Optical detector, optical receiver and optical transceiver

Abstract

To provide an optical detector and the like that can suppress backflow by residual light of a photodetector.SOLUTION: An optical detector has: a photodetector that has two or more ports converting signal light to an electric signal; and a mode filter that has two more ports. The photodetector has: a first port to which the signal light of a first mode is input; a second port to which the signal light of a second mode is input. The mode filter has: a third port that is connected to the first port; and a fourth port that is connected to the second port. The mode filter is configured to: transmit the signal light of the first mode for the first port, and output the signal light thereof from the third port; transmit the signal light of the second mode for the second port, and output the signal light thereof from the fourth port; remove the signal light of the second mode from the first port to be input from the third port; and remove the signal light of the first mode from the second port to be input from the fourth port.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an optical detector, an optical receiver, and an optical transceiver. [Background technology]

[0002] In recent years, coherent optical communication technology has been used to achieve high-speed, large-capacity communication in optical fiber networks. In an optical receiver using coherent optical communication technology, the received light, whose optical power has decreased due to long-distance transmission, is mixed with local light generated inside the optical receiver, and the mixed received light is then converted into electric current in a photo detector (PD). As a result, the optical receiver enables high-sensitivity reception by reducing the effects of thermal noise generated after the PD.

[0003] However, although the receiving sensitivity of an optical receiver increases as the optical power of the local light increases, when a large optical power is input to the PD, the space-charge effect limits the response speed of the PD, making high-speed communication difficult.

[0004] In the space charge effect, when light input into a PD generates electron-hole pairs, the electric field generated in the PD by these carriers acts in a direction that cancels out the bias applied to the PD, reducing the applied bias and weakening the force that carries carriers inside the PD out of the PD, which results in a decrease in the response speed of the PD.

[0005] In addition, the space charge effect becomes more pronounced when a large number of electron-hole pairs are generated in the PD, in other words, when the input power is large. Therefore, the space charge effect can be mitigated by reducing the input power.

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

[0007] In a PD, light is converted into electron-hole pairs as it propagates through it, so the optical power decreases the further away from the optical input section of the PD. Therefore, if the input optical power is divided into two and then input from two directions into the PD, the optical intensity per cross section can be reduced to about half, reducing the impact of the space charge effect. As a result, even if the optical power input to the PD is high, degradation of the PD's response speed can be suppressed.

[0008] The PD may be, for example, a waveguide type PD. The waveguide type PD has a PD and an optical waveguide that guides light, which are integrated on the same substrate, and performs light-to-current conversion in the PD by guiding light from the waveguide to the PD.

[0009] A waveguide-type PD is composed of an SOI (Silicon-On-Insulator) layer on an SOI substrate, and Ge epitaxially grown on top of it. In the communication wavelength band, for example the C-band, the band gap of Ge is narrower than that of Si, so light propagating through Ge is converted into electron-hole pairs in the Ge, and photoelectric conversion can be achieved by extracting these pairs using doped Si or electrodes. The layer that converts light, such as Ge, into electron-hole pairs is called the absorption layer. The absorption layer has a band gap different from that of the optical waveguide, and is the part that absorbs the input light and converts it into electron-hole pairs.

[0010] As described above, the waveguide-type PD has an optical waveguide and a PD. The optical waveguide has a Si substrate, a BOX (Buried Oxide) layer stacked 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 lithographically etching the Si of the SOI layer of the SOI substrate formed of the Si substrate and the BOX layer. The PD has 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 and absorbs the signal light from the optical waveguide to convert the signal light into electron-hole pairs. [Prior art documents] [Patent documents]

[0011]

Patent Document 1

Non-licensed literature

[0012] [Non-licensed document 1] A. Beling, X. Xie and JC Campbell, “High-power high-linearity photodiodes”, Optica, vol. 3, no. 3, pp. 328-338, 2016.

Non-licensed Document 2

Non-licensed Document 4

Non-licensed Document 5

[0013] In a waveguide PD, when light is input from two directions, the light that is not converted into electron-hole pairs in the absorption layer of the PD and remains is output from a port different from the input. As a result, the residual light output from the different port flows back and becomes reflected light. If the reflected light is further reflected by an external optical device, Fabry-Perot resonance occurs, and the photocurrent fluctuates due to changes in wavelength and temperature, reducing the stability of the optical receiver.

[0014] The main cause of this backflow due to residual light in the absorption layer of a PD is light that cannot be absorbed in the Ge absorption layer and passes through the Ge absorption layer as residual light.

[0015] Therefore, in a waveguide PD, it is possible to reduce the residual light by increasing the length of the Ge absorption layer so that all light can be absorbed. However, considering that a PD operates as a capacitor in an electric circuit, as the length of the absorption layer increases, the capacitance increases, and the RC time constant increases. As a result, the response speed is limited.

[0016] One object of the present invention is to provide a photodetector etc. capable of suppressing backflow due to residual light in a PD. [Means for solving the problem]

[0017] A photodetector according to one embodiment includes a photodetector having two or more ports for converting a signal light into an electrical signal, and a mode filter having two or more ports. The photodetector has a first port to which a signal light of a first mode is input, and a second port to which a signal light of a second mode is input. The mode filter has a third port connected to the first port, and a fourth port connected to the second port. The mode filter transmits the signal light of the first mode to the first port and outputs it from the third port, and transmits the signal light of the second mode to the second port and outputs it from the fourth port. The mode filter removes the signal light of the second mode from the first port that is input from the third port, and removes the signal light of the first mode from the second port that is input from the fourth port. Effect of the Invention

[0018] According to one aspect, backflow due to residual light in a photodetector can be suppressed. [Brief description of the drawings]

[0019] [Figure 1] FIG. 1 is a block diagram illustrating an example of a photodetector according to a first embodiment. [Figure 2A] FIG. 2A is a perspective view showing an example of a waveguide-type PD. [Figure 2B] FIG. 2B is a schematic plan view showing an example of a waveguide-type PD with a portion omitted. [Diagram 3] FIG. 3 is a schematic cross-sectional view taken along line AA shown in FIG. 2B. [Figure 4] FIG. 4 is a block diagram illustrating an example of a photodetector according to the second embodiment. [Diagram 5] FIG. 5 is an explanatory diagram illustrating an example of the fourth mode removal unit. [Figure 6]FIG. 6 is an explanatory diagram illustrating an example of the third mode removal unit. [Figure 7] FIG. 7 is a schematic cross-sectional view taken along line BB shown in FIG. [Figure 8] FIG. 8 is a block diagram illustrating an example of a photodetector according to the third embodiment. [Figure 9A] FIG. 9A is an explanatory diagram illustrating an example of the fifth mode removal unit. [Figure 9B] FIG. 9B is an explanatory diagram illustrating an example of the sixth mode removal section. [Figure 10] FIG. 10 is a block diagram illustrating an example of a photodetector according to the fourth embodiment. [Figure 11] FIG. 11 is an explanatory diagram illustrating an example of a mode conversion unit. [Figure 12A] FIG. 12A is an explanatory diagram showing an example of the operation of the mode removal unit at the time of input. [Figure 12B] FIG. 12B is an explanatory diagram showing an example of the operation of the mode removal unit when a reverse flow occurs. [Figure 13] FIG. 13 is a block diagram illustrating an example of a photodetector according to the fifth embodiment. [Figure 14] FIG. 14 is an explanatory diagram showing an example of the transition of light in the photodetector of the fifth embodiment. [Figure 15] FIG. 15 is an explanatory diagram illustrating an example of the mode removal unit. [Figure 16A] FIG. 16A is an explanatory diagram showing an example of a two-input waveguide type PD. [Figure 16B] FIG. 16B is an explanatory diagram showing an example of a three-input waveguide PD. [Figure 16C] FIG. 16C is an explanatory diagram showing an example of a four-input waveguide PD. [Figure 17] FIG. 17 is a block diagram illustrating an example of a photodetector according to the sixth embodiment. [Figure 18A] FIG. 18A is an explanatory diagram showing an example of a waveguide type PD. [Figure 18B] FIG. 18B is an explanatory diagram showing an example of a waveguide type PD. [Figure 18C] FIG. 18C is an explanatory diagram showing an example of a waveguide type PD. [Figure 18D] FIG. 18D is an explanatory diagram showing an example of a waveguide type PD. [Figure 18E] FIG. 18E is an explanatory diagram showing an example of a waveguide type PD. [Figure 19] FIG. 19 is an explanatory diagram showing an example of an optical transceiver that employs a photodetector. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0020] Hereinafter, examples of the photodetector and the like disclosed in the present application will be described in detail with reference to the drawings. Note that the disclosed technology is not limited to these examples. In addition, the following examples may be appropriately combined as long as no contradiction occurs. EXAMPLES

[0021] FIG. 1 is a block diagram showing an example of a photodetector 1 of Example 1, FIG. 2A is a perspective view showing an example of a waveguide-type PD2, and FIG. 2B is a schematic plan view showing an example of a waveguide-type PD2 with a portion omitted. The photodetector 1 shown in FIG. 1 has a waveguide-type PD2 and a mode filter 3. The mode filter 3 has a first mode removal section 3A and a second mode removal section 3B. The waveguide-type PD2 has 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.

[0022] The first mode elimination unit 3A has 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 transmits a first mode signal light from the first input port 3A1 and eliminates a second mode signal light from the first output port 3A2.

[0023] The second mode elimination unit 3B has 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 transmits the signal light of the second mode from the second input port 3B1 and eliminates the signal light of the first mode from the second output port 3B2. The signal light of the first mode is a signal light of a second mode different from the first mode.

[0024] The absorption layer 12 converts the signal light of the first mode input from the first port 2A1 into electron-hole pairs, and also converts the signal light of the second mode input from the second port 2A2 into electron-hole pairs.

[0025] For convenience of explanation, the first filter 30A is exemplified as removing the signal light of the second mode, but may be modified as appropriate to remove signal light other than the signal light of the first mode. Also, the second filter 30B is exemplified as removing the signal light of the first mode, but may be modified as appropriate to remove signal light other than the signal light of the second mode.

[0026] Fig. 3 is a schematic cross-sectional view of the AA line shown in Fig. 2B. The waveguide-type PD 2 shown in Figs. 2A, 2B, and 3 has a Si substrate 13, a BOX (Buried Oxide) 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 made of Si or the like. The BOX layer 14 is, for example, a layer of SiO2. The waveguide layer 11 is, for example, a waveguide layer made of Si. The buffer layer 15 is, for example, a layer of SiO2. The absorption layer 12 is, for example, an absorption layer made of Ge (Germanium).

[0027] The waveguide layer 11 has 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 has a p-doped region 11C1, an n-doped region 11C2, and an undoped region 11C3 disposed between the p-doped region 11C1 and the n-doped region 11C2. The absorption layer 12 is disposed on the undoped region 11C3, a part of the p-doped region 11C1 connected to the undoped region 11C3, and a part 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. Note that the dimensions exemplified here are merely set for calculation by the finite-difference time-domain method described later, and can be changed as appropriate.

[0028] However, in the absorption layer 12, if a part of the signal light of the first mode inputted from the first port 2A1 is not absorbed and remains, the remaining signal light of the first mode will be outputted from the second port 2A2 to the second input waveguide 11B. Therefore, the second filter 30B in the second mode removal unit 3B will remove the signal light of the first mode inputted from the second output port 3B2. Also, in the absorption layer 12, if a part of the signal light of the second mode inputted from the second port 2A2 is not absorbed and remains, the remaining signal light of the second mode will be outputted from the first port 2A1 to the first input waveguide 11A. Therefore, the first filter 30A in the first mode removal unit 3A will remove the signal light of the second mode inputted from the first output port 3A2. As a result, the photodetector 1 can suppress backflow due to the remaining light in the absorption layer 12.

[0029] The photodetector 1 of the first embodiment has a waveguide-type PD2 and a mode filter 3. The waveguide-type PD2 has a first port 2A1 to which a signal light of a first mode is input, and a second port 2A2 to which a signal light of a second mode is input. The mode filter 3 has a first mode elimination unit 3A and a second mode elimination unit 3B. The first mode elimination unit 3A transmits the signal light of the first mode to the first port 2A1, and eliminates the signal light of the second mode from the first port 2A1. The second mode elimination unit 3B transmits the signal light of the second mode to the second port 2A2, and eliminates the signal light of the first mode from the second port 2A2. As a result, the photodetector 1 can suppress backflow due to residual light in the absorption layer 12. In other words, the signal light intensity per cross section input to the waveguide-type PD2 is reduced, which prevents deterioration in the response speed of the waveguide-type PD2 and suppresses noise caused by the backflow of residual light that cannot be absorbed by the waveguide-type PD2. EXAMPLES

[0030] FIG. 4 is a block diagram showing an example of a photodetector 1A according to a second embodiment. For convenience of explanation, the same components as those of the photodetector 1 according to the first embodiment are denoted by the same reference numerals, and the explanation of the overlapping components and operations will be omitted. The photodetector 1A according to the second embodiment shown in FIG. 4 includes a branching unit 4, a third mode removal unit 3C, a fourth mode removal unit 3D, and a waveguide-type PD2. The branching unit 4 includes an input port 4A1, an output port 4A2, and an output port 4A3, and outputs a first mode signal light from the input port 4A1 to the output port 4A2 and the output port 4A3. Consider the case where the first mode signal light is TE0 and the second mode signal light is TM0. Here, TE0 and TM0 refer to the waveguide modes (TE mode, TM mode) in which the main component of the electric field is horizontal or vertical to the substrate, respectively, in which the effective refractive index is maximum.

[0031] The third mode removal unit 3C has 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 a second mode signal light (TM0 signal light), and removes the TE0 signal light input from the third output port 3C2.

[0032] The fourth mode elimination unit 3D has a fourth input port 3D1, a fourth output port 3D2, and a fourth filter 30D. The fourth filter 30D transmits and outputs the TE0 signal light input from the fourth input port 3D1, and eliminates the TM0 signal light input from the fourth output port 3D2.

[0033] The absorption layer 12 converts the TM0 signal light input from the first port 2A1 into electron-hole pairs, and also converts the TE0 signal light input from the second port 2A2 into electron-hole pairs. However, in the absorption layer 12, if a portion of the TM0 signal light input from the first port 2A1 cannot be absorbed and remains, the remaining TM0 signal light is output from the second port 2A2. Therefore, the fourth filter 30D in the fourth mode elimination unit 3D eliminates the TM0 signal light input from the fourth output port 3D2.

[0034] Furthermore, in the case where a portion of the TE0 signal light input from the second port 2A2 cannot be absorbed and remains in the absorption layer 12, the remaining TE0 signal light is output from the first port 2A1. Therefore, the third filter 30C in the third mode elimination unit 3C eliminates the TE0 signal light input from the third output port 3C2. As a result, the photodetector 1 can suppress backflow due to the residual light in the absorption layer 12.

[0035] That is, the fourth mode elimination unit 3D can suppress backflow of the TM0 signal light to the branching unit 4 by removing the TM0 signal light remaining in the absorption layer 12. Furthermore, the third mode elimination unit 3C can suppress backflow of the TE0 signal light to the branching unit 4 by removing the TE0 signal light remaining in the absorption layer 12.

[0036] FIG. 5 is an explanatory diagram showing an example of the fourth mode elimination unit 3D. The fourth filter 30D in the fourth mode elimination unit 3D is an adiabatic converter 30D1 configured by arranging a tapered first waveguide 31D and a tapered second waveguide 32D in parallel. The parallel section L of the adiabatic converter 30D1 in which the first waveguide 31D and the second waveguide 32D are arranged in parallel has 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 at the start point and the end point is the same.

[0037] When the signal light of TE0 and TM0 is inputted from the fourth output port 3D2 of the first waveguide 31D, the adiabatic converter 30D1 transmits and outputs the signal light of TE0 from the fourth input port 3D1 of the first waveguide 31D. Also, the adiabatic converter 30D1 gradually transfers the signal light of TM0 from the first waveguide 31D to the second waveguide 32D, and separates and outputs the signal light of TM0 from the second waveguide 32D. Since the fourth mode removal unit 3D has reversibility, when the signal light of TE0 is inputted from the fourth input port 3D1 of the first waveguide 31D, the signal light of TE0 is transmitted and outputted from the fourth output port 3D2 of the first waveguide 31D.

[0038] FIG. 6 is an explanatory diagram showing an example of the third mode elimination unit 3C, and FIG. 7 is a schematic cross-sectional view of the line BB shown in FIG. 6. The third filter 30C in the third mode elimination unit 3C shown in FIG. 6 and FIG. 7 has an adiabatic converter 30C1 and a tapered waveguide 33C. The third mode elimination unit 3C has 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 signal light of TE0 and the signal light of TE1. The tapered waveguide 33C is a waveguide that adiabatically converts the signal light of TE1 to the signal light of TM0.

[0039] 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 spatially separates the input TM0 signal light and the TE0 signal light. The adiabatic converter 30C1 arranges the first waveguide 31C and the second waveguide 32C in parallel, and the third input port 3C1 is the input port of the first waveguide 31C, and the input port of the second waveguide 32C is the discard port 3C3. When the adiabatic converter 30C1 inputs the TE0 signal light to the third input port 3C1, the TE0 signal light transitions from the first waveguide 31C to the second waveguide 32C, thereby converting the TE0 signal light to the TE1 signal light. Furthermore, the tapered waveguide 33C converts the TE1 signal light to the TM0 signal light and outputs it from the third output port 3C2. In addition, since the third mode elimination unit 3C has reversibility, when the TE0' signal light is input from the third output port 3C2 of the tapered waveguide 33C, the TE0' signal light is removed by transmitting and outputting the TE0' signal light to the discard port 3C3. Note that the TE0' signal light is the TE0 signal light input from the third output port 3C2.

[0040] Here, a comparison result between the photodetector 1A of the second embodiment and the comparative example will be described. In the comparative example, the branching unit 4 and the waveguide-type PD2 are directly connected without the third mode removal unit 3C and the fourth mode removal unit 3D. In the comparative example, the output port 4A2 of the branching unit 4 and the first port 2A1 of the waveguide-type PD2 are connected by the first input waveguide 11A to input the signal light of TE0 to the first port 2A1, and the output port 4A3 of the branching unit 4 and the second port 2A2 are connected by the second input waveguide 11B to input the signal light of TE0 to the second port 2A2. In the waveguide-type PD2 of FIG. 2A, FIG. 2B, and FIG. 3, the transmittance of TE0 of the absorption layer 12 when the signal light of TE0 is input to the first input waveguide 11A and the signal light of TM0 is input to the second input waveguide 11B was calculated by the finite difference time domain method. The parameters of the waveguide type PD2 are as follows: 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. 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 wavelength is 1.52 μm, the refractive index of the Ge material used in the absorption layer 12 is 4.278+i0.0245, the refractive index of the Si material used in the Si substrate 13 is 3.45, and the refractive index of the SiO2 material used in the BOX layer 14 and the buffer layer 15 is 1.44.

[0041] In the photodetector of the comparative example, the ratio of the signal light of TE0 output from the first input waveguide 11A was calculated when the signal light of TE0 was input to the second input waveguide 11B. As a result, the output ratio was -24.15 dB. However, in the waveguide type PD2, the optical power that flows back through the absorption layer 12 is multiplexed at the branching section 4, so the ratio of the output from the branching section 4 to the input power to the branching section 4 is -24.15 dB at maximum. The cross-sectional structure of the waveguide type PD2 is not limited to that shown in FIG. 3, and in a configuration having multiple ports, the light that cannot be absorbed by the absorption layer is output from another port regardless of the cross-sectional structure. Therefore, regardless of the cross-sectional structure adopted, the transmitted light due to the backflow caused by the residual light may occur.

[0042] Furthermore, when the photodetector 1A of the second embodiment is used to input a first mode signal light as TE0, a second mode signal light as TM0, and the signal lights of TE0 and TM0 to the absorption layer 12, it is considered that mode conversion occurs in the waveguide-type PD 2. In this case, it is considered that the effect of arranging the third mode elimination unit 3C and the fourth mode elimination unit 3D is lost.

[0043] In the photodetector 1A, when a TE0 signal light is input to the second input waveguide 11B, the rate at which the TE0 signal light is output from the first input waveguide 11A is -24.15 dB. Furthermore, the rate at which the TM0 signal light is output from the first input waveguide 11A, i.e., the rate at which the mode is converted, is -78.61 dB. Furthermore, when a TM0 signal light is input to the first input waveguide 11A, the rate at which the TM0 signal light is output from the second input waveguide 11B is -34.62 dB. Furthermore, the rate at which the TE0 signal light is output from the second input waveguide 11B, i.e., the rate at which the mode is converted, is -73.82 dB.

[0044] As a result, it is possible to eliminate the backflow of transmitted light due to unnecessary residual light from the output to the same mode by using the third mode elimination section 3C and the fourth mode elimination section 3D. Moreover, since the mode conversion in the waveguide-type PD 2 is negligible, the unnecessary transmitted light can be eliminated by adding a mode elimination structure. In addition, in a waveguide structure that is symmetrical in any plane along the light propagation direction, the conversion between different polarized waves is very small, so it is particularly preferable to use different polarized waves in such a waveguide structure.

[0045] The photodetector 1A of the second embodiment has a third mode elimination unit 3C and a fourth mode elimination unit 3D. The fourth mode elimination unit 3D transmits the TE0 signal light from the branching unit 4 and eliminates the TM0 signal light from the second port 2A2. The third mode elimination unit 3C converts the TE0 signal light from the branching unit 4 to a TM0 signal light, transmits the converted TM0 signal light, and eliminates the TE0 signal light from the first port 2A1. As a result, the photodetector 1A can suppress backflow due to residual light in the absorption layer 12.

[0046] The fourth mode elimination unit 3D has an adiabatic converter 30D1 that transmits the TE0 signal light from the branching unit 4 and removes the TM0 signal light from the second port 2A2. The third mode elimination unit 3C has an adiabatic converter 30C1 that converts the TE0 signal light from the branching unit 4 to a TM0 signal light, and a tapered waveguide 33C that transmits 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 backflow due to residual light in the absorption layer 12.

[0047] For convenience of explanation, the photodetector 1A of the second embodiment uses the TE0 signal light as the first mode signal light and the TM0 signal light as the second mode signal light, but the present invention is not limited to this and can be modified as appropriate. Therefore, a photodetector 1B of the third embodiment will be described, which uses the TE0 signal light as the first mode signal light and the TE1 signal light as the second mode signal light. EXAMPLES

[0048] FIG. 8 is a block diagram showing an example of a photodetector 1B according to a third embodiment. For convenience of explanation, the same components as those of the photodetector 1A according to the second embodiment are denoted by the same reference numerals, and explanations of the overlapping components and operations are omitted. The photodetector 1B shown in FIG. 8 is different from the photodetector 1A shown in FIG. 4 in that a fifth mode elimination unit 3E is arranged instead of the third mode elimination unit 3C, and a sixth mode elimination unit 3N is arranged instead of the fourth mode elimination unit 3D. The fifth mode elimination unit 3E uses an adiabatic converter 30E capable of coupling the signal light of TE0 and the signal light of TE1.

[0049] FIG. 9A is an explanatory diagram showing an example of a fifth mode elimination unit 3E. The fifth mode elimination unit 3E shown in FIG. 9A is an adiabatic converter 30E. The adiabatic converter 30E has a first waveguide 31E and a second waveguide 32E arranged in parallel. The adiabatic converter 30E has 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 narrows from the input port 31E1 to the output port 31E2. The second waveguide 32E has a structure in which the waveguide width gradually narrows from the output port 32E2 to the input port 32E1.

[0050] When the adiabatic converter 30E receives a TE0 signal light from the input port 31E1 of the first waveguide 31E, the adiabatic converter 30E gradually transitions from the TE0 signal light to the TE1 signal light from the first waveguide 31E to the output side of the second waveguide 32E in an adiabatic manner. Then, the adiabatic converter 30E outputs the TE1 signal light from the output port 32E2 of the second waveguide 32E. When the adiabatic converter 30E receives a TE0 signal light from the input port 32E1 of the second waveguide 32E, the adiabatic converter 30E transmits and outputs the TE0 signal light from the output port 32E2 of the second waveguide 32E.

[0051] Considering the reversibility of the adiabatic converter 30E of the fifth mode elimination unit 3E, the signal light of TE1 input from the output port 32E2 of the second waveguide 32E is converted to the signal light of TE0 and output from the input port 31E1 of the first waveguide 31E. The signal light of TE0 input from the output port 32E2 of the second waveguide 32E is output as it is from the input port 32E1 of the second waveguide 32E, thereby eliminating the signal light of TE0.

[0052] The adiabatic converter 30E functions as a mode conversion remover by connecting the input port 32E1 (3E1) of the first waveguide 31E to the branching section 4 and connecting the output port 32E2 (3E2) of the second waveguide 32E to the first port 2A1 of the waveguide-type PD2. In this case, the signal light of TE0 input to the output port 32E2 (3E2) of the second waveguide 32E, that is, the signal light of TE0 transmitted through the waveguide-type PD2, is output from the input port 32E1 (3E1) of the second waveguide 32E. Therefore, in order to remove this unnecessary light, the input port 32E1 (3E1) of the second waveguide 32E is optically terminated. When the input port 32E1 of the second waveguide 32E is optically terminated, the input port 32E1 may be, for example, tapered or connected to a doped waveguide. Also, a monitor PD for monitoring unnecessary light may be connected as an optical terminal, and this can be changed as appropriate.

[0053] FIG. 9B is an explanatory diagram showing an example of the sixth mode elimination unit 3N. The sixth filter 30N in the sixth mode elimination unit 3N is an adiabatic converter 30N1 configured by arranging a tapered first waveguide 31N and a tapered second waveguide 32N in parallel. The parallel section L of the adiabatic converter 30N1 in which the first waveguide 31N and the second waveguide 32N are in parallel has 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 at the start point and the end point is the same.

[0054] When the signal light of TE0 and TE1 is inputted from the sixth output port 3N2 of the first waveguide 31N, the adiabatic converter 30N1 transmits and outputs the signal light of TE0 from the sixth input port 3N1 of the first waveguide 31N. The adiabatic converter 30N1 also gradually transfers the signal light of TE1 from the first waveguide 31N to the second waveguide 32N, and separates and outputs the signal light of TE1 from the second waveguide 32N. Since the sixth mode elimination unit 3N has reversibility, when the signal light of TE0 is inputted from the sixth input port 3N1 of the first waveguide 31N, the signal light of TE0 is transmitted and outputted from the sixth output port 3N2 of the first waveguide 31N.

[0055] The adiabatic converter 30N1 in the sixth mode elimination unit 3N connects the sixth input port 3N1 of the first waveguide 31N to the branching unit 4, and connects the sixth output port 3N2 of the first waveguide 31N to the waveguide-type PD2. In this case, the signal light of TE1 input to the sixth output port 3N2 of the first waveguide 31N and passing through the waveguide-type PD2 is optically terminated at the input port 3N3 of the second waveguide 32N. When the input port 3N3 of the second waveguide 32N is optically terminated, the input port 3N3 may have a tapered structure, or may be connected to a doped waveguide, for example. In addition, a monitor PD for monitoring unnecessary light may be connected as the optical termination, and can be changed as appropriate.

[0056] When the photodetector 1B of the third embodiment is used, the signal light of the first mode is TE0, the signal light of the second mode is TE1, and the signal light of TE0 and TE1 is input to the absorption layer 12, it is considered that mode conversion occurs in the waveguide-type PD 2. In this case, it is considered that the effect of arranging the fifth mode elimination unit 3E and the sixth mode elimination unit 3N is lost. Therefore, in the waveguide-type PD 2 of FIG. 3, the transmittance of the absorption layer 12 when the signal light of TE0 is input to the first input waveguide 11A and the signal light of TE1 is input to the second input waveguide 11B is calculated by the finite difference time domain method.

[0057] When TE0 signal light is input to the second input waveguide 11B, the rate at which TE0 signal light is output from the first input waveguide 11A is -24.15 dB. Furthermore, the rate at which TE1 signal light is output from the first input waveguide 11A, i.e., the rate at which mode is converted, is -65.27 dB. When TE1 signal light is input to the first input waveguide 11A, the rate at which TE1 signal light is output from the second input waveguide 11B is -23.36 dB. Furthermore, the rate at which TE0 signal light is output from the second input waveguide 11B, i.e., the rate at which mode is converted, is -63.68 dB.

[0058] As a result, it is possible to eliminate the backflow of transmitted light due to unnecessary residual light from the output to the same mode by using the fifth mode elimination unit 3E and the sixth mode elimination unit 3N. Moreover, since the mode conversion in the waveguide-type PD 2 is negligible, the unnecessary transmitted light can be eliminated by adding a mode elimination structure. In addition, in a waveguide structure that is symmetrical in any plane along the light propagation direction, the conversion between even-order modes and odd-order modes in the same polarization becomes very small, so it is preferable to use modes that are related to each other, especially in such a waveguide structure.

[0059] The photodetector 1B of the third embodiment has a fifth mode elimination unit 3E and a sixth mode elimination unit 3N. The sixth mode elimination unit 3N transmits the TE0 signal light from the branching unit 4 and eliminates the TE1 signal light from the second port 2A2. The fifth mode elimination unit 3E converts the TE0 signal light from the branching unit 4 to a TE1 signal light, transmits the converted TE1 signal light, and eliminates the TE0 signal light from the first port 2A1. As a result, the photodetector 1B can suppress backflow due to residual light in the absorption layer 12.

[0060] The sixth filter 30N in the sixth mode elimination unit 3N has an adiabatic converter 30N1 that transmits the TE0 signal light from the branching unit 4 and eliminates the TE1 signal light from the second port 2A2. The adiabatic converter 30E in the fifth mode elimination unit 3E converts the TE0 signal light from the branching unit 4 into a TE1 signal light and outputs it, and eliminates the TE0 signal light from the first port 2A1. As a result, backflow due to residual light in the absorption layer 12 can be suppressed.

[0061] For ease of explanation, the third mode elimination section 3C has the structure shown in Fig. 6, the fourth mode elimination section 3D has the structure shown in Fig. 5, the fifth mode elimination section 3E has the structure shown in Fig. 9A, and the sixth mode elimination section 3N has the structure shown in Fig. 9B. However, the present invention is not limited to these structures and can be modified as appropriate.

[0062] In addition, the photodetector 1A of the second embodiment is exemplified as having the third mode elimination unit 3C and the fourth mode elimination unit 3D, but the present invention is not limited to this and can be modified as appropriate. Therefore, the embodiment will be described below as a photodetector 1C of a fourth embodiment. EXAMPLES

[0063] FIG. 10 is a block diagram showing an example of a photodetector 1C according to a fourth embodiment. The same components as those of the photodetector 1 according to the first embodiment are denoted by the same reference numerals, and the description of the overlapping components and operations will be omitted. The photodetector 1C shown in FIG. 10 has a mode conversion unit 3F, a mode removal unit 3G, and a waveguide-type PD2. The mode conversion unit 3F has an input port 3F1, an output port 3F2, and a conversion unit 30F. When the signal light of TE0 is input from the input port 3F1, the conversion unit 30F converts the signal light of TE0 into signal light of TE0 and TE1. That is, when the signal light of TE0 is input from the input port 3F1, the conversion unit 30F converts the signal light of TE0 into signal light in which TE0 and TE1 are mixed. The mode removal unit 3G has an input port 3G1, a first output port 3G2, a second output port 3G3, and a filter 30G.

[0064] 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 PD2. The second output port 3G3 is connected to the second input waveguide 11B of the waveguide type PD2. The filter 30G transmits and outputs the TE0 signal light from the input port 3G1 to the first output port 3G2, and removes the TE1 signal light flowing backward from the first output port 3G2. The filter 30G transmits and outputs the TE1 signal light from the input port 3G1 to the second output port 3G3, and removes the TE0 signal light flowing backward from the second output port 3G3.

[0065] The absorption layer 12 in the waveguide-type PD2 receives the signal light TE0 from the first input waveguide 11A and converts the signal light TE0 into electron-hole pairs, and receives the signal light TE1 from the second input waveguide 11B and converts the signal light TE1 into electron-hole pairs.

[0066] When a portion 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 elimination unit 3G eliminates the TE0 signal light input from the second input waveguide 11B. When a portion 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 elimination unit 3G eliminates the TE1 signal light input from the first input waveguide 11A.

[0067] Fig. 11 is an explanatory diagram showing an example of a mode converter 3F. A converter 30F in the mode converter 3F shown in Fig. 11 is configured with, for example, a Y-branch waveguide 30F1. When a signal light of TE0 is input, the Y-branch waveguide 30F1 outputs signal light of TE0 and TE1.

[0068] FIG. 12A is an explanatory diagram showing an example of the operation of the mode removal unit 3G at the time of input, and FIG. 12B is an explanatory diagram showing an example of the operation of the mode removal unit 3G at the time of reverse flow. The mode removal unit 3G shown in FIG. 12A is, for example, configured with a directional coupler 30G1. The directional coupler 30G1 arranges a linear first waveguide 31G and a linear second waveguide 32G in parallel. When the signal light of TE0 and TE1 is input from the first waveguide 31G, the directional coupler 30G1 outputs the signal light of TE0 from the first output port 3G2 of the first waveguide 31G to the first input waveguide 11A. Furthermore, the directional coupler 30G1 outputs the signal light of TE1 from the second output port 3G3 of the second waveguide 32G to the second input waveguide 11B.

[0069] 12B inputs the TE1 signal light from the first output port 3G2 of the first waveguide 31G, and inputs the TE0 signal light 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 an optical terminal connected to the input port of the second waveguide 32G. As a result, the TE1 signal light and the TE0 signal light that flow back from the waveguide-type PD2 can be removed.

[0070] The photodetector 1C of the fourth embodiment has 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 and outputs them to the mode removal unit 3G. The mode removal unit 3G transmits 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 PD2. The mode removal unit 3G transmits 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 PD2. As a result, the photodetector 1C can remove the TE1 signal light and the TE0 signal light that flow back from the absorption layer 12.

[0071] The mode conversion unit 3F has a Y-branch waveguide 30F1 that outputs the TE0 signal light to the TE0 signal light and the TE1 signal light. The mode removal unit 3G has a directional coupler 30G1 that transmits the TE0 signal light from the Y-branch waveguide 30F1 and removes the TE1 signal light from the first port 2A1. Furthermore, the directional coupler 30G1 transmits the TE1 signal light from the Y-branch waveguide 30F1 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 back from the absorption layer 12.

[0072] In the photodetector 1C of the fourth embodiment, the signal light of TE0 and TE1 is input to the waveguide type PD2, but the present invention is not limited to this and can be modified as appropriate. For example, the signal light of TE0 and TM0 may be input to the waveguide type PD2, and this embodiment will be described below as the photodetector 1D of the fifth embodiment. EXAMPLES

[0073] FIG. 13 is a block diagram showing an example of a photodetector 1D according to a fifth embodiment. The same components as those of the photodetector 1C according to the fourth embodiment are denoted by the same reference numerals, and the description of the overlapping components and operations will be omitted. The photodetector 1D according to the fifth embodiment has a mode conversion unit 3F, a mode removal unit 3H, and a waveguide-type PD2. The mode conversion unit 3F has an input port 3F1, an output port 3F2, and a conversion unit 30F. When the signal light of TE0 is input from the input port 3F1, the conversion unit 30F converts the signal light of TE0 into signal light of TE0 and TE1. That is, when the signal light of TE0 is input from the input port 3F1, the conversion unit 30F converts the signal light of TE0 into signal light in which TE0 and TE1 are mixed. The mode removal unit 3H has an input port 3H1, a first output port 3H2, a second output port 3H3, and a filter 30H.

[0074] 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 PD2. The second output port 3H3 is connected to the second input waveguide 11B of the waveguide type PD2. The filter 30H converts the TE1 signal light from the input port 3H1 into a TM0 signal light and outputs it to the second output port 3H3, and removes the TE0 signal light flowing backward from the second port 2A2. The filter 30H transmits and outputs the TE0 signal light from the input port 3H1 to the second output port 3H3, and removes the TM0 signal light flowing backward from the first port 2A1.

[0075] Fig. 14 is an explanatory diagram showing an example of the transition of light in the photodetector 1D of the fifth embodiment. The conversion unit 30F shown in Fig. 14 is, for example, configured with a Y-branch waveguide 30F1. The Y-branch waveguide 30F1 converts the signal light TE0 input from the input port 3F1 into signal light TE0 and TE1, and outputs the converted signal light TE0 and TE1 from the output port 3F2 to the mode removal unit 3H.

[0076] Fig. 15 is an explanatory diagram showing an example of a mode removal section 3H. The mode removal section 3H shown in Fig. 15 has a conversion section 3P and an adiabatic converter 3Q. The conversion section 3P has 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 serves as an input port 3H1 of the mode removal section 3H. The output port 3P2 is connected to the output stage of the tapered waveguide 31P and is a port connected to the adiabatic converter 3Q.

[0077] The tapered waveguide 31P is a tapered waveguide in which the effective refractive indexes of the TE1 signal light and the TM0 signal light are reversed between the input cross section and the output cross section, and the cross section of the material refractive index is asymmetric vertically. The tapered waveguide 31P is provided with a portion (slab portion) that is thinner than the core so that it is asymmetric vertically, but the waveguide structure can be efficiently changed by adding a structure that generates a tapered slab portion to that portion.

[0078] The tapered waveguide 31P converts the TE1 signal light input from the input port 3P1 into a TM0 signal light, and outputs the converted TM0 signal light from the output port 3P2. The tapered waveguide 31P also transmits the TE0 signal light input from the input port 3P1, 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 a TE1 signal light, and outputs the converted TE1 signal light from the input port 3P1. The tapered waveguide 31P also transmits the TE0 signal light input from the output port 3P2, and outputs the transmitted TE0 signal light from the input port 3P1.

[0079] The adiabatic converter 3Q is an adiabatic converter configured by arranging a tapered first waveguide 31Q and a tapered second waveguide 32Q in parallel. The parallel section L of the adiabatic converter 3Q in which the first waveguide 31Q and the second waveguide 32Q are in parallel has 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 at the start point and the end point is the same.

[0080] When the signal light of TE0 and TM0 is inputted to the first input port 3Q1 of the first waveguide 31Q, the adiabatic converter 3Q transmits and outputs the signal light of TE0 from the first output port 3Q1 of the first waveguide 31Q. Also, the adiabatic converter 3Q gradually transfers the signal light of TM0 from the first waveguide 31Q to the second waveguide 32Q, and separates and outputs the signal light of TM0 from the second output port 3Q3 of the second waveguide 32Q.

[0081] Since the adiabatic converter 3Q has reversibility, when the signal light of TM0 is inputted from the first output port 3Q2 of the first waveguide 31Q, the signal light of TM0 is gradually transferred from the first waveguide 31Q to the second waveguide 32Q, and the signal light of TM0 is outputted from the second input port 3Q4 of the second waveguide 32Q. That is, the adiabatic converter 3Q inputs the signal light of TM0 that flows back from the waveguide-type PD2 from the first output port 3Q2 of the first waveguide 31Q. Then, the adiabatic converter 3Q gradually transfers the signal light of TM0 from the first waveguide 31Q to the second waveguide 32Q, and outputs the signal light of TM0 from the second input port 3Q4 of the second waveguide 32Q. As a result, the signal light of TM0 that flows back from the waveguide-type PD2 can be removed.

[0082] Furthermore, when the adiabatic converter 3Q receives the TE0 signal light from the second output port 3Q3 of the second waveguide 32Q, the adiabatic converter 3Q transmits and outputs the TE0 signal light from the second input port 3Q4 of the second waveguide 32Q. That is, the adiabatic converter 3Q receives the TE0 signal light flowing backward from the waveguide-type PD2 from 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 backward from the waveguide-type PD2 can be removed.

[0083] The photodetector 1D of the fifth embodiment has a mode conversion unit 3F and a mode removal unit 3H. The mode conversion unit 3F converts the TE0 signal light into TE0 signal light and TE1 signal light and outputs them to the mode removal unit 3H. The mode removal unit 3H transmits 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 TM0 signal light and removes the TE0 signal light from the second port 2A2. As a result, the photodetector 1D can remove the TE1 signal light and the TE0 signal light that flow back from the waveguide-type PD2.

[0084] The mode conversion unit 3F has a Y-branch waveguide 30F1 that converts the TE0 signal light into TE0 signal light and TE1 signal light and outputs them to the mode removal unit 3H. The mode removal unit 3H has a conversion unit 3P and an adiabatic converter 3Q. The tapered waveguide 31P in the conversion unit 3P transmits the TE0 signal light from the Y-branch waveguide 30F1 and converts the TE1 signal light from the Y-branch waveguide 30F1 into TM0 signal light. The adiabatic converter 3Q transmits the TM0 signal light from the conversion unit 3P and removes the TE0 signal light from the second port 2A2, and transmits the TE0 signal light from the conversion unit 3P and removes the TM0 signal light from the first port 2A1. As a result, the photodetector 1D can remove the TE1 signal light and the TE0 signal light that flow back from the waveguide-type PD2.

[0085] Although the waveguide-type PD2 has two ports, the first port 2A1 and the second port 2A2, three or more ports may be used. In this case, a mode elimination section is provided for each port, and each mode elimination section transmits signal light of a different mode and outputs it to the waveguide-type PD2, and eliminates signal light of a mode flowing backward from the absorption layer 12 in the waveguide-type PD2.

[0086] For convenience of explanation, the waveguide-type PD2 has a first port 2A1 connected to the first input waveguide 11A and a second port 2A2 connected to the second input waveguide 11B, and a two-input structure is illustrated, but the present invention is not limited to this and can be modified as appropriate. Also, the waveguide-type PD2 has a first port 2A1 and a second port 2A2 arranged in a straight line, but the present invention can be modified as appropriate.

[0087] Fig. 16A is an explanatory diagram showing an example of a two-input waveguide PD2B. The waveguide PD2B shown in Fig. 16A may have a second input port 2B2 arranged at a position 90 degrees from the first input port 2B1. The first input port 2B1 inputs a signal light of a first mode and outputs a signal light of a second mode remaining in the absorption layer 12. The second input port 2B2 inputs a signal light of a second mode and outputs a signal light of the first mode remaining in the absorption layer 12. The waveguide PD2 of this embodiment may be replaced with a waveguide PD2B.

[0088] Fig. 16B is an explanatory diagram showing an example of a three-input waveguide-type PD2C. The waveguide-type PD2C shown in Fig. 16B is a three-input waveguide-type PD having a first input port 2C1, a second input port 2C2, and a third input port 2C3 arranged at a position at 90 degrees with respect to the first input port 2C1 or the second input port 2C2. The waveguide-type PD2 of this embodiment may be replaced with a waveguide-type PD2C.

[0089] FIG. 16C is an explanatory diagram showing an example of a four-input waveguide type PD2D. The waveguide type PD2D shown in FIG. 16C has a first port 2D1 and a second port 2D2. Furthermore, the waveguide type PD2D has a third port 2D3 arranged at a position at 90 degrees with respect to the first port 2D1 or the second port 2D2, and a fourth port 2D4 arranged linearly with respect to the third port 2D3. That is, the waveguide type PD2D is a four-input waveguide type PD. The waveguide type PD2 of this embodiment may be replaced with a waveguide type PD2D. Therefore, a photodetector 1E employing a four-input waveguide type PD2D will be described below as Example 6. EXAMPLES

[0090] Fig. 17 is a block diagram showing an example of a photodetector 1E of the sixth embodiment. The photodetector 1E shown in Fig. 17 has a first branching section 4B, an eleventh mode removal section 3J, a twelfth mode removal section 3K, a second branching section 4C, a thirteenth mode removal section 3L, and a fourteenth mode removal section 3M. The first branching section 4B has an input port 4B1, a first output port 4B2, and a second output port 4B3, and branches and outputs a signal light of a first mode to the first output port 4B2 and the second output port 4B3.

[0091] The eleventh mode elimination unit 3J has an eleventh input port 3J1, an eleventh filter 30J, and an eleventh output port 3J2. The eleventh filter 30J converts the first mode signal light input from the eleventh input port 3J1 into the second mode signal light, and eliminates the first mode signal light input from the eleventh output port 3J2. The twelfth mode elimination unit 3K has a twelfth input port 3K1, a twelfth filter 30K, and a twelfth output port 3K2. The twelfth filter 30K transmits and outputs the first mode signal light input from the twelfth input port 3K1, and eliminates the second mode signal light input from the twelfth output port 3K2.

[0092] The waveguide-type PD2D has a first port 2D1 connected to the first input waveguide 11A and a second port 2D2 connected to the second input waveguide 11B. Furthermore, the waveguide-type PD2D has 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.

[0093] The absorption layer 12 converts the second mode signal light input from the first port 2D1 into electron-hole pairs, and also converts the first mode signal light input from the second port 2D2 into electron-hole pairs. However, if the absorption layer 12 cannot absorb a portion of the second mode signal light input from the first port 2D1 and remains, the remaining second mode signal light is output from the second port 2D2 to the second input waveguide 11B. Therefore, the twelfth filter 30K in the twelfth mode removal unit 3K removes the second mode signal light from the twelfth output port 3K2.

[0094] Furthermore, if a portion of the first mode signal light input from the second port 2D2 cannot be absorbed and remains in the absorption layer 12, the remaining first mode signal light is output from the first port 2D1 to the first input waveguide 11A. Therefore, the eleventh filter 30J in the eleventh mode elimination unit 3J eliminates the first mode signal light from the eleventh output port 3J2.

[0095] The second branching unit 4C has an input port 4C1, a first output port 4C2 and a second output port 4C3, and branches and outputs the third mode signal light to the first output port 4C2 and the second output port 4C3.

[0096] The thirteenth mode removal unit 3L has a thirteenth input port 3L1, a thirteenth filter 30L, and a thirteenth output port 3L2. The thirteenth filter 30L converts the third mode signal light input from the thirteenth input port 3L1 into a fourth mode signal light, and removes the third mode signal light from the thirteenth output port 3L2. The fourteenth mode removal unit 3M has a fourteenth input port 3M1, a fourteenth filter 30M, and a fourteenth output port 3M2. The fourteenth filter 30M transmits and outputs the third mode signal light input from the fourteenth input port 3M1, and removes the fourth mode signal light 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 also converts the signal light of the third mode input from the fourth port 2D4 into electron-hole pairs.

[0097] However, when a portion of the signal light of the fourth mode input from the third port 2D3 cannot be absorbed and remains in the absorption layer 12, the remaining signal light of the fourth mode is output from the fourth port 2D4 to the fourth input waveguide 11D11. The fourteenth filter 30M in the fourteenth mode elimination unit 3M eliminates the signal light of the fourth mode from the fourteenth output port 3M2.

[0098] Furthermore, when a portion of the third mode signal light input from the fourth port 2D4 cannot be absorbed and remains in the absorption layer 12, the remaining fourth mode signal light is output from the third port 2D3 to the third input waveguide 11C11. The thirteenth filter 30L in the thirteenth mode removal unit 3L removes the third mode signal light from the thirteenth output port 3L2. As a result, the photodetector 1E can suppress transmitted light caused by the backflow of the remaining light in the absorption layer 12.

[0099] The photodetector 1E of the sixth embodiment includes an eleventh mode elimination unit 3J, a twelfth mode elimination unit 3K, a thirteenth mode elimination unit 3L, and a fourteenth mode elimination unit 3M. The eleventh mode elimination unit 3J transmits the second mode signal light to the first port 2D1 and eliminates the first mode signal light from the first port 2D1. The twelfth mode elimination unit 3K transmits the first mode signal light to the second port 2D2 and eliminates the second mode signal light from the second port 2D2. The thirteenth mode elimination unit 3L transmits the fourth mode signal light to the third port 2D3 and eliminates the third mode signal light from the third port 2D3. The fourteenth mode elimination unit 3M transmits the signal light of the third mode to the fourth port 2D4 and eliminates the signal light of the fourth mode from the fourth port 2D4. As a result, the photodetector 1E can suppress the signal light intensity per cross section input to the four-input waveguide-type PD2D, suppress deterioration of the response speed of the waveguide-type PD2D, and suppress noise due to backflow of residual light that could not be absorbed by the waveguide-type PD2D.

[0100] As described above, specific configurations have been described in the embodiments, but the conversion into different modes and the spatial separation of different modes are not limited to a specific method, and can be modified as appropriate.

[0101] The cross-sectional structure of the waveguide-type PD2 is exemplified as shown in Fig. 3, but is not limited to this and can be modified as appropriate. Figs. 18A to 18E are explanatory diagrams showing an example of the waveguide-type PD2.

[0102] The waveguide type PD2E shown in FIG. 18A has 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, for example, a waveguide layer made of Si or Si3N4. The buffer layer is, for example, a layer of SiO2. The waveguide layer 11 has a first input waveguide 11A1 and a second input waveguide 11B1. The absorption layer 12E is disposed on the BOX layer 14. The absorption layer 12 is, for example, an absorption layer made of Ge (Germanium). The first input waveguide 11A1 and the second input waveguide 11B1 are disposed on the BOX layer 14. The waveguide type PD2 of this embodiment can be replaced with a waveguide type PD2E to obtain the same effect.

[0103] The waveguide-type PD2F shown in FIG. 18B has 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 has 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 disposed on the waveguide 11C21. The first input waveguide 11A2 and the second input waveguide 11B2 are disposed on the BOX layer 14. The waveguide-type PD2 of this embodiment can be replaced with a waveguide-type PD2F to obtain the same effect.

[0104] The waveguide-type PD2G shown in Fig. 18C has 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 disposed on the waveguide layer 11. Furthermore, a waveguide 11G composed of a first input waveguide 11A3 and a second input waveguide 11B3 is disposed on the absorption layer 12G. The waveguide-type PD2 of this embodiment can be replaced with a waveguide-type PD2G to obtain the same effect.

[0105] The waveguide-type PD2H shown in Fig. 18D has 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 disposed on the waveguide layer 11. Furthermore, a waveguide 11H composed of a first input waveguide 11A4 and a second input waveguide 11B4 is disposed on the BOX layer 14 so as to run parallel to the absorption layer 12H at a predetermined interval. The waveguide-type PD2 of this embodiment can be replaced with a waveguide-type PD2H to obtain the same effect.

[0106] The waveguide-type PD2J shown in Fig. 18E has 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 disposed on the waveguide layer 11. Furthermore, a first input waveguide 11A5 and a second input waveguide 11B5 are disposed on the waveguide layer 11 so as to run in parallel with the absorption layer 12J, with the absorption layer 12J sandwiched therebetween. The waveguide-type PD2 of this embodiment can be replaced with a waveguide-type PD2J to obtain the same effect.

[0107] The mode filter 3 spatially separates the residual light to be removed and then outputs it from a port different from the input port, but if reflection occurs further down the line, the light may return to the waveguide-type PD 2, impairing stable operation. Therefore, by providing an optical termination at a port different from the input section, it is possible to reduce the reflection of light that has such adverse effects.

[0108] In addition, the optical termination may be a structure in which the waveguide is doped to absorb light, a structure in which the waveguide is tapered in the direction of light propagation to radiate light outside the waveguide, or a structure in which Ge is deposited on the waveguide to absorb light, and these structures can be modified as appropriate.

[0109] The waveguide may be, for example, a rib waveguide, a ridge waveguide, a rectangular waveguide, or a high mesa waveguide. A rib waveguide is preferable because light also seeps into the slab portion, so it is less affected by roughness of the core sidewall and allows low-loss propagation. A rectangular waveguide is preferable because it has strong light confinement and therefore reduces loss even if the bending radius R is made small. The waveguide may also be a low-loss bent waveguide, and can be changed as appropriate.

[0110] The waveguide may be a PLC with both the core and clad being SiO2, an InP waveguide, or a GaAs waveguide. It may also be a Si waveguide with a core of Si, a lower clad being SiO2, and an upper clad being SiO2 or air, SiN, or the like. When the waveguide is a Si waveguide, the relative refractive index difference is large, so that light is confined strongly, and therefore a low-loss bending waveguide can be realized even with a small bending radius R, which is preferable because it allows the optical device to be miniaturized. The absorption layer 12 may be, for example, SiGe in addition to Ge.

[0111] Next, an optical transceiver 50 employing the photodetector 1 of this embodiment will be described. FIG. 19 is an explanatory diagram showing an example of an optical transceiver 50 employing the photodetector 1. The optical transceiver 50 shown in FIG. 19 includes an optical transmitter / receiver 51 and a DSP (Digital Signal Processor) 52. The optical transmitter / receiver 51 includes an optical modulator element 54, a driver circuit 55, an optical receiver element 56, and a TIA (Transimpedance Amplifier) ​​57. The DSP 52 controls the entire optical transmitter / receiver 51. The DSP 52 is an electrical component that performs digital signal processing, such as IQ modulation processing of a transmission signal and demodulation processing of a reception signal.

[0112] The DSP 52 executes processing such as encoding of 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.

[0113] The optical receiver element 56 incorporates a photodetector 1 including a waveguide type PD 2 that converts the signal light into an electric signal. The TIA 57 amplifies the electric signal after the electric conversion and outputs the amplified electric signal to the DSP 52. The DSP 52 performs processing such as decoding of the electric signal obtained from the TIA 57 to obtain received data.

[0114] For ease of explanation, the optical transceiver 50 has been illustrated as incorporating an optical modulator element 54 and an optical receiver element 56, but the optical transceiver 50 may be an optical receiver incorporating only the optical receiver element 56, and may be modified as appropriate.

[0115] Furthermore, each component of each unit shown in the figure does not necessarily have to be physically configured as shown in the figure. In other words, the specific form of distribution and integration of each unit is not limited to that shown in the figure, and all or part of them can be functionally or physically distributed and integrated in any unit depending on various loads, usage conditions, etc.

[0116] Furthermore, the various processing functions performed by each device may be executed in whole or in part on a CPU (Central Processing Unit) (or a microcomputer such as an MPU (Micro Processing Unit) or an MCU (Micro Controller Unit)). It goes without saying that the various processing functions may be executed in whole or in part on a program analyzed and executed by a CPU (or a microcomputer such as an MPU or an MCU), or on hardware using wired logic.

[0117] The following supplementary notes are further disclosed regarding the above-described embodiments including the present example.

[0118] (Note 1) A photodetector having a photodetector with two or more ports for converting a signal light into an electrical signal, and a mode filter having two or more ports, The photodetector includes: a first port to which a signal light of a first mode is input; a second port to which a second mode signal light is input; The mode filter comprises: a third port connected to the first port; and a fourth port connected to the second port; The mode filter comprises: transmitting the first mode signal light to the first port and outputting it from the third port; transmitting the second mode signal light to the second port and outputting it from the fourth port; removing the second mode signal light from the first port which is input from the third port; The signal light of the first mode input from the second port is removed from the fourth port. 1. A photodetector comprising:

[0119] (Appendix 2) The mode filter is transmitting the first mode signal light to the first port and outputting it from the third port; transmitting the second mode signal light to the second port and outputting it from the fourth port; removing signal light including the second mode signal light other than the first mode signal light from the first port inputted from the third port; Removing the signal light including the first mode signal light other than the second mode signal light from the second port inputted from the fourth port. 2. The photodetector of claim 1 .

[0120] (Appendix 3) The mode filter is a first mode removal unit connected to the third port, transmitting the signal light of the first mode and removing the signal light of the second mode from the first port; a second mode removal unit connected to the fourth port, transmitting the second mode signal light and removing the first mode signal light from the second port; 2. The photodetector according to claim 1, comprising:

[0121] (Appendix 4) The mode filter is a branching unit that branches the signal light of the first mode, The first mode removal unit includes: transmits the signal light of the first mode from the branching portion and removes the signal light of the second mode from the second port; The second mode removal unit includes: 4. The optical detector according to claim 3, wherein the optical signal of the first mode from the branching section is converted into the optical signal of the second mode, the optical signal of the second mode is transmitted, and the optical signal of the first mode from the first port is removed.

[0122] (Additional Note 5) The first mode elimination unit is a first adiabatic converter that transmits the first mode signal light from the branch and rejects the second mode signal light from the second port; The second mode removal unit includes: a second adiabatic converter that converts the signal light of the first mode from the branching unit into signal light of the second mode; a tapered waveguide that transmits the signal light of the second mode converted by the second adiabatic converter and removes the signal light of the first mode from the first port; 5. The photodetector according to claim 4, comprising:

[0123] (Appendix 6) The mode filter is a mode conversion unit 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 and outputs the converted signal light; a mode elimination unit having a third port connected to the first port and a fourth port connected to the second port, The mode removal unit includes: transmitting the first mode signal light to the first port and outputting it from the third port; transmitting the second mode signal light to the second port and outputting it from the fourth port; removing the second mode signal light from the first port which is input from the third port; The signal light of the first mode input from the second port is removed from the fourth port. 2. The photodetector of claim 1 .

[0124] (Additional Note 7) The mode conversion unit is 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 and outputs the converted signal light, The mode removal unit includes: a directional coupler that transmits the signal light of the first mode from the Y-branch waveguide and removes the signal light of the second mode from the first port, and also transmits the signal light of the second mode from the Y-branch waveguide and removes the signal light of the first mode from the second port; 7. The photodetector according to claim 6,

[0125] (Appendix 8) The mode filter is a mode conversion unit 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 and outputs the converted signal light; a mode elimination unit having a fifth port connected to the first port and a sixth port connected to the second port, The mode removal unit includes: transmitting the signal light of the first mode to the first port and outputting it from the fifth port; transmitting the second mode signal light to the second port and outputting it from the sixth port; removing the second mode signal light from the first port which is input from the fifth port; The signal light of the first mode input from the second port is removed from the sixth port. 2. The photodetector of claim 1 .

[0126] (Additional Note 9) The mode conversion unit is 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 and outputs the converted signal light, The mode removal unit includes: a tapered waveguide that transmits 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; an adiabatic converter that transmits the signal light of the third mode from the tapered waveguide and removes the signal light of the first mode from the second port, and transmits the signal light of the first mode from the tapered waveguide and removes the signal light of the third mode from the first port; 9. The photodetector according to claim 8, comprising:

[0127] (Supplementary Note 10) The photodetector comprises: a fifth port to which the third mode signal light is input; a sixth port to which the fourth mode signal light is input; The mode filter comprises: a seventh port connected to the fifth port; and an eighth port connected to the sixth port; The mode filter comprises: transmitting the third mode signal light to the fifth port and outputting it from the seventh port; transmitting the fourth mode signal light to the sixth port and outputting it from the eighth port; removing the fourth mode signal light from the fifth port which is input from the seventh port; The third mode signal light from the sixth port inputted from the eighth port is removed. 2. The photodetector of claim 1 .

[0128] (Appendix 11) An optical receiver having an optical receiver element for receiving an electrical signal from an optical signal using light, The optical receiver element includes: a photodetector having two or more ports for converting the signal light into an electrical signal, and a mode filter having two or more ports; The photodetector includes: a first port to which a signal light of a first mode is input; a second port to which a second mode signal light is input; The mode filter comprises: a third port connected to the first port; and a fourth port connected to the second port; The mode filter comprises: transmitting the first mode signal light to the first port and outputting it from the third port; transmitting the second mode signal light to the second port and outputting it from the fourth port; removing the second mode signal light from the first port which is input from the third port; The signal light of the first mode input from the second port is removed from the fourth port. 1. An optical receiver comprising:

[0129] (Appendix 12) An optical transceiver having an optical modulator element for optically modulating light using a transmission signal to transmit a transmission light, and an optical receiver element for receiving a reception signal from the reception light using light, The optical receiver element comprises: a photodetector having two or more ports and a mode filter having two or more ports, the photodetector converting the received light into a received signal; The photodetector includes: a first port to which a signal light of a first mode is input; a second port to which a second mode signal light is input; The mode filter comprises: a third port connected to the first port; and a fourth port connected to the second port; The mode filter comprises: transmitting the first mode signal light to the first port and outputting it from the third port; transmitting the second mode signal light to the second port and outputting it from the fourth port; removing the second mode signal light from the first port which is input from the third port; The signal light of the first mode input from the second port is removed from the fourth port. 1. An optical transceiver comprising: [Explanation of symbols]

[0130] 1 Photodetector 2 Waveguide PD 2A1 1st port 2A2 Second Port 3 mode filter 3A First Mode Rejection Section 3B Second mode rejection section 3A1 output port 3B2 output port 3F Mode conversion section 3G mode rejection unit 4 Branching section

Claims

1. A photodetector that converts a signal light into an electrical signal and has a photodetector having two or more ports and a mode filter having two or more ports, The photodetector includes: a first port to which a signal light of a first mode is input; 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; The mode filter comprises: transmitting the first mode signal light to the first port and outputting it from the third port; transmitting the second mode signal light to the second port and outputting it from the fourth port; removing the second mode signal light from the first port which is input from the third port; The signal light of the first mode input from the second port is removed from the fourth port.

1. A photodetector comprising:

2. The mode filter comprises: a first mode removal unit connected to the third port, transmitting the signal light of the first mode and removing the signal light of the second mode from the first port; a second mode removal unit connected to the fourth port, transmitting the second mode signal light and removing the first mode signal light from the second port; 2. The photodetector according to claim 1, further comprising:

3. The mode filter comprises: a branching unit that branches the signal light of the first mode, The first mode removal unit includes: transmitting the signal light of the first mode from the branching portion and removing the signal light of the second mode from the second port; The second mode removal unit includes:

3. The photodetector according to claim 2, wherein the signal light of the first mode from the branching section is converted to the signal light of the second mode, the converted signal light of the second mode is transmitted, and the signal light of the first mode from the first port is removed.

4. The first mode removal unit includes: a first adiabatic converter that transmits the first mode signal light from the branch and rejects the second mode signal light from the second port; The second mode removal unit includes: a second adiabatic converter that converts the signal light of the first mode from the branching unit into signal light of the second mode; a tapered waveguide that transmits the signal light of the second mode converted by the second adiabatic converter and removes the signal light of the first mode from the first port; 4. The photodetector according to claim 3, further comprising:

5. The mode filter comprises: a mode conversion unit 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 and outputs the converted signal light; a mode elimination unit having a third port connected to the first port and a fourth port connected to the second port, The mode removal unit includes: transmitting the first mode signal light to the first port and outputting it from the third port; transmitting the second mode signal light to the second port and outputting it from the fourth port; removing the second mode signal light from the first port which is input from the third port; The signal light of the first mode input from the second port is removed from the fourth port.

2. The photodetector according to claim 1 .

6. The mode conversion unit is 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 and outputs the converted signal light, The mode removal unit includes: a directional coupler that transmits the signal light of the first mode from the Y-branch waveguide and removes the signal light of the second mode from the first port, and also transmits the signal light of the second mode from the Y-branch waveguide and removes the signal light of the first mode from the second port; 6. The photodetector according to claim 5.

7. The mode filter comprises: a mode conversion unit 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 and outputs the converted signal light; a mode elimination unit having a fifth port connected to the first port and a sixth port connected to the second port, The mode removal unit includes: transmitting the first mode signal light to the first port and outputting it from the fifth port; transmitting the second mode signal light to the second port and outputting it from the sixth port; removing the second mode signal light from the first port which is input from the fifth port; The signal light of the first mode input from the second port is removed from the sixth port.

2. The photodetector according to claim 1 .

8. The mode conversion unit is 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 and outputs the converted signal light, The mode removal unit includes: a tapered waveguide that transmits 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; an adiabatic converter that transmits the signal light of the third mode from the tapered waveguide and removes the signal light of the first mode from the second port, and transmits the signal light of the first mode from the tapered waveguide and removes the signal light of the third mode from the first port; 8. The photodetector according to claim 7, further comprising:

9. 1. An optical receiver having an optical receiver element for receiving an electrical signal from an optical signal using light, The optical receiver element comprises: a photodetector having two or more ports and a mode filter having two or more ports, the photodetector converting the signal light into an electrical signal; The photodetector includes: a first port to which a signal light of a first mode is input; 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; The mode filter comprises: transmitting the first mode signal light to the first port and outputting it from the third port; transmitting the second mode signal light to the second port and outputting it from the fourth port; removing the second mode signal light from the first port which is input from the third port; The signal light of the first mode input from the second port is removed from the fourth port.

1. An optical receiver comprising:

10. An optical transceiver having an optical modulator element that uses a transmission signal to optically modulate light and transmits the transmission light, and an optical receiver element that uses light to receive a reception signal from the reception light, The optical receiver element comprises: a photodetector having two or more ports and a mode filter having two or more ports, the photodetector converting the received light into a received signal; The photodetector includes: a first port to which a signal light of a first mode is input; 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; The mode filter comprises: transmitting the first mode signal light to the first port and outputting it from the third port; transmitting the second mode signal light to the second port and outputting it from the fourth port; removing the second mode signal light from the first port which is input from the third port; The signal light of the first mode input from the second port is removed from the fourth port.

1. An optical transceiver comprising:

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