Photodetector and photoreceiver

Abstract

To provide a photodetection device and an optical receiver that can enhance the reliability of an optical communication system.SOLUTION: A photodetection device 10 includes: a light receiving waveguide 40 that is connected to a first input waveguide 20 into which a first optical signal is input and a second input waveguide 30 into which a second optical signal is input; and a light receiving section 41 that outputs an electric signal according to the intensity of a combined signal of the first optical signal and the second optical signal input into the light receiving waveguide 40. The light receiving section 41 is configured to reduce the intensity of an optical signal returning in the opposite direction to a first direction in which the first optical signal propagates in the first input waveguide 20 and the intensity of an optical signal returning in the opposite direction to a second direction in which the second optical signal propagates in the second input waveguide 30.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to optical detection devices and optical receivers. [Background technology]

[0002] An optical receiver is known that separates different polarization components into two waveguides, demultiplexes the light into wavelengths in each waveguide, and detects the light of a specific wavelength demultiplexed in each waveguide with one photodetector (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] U.S. Patent No. 10,488,593 Summary of the Invention [Problem to be solved by the invention]

[0004] When light is input from two waveguides to one photodetector, the two waveguides are connected to the photodetector. In a photodetector with two connected waveguides, light propagating from one waveguide to the photodetector may pass through the photodetector and propagate in the other waveguide as returned light in the direction opposite to the input direction. When the photodetector is used in an optical communication system, the returned light may destabilize the optical communication system and reduce its reliability.

[0005] An object of the present disclosure is to provide a photodetector and an optical receiver that can improve the reliability of optical communication systems. [Means for solving the problem]

[0006] An optical detection device according to an embodiment of the present disclosure includes a receiving waveguide and a light receiving unit. The receiving waveguide is connected to a first input waveguide to which a first optical signal is input and a second input waveguide to which a second optical signal is input. The light receiving unit outputs an electrical signal corresponding to the intensity of a combined signal of the first optical signal and the second optical signal input to the receiving waveguide. The light receiving unit is configured to reduce the intensity of an optical signal returning in the first input waveguide in a direction opposite to a first direction in which the first optical signal propagates, and the intensity of an optical signal returning in the second input waveguide in a direction opposite to a second direction in which the second optical signal propagates.

[0007] An optical receiver according to an embodiment of the present disclosure includes an optical circuit and a photodetector. The optical circuit separates an input optical signal into a first optical signal propagating in TE mode and a second optical signal propagating in TM mode. The photodetector outputs an electrical signal corresponding to the intensities of the first and second optical signals input from the optical circuit. The photodetector includes a receiving waveguide and a photodetector. The receiving waveguide is connected to a first input waveguide to which the first optical signal is input and a second input waveguide to which the second optical signal is input. The photodetector outputs an electrical signal corresponding to the intensity of a combined signal of the first and second optical signals input to the receiving waveguide. The photodetector is configured to reduce the intensity of an optical signal returning in the first input waveguide in a direction opposite to the first direction in which the first optical signal propagates and the intensity of an optical signal returning in the second input waveguide in a direction opposite to the second direction in which the second optical signal propagates. [Effects of the Invention]

[0008] According to an embodiment of the present disclosure, the reliability of an optical communication system can be improved. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a block diagram illustrating an example of the configuration of an optical receiver according to an embodiment. [Figure 2] FIG. 2 is a plan view illustrating an example of the configuration of a photodiode according to an embodiment. [Figure 3] 3 is an example of a cross-sectional view taken along line AA in FIG. 2. [Figure 4] 3 is another example of a cross-sectional view taken along line AA in FIG. 2. [Figure 5] FIG. 10 is a plan view showing the configuration of a photodiode according to a comparative example. [Figure 6] FIG. 10 is a block diagram showing a configuration of an optical receiver according to a comparative example. [Figure 7] 3 is a plan view showing an example of the configuration of a photodiode in the configuration example of FIG. 2, in which the normal direction of the side surface of the photodiode is inclined with respect to the traveling direction of light. [Figure 8] 3 is a plan view showing an example of the configuration of a photodiode in the configuration example of FIG. 2, in which the side surface of the photodiode has a convex portion. [Figure 9] 3 is a plan view showing an example of the configuration of a photodiode in the configuration example of FIG. 2, in which the side surface of the photodiode has a convex portion, and the axis of the convex portion is inclined with respect to the traveling direction of light. [Figure 10] 10 is a plan view showing an example of the configuration of a photodiode in which the traveling direction of light in the first waveguide and the traveling direction of light in the second waveguide are perpendicular to each other. FIG. [Figure 11] 10 is a plan view showing an example of a configuration in which the direction of light travelling through the first waveguide and the direction of light travelling through the second waveguide are orthogonal to each other, and the normal direction to the side surface of the photodiode is inclined with respect to the direction of light travelling. FIG. [Figure 12] 10 is a plan view showing an example of a configuration in which the direction of light travelling through the first waveguide and the direction of light travelling through the second waveguide are perpendicular to each other, and the side surface of the photodiode has a convex portion. FIG. [Figure 13] FIG. 10 is a plan view showing an example of a configuration in which the light traveling direction in the first waveguide and the light traveling direction in the second waveguide are perpendicular to each other, the side surface of the photodiode has a convex portion, and the axis of the convex portion is inclined with respect to the light traveling direction. [Figure 14] 10 is a plan view showing an example of the configuration of a photodiode connected to a first waveguide and a second waveguide so that multimode interference occurs in a light-receiving waveguide. FIG. [Figure 15]10 is a plan view showing an example of the intensity distribution of light that appears due to multimode interference occurring in the receiving waveguide when no light receiving section is arranged in the receiving waveguide. FIG. [Figure 16] FIG. 10 is a plan view showing an example of the light intensity distribution when the size of the light receiving section is set so that the intensity of light incident from the first waveguide and propagating to the second waveguide is reduced to less than a predetermined intensity. [Figure 17] FIG. 10 is a plan view showing an example of the light intensity distribution when the intensity of light incident from the first waveguide that propagates to the second waveguide is equal to or greater than a predetermined intensity by changing the size of the light receiving section. [Figure 18] FIG. 10 is a plan view showing an example of the configuration of a photodiode when the direction of light travelling in the first waveguide and the direction of light travelling in the second waveguide are orthogonal to each other and the first waveguide and the second waveguide are connected so that multimode interference occurs in the light receiving waveguide. [Figure 19] FIG. 19 is a plan view showing an example of the configuration of a photodiode in the example of FIG. 18, further including a terminating waveguide at each of an end opposite to the side where the first waveguide is connected and an end opposite to the side where the second waveguide is connected. [Figure 20] FIG. 20 is a plan view showing an example of the configuration of a photodiode in the example of FIG. 19, in which a notch is formed on each of the side to which the first waveguide is connected and the side to which the second waveguide is connected. [Figure 21] FIG. 10 is a plan view showing an example of the configuration of a photodiode in which the distance from the end of the light receiving waveguide connected to the first waveguide to the light receiving section is different from the distance from the end of the light receiving waveguide connected to the second waveguide to the light receiving section. [Figure 22] FIG. 22 is a plan view showing an example of the configuration of a photodiode in the example of FIG. 21 further including a terminating waveguide at each of an end opposite to the side where the first waveguide is connected and an end opposite to the side where the second waveguide is connected. [Figure 23]23 is a plan view showing an example of the configuration of a photodiode in the example of FIG. 22, in which a notch is formed on each of the side to which the first waveguide is connected and the side to which the second waveguide is connected. FIG. [Figure 24] 15 is a plan view showing an example of the configuration of a photodiode in the configuration example of FIG. 14, in which the normal direction to the side surface of the photodiode is inclined with respect to the traveling direction of light. [Figure 25] 15 is a plan view showing an example of the configuration of a photodiode in the configuration example of FIG. 14, in which the side surface of the photodiode has a convex portion. [Figure 26] 15 is a plan view showing an example of the configuration of the photodiode in the configuration example of FIG. 14, in which the side surface of the photodiode has a convex portion, and the axis of the convex portion is inclined with respect to the traveling direction of light. [Figure 27] 15 is a plan view showing an example of the configuration of a photodiode in the configuration example of FIG. 14, in which the optical axes of the first waveguide and the second waveguide are misaligned with the optical axis passing through the light receiving section. FIG. [Figure 28] FIG. 10 is a block diagram showing an example of the configuration of an optical receiver having an edge coupler as an input section. [Figure 29] FIG. 10 is a block diagram showing an example of the configuration of an optical receiver having a two-dimensional grating coupler as an input section. [Figure 30] FIG. 29 is a block diagram showing an example of the configuration of an optical receiver in the case where a delay device is further provided in the example of the configuration of FIG. 28. [Figure 31] 31 is a block diagram showing an example of the configuration of an optical receiver in the configuration example of FIG. 30 in which the polarization splitter rotator is replaced with a polarization splitter. [Figure 32] FIG. 31 is a block diagram showing an example of the configuration of an optical receiver in the example of the configuration of FIG. 30 in which the delay device connected before the photodiode is replaced with a variable attenuator. [Figure 33] FIG. 33 is a block diagram showing an example of the configuration of an optical receiver in the case where a delay device is further provided between the variable attenuator connected before the photodiode and the photodiode in the example of the configuration of FIG. 32. DETAILED DESCRIPTION OF THE INVENTION

[0010] Direct modulation and direct detection (DDM) techniques are widely used in optical communications within data centers due to the simplicity and low power consumption of digital signal processors. However, increasing traffic in optical communications within data centers necessitates higher-density data transmission. To address this demand, silicon-based optical integrated circuits, particularly optical transceivers with compact wavelength-division multiplexing (WDM) circuits, are being considered. Among these, silicon-based WDM circuits include serial Mach-Zehnder interferometers and arrayed-waveguide gratings. Both WDM circuits exhibit significant polarization-dependent characteristics. Meanwhile, existing optical fiber networks within data centers widely use single-mode optical fibers. However, these optical fibers lack polarization-maintaining properties. Therefore, the polarization of light changes randomly every time the light passes through a bend or splice in the wiring. Therefore, to ensure consistent optical receiver characteristics for any polarization, a polarization splitter rotator is required before the optical circuit to separate the incident light into transverse electric (TE) and transverse magnetic (TM) components and then feed each into a separate WDM circuit. In the direct modulation direct polarization system, after polarization to either TE or TM, the output of the wavelength division multiplexing optical circuit is received by a photodiode and the sum of the outputs of the photodiodes corresponding to each polarization component must be detected.

[0011] When germanium photodiodes are used in a direct modulation and direct detection system, separate photodiodes are provided to detect the TE polarization component and the TM polarization component in order to detect the sum of the photodiode outputs corresponding to the TE and TM polarization components. The outputs of the photodiodes detecting the TE polarization component and the TM polarization component are then input in parallel to a transimpedance amplifier and added together to obtain the sum of the photodiode outputs corresponding to the TE and TM polarization components. However, in this case, the total parasitic capacitance of the two photodiodes connected in parallel is 2×Cpd, compared to the parasitic capacitance Cpd of a single photodiode. This increases the RC time constant due to the parasitic capacitance and the series resistance of the circuit, slowing the response speed of the photodiode.

[0012] On the other hand, it is also possible to input the output of each photodiode separately to a transimpedance amplifier and sum the voltages of the transimpedance amplifiers using an analog or digital circuit. In this case, the response speed of the photodiodes is fast because it corresponds to the parasitic capacitance Cpd of each photodiode, but an amplifier or DSP (Digital Signal Processor) is required to output the sum of the voltages. This increases the circuit size and power consumption.

[0013] The light detection device may be configured to detect each polarization component with a single photodiode instead of detecting each polarization component with a separate photodiode. For example, as disclosed in the aforementioned Patent Document 1 (U.S. Pat. No. 10,488,593), a system may use a two-dimensional grating coupler to separate different polarization components Px and Py into two waveguides, and then couple only light of a specific wavelength into the next two waveguides via a Bragg reflector. The two coupled waveguides are connected to a single photodiode. The single photodiode outputs the sum of the light intensities of the two polarization components as a photocurrent. This avoids increases in the parasitic capacitance of the photodiode, circuit size, or power consumption.

[0014] However, light reflected from the Bragg reflector or light not completely absorbed by the photodiode may return to the optical communication system as feedback light, which can destabilize the operation of the laser light source used for communication.

[0015] Furthermore, a different configuration is disclosed in the following document A. Document A: IEEE, Photonics Technology Letters, 31, 287 (2019)

[0016] The device described in the above-mentioned document A uses a two-dimensional grating coupler to couple different polarization components Px and Py to each waveguide, and then uses ring filters installed on each waveguide to couple only light of a specific wavelength to the next two waveguides. The two coupled waveguides are connected to the two-dimensional grating coupler. The two-dimensional grating coupler outputs the input light above the substrate. A photodiode placed above the two-dimensional grating coupler then converts the light output above the substrate into an electrical signal. The device described in the above-mentioned document A can output the sum of the optical intensities of the two polarized waves using a single photodiode, as in Patent Document 1. However, there is a possibility that feedback light from the grating coupler on the light receiving element side may cause instability in the operation of the communication laser light source.

[0017] Furthermore, the structure of Reference A requires a photodiode to be placed above the two-dimensional grating coupler. This makes it impossible to use the photodiodes commonly used in optical communications technology, i.e., strained germanium epitaxially grown on a flat top silicon layer, and complicates the manufacturing process. Furthermore, the structure in which light passes vertically through the epitaxial layers that make up the photodiode reduces the sensitivity of the photodiode.

[0018] A photodiode 10 (see FIG. 2, etc.) according to an embodiment of the present disclosure is configured to output the sum of the optical intensities of two polarized lights, TE and TM, using a single photodiode. An optical receiver 1 using the photodiode 10 is configured as a polarization-independent optical receiver. The photodiode 10 according to an embodiment of the present disclosure can reduce the parasitic capacitance of the photodiode and can reduce optical feedback so that an electrical circuit for adding the outputs of two photodiodes corresponding to the two polarized light components is not required.

[0019] An optical receiver 1 (see FIG. 1, etc.) according to an embodiment of the present disclosure may be used in combination with a configuration for transmitting an optical signal in an optical communication system. The configuration for transmitting an optical signal may include a light source and a modulator.

[0020] The light source may include, for example, a semiconductor laser such as a laser diode (LD) or a vertical cavity surface emitting laser (VCSEL). The light source may include a device that emits electromagnetic waves of various wavelengths, not limited to visible light. The modulator modulates the electromagnetic waves by changing their intensity. The modulator may, for example, pulse-modulate the electromagnetic waves.

[0021] The optical signal transmitting configuration may further include a signal input unit. The signal input unit receives a signal input from an external device or the like. The signal input unit may include, for example, a D / A converter. The signal input unit outputs a signal to a modulator. The modulator modulates an electromagnetic wave based on the signal acquired by the signal input unit.

[0022] The optical receiver 1 using the photodiode 10 can reduce the amount of light returned from the optical receiver 1 to the optical communication system, thereby realizing a polarization-independent optical receiver that operates at high speed and with low power consumption. In addition, because the photodiode can be formed on a flat silicon waveguide, strained germanium epitaxially grown on silicon can be used as the photodiode, and because the photodiode absorbs light propagating through the waveguide, light can be absorbed over a sufficiently long distance, thereby increasing the light receiving sensitivity.

[0023] (Configuration example of optical receiver 1) The optical receiver 1 is configured to receive an optical signal. An example configuration of the optical receiver 1 according to an embodiment of the present disclosure will be described below.

[0024] As shown in FIG. 1, the optical receiver 1 includes an input unit 81, a polarizing splitter rotator (PSR) 82, a demultiplexer (demultiplexer or DEMUX) 83, and a photodiode (PD) 10.

[0025] The input unit 81 is configured to receive an input of an optical signal generated by the above-mentioned modulator or the like. The polarization splitter rotator 82 separates the TE mode optical signal and the TM mode optical signal contained in the input optical signal, and converts the TM mode optical signal into a TE mode optical signal. The demultiplexer 83 separates, by wavelength, the TE mode optical signal separated by the polarization splitter rotator 82 and the TE mode optical signal converted from the TM mode optical signal separated by the polarization splitter rotator 82. In FIG. 1, the demultiplexer 83 is configured to separate the optical signal into light of n wavelengths represented by λ1 to λn.

[0026] The photodiodes 10 include photodiodes 10-1 to 10-n corresponding to light of each wavelength. Each photodiode 10 is connected to a demultiplexer 83 that separates a TE mode optical signal by wavelength, and a demultiplexer 83 that separates a TE mode optical signal converted from a TM mode optical signal by wavelength. In other words, each photodiode 10 has a port that accepts input of light separated from the TE mode optical signal, and a port that accepts input of light separated from the TE mode optical signal converted from the TM mode optical signal. Each photodiode 10 is configured to detect both light of each wavelength separated from the TE mode optical signal and light of each wavelength separated from the TE mode optical signal converted from the TM mode optical signal.

[0027] As will be described later, the photodiode 10 is configured so that an optical signal input to one port is unlikely to be transmitted to the other port. In other words, the optical signal is unlikely to return toward the source of the optical signal. If the optical signal returns toward the source of the optical signal, the light source or modulator of the source of the optical signal may become unstable. Therefore, by reducing the returning light, the light source or modulator of the source of the optical signal can be protected. As a result, the reliability of the optical communication system can be improved.

[0028] (Configuration example of photodiode 10) 2 and 3, a photodiode 10 according to an embodiment of the present disclosure includes a light-receiving waveguide 40 and a light-receiving section 41. The light-receiving waveguide 40 is connected to a first input waveguide 20 and a second input waveguide 30.

[0029] In the first input waveguide 20, an optical signal propagates in the positive direction of the Z axis and is input to the receiving waveguide 40. The first input waveguide 20 may include a tapered section 22 whose width changes continuously in a direction (X-axis direction) intersecting the propagation direction of the optical signal. The tapered section 22 is configured so that the width of the first input waveguide 20 matches the width of the receiving waveguide 40 at the position where the first input waveguide 20 and the receiving waveguide 40 are connected.

[0030] In the second input waveguide 30, an optical signal propagates in the negative direction of the Z axis and is input to the receiving waveguide 40. The second input waveguide 30 may include a tapered section 32 whose width continuously changes in a direction (X-axis direction) intersecting the propagation direction of the optical signal. The tapered section 32 is configured so that the width of the second input waveguide 30 matches the width of the receiving waveguide 40 at the position where the second input waveguide 30 and the receiving waveguide 40 are connected.

[0031] The receiving waveguide 40, the first input waveguide 20, and the second input waveguide 30 are arranged side by side in the Z-axis direction on the substrate 50. The first input waveguide 20 and the second input waveguide 30 extend in the Z-axis direction. The receiving waveguide 40, the first input waveguide 20, and the second input waveguide 30 are arranged on a first insulating layer 51 on the substrate 50, and are surrounded by the first insulating layer 51 and the second insulating layer 52 in a cross-sectional view as illustrated in FIG. 3. The receiving waveguide 40, the first input waveguide 20, and the second input waveguide 30 function as cores of the waveguides. The first insulating layer 51 and the second insulating layer 52 function as claddings of the waveguides.

[0032] The light-receiving section 41 is located on the light-receiving waveguide 40. The light-receiving section 41 functions as the core of the waveguide together with the light-receiving waveguide 40. In the area where the light-receiving section 41 is located, the core has a structure in which the light-receiving section 41 and the light-receiving waveguide 40 are stacked, and extends overall in the positive direction of the Y-axis. The core in which the light-receiving section 41 and the light-receiving waveguide 40 are stacked is surrounded by a first insulating layer 51, a second insulating layer 52, and a third insulating layer 53. The third insulating layer 53 functions as the cladding of the waveguide together with the first insulating layer 51 and the second insulating layer 52.

[0033] Light propagating through the light-receiving waveguide 40 propagates to the light-receiving section 41. Electrons in the light-receiving section 41 are excited by the energy of the light propagating to the light-receiving section 41. As a result, the potential of the light-receiving section 41 becomes lower than the potential of the light-receiving waveguide 40. That is, a potential difference occurs between the light-receiving section 41 and the light-receiving waveguide 40. The photodiode 10 outputs a photocurrent, which flows due to the potential difference generated between the light-receiving section 41 and the light-receiving waveguide 40, to an external device. The photocurrent corresponds to an electrical signal that corresponds to the combined intensity of the optical signals input to the first input waveguide 20 and the second input waveguide 30.

[0034] The configuration of the photodiode 10 according to this embodiment can be rephrased as follows: The optical signal input to the first input waveguide 20 is also referred to as the first optical signal. The direction in which the first optical signal is input (the positive direction of the Z axis) is also referred to as the first direction. The optical signal input to the second input waveguide 30 is also referred to as the second optical signal. The direction in which the second optical signal is input (the negative direction of the Z axis) is also referred to as the second direction. The receiving optical waveguide 40 is connected to the first input waveguide 20 and the second input waveguide 30. The receiving unit 41 outputs an electrical signal corresponding to the intensity of a signal obtained by combining the first optical signal and the second optical signal input to the receiving optical waveguide 40. The receiving unit 41 is configured to reduce the intensity of an optical signal returning in the first input waveguide 20 in the direction opposite to the first direction in which the first optical signal propagates, and the intensity of an optical signal returning in the second input waveguide 30 in the direction opposite to the second direction in which the second optical signal propagates.

[0035] <Materials of each component of the photodiode 10> The substrate 50 may be configured to include a semiconductor such as silicon, a conductor such as metal, or an insulator such as glass or resin. In this embodiment, the substrate 50 is made of silicon (Si), but is not limited to this and may be made of various other materials.

[0036] The light-receiving waveguide 40, the light-receiving section 41, the first input waveguide 20, and the second input waveguide 30, which function as cores, and the first insulating layer 51, the second insulating layer 52, and the third insulating layer 53, which function as claddings, may be configured to contain a dielectric. The light-receiving waveguide 40, the light-receiving section 41, the first input waveguide 20, and the second input waveguide 30 are also referred to as dielectric lines. The materials of the core and the cladding are determined so that the relative dielectric constant of the core is greater than the relative dielectric constant of the cladding. In other words, the materials of the core and the cladding are determined so that the refractive index of the core with respect to the propagating light is greater than the refractive index of the cladding. In this way, light propagating through the core can be totally reflected at the boundary with the cladding. As a result, loss of the optical signal propagating through the core can be reduced.

[0037] In this embodiment, the material of the light-receiving waveguide 40, the first input waveguide 20, and the second input waveguide 30, which function as cores, is silicon (Si). The material of the core is not limited to the above example, and various other materials may be used.

[0038] The material of the light-receiving section 41, which functions as a core together with the light-receiving waveguide 40, is germanium (Ge). The light-receiving section 41 is configured as a pin diode having a germanium pin structure formed by epitaxial growth on the light-receiving waveguide 40. The configuration of the light-receiving section 41 is not limited to the example described above.

[0039] In this embodiment, the first insulating layer 51, the second insulating layer 52, and the third insulating layer 53, which function as claddings, are made of silica glass or silicon oxide (SiO2). The cladding material is not limited to the above examples and may be various other materials. The relative dielectric constant of silicon is approximately 12. The relative dielectric constant of germanium is approximately 16. The relative dielectric constant of silica glass is approximately 2. Silicon can propagate electromagnetic waves having near-infrared wavelengths of approximately 1.2 μm to approximately 6 μm with low loss. When the receiving waveguide 40, the first input waveguide 20, and the second input waveguide 30 are made of silicon, they can propagate electromagnetic waves having wavelengths in the 1.3 μm band or the 1.55 μm band used in optical communications with low loss.

[0040] The relative dielectric constants of the core and the cladding may be made larger than that of air. By making the relative dielectric constants of the core and the cladding larger than that of air, leakage of electromagnetic waves from the photodiode 10 can be reduced. As a result, loss due to radiation of electromagnetic waves from the photodiode 10 to the outside can be reduced.

[0041] The receiving waveguide 40, the first input waveguide 20 and the second input waveguide 30, which function as cores, may satisfy the wave-guiding conditions in a single mode or in a multimode.

[0042] The photodiode 10 may further include an anode electrode 43 connected to the light-receiving portion 41 through a via electrode 45, and a cathode electrode 44 connected to the electrode region 42 of the light-receiving waveguide 40 through the via electrode 45. A photocurrent flowing between the light-receiving portion 41 and the light-receiving waveguide 40 flows between the anode electrode 43 and the cathode electrode 44, and is output to an external device. The anode electrode 43, the cathode electrode 44, and the via electrode 45 may be composed of a material including a metal or a semiconductor. The electrode region 42 may be formed as a doped region in which impurities are implanted into silicon constituting the light-receiving waveguide 40 in order to reduce electrical resistance between the via electrode 45 and the light-receiving waveguide 40.

[0043] <Other Configuration Examples of the Photodiode 10> 4, the photodiode 10 may be configured such that a second insulating layer 52 is sandwiched between the light-receiving section 41 and the light-receiving waveguide 40. In this case, the electrode region 42 is formed in the light-receiving section 41. The second insulating layer 52 is configured to have a thickness equal to or less than a predetermined value so that light propagating into the light-receiving waveguide 40 can pass through the second insulating layer 52 and propagate to the light-receiving section 41. The thin second insulating layer 52 on the light-receiving waveguide 40 can protect the light-receiving waveguide 40 when the light-receiving section 41 is formed.

[0044] <Comparative Example> 5, the photodiode 90 according to the comparative example includes a light-receiving waveguide 940, a light-receiving portion 941, an electrode region 942, a cathode electrode 943, an anode electrode 944, and a substrate 950. The light-receiving waveguide 940 is connected to the input waveguide 920.

[0045] In the input waveguide 920, the optical signal travels in the positive direction of the Z axis and is input to the receiving waveguide 940. The input waveguide 920 includes a tapered section 922 whose width changes continuously in a direction (X axis direction) intersecting the traveling direction of the optical signal. The receiving waveguide 940 and the input waveguide 920 are arranged side by side in the Z axis direction on a substrate 950. The input waveguide 920 extends in the Z axis direction. The receiving section 941 is located above the receiving waveguide 940.

[0046] Light propagating through the light-receiving waveguide 940 propagates to the light-receiving section 941. Electrons in the light-receiving section 941 are excited by the energy of the light propagating to the light-receiving section 941. As a result, the potential of the light-receiving section 941 becomes lower than the potential of the light-receiving waveguide 940. That is, a potential difference occurs between the light-receiving section 941 and the light-receiving waveguide 940. The photodiode 90 outputs a photocurrent flowing between the cathode electrode 943 and the anode electrode 944 due to the potential difference generated between the light-receiving section 941 and the light-receiving waveguide 940 to an external device. The photocurrent corresponds to an electrical signal according to the intensity of one optical signal input to one input waveguide 920.

[0047] As shown in FIG. 6, the optical receiver 9 according to the comparative example includes an input unit 981, a polarization splitter rotator (PSR) 982, a demultiplexer (DEMUX) 983, a photodiode (PD) 90, and transimpedance amplifiers (TIA) 93-1 to 93-n.

[0048] The input unit 981 accepts an input optical signal. The polarization splitter rotator 982 separates the TE mode optical signal and the TM mode optical signal contained in the input optical signal, and converts the TM mode optical signal into a TE mode optical signal. The demultiplexer 983 separates, by wavelength, the TE mode optical signal separated by the polarization splitter rotator 982 and the TE mode optical signal converted from the TM mode optical signal separated by the polarization splitter rotator 982. The demultiplexer 983 separates the optical signal into light of n wavelengths represented by λ1 to λn.

[0049] The photodiode 90 includes photodiodes 91-1 to 91-n connected to a demultiplexer 983 that separates the TE mode optical signal by wavelength. The photodiode 90 also includes photodiodes 92-1 to 92-n connected to a demultiplexer 983 that separates the optical signal converted from TM mode to TE mode by the polarization splitter rotator 82 by wavelength. The transimpedance amplifier 93-1 outputs an electrical signal V1 obtained by adding together the electrical signal output from the photodiode 91-1 and the electrical signal output from the photodiode 92-1. The transimpedance amplifier 93-n outputs an electrical signal Vn obtained by adding together the electrical signal output from the photodiode 91-n and the electrical signal output from the photodiode 92-n. In other words, in the optical receiver 9 according to the comparative example, the photodiode 90 can only output an electrical signal corresponding to the intensity of one optical signal, and therefore transimpedance amplifiers 93-1 to 93-n that add together the electrical signals output from the photodiodes 91-1 to 92-1 to 92-n are required.

[0050] Here, when electrical signals from two photodiodes 90 are input in parallel to one transimpedance amplifier 93, the parasitic capacitance of the photodiode 90 doubles. The increased parasitic capacitance increases the RC time constant. The increased RC time constant slows the response of the photodiode 90. Furthermore, when electrical signals from one photodiode 90 are input to one transimpedance amplifier 93 and the outputs of the transimpedance amplifiers 93 are added together in a separate analog or digital circuit, the circuit size and power consumption increase.

[0051] On the other hand, the photodiode 10 according to this embodiment is configured to output an electrical signal corresponding to the combined intensity of two optical signals. The optical receiver 1 using the photodiode 10 according to this embodiment does not require a configuration in which two photodiodes 10 are connected in parallel, and therefore can maintain a high response speed. Furthermore, the optical receiver 1 using the photodiode 10 according to this embodiment does not require a circuit for adding two electrical signals. As a result, an increase in circuit size or power consumption can be avoided.

[0052] As another comparative example, consider a photodiode that outputs an electrical signal corresponding to the intensities of two optical signals, configured so that the two optical signals are simply incident on a single optical receiver. In this configuration, the optical signal incident on the optical receiver may be reflected and become a return light. Also, an optical signal input from one of two ports that accepts two optical signals may pass through the optical receiver and propagate to the other port, becoming a return light. If the return light optical signal returns to the light source or modulator, the operation of the light source, modulator, etc. may become unstable.

[0053] On the other hand, the photodiode 10 according to this embodiment is configured to reduce the optical signal returning to each of the first input waveguide 20 and the second input waveguide 30. In other words, the photodiode 10 according to this embodiment is configured to reduce the returning light. According to the optical receiver 1 using the photodiode 10 according to this embodiment, the returning light can be reduced, thereby protecting the operation of the light source, modulator, etc. As a result, the reliability of the optical communication system can be improved.

[0054] (Configuration example of photodiode 10 that reduces feedback light) As described above, the photodiode 10 according to this embodiment is configured so that an optical signal input to one port is unlikely to be transmitted to the other port. In other words, the photodiode 10 according to this embodiment is configured to reduce feedback light. Below, configuration examples for reducing feedback light, including a linear type, a cross type, and a multi-mode interference (MMI) type, will be described.

[0055] <Linear type> 2, the linear photodiode 10 is configured such that the first input waveguide 20 and the second input waveguide 30 are positioned along a single straight line (Z-axis). In other words, a first direction in which the first optical signal input from the first input waveguide 20 propagates and a second direction in which the second optical signal input from the second input waveguide 30 propagate may be substantially parallel to each other.

[0056] The optical signals propagating from the first input waveguide 20 and the second input waveguide 30 propagate to the light receiving unit 41 and are absorbed by the light receiving unit 41. Here, a portion of the optical signal propagating from the first input waveguide 20 in the positive direction of the Z axis may be reflected at the end of the light receiving unit 41 on the negative side of the Z axis to become return light to the first input waveguide 20. Also, a portion of the optical signal propagating from the second input waveguide 30 in the negative direction of the Z axis may be reflected at the end of the light receiving unit 41 on the positive side of the Z axis to become return light to the second input waveguide 30.

[0057] 7 , the light receiving unit 41 of the photodiode 10 according to this embodiment may be configured such that a normal 411A of an end face 411 connected to the first input waveguide 20 is inclined with respect to the optical axis 20A of the first input waveguide 20. Furthermore, the light receiving unit 41 may be configured such that a normal 412A of an end face 412 connected to the second input waveguide 30 is inclined with respect to the optical axis 30A of the second input waveguide 30. By inclining the end faces 411 and 412 of the light receiving unit 41 with respect to the optical axes 20A and 30A, optical signals reflected by the end faces 411 and 412 propagate in directions deviated from the optical axes 20A and 30A, making it difficult for them to return to the first input waveguide 20 and the second input waveguide 30. Furthermore, when the normal 411A is inclined with respect to the optical axis 20A, the traveling direction of light incident on the light receiving unit 41 is inclined with respect to the optical axis 20A due to refraction of light. In this case, the proportion of light that passes through the light receiving section 41 without being absorbed by the light receiving section 41 and that enters the opposing second input waveguide 30 as returned light can be reduced.

[0058] 8, the light receiving unit 41 of the photodiode 10 according to this embodiment may have a convex portion 413 on the end face 411. The light receiving unit 41 may have a convex portion 414 on the end face 412. The convex portions 413 and 414 may have an inverse tapered shape in plan view (when viewed from the Y-axis direction) such that the width in the X-axis direction increases in the direction in which the optical signal propagates (the width in the X-axis direction decreases in the direction away from the light receiving unit 41). By having the convex portions 413 and 414 on the end faces 411 and 412, the optical signal propagating to be input to the light receiving unit 41 is less likely to be reflected in the direction opposite to the input by the portions of the convex portions 413 and 414 that are narrow in the X-axis direction. As a result, the optical signal is less likely to return to the first input waveguide 20 and the second input waveguide 30.

[0059] In the example of FIG. 8 , the axis 413A of the convex portion 413 is aligned with the direction of the optical axis 20A of the first input waveguide 20 (the Z-axis direction). Furthermore, the axis 414A of the convex portion 414 is aligned with the direction of the optical axis 30A of the second input waveguide 30 (the Z-axis direction). As illustrated in FIG. 9 , the axis 413A of the convex portion 413 may be inclined with respect to the direction of the optical axis 20A (the Z-axis direction). Furthermore, the axis 414A of the convex portion 414 may be inclined with respect to the direction of the optical axis 30A (the Z-axis direction). In this way, the optical signals reflected at the tips of the convex portions 413 and 414 propagate in directions deviated from the optical axes 20A and 30A, and are less likely to return to the first input waveguide 20 and the second input waveguide 30.

[0060] As described above, in the linear photodiode 10, the shapes of the end faces 411 and 412 of the light receiving section 41 are set so as to reduce the return light caused by reflection at the end faces 411 and 412.

[0061] <Cross type> 10, the cross-type photodiode 10 is configured so that the first input waveguide 20 and the second input waveguide 30 cross each other. In other words, the angle formed between the first direction in which the first optical signal input from the first input waveguide 20 propagates and the second direction in which the second optical signal input from the second input waveguide 30 propagates may be within a predetermined range including 90 degrees.

[0062] Optical signals propagating from the first input waveguide 20 and the second input waveguide 30 propagate to the light-receiving section 41 and are absorbed by the light-receiving section 41. Here, a portion of the optical signal propagating from the first input waveguide 20 in the positive direction of the Z axis passes through the light-receiving section 41 (without being absorbed by the light-receiving section 41) and continues propagating in the positive direction of the Z axis. However, because the second input waveguide 30 intersects with the first input waveguide 20, the optical signal propagating from the first input waveguide 20 in the positive direction of the Z axis is less likely to propagate to the second input waveguide 30 even after passing through the light-receiving section 41. In addition, a portion of the optical signal propagating from the second input waveguide 30 in the positive direction of the X axis passes through the light-receiving section 41 (without being absorbed by the light-receiving section 41) and continues propagating in the positive direction of the X axis. However, because the first input waveguide 20 intersects with the second input waveguide 30, an optical signal propagating from the second input waveguide 30 in the positive direction of the X-axis is less likely to propagate to the first input waveguide 20 even after passing through the light-receiving unit 41. As described above, because the first input waveguide 20 and the second input waveguide 30 intersect, an optical signal input from one waveguide is less likely to propagate to the other waveguide and become returned light even after passing through the light-receiving unit 41.

[0063] 10, the light receiving unit 41 may be configured such that a normal 411A of an end face 411 connected to the first input waveguide 20 is inclined with respect to the optical axis 20A of the first input waveguide 20, as illustrated in Fig. 11. Also, the light receiving unit 41 may be configured such that a normal 412A of an end face 412 connected to the second input waveguide 30 is inclined with respect to the optical axis 30A of the second input waveguide 30.

[0064] By tilting the end faces 411 and 412 of the light receiving unit 41 with respect to the optical axes 20A and 30A, the optical signals reflected by the end faces 411 and 412 propagate in directions deviated from the optical axes 20A and 30A, and are less likely to return to the first input waveguide 20 and the second input waveguide 30. In this way, the return light caused by the reflection of the optical signals at the end faces 411 and 412 can be reduced.

[0065] 10, the light receiving section 41 may have a convex portion 413 on the end face 411, as illustrated in FIG. 12. The light receiving section 41 may have a convex portion 414 on the end face 412. The convex portions 413 and 414 may have an inverse tapered shape in a plan view (when viewed from the Y-axis direction) such that the width in the X-axis direction increases toward the direction in which the optical signal propagates (the width in the X-axis direction decreases toward the direction away from the light receiving section 41). By having the convex portions 413 and 414 on the end faces 411 and 412, the optical signal propagating to be input to the light receiving section 41 is less likely to be reflected in the direction opposite to the input by the portions of the convex portions 413 and 414 that are narrow in the X-axis direction. As a result, the optical signal is less likely to return to the first input waveguide 20 and the second input waveguide 30.

[0066] 12, the axis 413A of the convex portion 413 is aligned with the direction of the optical axis 20A of the first input waveguide 20 (Z-axis direction). Furthermore, the axis 414A of the convex portion 414 is aligned with the direction of the optical axis 30A of the second input waveguide 30 (Z-axis direction). As illustrated in FIG. 13, the axis 413A of the convex portion 413 may be inclined with respect to the direction of the optical axis 20A (Z-axis direction). Furthermore, the axis 414A of the convex portion 414 may be inclined with respect to the direction of the optical axis 30A (Z-axis direction). In this way, the optical signals reflected at the tips of the convex portions 413 and 414 propagate in directions deviated from the optical axes 20A and 30A, and are less likely to return to the first input waveguide 20 and the second input waveguide 30.

[0067] As described above, the cross-type photodiode 10 can reduce the return light caused by the optical signal transmitted through the light-receiving portion 41. Furthermore, the shapes of the end faces 411 and 412 of the light-receiving portion 41 may be set so as to reduce the return light caused by reflection at the end faces 411 and 412 of the light-receiving portion 41.

[0068] The configuration of the cross-type photodiode 10 can be rephrased as follows. The end face 411 is also referred to as the first face. The end face 412 is also referred to as the second face. The light receiving section 41 has a first face located on the first direction side and a second face located on the second direction side. At least a portion of the first face may include a face having a normal direction that is non-parallel to the first direction. Furthermore, at least a portion of the second face may include a face having a normal direction that is non-parallel to the second direction.

[0069] <Multi-mode interference type (MMI type)> 14, the MMI photodiode 10 is configured so that an optical signal propagated from the first input waveguide 20 to the receiving waveguide 40 and an optical signal propagated from the second input waveguide 30 to the receiving waveguide 40 cause multimode interference in the receiving waveguide 40. In other words, the MMI photodiode 10 is configured so that the receiving waveguide 40 serves as a multimode waveguide and operates as a multimode interference system.

[0070] Specifically, in the MMI photodiode 10, the first input waveguide 20 and the receiving waveguide 40 are discontinuously connected. Also, the second input waveguide 30 and the receiving waveguide 40 are discontinuously connected. The state in which the first input waveguide 20 and the receiving waveguide 40 are discontinuously connected may correspond to a state in which the width of the first input waveguide 20 and the width of the receiving waveguide 40 are different at the connection portion between the first input waveguide 20 and the receiving waveguide 40. In other words, the state in which the first input waveguide 20 and the receiving waveguide 40 are discontinuously connected may correspond to a state in which the first input waveguide 20 without the tapered portion 22 (see FIG. 2) is connected to the receiving waveguide 40.

[0071] The state in which the first input waveguide 20 and the receiving waveguide 40 are discontinuously connected may correspond to a state in which the rate of change of the width of the receiving waveguide 40 relative to the width of the first input waveguide 20 when traveling along the first direction is equal to or greater than a predetermined value. The rate of change is calculated as half the amount of change in the width of the waveguide when traveling a unit distance along the first direction. The reason for setting the rate to half the amount of change in the width of the waveguide is that the shape of the end of the waveguide is considered to be line-symmetric with respect to the first direction. The predetermined value may be set to, for example, 1. When the predetermined value is 1, the direction in which the line representing the end of the receiving waveguide 40 extends forms a 45-degree angle with the first direction. The state in which the second input waveguide 30 and the receiving waveguide 40 are discontinuously connected may correspond to a state in which the rate of change of the width of the receiving waveguide 40 relative to the width of the second input waveguide 30 when traveling along the second direction is equal to or greater than a predetermined value. In other words, this can correspond to a state in which the rate of change of the width of the receiving waveguide 40 relative to the widths of the first input waveguide 20 and the second input waveguide 30 when traveling along the propagation direction of the optical signal is equal to or greater than a predetermined value.

[0072] In the MMI photodiode 10, the light receiving section 41 is disposed at the light collecting position of the multi-mode waveguide, so that the light receiving section 41 can absorb the optical signal with high efficiency.

[0073] In the MMI photodiode 10, the intensity of an optical signal propagating from the first input waveguide 20 to the receiving waveguide 40 may be distributed in the receiving waveguide 40 as shown in FIG. 15. In FIG. 15, the first input waveguide 20 and the receiving waveguide 40 are connected at a position Z=0. The second input waveguide 30 and the receiving waveguide 40 are connected at a position Z=L. The light receiving section 41 is located at the center of the receiving waveguide 40, at a position Z=L / 2.

[0074] In Fig. 15, areas with high brightness (areas close to white) represent areas where the intensity of the optical signal is high. The positions where the intensity of the optical signal is high are the light-condensing positions of the multi-mode waveguide. In Fig. 15, the light-receiving unit 41 is disposed at the light-condensing position.

[0075] The intensity distribution of the optical signal illustrated in FIG. 15 represents the intensity distribution of the optical signal when the light-receiving portion 41 is not disposed in the light-receiving waveguide 40. When the light-receiving portion 41 is not disposed, the light is condensed again even at the position of Z = L, and the intensity of the optical signal becomes high. Here, when the light-receiving portion 41 having a refractive index close to that of the light-receiving waveguide 40 is disposed in the light-receiving waveguide 40, as illustrated in FIG. 16, the intensity of the optical signal becomes low in the region of Z > L / 2. Further, by changing the condensing point of the MMI to a position of Z < L, the intensity of the optical signal at the position of the second input waveguide 30 (the position of Z = L) is greatly reduced. By greatly reducing the intensity of the optical signal at the position of Z = L, among the optical signals propagated from the first input waveguide 20, the optical signals that are transmitted through the second input waveguide 30 and become return light are greatly reduced. In the configuration illustrated in FIG. 16, the intensity of the return light to the second input waveguide 30 is -20 dB or less with respect to the intensity of the optical signal propagated from the first input waveguide 20 when there is no light absorption in the light-receiving portion 41. When considering the light absorption by the germanium constituting the light-receiving portion 41, the intensity of the return light to the second input waveguide 30 is -30 dB or less with respect to the intensity of the optical signal propagated from the first input waveguide 20.

[0076] If the length of the light-receiving section 41 in the Z-axis direction is longer than that of the configuration illustrated in FIG. 16 , there exists a length over which the light transmitted through the light-receiving section 41 is again confined in the receiving light waveguide 40. As a result, as illustrated in FIG. 17 , the intensity of the optical signal in the region Z>L / 2 becomes higher than the intensity illustrated in FIG. 16 . Furthermore, as the light-focusing point of the MMI becomes closer to the position Z=L, the intensity of the optical signal at the position Z=L becomes higher than the intensity illustrated in FIG. 16 . From the above, the dimensions of the light-receiving section 41 in the MMI photodiode 10 affect the return light resulting from transmission through the light-receiving section 41. In other words, when the dimension of the light-receiving section 41 in the Z-axis direction is appropriate, the component of the light transmitted in the Z-axis direction that exits from above and below (the Y-axis direction) the receiving light waveguide 40 increases. As a result, the intensity of the light confined in the receiving light waveguide 40 becomes smaller in the region Z>L / 2. On the other hand, if the dimension of the light-receiving section 41 in the Z-axis direction is different from the appropriate length, the length may be such that light is just coupled into the receiving light waveguide 40 at the end of the light-receiving section 41. In this case, the intensity of light propagating through the receiving light waveguide 40 increases in the region where Z>L / 2, as shown in FIG. 17 . As a result, optical interference similar to that of MMI also occurs in the Y-axis direction in the region where the light-receiving section 41 is present. As a result of the optical interference, depending on the length of the light-receiving section 41, the optical intensity may increase at the transmission-side end of the light-receiving section 41 at the height of the receiving light waveguide 40, as shown in FIG. 17 , or decrease as shown in FIG. 16 .

[0077] 16 and 17, when the dimension of the light receiving section 41 in the Z-axis direction is small, the intensity of the optical signal transmitted to the second input waveguide 30 is low. This shows that in the MMI photodiode 10, it is possible to achieve both an increase in the absorption rate in the light receiving section 41 and a reduction in the size of the light receiving section 41.

[0078] 18, the first input waveguide 20 and the second input waveguide 30 may also intersect with each other. When the first input waveguide 20 and the second input waveguide 30 intersect with each other, an optical signal propagating from the first input waveguide 20 and passing through the light-receiving section 41 propagates to a portion of the light-receiving waveguide 40 to which the second input waveguide 30 is not connected. As a result, even when the optical signal propagating from the first input waveguide 20 passes through the light-receiving section 41, the return light propagating to the second input waveguide 30 can be reduced.

[0079] 19 , the receiving waveguide 40 may include a terminating waveguide 46 at the end of the propagation direction of the optical signal from the first input waveguide 20 and at the end of the propagation direction of the optical signal from the second input waveguide 30. The terminating waveguide 46 located at the end of the propagation direction of the optical signal from the first input waveguide 20 (on the opposite side of the receiving waveguide 40 from the first input waveguide 20 along the first direction) is also referred to as a first terminating waveguide. The terminating waveguide 46 located at the end of the propagation direction of the optical signal from the second input waveguide 30 (on the opposite side of the receiving waveguide 40 from the second input waveguide 30 along the second direction) is also referred to as a second terminating waveguide. The receiving waveguide 40 may have at least one of a first terminating waveguide or a second terminating waveguide.

[0080] When the receiving waveguide 40 includes a terminating waveguide 46, the dimensions of the receiving waveguide 40 may be set so that the portion to which the terminating waveguide 46 is connected becomes the light collecting position. The terminating waveguide 46 may include an antenna that radiates electromagnetic waves. The terminating waveguide 46 may include an absorbing member that absorbs electromagnetic waves. The terminating waveguide 46 may be disposed at an end of the receiving waveguide 40 in the width direction, as illustrated in FIG. 19 . The terminating waveguide 46 may be disposed over the entire width of the receiving waveguide 40, or may be disposed at any position in the width direction of the receiving waveguide 40.

[0081] 20 , the receiving waveguide 40 may have notches 47 between the portion where the first input waveguide 20 is connected and the portion where the light receiving unit 41 is provided, and between the portion where the second input waveguide 30 is connected and the portion where the light receiving unit 41 is provided. The notches 47 are configured to prevent the propagation of optical signals other than the optical signals collected by the light receiving unit 41 among the optical signals propagated from the first input waveguide 20 and the second input waveguide 30. By doing so, the intensity of the optical signals passing through the light receiving unit 41 can be reduced.

[0082] The MMI photodiode 10 may be configured so that the position of the light receiving section 41 in the light receiving waveguide 40 is offset from the center of the light receiving waveguide 40 in the traveling direction (Z-axis direction) of the optical signal. As illustrated in Fig. 21 , a distance L1 from the connection position between the first input waveguide 20 and the light receiving waveguide 40 to the position of the light receiving section 41 may be different from a distance L2 from the connection position between the second input waveguide 30 and the light receiving waveguide 40 to the position of the light receiving section 41. L1 and L2 may be set to match the period at which the focusing position of the optical signal appears in the Z-axis direction.

[0083] When L1 and L2 are different, the polarization mode of the optical signal propagating from the first input waveguide 20 may be different from the polarization mode of the optical signal propagating from the second input waveguide 30. Specifically, the optical signal propagating from the first input waveguide 20 may be an optical signal in TE mode, and the optical signal propagating from the second input waveguide 30 may be an optical signal in TM mode. In this case, L1 and L2 may be set according to the focusing positions of the optical signals propagating in each polarization mode.

[0084] When the polarization mode of the optical signal propagating from the first input waveguide 20 differs from the polarization mode of the optical signal propagating from the second input waveguide 30, the receiving waveguide 40 can be configured to collect the optical signal propagating from the first input waveguide 20 at a position close to the end in the width direction at the end on the side connected to the second input waveguide 30. By doing so, the optical signal propagating from the first input waveguide 20 that propagates to the second input waveguide 30 and becomes returned light can be reduced.

[0085] 22 , the receiving waveguide 40 may include a terminating waveguide 46 at the end in the propagation direction of the optical signal from the first input waveguide 20 (at the end on the side connected to the second input waveguide 30). The terminating waveguide 46 may be connected at a position where the optical signal propagated from the first input waveguide 20 is focused at the end on the side connected to the second input waveguide 30. The receiving waveguide 40 may also include a terminating waveguide 46 at the end in the propagation direction of the optical signal from the second input waveguide 30 (at the end on the side connected to the first input waveguide 20). The terminating waveguide 46 may be connected at a position where the optical signal propagated from the second input waveguide 30 is focused at the end on the side connected to the first input waveguide 20.

[0086] 23 , the receiving waveguide 40 may have notches 47 between the portion where the first input waveguide 20 is connected and the portion where the receiving unit 41 is provided, and between the portion where the second input waveguide 30 is connected and the portion where the receiving unit 41 is provided. The notches 47 are configured to prevent the propagation of optical signals other than the optical signals collected by the receiving unit 41 among the optical signals propagating from the first input waveguide 20 and the second input waveguide 30. By doing so, the intensity of the optical signals passing through the receiving unit 41 can be reduced.

[0087] 24 , the light receiving unit 41 may be configured such that a normal 411A of an end face 411 connected to the first input waveguide 20 is inclined with respect to the optical axis 20A of the first input waveguide 20. Furthermore, the light receiving unit 41 may be configured such that a normal 412A of an end face 412 connected to the second input waveguide 30 is inclined with respect to the optical axis 30A of the second input waveguide 30. By inclining the end faces 411 and 412 of the light receiving unit 41 with respect to the optical axes 20A and 30A, the optical signals reflected by the end faces 411 and 412 propagate in directions deviated from the optical axes 20A and 30A, and are less likely to return to the first input waveguide 20 and the second input waveguide 30. In this way, the return light caused by the reflection of the optical signals at the end faces 411 and 412 can be reduced.

[0088] 25, the light receiving unit 41 may have a convex portion 413 on the end face 411. The light receiving unit 41 may have a convex portion 414 on the end face 412. The convex portions 413 and 414 may have an inverted tapered shape in a plan view (when viewed from the Y-axis direction) such that the width in the X-axis direction increases toward the direction in which the optical signal propagates (the width in the X-axis direction decreases toward the direction away from the light receiving unit 41). By having the convex portions 413 and 414 on the end faces 411 and 412, the optical signal propagating to be input to the light receiving unit 41 is less likely to be reflected in the direction opposite to the input by the portions of the convex portions 413 and 414 that are narrow in the X-axis direction. As a result, the optical signal is less likely to return to the first input waveguide 20 and the second input waveguide 30.

[0089] 25, the axis 413A of the convex portion 413 is aligned with the direction of the optical axis 20A of the first input waveguide 20 (the Z-axis direction). Furthermore, the axis 414A of the convex portion 414 is aligned with the direction of the optical axis 30A of the second input waveguide 30 (the Z-axis direction). As illustrated in FIG. 26, the axis 413A of the convex portion 413 may be inclined with respect to the direction of the optical axis 20A (the Z-axis direction). Furthermore, the axis 414A of the convex portion 414 may be inclined with respect to the direction of the optical axis 30A (the Z-axis direction). In this way, the optical signals reflected at the tips of the convex portions 413 and 414 propagate in directions deviated from the optical axes 20A and 30A, and are less likely to return to the first input waveguide 20 and the second input waveguide 30.

[0090] 27, the MMI photodiode 10 may be configured such that the optical axis 20A of the first input waveguide 20 and the optical axis 30A of the second input waveguide 30 are offset from the central axis 40A of the receiving light waveguide 40 in a direction (X-axis direction) that intersects with the propagation direction of the optical signal (Z-axis direction). In Fig. 27, the light receiving section 41 is disposed on the central axis 40A of the receiving light waveguide 40, but may be disposed so as to deviate from the central axis 40A in the width direction (X-axis direction).

[0091] As described above, the MMI photodiode 10 can reduce the return light caused by the optical signal transmitted through the light receiving portion 41. Furthermore, the shapes of the end faces 411 and 412 of the light receiving portion 41 may be set so as to reduce the return light caused by reflection at the end faces 411 and 412 of the light receiving portion 41.

[0092] <Example of Manufacturing Method of Photodiode 10> An example of a method for manufacturing the photodiode 10 according to the present disclosure will now be described.

[0093] As an example of a manufacturing method, the material of the receiving waveguide 40, the first input waveguide 20, and the second input waveguide 30 may be, for example, silicon. The receiving waveguide 40, the first input waveguide 20, and the second input waveguide 30 may be formed by processing the top silicon layer of a silicon-on-insulator (SOI) substrate using photolithography and reactive ion etching. Each waveguide may be formed as a buried channel type waveguide in which the silicon other than the waveguide is completely etched away. Each waveguide may be formed as a rib type optical waveguide in which a certain thickness of silicon remains. After etching the silicon, a silicon oxide layer is deposited by chemical vapor deposition. The deposited silicon oxide layer is then planarized by chemical mechanical polishing or the like.

[0094] By photolithography and ion implantation, phosphorus ions are implanted into the region of the light-receiving waveguide 40 where the light-receiving portion 41 will be formed and into the region that will become the electrode region 42. Subsequently, by photolithography and reactive ion etching or wet etching using hydrofluoric acid or the like, the oxide film on the silicon in the region where the light-receiving portion 41 will be formed is removed.

[0095] Germanium is epitaxially grown using metal-organic vapor phase epitaxy (MOVPE) or other methods. During this process, germanium is deposited only in the areas where silicon is exposed. By varying the dopant used during epitaxial growth, the germanium layer can be deposited in order from the silicon substrate side to n-type, undoped, and p-type, forming a pin diode perpendicular to the substrate.

[0096] The silicon oxide film is removed and phosphorus ions are implanted in the region that will become the electrode region 42 in the light-receiving waveguide 40. Boron ions are implanted into the upper part of the germanium layer.

[0097] A silicon oxide film is deposited by chemical vapor deposition and planarized by chemical mechanical polishing. The silicon oxide film on the light receiving portion 41 and the electrode region 42 is removed by photolithography and reactive ion etching. A via electrode 45 is formed by filling with tungsten by chemical vapor deposition. An aluminum thin film is deposited by sputtering. Wiring including an anode electrode 43 and a cathode electrode 44 is formed by processing the aluminum thin film by photolithography and reactive ion etching.

[0098] As another example of a manufacturing method, the light-receiving waveguide 40, the first input waveguide 20, and the second input waveguide 30 may be formed in the substrate 50, and after the waveguides are buried and planarized with a silicon oxide film, a compound semiconductor substrate manufactured in a separate process may be bonded to the substrate. The compound semiconductor substrate may be, for example, an InP substrate. In the InP substrate, a pin layer containing InGaAs and InAlGaAs is formed as the light-receiving portion 41 by epitaxial growth using MOVPE. A silicon oxide film is formed on the top surface by PECVD (plasma-enhanced chemical vapor deposition), and the surface is planarized by chemical mechanical polishing.

[0099] The substrate 50 on which the waveguide is formed and the InP substrate on which the light-receiving portion 41 is formed are bonded by a dehydration condensation process. The InP substrate is removed by etching using hydrochloric acid or the like. As a result, only the light-receiving portion 41 remains bonded to the substrate 50. Only the contact layer of the light-receiving portion 41 remains by photolithography and reactive ion etching. Unnecessary portions of the contact layer are removed by photolithography and reactive ion etching or wet etching. Metal electrodes are formed on the p-electrode and n-electrode by photolithography and lift-off. Examples of metal electrodes that can be used include stacked films of Au / Pt / Ti, Au / Pt / Ti, or Au / Zn / Ti. A silicon oxide film is deposited by PECVD and planarized by CMP (Chemical Mechanical Polishing). Wiring including anode electrode 43 and cathode electrode 44 is formed on the silicon oxide film to connect via electrode 45 to the p-electrode and n-electrode.

[0100] (Configuration example of optical receiver 1) As shown in FIG. 28 , an optical receiver 1 may be configured using a photodiode 10 according to the present disclosure. The optical receiver 1 includes an edge coupler 811 as an input unit 81. The edge coupler 811 is configured to input light from a chip end face. The optical receiver 1 further includes a polarization splitter rotator (PSR) 82 and a demultiplexer (DEMUX) 83. In the optical receiver 1, circuits that process the optical signal before inputting it to the photodiode 10 are collectively referred to as an optical circuit. The optical circuit separates the optical signal input to the optical receiver 1 into an optical signal propagating in TE mode and an optical signal propagating in TM mode. In the optical receiver 1 according to this embodiment, the optical signal propagating in TE mode is assumed to correspond to a first optical signal. The optical signal propagating in TM mode is assumed to correspond to a second optical signal.

[0101] In the optical receiver 1, an optical signal containing a mixture of TE mode optical signals and TM mode optical signals and containing n wavelengths is input to the edge coupler 811. The polarization splitter rotator 82 separates the TE mode optical signals from the TM mode optical signals. The polarization splitter rotator 82 also converts the TM mode optical signals into TE mode optical signals. The demultiplexer 83 further separates each of the optical signals separated by the polarization splitter rotator 82 into n wavelengths.

[0102] Signals obtained by separating a TE mode optical signal for each wavelength and signals obtained by converting a TM mode optical signal into a TE mode optical signal and separating it for each wavelength are input to each of the photodiodes 10-1 to 10-n. As a result, each of the photodiodes 10-1 to 10-n outputs an electrical signal corresponding to the intensity of the signals obtained by separating the optical signal, which is a mixture of TE mode and TM mode and is input to the edge coupler 811, for each wavelength.

[0103] The optical receiver 1 according to this embodiment may further include a transimpedance amplifier that converts the electrical signal output from the photodiode 10. Compared to a case where two photodetectors are used to detect TE mode optical signals and TM mode optical signals for each wavelength, the optical receiver 1 according to this embodiment can detect optical signals with one photodiode for each wavelength. This can achieve higher speeds by reducing parasitic capacitance, or reduced power consumption and circuit size.

[0104] As shown in FIG. 29, the optical receiver 1 may include a two-dimensional grating coupler (2DGC) 812 as an input section 81. The two-dimensional grating coupler 812 splits an input optical signal into two polarization components Px and Py and outputs each polarization component as an optical signal polarized in TE mode. The demultiplexer 83 splits the optical signal of each polarization component by wavelength. A signal obtained by splitting the optical signal of polarization component Px by wavelength and a signal obtained by splitting the optical signal of polarization component Py by wavelength are input to each of the photodiodes 10-1 to 10-n. As a result, the photodiodes 10-1 to 10-n output electrical signals corresponding to the intensities of the signals obtained by splitting the optical signal, which is a mixture of TE mode and TM mode and is input to the two-dimensional grating coupler 812, by wavelength. If the optical receiver 1 includes the two-dimensional grating coupler 812, it is not necessary to include the polarization splitter rotator 82.

[0105] 30, the optical receiver 1 may further include delay devices 84 between the polarization splitter rotator 82 and each of the two demultiplexers 83, and between the demultiplexer 83 and each of the n photodiodes 10-1 to 10-n. The delay devices 84 delay the propagation of the optical signal. The optical receiver 1 compensates for deviations in the delay of the optical signal caused by manufacturing errors in the waveguides using the delay devices 84. By including the delay devices 84, the optical receiver 1 can reduce jitter in the signal output from the photodiode 10, which is a combination of the TE mode optical signal and the TE mode optical signal converted from the TM mode optical signal.

[0106] The delay device 84 may be configured as a waveguide having a predetermined length, and the effective refractive index of the waveguide may be adjustable by a heater. The delay device 84 may be configured as a phase modulator having a predetermined length, and the amount of phase modulation may be adjustable by applying a voltage.

[0107] 31, in the optical receiver 1, the polarization splitter rotator 82 may be replaced with a polarization splitter (PS) 822. The polarization splitter 822 separates an input optical signal into an optical signal in TE mode and an optical signal in TM mode. The propagation velocities of the optical signal in TE mode and the optical signal in TM mode are different from each other. The optical receiver 1 may include a delay device 84 to compensate for the difference in delay between the optical signal in TE mode and the optical signal in TM mode.

[0108] 32, the delay devices 84 connected between the demultiplexer 83 and each of the n photodiodes 10-1 to 10-n in the optical receiver 1 illustrated in FIG. 28 may be replaced with variable optical attenuators (VOAs) 85. The variable optical attenuators 85 may include, for example, silicon pin diodes. The variable optical attenuators 85 absorb light by injecting current into them, thereby attenuating the optical intensity. By adjusting the current injected into each variable optical attenuator 85, optical losses occurring in the polarization splitter rotator 82 or the demultiplexer 83 can be compensated for. Therefore, even if the optical losses in the polarization splitter rotator 82 or the demultiplexer 83 are not uniform due to differences in the polarization or wavelength of the optical signal, the optical receiving sensitivity of optical signals of any polarization or wavelength can be made closer to uniform by reducing the current value of the variable optical attenuator 85 through which optical signals with large optical loss pass and increasing the current value of the variable optical attenuator 85 through which optical signals with small optical loss pass.

[0109] As shown in FIG. 33, the optical receiver 1 may include both a variable optical attenuator 85 and a delay device 84 between the demultiplexer 83 and each of the n photodiodes 10-1 to 10-n.

[0110] As described above, the optical receiver 1 according to this embodiment can detect an optical signal using the photodiode 10 configured to reduce returning light. By reducing the returning light, the optical signal returning to the input section 81 can be reduced. By reducing the optical signal returning to the input section 81, stable operation of the light source or modulator that transmits the optical signal to the input section 81 can be maintained. As a result, the reliability of the optical communication system using the optical receiver 1 can be improved.

[0111] <Photodetector> The photodiode 10 according to the present disclosure is used as a photodetector that detects optical signals. The photodetector is not limited to the photodiode 10, and other light-receiving elements such as phototransistors may also be used. Even when other light-receiving elements are used as the photodetector, the reliability of the optical communication system can be improved by configuring the device to reduce return light.

[0112] Although the embodiments of the present disclosure have been described based on the drawings and examples, it should be noted that those skilled in the art could make various modifications or alterations based on the present disclosure. Therefore, it should be noted that these modifications or alterations are included in the scope of the present disclosure. For example, the functions included in each component can be rearranged so as not to be logically inconsistent, and multiple components can be combined into one or divided. It should be understood that these modifications are also included in the scope of the present disclosure.

[0113] In this disclosure, the terms "first" and "second" are identifiers for distinguishing the configuration. In this disclosure, the configurations distinguished by terms such as "first" and "second" can have their numbers interchanged. For example, the first input waveguide 20 can have its identifiers "first" and "second" interchanged with the second input waveguide 30. The identifiers are interchanged simultaneously. The configurations remain distinguished even after the identifiers are interchanged. The identifiers may be deleted. A configuration from which an identifier has been deleted is distinguished by a symbol. The identifiers "first" and "second" in this disclosure should not be used solely to interpret the order of the configurations or to justify the existence of an identifier with a smaller number.

[0114] In this disclosure, the X-axis, Y-axis, and Z-axis are provided for convenience of explanation and may be interchanged. The configurations according to this disclosure have been described using a Cartesian coordinate system formed by the X-axis, Y-axis, and Z-axis. The positional relationship between the components according to this disclosure is not limited to an orthogonal relationship.

[0115] In one embodiment, (1) an optical detection device includes a first input waveguide to which a first optical signal is input, a light-receiving waveguide connected to a second input waveguide to which a second optical signal is input, and a light-receiving unit that outputs an electrical signal corresponding to the intensity of a combined signal of the first optical signal and the second optical signal input to the light-receiving waveguide, wherein the light-receiving unit is configured to reduce the intensity of an optical signal returning in the first input waveguide in a direction opposite to a first direction in which the first optical signal propagates, and the intensity of an optical signal returning in the second input waveguide in a direction opposite to a second direction in which the second optical signal propagates.

[0116] (2) In the photodetector device of (1) above, the light-receiving waveguide may be a multi-mode interference type optical waveguide.

[0117] (3) In the photodetector device of (2) above, the receiving waveguide may be connected to the first input waveguide such that a width of the first input waveguide and a width of the receiving waveguide change discontinuously along the first direction, and may be connected to the second input waveguide such that a width of the second input waveguide and a width of the receiving waveguide change discontinuously along the second direction.

[0118] (4) In the photodetector device of (3) above, a rate of change in the width of the light-receiving waveguide relative to the width of each of the first input waveguide and the second input waveguide may be equal to or greater than a predetermined value.

[0119] (5) In any one of the photodetector devices (2) to (4) above, the receiving waveguide may have at least one of a first terminating waveguide connected to the opposite side of the first input waveguide along the first direction, or a second terminating waveguide connected to the opposite side of the second input waveguide along the second direction.

[0120] (6) In the photodetector device according to any one of (1) to (5) above, the first direction and the second direction may be substantially parallel to each other.

[0121] (7) In the photodetector device according to any one of (1) to (5) above, the angle formed between the first direction and the second direction may be within a predetermined range including 90 degrees.

[0122] (8) In the photodetector device of any one of (1) to (7), the light receiving unit may have a first surface located on the first direction side and a second surface located on the second direction side. The first surface may include a surface having a normal direction that is non-parallel to the first direction, at least in part. The second surface may include a surface having a normal direction that is non-parallel to the second direction, at least in part.

[0123] In one embodiment, (9) an optical receiver includes an optical circuit that separates an input optical signal into a first optical signal propagating in TE mode and a second optical signal propagating in TM mode, and an optical detection device that outputs an electrical signal corresponding to the intensities of the first optical signal and the second optical signal input from the optical circuit. The optical detection device includes an optical receiving waveguide connected to a first input waveguide to which the first optical signal is input and a second input waveguide to which the second optical signal is input, and an optical receiving unit that outputs an electrical signal corresponding to the intensity of a combined signal of the first optical signal and the second optical signal input to the optical receiving waveguide. The optical receiving unit is configured to reduce the intensity of an optical signal returning in the first input waveguide in a direction opposite to the first direction in which the first optical signal propagates, and the intensity of an optical signal returning in the second input waveguide in a direction opposite to the second direction in which the second optical signal propagates.

[0124] (10) In the optical receiver of (9) above, the optical circuit may include an edge coupler that accepts an input of an optical signal, a polarization splitter rotator that separates the optical signal input to the edge coupler into the first optical signal and the second optical signal and converts the second optical signal from TM mode to TE mode, and a demultiplexer that separates each of the first optical signal and the second optical signal into optical signals of each wavelength.

[0125] (11) In the optical receiver of (9) above, the optical circuit may include a two-dimensional grating coupler that receives an input optical signal, separates it into the first optical signal and the second optical signal, and converts the second optical signal from TM mode to TE mode, and a demultiplexer that separates each of the first optical signal and the second optical signal into optical signals of each wavelength.

[0126] (12) In the optical receiver of (10) or (11) above, the optical circuit may further include a delay device.

[0127] (13) In the optical receiver of (12) above, the optical circuit may further include a variable optical attenuator. [Explanation of symbols]

[0128] 1 Optical receiver (81: input section, 811: edge coupler, 812: two-dimensional grating coupler (2DGC), 82: polarization splitter rotator (PSR), 822: polarization splitter (PS), 83: demultiplexer (DEMUX), 84: delay, 85: variable optical attenuator (VOA)) 10 Photodiode 20 first input waveguide (20A: optical axis, 22: tapered portion) 30 second input waveguide (30A: optical axis, 32: tapered portion) 40: light-receiving waveguide (40A: central axis, 41: light-receiving portion, 411, 412: end faces, 413, 414: convex portions, 42: electrode region, 43: anode electrode, 44: cathode electrode, 45: via electrode, 46: termination waveguide, 47: notch) 50 substrate (51 to 53: first to third insulating layers)

Claims

1. a multi-mode interference type receiving waveguide connected to a first input waveguide to which a first optical signal is input and a second input waveguide to which a second optical signal is input; a light receiving section that outputs an electrical signal corresponding to the intensity of a signal obtained by combining the first optical signal and the second optical signal input to the light receiving waveguide; Equipped with the light receiving unit is disposed on the receiving waveguide and at a light focusing position when the receiving waveguide operates as a multi-mode interference system, and is configured to reduce the intensity of an optical signal returning in the first input waveguide in a direction opposite to a first direction in which the first optical signal propagates, and the intensity of an optical signal returning in the second input waveguide in a direction opposite to a second direction in which the second optical signal propagates.

2. 2. The photodetector according to claim 1, wherein the receiving waveguide is connected to the first input waveguide such that a width of the first input waveguide and a width of the receiving waveguide vary discontinuously along the first direction, and the receiving waveguide is connected to the second input waveguide such that a width of the second input waveguide and a width of the receiving waveguide vary discontinuously along the second direction.

3. 3. The photodetector according to claim 2, wherein a rate of change in width of said light-receiving waveguide relative to the width of each of said first input waveguide and said second input waveguide is equal to or greater than a predetermined value.

4. An optical detection device as described in Claim 3, wherein the predetermined value is 1 or more.

5. 2. The photodetector device of claim 1, wherein the receiving waveguide has at least one of a first terminating waveguide connected to an opposite side of the first input waveguide along the first direction and a second terminating waveguide connected to an opposite side of the second input waveguide along the second direction.

6. The light detection device according to claim 1 , wherein the first direction and the second direction are substantially parallel to each other.

7. The light detection device according to claim 1 , wherein the angle formed between the first direction and the second direction is within a predetermined range including 90 degrees.

8. the light receiving unit has a first surface positioned on the side in the first direction and a second surface positioned on the side in the second direction, the first surface includes, at least in part, a surface having a normal direction that is non-parallel to the first direction; The light detection device according to claim 1 , wherein the second surface includes, at least in part, a surface having a normal direction that is non-parallel to the second direction.

9. an optical circuit that separates an input optical signal into a first optical signal propagating in a TE mode and a second optical signal propagating in a TM mode; a photodetector that outputs an electrical signal corresponding to the intensities of the first optical signal and the second optical signal input from the optical circuit; Equipped with The photodetector device a multimode interference type receiving waveguide connected to a first input waveguide to which the first optical signal is input and a second input waveguide to which the second optical signal is input; a light receiving section that outputs an electrical signal corresponding to the intensity of a signal obtained by combining the first optical signal and the second optical signal input to the light receiving waveguide; Equipped with the light receiving unit is disposed on the light receiving waveguide and at a light collecting position when the light receiving waveguide operates as a multi-mode interference system, and is configured to reduce the intensity of an optical signal returning in the first input waveguide in a direction opposite to a first direction in which the first optical signal propagates, and the intensity of an optical signal returning in the second input waveguide in a direction opposite to a second direction in which the second optical signal propagates. Optical receiver.

10. 10. The optical receiver of claim 9, wherein the optical circuit comprises an edge coupler that accepts an input of an optical signal, a polarization splitter rotator that separates the optical signal input to the edge coupler into the first optical signal and the second optical signal and converts the second optical signal from TM mode to TE mode, and a demultiplexer that separates each of the first optical signal and the second optical signal into optical signals of each wavelength.

11. 10. The optical receiver of claim 9, wherein the optical circuit comprises: a two-dimensional grating coupler that receives an input optical signal, separates it into the first optical signal and the second optical signal, and converts the second optical signal from TM mode to TE mode; and a demultiplexer that separates each of the first optical signal and the second optical signal into optical signals of respective wavelengths.

12. 12. The optical receiver according to claim 10, wherein the optical circuit further includes a delay device.

13. The optical receiver of claim 12 , wherein the optical circuit further includes a variable optical attenuator.

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