Light detection device and light receiver

Abstract

The optical detection device includes: a light 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; and a light receiving unit 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. The light receiving unit is configured so as to reduce the intensity of an optical signal returned in the first input waveguide in the opposite direction to a first direction in which the first optical signal propagates and the intensity of an optical signal returned in the second input waveguide in the opposite direction to a second direction in which the second optical signal propagates.

Description

[0001] Cross-reference to related applications

[0002] This application claims the priority of Japanese Patent Application No. 2022-76677 (filed on May 6, 2022), and all the disclosed contents of that application are incorporated herein by reference. Technical Field

[0003] The present disclosure relates to a light detection device and a light receiver. Background Art

[0004] An optical receiver is known that separates different polarized light components into two waveguides, demultiplexes the light by wavelength in each waveguide, and detects the demultiplexed light of a specific wavelength in each waveguide using a photodetector (for example, see Patent Document 1).

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: U.S. Patent No. 10,488,593 Summary of the invention

[0008] An optical detection device according to one embodiment of the present disclosure includes a light receiving waveguide and a light receiving unit. The light receiving waveguide is connected to a first input waveguide for inputting a first light signal and a second input waveguide for inputting a second light signal. The light receiving unit outputs an electrical signal corresponding to the intensity of a signal obtained by combining the first light signal and the second light signal input to the light receiving waveguide. The light receiving unit is configured to reduce the intensity of a light signal that is returned in the first input waveguide in the opposite direction of the first direction in which the first light signal propagates and the intensity of a light signal that is returned in the second input waveguide in the opposite direction of the second direction in which the second light signal propagates.

[0009] An optical receiver of one embodiment of the present disclosure has an optical circuit and an optical detection device. The optical circuit separates the input optical signal into a first optical signal propagating in a TE mode and a second optical signal propagating in a TM mode. The optical detection device outputs an electrical signal corresponding to the intensity of the first optical signal and the second optical signal input from the optical circuit. The optical detection device has a light receiving waveguide and a light receiving unit. The light receiving waveguide is connected to a first input waveguide for inputting the first optical signal and a second input waveguide for inputting the second optical signal. The light receiving unit 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. The light receiving unit is configured to reduce the intensity of an optical signal returned in the first input waveguide in the opposite direction of the first direction in which the first optical signal propagates and the intensity of an optical signal returned in the second input waveguide in the opposite direction of the second direction in which the second optical signal propagates. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] Figure 1 This is a block diagram showing a configuration example of an optical receiver according to one embodiment.

[0011] Figure 2 It is a plan view showing a structural example of a photodiode according to one embodiment.

[0012] Figure 3 yes Figure 2 An example of an AA cross-sectional view.

[0013] Figure 4 yes Figure 2 Another example of an AA cross-sectional view of FIG.

[0014] Figure 5 It is a top view showing the structure of a photodiode of a comparative example.

[0015] Figure 6 : is a block diagram showing the structure of an optical receiver of a comparative example.

[0016] Figure 7 It means in Figure 2 A top view of a structural example of a photodiode in which the normal direction of the side surface of the photodiode is inclined with respect to the direction in which light travels.

[0017] Figure 8 It means in Figure 2 A top view of a structural example of a photodiode in which a side surface of the photodiode has a convex portion in the structural example.

[0018] Fig. 9 It means in Figure 2A top view of a structural example of a photodiode in which a side surface of the photodiode has a convex portion and an axis of the convex portion is inclined with respect to the direction in which light travels.

[0019] Fig.10 This is a plan view showing a configuration example of a photodiode in which the traveling direction of light in a first waveguide and the traveling direction of light in a second waveguide are perpendicular to each other.

[0020] Fig.11 This is a plan view showing a structural example in which the light propagation direction of the first waveguide is orthogonal to the light propagation direction of the second waveguide and the normal direction of the side surface of the photodiode is inclined with respect to the light propagation direction.

[0021] Fig.12 This is a plan view showing a structural example in which the traveling direction of light in the first waveguide is perpendicular to the traveling direction of light in the second waveguide and the side surface of the photodiode has a convex portion.

[0022] Fig.13 This is a plan view showing a structural example in which the light traveling direction of the first waveguide is orthogonal to the light traveling direction of the second waveguide, and 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.

[0023] Fig.14 It is a plan view showing a configuration example of a photodiode connected to a first waveguide and a second waveguide so as to generate multi-mode interference in a light-receiving waveguide.

[0024] Fig.15 This is a plan view showing an example of intensity distribution of light that occurs due to multi-mode interference in the light receiving waveguide when no light receiving unit is provided in the light receiving waveguide.

[0025] Fig.16 This is a plan view showing an example of light intensity distribution when the size of the light receiving section is set so that the intensity of light propagating from the first waveguide to the second waveguide is reduced to less than a predetermined intensity.

[0026] Fig.17 This is a plan view showing an example of light intensity distribution when the intensity of light propagating from the first waveguide to the second waveguide becomes equal to or higher than a predetermined intensity by changing the size of the light receiving portion.

[0027] Fig.18 This is a plan view showing a configuration example of a photodiode in which the light propagation direction of the first waveguide is orthogonal to the light propagation direction of the second waveguide and the photodiode is connected to the first waveguide and the second waveguide so as to generate multi-mode interference in the light receiving waveguide.

[0028] Fig.19 It means in Fig.18A top view of a structural example of a photodiode in which a terminal waveguide is further provided at an end portion on the opposite side to a side connected to a first waveguide and an end portion on the opposite side to a side connected to a second waveguide, respectively.

[0029] Fig. 20 It means in Fig.19 A top view of a structural example of a photodiode in which a notch is formed on each of a side connected to a first waveguide and a side connected to a second waveguide in a structural example.

[0030] Fig.21 It is a plan view showing a configuration example of a photodiode in which the distance from the end connected to the first waveguide to the light receiving portion and the distance from the end connected to the second waveguide to the light receiving portion are different in the light receiving waveguide.

[0031] Fig. 22 It means in Fig.21 A top view of a structural example of a photodiode in which a terminal waveguide is further provided at an end portion on the opposite side to a side connected to a first waveguide and an end portion on the opposite side to a side connected to a second waveguide, respectively.

[0032] Fig.23 It means in Fig. 22 A top view of a structural example of a photodiode in which a notch is formed on each of a side connected to the first waveguide and a side connected to the second waveguide in a structural example.

[0033] Fig.24 It means in Fig.14 FIG. 1 is a top view of a structural example of a photodiode in which the normal direction of the side surface of the photodiode is inclined with respect to the traveling direction of light.

[0034] Fig.25 It means in Fig.14 A top view of a structural example of a photodiode in which a side surface of the photodiode has a convex portion in the structural example.

[0035] Fig.26 It means in Fig.14 A top view of a structural example of a photodiode in which a side surface of the photodiode has a convex portion and an axis of the convex portion is inclined with respect to the direction in which light travels.

[0036] Fig. 27 It means in Fig.14 A top view of a structural example of a photodiode in which the optical axes of the first waveguide and the second waveguide and the optical axis passing through the light receiving unit are offset in the structural example.

[0037] Fig.28This is a block diagram showing a configuration example of an optical receiver having an edge coupler as an input unit.

[0038] Fig.29 This is a block diagram showing a configuration example of an optical receiver having a two-dimensional grating coupler as an input unit.

[0039] Fig.30 It means in Fig.28 A block diagram of a structural example of an optical receiver in which the structural example also includes a delay device.

[0040] Fig.31 It means in Fig.30 A block diagram of a configuration example of an optical receiver in which the polarization separation rotator is replaced by a polarization separator in the configuration example.

[0041] Fig.32 It means in Fig.30 A block diagram of a configuration example of an optical receiver in which a delay device connected before a photodiode in a configuration example is replaced with a variable attenuator.

[0042] Fig.33 It means in Fig.32 A block diagram of a configuration example of an optical receiver in which a variable attenuator connected before a photodiode is provided in the configuration example and a delay device is further provided between the photodiode and the photodiode. DETAILED DESCRIPTION

[0043] In the case where light is input from two waveguides to one photodetector, two waveguides are connected to the photodetector. In the photodetector connected to the two waveguides, light propagated from one waveguide to the photodetector passes through the photodetector and propagates in the other waveguide as return light in the opposite direction of the input direction. In the case where the photodetector is used in an optical communication system, the return light may make the optical communication system unstable and reduce the reliability of the optical communication system. According to an optical detection device and an optical receiver of one embodiment of the present disclosure, the reliability of the optical communication system can be improved.

[0044] In optical communications within data centers, direct modulation and direct detection methods are widely used because digital signal processors are simple and low-power. On the other hand, as the volume of optical communications within data centers increases, higher-density data transmission is required, and optical transceivers with optical integrated circuits using silicon, especially small wavelength-multiplexed optical circuits, are being studied. Among them, as a wavelength-multiplexed optical circuit implemented by silicon, a series Mach-Zehnder interference system or an arrayed waveguide grating can be used. These wavelength-multiplexed optical circuits all have the characteristic that the characteristics change greatly according to the polarization wave of the light. On the other hand, single-mode optical fibers are widely used in existing optical fiber networks in data centers. However, this optical fiber does not have a polarization-maintaining characteristic. Therefore, whenever light passes through a curved part or a connecting part of the wiring, the polarization wave of the light changes randomly. Therefore, in order to make the characteristics of the optical receiver uniform for any polarization wave, it is necessary to set a polarization separation rotator at the front stage of the optical circuit to separate the incident light into TE (Transverse Electric: transverse electric field) components and TM (Transverse Magnetic: transverse magnetic field) components, and make them respectively incident on their respective wavelength-multiplexed optical circuits. In the direct modulation direct polarization method, after polarizing light into either TE or TM, it is necessary to receive the output of the wavelength multiplexing optical circuit with a photodiode and detect the sum of the outputs of the photodiodes corresponding to the respective polarization components.

[0045] In the direct modulation direct detection method, when a germanium photodiode is used, in order to detect the sum of the outputs of the photodiodes corresponding to the polarization wave components of TE and TM, a photodiode for detecting the polarization wave component of TE and a photodiode for detecting the polarization wave component of TM are respectively provided. Then, the output of the photodiode for detecting the polarization wave component of TE and the output of the photodiode for detecting the polarization wave component of TM are input in parallel to the transimpedance amplifier and added, thereby obtaining the sum of the outputs of the photodiodes corresponding to the polarization wave components of TE and TM. However, in this case, for the parasitic capacitance Cpd of a single photodiode, the total parasitic capacitance of the two photodiodes connected in parallel is 2×Cpd. Therefore, the RC time constant caused by the parasitic capacitance and the series resistance of the circuit becomes larger, and the response speed of the photodiode is reduced.

[0046] On the other hand, the output of each photodiode can be input to a transimpedance amplifier, and the sum of the voltages of the transimpedance amplifier can be calculated using an analog circuit or a digital circuit. In this case, the response speed of the photodiode is the speed corresponding to the parasitic capacitance Cpd of a single photodiode, so it becomes high-speed, but an amplifier or DSP (Digital Signal Processor) is required to output the sum of the voltages. Therefore, the circuit size increases and the power consumption increases.

[0047] The device for detecting light can be configured to detect each polarization wave component by one photodiode instead of each photodiode detecting each polarization wave component. For example, as disclosed in the above-mentioned patent document 1 (U.S. Patent No. 10,488,593 specification), after different polarization light components Px and Py are separated into two waveguides by a two-dimensional grating coupler, the system can couple only light of a specific wavelength to the following two waveguides via a Bragg reflector. The two waveguides at the coupling destination are connected to a single photodiode. A single photodiode outputs the sum of the light intensities of the two polarization light components as a photocurrent. Thus, it is possible to avoid increasing the parasitic capacitance, circuit size or power consumption of the photodiode.

[0048] However, the reflected light from the Bragg reflector or the light not absorbed by the photodiode returns to the optical communication system as return light. The return light is a major factor that causes the operation of the laser light source for communication to become unstable.

[0049] In addition, Document A listed below discloses a different structure.

[0050] Document A: IEEE,Photonics Technology Letters,31,287(2019)

[0051] The device described in the above document A couples different polarized light components Px and Py to the waveguide respectively through a two-dimensional grating coupler, and only couples the light of a specified wavelength to the following two waveguides through a ring filter arranged on each waveguide. The two waveguides at the coupling destination are connected to the two-dimensional grating coupler. The two-dimensional grating coupler emits the input light above the substrate. Then, the photodiode arranged above the two-dimensional grating coupler converts the light emitted above the substrate into an electrical signal. The device described in the above document A is capable of outputting the total light intensity of two polarized waves from a single photodiode, similarly to patent document 1. However, it is possible that return light from the grating coupler on the light receiving element side may be generated, making the operation of the communication laser light source unstable.

[0052] In addition, in the structure of document A, a photodiode needs to be provided above the two-dimensional grating coupler. In this case, the photodiode generally used in the field of optical communication technology, that is, strained germanium epitaxially grown on a flat top silicon layer, cannot be used, and the manufacturing process becomes complicated. Furthermore, since light passes vertically through the structure of the epitaxial layer constituting the photodiode, the sensitivity of the photodiode becomes low.

[0053] The photodiode 10 (see Figure 2The optical receiver 1 using the photodiode 10 is configured as a polarization-independent optical receiver. The photodiode 10 according to one embodiment of the present disclosure can reduce the return light, keep the parasitic capacitance of the photodiode small, and eliminate the need for a circuit that adds the outputs of the two photodiodes corresponding to the two polarization wave components.

[0054] An optical receiver 1 (see Figure 1 The optical signal transmitting structure may include a light source and a modulator.

[0055] The light source may include, for example, a semiconductor laser such as an LD (Laser Diode) or a VCSEL (Vertical Cavity Surface Emitting LASER). The light source is not limited to visible light, and may include a device that emits electromagnetic waves of various wavelengths. The modulator modulates by varying the intensity of the electromagnetic wave. The modulator may, for example, pulse modulate the electromagnetic wave.

[0056] The structure for transmitting the optical signal may further include a signal input unit. The signal input unit receives input of a signal 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 the electromagnetic wave based on the signal acquired by the signal input unit.

[0057] The optical receiver 1 using the photodiode 10 can reduce the return light from the optical receiver 1 to the optical communication system, and thus can realize a polarization-independent optical receiver that operates at high speed and low power consumption. In addition, since the photodiode can be formed on a flat silicon waveguide, strained germanium epitaxially grown on silicon can be used as a photodiode, and since the photodiode absorbs light propagating in the waveguide, it can absorb light over a sufficiently long distance, and the light receiving sensitivity can be improved.

[0058] (Configuration Example of Optical Receiver 1)

[0059] The optical receiver 1 is configured to receive an optical signal. Hereinafter, a configuration example of the optical receiver 1 according to an embodiment of the present disclosure will be described.

[0060] like Figure 1 As shown, the optical receiver 1 includes an input unit 81 , a polarizing splitter rotator (PSR: Polarizing Splitter Rotator) 82 , a demultiplexer (multiplexer or DEMUX) 83 , and a photodiode (PD: Photo Diode) 10 .

[0061] 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 separation 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 each of the TE mode optical signal separated by the polarization separation rotator 82 and the TE mode optical signal converted from the TM mode optical signal separated by the polarization separation rotator 82 for each wavelength. Figure 1 In FIG. 8 , the wavelength splitter 83 is configured to separate the optical signal into light of n wavelengths represented by λ1 to λn.

[0062] The photodiode 10 includes photodiodes 10-1 to 10-n corresponding to the light of each wavelength. Each photodiode 10 is connected to a demultiplexer 83 that separates the TE mode optical signal for each wavelength and a demultiplexer 83 that separates the TE mode optical signal converted from the TM mode optical signal for each wavelength. In other words, each photodiode 10 has: a port that receives input of light separated from the TE mode optical signal; and a port that receives input of light separated from the TE mode optical signal converted from the TM mode optical signal. Each photodiode 10 is configured to detect the light of each wavelength separated from the TE mode optical signal and the light of each wavelength separated from the TE mode optical signal converted from the TM mode optical signal.

[0063] As described later, the photodiode 10 is configured so that an optical signal input to one port is difficult to pass through another port. That is, it is difficult for the optical signal to return toward the transmission source of the optical signal. Since the optical signal returns toward the transmission source of the optical signal, the light source or modulator of the transmission source of the optical signal becomes unstable. Therefore, by reducing the return light, the light source or modulator of the transmission source of the optical signal can be protected. As a result, the reliability of the optical communication system can be improved.

[0064] (Structural Example of Photodiode 10)

[0065] like Figure 2 and Figure 3 As shown, the photodiode 10 according to one embodiment of the present disclosure includes a light receiving waveguide 40 and a light receiving unit 41. The light receiving waveguide 40 is connected to the first input waveguide 20 and the second input waveguide 30.

[0066] In the first input waveguide 20, the optical signal travels in the positive direction of the Z axis and is input to the light receiving waveguide 40. The first input waveguide 20 may include a tapered portion 22 whose width continuously changes in a direction (X axis direction) intersecting the traveling direction of the optical signal. The tapered portion 22 is configured so that the width of the first input waveguide 20 is consistent with the width of the light receiving waveguide 40 at a position where the first input waveguide 20 and the light receiving waveguide 40 are connected.

[0067] In the second input waveguide 30, the optical signal travels in the negative direction of the Z axis and is input to the light receiving waveguide 40. The second input waveguide 30 may include a tapered portion 32 whose width continuously changes in a direction (X axis direction) intersecting the traveling direction of the optical signal. The tapered portion 32 is configured so that the width of the second input waveguide 30 is consistent with the width of the light receiving waveguide 40 at a position where the second input waveguide 30 and the light receiving waveguide 40 are connected.

[0068] The receiving waveguide 40, the first input waveguide 20, and the second input waveguide 30 are arranged in a row 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 the first insulating layer 51 on the substrate 50. Figure 3 In the cross-sectional view shown in the example, they are surrounded by the first insulating layer 51 and the second insulating layer 52. The light receiving waveguide 40, the first input waveguide 20, and the second input waveguide 30 function as the core of the waveguide. The first insulating layer 51 and the second insulating layer 52 function as the cladding of the waveguide.

[0069] The light receiving portion 41 is located on the light receiving waveguide 40. The light receiving portion 41 functions as the core of the waveguide together with the light receiving waveguide 40. In the range where the light receiving portion 41 is located, the core has a structure in which the light receiving portion 41 and the light receiving waveguide 40 are stacked, and as a whole, it extends in the positive direction of the Y axis. The core in which the light receiving portion 41 and the light receiving waveguide 40 are stacked is surrounded by the first insulating layer 51, the second insulating layer 52, and the 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.

[0070] The light propagated through the light receiving waveguide 40 propagates to the light receiving portion 41. The electrons of the light receiving portion 41 are excited by the energy of the light propagated to the light receiving portion 41. As a result, the potential of the light receiving portion 41 becomes lower than the potential of the light receiving waveguide 40. That is, a potential difference is generated between the light receiving portion 41 and the light receiving waveguide 40. The photodiode 10 outputs a photocurrent flowing due to the potential difference generated between the light receiving portion 41 and the light receiving waveguide 40 to an external device. The photocurrent corresponds to an electrical signal corresponding to the intensity of the light signals respectively input to the first input waveguide 20 and the second input waveguide 30 combined.

[0071] In other words, the structure of the photodiode 10 of the present embodiment is 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 light receiving waveguide 40 is connected to the first input waveguide 20 and the second input waveguide 30. The light receiving unit 41 outputs an electrical signal corresponding to the signal strength obtained by combining the first optical signal and the second optical signal input to the light receiving waveguide 40. The light receiving unit 41 is configured to reduce the intensity of the optical signal returned in the first input waveguide 20 in the opposite direction of the first direction in which the first optical signal propagates, and the intensity of the optical signal returned in the second input waveguide 30 in the opposite direction of the second direction in which the second optical signal propagates.

[0072] <Material of Each Structural Component of the Photodiode 10>

[0073] The substrate 50 may be formed of a semiconductor such as silicon, a conductor such as metal, an insulator such as glass or resin, etc. In the present embodiment, the substrate 50 is silicon (Si), but is not limited thereto and may be other various materials.

[0074] The light receiving waveguide 40, the light receiving part 41, the first input waveguide 20 and the second input waveguide 30 functioning as the core, and the first insulating layer 51, the second insulating layer 52 and the third insulating layer 53 functioning as the cladding can be constructed to include a dielectric. The light receiving waveguide 40, the light receiving part 41, the first input waveguide 20 and the second input waveguide 30 are also called dielectric lines. The material of the core and the cladding is determined so that the relative dielectric constant of the core is greater than the relative dielectric constant of the cladding. In other words, the material of the core and the cladding is determined so that the refractive index of the core for the propagated light is greater than the refractive index of the cladding for the propagated light. In this way, the light propagating in the core can be totally reflected at the boundary with the cladding. As a result, the loss of the optical signal propagating in the core can be reduced.

[0075] In this embodiment, the light receiving waveguide 40, the first input waveguide 20, and the second input waveguide 30, which function as cores, are made of silicon (Si). The material of the core is not limited to the above example, and may be other types of materials.

[0076] The light receiving unit 41 that functions as a core together with the light receiving waveguide 40 is made of germanium (Ge). The light receiving unit 41 is configured as a pin diode having a germanium pin structure formed by epitaxial growth on the light receiving waveguide 40. The structure of the light receiving unit 41 is not limited to the above example.

[0077] In the present embodiment, the material of the first insulating layer 51, the second insulating layer 52 and the third insulating layer 53 functioning as the cladding is quartz glass or a silicon oxide film (SiO2). The material of the cladding is not limited to the above examples, and may be other types of materials. The relative dielectric constant of silicon is about 12. The relative dielectric constant of germanium is about 16. The relative dielectric constant of quartz glass is about 2. Silicon can propagate electromagnetic waves having a near-infrared wavelength of about 1.2 μm to about 6 μm with low loss. When the light receiving waveguide 40, the first input waveguide 20 and the second input waveguide 30 are composed of silicon, electromagnetic waves having a wavelength of 1.3 μm band or 1.55 μm band used in optical communication can be propagated with low loss.

[0078] The relative dielectric constant of the core and the cladding may be greater than the relative dielectric constant of air. By making the relative dielectric constant of the core and the cladding greater than the relative dielectric constant of air, it is possible to reduce the leakage of electromagnetic waves from the photodiode 10. As a result, it is possible to reduce the loss caused by the emission of electromagnetic waves from the photodiode 10 to the outside.

[0079] The light receiving waveguide 40 , the first input waveguide 20 , and the second input waveguide 30 functioning as cores may satisfy single-mode waveguide conditions or multi-mode waveguide conditions.

[0080] The photodiode 10 may further include an anode electrode 43 connected to the light receiving portion 41 via a through-hole electrode 45, and a cathode electrode 44 connected to an electrode region 42 of the light receiving waveguide 40 via the through-hole electrode 45. The 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, thereby being output to an external device. The anode electrode 43, the cathode electrode 44, and the through-hole electrode 45 may be formed to include metal or semiconductor, etc. The electrode region 42 may be formed as a doped region in which impurities are injected into silicon constituting the light receiving waveguide 40 to reduce the resistance between the through-hole electrode 45 and the light receiving waveguide 40.

[0081] <Other structural examples of the photodiode 10 >

[0082] like Figure 4 As shown in FIG. 1 , the photodiode 10 is configured such that a second insulating layer 52 is sandwiched between the light receiving portion 41 and the light receiving waveguide 40. In this case, the electrode region 42 is formed on the light receiving portion 41. The second insulating layer 52 is configured to have a thickness not greater than a predetermined value so that light propagating in the light receiving waveguide 40 can propagate toward the light receiving portion 41 through the second insulating layer 52. By having a relatively thin second insulating layer 52 on the light receiving waveguide 40, the light receiving waveguide 40 can be protected when the light receiving portion 41 is formed.

[0083] <Comparative Example>

[0084] like Figure 5As shown, the photodiode 90 of 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.

[0085] In the input waveguide 920, the optical signal travels in the positive direction of the Z axis and is input to the light receiving waveguide 940. The input waveguide 920 includes a tapered portion 922 whose width continuously changes in a direction intersecting the traveling direction of the optical signal (X axis direction). The light receiving waveguide 940 and the input waveguide 920 are arranged in the Z axis direction on the substrate 950. The input waveguide 920 extends in the Z axis direction. The light receiving portion 941 is located on the light receiving waveguide 940.

[0086] The light propagated through the light receiving waveguide 940 propagates to the light receiving portion 941. The electrons of the light receiving portion 941 are excited by the energy of the light propagated to the light receiving portion 941. As a result, the potential of the light receiving portion 941 is lower than the potential of the light receiving waveguide 940. That is, a potential difference is generated between the light receiving portion 941 and the light receiving waveguide 940. The photodiode 90 uses the potential difference generated between the light receiving portion 941 and the light receiving waveguide 940 to output the photocurrent flowing between the cathode electrode 943 and the anode electrode 944 to an external device. The photocurrent corresponds to an electrical signal corresponding to the intensity of the optical signal input to one input waveguide 920.

[0087] like Figure 6 As shown, the optical receiver 9 of the comparative example includes an input unit 981, a polarization separation rotator (PSR) 982, a demultiplexer (DEMUX) 983, a photodiode (PD) 90, and transimpedance amplifiers (TIA: Transimpedance Amplifier) ​​93-1 to 93-n.

[0088] The input unit 981 receives an input of an optical signal. The polarization separation 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 the TE mode optical signal. The wavelength splitter 983 separates each of the TE mode optical signal separated by the polarization separation rotator 982 and the TE mode optical signal converted from the TM mode optical signal separated by the polarization separation rotator 982 for each wavelength. The wavelength splitter 983 separates the optical signal into n types of light of wavelengths represented by λ1 to λn.

[0089] The photodiode 90 includes photodiodes 91-1 to 91-n connected to a demultiplexer 983 for separating the optical signal of the TE mode for each wavelength. In addition, the photodiode 90 includes photodiodes 92-1 to 92-n connected to a demultiplexer 983 for separating the optical signal converted from the TM mode to the TE mode in the polarization separation rotator 82 for each wavelength. The transimpedance amplifier 93-1 outputs an electrical signal V1 obtained by adding the electrical signal output from the photodiode 91-1 and the electrical signal output from the photodiode 92-1. In addition, the transimpedance amplifier 93-n outputs an electrical signal Vn obtained by adding the electrical signal output from the photodiode 91-n and the electrical signal output from the photodiode 92-n. That is, in the optical receiver 9 of the comparative example, since the photodiode 90 can only output an electrical signal corresponding to the intensity of one optical signal, the transimpedance amplifiers 93-1 to 93-n for adding the electrical signals output from the photodiodes 91-1 to 91-n and the photodiodes 92-1 to 92-n are required.

[0090] Here, by outputting the electrical signals of two photodiodes 90 in parallel to one transimpedance amplifier 93, the parasitic capacitance of the photodiode 90 is doubled. By increasing the parasitic capacitance, the RC time constant is increased. By increasing the RC time constant, the response of the photodiode 90 is slowed down. In addition, when the electrical signal of one photodiode 90 is input to one transimpedance amplifier 93 and the outputs of each transimpedance amplifier 93 are added by another analog circuit or digital circuit, the circuit size increases and the power consumption increases.

[0091] On the other hand, the photodiode 10 of the present embodiment is configured to output an electrical signal corresponding to the intensity of the two optical signals combined. The optical receiver 1 using the photodiode 10 of the present embodiment does not need a structure in which two photodiodes 10 are connected in parallel, so the response speed can be maintained. In addition, the optical receiver 1 using the photodiode 10 of the present embodiment does not need a circuit for adding two electrical signals. As a result, an increase in circuit size or an increase in power consumption can be avoided.

[0092] As another comparative example, a photodiode that outputs an electrical signal corresponding to the intensity of two optical signals is configured to allow the two optical signals to be incident on only one light receiving portion. In this structure, the optical signal incident on the light receiving portion is reflected and becomes return light. In addition, the optical signal input from one of the two ports receiving the input of the two optical signals is transmitted through the light receiving portion to the other port and becomes return light. In the case where the optical signal that becomes return light returns to the light source or modulator, the operation of the light source or modulator becomes unstable.

[0093] On the other hand, the photodiode 10 of the present embodiment is configured to reduce the optical signals returned to the first input waveguide 20 and the second input waveguide 30, respectively. That is, the photodiode 10 of the present embodiment is configured to reduce the return light. According to the optical receiver 1 using the photodiode 10 of the present embodiment, the return light can be reduced, and the operation of the light source or the modulator can be protected. As a result, the reliability of the optical communication system can be improved.

[0094] (Configuration Example of Photodiode 10 for Reducing Return Light)

[0095] As described above, the photodiode 10 of the present embodiment is configured so that an optical signal input to one port is difficult to pass through the other port. That is, the photodiode 10 of the present embodiment is configured to reduce return light. Hereinafter, as a structural example for reducing return light, the structural examples of the linear type, the cross type, and the multi-mode interference (MMI: Multi-Mode Interference) type are described.

[0096] <Linear type>

[0097] like Figure 2 As shown, the linear photodiode 10 is configured such that the first input waveguide 20 and the second input waveguide 30 are arranged along a straight line (Z axis). In other words, the first direction in which the first optical signal input from the first input waveguide 20 propagates can be substantially parallel to the second direction in which the second optical signal input from the second input waveguide 30 propagates.

[0098] The optical signals propagated 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 part of the optical signal propagated from the first input waveguide 20 in the positive direction of the Z axis can be reflected at the end of the light receiving unit 41 on the negative direction of the Z axis to become the return light of the first input waveguide 20. In addition, a part of the optical signal propagated from the second input waveguide 30 in the negative direction of the Z axis can be reflected at the end of the light receiving unit 41 on the positive direction of the Z axis to become the return light of the second input waveguide 30.

[0099] like Figure 7As shown, the light receiving portion 41 of the photodiode 10 of the present embodiment may be configured such that a normal line 411A of an end surface 411 on the side connected to the first input waveguide 20 is inclined relative to the optical axis 20A of the first input waveguide 20. In addition, the light receiving portion 41 may be configured such that a normal line 412A of an end surface 412 on the side connected to the second input waveguide 30 is inclined relative to the optical axis 30A of the second input waveguide 30. Since the end surfaces 411 and 412 of the light receiving portion 41 are inclined relative to the optical axes 20A and 30A, the optical signal reflected by the end surfaces 411 and 412 propagates in a direction deviating from the optical axes 20A and 30A, and is difficult to return to the first input waveguide 20 and the second input waveguide 30. In addition, when the normal line 411A is inclined relative to the optical axis 20A, the traveling direction of the light incident on the light receiving portion 41 is inclined relative to the optical axis 20A due to the refraction of the light. In this case, the ratio of the light that passes through the light receiving section 41 without being absorbed by the light receiving section 41 and enters the opposing second input waveguide 30 as return light can be reduced.

[0100] In addition, if Figure 8 As shown in the example, the light receiving portion 41 of the photodiode 10 of the present embodiment may have a convex portion 413 on the end surface 411. In addition, the light receiving portion 41 may have a convex portion 414 on the end surface 412. The convex portions 413 and 414 may have an inverted tapered shape in which the width in the X-axis direction expands toward the direction in which the optical signal propagates (the width in the X-axis direction narrows toward the direction away from the light receiving portion 41) when viewed from above (when viewed from the Y-axis direction). By having the convex portions 413 and 414 on the end surfaces 411 and 412, the optical signal propagating in a manner input to the light receiving portion 41 is less likely to be reflected in the opposite direction of the input at the portion where the width in the X-axis direction of the convex portions 413 and 414 is narrow. As a result, the optical signal is less likely to return to the first input waveguide 20 and the second input waveguide 30.

[0101] exist Figure 8 In the example of FIG. 4 , the axis 413A of the protrusion 413 is along the direction of the optical axis 20A of the first input waveguide 20 (Z-axis direction). In addition, the axis 414A of the protrusion 414 is along the direction of the optical axis 30A of the second input waveguide 30 (Z-axis direction). Fig. 9 As shown in the example, the axis 413A of the convex portion 413 may be inclined relative to the direction of the optical axis 20A (Z-axis direction). In addition, the axis 414A of the convex portion 414 may be inclined relative to the direction of the optical axis 30A (Z-axis direction). In this way, the optical signal reflected by the top of the convex portions 413 and 414 propagates in a direction deviating from the optical axes 20A and 30A, and is difficult to return to the first input waveguide 20 and the second input waveguide 30.

[0102] As described above, in the linear photodiode 10 , the shapes of the end faces 411 and 412 are set so as to reduce the return light generated by reflection at the end faces 411 and 412 of the light receiving portion 41 .

[0103] <Cross type>

[0104] like Fig.10 As shown in the example, the crossed photodiode 10 is configured such that the first input waveguide 20 crosses the second input waveguide 30. In other words, the angle formed by 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 can be within a specified range including 90 degrees.

[0105] 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 part of the optical signal propagating from the first input waveguide 20 in the positive direction of the Z axis passes through the light receiving unit 41 (is not absorbed by the light receiving unit 41) and directly propagates in the positive direction of the Z axis. However, since 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 difficult to propagate to the second input waveguide 30 even if it passes through the light receiving unit 41. In addition, a part of the optical signal propagating from the second input waveguide 30 in the positive direction of the X axis passes through the light receiving unit 41 (is not absorbed by the light receiving unit 41) and directly propagates in the positive direction of the X axis. However, since the first input waveguide 20 intersects with the second input waveguide 30, the optical signal propagating from the second input waveguide 30 in the positive direction of the X axis is difficult to propagate to the first input waveguide 20 even if it passes through the light receiving unit 41. As described above, since the first input waveguide 20 and the second input waveguide 30 intersect, an optical signal input from one waveguide is unlikely to propagate to the other waveguide and become return light even if it passes through the light receiving unit 41 .

[0106] exist Fig.10 In the cross-type photodiode 10 shown, Fig.11 As shown in the example, the light receiving unit 41 may be configured such that a normal line 411A of an end surface 411 on the side connected to the first input waveguide 20 is inclined relative to the optical axis 20A of the first input waveguide 20. In addition, the light receiving unit 41 may be configured such that a normal line 412A of an end surface 412 on the side connected to the second input waveguide 30 is inclined relative to the optical axis 30A of the second input waveguide 30.

[0107] Since the end faces 411 and 412 of the light receiving unit 41 are inclined relative to the optical axes 20A and 30A, the optical signals reflected by the end faces 411 and 412 propagate in directions deviating 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 signal at the end faces 411 and 412 can be reduced.

[0108] In addition, Fig.10 In the cross-type photodiode 10 shown, Fig.12As shown in the example, the light receiving unit 41 may have a convex portion 413 on the end surface 411. In addition, the light receiving unit 41 may have a convex portion 414 on the end surface 412. The convex portions 413 and 414 may have an inverted tapered shape in which the width in the X-axis direction expands toward the direction in which the optical signal propagates (the width in the X-axis direction narrows toward the direction away from the light receiving unit 41) when viewed from above (when viewed from the Y-axis direction). By having the convex portions 413 and 414 on the end surfaces 411 and 412, the optical signal propagating in a manner input to the light receiving unit 41 is less likely to be reflected in the opposite direction of the input at the narrower width of the convex portions 413 and 414 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.

[0109] exist Fig.12 In the example of FIG. 4 , the axis 413A of the protrusion 413 is along the direction of the optical axis 20A of the first input waveguide 20 (Z-axis direction). In addition, the axis 414A of the protrusion 414 is along the direction of the optical axis 30A of the second input waveguide 30 (Z-axis direction). Fig.13 As shown in the example, the axis 413A of the convex portion 413 may be inclined relative to the direction of the optical axis 20A (Z-axis direction). In addition, the axis 414A of the convex portion 414 may be inclined relative to the direction of the optical axis 30A (Z-axis direction). In this way, the optical signal reflected by the top of the convex portions 413 and 414 propagates in a direction deviating from the optical axes 20A and 30A, and is difficult to return to the first input waveguide 20 and the second input waveguide 30.

[0110] As described above, the cross-type photodiode 10 can reduce return light caused by the optical signal transmitted through the light receiving unit 41. In addition, the shapes of the end faces 411 and 412 can be set so as to reduce return light generated by reflection from the end faces 411 and 412 of the light receiving unit 41.

[0111] The structure of the cross-type photodiode 10 can be expressed 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 portion 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 includes a face having a normal direction that is non-parallel to the first direction. In addition, at least a portion of the second face includes a face having a normal direction that is non-parallel to the second direction.

[0112] <Multi-mode interference type (MMI type)>

[0113] like Fig.14 As shown, the MMI-type photodiode 10 is configured so that the optical signal propagated from the first input waveguide 20 to the light receiving waveguide 40 and the optical signal propagated from the second input waveguide 30 to the light receiving waveguide 40 generate multi-mode interference in the light receiving waveguide 40. In other words, the MMI-type photodiode 10 is configured so that the light receiving waveguide 40 becomes a multi-mode waveguide and operates as a multi-mode interference system.

[0114] Specifically, in the MMI type photodiode 10, the first input waveguide 20 and the light receiving waveguide 40 are not continuously connected. In addition, the second input waveguide 30 and the light receiving waveguide 40 are not continuously connected. The state in which the first input waveguide 20 and the light receiving waveguide 40 are not continuously connected may correspond to a state in which the width of the first input waveguide 20 and the width of the light receiving waveguide 40 at the connection portion of the first input waveguide 20 and the light receiving waveguide 40 are different. In other words, the state in which the first input waveguide 20 and the light receiving waveguide 40 are not continuously connected may correspond to a state in which the tapered portion 22 (see Figure 2 ) is a state in which the first input waveguide 20 is connected to the light receiving waveguide 40.

[0115] The state in which the first input waveguide 20 and the light receiving waveguide 40 are discontinuously connected may correspond to a state in which the rate of change of the width of the light receiving waveguide 40 relative to the width of the first input waveguide 20 when advancing in the first direction is greater than a prescribed value. The rate of change is calculated as a value of 1 / 2 times the amount of change in the width of the waveguide when advancing a unit distance in the first direction. The reason for setting it to 1 / 2 times 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 the first direction as an axis. The prescribed value may be set to 1, for example. When the prescribed value is 1, the direction in which the line representing the end of the light receiving waveguide 40 extends is at an angle of 45 degrees relative to the first direction. The state in which the second input waveguide 30 and the light receiving waveguide 40 are discontinuously connected may correspond to a state in which the rate of change of the width of the light receiving waveguide 40 relative to the width of the second input waveguide 30 when advancing in the second direction is greater than a prescribed value. In other words, it may correspond to a state in which the rate of change of the width of the light receiving waveguide 40 relative to the width of each of the first input waveguide 20 and the second input waveguide 30 when advancing in the direction of travel of the optical signal is greater than a prescribed value.

[0116] In the MMI type photodiode 10 , the light receiving section 41 is disposed at the light focusing position of the multimode waveguide. Thus, the light signal is efficiently absorbed in the light receiving section 41 .

[0117] In the MMI type photodiode 10, the intensity of the optical signal propagating from the first input waveguide 20 to the light receiving waveguide 40 can be measured in the light receiving waveguide 40 as follows: Fig.15 As shown in the example. Fig.15 In FIG. 1 , the first input waveguide 20 and the light receiving waveguide 40 are connected at the position of Z = 0. The second input waveguide 30 and the light receiving waveguide 40 are connected at the position of Z = L. The light receiving unit 41 is arranged at the center of the light receiving waveguide 40, that is, at the position of Z = L / 2.

[0118] exist Fig.15 In the figure, the brightest part (the part close to white) indicates the part where the intensity of the optical signal increases. The position where the intensity of the optical signal is high is the focusing position of the multimode waveguide. Fig.15 In the embodiment, the light receiving unit 41 is arranged at a light focusing position.

[0119] like Fig.15 The intensity distribution of the optical signal shown in the example shows the intensity distribution of the optical signal when the light receiving portion 41 is not provided in the light receiving waveguide 40. When the light receiving portion 41 is not provided, the light is concentrated again at the position 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 provided in the light receiving waveguide 40, as shown in FIG. Fig.16 As shown in the example, the intensity of the optical signal decreases in the region of Z>L / 2. In addition, since the focal point of the MMI changes to the 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. Since the intensity of the optical signal at the position of Z=L is greatly reduced, the optical signal that passes through the second input waveguide 30 and becomes the return light among the optical signals propagated from the first input waveguide 20 is greatly reduced. Fig.16 In the illustrated structure, the intensity of the return light returning to the second input waveguide 30 is -20 dB or less relative 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. In consideration of the absorption of light by germanium constituting the light receiving portion 41, the intensity of the return light returning to the second input waveguide 30 is -30 dB or less relative to the intensity of the optical signal propagated from the first input waveguide 20.

[0120] Assume that the length of the light receiving portion 41 in the Z-axis direction is greater than Fig.16 In the case of the illustrated structure being long, there is a length at which the light transmitted through the light receiving portion 41 is confined again in the light receiving waveguide 40. As a result, Fig.17 As shown in the example, the intensity of the optical signal in the region Z>L / 2 is higher than Fig.16 In addition, the focal point of the MMI is close to the position Z = L, so the intensity of the optical signal at the position Z = L is higher than Fig.16 The intensity shown. As can be seen from the above, the size of the light receiving portion 41 of the MMI type photodiode 10 affects the return light caused by the transmission of the light receiving portion 41. In other words, when the size of the light receiving portion 41 in the Z-axis direction is an appropriate length, in the light transmitted in the Z-axis direction, the components emitted from the top and bottom (Y-axis direction) of the light receiving waveguide 40 increase. As a result, in the region of Z>L / 2, the intensity of light confined in the light receiving waveguide 40 becomes smaller. On the other hand, when the size of the light receiving portion 41 in the Z-axis direction is a length different from the appropriate length, the length may be such that the light is just coupled to the light receiving waveguide 40 at the end of the light receiving portion 41. In this case, as Fig.17As shown in FIG. 1 , the intensity of light propagating in the light receiving waveguide 40 increases in the region of Z>L / 2. As a phenomenon, in the region where the light receiving portion 41 is present, light interference similar to MMI also occurs in the Y-axis direction. As a result of light interference, at the end of the transmission side of the light receiving portion 41, at the height of the light receiving waveguide 40, the light intensity increases. Fig.17 The light intensity increases, or Fig.16 The light intensity shown decreases.

[0121] exist Fig.16 and Fig.17 In the comparison, when the size of the light receiving unit 41 in the Z-axis direction is small, the intensity of the optical signal transmitted to the second input waveguide 30 becomes low. Therefore, in the MMI type photodiode 10, it is possible to achieve both an increase in the absorption rate in the light receiving unit 41 and a reduction in the size of the light receiving unit 41.

[0122] In the MMI type photodiode 10, as Fig.18 As shown in the example, the first input waveguide 20 and the second input waveguide 30 may cross each other. When the first input waveguide 20 and the second input waveguide 30 cross each other, the optical signal transmitted through the light receiving unit 41 among the optical signals propagated from the first input waveguide 20 propagates to the 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 propagated from the first input waveguide 20 transmits through the light receiving unit 41, the return light propagating to the second input waveguide 30 can be reduced.

[0123] like Fig.19 As shown in the example, the receiving waveguide 40 may have a terminal waveguide 46 at the destination of the propagation direction of the optical signal from the first input waveguide 20 and the destination of the propagation direction of the optical signal from the second input waveguide 30. The terminal waveguide 46 arranged at the destination of the propagation direction of the optical signal from the first input waveguide 20 (located on the opposite side of the first input waveguide 20 along the first direction in the receiving waveguide 40) is also referred to as a first terminal waveguide. The terminal waveguide 46 arranged at the destination of the propagation direction of the optical signal from the second input waveguide 30 (located on the opposite side of the second input waveguide 30 along the second direction in the receiving waveguide 40) is also referred to as a second terminal waveguide. The receiving waveguide 40 may have at least one of the first terminal waveguide and the second terminal waveguide.

[0124] When the light receiving waveguide 40 has the terminal waveguide 46, the size of the light receiving waveguide 40 can be set so that the portion connected to the terminal waveguide 46 becomes the focusing position. The terminal waveguide 46 can be configured to include an antenna that emits electromagnetic waves. The terminal waveguide 46 can be configured to include an absorbing member that absorbs electromagnetic waves. Fig.19As shown in the example, the terminal waveguide 46 may be provided at an end in the width direction of the light receiving waveguide 40. The terminal waveguide 46 may be provided in the entire width direction of the light receiving waveguide 40 or may be provided at any position in the width direction of the light receiving waveguide 40.

[0125] like Fig. 20 As shown in the example, the light receiving waveguide 40 may have a cutout 47 between a portion connecting the first input waveguide 20 and a portion provided with the light receiving portion 41, and between a portion connecting the second input waveguide 30 and a portion provided with the light receiving portion 41. The cutout 47 is configured to prevent the propagation of optical signals other than the optical signals focused by the light receiving portion 41 among the optical signals propagated from the first input waveguide 20 and the second input waveguide 30. In this way, the intensity of the optical signal transmitted through the light receiving portion 41 can be reduced.

[0126] The MMI type photodiode 10 may be configured such that the light receiving portion 41 in the light receiving waveguide 40 is disposed at a position offset from the center of the light receiving waveguide 40 in the direction of travel of the optical signal (Z-axis direction). Fig.21 As shown in the example, the distance L1 from the connection position of the first input waveguide 20 and the light receiving waveguide 40 to the position of the light receiving unit 41 may be different from the distance L2 from the connection position of the second input waveguide 30 and the light receiving waveguide 40 to the position of the light receiving unit 41. L1 and L2 may also be set so that the focusing position of the optical signal coincides with the period appearing in the Z-axis direction.

[0127] When L1 and L2 are different, the polarization mode of the optical signal propagated from the first input waveguide 20 and the polarization mode of the optical signal propagated from the second input waveguide 30 may also be different. Specifically, the optical signal propagated from the first input waveguide 20 is an optical signal in the TE mode, and the optical signal propagated from the second input waveguide 30 may be an optical signal in the TM mode. In this case, L1 and L2 can be set according to the focusing position of the optical signal propagated in each polarization mode.

[0128] When the polarization mode of the optical signal propagated from the first input waveguide 20 is different from the polarization mode of the optical signal propagated from the second input waveguide 30, the receiving waveguide 40 is configured to collect the optical signal propagated from the first input waveguide 20 at a position close to the end in the width direction of the end portion on the side connected to the second input waveguide 30. In this way, the optical signal propagated from the first input waveguide 20 that propagates to the second input waveguide 30 and becomes return light can be reduced.

[0129] like Fig. 22As shown in the example, the light receiving waveguide 40 may also include a terminal waveguide 46 at the destination (the end portion on the side connected to the second input waveguide 30) of the propagation direction of the optical signal from the first input waveguide 20. The terminal waveguide 46 may be connected to a position where the optical signal propagated from the first input waveguide 20 is focused at the end portion on the side connected to the second input waveguide 30. In addition, the light receiving waveguide 40 may also include a terminal waveguide 46 at the destination (the end portion on the side connected to the first input waveguide 20) of the propagation direction of the optical signal from the second input waveguide 30. The terminal waveguide 46 may be connected to a position where the optical signal propagated from the second input waveguide 30 is focused at the end portion on the side connected to the first input waveguide 20.

[0130] like Fig.23 As shown in the example, the light receiving waveguide 40 may have a notch 47 between the portion connected to the first input waveguide 20 and the portion provided with the light receiving portion 41, and between the portion connected to the second input waveguide 30 and the portion provided with the light receiving portion 41. The notch 47 is configured to prevent the propagation of optical signals other than the optical signals focused by the light receiving portion 41 among the optical signals propagated from the first input waveguide 20 and the second input waveguide 30. In this way, the intensity of the optical signal transmitted through the light receiving portion 41 can be reduced.

[0131] like Fig.24 As shown in the example, the light receiving unit 41 may be configured such that a normal line 411A of an end surface 411 on the side connected to the first input waveguide 20 is inclined relative to the optical axis 20A of the first input waveguide 20. In addition, the light receiving unit 41 may be configured such that a normal line 412A of an end surface 412 on the side connected to the second input waveguide 30 is inclined relative to the optical axis 30A of the second input waveguide 30. Since the end surfaces 411 and 412 of the light receiving unit 41 are inclined relative to the optical axes 20A and 30A, the optical signal reflected at the end surfaces 411 and 412 propagates in a direction deviating from the optical axes 20A and 30A, and is 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 signal at the end surfaces 411 and 412 can be reduced.

[0132] In addition, if Fig.25 As shown in the example, the light receiving unit 41 may have a convex portion 413 on the end surface 411. In addition, the light receiving unit 41 may have a convex portion 414 on the end surface 412. The convex portions 413 and 414 may have an inverted tapered shape in which the width in the X-axis direction expands toward the direction in which the optical signal propagates (the width in the X-axis direction narrows toward the direction away from the light receiving unit 41) when viewed from above (when viewed from the Y-axis direction). By having the convex portions 413 and 414 on the end surfaces 411 and 412, the optical signal propagating in a manner input to the light receiving unit 41 is less likely to be reflected in the opposite direction of the input at the narrower width of the convex portions 413 and 414 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.

[0133] exist Fig.25 In the example of FIG. 4 , the axis 413A of the protrusion 413 is along the direction of the optical axis 20A of the first input waveguide 20 (Z-axis direction). In addition, the axis 414A of the protrusion 414 is along the direction of the optical axis 30A of the second input waveguide 30 (Z-axis direction). Fig.26 As shown in the example, the axis 413A of the convex portion 413 may be inclined relative to the direction of the optical axis 20A (Z-axis direction). In addition, the axis 414A of the convex portion 414 may be inclined relative to the direction of the optical axis 30A (Z-axis direction). In this way, the optical signal reflected by the top of the convex portions 413 and 414 propagates in a direction deviating from the optical axes 20A and 30A, and is difficult to return to the first input waveguide 20 and the second input waveguide 30.

[0134] like Fig. 27 As in the above example, 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 light receiving waveguide 40 in a direction (X-axis direction) intersecting the propagation direction (Z-axis direction) of the optical signal. Fig. 27 In the embodiment, the light receiving section 41 is arranged on the central axis 40A of the light receiving waveguide 40, but may be arranged offset from the central axis 40A in the width direction (X-axis direction).

[0135] As described above, the MMI photodiode 10 can reduce return light caused by the optical signal transmitted through the light receiving unit 41. In addition, the shapes of the end faces 411 and 412 can be set so as to reduce return light caused by reflection from the end faces 411 and 412 of the light receiving unit 41.

[0136] <Example of a method for manufacturing the photodiode 10>

[0137] Hereinafter, an example of a method for manufacturing the photodiode 10 of the present disclosure will be described.

[0138] As an example of a manufacturing method, the material of the light receiving waveguide 40, the first input waveguide 20, and the second input waveguide 30 may be, for example, silicon. The light 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 silicon other than the waveguide is completely etched. Each waveguide may be formed as a rib type optical waveguide in which silicon is retained to a certain thickness. After etching the silicon, a silicon oxide layer is deposited by chemical vapor deposition. The deposited silicon oxide layer is flattened by chemical mechanical polishing or the like.

[0139] Phosphorus ions are implanted into the region forming the light receiving portion 41 and the region forming the electrode region 42 of the light receiving waveguide 40 by photolithography and ion implantation. Next, the oxide film on silicon in the region forming the light receiving portion 41 is removed by photolithography and reactive ion etching or wet etching using hydrofluoric acid or the like.

[0140] The epitaxial growth of germanium is performed by MOVPE (metal-organic vapor phase epitaxy). At this time, germanium is deposited only in the area where silicon is exposed. In addition, the germanium layer is deposited in the form of n-type, undoped, and p-type from the silicon substrate side by changing the dopant during epitaxial growth, so that a pin diode can be formed in the direction perpendicular to the substrate.

[0141] The silicon oxide film is removed and phosphorus ions are implanted in a region to be the electrode region 42 in the light receiving waveguide 40. In addition, boron ions are implanted in the upper portion of the germanium layer.

[0142] A silicon oxide film is deposited by chemical vapor deposition and flattened by chemical mechanical polishing. The silicon oxide film on the upper part of the light receiving portion 41 and the electrode region 42 is removed by photolithography and reactive ion etching. Tungsten is buried by chemical vapor deposition to form a through-hole electrode 45. In addition, an aluminum film is deposited by sputtering. The aluminum film is processed by photolithography and reactive ion etching to form wiring including an anode electrode 43 and a cathode electrode 44.

[0143] In addition, as an example of another 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 embedded in a silicon oxide film and flattened, they are bonded to a compound semiconductor substrate manufactured in another process. The compound semiconductor substrate may be, for example, an InP substrate. In the InP substrate, a pin layer including InGaAs and InAlGaAs is formed as a light receiving portion 41 by epitaxial growth by MOVPE. A silicon oxide film is formed on the outermost surface by PECVD (plasma-enhanced chemical vapor deposition), and the surface is flattened by chemical mechanical polishing.

[0144] The substrate 50 formed with the waveguide and the InP substrate formed with the light receiving portion 41 are joined 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 in a state of being joined to the substrate 50. By photolithography and reactive ion etching, only the contact layer in the light receiving portion 41 remains. By photolithography and reactive ion etching or wet etching, the contact layer in unnecessary parts is removed. By photolithography and stripping, a metal film as an electrode is formed on the p-electrode and the n-electrode. As the metal of the electrode, a laminated film such as Au / Pt / Ti, Au / Pt / Ti or Au / Zn / Ti can be used. A silicon oxide film is deposited by PECVD and planarized by CMP (Chemical Mechanical Polishing). Wiring including an anode electrode 43 and a cathode electrode 44 is formed on the silicon oxide film to connect to the through-hole electrode 45 and the p-electrode and the n-electrode.

[0145] (Configuration Example of Optical Receiver 1)

[0146] like Fig.28 As shown, the optical receiver 1 can be configured to use the photodiode 10 of the present disclosure. The optical receiver 1 has an edge coupler 811 as an input portion 81. The edge coupler 811 is configured to input light from the chip end face. The optical receiver 1 also has a polarization separation rotator (PSR) 82 and a demultiplexer (DEMUX) 83. In the optical receiver 1, the circuits processed before the optical signal is input to the photodiode 10 are collectively referred to as optical circuits. The optical circuit separates the optical signal input to the optical receiver 1 into an optical signal propagating in a TE mode and an optical signal propagating in a TM mode. In the optical receiver 1 of the present embodiment, the optical signal propagating in the TE mode corresponds to the first optical signal. The optical signal propagating in the TM mode corresponds to the second optical signal.

[0147] In the optical receiver 1, an optical signal in which a TE mode optical signal and a TM mode optical signal are mixed and an optical signal of n wavelengths is input to the edge coupler 811. The polarization separation rotator 82 separates the TE mode optical signal and the TM mode optical signal. In addition, the polarization separation rotator 82 converts the TM mode optical signal into a TE mode optical signal. The splitter 83 further separates each optical signal separated by the polarization separation rotator 82 into n wavelengths.

[0148] A signal obtained by separating the TE mode optical signal for each wavelength and a signal obtained by converting the TM mode optical signal into the TE mode optical signal and separating the signal for each wavelength are input to each of the photodiodes 10-1 to n. As a result, each of the photodiodes 10-1 to n outputs an electrical signal corresponding to the intensity of the signal obtained by separating the TE mode and TM mode mixed optical signal input to the edge coupler 811 for each wavelength.

[0149] The optical receiver 1 of this embodiment may further include a transimpedance amplifier for converting the electrical signal output from the photodiode 10. Compared with the case where two light receiving elements are used for detecting the TE mode optical signal and the TM mode optical signal for each wavelength, the optical receiver 1 of this embodiment can detect the optical signal for each wavelength using one photodiode. In this way, it is possible to achieve a higher speed or a lower power consumption and a lower circuit size due to a reduction in parasitic capacitance.

[0150] like Fig.29 As shown, the optical receiver 1 may also have a two-dimensional grating coupler (2DGC: 2-dimensional grating couplers) 812 as an input unit 81. The two-dimensional grating coupler 812 separates the input optical signal into two polarization components Px and Py, and outputs each polarization component as an optical signal of a polarized wave in a TE mode. The demultiplexer 83 separates the optical signal of each polarization component according to each wavelength. A signal after the optical signal of the polarization component Px is separated according to each wavelength and a signal after the optical signal of the polarization component Py is separated according to each wavelength are input to each of the photodiodes 10-1~n. As a result, the photodiodes 10-1~n output an electrical signal corresponding to the signal intensity after the optical signal of the TE mode and the TM mode mixed input to the two-dimensional grating coupler 812 is separated according to each wavelength. When the optical receiver 1 has the two-dimensional grating coupler 812, it may not have the polarization separation rotator 82.

[0151] like Fig.30 As shown, the optical receiver 1 may further include a delay device 84 between the polarization separation rotator 82 and each of the two splitters 83 and between the splitter 83 and each of the n photodiodes 10-1 to 10-n. The delay device 84 delays the propagation of the optical signal. The optical receiver 1 compensates for the deviation of the delay of the optical signal caused by the manufacturing error of the waveguide by using the delay device 84. By including the delay device 84, the optical receiver 1 can reduce the jitter of the signal obtained by combining the TE mode optical signal output from the photodiode 10 and the TE mode optical signal obtained by converting the TM mode optical signal.

[0152] The retarder 84 may be configured as a waveguide having a predetermined length, for example, and the effective refractive index of the waveguide may be adjusted by a heater. The retarder 84 may be configured as a phase modulator having a predetermined length, and the phase modulation amount may be adjusted by applying a voltage.

[0153] like Fig.31As shown, in the optical receiver 1, the polarization light separation rotator 82 can be replaced by a polarization light splitter (PS: Polarizing Splitter) 822. The polarization light splitter 822 separates the input optical signal into a TE mode optical signal and a TM mode optical signal. The propagation speed of the TE mode optical signal and the propagation speed of the TM mode optical signal are different from each other. The optical receiver 1 can have a delay device 84 to compensate for the deviation of the delay of the TE mode optical signal and the TM mode optical signal.

[0154] like Fig.32 As shown, in Fig.28 In the optical receiver 1 shown in the example, the delay device 84 connected between the demultiplexer 83 and each of the n photodiodes 10-1 to 10-n can be replaced with a variable optical attenuator (VOA) 85. The variable optical attenuator 85 can be configured to include, for example, a silicon pin diode. The variable optical attenuator 85 absorbs light by injecting current to attenuate the light intensity. By adjusting the current injected into each variable optical attenuator 85, it is possible to compensate for the optical loss generated in the polarization separation rotator 82 or the demultiplexer 83. Therefore, even if the optical loss in the polarization separation rotator 82 or the demultiplexer 83 is not uniform due to the difference in polarization or wavelength of the optical signal, by reducing the current value of the variable optical attenuator 85 through which the optical signal under the condition of large optical loss passes and increasing the current value of the variable optical attenuator 85 through which the optical signal under the condition of small optical loss passes, the light receiving sensitivity of the optical signal of any polarization or wavelength can be made close to uniform.

[0155] like Fig.33 As shown, 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 n.

[0156] As described above, the optical receiver 1 of the present embodiment can detect an optical signal by the photodiode 10 configured to reduce the return light. By reducing the return light, the optical signal returning to the input unit 81 can be reduced. By reducing the optical signal returning to the input unit 81, the stable operation of the light source or modulator that sends the optical signal to the input unit 81 can be maintained. As a result, the reliability of the optical communication system using the optical receiver 1 can be improved.

[0157] <Light detection device>

[0158] The photodiode 10 disclosed in the present invention is used as a light detection device for detecting light signals. As a light detection device, it is not limited to the photodiode 10, and other light receiving elements such as phototransistors can also be used. When other light receiving elements are used as light detection devices, the reliability of the optical communication system can be improved by reducing the return light.

[0159] The present disclosure is described based on the drawings and embodiments, but it should be noted that it is easy for those skilled in the art to make various deformations or modifications based on the present disclosure. Therefore, it should be noted that these deformations or modifications are also included in the scope of the present invention. For example, the functions included in each functional unit can be reconfigured in a logically non-contradictory manner. Multiple functional units can be combined into one or divided. It should be understood that these are also included in the scope of the present disclosure.

[0160] In the present disclosure, the records such as "first" and "second" are identifiers for distinguishing the structure. The structures distinguished by the records such as "first" and "second" in the present disclosure can exchange the numbers in the structure. For example, the first input waveguide 20 can exchange "first" and "second" as identifiers with the second input waveguide 30. The exchange of identifiers is performed simultaneously. The structure is also distinguished after the exchange of identifiers. The identifier can also be deleted. The structure from which the identifier is deleted is distinguished by the figure mark. The order of the structure should not be interpreted solely based on the records of the identifiers such as "first" and "second" in the present disclosure or become the basis for the existence of identifiers with small numbers.

[0161] In the present disclosure, the X-axis, Y-axis, and Z-axis are provided for convenience of description and may be replaced with each other. The structure of the present disclosure is described using an orthogonal coordinate system consisting of the X-axis, Y-axis, and Z-axis. The positional relationship of each structure of the present disclosure is not limited to an orthogonal relationship.

[0162] Description of Reference Numerals

[0163] 1 Optical receiver (81: input unit, 811: edge coupler, 812: two-dimensional grating coupler (2DGC), 82: polarization separation rotator (PSR), 822: polarization separation (PS), 83: demultiplexer (DEMUX), 84: delay device, 85: variable optical attenuator (VOA))

[0164] 10 Photodiode

[0165] 20 first input waveguide (20A: optical axis, 22: tapered portion)

[0166] 30 Second input waveguide (30A: optical axis, 32: tapered portion)

[0167] 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: through-hole electrode, 46: terminal waveguide, 47: cutout)

[0168] 50 substrate (51-53: first to third insulating layers)

Claims

1. A light detection device, wherein: have: A receiving waveguide connected to the first input waveguide for inputting the first optical signal and the second input waveguide for inputting the second optical signal; as well as a light receiving unit that outputs an electrical signal corresponding to the intensity of a signal obtained by combining the first light signal and the second light signal input to the light 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.

2. The light detection device according to claim 1, wherein: The light receiving waveguide is a multi-mode interference type optical waveguide.

3. The light detection device according to claim 2, wherein: The receiving waveguide is connected to the first input waveguide in a manner that the width of the first input waveguide and the width of the receiving waveguide change discontinuously along the first direction, and is connected to the second input waveguide in a manner that the width of the second input waveguide and the width of the receiving waveguide change discontinuously along the second direction.

4. The light detection device according to claim 3, wherein: A change rate of the width of the light receiving waveguide with respect to each width of the first input waveguide and the second input waveguide is equal to or greater than a predetermined value.

5. The light detection device according to any one of claims 2 to 4, wherein: The light receiving waveguide includes at least one of a first terminal waveguide connected to the opposite side of the first input waveguide along the first direction and a second terminal waveguide connected to the opposite side of the second input waveguide along the second direction.

6. The light detection device according to any one of claims 1 to 5, wherein: The first direction and the second direction are substantially parallel.

7. The light detection device according to any one of claims 1 to 5, wherein: An angle formed by the first direction and the second direction is within a prescribed range including 90 degrees.

8. The light detection device according to any one of claims 1 to 7, wherein: The light receiving portion has a first surface located on the first direction side and a second surface located on the second direction side. at least a portion of the first surface includes a surface having a normal direction that is non-parallel to the first direction, At least a portion of the second surface includes a surface having a normal direction that is non-parallel to the second direction.

9. An optical receiver, wherein: have: 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; as well as a light detection device that outputs an electrical signal corresponding to the intensity of the first light signal and the second light signal input from the optical circuit; The light detection device comprises: A receiving waveguide connected to a first input waveguide inputting the first optical signal and a second input waveguide inputting the second optical signal; as well as a light receiving unit that outputs an electrical signal corresponding to the intensity of a signal obtained by combining the first light signal and the second light signal input to the light 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.

10. The optical receiver according to claim 9, wherein The optical circuit has: Edge coupler, accepting input of optical signal; a polarization separation rotator, which 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 a TM mode to a TE mode; and The wavelength splitter separates the first optical signal and the second optical signal into optical signals of each wavelength.

11. The optical receiver according to claim 9, wherein The optical circuit has: a two-dimensional grating coupler that receives an input of an optical signal and separates the optical signal into the first optical signal and the second optical signal, and converts the second optical signal from a TM mode to a TE mode; and The wavelength splitter separates the first optical signal and the second optical signal into optical signals of each wavelength.

12. The optical receiver according to claim 10 or 11, wherein: The optical circuit also includes a delay device.

13. The optical receiver according to claim 12, wherein: The optical circuit also includes a variable optical attenuator.

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