion 110 along the light propagation direction is shorter than the length of the polarization conversion section 100, i.e., the length of the terrace portion 120 along the light propagation direction, and the ridge portion 110 ends midway along the longitudinal direction of the input-side polarization conversion section 100. Therefore, the output end does not have the ridge portion 110 and is configured only by the terrace portion 120.

[0037] The wide portion 112 is configured to have a constant width or to increase in width from the input portion 400 side toward the tapered portion 114. For example, the width of the end of the wide portion 112 connected to the input portion 400 is 300 nm, and the width of the end of the wide portion 112 connected to the tapered portion 114 is 350 nm.

[0038] The tapered portion 114 is configured so that its width narrows from the wide portion 112 side toward the grating portion 200. The width of the end of the tapered portion 114 that is connected to the wide portion 112 is equal to the width of the end of the wide portion 112 that is connected to the tapered portion 114, and the width of the end (tip) of the tapered portion 114 on the grating portion 200 side is equal to or greater than the minimum width that can be produced. The width of this tip is, for example, 200 nm.

[0039] Here, the width of the rib waveguide (the combined width of the ridge portion 110 and the terrace portion 120) at the end (input end) of the input-side polarization conversion unit 100 connected to the input unit 400 is, for example, 500 nm. Also, the width of the rib waveguide (the width of the terrace portion 120) at the end (output end) on the grating unit 200 side is 1100 nm.

[0040] As described above, the width of the rib waveguide at the input end is 500 nm, which is larger than the 300 nm width of the ridge portion 110. In this way, the width of the terrace portion 120 is an infinitesimal portion, that is, there is no portion on either side of the ridge portion 110 where the terrace portion 120 is not formed, and the structure is such that the terrace portion 120 is provided on both sides of the ridge portion 110. In addition, the terrace portion 120 is formed with a width equal to or greater than the minimum width that can be produced, and for example, at the input end, it is formed with a width of 100 nm on both sides of the ridge portion 110.

[0041] A tapered structure that has been generally proposed is a structure that gradually decreases to an infinitesimal width that is smaller than the minimum width that can be produced, but it is difficult to actually produce it.

[0042] In contrast, the optical waveguide element of the present invention does not have a tapered structure in which the width gradually decreases to an infinitesimal width, and even the narrowest part is equal to or larger than the minimum width that can be produced, making it easy to produce.

[0043] This structure of the input side polarization conversion section 100 ensures sufficient width for the terrace portion 120 and the tapered portion 114 for fabrication, and has been found to provide good characteristics through simulations.

[0044] The operation of the input-side polarization conversion unit 100 will be described with reference to Fig. 3. Fig. 3 is a diagram for explaining the operation of the input-side polarization conversion unit 100, and is a diagram for explaining the relationship between the propagation distance and the equivalent refractive index in the polarization conversion unit 100 for the region where the ridge portion 110 is provided. In Fig. 3, the horizontal axis represents the propagation distance, and the vertical axis represents the equivalent refractive index.

[0045] As the propagation distance increases, i.e., as light propagates toward the grating section 200, the equivalent refractive index of the TE1 mode (TE1) increases. On the other hand, as the propagation distance increases, i.e., as light propagates toward the grating section 200, the equivalent refractive index of the TM fundamental mode (TM0) decreases. If the ridge section 110 is designed so that the equivalent refractive index of the TE1 mode and the equivalent refractive index of the TM fundamental mode are equal near the middle of the propagation direction, polarization conversion occurs between the TM fundamental mode and the TE1 mode.

[0046] The equivalent refractive indexes of the TM fundamental mode and the TE first-order mode are closest when the width of the ridge portion 110 is 350 nm and the width of the rib waveguide is 700 nm, near the region where the wide portion 112 and the tapered portion 114 are connected in the longitudinal direction.

[0047] When light in the TE fundamental mode and the TM fundamental mode is input from the input unit 400 to the input-side polarization conversion unit 100, the TE fundamental mode passes through the input-side polarization conversion unit 100 as is and is sent as TE polarization to the grating unit 200 while remaining in the TE fundamental mode. On the other hand, the TM fundamental mode is converted to the TE first-order mode by the input-side polarization conversion unit 100 and then sent as TE polarization to the grating unit 200.

[0048] The input section 400 is provided with a width of, for example, 440 nm, which satisfies the single-mode condition, but the width changes toward the end connected to the polarization conversion section 100. The input waveguide 300 is generated as a thick portion with the same thickness as the ridge portion.

[0049] FIG. 1 shows an example in which the width of the end of the input section 400 connected to the input polarization conversion section 100 is 300 nm, which is the width of the end of the ridge section 110 connected to the input waveguide 300. In this case, the width of the input section 400 narrows toward the end connected to the input polarization conversion section 100. Note that the width of the end of the input section 400 connected to the input polarization conversion section 100 may be smaller than the width of the end of the rib waveguide connected to the input section 400 and larger than the width of the end of the ridge section 110 connected to the input section 400, for example, 450 nm. In this case, the width of the input waveguide 300 widens toward the end connected to the polarization conversion section 100. Note that the input waveguide 300 may be connected to the polarization conversion section 100 while maintaining its width of 440 nm, which satisfies the single-mode condition.

[0050] The input light propagating through the input unit 400 toward the input-side polarization conversion unit 100 is input to the input-side polarization conversion unit 100. The TE fundamental mode of the input light passes through the input-side polarization conversion unit 100 as is and is sent to the grating unit 200. On the other hand, the TM fundamental mode of the input light is converted to the TE first-order mode by the input-side polarization conversion unit 100 and then sent to the grating unit 200.

[0051] The grating section 200 has the same thickness as the terrace section 120, 150 nm in this example, and has antisymmetric gratings on both sides. Light of a desired wavelength determined by the grating period is diffracted and reflected by this grating section 200. The reflected light diffracted and reflected by the grating section 200 is sent to the input-side polarization conversion section 100.

[0052] If the widths of the input-side polarization conversion section 100 and the grating section 200 are different, the two can be connected by a tapered waveguide 902 that is the same thickness as the terrace section 120 of the input-side polarization conversion section 100 and the grating section 200, i.e., a thin tapered waveguide 902.

[0053] In this example, the waveguide width of the grating section 200 is 600 nm, and the grating is formed by 250 nm unevenness on the side surface. The grating period Λ is 420 nm, and near the start and end of the grating, the unevenness of the grating is small to prevent unnecessary diffraction, making it a so-called apodized structure.

[0054] The TE fundamental mode of the desired wavelength sent to the grating section 200 is converted to the TE first-order mode and reflected because the grating is antisymmetric. This reflected light in the TE first-order mode is converted to the TM fundamental mode by the input-side polarization conversion section 100 and sent to the input section 400. On the other hand, the input light in the TE first-order mode of the desired wavelength sent to the grating section 200 is converted to the TE fundamental mode and reflected. This reflected light in the TE fundamental mode passes through the input-side polarization conversion section 100 as is and sent to the input section 400.

[0055] Furthermore, of the input light sent to the grating section 200, light other than that of the desired wavelength passes through the grating section 200 as is and is sent to the output-side polarization conversion section 300. The output-side polarization conversion section 300 has the same configuration as the input-side polarization conversion section 100, and is provided in the opposite direction to the input-side polarization conversion section 100 with respect to the propagation direction, i.e., facing each other.

[0056] The output-side polarization conversion section 300 has a so-called rib waveguide structure, which is configured with ridge portions and terrace portions. The ridge portions are thick portions that extend along the longitudinal direction (the direction of light propagation). The terrace portions are thin portions. The ends of the ridge portions 110 and terrace portions 120 on the side where light is output are provided in the same plane perpendicular to the direction of light propagation.

[0057] The rib waveguide, particularly the terrace portion, is configured so that its width becomes thinner (narrower) from the grating section 200 toward the output waveguide 500 along the longitudinal direction.

[0058] The ridge portion is configured to have a tapered portion and a wide portion along the longitudinal direction, in that order from the grating section 200 side. The length of the ridge portion is shorter than the length of the polarization conversion section, i.e., the length of the terrace portion, and the end of the output-side polarization conversion section 300 on the grating section 200 side does not have a ridge portion, but is provided halfway along the longitudinal direction. Therefore, the end of the output-side polarization conversion section 300 on the grating section 200 side does not have a ridge portion and is composed only of terrace portions.

[0059] The input light in the TE fundamental mode sent to the output side conversion unit 300 passes through the output side polarization conversion unit 300 as is and is sent to the output unit 500. On the other hand, the input light in the TE first-order mode is converted to the TM fundamental mode and sent to the output unit 500.

[0060] Thus, in this optical waveguide element, input light in the TE fundamental mode remains in the TE fundamental mode, with light of the desired wavelength reflected by the input section 400, and light of wavelengths other than the desired wavelength being output to the output section 500. Furthermore, input light in the TM fundamental mode is converted to the TM fundamental mode as the TE first-order mode after the light of the desired wavelength is reflected and output to the input section 400, and light of wavelengths other than the desired wavelength passes through the grating section 200 as the TE first-order mode, is converted to the TM fundamental mode and output to the output section 500.

[0061] As described above, this optical waveguide element uses only TE polarization in the grating section 200 having wavelength selection function, and therefore can be used as a wavelength filter that operates independently of polarization, and does not require high manufacturing precision.

[0062] (Second waveguide element) An optical waveguide element according to a second embodiment of the present invention (hereinafter also referred to as a second optical waveguide element) will be described with reference to Fig. 4. Fig. 4 is a schematic diagram for explaining the second optical waveguide element. Fig. 2 is a schematic plan view of the second optical waveguide element, showing only the waveguide cores with the support substrate and lower cladding omitted.

[0063] In the following description, illustration and description of configurations that overlap with those of the first waveguide element may be omitted.

[0064] The second waveguide element has a grating section 202 that alternately connects antisymmetric grating sections 222, similar to the first waveguide element, and resonator sections 224, where no grating is formed. In this way, when the resonator section 224 is sandwiched between the antisymmetric grating sections 222, a resonator is formed with the resonator section 224 at its center, and resonates at a specific wavelength.

[0065] Furthermore, as shown in Figure 4, it is known that connecting two resonators can flatten the transmission wavelength peak and make the slope steeper.

[0066] The characteristics of the second optical waveguide element obtained using three-dimensional FDTD (Finite Difference Time Domain) will be described with reference to Fig. 5. Fig. 5 is a diagram showing the simulation results for the second optical waveguide element. In Fig. 5, the horizontal axis represents wavelength (unit: μm) and the vertical axis represents optical intensity (unit: dB).

[0067] Of the three antisymmetric grating sections 222, the length of the central antisymmetric grating section 222 was set to 51 periods, and the lengths of the other two antisymmetric grating sections 222 were set to 30 periods. In addition, the lengths of the two resonator sections 224 were both set to 37.5 periods.

[0068] In Fig. 5, the intensity of the input light input from the input section 400 is shown by curve I, the intensity of the reflected light output from the input section 400 is shown by curve II, and the intensity of the transmitted light output from the output section 500 is shown by curve III. As shown in Fig. 5, a characteristic unique to a resonator was obtained, in which multiple peaks were observed in the reflected light (II).

[0069] Furthermore, the results of a three-dimensional FDTD simulation showed that even if the widthwise position of the ridge portions of the input-side polarization conversion section 100 and the output-side polarization conversion section 300 is shifted by several tens of nanometers from the center of the rib waveguide, it does not have much effect on the polarization conversion.

[0070] (Third optical waveguide element) An optical waveguide element according to a third embodiment of the present invention (hereinafter also referred to as a third optical waveguide element) will be described with reference to Fig. 6. Fig. 6 is a schematic diagram for explaining an optical waveguide element. Fig. 6 is a schematic plan view of the third optical waveguide element, showing only the waveguide cores with the support substrate and lower cladding omitted.

[0071] In the first and second optical waveguide elements, the grating section is made of a waveguide core that is thinner than the ridge section of the rib waveguide, just like the terrace section of the rib waveguide. However, the process of thinning the thickness of the waveguide core may cause variations in thickness of the waveguide core.

[0072] In contrast, in the third optical waveguide element, the grating section 203 is composed of a waveguide core having the same thickness as the rib waveguide ridge section 110. That is, according to the third optical waveguide element, the grating section 203 is composed of a thick waveguide, so that a process for thinning the waveguide thickness of the grating section 203 is not required, and thickness variations in the grating section 203 can be suppressed.

[0073] Except for the thickness of the grating section 203 and the connection between the input-side polarization conversion section and the output-side polarization conversion section, respectively, and the grating section 203 using the thickness conversion structure, the first optical waveguide element and the second optical waveguide element can be configured in the same way, and therefore, redundant explanations may be omitted.

[0074] In order to connect the input side polarization conversion section 100 having a thin waveguide thickness to the grating section 203 having a thick waveguide thickness, a thickness conversion structure 600 is provided between the grating section 203 and the input side polarization conversion section 100.

[0075] In the thickness conversion structure 600, for example, a tapered section 612 having a thin waveguide thickness connected to the terrace portion 120 of the input-side polarization conversion section 100 and a tapered section 622 having a thick waveguide thickness connected to the grating section 203 are provided at a distance from each other and facing each other. The tapered section 612 having a thin waveguide thickness connected to the input-side polarization conversion section 100 narrows in width along the propagation direction of the input light, while the tapered section 622 having a thick waveguide thickness connected to the grating section 622 widens in width along the propagation direction of the input light.

[0076] Furthermore, sub-waveguides 632 are provided at positions sandwiching tapered section 612 with a thin waveguide thickness and tapered section 622 with a thick waveguide thickness. The sub-waveguide 632 has the same waveguide thickness as the grating section 203. The sub-waveguide 632 is configured by connecting a portion arranged in parallel with tapered section 612 with a thin waveguide thickness, which widens in the propagation direction of the input light, unlike tapered section 612 with a thin waveguide thickness, and a portion arranged in parallel with tapered section 622 with a thick waveguide thickness, which narrows in the propagation direction of the input light, unlike tapered section 622 with a thick waveguide thickness. The tapered section 612 with a thin waveguide thickness and the sub-waveguides 632 provided on both sides thereof form a Y-branch structure. Furthermore, the tapered portion 622, which has a thick waveguide thickness, and the sub-waveguides 632 provided on both sides thereof form an inverted Y-branch structure. Note that, although an example will be described here in which the sub-waveguide 632 is configured to have the same waveguide thickness as the grating portion 203, the thickness of the sub-waveguide 632 may be the same as the thickness of the tapered portion 612, which has a thin waveguide thickness.

[0077] The tapered section 612 with a thin waveguide thickness and the sub-waveguides 632 provided on both sides thereof are configured with dimensions that allow the TE fundamental mode and the TE first-order mode to be phase-matched. As a result, the TE fundamental mode and the TE first-order mode propagating through the tapered section 612 with a thin waveguide thickness transition to a symmetric mode and an antisymmetric mode propagating through the sub-waveguide 632.

[0078] Furthermore, the symmetric mode and antisymmetric mode propagating through the sub-waveguide 632 are converted into the TE fundamental mode and TE first-order mode propagating through the tapered portion 622 where the waveguide thickness is thin.

[0079] The tapered sections 612 and 622 and the sub-waveguide 632 can be based on any suitable conventionally known design, but the width of the tip is formed to the minimum width that can be produced, for example, 150 nm. The widest part of the sub-waveguide is formed to, for example, 440 nm, which satisfies the single-mode condition.

[0080] Here, we have explained an example in which the input-side polarization conversion section 100 and the grating section 203 are connected by a thickness conversion structure 600, but the output-side polarization conversion section 300 and the grating section 203 can also be connected in a similar manner, so duplicate explanations will be omitted.

[0081] (Fourth optical waveguide element) An optical waveguide element according to a fourth embodiment of the present invention (hereinafter also referred to as a fourth optical waveguide element) will be described with reference to Fig. 7. Fig. 7 is a schematic diagram for explaining the fourth optical waveguide element. Fig. 7(A) is a schematic plan view of the fourth optical waveguide element, showing only the waveguide cores with the support substrate and lower clad omitted. Fig. 7(B) is a diagram showing a cut end surface of the fourth optical waveguide element taken along line AA, showing only the waveguide cores with the support substrate and lower clad omitted.

[0082] In the fourth optical waveguide element, the grating section 204 is made of silicon nitride (SiN). In this case, the input polarization conversion section and the output polarization conversion section are formed with silicon optical waveguide cores, and then an upper clad covering the input polarization conversion section and the output polarization conversion section is formed on the lower clad 20. Furthermore, the grating section 204 is formed on the upper clad with a SiN waveguide core that is 2 μm wide and 400 nm thick. The grating formed in the SiN waveguide of the grating section 204 is formed with an unevenness of, for example, 500 nm.

[0083] If the grating section 204 is made of a SiN waveguide core, the temperature dependency of the diffracted wavelength can be reduced.

[0084] A transition terrace 710 is provided on the grating section 204 side of the input-side polarization conversion section 100. The transition terrace 710 is an extension of the terrace 120 of the input-side polarization conversion section 100, and is formed so that its width narrows in the direction toward the grating section 204. The width of the tip of the transition terrace 710 on the grating section side is the smallest width that can be made, for example, 150 nm.

[0085] In the region where the transitional terrace portion 710 is formed, a transitional SiN portion 720 is provided so as to overlap the Si waveguide core. The transitional SiN portion 720 is a linear waveguide portion that extends the grating portion 204 toward the input-side polarization conversion portion 100. The distance in the thickness direction between the transitional terrace portion 710 and the transitional SiN portion 720 is, for example, 400 nm.

[0086] The transition section 700, which comprises a transition terrace section 710 and a transition SiN section 720, is where the light transitions between the silicon waveguide core and the SiN waveguide core.

[0087] Here, the input polarization conversion section 100 and the grating section 204 have been described, but the output polarization conversion section 300 and the grating section 204 can also be connected in the same way, so duplicated description will be omitted.

[0088] Furthermore, except for the fact that the grating section 204 is made of a SiN waveguide and the connection between the input-side polarization conversion section and the output-side polarization conversion section and the grating section 204, the grating section 204 can be configured in the same way as the first optical waveguide element and the second optical waveguide element, and therefore redundant explanations may be omitted.

[0089] According to the present invention, which has been explained above using the first to fourth optical waveguide elements as examples, it is possible to obtain an element that operates polarization-independently and has a specified wavelength band, using fewer elements and without requiring high manufacturing precision. [Explanation of symbols]

[0090] 10 Support substrate 20 Clad 22 Lower Cladding 24 Upper Cladding 30 Waveguide core 100 Input side polarization conversion section 110 Ridge area 112 Wide section 114 Tapered part 120 Terrace area 200, 202, 203, 204 Grating section 300 Output polarization conversion section 400 Input section 500 Output Unit 600 thickness conversion structure 612, 622 tapered section 632 Sub-waveguide 700 Transition 710 Transitional Terrace 720 Transitional SiN section

Claims

1. An optical waveguide element in which a waveguide core is formed on a lower clad, and the waveguide core is embedded in a clad composed of the lower clad and an upper clad, an input polarization conversion unit, a grating unit, and an output polarization conversion unit arranged in series in this order; the input polarization conversion unit and the output polarization conversion unit are provided opposite to each other, the input polarization conversion unit and the output polarization conversion unit are each composed of a ridge portion and a terrace portion, the terrace portion having a thickness in a direction perpendicular to an upper surface of the lower cladding that is thinner than that of the ridge portion, On the input polarization conversion side, the ends of the ridge portion and the terrace portion on the light input side are provided in the same plane perpendicular to the light propagation direction, The length of the ridge portion along the light propagation direction is shorter than the length of the terrace portion along the light propagation direction, The ridge portion has a wide portion formed to widen or have a uniform width along the propagation direction of the input light, and a tapered portion configured to narrow along the propagation direction of the input light, arranged in that order along the propagation direction of the input light, and at the end where the wide portion and the tapered portion are connected, the widths of the wide portion and the tapered portion are equal to each other, and the end on the narrower side of the tapered portion is configured to have a width equal to or greater than the minimum width that can be produced, The terrace portion is configured to have a width that increases along the propagation direction of the input light, and is formed with a width that is equal to or greater than the minimum width that can be created at the end portion on the light input side, On the output polarization conversion side, the ends of the ridge portion and the terrace portion from which light is output are provided in the same plane perpendicular to the direction of light propagation, The length of the ridge portion along the light propagation direction is shorter than the length of the terrace portion along the light propagation direction, The ridge portion is provided with a tapered portion configured to widen in width along the propagation direction of the input light, and a wide portion configured to narrow in width or to have a uniform width along the propagation direction of the input light, in that order along the propagation direction of the input light, and at the end where the wide portion and the tapered portion are connected, the widths of the wide portion and the tapered portion are equal to each other, and the end on the narrower side of the tapered portion is configured with a width equal to or greater than the minimum width that can be produced, the terrace portion is configured to have a width that narrows along the propagation direction of the input light, and is formed with a width that is equal to or greater than the minimum width that can be created at the end portion on the light output side; The grating section has antisymmetric gratings on both sides of the optical waveguide core. Optical waveguide element.

2. The grating portion is formed to the same thickness as the terrace portion. The optical waveguide element according to claim 1 .

3. a tapered waveguide formed to the same thickness as the terrace portion is provided between the input polarization conversion portion and the output polarization conversion portion and the grating portion, the widths of the tapered waveguide and the input-side polarization conversion unit at an end where the tapered waveguide and the input-side polarization conversion unit are connected are equal to each other; the widths of the tapered waveguide and the output-side polarization conversion unit at an end where the tapered waveguide and the output-side polarization conversion unit are connected are equal to each other; The widths of the tapered waveguide and the grating section at the end where the tapered waveguide and the grating section are connected are equal to each other. The optical waveguide element according to claim 2 .

4. the grating portion is formed to the same thickness as the ridge portion, A thickness conversion structure is provided between the input polarization conversion section and the output polarization conversion section and the grating section. The optical waveguide element according to claim 1 .

5. The thickness conversion structure is a tapered section having a thin waveguide thickness, the tapered section having the same thickness as the terrace section in a direction from the terrace section toward the grating section, the width of which decreases toward the grating section, and the width of the tip end of the tapered section being equal to or greater than the minimum width that can be produced; a tapered section having a thick waveguide thickness, the tapered section having the same thickness as the grating section in a direction from the grating section toward the terrace section, the width of which narrows toward the terrace section, and the width of the tip end of which is equal to or greater than the minimum width that can be produced; a pair of sub-waveguides, the same as the grating section, at positions sandwiching the tapered section with the thin waveguide thickness and the tapered section with the thick waveguide thickness; Equipped with 5. The optical waveguide element according to claim 4.

6. the grating section is formed of an optical waveguide core of a silicon nitride film, and the input polarization conversion section and the output polarization conversion section are formed of silicon optical waveguide cores, the silicon nitride film optical waveguide core is provided spaced apart from the silicon optical waveguide core in the thickness direction; a transition terrace section formed by extending a terrace portion and tapering in a direction toward the grating section is provided on the grating section side of the input polarization conversion section and the output polarization conversion section, and the width of the tip of the transition terrace section on the grating section side is the smallest width that can be created; In the region where the transition terrace portion is provided, a transition SiN portion is provided on the input-side polarization conversion portion and output-side polarization conversion portion side of the grating portion, the transition SiN portion being an extension of the grating portion. The optical waveguide element according to claim 1 .

7. The grating section is one or more resonance regions in which no grating is formed; the regions in which the gratings are formed and the resonance regions are alternately arranged along the longitudinal direction, One of the resonance regions or each of the plurality of resonance regions is sandwiched between regions in which a grating is formed. The optical waveguide element according to any one of claims 1 to 6.

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