Optical circuit

Abstract

An optical circuit according to the present disclosure can be connected with an optical fiber and comprises an optical signal processing region and an intermediate region of an end portion connected with the fiber. The intermediate region comprises a polarization converter and a polarization separator connected with the fiber. In addition, the optical circuit can include a first MFD converter between the polarization separator and the fiber, and a second MFD converter between the polarization converter and a waveguide of the optical signal processing region. The polarization separator separates input light from the fiber into a first polarized wave and a second polarized wave that are orthogonal to each other, and the polarization converter converts the separated second polarized wave into the first polarized wave. The separated first polarized wave and the polarization-converted first polarized wave are individually input to the same or corresponding separate optical signal processing circuits.

Description

optical circuit

[0001] The present invention relates to an optical signal processing device and an optical circuit.

[0002] In recent years, waveguide devices using SiN or Si as core materials have been adopted in optical communication networks and have become key devices. One of the challenges of these waveguide devices is the significant polarization dependency of their fundamental optical signal processing characteristics. For example, polarization-dependent loss (PDL) becomes significant when light propagates through a waveguide (see Non-Patent Document 1). Another major issue is the polarization dependency of the extinction wavelength of optical interferometers and other optical interference circuits due to the difference in refractive index (birefringence) between different polarizations.

[0003] To address the polarization dependency of waveguide devices, a method is known in which the main polarization state for optical signal processing is unified into one polarization. For example, by separating the polarization of input light into TE polarization and TM polarization and converting the TM polarization to TE polarization, optical signal processing is performed using only TE polarization, thereby eliminating the polarization dependency of waveguide devices.

[0004] Furthermore, compared to the refractive index difference between the core and cladding of typical optical fibers used in optical communications, the refractive index difference between the core and cladding of a waveguide with a SiN or Si core is large. As a result, there is a difference of more than 10 times between the mode-field diameter (MFD) of the propagation mode of light propagating through the waveguide and the MFD of the fiber. If a waveguide device is directly connected to a fiber, a large connection loss will occur (Non-Patent Document 2).

[0005] In order to reduce the connection loss due to MFD mismatch, a method has been reported in which a waveguide element (interposer) with an intermediate MFD is placed between a fiber and a waveguide with a high refractive index difference, such as a Si-core waveguide (Non-Patent Document 3). By introducing input light from the fiber into the waveguide via the interposer, the MFD mismatch at each connection surface can be reduced, and the connection loss of the entire optical circuit can be suppressed.

[0006] Keijiro SUZUKI, Ryotaro KONOIKE, Satoshi SUDA, Hiroyuki MATSUURA, Shu NAMIKI, Hitoshi KAWASHIMA, Kazuhiro IKEDA, Strictly Non-Blocking Silicon Photonics Switches, IEICE Transactions on Electronics, 2020, Volume E103.C, Issue 11, Pages 627-634Arnab Dewanjee, Jan Niklas Caspers, James Stewart Aitchison, and Mo. Mojahedi, "Demonstration of a compact bilayer inverse taper coupler for Si-photonics with enhanced polarization insensitivity," Opt. Express 24, 28194-28203 (2016)J. Hasegawa et al., "32-Port 5.5%-Δ Silica-Based Connecting Device for Low-Loss Coupling between SMFs and Silicon Waveguides," 2018 Optical Fiber Communications Conference and Exposition (OFC), San Diego, CA, USA, 2018, pp. 1-3.

[0007] However, waveguides with SiN or Si cores have large propagation losses and losses as optical elements, and losses caused by polarization separation and polarization conversion are issues. Furthermore, when the MFD difference between the connection surfaces of the above-mentioned interposers is large, multiple types of interposers must be cascaded to gradually change the MFD. Furthermore, the polarization separation and conversion functions have had to be performed using waveguides with high refractive index differences. There is a demand for optical circuit configurations that suppress the polarization dependency issues in optical circuits connected to fibers and the connection losses caused by MFD mismatch.

[0008] One embodiment of the present invention is an optical circuit to be connected to a fiber, comprising: an intermediate region in which a waveguide having a first refractive index difference is formed in a first substrate and to which the fiber can be optically connected; and an optical signal processing region in which a waveguide having a second refractive index difference larger than the first refractive index difference is formed in a second substrate and which has two identical or corresponding optical signal processing circuits, wherein the optical circuit is configured so that signal light propagates with the same polarization between the intermediate region and each of the two optical signal processing circuits.

[0009] An optical circuit is provided that is connected to a fiber and suppresses polarization dependent loss and connection loss due to MFD mismatch.

[0010] FIG. 1 is a diagram illustrating the configuration of an optical circuit of a first embodiment. FIG. 2 is a diagram illustrating the configuration of an optical circuit of a second embodiment. FIG. 3 is a diagram illustrating the configuration of an optical circuit of a third embodiment. FIG. 4 is a diagram illustrating the configuration of an optical circuit of a fourth embodiment. FIG. 5 is a diagram illustrating the configuration of a polarization separator using a waveguide structure in an optical circuit of the present disclosure. FIG. 6 is a diagram illustrating the configuration of a polarization converter using a waveguide structure in an optical circuit of the present disclosure. FIG. 7 is a diagram illustrating the configuration of a waveguide near the boundary of an intermediate region in an optical circuit of the present disclosure. FIG. 8 is a diagram illustrating a specific configuration of an intermediate region using a waveguide structure in an optical circuit of the present disclosure.

[0011] The optical circuit of the present disclosure is connectable to an optical fiber and includes an intermediate region at the end of the optical circuit that connects to the fiber and an optical signal processing region. The intermediate region includes a polarization splitter and a polarization converter connected to the fiber. The optical circuit may further include a first MFD converter between the polarization splitter and the fiber, and a second MFD converter between the polarization converter and the waveguide in the optical signal processing region. The polarization splitter splits input light from the fiber into orthogonal first and second polarizations, and the polarization converter converts the split second polarization into a first polarization. The split first polarization and the polarization-converted first polarization are input to separate optical signal processing circuits (blocks) having the same or corresponding optical signal processing functions. The first and second MFD converters in the intermediate region gradually reduce the MFD from the fiber toward the waveguide in the optical signal processing region of the optical circuit.

[0012] In the following description, the "refractive index difference Δ" is the refractive index of the core １ , the refractive index of the cladding n ２ When this is the case, the refractive index difference is expressed by the following equation: The refractive index difference is also called the relative refractive index difference.

[0013] The refractive index difference Δcircuit of the waveguide in the optical signal processing region of the optical circuit disclosed herein is greater than the refractive index difference Δfiber of the fiber connected to the optical circuit. The optical signal processing region of the optical circuit disclosed herein includes a waveguide having a higher refractive index difference than the fiber, for example, a waveguide with a SiN or Si core. In the following description, the optical circuit disclosed herein is shown connected to a fiber, but it should be noted that the main characteristic features are in the intermediate region. The following embodiments can be combined, some components can be omitted or modified, or additional components can be implemented.

[0014] In the following description, the optical circuit is described as a configuration (receiver) in which an optical signal is input from an external fiber, but a configuration (transmitter) in which an optical signal is output from the optical circuit is also possible.

[0015] [First Embodiment] FIG. 1 is a diagram illustrating the configuration of an optical circuit according to a first embodiment. (a) of FIG. 1 illustrates the configuration of the fiber and waveguides of the optical circuit 100-1 in a cross section (x-z plane) perpendicular to the substrate surface of the optical circuit 100-1. (b) of FIG. 1 illustrates a schematic diagram of the function of the optical circuit 100-1 in the substrate surface (y-z plane). The positional relationship between the cross-sectional view in (a) and the functional diagram in (b) roughly corresponds, but is not exact. Furthermore, (a) illustrates the cross-sectional structure of the waveguide along each waveguide, and is not a flat cross section. Furthermore, only a portion of the substrate is shown in the thickness direction, and the relationship to the height of the waveguide is not accurate. Furthermore, please note that the scale relationship between the thickness direction (x-axis) and the light propagation direction (z-direction) differs from that of an actual device.

[0016] Referring to FIG. 1A, optical circuit 100-1 comprises an intermediate region 20 formed on substrate 25 and an optical signal processing region 30 formed on substrate 35. In intermediate region 20, a waveguide consisting of cladding 5 and core 2 is formed on substrate 25, and has a refractive index difference ΔMid. In optical signal processing region 30, a waveguide consisting of cladding 6 and core 3 is formed, and has a refractive index difference Δcircuit. Intermediate region 20 is located at the end of the optical circuit, and is connected to fiber 10 at the end face of intermediate region 20. Fiber 10 consists of core 1 and cladding 4, and has a refractive index difference Δfiber. The refractive index differences of the respective portions have the following relationship: Δfiber < ΔMid < Δcircuit Equation (2)

[0017] As mentioned above, in conventional waveguide devices, an interposer is inserted between the fiber to mitigate losses due to MFD mismatch. Even for small-core optical fibers with a small MFD, the MFD of a silicon waveguide that satisfies the single-mode condition is several hundred nanometers. The MFD of single-mode fibers commonly used in optical communications is approximately 10 μm, further increasing the MFD mismatch with a silicon waveguide.

[0018] In the optical circuit of the present disclosure, an intermediate region 20 having an intermediate MFD is formed at the end of the optical circuit that is connected to the fiber. fiber and the MFD of the optical signal processing region 30 circuit MFD of the median value of Mid and the following relationship exists: fiber > MFD Mid > MFD circuit Formula (3)

[0019] By forming an intermediate region 20 with an intermediate MFD at the end of the optical circuit, it is possible to mitigate losses due to MFD mismatch. Furthermore, by configuring a polarization separator 21 and a polarization converter 22 within the intermediate region 20, it is possible to eliminate the polarization dependency of the basic characteristics related to optical signal processing. MidIn order to realize this, the substrate 25 of the intermediate region 20 and the substrate 35 of the optical signal processing region 30 can be made of different materials, and the two substrates can be bonded to create the optical circuit 100-1. Also, by using substrates made of the same material for the intermediate region 20 and the optical signal processing region 30, the optical circuit 100-1 can be made Mid The substrate 25 of the intermediate region 20 may be made of Si, SiO ２ The substrate 35 of the optical signal processing region 30 may be made of Si, SiO ２ etc.

[0020] 1B, within the substrate plane, the intermediate region 20 includes a polarization separator 21 and a polarization converter 22. One output port of the polarization separator 21 is connected to the polarization converter 22. The other output port of the polarization separator 21 and the output of the polarization converter 22 are connected to the optical signal processing region 30.

[0021] The polarization separator 21 separates an optical signal input from the fiber 10 into two orthogonal polarizations. For example, it separates input light 51, which contains a mixture of TE and TM polarizations, into TE polarization 52 and TM polarization 53. The polarization separator 21 can be realized, for example, by a polarization beam splitter using a dielectric multilayer film. If an element made of a material different from the substrate, such as a dielectric multilayer film, is used, the light must be temporarily output from the waveguide in the intermediate region 20. This configuration can cause loss of the optical signal. To reduce loss, the polarization separator 21 is preferably constructed solely with a waveguide structure. For example, a configuration is proposed in which a groove is placed only in one arm of a Mach-Zehnder interferometer (MZI) to utilize the birefringence difference between the arms (described later in the fifth embodiment). A configuration using an asymmetric directional coupler has also been proposed. These elements, constructed solely with a waveguide structure, can reduce loss in the polarization separator 21.

[0022] One output light of the polarization separator 21, for example, TM polarization 53, is input to the polarization converter 22. Like the polarization separator 21, the polarization converter 22 is preferably configured with a waveguide element that converts the polarization state of the input light (first polarization) into another polarization (second polarization) that is orthogonal to the input light. For example, a groove may be formed on one side of the waveguide to control the birefringence and optical axis of the waveguide, thereby controlling the polarization state and changing the angle of linear polarization (described later in the fifth embodiment). The polarization converter 22 only needs to have a polarization conversion function for one of the polarizations of the light split into two by the polarization separator 21.

[0023] The polarization converter 22 can adjust the two light beams separated by the polarization separator 21 to have the same polarization state. For example, as shown in FIG. 1B, when the input light is separated into TE polarization 52 and TM polarization 53 by the polarization separator 21, the TM polarization 53 can be polarization-converted to align the two light beams to TE polarization. Conversely, the TE polarization from the polarization separator 21 can be converted to TM polarization to align the two light beams to TM polarization.

[0024] By including the polarization separator 21 and the polarization converter 22, the polarization state of the input light to the optical signal processing region 30 can be set to a specific state. The input light, which contains a mixture of different polarizations from the fiber 10, is aligned to a specific polarization, and the same polarization is applied to the same or corresponding two optical signal processing blocks 31a and 31b. There is no need to consider polarization-dependent loss that occurs when connecting the external fiber 10 to the optical circuit 100-1, which is a waveguide device. Generally, when connecting a waveguide with a Si or SiN core to a waveguide or optical fiber with a quartz core, TE polarization tends to have a smaller connection loss than TM polarization. Therefore, by aligning the polarization state to TE polarization at the connection point between the intermediate region 20 and the optical signal processing region 30, the connection loss itself can be further reduced.

[0025] Since optical circuit 100-1 is configured to have an intermediate region 20 and can handle only a specific polarization state in optical signal processing region 30, an optical circuit with reduced polarization dependency can be realized. Therefore, the optical circuit of the present disclosure can be implemented as an optical circuit to be connected to a fiber, comprising: an intermediate region 20 in which a waveguide core 2 having a first refractive index difference is formed in a first substrate 25 and to which the fiber can be optically connected; and an optical signal processing region 30 in which a waveguide core 3 having a second refractive index difference larger than the first refractive index difference is formed in a second substrate 35 and which has two identical or corresponding optical signal processing circuits, and configured so that signal light propagates with the same polarization (TE polarization) between the intermediate region and each of the two optical signal processing circuits 31 a, 31 b.

[0026] 1B, the polarization separator 21 and the polarization converter 22 are shown as separate elements connected by a waveguide, but the entire intermediate region 20 can also be configured as an integrated unit. For example, by combining an asymmetric directional coupler and a tapered waveguide, the functions of the polarization separator 21 and the polarization converter 22 can be integrated into an integrated unit. A specific configuration example of the polarization separator 21 and the polarization converter 22 that utilizes a waveguide structure will be further described in the fifth embodiment.

[0027] In addition, in FIG. 1B, the polarization separator 21 and the polarization converter 22 are arranged in a generally straight line, but they can also be arranged so that the light propagation direction is non-linear by inserting a curved waveguide or the like between them.

[0028] The optical circuit 100-1 of this embodiment assumes that the input light 51 from the fiber 10 contains a mixture of multiple polarization states. One example is polarization multiplexed light that has propagated through a long-distance optical fiber and has an indefinite polarization state. However, the input light from the fiber 10 may contain only one polarization state.

[0029] Furthermore, although the optical circuit 100-1 of this embodiment is intended for a receiving operation in which input light is provided from the fiber 10 to the optical signal processing region 30, it can also be applied to a transmitting operation in which signal light is output from the optical signal processing region 30 to the fiber 10. For example, in the optical signal processing region 30, polarization multiplexed light can be output to the fiber 10 from light of a semiconductor laser (not shown) that outputs light in a specific linear polarization state. Furthermore, by providing the optical signal processing region 30 with a modulation function, a polarization multiplexed optical signal can also be output to the fiber 10.

[0030] In conventional optical circuits, the separation and conversion of TM and TE polarizations are performed in an optical signal processing region having a high refractive index difference waveguide. In contrast, in the optical circuit disclosed herein, a polarization splitter and a polarization converter are integrated into an intermediate region 20 having an intermediate refractive index difference ΔMid between the fiber and the optical signal processing region. As shown in equation (3), the refractive index difference ΔMid of the intermediate region 20 is smaller than the refractive index difference Δcircuit of the optical signal processing region 30, which performed polarization separation and polarization conversion in conventional technology. Generally, waveguides with a high refractive index difference have large manufacturing errors in various optical characteristics. On the other hand, waveguides with a smaller refractive index difference have relatively small variations in optical characteristics and can be manufactured stably. Furthermore, high refractive index difference waveguides have strong optical confinement, which increases the intensity of light propagating within the waveguide, thereby increasing the impact of unintended nonlinear effects.

[0031] Even if a polarization splitter or a polarization converter is fabricated in a waveguide with a high refractive index difference, the influence of the above-mentioned variations and nonlinear effects cannot be ignored in the waveguides surrounding them. By consolidating the polarization splitter 21 and the polarization converter 22 in the intermediate region 20 with a smaller refractive index difference ΔMid, as in the optical circuit of the present disclosure, the influence of manufacturing variations and nonlinear effects can be significantly suppressed.

[0032] Second Embodiment In the optical circuit of the first embodiment described above, the intermediate region 20 is provided with the polarization separator 21 and the polarization converter 22, but it may also be provided with a function of converting the mode diameter.

[0033] FIG. 2 is a diagram illustrating the configuration of an optical circuit according to a second embodiment. (a) of FIG. 2 shows the configuration of the fiber and optical circuit waveguides in a cross section (x-z plane) perpendicular to the substrate surface of the optical circuit 100-2. (b) of FIG. 2 shows a schematic diagram of the function of the optical circuit 100-2 in the substrate surface (y-z plane). The positional relationship between the cross-sectional view in (a) and the functional diagram in (b) roughly corresponds, but is not exact. Furthermore, (a) shows the cross-sectional structure of the waveguide along the waveguides of each component, and is not a flat cross section. It should be noted that the scale of each component in FIG. 2 differs from that of the actual device, as in FIG. 1.

[0034] 1 in that the optical circuit 100-2 further includes two MFD converters 23-1 and 23-2 in the intermediate region 20. Referring to the cross-sectional view of FIG. 2A, the optical circuit 100-2 includes an MFD converter 23-1 that gradually lowers the core height on the fiber 10 side of the intermediate region 20. In addition, the optical circuit 100-2 includes an MFD converter 23-2 that gradually lowers the core height from the polarization separator 21 and polarization converter 22 on the optical signal processing region 30 side of the intermediate region 20.

[0035] By providing the MFD converter 23-1, which is a waveguide whose core height changes in the light propagation direction, the MFD of the fiber 10 is converted to an MFD suitable for propagation through the polarization separator and polarization converter in the intermediate region 20. If light is input to the intermediate region 20 while the MFD is large, higher-order mode excitation may occur. The MFD converter 23-1 can suppress the adverse effects on optical characteristics caused by higher-order mode excitation. Furthermore, by providing the MFD converter 23-2, the MFD is converted to an extremely small MFD suitable for connection with the waveguide in the optical signal processing region 30. The MFD converter 23-1 reduces connection loss at the interface with the fiber 10, and the MFD converter 23-2 reduces connection loss at the interface with the optical signal processing region 30.

[0036] The MFD converters 23-1 and 23-2 in the optical circuit 100-2 in Figure 2 have core heights that are different from those in the other regions of the intermediate region 20. Such a structure in which the core height is inclined along the waveguide can be achieved by, for example, etching the core in multiple steps or using locally generated plasma. Specifically, the core is first etched to a height appropriate for the MFD converter 23-1 on the fiber side, creating the inclined core 2-1. Next, the core is etched to a height appropriate for the polarization separator 21 and the polarization converter 22. Furthermore, the core is etched to a height appropriate for the MFD converter 23-2 on the optical signal processing region 30, creating the inclined core 2-2. Waveguides with different core heights can be fabricated using a step-by-step etching process.

[0037] In the optical circuit of this embodiment, in addition to the configuration for eliminating polarization dependency, an MFD converter that eliminates MFD mismatch is included in the intermediate region 20. The MFD converter 23-2 on the optical signal processing region 30 side of the polarization separator 21 only needs to convert the MFD for the TE polarization. Because the size of the MFD also depends on polarization, the MFD converter 23-2 makes it possible to design an optimal optical circuit specialized for the TE polarization.

[0038] [Third Embodiment] The optical circuit of the second embodiment described above has MFD converters using waveguides with inclined core heights on both sides of the intermediate region 20. For reasons of the optical circuit configuration or process, the MFD converters may be provided on only one side, or may be replaced by other configurations.

[0039] FIG. 3 is a diagram illustrating the configuration of an optical circuit according to a third embodiment. (a) of FIG. 3 illustrates the configuration of the fiber and waveguides of the optical circuit in a cross section (x-z plane) perpendicular to the substrate surface of the optical circuit 100-3. (b) of FIG. 3 illustrates a schematic diagram of the function of the optical circuit 100-3 in the substrate surface (y-z plane). The positional relationship between the cross-sectional view in (a) and the functional diagram in (b) roughly corresponds, but is not exact. Furthermore, (a) illustrates the cross-sectional structure of the waveguide along the waveguide of each component, and is not a flat cross section. It should be noted that the scale of each component in FIG. 3 differs from that of the actual device, as in FIGS. 1 and 2.

[0040] The difference from the optical circuit 100-2 in Figure 2 is that the fiber-side MFD converter 23-1 is realized with a configuration different from that of a tilted core. Fabricating tilted cores in multiple locations requires time and cost. In the intermediate region 20 of the optical circuit 100-3, a core 2-2 with a tilted height is used only in the MFD converter 23-2 on the optical signal processing region 30 side. The MFD converter 23-1 on the fiber 10 side increases the MFD toward the fiber 10 without changing the core height, thereby suppressing the connection loss between the fiber 10 and the intermediate region 20. A tapered structure in which the core width gradually narrows toward the fiber 10 side is one example of a configuration in which the MFD increases toward the fiber 10 side. Furthermore, by configuring the waveguide in a segmented shape with repeated regions with and without cores, it is possible to weaken confinement and increase the MFD.

[0041] 3, only the MFD converter 23-2 on the optical signal processing region 30 side is realized with an inclined core, but conversely, only the MFD converter 23-1 on the fiber side may be realized with an inclined core.Furthermore, both MFD converters 23-1 and 23-2 may be realized by a method other than an inclined core.

[0042] [Fourth Embodiment] In the optical circuits of the above-described embodiments, an MFD converter is provided in the intermediate region 20, and the MFD of the waveguide in the intermediate region 20 is brought closer to the MFD of the fiber 10 and the MFD of the waveguide in the optical signal processing region 30. In order to reduce connection loss due to MFD mismatch, the fiber 10 or the optical signal processing region 30 may be provided with an MFD conversion function.

[0043] FIG. 4 is a diagram illustrating the configuration of an optical circuit according to a fourth embodiment. (a) of FIG. 4 illustrates the configuration of the fiber and waveguides of the optical circuit in a cross section (x-z plane) perpendicular to the substrate surface of the optical circuit 100-4. (b) of FIG. 4 illustrates the function of the optical circuit 100-4 in the substrate surface (y-z plane). The positional relationship between the cross-sectional view in (a) and the functional diagram in (b) roughly corresponds, but is not exact. Furthermore, (a) illustrates the cross-sectional structure of the waveguide along the waveguide of each component, and is not a flat cross section. It should be noted that the scale of each component in FIG. 4 differs from that of the actual device, as in FIGS. 1 to 3.

[0044] 2 and 3 are as follows: The intermediate region 20 of the optical circuit 100-4 has the same configuration as the intermediate region of the optical circuit 100-3 of the third embodiment. That is, the MFD converter 23-1 on the fiber 10 side has a different configuration from the inclined core, and only the MFD converter 23-2 on the optical signal processing region 30 side uses a core 2-2 with an inclined height.

[0045] A TEC fiber is disposed at the end of the fiber 10 connected to the optical circuit 100-4. A TEC (Thermally Expanded Core) fiber is a fiber in which the MFD of a single-mode optical fiber is locally expanded by two to three times, and can reduce connection loss.

[0046] The optical signal processing region 30 also includes an MFD converter 32 on the side of the intermediate region 20. Specifically, similar to the MFD converter 23-1 described in the third embodiment, a tapered structure can be used in which the width of the core 3 gradually narrows toward the intermediate region 20.

[0047] By combining the MFD converters of the second to fourth embodiments and bringing the MFDs closer from both sides at the boundaries between the fiber and the intermediate region and between the intermediate region and the optical signal processing region, connection loss can be further reduced.

[0048] Fifth Embodiment This embodiment presents a more specific configuration of the polarization splitter and polarization converter in the optical circuit of the present disclosure. As described in the first embodiment, the polarization splitter and polarization converter in the intermediate region 20 can be realized by utilizing a waveguide structure, thereby reducing loss. Furthermore, it is undesirable to increase the number of processes required to fabricate the polarization splitter and polarization converter; therefore, it is desirable to have a configuration that allows both the polarization converter and the polarization splitter to be fabricated within the same process.

[0049] FIG. 5 is a diagram showing the configuration of a polarization splitter utilizing a waveguide structure in an optical circuit according to the present disclosure. FIG. 5(a) shows the waveguide pattern of the polarization splitter as viewed from the substrate surface (y-z plane) of the optical circuit, and FIG. 5(b) shows a cross-sectional view (x-y plane) taken along line Vb-Vb. The polarization splitter 21 is an MZI consisting of cores 2a and 2b of two arm waveguides, with grooves 7a and 7b formed in one of the arm waveguides. Input light 51 is input to one port of the MZI. TE polarization 52 is output from one output port, and TM polarization 53 is output from the other output port. As shown in FIG. 5(b), the MZI-based polarization splitter 21 is composed of a cladding 5 and cores 2a and 2b formed on a substrate 25 in the intermediate region 20, and can be realized in the form of a waveguide.

[0050] FIG. 6 is a diagram showing the configuration of a polarization converter using a waveguide structure in an optical circuit according to the present disclosure. FIG. 6(a) shows the waveguide pattern of the polarization converter as viewed from the substrate surface (y-z plane) of the optical circuit, and FIG. 6(b) shows a cross-sectional view (x-y plane) taken along line VIb-VIb. The polarization converter 22 achieves polarization conversion by forming a groove 7c in the cladding along the core 2c of a single waveguide. As shown in FIG. 6(b), the MZI-based polarization converter 22 is composed of a cladding 5 and a core 2c formed on a substrate 25 in the intermediate region 20, and can be realized in the form of a waveguide, similar to the polarization separator 21 shown in FIG. 5.

[0051] By adopting a configuration in which grooves are formed in the clad around the waveguide, both the polarization separator 21 and the polarization converter 22 can be processed in a single groove forming process. As will be described later in the seventh embodiment, when a polarization converter in which a groove is formed in the clad around the waveguide is combined with a polarization separator in which a groove is formed in only one arm of the MZI, the groove can also be shared.

[0052] Sixth Embodiment This embodiment presents the configuration of a waveguide in an intermediate region in an optical circuit according to the present disclosure. As shown in each of the optical circuits illustrated in FIGS. 1 to 4 , the optical signal processing region 30 includes at least two identical or corresponding optical signal processing blocks 31a, 31b. A separated TE polarization 52 is input via a waveguide to one of the optical signal processing blocks, and a converted TE polarization 54 is input via another waveguide to the other of the optical signal processing blocks. Because there are multiple waveguides connecting the intermediate region 20 and the optical signal processing region 30, an optical circuit connecting to multiple fibers will have a large chip size. To reduce the chip size of the optical circuit, it is preferable that the distance between the two waveguides is narrow.

[0053] FIG. 7 is a diagram showing a waveguide configuration near the boundary of the intermediate region in the optical circuit of the present disclosure. FIG. 7 corresponds to an enlarged view of the vicinity of the MFD converter 23-2 in FIG. 2, in which the core height is gradually reduced. FIG. 7(a) shows the waveguide pattern viewed from the substrate surface near the boundary between the intermediate region 20 and the optical signal processing region 30, and FIG. 7(b) shows a cross section taken along the core 2-2 of one waveguide. The two TE polarized waves from the polarization splitter and polarization converter in the intermediate region 20 have their spacing adjusted by two bent waveguides and are passed to the optical signal processing region 30.

[0054] In order to configure a polarization splitter or polarization converter using a waveguide structure within the intermediate region 20, it is difficult to design the waveguides in close proximity. A spacing adjustment region 26 is provided near the boundary of the intermediate region 20 with the optical signal processing region 30, to adjust the waveguide spacing. As long as the cores 2-2 of the two waveguides, including the bent portion of the spacing adjustment region 26, have the same length and symmetrical shapes, there is no problem even if the core shape changes midway and the equivalent refractive index changes. The size of the intermediate region 20 can be reduced by gradually lowering the core height while bending in the spacing adjustment region 26. The spacing adjustment region 26 corresponds to the MFD converter 23-1.

[0055] [Seventh Embodiment] Figure 8 is a diagram showing a specific configuration of an intermediate region having a waveguide structure in an optical circuit according to the present disclosure. Figure 8 shows the waveguide patterns of a polarization converter and a polarization splitter viewed from the substrate surface of the optical circuit. The configuration of the intermediate region in Figure 8 combines a polarization splitter 21 having the waveguide structure shown in Figure 5 with a polarization converter 22 having the waveguide structure shown in Figure 6. Two grooves 7a and 7c are formed along the core 2a of one arm waveguide of the polarization splitter 21. The core 2c of the waveguide of the polarization converter 22 is formed by folding back from one output port of the MZI of the polarization splitter 21. One of the grooves 7c is also used as a groove formed along the core 2c of the waveguide of the polarization converter 22. The TM polarization 53 separated by the polarization splitter 21 is directly converted to TE polarization by the polarization converter 22.

[0056] 8, the element size can be reduced by using one of the two grooves in the polarization separator 21 as the groove for the polarization converter 22. By making the groove depths in the polarization converter and the polarization separator the same, the grooves can be formed simultaneously.

[0057] In the above-described embodiments, the optical circuit configuration has been described with respect to the operation in which signal light is input from the fiber 10 and provided to the optical signal processing region 30. It should be noted that it is also possible to reverse the direction of the optical signal, so that signal light is input from the optical signal processing region 30 and provided to the fiber 10, and in this case the polarization separator 21 operates as a polarization multiplexer. Generally, due to the reciprocity of optical circuits, a polarization separator can operate as a polarization multiplexer if the propagation direction of light is reversed. Similarly, in a polarization converter, if the propagation direction of light is reversed, the reverse polarization conversion is performed.

[0058] In addition, although the above-described embodiments show examples in which the fiber is connected to the end face of the optical circuit, the present disclosure can also be applied to configurations in which the optical connection or coupling is performed perpendicular or obliquely to the substrate surface within the intermediate region. As described above in detail, the optical circuit of the present disclosure makes it possible to suppress polarization dependent loss and connection loss due to MFD mismatch.

[0059] The present invention can be used in optical signal processing.

Claims

1. An optical circuit to be connected to a fiber, comprising: an intermediate region in which a waveguide having a first refractive index difference is formed on a first substrate and to which the fiber can be optically connected; and an optical signal processing region in which a waveguide having a second refractive index difference larger than the first refractive index difference is formed on a second substrate and which has two identical or corresponding optical signal processing circuits, wherein signal light propagates with the same polarization between the intermediate region and each of the two optical signal processing circuits.

2. The optical circuit according to claim 1, wherein the intermediate region includes a polarization separator that separates input light from the fiber into a first polarization and a second polarization orthogonal to the first polarization, and a polarization converter that converts the second polarization into the first polarization, and the separated first polarization from the polarization separator and the converted first polarization from the polarization converter are input to the two optical signal processing circuits.

3. The optical circuit according to claim 1, wherein the intermediate region includes a polarization converter that converts input light of a first polarization from one of the two optical signal processing circuits into a second polarization orthogonal to the first polarization, and a polarization multiplexer that multiplexes input light of the first polarization from the other of the two optical signal processing circuits with the converted second polarization from the polarization converter, and the multiplexed optical signal from the polarization multiplexer is output to the fiber.

4. The optical circuit according to claim 2 or 3, further comprising at least one of: a first MFD converter that brings the mode field diameter (MFD) of the fiber closer to the MFD of the waveguide in the intermediate region; or a second MFD converter that brings the MFD of the waveguide in the intermediate region closer to the MFD of the waveguide in the optical signal processing region.

5. The optical circuit according to claim 4, wherein the first MFD converter or the second MFD converter is a waveguide whose core height changes in the light propagation direction.

6. The optical circuit according to claim 2, wherein the first substrate and the second substrate are made of the same material, the first polarization is TE polarization, and the second polarization is TM polarization.

7. The optical circuit according to claim 2, wherein the polarization separator is a Mach-Zehnder interferometer (MZI) having two arm waveguides, the MZI having two grooves formed in the clad along one arm waveguide, and the polarization converter is connected to one output port of the MZI, is a waveguide parallel to the one arm waveguide, and shares one of the two grooves.

8. The optical circuit according to claim 3, wherein the polarization multiplexer is a Mach-Zehnder interferometer (MZI) having two arm waveguides, the MZI having two grooves formed in the clad along one arm waveguide, and the polarization converter is connected to one input port of the MZI, is a waveguide parallel to the one arm waveguide, and shares one of the two grooves.

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