Optical device

Abstract

The present invention comprises one or a plurality of optical interferometers including a plurality of arm waveguides (216) that guide respective light split at optical splitting units (330a, 331a, and 332a) to optical multiplexing units (332b, 331b, and 219), the light undergoing different phase changes between different arm waveguides. The optical splitting units and the optical multiplexing units are arranged in a direction different from the direction in which the end face of the optical splitting unit (330a, 331a or 332a) from which the light is outputted and the end face of the optical multiplexing unit (332b, 331b or 219) to which the light is inputted face each other. Each of the arm waveguides (216) has a shape along a path that includes a curved portion in accordance with the different direction mentioned above between the output end face of the optical splitting unit and the input end face of the optical multiplexing unit. The optical path length difference between different arm waveguides on the path including the curved portion is less than or equal to the wavelength of the light.

Description

Optical device

[0001] The present disclosure relates to an optical device.

[0002] With the expansion of data communication demands such as those of the Internet, higher capacity is required for optical communication networks. Furthermore, in recent years, the communication volume between data centers has been increasing exponentially, so there is a demand for increasing the transmission capacity of optical transceivers or miniaturizing them to improve space utilization efficiency. For example, in medium- and long-distance transmission, coherent optical communication that can ensure a sufficient transmission distance is mainly used.

[0003] Yusuke Nasu, Shogo Yamanaka, “Ultracompact Silicon Photonics Coherent Optical Subassembly for Ultrahigh-capacity Optical Communication”, NTT Technical Review, Vol. 18 No. 10 Oct. 2020.

[0004] For example, wavelength dependence may occur in an optical device due to an optical component (for example, an optical interferometer) arranged in an optical waveguide circuit that can be used for coherent optical communication.

[0005] One non-limiting object of the present disclosure is to provide an optical device capable of suppressing wavelength dependence.

[0006] An optical device according to one aspect of the present disclosure comprises one or more optical interferometers including an optical branching section, an optical multiplexing section, and a plurality of arm waveguides that guide each of the light branched in the optical branching section to the optical multiplexing section, wherein the light undergoes different phase changes between different arm waveguides, wherein the optical branching section and the optical multiplexing section are arranged in orientations different from the direction in which the output end face of the light from the optical branching section and the input end face of the light to the optical multiplexing section face each other, each of the arm waveguides has a shape along a path including a curved portion corresponding to the different orientation between the output end face of the optical branching section and the input end face of the optical multiplexing section, and the difference in optical path length between different arm waveguides in the path including the curved portion is less than or equal to the wavelength of the light.

[0007] This is a block diagram showing an exemplary configuration of a coherent transceiver. This is a block diagram showing an exemplary configuration of an optical device according to Embodiment 1. This is a block diagram showing another exemplary configuration of an optical device according to Embodiment 1. This is a cross-sectional view showing an exemplary schematic configuration of a connection part in an optical device according to Embodiment 2. This is a schematic exploded view of the configuration exemplified in Figure 4. This is a cross-sectional view showing a partially enlarged view of the configuration exemplified in Figure 4. This is a perspective view showing an exemplary configuration of a silicon waveguide substrate according to Modification 1 of Embodiment 2. This is a perspective view showing an exemplary configuration of a silicon waveguide substrate according to Modification 2 of Embodiment 2. This is a schematic diagram showing an exemplary manufacturing process of a silicon waveguide substrate according to Embodiment 3. This is a schematic diagram showing an exemplary manufacturing process of an optical phase modulation unit according to Embodiment 3. This is a schematic diagram showing an exemplary connection process between a silicon waveguide substrate and an optical phase modulation unit according to Embodiment 3. This is a cross-sectional view showing an exemplary schematic configuration of a connection part in an optical device according to Embodiment 4.

[0008] Embodiments will be described in detail below with reference to the drawings. However, the accompanying drawings and the following description are provided for the benefit of those skilled in the art to fully understand this disclosure and are not intended to limit the subject matter described in the claims. Furthermore, unnecessary details may be omitted. For example, detailed descriptions of already well-known matters or redundant descriptions of substantially identical configurations may be omitted.

[0009] Furthermore, in the drawings, identical or corresponding elements are appropriately denoted by the same reference numeral. The drawings are schematic, and the dimensional relationships or ratios of each element may differ from reality. There may also be differences in dimensional relationships or ratios between drawings. When numerical values ​​are given in the following explanation, these values ​​are merely examples, and other values ​​may be used additionally or as substitutes.

[0010] <Overview> Figure 1 is a block diagram showing an exemplary configuration of a coherent transceiver. As shown in Figure 1, the coherent transceiver includes, for example, a digital signal processing unit (DSP) 101, a local oscillator light source 104, an optical receiving unit 102, an optical transmitting unit (or optical modulation unit) 103, and an optical splitter 110.

[0011] The coherent transceiver illustrated in Figure 1 operates, for example, as follows: First, focusing on the transmitting side, a continuous wave (CW) light of any wavelength output from the local oscillator light source 104 is split into two at the optical splitter 110, and one of the split CW lights is input to the optical modulation unit 103.

[0012] The CW light input to the optical modulation unit 103 is further split into two at the optical splitter 110a, and each of the split CW light signals is input to two orthogonal (in-phase / quadrature-phase, IQ) modulators 109, which are shown with hatching in Figure 1.

[0013] Each of the two IQ modulators 109 modulates the CW light input from the optical splitter 108 according to the electrical modulation signal provided from the DSP 101. The modulation scheme is not limited, but could be a multi-level modulation scheme such as QPSK (Quadrature Phase Shift Keying) or QAM (Quadrature Amplitude Modulation).

[0014] The output modulated signal light from one of the two IQ modulators 109 is polarized by 90° in a Polarization Beam Combiner Rotator (PBCR) 109a, then polarized and combined with the output modulated signal light from the other IQ modulator 109, and output from the output port 111.

[0015] On the other hand, focusing on the receiving side of the coherent transceiver, the optical receiving unit 102 receives the received signal light from the input port 114 and the other CW light that has been split into two by the optical splitter 110.

[0016] The received signal light is separated into two orthogonal polarizations by a polarization beam splitter (PBS 108), and the polarization direction of one of these is rotated by 90° by a polarization rotator (PR) 108a. This 90° rotation causes the polarization direction of the polarized received signal light to match the polarization direction of the CW light.

[0017] Each polarization-separated received signal light is optically demodulated by the 90° optical hybrid 112, which is hatched in Figure 1, and in-phase and quadrature-phase components are extracted for each quadrature polarization component of the received signal light. Each of the extracted components is then individually received by a photodetector (PD) 113, and the transmitted signal information is reconstructed in the DSP 101 based on the electrically demodulated signal corresponding to the intensity of each received light.

[0018] Coherent transceivers are broadly classified into two types (or configurations) based on their transmission distance. One configuration is suitable for long-distance transmission and, for example, uses an optical front end called IC-TROSA (Integrated Coherent Transmitter Receiver Sub-Assembly).

[0019] IC-TROSA includes an optical modulation section using a material system capable of forming a phase shifter that enables high-speed phase modulation, such as indium phosphide (InP) or lithium niobate (LiNbO3,LN), and an optical module enclosed in a hermetically sealed package with an InP-based optical receiver. The optical module may also integrate a local oscillator light source 104.

[0020] In IC-TROSA, for example, the optical modulation chip 106 and the optical receiving chip 105, shown by the dashed line in Figure 1, are integrated into an optical waveguide circuit, and PBS 108, PR 108a, PBCR 109a, etc., are constructed by a spatial optical system using microoptics.

[0021] Another configuration of the coherent transceiver is one used, for example, for transmission over relatively short distances (around 100 km), in which the optical transmitting unit and the optical receiving unit are configured on a single silicon photonics chip 107.

[0022] In this configuration, the optical modulator for transmission and the PD for reception are integrated on a single silicon photonics chip 107, making it possible to realize a compact coherent transceiver (for example, Non-Patent Document 1).

[0023] Incidentally, the demand for faster optical transceivers to meet the needs of inter-data center communication is increasing year by year, and for example, it is expected that a capacity or transmission rate of 1.6 terabits per second (Tbps) or more will be required from the mid to late 2020s.

[0024] A 1.6 Tbps data center inter-data center transceiver can efficiently accommodate 400 Gbps class client signals generated within a data center. Since the transmission distance between data centers can be as long as 100 km depending on the distance between them, a digital coherent method is expected to be effective.

[0025] On the other hand, in order to achieve transmission distances exceeding 100 km at transmission speeds of 1.6 Tbps or higher, a signal baud rate of around 260 Gbaud may be required. In contrast, the bandwidth required for an optical modulator is around 130 GHz.

[0026] However, optical modulator configurations using highly integrated silicon photonics have a low modulation bandwidth of at most around 70 GHz, making it difficult to achieve the modulation performance required for capacities or transmission rates of 1.6 Tbps or more.

[0027] In contrast, the IC-TROSA configuration described above uses InP or LN as the modulation element, which may allow for the achievement of a sufficient modulation bandwidth. However, as previously mentioned, this configuration is unsuitable for miniaturization or cost reduction because the PBS, PR, and / or PBCR are constructed using a spatial optical system such as micro-optics.

[0028] For example, if the optical transmission function is configured using an LN and the optical reception function is configured using InP, which is a different material, then two optical waveguide circuits (or chips) with different material systems will be integrated into a single optical module (or optical device).

[0029] However, wavelength dependence can occur in optical modules due to optical components (e.g., optical interferometers) placed in each optical waveguide circuit. Therefore, the following describes several embodiments of optical devices that can suppress wavelength dependence.

[0030] <Embodiment 1> Figure 2 is a block diagram showing an exemplary configuration of an optical device according to Embodiment 1. "Optical device" may be read as "optical signal processing device". The optical device illustrated in Figure 2 includes, as an example of an optical transmission unit, an optical distribution unit 201, an optical merging unit 202, and an optical phase modulation unit 203. This optical transmission unit may correspond to, for example, the optical transmission unit 103 shown in Figure 1.

[0031] The optical distribution unit 201 and the optical converging unit 202 are, for example, waveguide circuits (e.g., PLCs) in which the waveguides are made of silicon (Si). In contrast, the optical phase modulation unit 203 is a waveguide circuit (e.g., PLCs) in which the waveguides are made of a material system other than silicon, such as LN or InP.

[0032] In other words, the configuration of the optical transmission unit illustrated in Figure 2 is a hybrid configuration in which the optical distribution unit 201, the optical merging unit 202, and the optical phase modulation unit 203 each have waveguides made of different material systems. Furthermore, in Figure 2, the configuration of the optical receiving unit, which may correspond to, for example, the optical receiving unit 102 illustrated in Figure 1, can also be a hybrid configuration including waveguides of different materials, similar to the optical transmission unit.

[0033] The optical distribution unit 201 includes, as an example, an input waveguide 211, a first optical branching unit 212a, second optical branching units 213a and 213b, and third optical branching units 214a to 214d.

[0034] The light input to the input waveguide 211 (for example, CW light from the local oscillator light source 104 shown in Figure 1) is split into two at the first optical branching section 212a, and then finally split into eight via the second optical branching sections 213a and 213b, and the third optical branching sections 214a to 214d.

[0035] Note that the number of branched lights is 2 N When expressed as (where N is an integer greater than or equal to 1), the optical distribution unit 201 uses N-stage cascaded optical branching units to divide the input light from the input waveguide 211 into two NThis can be generalized to an optical element that branches (or distributes or separates) the light into individual beams. Each of the branched beams is input to the optical phase modulation unit 203 via the first connection unit 215a.

[0036] The optical phase modulation unit 203 includes, as an example, eight waveguides 216a to 216h corresponding to each of the branched optical beams. When the waveguides 216a to 216h are not distinguished, the identification letters "a" to "h" may be omitted, and they may be abbreviated as "waveguide 216". Similarly, identification letters such as "a" and "b" attached to the symbols of other components may also be omitted in their notation.

[0037] Of the eight waveguides 216, two adjacent waveguides 216 each correspond to arm waveguides (or interference arms) that constitute an optical interferometer (for example, a Mach-Zehnder interferometer (MZI)). A phase shifter, not shown in Figure 2, is provided for one or both of the two arm waveguides 216 that constitute the MZI.

[0038] By applying an electrical modulation signal to each phase shifter, the phase of the light propagating through the arm waveguide 216 changes according to the electrical modulation signal, thereby imparting phase modulation to the propagating light. For convenience, each of the arm waveguides 216 may be referred to as a "phase shifter waveguide 216" below. The light propagating through each arm waveguide 216 is input to the optical confluence unit 202 via the second connection unit 215b.

[0039] The optical converging section 202 includes, as an example, first optical multiplexers 217a to 217d, second optical multiplexers 218a and 218b, a PBCR 219, and an output waveguide 220.

[0040] The eight light beams input to the optical converging unit 202 through each arm waveguide 216 of the optical phase modulation unit 203 are combined two by two in each of the first optical multiplexing units 217a to 217d to form four (2 N-1 The combined wave light of ) is output.

[0041] The four combined light waves are combined two at each of the second optical combined sections 218a and 218b to form two (2 N-2The combined optical wave of ) is output. The two combined optical waves are combined in the PBCR 219 and then output from the output waveguide 220.

[0042] Here, the waveguide region sandwiched between the second optical branching section 213a and the second optical multiplexing section 218a constitutes the IQ modulator 209a for the first polarization state (SOP1). Also, the waveguide region sandwiched between the second optical branching section 213b and the second optical multiplexing section 218b constitutes the IQ modulator 209b for the second polarization state (SOP2).

[0043] The PBCR 219 generates a polarization multiplexed coherent signal light by polarization combining the signal lights of SOP1 and SOP2 output from each of the IQ modulators 209a and 209b, and outputs it to the output waveguide 220.

[0044] Further, phase shifters 221a to 221d and phase shifters 222a and 222b formed on the silicon waveguide are provided in the above-described MZI and IQ modulators 209a and 209b. In FIG. 2, the phase shifters 221 and 222 are provided in the optical multiplexing section 202, but alternatively or additionally, they may be provided on the silicon waveguide of the optical distribution section 201.

[0045] FIG. 2 shows an example in which the optical distribution section 201 and the optical multiplexing section 202 are configured as different waveguide circuits, but each of the sections 201 and 202 may be integrated (or integrated) into a single waveguide circuit as shown in FIG. 3, for example.

[0046] That is, functions corresponding to those of the optical distribution section 201 and the optical multiplexing section 202 illustrated in FIG. 2 may be provided on a single silicon waveguide substrate 301 as shown in FIG. 3. In this case, the phase shifter waveguide 216 constituting the optical phase modulation section 302 illustrated in FIG. 2 is formed such that each input end face and output end face are present or located at a single connection portion 315 (in other words, one side of the silicon waveguide substrate 301) as shown in FIG. 3.

[0047] Such an arrangement is an example in which the output end faces of the light from the optical distribution unit 201 (or optical branching units 330a, 331a, and 332a) and the input end faces of the light to the optical converging unit 202 (or optical wave converging units 331b and 332b) are oriented in directions different from the directions in which they face each other.

[0048] In Figure 3, one optical branching section 330a, two optical branching sections 331a, and four optical branching sections 332a may correspond to the optical branching sections 212a, 213a, and 213b, and 214a to 214d, respectively, that constitute the optical distribution section 201 illustrated in Figure 2.

[0049] Furthermore, the four optical multiplexers 332b and the two optical multiplexers 331b may correspond to the optical multiplexers 217a to 217d and optical multiplexers 218a and 218b that constitute the optical confluence unit 202 illustrated in Figure 2, respectively.

[0050] In Figure 3, the phase shifter waveguide 216 is shown having a shape that makes a U-turn relative to the connection portion 315, but it may also have a shape that makes a W-turn, or it may have even more reciprocating structures.

[0051] By the way, as shown in Figure 3, when the input end face and output end face of the phase shifter waveguide 216 constituting the optical phase modulation unit 302 are placed on the same connection part (or connection end face) 315, a curved portion is created in each of the phase shifter waveguides 216.

[0052] In other words, each of the phase shifter waveguides 216 has a shape that follows a path including a curved portion corresponding to the arrangement relationship between the output end face of the optical distribution section 201 and the input end face of the optical confluence section 202 on the silicon waveguide substrate 301. As a result, a difference in optical path length occurs between the waveguides 216 located on the outside and the waveguides 216 located on the inside.

[0053] If there is a difference in optical path length between the phase shifter waveguides 216 that constitute the MZI and IQ modulator described above, wavelength dependence due to the MZI may occur. Therefore, by minimizing the difference in optical path length between the phase shifter waveguides 216 (for example, to less than or equal to the wavelength λ of the guided light), the wavelength dependence can be eliminated or reduced.

[0054] For example, in the MZI on the silicon waveguide substrate 301 in Figure 3, among the pair of arm waveguides 216, the inner arm waveguide 216 in a folded structure (or bending structure) such as a U-turn has a shorter optical path length than the outer arm waveguide 216.

[0055] Therefore, the inner arm waveguide 216 may be fitted with curved waveguides 333, 334, and 335, each having one or more curved sections, to match the optical path length of the outer arm waveguide 216. Each of the curved waveguides 333, 334, and 335 corresponds to a waveguide for adjusting the optical path length and may be conveniently referred to as an "equilinear waveguide".

[0056] Focusing on the MZI, which is formed by the waveguide region sandwiched between the optical branching section 332a and the optical multiplexing section 332b, waveguides 334a and 334b ​​can equalize the optical path lengths of the inner arm waveguide 216 and the outer arm waveguide 216. Furthermore, in the optical phase modulation section 302, waveguide 335 can equalize the optical path lengths of the inner arm waveguide 216 and the outer arm waveguide 216.

[0057] Similarly, when considering the IQ modulator formed by the waveguide region sandwiched between the optical branching section 331a and the optical multiplexing section 331b, the waveguides 333a and 333b can make the optical path lengths of the inner arm waveguide 216 and the outer arm waveguide 216 equal.

[0058] As described above, by making the optical path lengths of each pair of arm waveguides 216 in each MZI equal in length, wavelength dependence caused by the MZI can be eliminated or reduced. Similarly, wavelength dependence caused by the IQ modulator, which is formed by the waveguide region sandwiched between the optical branching section 331a and the optical multiplexing section 331b, can also be eliminated or reduced.

[0059] Furthermore, since the inner arm waveguide is composed of a curved waveguide, the optical waveguide loss may increase relative to the propagation light in the outer arm waveguide. In order to increase the extinction ratio of the MZI, the branching ratio in the optical branching section 331a and / or the multiplexing ratio in the optical multiplexing section 331b that constitute the MZI may be shifted from 50% so that the ultimately obtained multiplexed optical power is equal between each arm waveguide. Alternatively or additionally, the losses of the inner arm waveguide and the outer arm waveguide may be controlled to be equivalent by means such as providing a waveguide offset in the outer arm waveguide.

[0060] Furthermore, although the above-described embodiment 1 describes an example in which a curved waveguide is applied to the inner arm waveguide, the equalization of the optical path lengths between the arm waveguides constituting the MZI and / or IQ modulator may be achieved by other means or methods.

[0061] As described in Embodiment 1, the optical modulation function related to information transmission is implemented in the optical phase modulation section 203 or 303, which is composed of a material system capable of high-speed operation. On the other hand, other functions such as optical branching, optical multiplexing, polarization rotation, and / or polarization synthesis are integrated into the silicon waveguide substrate 301 (optical distribution section 201 and / or optical confluence section 202), which is composed of other material systems suitable for realizing each individual function. Therefore, this can contribute to high-speed optical modulation and miniaturization of optical devices.

[0062] In the embodiment 1 described above, an example of arrangement was shown in which the input and output end faces of the phase shifter waveguide 216 constituting the optical phase modulation unit 302 are located at a single connection part 315, as illustrated in Figure 3. However, the disclosure is not limited thereto.

[0063] For example, the optical distribution unit 201 and the optical converging unit 202 may be arranged in such a positional relationship that the normal to the output end face of the optical distribution unit 201 and the normal to the input end face of the optical converging unit 202 intersect each other (for example, forming an angle greater than 0° and less than 180°). In other words, the present disclosure can be similarly applied to arrangements in which the optical distribution unit 201 and the optical converging unit 202 are connected by an arm waveguide 216 along a non-linear path.

[0064] Furthermore, while silicon was given as an example of the material system for the waveguide constituting the optical distribution section 201 and the optical confluence section 202 in Embodiment 1, this disclosure is not limited to this. For example, other material systems such as quartz (SiO2), silicon nitride (Si3N4), or silicon oxynitride (SiON) may be applied to the waveguide, as long as they are suitable for realizing passive functions such as optical branching, optical confluence, polarization rotation, and polarization synthesis.

[0065] <Embodiment 2> In Embodiment 2, an example of the connection structure between the silicon waveguide substrate 301 and the optical phase modulation unit 302, as described in Embodiment 1, will be explained with reference to Figures 4 and 5.

[0066] Figure 4 is a cross-sectional view showing an exemplary schematic configuration of the connection portion 315 illustrated in Figure 3, and Figure 5 is a schematic exploded view of the configuration illustrated in Figure 4. In Embodiment 2, the explanation will be based on the configuration illustrated in Figure 3, but the same connection structure as illustrated in Figures 4 and 5 can also be applied to the configuration illustrated in Figure 2.

[0067] As shown in Figure 4, the silicon waveguide substrate 301 described above is an example of a first optical waveguide circuit, and for example, it has a silicon substrate 1001, a quartz undercladding layer 1002 formed on the silicon substrate 1001, a silicon waveguide 1003 formed on the undercladding layer 1002, and a quartz overcladding layer 1004 formed to cover the undercladding layer 1002 and the silicon waveguide 1003.

[0068] The silicon waveguide 1003 corresponds to an example of a first optical waveguide constructed (or formed) by embedding silicon, an example of a first material system, within an overcladding layer 1004, which is an example of a first layer.

[0069] Furthermore, as illustrated in Figures 4 and 5, the overcladding layer 1004 may be provided with, for example, a region (recess) 1007 on its upper surface that is partially recessed toward the silicon waveguide 1003. The distance between the silicon waveguide 1003 and the LN waveguide 1026, described later, is determined according to the depth of the recess 1007.

[0070] Therefore, by adjusting the depth of the recess 1007, it becomes possible to bring the spatial distance between the silicon waveguide 1003 and the LN waveguide 1026 as close as possible, thereby promoting (for example, maximizing) optical coupling (guided mode coupling).

[0071] Furthermore, on the upper surface of the overcladding layer 1004, for example, electrical wiring 1005, which serves as a conductive layer extending parallel to the direction of light propagation in the silicon waveguide 1003, is provided on both sides of the silicon waveguide 1003.

[0072] The electrical wiring 1005 is an example of the first electrical wiring layer and may be provided in a region that avoids the region (or structure) where the silicon waveguide 1003 and the LN waveguide 1026 (described later) are optically coupled, for example, on both sides of the recess 1007. As a non-limiting example, metallic materials such as aluminum or gold may be used for the electrical wiring 1005.

[0073] Furthermore, a fixing layer (or connecting layer) 1006 is provided on top of the electrical wiring 1005 for, for example, bonding and fixing the optical phase modulation unit 302 (or 203). A conductive paste, such as solder or silver paste, may be applied to the fixing layer 1006.

[0074] The optical phase modulation unit 302 is an example of a second optical waveguide circuit and includes, for example, a substrate 1021, a quartz undercladding layer 1022 formed on the substrate 1021, and an LN waveguide layer 1023 formed on the undercladding layer 1022.

[0075] In the LN waveguide layer 1023, a rib structure 1026 defining the waveguide is formed, as an example. Note that the "rib structure" in the LN waveguide layer 1023 may be appropriately replaced with "LN rib waveguide," "LN waveguide," or "rib waveguide."

[0076] The LN waveguide 1026 is an example of a second optical waveguide and is composed (or formed) of LN, which is an example of a second material system different from silicon, which is an example of a first material system, and is composed in an LN waveguide layer 1023, which is a second layer different from the first layer in which the silicon waveguide 1003 is composed.

[0077] Furthermore, on the surface of the LN waveguide layer 1023, for example, electrical wiring 1025, which serves as a conductive layer, is provided on both sides of the LN waveguide 1026, extending parallel to the direction of light propagation in the LN waveguide 1026.

[0078] The electrical wiring 1025 is an example of a second conductive layer that is electrically connected to the first conductive layer, the electrical wiring 1005, on the silicon waveguide circuit 301 side. For example, the electrical wiring 1025 may be provided in a region that avoids the region (or structure) where the LN waveguide 1026 and the silicon waveguide 1003 are optically coupled, for example, at positions corresponding to both sides of the recess 1007 on the silicon waveguide circuit 301 side. Similar to the electrical wiring 1005 on the silicon waveguide substrate 301 side, a metallic material such as aluminum or gold may be used for the electrical wiring 1025.

[0079] As illustrated in Figure 5, the silicon waveguide substrate 301 and the optical phase modulation unit 302 are connected in close contact with each other, with the surfaces on which the silicon waveguide 1003 and the LN waveguide 1026 are provided facing each other.

[0080] For example, the LN waveguide 1026 in the optical phase modulation section 302 is in close contact with the overcladding layer 1004 located above the silicon waveguide 1003. Exemplarily, the LN waveguide 1026 is aligned along the silicon waveguide 1003, located in the lower region of the recess 1007 described later, which is provided on the upper surface of the overcladding layer 1004, so that a portion of it (for example, the tapered structure 1032 described later) extends above the silicon waveguide 1003, and is fixed in close contact with the recess 1007.

[0081] Furthermore, the electrical wiring 1005 on the silicon waveguide substrate 301 side is in close contact with the electrical wiring 1025 on the optical phase modulation unit 302 side via a conductive fixed layer 1006, thereby enabling current to flow between the silicon waveguide substrate 301 and the optical phase modulation unit 302.

[0082] Here, as illustrated in Figure 5, the silicon waveguide 1003 and the LN waveguide 1026 may each be provided with tapered structures 1031 and 1032 to induce adiabatic photocoupling between the two waveguides 1003 and 1026.

[0083] Note that "tapered structure" is an example of a spot size converter (SSC), and may be interchanged with terms such as "tapered section," "tapered region," "SSC section," or "SSC region."

[0084] The tapered structures 1031 and 1032 are structures in which the waveguide width that defines the propagation mode changes in the direction of light propagation. As shown in Figure 5, two adjacent waveguides 1003 and 1026 have tapered structures 1031 and 1032 in which the change in waveguide width in the direction of light propagation is reversed relative to each other.

[0085] For example, in the tapered structure 1031 of one waveguide 1003, the waveguide width gradually decreases in the direction of light propagation, while in the tapered structure 1032 of the other waveguide 1026, the waveguide width gradually increases in the direction of light propagation.

[0086] The LN waveguide 1026 of the optical phase modulation section 302 is aligned such that its tapered structure 1032 and the tapered structure 1031 of the silicon waveguide 1003 located in the lower region of the recess 1007 are spatially close to each other in the vertical direction and overlap when viewed from above, and is then tightly fixed to the upper surface of the recess 1007.

[0087] In this case, optical coupling between waveguides 1003 and 1026 occurs as follows. For example, when light propagating through silicon waveguide 1003 reaches tapered structure 1031, the beam diameter of the light wave expands as the waveguide width holding the light wave decreases, promoting mode coupling to the adjacent LN waveguide 1026.

[0088] The tapered structure 1032 of the LN waveguide 1026 overlaps with the beam of light waves propagating through the tapered structure 1031 of the silicon waveguide 1003 in a top view, for example, and the waveguide width gradually expands in the propagation direction. Therefore, unlike the tapered structure 1031 of the silicon waveguide 1003, the beam diameter of the light waves propagating through the LN waveguide 1026 gradually decreases in the direction of light propagation.

[0089] In the region where the propagation constants of tapered structures 1031 and 1032 are identical or so closely aligned that they can be considered substantially identical, the mode coupling efficiency of the light wave energy from waveguide 1003 to LN waveguide 1026 is maximized, thereby promoting optical coupling.

[0090] Thus, the tapered structures 1031 and 1032 have a structure that causes the propagation constants of each to gradually approach each other, and then gradually move apart after reaching their closest point. Therefore, it is possible to prevent or suppress the re-transition of light that has transitioned from one waveguide to the other at the connection point 315 back to the other waveguide. Consequently, tolerance to manufacturing errors during mounting can be improved.

[0091] Optical coupling via such tapered structures 1031 and 1032 enables an adiabatic optical connection that promotes optical coupling by slowing down changes in the light propagation mode. Therefore, for example, it is possible to reduce optical coupling loss at the connection portion 315 between the silicon waveguide substrate 301 and the optical phase modulation unit 302.

[0092] Figure 6 is a partially enlarged cross-sectional view showing the configuration illustrated in Figure 4. The parameters illustrated in Figure 6 are as follows:

[0093] Tc: Core height of silicon waveguide 1003 Te: Depth of recess 1007 To: Height from the top surface of silicon waveguide 1003 to the surface of recess 1007 Ts: Thickness of the slab region of LN waveguide layer 1023 Tr: Height of rib structure 1026 tws: Thickness of electrical wiring 1005 Twl: Thickness of electrical wiring 1025 ts: Thickness of fixed layer 1006

[0094] Here, for example, if the condition expressed by the following equation (1) is satisfied, the silicon waveguide substrate 301 and the optical phase modulation unit 302 can be optically and electrically coupled in close contact at the connection part 315, as described above. tw1 + ts + tws = Tr - Te (1)

[0095] Equation (1) means that the total thickness of the electrical wiring 1005, electrical wiring 1025, and the fixed layer 1006 is equal to the difference between the height of the rib structure 1026 of the LN waveguide layer 1023 and the depth of the recess 1007.

[0096] In the above-described embodiment 2, an example was given in which silicon was used for the substrate 301 and LN was used for the optical phase modulation unit 302, but this disclosure is not limited to these examples. For example, the substrate 301 may be made of a material system suitable for realizing passive functions such as optical branching, optical multiplexing, polarization rotation, and polarization synthesis, such as quartz, silicon nitride, or silicon oxynitride. Alternatively, the optical phase modulation unit 302 may be made of a material system suitable for realizing optical modulation functions, such as InP or lanthanum-doped lead zirconate titanate (PLZT), or polymer-based electro-optical materials (EO polymers).

[0097] <Modification 1 of Embodiment 2> Embodiment 2 describes an example in which tapered structures 1031 and 1032, as examples of SSCs, are provided in both waveguides 1003 and 1026, respectively, for optical coupling between the silicon waveguide 1003 and the LN waveguide 1026.

[0098] However, as previously described, if an optical coupling means different from the tapered structure can be applied to the connection portion 315, it is possible to achieve efficient optical coupling and suppression of feedback recombination by adjusting the mode coupling efficiency of the optical wave energy between the waveguides.

[0099] For example, as shown in Figure 7, the silicon waveguide 1003 may be provided with a plurality of waveguide segments 1041 instead of the tapered structure 1031, such that they are partially discontinuous in the direction of light propagation and the occupancy rate of the waveguide core gradually decreases in the direction of light propagation. The plurality of waveguide segments 1041 may be conveniently referred to as a segment structure 1041. Furthermore, a shape combining such a segment structure 1041 and the tapered structure 1031 described above may be applied to the silicon waveguide 1003.

[0100] Furthermore, for the LN waveguide 1026 on the optical phase modulation section 302 side, the segment structure 1041 described above may be applied to the LN waveguide 1026 instead of the tapered structure 1032, or in combination with the tapered structure 1032.

[0101] <Modification 2 of Embodiment 2> In Embodiment 2 described above, tapered structures 1031 and 1032 were described as structural examples of SSCs that expand the beam diameter of light waves in the direction of light propagation. However, as another structural example of an SSC, a trident structure 1052 as shown in Figure 8 may be adopted.

[0102] The Trident structure 1052, as an example, has a tapered structure 1031 of a silicon waveguide 1003 on a recess 1007, and two sub-waveguides 1051 provided in close proximity and without physical contact on either side of the tapered structure 1031.

[0103] Each of the sub-waveguides 1051 is, for example, formed from the same material as the silicon waveguide 1003 and has a tapered structure oriented in the same direction as the tapered structure 1031. By providing the sub-waveguides 1051 in close proximity to both sides of the tapered structure 1031, optical perturbations are applied to the light waves propagating through the tapered structure 1031, making it possible to gradually increase the beam diameter in the direction of light wave propagation.

[0104] Furthermore, such a trident structure 1052 may also be applied as an alternative structure to the tapered structure 1032 for the LN waveguide 1026 in the connection portion 315 on the optical phase modulation section 302 side.

[0105] By applying a trident structure 1052, in which the beam diameter gradually expands in the direction of light wave propagation, to the connection section 315, it is possible to improve the tolerance of manufacturing errors to misalignment between the silicon waveguide 1003 and the LN waveguide 1026 in the connection section 315.

[0106] Furthermore, the trident structure 1052 may be applied in combination with one or both of the tapered structure 1031 (1032) and the segment structure 1041 described above.

[0107] <Embodiment 3> Embodiment 3 describes a method for creating (or manufacturing) an optical device that satisfies the optical and electrical connection between the silicon waveguide substrate 301 and the optical phase modulation unit 302 described above in Embodiment 2.

[0108] In the following description of the manufacturing method, detailed procedures for processing techniques such as photolithography or etching that can be used to form waveguides or electrical wiring will be omitted, and the structural changes of the silicon waveguide substrate 301 and the optical phase modulation unit 302 will be described.

[0109] Processing techniques may include, but are not limited to, photolithography using photoresists, reactive ion etching, and / or planarization by CMP (Chemical Mechanical Polishing).

[0110] <Process for Manufacturing Silicon Waveguide Substrate 301> Figure 9 is a schematic diagram showing an exemplary process (or procedure) for manufacturing the silicon waveguide substrate 301 described above. As illustrated in Figure 9, in step 1, an underclad layer 1002 is formed on the silicon substrate 1001, and a silicon layer 1003a is formed on the underclad layer 1002. Step 1 may also be a step of preparing a laminate in which the silicon substrate 1001, underclad layer 1002, and silicon layer 1003a are formed.

[0111] Next, in step 2, the silicon waveguide 1003 is formed by processing the silicon layer 1003a into the desired waveguide shape using, for example, photolithography or a combination of electron beam exposure and dry etching.

[0112] Subsequently, in step 3, an overcladding layer 1004 is formed so as to cover the undercladding layer 1002 and the silicon waveguide 1003.

[0113] In step 4, recesses 1007 are partially formed in the overclad layer 1004, as illustrated in Figures 4 and 5. The recesses 1007 are processed to a depth (for example, about 0.4 μm to 1.0 μm) that is close enough for the silicon waveguide 1003 to optically couple with the LN waveguide 1026 when the silicon waveguide substrate 301 and the optical phase modulation section 302 are in close contact, as illustrated in Figures 4 and 5.

[0114] In step 5, electrical wiring 1005 extending along the silicon waveguide 1003 is formed on the overclad layer 1004 in the regions on both sides of the recess 1007 by appropriate processing techniques such as photolithography, dry etching, or lift-off.

[0115] In step 6, on the upper surface of each electrical wiring 1005, a solder film, as an example of a fixed layer 1006, is formed in the portion area connected to the electrical wiring 1025 on the optical phase modulation unit 302 side, for example, by a method such as vapor deposition. The thickness ts0 of the formed solder 1006 may be thicker than, for example, the thickness ts of the fixed layer 1006 after close connection as illustrated in Figure 6.

[0116] <Manufacturing Process for Optical Phase Modulation Unit 302> Figure 10 is a schematic diagram showing an exemplary manufacturing process (or procedure) for the optical phase modulation unit 302. In Figure 10, in step 1, an underclad layer 1022 is formed on the substrate 1021, and an LN layer 1023a is formed on the underclad layer 1022. The substrate 1021 may be a silicon substrate, a glass substrate, or a substrate of other dielectric material. Step 1 may also be a step of preparing a laminate on which the substrate 1021, the underclad layer 1022, and the LN layer 1023a are formed.

[0117] In step 2, the LN layer 1023a is processed using a processing technique such as photolithography or reactive dry etching to form an LN waveguide layer 1023 having a rib structure 1026. The region of the LN waveguide layer 1023 excluding the rib structure 1026 corresponds to the slab region.

[0118] Furthermore, after the formation of the rib structure 1026, in step 2a, which is not essential, a protective layer 1301 may be formed (for example, by film deposition) to cover the LN waveguide layer 1023 including the rib structure 1026. As a non-limiting example, the protective layer 1301 may be made of quartz or silicon nitride.

[0119] In step 3, similar to step 5 in Figure 9, electrical wiring 1025 extending along the rib structure 1026 is formed on the slab regions 1023 on both sides of the rib structure 1026 by appropriate processing techniques such as photolithography, dry etching, or lift-off.

[0120] Furthermore, when the protective layer 1301 is formed, the electrical wiring 1025 may be provided on the protective layer 1301, or after the protective layer 1301 is formed, the protective layer 1301 in the area where the electrical wiring 1025 is provided may be removed, and the electrical wiring 1025 may be provided in the removed area. Alternatively, the protective layer 1301 may be formed after the electrical wiring 1025 is formed, and the protective layer 1301 present on the area of ​​the electrical wiring 1025 may be removed. In this case, after the removal of the protective layer 1301, metal for stretching the electrical wiring 1025 may be further deposited.

[0121] <Connection between silicon waveguide substrate 301 and optical phase modulation unit 302> Figure 11 is a schematic diagram showing an example of a process for connecting the silicon waveguide substrate 301, which was manufactured by steps 1 to 6 as illustrated in Figure 9, and the optical phase modulation unit 302, which was manufactured by steps 1 to 3 (step 2a may also be included) as illustrated in Figure 10.

[0122] As shown in Figure 11, the side of the optical phase modulation section 302, which was manufactured as illustrated in Figure 10, on the side where the LN waveguide 1026 is located is pressed and brought into close contact with the side of the silicon waveguide substrate 301 on the side where the silicon waveguide 1003 is located.

[0123] During contact, the optical phase modulation unit 302 is aligned with respect to the silicon waveguide substrate 301 such that, for example, the LN waveguide 1026 and the electrical wiring 1025 are located above the fixed layer 1006 on the silicon waveguide 1003 and the electrical wiring 1005, respectively.

[0124] In other words, the silicon waveguide substrate 301 and the optical phase modulation unit 302 are aligned and in close contact with each other so that, in a top view, at least the silicon waveguide 1003 and the LN waveguide 1026 overlap each other at least partially.

[0125] Furthermore, during adhesion, the fixed layer 1006, which is solder, is heated to a temperature at which it melts and softens. When the fixed layer 1006 is softened by heating, the optical phase modulation section 302 is pressed against the silicon waveguide substrate 301, causing the thickness of the fixed layer 1006 to gradually decrease from its initial thickness ts0.

[0126] Finally, the optical phase modulation unit 302 is pressed until the surface of the LN waveguide 1026 contacts the surface of the recess 1007 on the silicon waveguide substrate 301 side, and then the heating of the fixed layer 1006 is terminated. Therefore, the thickness of the fixed layer 1006 is ultimately reduced from ts0 to ts. For this reason, in the aforementioned step 6 illustrated in Figure 9, a fixed layer 1006 with a thickness of ts0 > ts is formed.

[0127] This process allows the silicon waveguide substrate 301 and the optical phase modulation unit 302 to be connected to each other in close optical and electrical contact. Furthermore, since mechanical fixing by the fixed layer 1006 is achieved along with the close optical and electrical contact, the mounting process can be reduced compared to when optical, electrical, and mechanical connections are performed individually. Therefore, it is possible to simplify the mounting method and improve the manufacturability of the hybrid optical device including the silicon waveguide substrate 301 and the optical phase modulation unit 302.

[0128] In the above-described embodiment 3, the fixed layer 1006 is shown to be provided on the electrical wiring 1005 on the silicon waveguide substrate 301 side, but it may also be provided on the electrical wiring 1025 on the optical phase modulation unit 302 side.

[0129] Furthermore, in the above-described embodiment 3, an example was shown in which the optical phase modulation unit 302 is pressed against the silicon waveguide substrate 301 to create a close connection. However, conversely, the silicon waveguide substrate 301 may be pressed against the optical phase modulation unit 302 to create a close connection.

[0130] <Embodiment 4> In embodiments 1 to 3 described above, an example was described in which the electrical wiring 1025 of the optical phase modulation section 302 is formed on the surface of the LN waveguide layer 1023. In this configuration, electric field lines tend to concentrate in the LN layer, which has a higher dielectric constant than other material layers, so efficient electro-optic modulation can be achieved.

[0131] On the other hand, mechanical strength can be improved by increasing the thickness of one or both of the electrical wirings 1005 and 1025, and / or the fixed layer 1006. For example, the difference in thermal expansion coefficients between the silicon waveguide substrate 301 and the optical phase modulation section 302 can be absorbed by the electrical wirings 1005, 1025, and / or the fixed layer 1006, which are made of a material with higher plasticity compared to other material layers. Therefore, it is possible to expand the ambient temperature range in which the optical device can be used.

[0132] Figure 12 is a cross-sectional view showing an exemplary schematic configuration of the connection portion 315 illustrated in Figure 3, according to Embodiment 4. The configuration illustrated in Figure 12 differs from the configuration illustrated in the cross-sectional view of Figure 4 in that at least a portion of the slab area excluding the rib structure 1026 of the LN waveguide layer 23 is removed, and the electrical wiring 1025 is provided on the surface of the undercladding layer 1022.

[0133] According to the configuration illustrated in Figure 12, for example, the thickness of one or both of the electrical wirings 1005 and 1025, and / or the thickness of the fixed layer 1006 can be increased compared to the configurations illustrated in Embodiments 1 to 3 described above. Therefore, the mechanical strength of the optical device is improved, contributing to improved reliability of the optical device, for example, by expanding the usable ambient temperature range.

[0134] <Clarification of Terms> The terms “connection” or “joining” as used in this disclosure may be understood to mean any direct or indirect “connection” or “joining” between two or more elements. For example, an indirect “connection” or “joining” between two elements that are connected or joined to each other, with one or more intermediate elements interposed between them, may also be understood to be included in the term. The “connection” or “joining” between elements may be either optical or electrical.

[0135] Any reference to elements with designations such as "first..." or "second..." does not limit the quantity or order of those elements. These designations are merely a convenient way of distinguishing between two or more elements. For example, references to the first and second elements do not imply that only two elements can be adopted, nor do they imply that the first element must take precedence over the second element in any physical quantity.

[0136] While the Disclosure has been described in detail above, it will be apparent to those skilled in the art that the purpose and scope of this Disclosure are not limited to what has been described herein. This Disclosure can be implemented in modified and altered forms without exceeding the purpose and scope of this Disclosure as defined by the claims. Therefore, the descriptions in this Disclosure are for illustrative purposes only and are not intended to be restrictive in any way to the purpose and scope of this Disclosure.

[0137] This disclosure is useful for technologies that use optical devices, such as optical communication systems or optical information processing systems.

[0138] 201 Optical distribution section 202 Optical confluence section 203, 302 Optical phase modulation section 209a, 209b IQ modulator 211 Input waveguide 212a First optical branching section 213a, 213b Second optical branching section 214a-214d Third optical branching section 215a First connection section 216a-216h Waveguide 217a-217d First optical multiplexing section 218a, 218b Second optical multiplexing section 219 PBCR 220 Output waveguide 221a-221d, 222a, 222b Phase shifter 301 Silicon waveguide substrate 315 Connection section 330a, 331a, 332a Optical branching section 331b, 332b Optical multiplexing section 333a, 333b, 335 Waveguides 1001 Silicon substrate 1002 Underclad layer 1003 Silicon waveguide 1003a Silicon layer 1004 Overclad layer 1005, 1025 Electrical wiring (conductive layer) 1006 Fixed layer (or connecting layer) 1007 Recess 1021 Substrate 1022 Underclad layer 1023 LN waveguide layer 1023a LN layer 1026 Rib structure (LN waveguide) 1031, 1032 Tapered structure 1041 Waveguide segment 1051 Subwaveguide 1052 Trident structure

Claims

1. An optical interferometer comprising one or more optical interferometers including: an optical branching section; an optical multiplexing section; and a plurality of arm waveguides for guiding each of the light branched in the optical branching section to the optical multiplexing section, wherein the light undergoes different phase changes between different arm waveguides, wherein the optical branching section and the optical multiplexing section are arranged in directions different from the direction in which the output end face of the light from the optical branching section and the input end face of the light to the optical multiplexing section face each other, and each of the arm waveguides has a shape along a path including a curved portion corresponding to the different directions between the output end face of the optical branching section and the input end face of the optical multiplexing section, and the difference in optical path length between different arm waveguides in the path including the curved portion is less than or equal to the wavelength of the light.

2. The optical device according to claim 1, wherein one of the pair of arm waveguides in the optical interferometer has an isolength waveguide to match the optical path length of the other arm waveguide.

3. The optical device according to claim 1, wherein the optical branching section and the optical multiplexing section are provided on a first substrate to constitute a first optical waveguide circuit, each of the arm waveguides is provided on a second substrate to constitute a second optical waveguide circuit, the output end face of the optical branching section and the input end face of the optical multiplexing section are arranged to be located at the connection point between the first optical waveguide circuit and the second optical waveguide circuit, and the shape of the arm waveguide along the path is the shape along the path that folds the light back from the output end face of the optical branching section to the input end face of the optical multiplexing section.

4. The optical device according to claim 3, wherein the first optical waveguide circuit includes a cladding layer containing first optical waveguides that constitute the optical branching section and the optical multiplexing section, respectively, and a recess that is partially recessed from the upper surface of the cladding layer toward the first optical waveguide, and the second optical waveguide circuit has a second optical waveguide with a rib structure that forms the arm waveguide, and the rib structure is tightly fixed in the recess.

5. The optical device according to claim 4, wherein the first optical waveguide has a first spot size conversion unit that gradually increases the beam diameter of light propagating through the first optical waveguide in the direction of light propagation, and the second optical waveguide has a second spot size conversion unit that gradually decreases the beam diameter of light propagating through the second optical waveguide in the direction of light propagation.

6. The optical device according to claim 4, wherein the first optical waveguide circuit has a first conductive layer in a region avoiding the recess, and the second optical waveguide circuit has a second conductive layer electrically connected to the first conductive layer in a region avoiding the recess.

7. The optical device according to claim 6, further comprising a fixing layer of solder or conductive paste for bonding and fixing the first conductive layer and the second conductive layer.

8. The optical device according to claim 4, wherein the material system of the first optical waveguide is silicon or quartz, and the material system of the second optical waveguide is lithium niobate or indium phosphide.

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