Optical detector, optical receiver, and optical transceiver

Abstract

The invention relates to an optical detector, an optical receiver and an optical transceiver. The photodetector includes a photodetector that converts signal light into an electrical signal, and a mode filter. The photodetector includes: a first port to which signal light of a first mode is input; and a second port to which the signal light of the second mode is input. The mode filter includes a third port connected to the first port, and a fourth port connected to the second port. The mode filter outputs the signal light of the first mode toward the first port through the third port, outputs the signal light of the second mode toward the second port through the fourth port, and removes the signal light of the second mode from the first port input from the third port. And removing the signal light of the first mode from the second port input from the fourth port.

Description

Technical Field

[0001] The embodiments discussed herein relate to optical detectors, optical receivers, and optical transceivers. Background Art

[0002] In recent years, high-speed, high-capacity communications have been realized in optical fiber communication networks using coherent optical communication technology. In an optical receiver using coherent optical communication technology, after mixing the received light whose optical power is reduced due to long-distance transmission with the local light emission generated inside the optical receiver, the mixed received light is converted into an electric current by a photodetector (PD). As a result, the optical receiver achieves high-sensitivity reception by reducing the influence of thermal noise generated after the PD.

[0003] However, in an optical receiver, the receiving sensitivity increases with the increase of the optical power of local light emission, but when a large optical power is input to the PD, the response speed of the PD is limited by the space charge effect, making it difficult to perform high-speed communication.

[0004] In the space charge effect, when light input into the PD generates electron-hole pairs, the electric field generated by these carriers in the PD will offset the bias applied to the PD, reducing the applied bias and weakening the force that transports the carriers in the PD to the outside of the PD. As a result, the response speed of the PD decreases.

[0005] In addition, when the number of electron-hole pairs generated in the PD is large, in other words, in a case where the input power is large, the space charge effect becomes significant, and therefore, by reducing the input power, the space charge effect can be mitigated.

[0006] As a method of reducing input power to mitigate the space charge effect, for example, a method is known in which optical power is divided before being input to a PD and each light beam is input to the PD from two directions.

[0007] In the PD, light is converted into electron-hole pairs during propagation, so that the optical power decreases as it moves away from the light input unit of the PD. Therefore, in the case where the optical power to be input is divided into two parts and input to the PD from two directions, the light intensity of each cross section can be suppressed to about half, so that the influence of the space charge effect can be reduced. As a result, even in the case where the optical power input to the PD is high, the degradation of the PD response speed can be suppressed.

[0008] For example, a waveguide type PD is adopted as the PD. The waveguide type PD includes a PD and an optical waveguide for guiding light integrated on the same substrate, and guides light from the waveguide to the PD to perform light / current conversion by the PD.

[0009] The waveguide type PD is composed of a silicon-on-insulator (SOI) substrate SOI layer and Ge epitaxially grown thereon. In the communication band (e.g., C-band), the band gap of Ge is narrower than that of Si, so that light propagating in Ge is converted into electron-hole pairs in Ge, and photoelectric conversion is achieved by extracting the electron-hole pairs by doped Si or electrodes. A layer like Ge that is used to convert light into electron-hole pairs is called an absorption layer. The absorption layer is a portion having a band gap different from the band gap of the optical waveguide, and absorbs input light to be converted into electron-hole pairs.

[0010] As described above, the waveguide type PD includes an optical waveguide and a PD. The optical waveguide includes a Si substrate, a buried oxide layer (BOX) layer laminated on the Si substrate, a Si optical waveguide formed on the BOX layer, and a buffer layer covering the BOX layer and the optical waveguide. The Si optical waveguide is formed on the BOX layer by photolithographically etching the Si of the SOI layer of the SOI substrate formed by the Si substrate and the BOX layer. The PD includes a Si substrate, a BOX layer, a Ge absorption layer, and a buffer layer covering the BOX layer and the absorption layer. The absorption layer is connected to the optical waveguide, absorbs the signal light from the optical waveguide, and converts the signal light into electron-hole pairs.

[0011] For example, the related technology is described in the following documents: U.S. Patent Application Publication No. 2019 / 0391006; A. Beling, X. Xie and JC Campbell, "High-power high-linearity photodiodes", Optica, Vol. 3, No. 3, pp. 328-338, 2016; Jishi Cui et al., "The dual-injection Ge-on-Si photodetectors with high saturation power by optimizing light field distribution", Optics Communications, Vol. 480, February 1, 2021, 126467; Xiao Hu et al., "High-speed and high-power germanium photodetector with a lateral silicon nitride waveguide", Photonics Research, Vol. Daigao Chen et al., “Broadband, Fabrication-Tolerant Polarization Beam Splitters Based on a Tapered Directional Coupler,” IEEE Photonics Research, Vol. 9, No. 5, pp. 749-756 (2021); Daigao Chen et al., “Broadband, Fabrication-Tolerant Polarization Beam Splitters Based on a Tapered Directional Coupler,” IEEE Photonics Technology Letters, Vol. 28, No. 19, October 1, 2016; Yi Zhang et al., “A high-responsivity photodetector absent metal-germanium direct contact,” Optics Express, Vol. 22, No. 9, pp. 11367-11375 (2014); and Chia-Ming Chang et al., “High-power doubly-fed traveling-wave photodetector circuit in silicon photonics,” Optics Express, Vol. 22, No. 9, pp. 11367-11375 (2014). Express, vol. 23, August 24, 2015, 22857.

[0012] In the case where light is input to a waveguide type PD from two directions, residual light that is not converted into electron-hole pairs in the absorption layer in the PD is output from a port different from the input port. As a result, the residual light output from a different port flows in the opposite direction to become reflected light. When the reflected light is further reflected in an external optical device, Fabry-Perot resonance is caused, the photocurrent changes due to changes in wavelength, temperature, etc., and the stability of the optical receiver is reduced.

[0013] This backflow of residual light in the absorption layer of the PD is mainly caused by the light that is not completely absorbed by the Ge absorption layer passing through the Ge absorption layer as residual light.

[0014] Therefore, in a waveguide PD, residual light can be reduced by increasing the length of the Ge absorption layer so that all the light beam is absorbed. However, considering that the PD is used as a capacitor in an electronic circuit, as the length of the absorption layer increases, the capacitance also increases, and the RC time constant also increases. As a result, the response speed is limited.

[0015] Therefore, an object of one aspect of an embodiment of the present invention is to provide a photodetector or the like capable of suppressing the backflow of residual light in a PD. Summary of the invention

[0016] According to one aspect of the embodiment, a light detector includes a photodetector and a mode filter. The photodetector converts signal light into an electrical signal and includes two or more ports. The mode filter includes two or more ports. The photodetector includes: a first port, to which signal light of a first mode is input; and a second port, to which signal light of a second mode is input. The mode filter includes a third port connected to the first port, and a fourth port connected to the second port. The mode filter allows signal light of a first mode toward the first port to pass through to be output from the third port, and allows signal light of a second mode toward the second port to pass through to be output from the fourth port. The mode filter removes signal light of a second mode to be input from the third port, which is output from the first port. The mode filter removes signal light of a first mode to be input from the fourth port, which is output from the second port. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a block diagram illustrating an example of a photodetector according to the first embodiment;

[0018] Figure 2A is a perspective view illustrating an example of a waveguide type PD;

[0019] Figure 2B is a schematic plan view illustrating an example of a waveguide type PD which is partially omitted;

[0020] Figure 3 is along Figure 2B A schematic cross-sectional view of line AA is shown;

[0021] Figure 4 is a block diagram illustrating an example of a photodetector according to a second embodiment;

[0022] Figure 5 is an explanatory diagram illustrating an example of a fourth pattern removing unit;

[0023] Figure 6 is an explanatory diagram illustrating an example of a third pattern removal unit;

[0024] Figure 7 is along Figure 6 A schematic cross-sectional view of line BB is shown;

[0025] Figure 8 is a block diagram illustrating an example of a photodetector according to a third embodiment;

[0026] Fig.9A is an explanatory diagram illustrating an example of a fifth pattern removing unit;

[0027] Fig. 9B is an explanatory diagram illustrating an example of a sixth pattern removing unit;

[0028] Fig.10 is a block diagram illustrating an example of a photodetector according to a fourth embodiment;

[0029] Fig.11 is an explanatory diagram illustrating an example of a mode conversion unit;

[0030] Fig. 12A is an explanatory diagram illustrating an example of the operation of the pattern removal unit at the time of input;

[0031] Fig. 12B is an explanatory diagram illustrating an example of the operation of the pattern removal unit at the time of reflow;

[0032] Fig.13 is a block diagram illustrating an example of a photodetector according to a fifth embodiment;

[0033] Fig.14 is an explanatory diagram illustrating an example of transition of light in a photodetector according to a fifth embodiment;

[0034] Fig.15 is an explanatory diagram illustrating an example of a pattern removal unit;

[0035] Fig.16A is an explanatory diagram illustrating an example of a dual-input waveguide type PD;

[0036] Fig. 16B is an explanatory diagram illustrating an example of a three-input waveguide type PD;

[0037] Fig. 16C is an explanatory diagram illustrating an example of a four-input waveguide type PD;

[0038] Fig.17 is a block diagram illustrating an example of a photodetector according to a sixth embodiment;

[0039] Fig.18A is an explanatory diagram illustrating an example of a waveguide type PD;

[0040] Fig.18B is an explanatory diagram illustrating an example of a waveguide type PD;

[0041] Fig.18C is an explanatory diagram illustrating an example of a waveguide type PD;

[0042] Fig.18D is an explanatory diagram illustrating an example of a waveguide type PD;

[0043] Fig.18E is an explanatory diagram illustrating an example of a waveguide type PD; and

[0044] Fig.19 is an explanatory diagram illustrating an example of an optical transceiver employing a photodetector. DETAILED DESCRIPTION

[0045] Preferred embodiments of the present invention will be described with reference to the accompanying drawings. The disclosed technology is not limited by the embodiments. The embodiments described below can be appropriately combined with each other without causing contradictions.

[0046] (a) First Embodiment

[0047] Figure 1 is a block diagram illustrating an example of a photodetector 1 according to the first embodiment, Figure 2A is a perspective view illustrating an example of a waveguide type PD 2, and Figure 2B : is a schematic plan view (which is partially omitted) illustrating an example of the waveguide type PD 2 . Figure 1 The photodetector 1 shown includes a waveguide type PD 2 and a mode filter 3. The mode filter 3 includes a first mode removal unit 3A and a second mode removal unit 3B. The waveguide type PD 2 includes a first port 2A1 connected to a first input waveguide 11A, a second port 2A2 connected to a second input waveguide 11B, and an absorption layer 12.

[0048] The first mode removal unit 3A includes a first input port 3A1, a first output port 3A2, and a first filter 30A. The first output port 3A2 is connected to the first input waveguide 11A. The first filter 30A is a filter that passes the first mode signal light from the first input port 3A1 and removes the second mode signal light from the first output port 3A2.

[0049] The second mode removal unit 3B includes a second input port 3B1, a second output port 3B2, and a second filter 30B. The second output port 3B2 is connected to the second input waveguide 11B. The second filter 30B is a filter that allows the signal light of the second mode from the second input port 3B1 to pass and removes the signal light of the first mode from the second output port 3B2. The signal light of the first mode is the signal light of the second mode different from the first mode.

[0050] The absorption layer 12 converts the signal light of the first mode input from the first port 2A1 into electron-hole pairs, and converts the signal light of the second mode input from the second port 2A2 into electron-hole pairs.

[0051] For the sake of convenience, the case where the first filter 30A removes the signal light of the second mode is taken as an example, but signal light other than the signal light of the first mode may also be removed. This embodiment may be appropriately changed. In addition, the case where the second filter 30B removes the signal light of the first mode is taken as an example, but signal light other than the signal light of the second mode may also be removed. This embodiment may be appropriately changed.

[0052] Figure 3 is along Figure 2B Schematic cross-section along line AA is shown. Figure 2A , Figure 2B and Figure 3 The waveguide type PD 2 shown in the figure includes a Si substrate 13, a buried oxide layer (BOX) layer 14 laminated on the Si substrate 13, a waveguide layer 11 laminated on the BOX layer 14, and a buffer layer 15 covering the waveguide layer 11. The Si substrate 13 is a support substrate formed of Si or the like. The BOX layer 14 is, for example, a SiO2 layer. The waveguide layer 11 is, for example, a waveguide layer formed of Si. The buffer layer 15 is, for example, a SiO2 layer. The absorption layer 12 is, for example, an absorption layer formed of germanium (Ge).

[0053] The waveguide layer 11 includes a first input waveguide 11A, a second input waveguide 11B, and a waveguide 11C connecting the first input waveguide 11A and the second input waveguide 11B. The waveguide 11C includes a p-doped region 11C1, an n-doped region 11C2, and an undoped region 11C3 located between the p-doped region 11C1 and the n-doped region 11C2. The absorption layer 12 is disposed over the undoped region 11C3, a portion of the p-doped region 11C1 connected to the undoped region 11C3, and a portion of the n-doped region 11C2 connected to the undoped region 11C3. The first input waveguide 11A and the second input waveguide 11B are disposed on the BOX layer 14. The thickness of the waveguide layer 11 is, for example, 0.22 μm, the width of the undoped region 11C3 is, for example, 0.4 μm, the thickness of the absorption layer 12 is, for example, 0.3 μm, the width of the absorption layer 12 is, for example, 1.0 μm, and the length of the absorption layer 12 is, for example, 40 μm. The dimensions exemplified here are set only for calculation of a Finite-Difference Time-Domain method (described later) and can be changed appropriately.

[0054] However, in the case where a portion of the signal light of the first mode input from the first port 2A1 is not completely absorbed by the absorption layer 12 but remains therein, the remaining signal light of the first mode is output to the second input waveguide 11B through the second port 2A2. Therefore, the second filter 30B in the second mode removal unit 3B removes the signal light of the first mode input from the second output port 3B2. In addition, in the case where a portion of the signal light of the second mode input from the second port 2A2 is not completely absorbed by the absorption layer 12 but remains therein, the remaining signal light of the second mode is output to the first input waveguide 11A through the first port 2A1. Therefore, the first filter 30A in the first mode removal unit 3A removes the signal light of the second mode input from the first output port 3A2. As a result, the optical detector 1 is able to suppress the backflow of the residual light in the absorption layer 12.

[0055] The photodetector 1 according to the first embodiment includes a waveguide type PD 2 and a mode filter 3. The waveguide type PD 2 includes a first port 2A1 into which a signal light of a first mode is input and a second port 2A2 into which a signal light of a second mode is input. The mode filter 3 includes a first mode removal unit 3A and a second mode removal unit 3B. The first mode removal unit 3A transfers the signal light of a first mode to the first port 2A1 and removes the signal light of a second mode from the first port 2A1. The second mode removal unit 3B transfers the signal light of a second mode to the second port 2A2 and removes the signal light of a first mode from the second port 2A2. As a result, the photodetector 1 can suppress the backflow of residual light in the absorption layer 12. That is, it is possible to suppress the intensity of the signal light per unit cross section input to the waveguide type PD 2, suppress the degradation of the response speed of the waveguide type PD 2, and suppress the noise caused by the backflow of the residual light not absorbed by the waveguide type PD 2.

[0056] (b) Second Embodiment

[0057] Figure 4 1 is a block diagram illustrating an example of a photodetector 1A according to the second embodiment. For convenience of explanation, the same configurations as those of the photodetector 1 in the first embodiment are denoted by the same reference numerals, and descriptions on the duplicated configurations and operations will not be repeated. Figure 4 The photodetector 1A of the second embodiment shown includes a branching unit 4, a third mode removal unit 3C, a fourth mode removal unit 3D, and a waveguide type PD 2. The branching unit 4 includes an input port 4A1, an output port 4A2, and an output port 4A3, and outputs the signal light of the first mode from the input port 4A1 to the output port 4A2 and the output port 4A3 in a branching manner. It is assumed that the signal light of the first mode is TE0, and it is assumed that the signal light of the second mode is TM0. Here, TE0, TM0 each indicate a waveguide mode having the largest effective refractive index among the waveguide modes (TE mode, TM mode) in which the direction of the main component of the electric field becomes parallel to or perpendicular to the substrate.

[0058] The third mode removal unit 3C includes a third input port 3C1, a third output port 3C2, and a third filter 30C. The third filter 30C converts the first mode signal light (TE0 signal light) input from the third input port 3C1 into the second mode signal light (TM0 signal light), and removes the TE0 signal light input from the third output port 3C2.

[0059] The fourth mode removal unit 3D includes a fourth input port 3D1, a fourth output port 3D2, and a fourth filter 30D. The fourth filter 30D passes and outputs the TE0 signal light input from the fourth input port 3D1, and removes the TM0 signal light input from the fourth output port 3D2.

[0060] The absorption layer 12 converts the TM0 signal light input from the first port 2A1 into electron-hole pairs, and converts the TE0 signal light input from the second port 2A2 into electron-hole pairs. However, in the case where a portion of the TM0 signal light input from the first port 2A1 is not completely absorbed by the absorption layer 12 but remains therein, the remaining TM0 signal light is output from the second port 2A2. Therefore, the fourth filter 30D in the fourth mode removal unit 3D removes the TM0 signal light input from the fourth output port 3D2.

[0061] In addition, in the case where a portion of the TE0 signal light input from the second port 2A2 is not completely absorbed by the absorption layer 12 but remains therein, the remaining TE0 signal light is output from the first port 2A1. Therefore, the third filter 30C in the third mode removal unit 3C removes the TE0 signal light input from the third output port 3C2. As a result, the optical detector 1 can suppress the backflow of the residual light in the absorption layer 12.

[0062] That is, the fourth mode removal unit 3D can suppress the backflow of TM0 signal light to the branch unit 4 by removing the TM0 signal light remaining in the absorption layer 12. In addition, the third mode removal unit 3C can suppress the backflow of TE0 signal light to the branch unit 4 by removing the TE0 signal light remaining in the absorption layer 12.

[0063] Figure 5 : is an explanatory diagram illustrating an example of a fourth mode removal unit 3D. The fourth filter 30D in the fourth mode removal unit 3D is an adiabatic converter 30D1 configured by arranging a first waveguide 31D having a tapered shape and a second waveguide 32D having a tapered shape in parallel with each other. In a parallel section L of the adiabatic converter 30D1 in which the first waveguide 31D is parallel to the second waveguide 32D, there are a start point and an end point. The width W1 of the first waveguide 31D at the start point is equal to the width W1 of the second waveguide 32D at the end point. The width W2 of the first waveguide 31D at the end point is equal to the width W2 of the second waveguide 32D at the start point. The interval G between the first waveguide 31D and the second waveguide 32D is the same at the start point and the end point.

[0064] In the case where a beam of TE0 signal light and TM0 signal light is input from the fourth output port 3D2 of the first waveguide 31D, the adiabatic converter 30D1 allows the TE0 signal light to pass through and output from the fourth input port 3D1 of the first waveguide 31D. The adiabatic converter 30D1 allows the TM0 signal light to gradually migrate from the first waveguide 31D to the second waveguide 32D, and outputs the TM0 signal light alone from the second waveguide 32D. The fourth mode removal unit 3D has reversibility, so that, in the case where the TE0 signal light is input from the fourth input port 3D1 of the first waveguide 31D, the fourth mode removal unit 3D allows the TE0 signal light to pass through and output from the fourth output port 3D2 of the first waveguide 31D.

[0065] Figure 6 is an explanatory diagram illustrating an example of the third pattern removal unit 3C, and Figure 7 is along Figure 6 A schematic cross-sectional view along line BB is shown. Figure 6 and Figure 7 The third filter 30C in the third mode removal unit 3C shown in the figure includes an adiabatic converter 30C1 and a tapered waveguide 33C. The third mode removal unit 3C includes a Si substrate 13, a BOX layer 14 laminated on the Si substrate 13, an adiabatic converter 30C1 and a tapered waveguide 33C laminated on the BOX layer 14, and a buffer layer 15 covering the BOX layer 14, the adiabatic converter 30C1 and the tapered waveguide 33C. The adiabatic converter 30C1 can couple the TE0 signal light with the TE1 signal light. The tapered waveguide 33C is a waveguide that adiabatically converts the TE1 signal light into the TM0 signal light.

[0066] The third mode removal unit 3C converts the TE0 signal light and the TM0 signal light using the adiabatic converter 30C1 and the tapered waveguide 33C, and the input TM0 signal light and the TE0 signal light can be separated from each other in space. In the adiabatic converter 30C1, the first waveguide 31C and the second waveguide 32C are arranged in parallel with each other, so that the third input port 3C1 becomes the input port of the first waveguide 31C, and the input port of the second waveguide 32C becomes the discard port 3C3. In the adiabatic converter 30C1, when the TE0 signal light is input to the third input port 3C1, the TE0 signal light migrates from the first waveguide 31C to the second waveguide 32C to convert the TE0 signal light into the TE1 signal light. In addition, the tapered waveguide 33C converts the TE1 signal light into the TM0 signal light to output the TM0 signal light from the third output port 3C2. The third mode removal unit 3C has reversibility, so that when the TE0' signal light is input from the third output port 3C2 of the tapered waveguide 33C, the third mode removal unit 3C allows the TE0' signal light to pass and outputs it to the discard port 3C3 to remove the TE0' signal light. The TE0' signal light is the TE0 signal light input from the third output port 3C2.

[0067] The following describes the results of the comparison between the optical detector 1A according to the second embodiment and the comparative example. In the comparative example, the branch unit 4 is directly connected to the waveguide type PD 2 without using the third mode removal unit 3C and the fourth mode removal unit 3D. In the comparative example, the output port 4A2 of the branch unit 4 is connected to the first port 2A1 of the waveguide type PD 2 through the first input waveguide 11A to input the TE0 signal light to the first port 2A1, and the output port 4A3 of the branch unit 4 is connected to the second port 2A2 through the second input waveguide 11B to input the TE0 signal light to the second port 2A2. Figure 2A , Figure 2B and Figure 3 For the waveguide type PD 2 in FIG. 1 , the transmittance of the absorption layer 12 to TE0 when the TE0 signal light is input to the first input waveguide 11A and the TM0 signal light is input to the second input waveguide 11B is calculated by using the finite difference time domain method. As parameters of the waveguide type PD 2, it is assumed that the thickness of the waveguide layer 11 is, for example, 0.22 μm, the width of the undoped region 11C3 is, for example, 0.4 μm, and the thickness of the absorption layer 12 is, for example, 0.3 μm. It is assumed that the width of the absorption layer 12 is, for example, 1.0 μm, and the length of the absorption layer 12 is, for example, 40 μm. It is assumed that the wavelength is 1.52 μm, the material refractive index of Ge used in the absorption layer 12 is 4.278+i0.0245, the material refractive index of Si used in the Si substrate 13 is 3.45, and the material refractive index of SiO2 used in the BOX layer 14 and the buffer layer 15 is 1.44.

[0068] Regarding the photodetector according to the comparative example, the ratio of the TE0 signal light output from the first input waveguide 11A in the case where the TE0 signal light is input to the second input waveguide 11B is calculated. The result is that the output ratio is -24.15 dB. However, in the waveguide type PD 2, the optical power that passes through the absorption layer 12 and flows in the reverse direction is multiplexed at the branch unit 4, so that the ratio of the power output from the branch unit 4 relative to the power input to the branch unit 4 is -24.15 dB at maximum. The cross-sectional structure of the waveguide type PD 2 is not limited to Figure 3 In a configuration including a plurality of ports, light not absorbed by the absorption layer is output from another port having any cross-sectional structure. Therefore, even if any cross-sectional structure is adopted, through light caused by the backflow of residual light may be generated.

[0069] In the case of using the photodetector 1A according to the second embodiment, assuming that the signal light of the first mode is TE0, assuming that the signal light of the second mode is TM0, and inputting the light beams of the TE0 signal light and the TM0 signal light to the absorption layer 12, mode conversion may be caused in the waveguide type PD 2. In this case, the effect of providing the third mode removal unit 3C and the fourth mode removal unit 3D may be lost.

[0070] In the optical detector 1A, when the TE0 signal light is input to the second input waveguide 11B, the ratio of the TE0 signal light output from the first input waveguide 11A is -24.15 dB. In addition, the ratio of the TM0 signal light output from the first input waveguide 11A (i.e., the ratio of mode conversion) is -78.61 dB. When the TM0 signal light is input to the first input waveguide 11A, the ratio of the TM0 signal light output from the second input waveguide 11B is -34.62 dB. In addition, the ratio of the TE0 signal light output from the second input waveguide 11B (i.e., the ratio of mode conversion) is -73.82 dB.

[0071] As a result, naturally, with respect to the output to the same mode, the through light caused by the backflow of the unnecessary residual light can be removed by using the third mode removal unit 3C and the fourth mode removal unit 3D. In addition, the mode conversion in the waveguide type PD 2 can be ignored, so that the unnecessary through light can be removed by adding the mode removal structure. In the waveguide structure that is symmetrical within the optional plane along the traveling direction of the light, the conversion between the different polarized waves becomes very small, so it is preferable to use different polarized waves in such a waveguide structure.

[0072] The optical detector 1A according to the second embodiment includes a third mode removal unit 3C and a fourth mode removal unit 3D. The fourth mode removal unit 3D allows the TE0 signal light from the branch unit 4 to pass through, and removes the TM0 signal light from the second port 2A2. The third mode removal unit 3C converts the TE0 signal light from the branch unit 4 into the TM0 signal light, allows the converted TM0 signal light to pass through, and removes the TE0 signal light from the first port 2A1. As a result, the optical detector 1A can suppress the backflow of the residual light in the absorption layer 12.

[0073] The fourth mode removal unit 3D includes an adiabatic converter 30D1 that passes the TE0 signal light from the branch unit 4 and removes the TM0 signal light from the second port 2A2. The third mode removal unit 3C includes an adiabatic converter 30C1 that converts the TE0 signal light from the branch unit 4 into the TM0 signal light, and a tapered waveguide 33C that passes the TM0 signal light converted by the adiabatic converter 30C1 and removes the TE0 signal light from the first port 2A1. As a result, the photodetector 1A can suppress the backflow of the residual light in the absorption layer 12.

[0074] For ease of explanation, a case where the optical detector 1A according to the second embodiment uses TE0 signal light as the signal light of the first mode and uses TM0 signal light as the signal light of the second mode is taken as an example, but the embodiment is not limited thereto and can be appropriately changed. The following describes an optical detector 1B according to a third embodiment, which uses the signal light of the first mode as the TE0 signal light and uses the signal light of the second mode as the TE1 signal light.

[0075] (c) Third Embodiment

[0076] Figure 8 1B is a block diagram illustrating an example of a photodetector 1B according to the third embodiment. For convenience of explanation, the same configurations as those of the photodetector 1A in the second embodiment are denoted by the same reference numerals, and descriptions on the duplicated configurations and operations are not repeated. Figure 8 The photodetector 1B shown is Figure 4 The optical detector 1A shown is different in that a fifth mode removal unit 3E is provided instead of the third mode removal unit 3C, and a sixth mode removal unit 3N is provided instead of the fourth mode removal unit 3D. The fifth mode removal unit 3E uses an adiabatic converter 30E, which can couple the TE0 signal light with the TE1 signal light.

[0077] Fig.9A is an explanatory diagram illustrating an example of the fifth pattern removing unit 3E. Fig.9AThe fifth mode removal unit 3E illustrated in the figure is an adiabatic converter 30E. In the adiabatic converter 30E, a first waveguide 31E and a second waveguide 32E are arranged in parallel with each other. The adiabatic converter 30E includes an input port 31E1 (3E1) and an output port 31E2 (3E2) of the first waveguide 31E, and an input port 32E1 (3E1) and an output port 32E2 (3E2) of the second waveguide 32E. The first waveguide 31E has a structure in which the waveguide width gradually decreases from the input port 31E1 to the output port 31E2. The second waveguide 32E has a structure in which the waveguide width gradually decreases from the output port 32E2 to the input port 32E1.

[0078] When TE0 signal light is input from the input port 31E1 of the first waveguide 31E, the adiabatic converter 30E gradually and adiabatically converts the TE0 signal light into TE1 signal light from the first waveguide 31E to the output side of the second waveguide 32E. Then, the adiabatic converter 30E outputs the TE1 signal light from the output port 32E2 of the second waveguide 32E. When TE0 signal light is input from the input port 32E1 of the second waveguide 32E, the adiabatic converter 30E passes the TE0 signal light and outputs it from the output port 32E2 of the second waveguide 32E.

[0079] Considering the reversibility of the adiabatic converter 30E of the fifth mode removing unit 3E, the TE1 signal light input from the output port 32E2 of the second waveguide 32E is converted into the TE0 signal light, which is output from the input port 31E1 of the first waveguide 31E. The TE0 signal light input from the output port 32E2 of the second waveguide 32E is directly output from the input port 32E1 of the second waveguide 32E to remove the TE0 signal light.

[0080] The adiabatic converter 30E functions as a mode conversion removal unit by connecting the input port 32E1 (3E1) of the first waveguide 31E to the branch unit 4, and connecting the output port 32E2 (3E2) of the second waveguide 32E to the first port 2A1 of the waveguide type PD 2. In this case, the TE0 signal light input to the output port 32E2 (3E2) of the second waveguide 32E (i.e., the TE0 signal light passing through the waveguide type PD 2) is output from the input port 32E1 (3E1) of the second waveguide 32E. Therefore, in order to remove such unnecessary light, the input port 32E1 (3E1) of the second waveguide 32E is made an optical terminal. In the case of making the input port 32E1 of the second waveguide 32E an optical terminal, for example, the input port 32E1 may have a tapered structure that gradually tapers, or may be connected to a doped waveguide. As an optical terminal, a monitor PD for monitoring unnecessary light may be connected, and the embodiment may be appropriately changed.

[0081] Fig. 9B : is an explanatory diagram illustrating an example of the sixth mode removal unit 3N. The sixth filter 30N in the sixth mode removal unit 3N is an adiabatic converter 30N1 configured by arranging a first waveguide 31N having a tapered shape and a second waveguide 32N having a tapered shape in parallel to each other. In a parallel section L of the adiabatic converter 30N1 in which the first waveguide 31N is parallel to the second waveguide 32N, there are a start point and an end point. The width W1 of the first waveguide 31N at the start point is equal to the width W1 of the second waveguide 32N at the end point. The width W2 of the first waveguide 31N at the end point is equal to the width W2 of the second waveguide 32N at the start point. The interval G between the first waveguide 31N and the second waveguide 32N is the same at the start point and the end point.

[0082] When the beams of TE0 and TE1 signal lights are input from the sixth output port 3N2 of the first waveguide 31N, the adiabatic converter 30N1 allows the TE0 signal light to pass through and output from the sixth input port 3N1 of the first waveguide 31N. The adiabatic converter 30N1 allows the TE1 signal light to gradually migrate from the first waveguide 31N to the second waveguide 32N, and outputs the TE1 signal light alone from the second waveguide 32N. The sixth mode removal unit 3N has reversibility, so that, in the case where the TE0 signal light is input from the sixth input port 3N1 of the first waveguide 31N, the sixth mode removal unit 3N allows the TE0 signal light to pass through and output from the sixth output port 3N2 of the first waveguide 31N.

[0083] In the adiabatic converter 30N1 in the sixth mode removal unit 3N, the sixth input port 3N1 of the first waveguide 31N is connected to the branch unit 4, and the sixth output port 3N2 of the first waveguide 31N is connected to the waveguide type PD 2. In this case, the TE1 signal light that has been input to the sixth output port 3N2 of the first waveguide 31N and passed through the waveguide type PD 2 reaches the input port 3N3 of the second waveguide 32N as an optical terminal. In the case of making the input port 3N3 of the second waveguide 32N an optical terminal, for example, the input port 3N3 may be made to have a tapered structure that is gradually reduced, or may be connected to a doped waveguide, etc. As an optical terminal, a monitor PD for monitoring unnecessary light may be connected, and the embodiment may be appropriately changed.

[0084] In the case of using the photodetector 1B according to the third embodiment, assuming that the signal light of the first mode is TE0, assuming that the signal light of the second mode is TE1, and the light beams of the TE0 and TE1 signal lights are input to the absorption layer 12, mode conversion may be caused in the waveguide type PD 2. In this case, the effect of providing the fifth mode removal unit 3E and the sixth mode removal unit 3N may be lost. Therefore, regarding Figure 3In the waveguide type PD 2, the transmittance of the absorption layer 12 in the case where TE0 signal light is input to the first input waveguide 11A and TE1 signal light is input to the second input waveguide 11B is calculated by using the finite difference time domain method.

[0085] When the TE0 signal light is input to the second input waveguide 11B, the ratio of the TE0 signal light output from the first input waveguide 11A is -24.15 dB. In addition, the ratio of the TE1 signal light output from the first input waveguide 11A (i.e., the mode conversion ratio) is -65.27 dB. In the case where the TE1 signal light is input to the first input waveguide 11A, the ratio of the TE1 signal light output from the second input waveguide 11B is -23.36 dB. In addition, the ratio of the TE0 signal light output from the second input waveguide 11B (i.e., the mode conversion ratio) is -63.68 dB.

[0086] Therefore, naturally, regarding the output to the same mode, the through light caused by the unnecessary residual light backflow can be removed by using the fifth mode removal unit 3E and the sixth mode removal unit 3N. In addition, the mode conversion in the waveguide type PD 2 is negligible, so that the unnecessary through light can be removed by adding the mode removal structure. In the waveguide structure symmetrical within the optional plane along the traveling direction of the light, the conversion between the even-order mode and the odd-order mode having the same polarization wave becomes very small, so it is preferable to use these related modes in such a waveguide structure in particular.

[0087] The optical detector 1B according to the third embodiment includes a fifth mode removal unit 3E and a sixth mode removal unit 3N. The sixth mode removal unit 3N allows the TE0 signal light from the branch unit 4 to pass through, and removes the TE1 signal light from the second port 2A2. The fifth mode removal unit 3E converts the TE0 signal light from the branch unit 4 into TE1 signal light, allows the converted TE1 signal light to pass through, and removes the TE0 signal light from the first port 2A1. As a result, the optical detector 1B can suppress the backflow of the residual light in the absorption layer 12.

[0088] The sixth filter 30N in the sixth mode removal unit 3N includes an adiabatic converter 30N1 that passes the TE0 signal light from the branch unit 4 and removes the TE1 signal light from the second port 2A2. The adiabatic converter 30E in the fifth mode removal unit 3E converts the TE0 signal light from the branch unit 4 into the TE1 signal light for output, and removes the TE0 signal light from the first port 2A1. As a result, the backflow of the residual light in the absorption layer 12 can be suppressed.

[0089] For ease of explanation, Figure 6 The structure shown is an example of the third mode removal unit 3C. Figure 5 The structure shown is an example of the fourth mode removal unit 3D. Fig.9A The structure shown is taken as an example of the fifth mode removal unit 3E, and is Fig. 9B The structure shown is taken as an example of the sixth pattern removing unit 3N. However, the structure is not limited thereto and may be changed appropriately.

[0090] Regarding the photodetector 1A according to the second embodiment, the case where the third mode removal unit 3C and the fourth mode removal unit 3D are provided is exemplified, but the embodiment is not limited thereto and can be appropriately changed. Therefore, the embodiment described below is as a photodetector 1C according to the fourth embodiment.

[0091] (d) Fourth Embodiment

[0092] Fig.10 1 is a block diagram illustrating an example of a photodetector 1C according to the fourth embodiment. The same configurations as those of the photodetector 1 in the first embodiment are denoted by the same reference numerals, and descriptions on the duplicate configurations and operations will not be repeated. Fig.10 The photodetector 1C illustrated in FIG. 1 includes a mode conversion unit 3F, a mode removal unit 3G, and a waveguide type PD 2. The mode conversion unit 3F includes an input port 3F1, an output port 3F2, and a conversion unit 30F. When TE0 signal light is input from the input port 3F1, the conversion unit 30F converts the TE0 signal light into a beam of TE0 and TE1 signal light. That is, when TE0 signal light is input from the input port 3F1, the conversion unit 30F converts the TE0 signal light into a signal light in which TE0 and TE1 are mixed. The mode removal unit 3G includes an input port 3G1, a first output port 3G2, a second output port 3G3, and a filter 30G.

[0093] The input port 3G1 is connected to the output port 3F2 of the mode conversion unit 3F. The first output port 3G2 is connected to the first input waveguide 11A of the waveguide type PD 2. The second output port 3G3 is connected to the second input waveguide 11B of the waveguide type PD 2. The filter 30G passes the TE0 signal light from the input port 3G1 and outputs it to the first output port 3G2, and removes the TE1 signal light flowing in the reverse direction from the first output port 3G2. The filter 30G passes the TE1 signal light from the input port 3G1 and outputs it to the second output port 3G3, and removes the TE0 signal light flowing in the reverse direction from the second output port 3G3.

[0094] The absorption layer 12 in the waveguide type PD 2 receives TE0 signal light input from the first input waveguide 11A and converts the TE0 signal light into electron-hole pairs, and receives TE1 signal light input from the second input waveguide 11B and converts the TE1 signal light into electron-hole pairs.

[0095] When a part of the TE0 signal light from the first input waveguide 11A remains, the absorption layer 12 outputs the remaining TE0 signal light to the second input waveguide 11B. The filter 30G in the mode removal unit 3G removes the TE0 signal light input from the second input waveguide 11B. When a part of the TE1 signal light from the second input waveguide 11B remains, the absorption layer 12 outputs the remaining TE1 signal light to the first input waveguide 11A. The filter 30G in the mode removal unit 3G removes the TE1 signal light input from the first input waveguide 11A.

[0096] Fig.11 is an explanatory diagram illustrating an example of the mode conversion unit 3F. Fig.11 The conversion unit 30F in the illustrated mode conversion unit 3F is configured by, for example, a Y-branch waveguide 30F1. In the case where TE0 signal light is input, the Y-branch waveguide 30F1 outputs beams of TE0 signal light and TE1 signal light.

[0097] Fig. 12A is an explanatory diagram illustrating an example of the operation of the pattern removal unit 3G at the time of input, and Fig. 12B is an explanatory diagram illustrating an example of the operation of the pattern removing unit 3G at the time of reflow. Fig. 12A The mode removal unit 3G shown in FIG. 1 is configured by, for example, a directional coupler 30G1. In the directional coupler 30G1, a first waveguide 31G having a linear shape and a second waveguide 32G having a linear shape are arranged in parallel with each other. In the case where a beam of TE0 signal light and TE1 signal light is input from the first waveguide 31G, the directional coupler 30G1 outputs the TE0 signal light from the first output port 3G2 of the first waveguide 31G to the first input waveguide 11A. In addition, the directional coupler 30G1 outputs the TE1 signal light from the second output port 3G3 of the second waveguide 32G to the second input waveguide 11B.

[0098] exist Fig. 12B In the directional coupler 30G1 shown, the TE1 signal light is input from the first output port 3G2 of the first waveguide 31G, and the TE0 signal light is input from the second output port 3G3 of the second waveguide 32G. In this case, the directional coupler 30G1 outputs the TE1 signal light and the TE0 signal light to the optical terminal connected to the input port of the second waveguide 32G. As a result, the TE1 signal light and the TE0 signal light flowing in the reverse direction from the waveguide type PD 2 can be removed.

[0099] The photodetector 1C according to the fourth embodiment includes a mode conversion unit 3F and a mode removal unit 3G. The mode conversion unit 3F converts the TE0 signal light into the TE0 signal light and the TE1 signal light to output to the mode removal unit 3G. The mode removal unit 3G passes the TE0 signal light from the mode conversion unit 3F and removes the TE1 signal light from the first port 2A1 of the waveguide type PD 2. The mode removal unit 3G passes the TE1 signal light from the mode conversion unit 3F and removes the TE0 signal light from the second port 2A2 of the waveguide type PD 2. As a result, the photodetector 1C can remove the TE1 signal light and the TE0 signal light that flow in the reverse direction from the absorption layer 12.

[0100] The mode conversion unit 3F includes a Y-branch waveguide 30F1 that outputs the TE0 signal light as the TE0 signal light and the TE1 signal light. The mode removal unit 3G includes a directional coupler 30G1 that allows the TE0 signal light from the Y-branch waveguide 30F1 to pass and removes the TE1 signal light from the first port 2A1. In addition, the directional coupler 30G1 allows the TE1 signal light from the Y-branch waveguide 30F1 to pass and removes the TE0 signal light from the second port 2A2. As a result, the photodetector 1C can remove the TE1 signal light and the TE0 signal light that flow in the reverse direction from the absorption layer 12.

[0101] Regarding the optical detector 1C according to the fourth embodiment, the following case is taken as an example: the light beams of TE0 signal light and TE1 signal light are input to the waveguide type PD 2, but the embodiment is not limited to this and can be appropriately changed. For example, the light beams of TE0 signal light and TE1 signal light can be input to the waveguide type PD 2, and its embodiment will be described below as the optical detector 1D according to the fifth embodiment.

[0102] (e) Fifth Embodiment

[0103] Fig.13 1 is a block diagram illustrating an example of a photodetector 1D according to a fifth embodiment. The same configuration as the photodetector 1C in the fourth embodiment is denoted by the same reference numerals, and the description of the repeated configuration and operation will not be repeated. The photodetector 1D according to the fifth embodiment includes a mode conversion unit 3F, a mode removal unit 3H, and a waveguide type PD 2. The mode conversion unit 3F includes an input port 3F1, an output port 3F2, and a conversion unit 30F. When TE0 signal light is input from the input port 3F1, the conversion unit 30F converts the TE0 signal light into a beam of TE0 signal light and TE1 signal light. That is, when TE0 signal light is input from the input port 3F1, the conversion unit 30F converts the TE0 signal light into a signal light mixed with TE0 and TE1. The mode removal unit 3H includes an input port 3H1, a first output port 3H2, a second output port 3H3, and a filter 30H.

[0104] The input port 3H1 is connected to the output port 3F2 of the mode conversion unit 3F. The first output port 3H2 is connected to the first input waveguide 11A of the waveguide type PD 2. The second output port 3H3 is connected to the second input waveguide 11B of the waveguide type PD 2. The filter 30H converts the TE1 signal light from the input port 3H1 into the TM0 signal light to output to the second output port 3H3, and removes the TE0 signal light flowing in the reverse direction from the second port 2A2. The filter 30H passes the TE0 signal light from the input port 3H1 and outputs it to the second output port 3H3, and removes the TM0 signal light flowing in the reverse direction from the first port 2A1.

[0105] Fig.14 is an explanatory diagram illustrating an example of light migration in a photodetector 1D according to the fifth embodiment. Fig.14 The conversion unit 30F shown in FIG. 3 is configured by, for example, a Y-branch waveguide 30F1. The Y-branch waveguide 30F1 converts the TE0 signal light input from the input port 3F1 into beams of TE0 signal light and TE1 signal light, and outputs the converted beams of TE0 signal light and TE1 signal light from the output port 3F2 to the mode removal unit 3H.

[0106] Fig.15 is an explanatory diagram illustrating an example of the pattern removing unit 3H. Fig.15 The mode removal unit 3H shown in FIG. 1 includes a conversion unit 3P and an adiabatic converter 3Q. The conversion unit 3P includes an input port 3P1, an output port 3P2, and a tapered waveguide 31P. The input port 3P1 is connected to the input stage of the tapered waveguide 31P and makes it an input port 3H1 of the mode removal unit 3H. The output port 3P2 is a port connected to the output stage of the tapered waveguide 31P and connected to the adiabatic converter 3Q.

[0107] The tapered waveguide 31P is a tapered waveguide in which the magnitude of the effective refractive index of the TE1 signal light and the TM0 signal light is reversed between the input section and the output section, and the cross section of the material refractive index becomes vertically asymmetric. In the tapered waveguide 31P, a portion whose height is lower than the core (slab portion) is provided to realize a vertically asymmetric shape, but the waveguide structure can be effectively changed by separately adding a structure that generates a slab portion having a tapered shape as the above-mentioned portion.

[0108] The tapered waveguide 31P converts the TE1 signal light input from the input port 3P1 into the TM0 signal light, and outputs the converted TM0 signal light from the output port 3P2. The tapered waveguide 31P allows the TE0 signal light input from the input port 3P1 to pass through, and outputs the transmitted TE0 signal light from the output port 3P2. The tapered waveguide 31P converts the TM0 signal light input from the output port 3P2 into the TE1 signal light, and outputs the converted TE1 signal light from the input port 3P1. The tapered waveguide 31P allows the TE0 signal light input from the output port 3P2 to pass through, and outputs the transmitted TE0 signal light from the input port 3P1.

[0109] The adiabatic converter 3Q is an adiabatic converter configured by arranging the first waveguide 31Q having a tapered shape and the second waveguide 32Q having a tapered shape in parallel to each other. In the parallel section L of the adiabatic converter 3Q where the first waveguide 31Q is parallel to the second waveguide 32Q, there are a start point and an end point. The width W1 of the first waveguide 31Q at the start point is equal to the width W1 of the second waveguide 32Q at the end point. The width W2 of the first waveguide 31Q at the end point is equal to the width W2 of the second waveguide 32Q at the start point. The interval G between the first waveguide 31Q and the second waveguide 32Q is the same at the start point and the end point.

[0110] When the TE0 signal light and the TM0 signal light are input from the first input port 3Q1 of the first waveguide 31Q, the adiabatic converter 3Q allows the TE0 signal light to pass through and output from the first output port 3Q2 of the first waveguide 31Q. The adiabatic converter 3Q allows the TM0 signal light to be gradually transferred from the first waveguide 31Q to the second waveguide 32Q, and outputs the TM0 signal light alone from the second output port 3Q3 of the second waveguide 32Q.

[0111] The adiabatic converter 3Q has reversibility, so that, in the case where the TM0 signal light is input from the first output port 3Q2 of the first waveguide 31Q, the adiabatic converter 3Q causes the TM0 signal light to gradually migrate from the first waveguide 31Q to the second waveguide 32Q, and outputs the TM0 signal light from the second input port 3Q4 of the second waveguide 32Q. That is, the adiabatic converter 3Q inputs the TM0 signal light flowing in the reverse direction from the waveguide type PD 2 through the first output port 3Q2 of the first waveguide 31Q. Then, the adiabatic converter 3Q causes the TM0 signal light to gradually migrate from the first waveguide 31Q to the second waveguide 32Q, and outputs the TM0 signal light from the second input port 3Q4 of the second waveguide 32Q. As a result, the TM0 signal light flowing in the reverse direction from the waveguide type PD 2 can be removed.

[0112] In the case where the TE0 signal light is input from the second output port 3Q3 of the second waveguide 32Q, the adiabatic converter 3Q passes the TE0 signal light and outputs it from the second input port 3Q4 of the second waveguide 32Q. That is, the adiabatic converter 3Q inputs the TE0 signal light flowing in the reverse direction from the waveguide type PD 2 through the second output port 3Q3 of the second waveguide 32Q. Then, the adiabatic converter 3Q outputs the TE0 signal light from the second input port 3Q4 of the second waveguide 32Q. As a result, the TE0 signal light flowing in the reverse direction from the waveguide type PD 2 can be removed.

[0113] The optical detector 1D according to the fifth embodiment includes a mode conversion unit 3F and a mode removal unit 3H. The mode conversion unit 3F converts the TE0 signal light into the TE0 signal light and the TE1 signal light to output to the mode removal unit 3H. The mode removal unit 3H passes the TE0 signal light from the mode conversion unit 3F and removes the TM0 signal light from the first port 2A1. The mode removal unit 3H converts the TE1 signal light from the mode conversion unit 3F into the TM0 signal light and removes the TE0 signal light from the second port 2A2. Therefore, the optical detector 1D can remove the TE1 signal light and the TE0 signal light flowing in the reverse direction from the waveguide type PD 2.

[0114] The mode conversion unit 3F includes a Y-branch waveguide 30F1 that converts the TE0 signal light into the TE0 signal light and the TE1 signal light to output to the mode removal unit 3H. The mode removal unit 3H includes a conversion unit 3P and an adiabatic converter 3Q. The tapered waveguide 31P in the conversion unit 3P passes the TE0 signal light from the Y-branch waveguide 30F1 and converts the TE1 signal light from the Y-branch waveguide 30F1 into the TM0 signal light. The adiabatic converter 3Q passes the TM0 signal light from the conversion unit 3P and removes the TE0 signal light from the second port 2A2, and passes the TE0 signal light from the conversion unit 3P and removes the TM0 signal light from the first port 2A1. As a result, the optical detector 1D can remove the TE1 signal light and the TE0 signal light that flow in the reverse direction from the waveguide type PD 2.

[0115] Taking the case where the waveguide type PD 2 includes two ports (i.e., the first port 2A1 and the second port 2A2) as an example, the number of ports may be three or more. In this case, a mode removal unit is provided for each port, and each mode removal unit passes signal light of each different mode to be output to the waveguide type PD 2, and removes signal light of a mode that flows in the reverse direction from the absorption layer 12 in the waveguide type PD 2.

[0116] For convenience of explanation, a dual input structure of a waveguide type PD 2 including a first port 2A1 connected to a first input waveguide 11A and a second port 2A2 connected to a second input waveguide 11B is taken as an example, but the embodiment is not limited thereto and may be appropriately changed. In addition, a waveguide type PD 2 in which the first port 2A1 and the second port 2A2 are set in a straight line shape is taken as an example, but the embodiment may be appropriately changed.

[0117] Fig.16A is an explanatory diagram illustrating an example of a dual-input waveguide type PD 2B. Fig.16A In the waveguide PD 2B shown, the second input port 2B2 may be set at a position 90 degrees relative to the first input port 2B1. Through the first input port 2B1, the signal light of the first mode is input, and the signal light of the second mode remaining in the absorption layer 12 is output. Through the second input port 2B2, the signal light of the second mode is input, and the signal light of the first mode remaining in the absorption layer 12 is output. The waveguide PD 2 in this embodiment can be replaced by a waveguide PD 2B.

[0118] Fig. 16B is an explanatory diagram illustrating an example of a three-input waveguide type PD 2C. Fig. 16B The waveguide PD 2C shown in FIG. 2 is a three-input waveguide PD including a first input port 2C1, a second input port 2C2, and a third input port 2C3 provided at a position 90 degrees relative to the first input port 2C1 or the second input port 2C2. The waveguide PD 2 according to the present embodiment may be replaced with the waveguide PD 2C.

[0119] Fig. 16C is an explanatory diagram illustrating an example of a four-input waveguide type PD 2D. Fig. 16C The waveguide PD 2D shown in the figure includes a first port 2D1 and a second port 2D2. In addition, the waveguide PD 2D includes a third port 2D3 provided at a position 90 degrees relative to the first port 2D1 or the second port 2D2, and a fourth port 2D4 arranged linearly relative to the third port 2D3. That is, the waveguide PD 2D is a four-input waveguide PD. The waveguide PD 2 according to the present embodiment can be replaced with the waveguide PD 2D. The optical detector 1E adopting the four-input waveguide PD 2D is described below as a sixth embodiment.

[0120] (f) Sixth Embodiment

[0121] Fig.17 is a block diagram illustrating an example of a photodetector 1E according to the sixth embodiment. Fig.17The optical detector 1E shown includes a first branch unit 4B, an eleventh mode removal unit 3J, a twelfth mode removal unit 3K, a second branch unit 4C, a thirteenth mode removal unit 3L, and a fourteenth mode removal unit 3M. The first branch unit 4B includes an input port 4B1, a first output port 4B2, and a second output port 4B3, and outputs the signal light of the first mode to the first output port 4B2 and the second output port 4B3 in a branching manner.

[0122] The eleventh mode removal unit 3J includes an eleventh input port 3J1, an eleventh filter 30J, and an eleventh output port 3J2. The eleventh filter 30J converts the signal light of the first mode input from the eleventh input port 3J1 into the signal light of the second mode, and removes the signal light of the first mode input from the eleventh output port 3J2. The twelfth mode removal unit 3K includes a twelfth input port 3K1, a twelfth filter 30K, and a twelfth output port 3K2. The twelfth filter 30K allows the signal light of the first mode input from the twelfth input port 3K1 to pass and output it, and removes the signal light of the second mode input from the twelfth output port 3K2.

[0123] The waveguide type PD 2D includes a first port 2D1 connected to the first input waveguide 11A and a second port 2D2 connected to the second input waveguide 11B. In addition, the waveguide type PD 2D includes a third port 2D3 connected to the third input waveguide 11C11, a fourth port 2D4 connected to the fourth input waveguide 11D11, and an absorption layer 12.

[0124] The absorption layer 12 converts the signal light of the second mode input from the first port 2D1 into electron-hole pairs, and converts the signal light of the first mode input from the second port 2D2 into electron-hole pairs. However, in the case where a part of the signal light of the second mode input from the first port 2D1 is not completely absorbed by the absorption layer 12 but remains therein, the remaining signal light of the second mode is output to the second input waveguide 11B through the second port 2D2. Therefore, the twelfth filter 30K in the twelfth mode removal unit 3K removes the signal light of the second mode from the twelfth output port 3K2.

[0125] In addition, in the case where a portion of the signal light of the first mode input from the second port 2D2 is not completely absorbed by the absorption layer 12 but remains therein, the remaining signal light of the first mode is output to the first input waveguide 11A through the first port 2D1. Therefore, the eleventh filter 30J in the eleventh mode removal unit 3J removes the signal light of the first mode from the eleventh output port 3J2.

[0126] The second branching unit 4C includes an input port 4C1 , a first output port 4C2 , and a second output port 4C3 , and outputs the signal light of the third mode to the first output port 4C2 and the second output port 4C3 in a branching manner.

[0127] The thirteenth mode removal unit 3L includes a thirteenth input port 3L1, a thirteenth filter 30L and a thirteenth output port 3L2. The thirteenth filter 30L converts the signal light of the third mode input from the thirteenth input port 3L1 into the signal light of the fourth mode, and removes the signal light of the third mode from the thirteenth output port 3L2. The fourteenth mode removal unit 3M includes a fourteenth input port 3M1, a fourteenth filter 30M and a fourteenth output port 3M2. The fourteenth filter 30M allows the signal light of the third mode input from the fourteenth input port 3M1 to pass and output it, and removes the signal light of the fourth mode from the fourteenth output port 3M2. The absorption layer 12 converts the signal light of the fourth mode input from the third port 2D3 into electron-hole pairs, and converts the signal light of the third mode input from the fourth port 2D4 into electron-hole pairs.

[0128] When a portion of the fourth mode signal light input from the third port 2D3 is not completely absorbed by the absorption layer 12 but remains therein, the remaining fourth mode signal light is output to the fourth input waveguide 11D11 through the fourth port 2D4. The fourteenth filter 30M in the fourteenth mode removal unit 3M removes the fourth mode signal light from the fourteenth output port 3M2.

[0129] In addition, in the case where a portion of the signal light of the third mode input from the fourth port 2D4 is not completely absorbed by the absorption layer 12 but remains therein, the remaining signal light of the third mode is output to the third input waveguide 11C11 through the third port 2D3. The thirteenth filter 30L in the thirteenth mode removal unit 3L removes the signal light of the third mode from the thirteenth output port 3L2. As a result, the photodetector 1E can suppress the passing light caused by the residual light backflow in the absorption layer 12.

[0130] The optical detector 1E according to the sixth embodiment includes an eleventh mode removal unit 3J, a twelfth mode removal unit 3K, a thirteenth mode removal unit 3L, and a fourteenth mode removal unit 3M. The eleventh mode removal unit 3J transfers the signal light of the second mode to the first port 2D1, and removes the signal light of the first mode from the first port 2D1. The twelfth mode removal unit 3K transfers the signal light of the first mode to the second port 2D2, and removes the signal light of the second mode from the second port 2D2. The thirteenth mode removal unit 3L transfers the signal light of the fourth mode to the third port 2D3, and removes the signal light of the third mode from the third port 2D3. The fourteenth mode removal unit 3M transfers the signal light of the third mode to the fourth port 2D4, and removes the signal light of the fourth mode from the fourth port 2D4. As a result, the optical detector 1E is able to suppress the signal light intensity per unit cross section input to the four-input waveguide type PD 2D, suppress the degradation of the response speed of the waveguide type PD 2D, and suppress the noise caused by the backflow of the residual light not absorbed by the waveguide type PD 2D.

[0131] As described above, specific configurations are described in the embodiments, but the method for converting one mode into a different mode and spatially separating the different modes from each other is not limited to a specific method, and the embodiments may be appropriately changed.

[0132] The cross-sectional structure of the waveguide type PD 2 is Figure 3 The structure shown is an example, but is not limited thereto and can be changed appropriately. 18A to 18E 2 is an explanatory diagram illustrating an example of the waveguide type PD 2 .

[0133] Fig.18A The waveguide type PD 2E shown includes a Si substrate 13, a BOX layer 14 laminated on the Si substrate 13, a waveguide layer 11 laminated on the BOX layer 14, and a buffer layer (not shown) laminated on the waveguide layer 11. The waveguide layer 11 is a waveguide layer formed of, for example, Si or Si3N4. The buffer layer is, for example, a SiO2 layer. The waveguide layer 11 includes a first input waveguide 11A1 and a second input waveguide 11B1. An absorption layer 12E is provided on the BOX layer 14. The absorption layer 12 is an absorption layer formed of, for example, germanium (Ge). The first input waveguide 11A1 and the second input waveguide 11B1 are provided on the BOX layer 14. Even if the waveguide type PD 2 according to the present embodiment is replaced with a waveguide type PD 2E, the same effect can be obtained.

[0134] Fig.18BThe waveguide type PD 2F shown includes a Si substrate 13, a BOX layer 14, a waveguide layer 11 laminated on the BOX layer 14, and a buffer layer (not shown) laminated on the waveguide layer 11. The waveguide layer 11 includes a first input waveguide 11A2, a second input waveguide 11B2, and a waveguide 11C21 connecting the first input waveguide 11A2 and the second input waveguide 11B2. The absorption layer 12 is provided on the waveguide 11C21. The first input waveguide 11A2 and the second input waveguide 11B2 are provided on the BOX layer 14. Even if the waveguide type PD 2 according to the present embodiment is replaced with a waveguide type PD 2F, the same effect can be obtained.

[0135] Fig.18C The waveguide type PD 2G shown includes a Si substrate 13, a BOX layer 14, a waveguide layer 11 laminated on the BOX layer 14, and a buffer layer (not shown) laminated on the waveguide layer 11. An absorption layer 12G is provided on the waveguide layer 11. In addition, a waveguide 11G composed of a first input waveguide 11A3 and a second input waveguide 11B3 is provided on the absorption layer 12G. Even if the waveguide type PD 2 according to the present embodiment is replaced with a waveguide type PD 2G, the same effect can be obtained.

[0136] Fig.18D The waveguide type PD 2H shown includes a Si substrate 13, a BOX layer 14, a waveguide layer 11 laminated on the BOX layer 14, and a buffer layer (not shown) laminated on the waveguide layer 11. An absorption layer 12H is provided on the waveguide layer 11. In addition, on the BOX layer 14, a waveguide 11H composed of a first input waveguide 11A4 and a second input waveguide 11B4 is provided in parallel with the absorption layer 12H at a predetermined interval. Even if the waveguide type PD 2 according to the present embodiment is replaced with a waveguide type PD 2H, the same effect can be obtained.

[0137] Fig.18E The waveguide type PD 2J shown includes a Si substrate 13, a BOX layer 14, a waveguide layer 11 laminated on the BOX layer 14, and a buffer layer (not shown) laminated on the waveguide layer 11. An absorption layer 12J is provided on the waveguide layer 11. Furthermore, on the waveguide layer 11, a first input waveguide 11A5 and a second input waveguide 11B5 are provided in parallel with the absorption layer 12J in a manner of sandwiching the absorption layer 12J. Even if the waveguide type PD 2 according to the present embodiment is replaced with a waveguide type PD 2J, the same effect can be obtained.

[0138] After spatially separating the residual light targeted for removal, the mode filter 3 outputs the residual light from a port different from the input port. However, if reflection occurs thereafter, the light may return to the waveguide PD 2 again and impair stable operation. Therefore, by providing an optical terminal at a port different from the input unit, light reflection that causes such an adverse effect can be reduced.

[0139] The optical terminal may have a structure in which the waveguide is doped to absorb light, a structure in which the waveguide has a tapered shape along the direction of light travel to radiate light to the outside of the waveguide, a structure in which Ge is accumulated on the waveguide to absorb light, etc., and the implementation method may be appropriately changed.

[0140] The waveguide may be, for example, a rib waveguide, a ridge waveguide, a rectangular waveguide, or a high-table waveguide. A rib waveguide is preferred because light seeps out to the slab portion, making it almost unaffected by the roughness of the side wall of the core, and low-loss propagation can be achieved. A rectangular waveguide is preferred because even if the bending radius R is reduced, the loss is reduced due to the strong light confinement. A low-loss curved waveguide may be used as the waveguide, and the embodiment may be appropriately changed.

[0141] The waveguide may be an InP waveguide, a GaAs waveguide, or a PLC in which both the core and the cladding are SiO2. The waveguide may be a Si waveguide in which the lower cladding is SiO2 and the upper cladding is SiO2 or air, SiN, etc. The case where the waveguide is a Si waveguide is preferred because the light confinement is strong due to the large relative refractive index difference, so that a low-loss curved waveguide with a small bending radius R can be realized, that is, the size of the optical device can be reduced. The absorption layer 12 may be, for example, SiGe, or may be Ge.

[0142] Next, an optical transceiver 50 employing the photodetector 1 according to the present embodiment is described below. Fig.19 is an explanatory diagram illustrating an example of an optical transceiver 50 employing the photodetector 1 . Fig.19 The optical transceiver 50 shown includes an optical transmitter-receiver 51 and a digital signal processor (DSP) 52. The optical transmitter-receiver 51 includes an optical modulator element 54, a driver circuit 55, an optical receiver element 56, and a transimpedance amplifier (TIA) 57. The DSP 52 controls the entire optical transmitter-receiver 51. The DSP 52 is an electronic component that performs digital signal processing to perform IQ modulation processing on a transmission signal and demodulation processing on a reception signal.

[0143] For example, the DSP 52 performs processing such as encoding on the transmission data, generates an electrical signal including the transmission data, and outputs the generated electrical signal to the driver circuit 55. The driver circuit 55 drives the optical modulator element 54 in response to the electrical signal from the DSP 52. The optical modulator element 54 optically modulates the signal light.

[0144] The optical receiver element 56 has a built-in optical detector 1 including a waveguide type PD 2 that electrically converts signal light. The TIA 57 amplifies the electrically converted electrical signal and outputs the amplified electrical signal to the DSP 52. The DSP 52 performs processing such as decoding on the electrical signal acquired from the TIA 57 to obtain reception data.

[0145] For convenience of explanation, a case where the optical transceiver 50 has a communication element 53 including an optical modulator element 54 and an optical receiver element 56 built therein is taken as an example, but the optical transceiver 50 may be an optical receiver having only the optical receiver element 56 built therein. The embodiment may be changed appropriately.

[0146] The constituent elements of the units shown in the figure are not necessarily physically configured as shown in the figure. That is, the specific forms of distribution and integration of the units are not limited to those shown in the figure. All or part of them can be functionally or physically distributed / integrated into any unit according to various loads, usage status, etc.

[0147] In addition, all or optional parts of various processing functions performed by each device may be performed on a central processing unit (CPU) (or a microcomputer such as a microprocessing unit (MPU) or a microcontroller unit (MCU)). Obviously, all or optional parts of various processing functions may be performed on a computer program analyzed and executed by the CPU (or a microcomputer such as an MPU or an MCU), or may be performed on hardware using wired logic.

[0148] According to one aspect, backflow of residual light in the photodetector can be suppressed.

Claims

1. A light detector, comprising: a photodetector that converts signal light into an electrical signal and includes two or more ports; as well as A mode filter comprising two or more ports, wherein: The photodetector comprises: a first port to which signal light of a first mode is input; and a second port to which the signal light of the second mode is input, The mode filter comprises: a third port connected to the first port; and a fourth port connected to the second port, and The mode filter: allowing the signal light of the first mode toward the first port to pass through to be output from the third port, allowing the signal light of the second mode toward the second port to pass through to be output from the fourth port, removing the signal light of the second mode input from the third port, the signal light of the second mode being output from the first port, and The signal light of the first mode input from the fourth port is removed, and the signal light of the first mode is output from the second port.

2. The optical detector according to claim 1, wherein The mode filter: allowing the signal light of the first mode toward the first port to pass through to be output from the third port, allowing the signal light of the second mode toward the second port to pass through to be output from the fourth port, removing signal light of the second mode from signal light including the first mode to be input from the third port, the signal light including the second mode being output from the first port, and The signal light including the signal light of the first mode is removed from the signal light including the second mode input from the fourth port, and the signal light including the signal light of the first mode is output from the second port.

3. The light detector according to claim 1, wherein The mode filter comprises: a first mode remover connected to the third port, allowing the signal light of the first mode to pass therethrough and removing the signal light of the second mode from the first port; and A second mode remover is connected to the fourth port, passes the signal light of the second mode, and removes the signal light of the first mode from the second port.

4. The light detector according to claim 3, wherein The mode filter: comprising a branching coupler for branching the signal light of the first mode, The first mode remover: passing the signal light of the first mode from the branch coupler and removing the signal light of the second mode from the second port, and The second mode remover: The signal light in the first mode from the branch coupler is converted into the signal light in the second mode, the converted signal light in the second mode is passed, and the signal light in the first mode from the first port is removed.

5. The light detector according to claim 4, wherein The first mode remover: comprising a first adiabatic converter that passes the signal light of the first mode from the branch coupler and removes the signal light of the second mode from the second port, and The second mode remover comprises a second adiabatic converter, the second adiabatic converter converting the signal light of the first mode from the branch coupler into the signal light of the second mode; as well as A tapered waveguide passes the signal light in the second mode converted by the second adiabatic converter and removes the signal light in the first mode from the first port.

6. The light detector according to claim 1, wherein The mode filter comprises: a mode converter that converts the signal light of the first mode into the signal light of the first mode and the signal light of the second mode for output; and a mode remover comprising a third port connected to the first port and a fourth port connected to the second port, and The pattern remover: allowing the signal light of the first mode toward the first port to pass through to be output from the third port, allowing the signal light of the second mode toward the second port to pass through to be output from the fourth port, removing the signal light of the second mode from the first port input from the third port, and The signal light of the first mode from the second port input from the fourth port is removed.

7. The light detector according to claim 6, wherein The mode converter: including a Y-branch waveguide that converts the signal light of the first mode into the signal light of the first mode and the signal light of the second mode for output, and The pattern remover: A directional coupler is included, which allows the signal light of the first mode from the Y-branch waveguide to pass and removes the signal light of the second mode from the first port, and allows the signal light of the second mode from the Y-branch waveguide to pass and removes the signal light of the first mode from the second port.

8. The light detector according to claim 1, wherein The mode filter comprises: a mode converter that converts the signal light of the first mode into the signal light of the first mode and the signal light of the second mode for output; and a mode remover comprising a fifth port connected to the first port and a sixth port connected to the second port, and The pattern remover: passing the signal light of the first mode toward the first port to output from the fifth port, passing the signal light of the second mode toward the second port to output from the sixth port, removing the signal light of the second mode input from the first port from the fifth port, and The signal light of the first mode from the second port input from the sixth port is removed.

9. The light detector according to claim 8, wherein The mode converter: including a Y-branch waveguide that converts the signal light of the first mode into the signal light of the first mode and the signal light of the second mode for output, and The pattern remover comprises: a tapered waveguide that passes the signal light of the first mode from the Y-branch waveguide and converts the signal light of the second mode from the Y-branch waveguide into signal light of a third mode; and an adiabatic converter that passes the third mode signal light from the tapered waveguide and removes the first mode signal light from the second port, and passes the first mode signal light from the tapered waveguide and removes the third mode signal light from the first port.

10. The light detector according to claim 1, wherein The photodetector comprises: a fifth port to which the signal light of the third mode is input; and a sixth port to which the signal light of the fourth mode is input, The mode filter comprises: a seventh port connected to the fifth port; and an eighth port connected to the sixth port, and The mode filter: passing the signal light of the third mode toward the fifth port to be output from the seventh port, passing the signal light of the fourth mode toward the sixth port to be output from the eighth port, removing the signal light of the fourth mode input from the fifth port from the seventh port, and The signal light of the third mode from the sixth port input from the eighth port is removed.

11. An optical receiver, comprising an optical receiver element for receiving an electrical signal from a signal light using light, wherein: The optical receiver element comprises: A light detector, the light detector comprising: a photodetector that converts the signal light into an electrical signal and includes two or more ports; and A mode filter comprising two or more ports, wherein: The photodetector comprises: a first port to which signal light of a first mode is input; and a second port to which the signal light of the second mode is input, The mode filter comprises: a third port connected to the first port; and a fourth port connected to the second port, and The mode filter: passing the signal light of the first mode toward the first port to be output from the third port, passing the signal light of the second mode toward the second port to output from the fourth port, removing the signal light of the second mode input from the third port, the signal light of the second mode being output from the first port, and The signal light of the first mode input from the fourth port is removed, and the signal light of the first mode is output from the second port.

12. An optical transceiver, comprising: an optical modulator element that transmits transmission light by optically modulating the light using a transmission signal; as well as an optical receiver element for receiving a reception signal from received light using light, wherein The optical receiver element comprises: A light detector, the light detector comprising: a photodetector that converts the received light into a received signal and includes two or more ports; and a mode filter, the mode filter comprising two or more ports, The photodetector comprises: a first port to which signal light of a first mode is input; and a second port to which the signal light of the second mode is input, The mode filter comprises: a third port connected to the first port; and a fourth port connected to the second port, and The mode filter: passing the signal light of the first mode toward the first port to be output from the third port, passing the signal light of the second mode toward the second port to output from the fourth port, removing the signal light of the second mode input from the third port, the signal light of the second mode being output from the first port, and The signal light of the first mode input from the fourth port is removed, and the signal light of the first mode is output from the second port.

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