Optical receiver

Abstract

Provided is a light receiver in which degradation of the operating band is less likely to occur even when the distance between elements constituting the light receiver has changed. A light receiver of one embodiment comprises: a light receiving element (PD); a bias circuit that applies a bias voltage; a transimpedance amplifier (TIA); at least one subcarrier on which the PD, the bias circuit, and the TIA are mounted; and at least one high frequency wiring board that connects an upper surface of the PD and an upper surface of the bias circuit, and / or the upper surface of the PD and an upper surface of the TIA.

Description

Technical Field

[0001] The present disclosure relates to an optical receiver, and more particularly to an implementation technique of an optical semiconductor module for an optical receiver.

Background Art

[0002] Conventionally, an optical receiver including a photodiode (PD) as a light-receiving element, a bias circuit for driving the PD, and a transimpedance amplifier (TIA) has been known (see, for example, Non-Patent Document 1). In particular, when the light-receiving element is an avalanche photodiode (APD), a separate bias circuit is required because it is difficult to drive with the supply voltage from the TIA.

[0003] FIG. 1 is a diagram showing a schematic configuration of a conventional optical receiver. FIG. 1(a) is a top view, and FIG. 1(b) is a cross-sectional view. The optical receiver 100 shown in FIG. 1 includes a PD 101, a bias circuit 102 for driving the PD 101, and a TIA 104 which is an electrical amplifier.

[0004] The PD 101 and the bias circuit 102 are disposed on a PD sub-carrier 103, and the TIA 104 is disposed on a TIA sub-carrier 105 different from the PD sub-carrier 103. The PD sub-carrier 103 and the TIA sub-carrier 105 are dielectric carriers.

[0005] The bias circuit 102 is a capacitor constituting the bias circuit, and applies a reverse bias to the connected PD 101. The TIA 104 converts the current from the connected PD 101 into a voltage. The bias circuit 102 (capacitor) located closest to the PD 101 affects the high-frequency characteristics of the optical receiver 100.

[0006] A certain distance is provided between the bias circuit 102 and PD101, and between PD101 and TIA104. PD101 and the bias circuit 102 are electrically connected by a metal wire 106a. Similarly, PD101 and TIA104 are electrically connected by a metal wire 106b.

[0007] The PD subcarrier 103 has a through hole, which forms an opening 103a. The PD 101 is positioned on the PD subcarrier 103 so that the signal light is incident on the light-receiving surface on the back side of the PD 101 through the opening 103a. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] HT Chen et. al., “Low-voltage waveguide Ge APD based high sensitivity 10Gb / s Si photonic receiver,” 2015 European Conference on Optical Communication, Tu1.3.4. [Overview of the Initiative]

[0009] As mentioned above, in order to transmit the photoelectrically converted signal in an optical receiver, the elements constituting the optical receiver must be electrically connected. Generally, metal wires are used for the electrical connections between each element. Since these metal wires affect the frequency response characteristics of the optical receiver, in order to operate the optical receiver over a wide bandwidth, it is necessary to implement wires of appropriate lengths that take into account the frequency characteristics of each element. For example, in optical receivers exceeding 100 Gbaud, it is said that implementing wires of around 100 μm introduces a slight peak in the frequency characteristics, thereby achieving a wide bandwidth. To implement wires of the optimal length, it is necessary to provide a corresponding distance between elements. In addition, variations in wire length due to implementation tolerances can cause changes in bandwidth. Therefore, in the case of wire implementation, there was a problem that it limited the degree of freedom in the structural design of the optical receiver.

[0010] This disclosure has been made in view of such problems, and its purpose is to provide an optical receiver in which the operating bandwidth is less likely to deteriorate even when the distance between the elements constituting the optical receiver changes.

[0011] To achieve this objective, an optical receiver according to one embodiment of the present invention comprises a photodetector (PD), a bias circuit for applying a bias voltage, a transimpedance amplifier (TIA), at least one subcarrier on which the PD, the bias circuit, and the TIA are mounted, and at least one high-frequency wiring board connecting at least one of the following: the top surface of the PD and the top surface of the bias circuit, or the top surface of the PD and the top surface of the TIA.

[0012] This configuration makes it possible to provide an optical receiver in which the operating bandwidth is less likely to deteriorate even when the distance between the elements constituting the optical receiver changes. [Brief explanation of the drawing]

[0013] [Figure 1] This diagram shows the schematic configuration of a conventional optical receiver, with (a) being a top view and (b) being a cross-sectional view. [Figure 2]This figure shows a schematic configuration of an optical receiver according to one embodiment of the present disclosure, where (a) is a top view, (b) is a cross-sectional view, (c) is a top view of the high-frequency waveguide substrate, and (d) is a cross-sectional view of the high-frequency waveguide substrate. [Figure 3] This figure shows a schematic configuration of an optical receiver according to one embodiment of the present disclosure, where (a) is a top view, (b) is a cross-sectional view, (c) is a top view of the high-frequency waveguide substrate, and (d) is a cross-sectional view of the high-frequency waveguide substrate. [Figure 4] This figure shows a schematic configuration of an optical receiver according to one embodiment of the present disclosure, where (a) is a top view, (b) is a cross-sectional view, (c) is a top view of the high-frequency waveguide substrate, and (d) is a cross-sectional view of the high-frequency waveguide substrate. [Figure 5] This figure shows a schematic configuration of an optical receiver according to one embodiment of the present disclosure, where (a) is a top view, (b) is a cross-sectional view, (c) is a top view of the high-frequency waveguide substrate, and (d) is a cross-sectional view of the high-frequency waveguide substrate. [Figure 6] This figure shows a schematic configuration of an optical receiver according to one embodiment of the present disclosure, where (a) is a top view, (b) is a cross-sectional view, (c) is a top view of the high-frequency waveguide substrate, and (d) is a cross-sectional view of the high-frequency waveguide substrate. [Figure 7] This figure shows a schematic configuration of an optical receiver according to one embodiment of the present disclosure, where (a) is a top view, (b) is a cross-sectional view, (c) is a top view of the high-frequency waveguide substrate, and (d) is a cross-sectional view of the high-frequency waveguide substrate. [Figure 8] This figure shows a schematic configuration of an optical receiver according to one embodiment of the present disclosure, where (a) is a top view, (b) is a cross-sectional view, (c) is a top view of the high-frequency waveguide substrate, and (d) is a cross-sectional view of the high-frequency waveguide substrate. [Figure 9] This figure shows the frequency response characteristics of the TIA output section of an optical receiver when receiving a 50 Gbaud signal. [Figure 10] This figure shows the frequency response characteristics of the TIA output section of an optical receiver when receiving a 100 Gbaud signal. [Modes for carrying out the invention]

[0014] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. The same or similar reference numerals indicate the same or similar elements, and repeated descriptions will be omitted. The numerical values exemplified in the following description are not limited, and other numerical values can be used as long as they do not deviate from the gist of the present disclosure. In this specification, the Z direction of the XY plane is referred to as up.

[0015] In the optical receiver according to an embodiment of the present disclosure, as shown in FIGS. 2 to 8, a high-frequency wiring board is used instead of a metal wire for part or all of the connections between the elements constituting the optical receiver. In the conventional optical receiver as described above, when the distance between the elements constituting the optical receiver deviates from the optimum value, there is a problem that the length of the metal wire changes and the bandwidth of the optical receiver deteriorates. In contrast, in the configuration of the optical receiver according to the embodiment of the present disclosure, by using a high-frequency wiring board, which is a composite of an impedance line and a high-impedance line, for the electrical connection between the elements, the bandwidth degradation of the optical receiver can be reduced.

[0016] (Embodiment 1) Referring to FIG. 2, the optical receiver according to Embodiment 1 of the present disclosure will be described. FIG. 2(a) is a top view of the optical receiver 200 of the present embodiment, and FIG. 2(b) is a cross-sectional view of the optical receiver 200. The optical receiver 200 shown in FIG. 2 includes a PD101, a bias circuit 102, a TIA104, a PD sub-carrier 203, and a TIA sub-carrier 105.

[0017] The PD101 and the bias circuit 102 are arranged on the upper surfaces of two opposing main surfaces (XY plane) of the PD sub-carrier 203, which is a dielectric carrier. The optical receiver is different from the PD sub-carrier 103 shown in FIG. 1, which has a step on the upper surface, in that the two opposing main surfaces of the PD sub-carrier 203 are substantially parallel.

[0018] The TIA104 is arranged on the upper surfaces of two opposing main surfaces (XY plane) of the TIA sub-carrier 105, which is a dielectric carrier.

[0019] Similar to the optical receiver 100 described with reference to FIG. 1, a certain distance is provided between the bias circuit 102 and the PD 101, and between the PD 101 and the TIA 104, respectively. The PD 101 and the TIA 104 are electrically connected by a metal wire 106b.

[0020] The optical receiver 200 shown in FIG. 2 is different from the optical receiver 100 in FIG. 1 in that a high-frequency wiring board 222 is used instead of the metal wire 106a for the electrical connection between the PD 101 and the bias circuit 102 (capacitor). The high-frequency wiring board 222 is a line in which an impedance line and a high-impedance line are combined by appropriately designing the structure and dielectric material. Further, the optical receiver 200 is configured such that the heights of the connection surface, which is the upper surface of the PD 101 bridged by the high-frequency wiring board 222, and the connection surface, which is the upper surface of the bias circuit 102, are the same. By polishing the PD 101, the heights of the connection surface of the PD 101 and the connection surface of the bias circuit 102 can be made the same.

[0021] The impedance of the transmission lines in the high-frequency printed circuit board 222 can be adjusted by changing the width (in the X-axis direction) of the transmission lines 223a in the high-frequency printed circuit board 222. Specifically, by providing a portion of the transmission line 223a with a narrower width, the impedance of that portion increases. By reducing the width of a portion of the transmission line 223a that is designed to have a width corresponding to a specific impedance (for example, 50Ω), that portion of the transmission line 223a becomes a transmission line with high impedance, i.e., a high-impedance transmission line. In this disclosure, the portion of the transmission line 223a with a width corresponding to a specific impedance is referred to as an impedance transmission line. A high-frequency printed circuit board 222 that combines the impedance transmission lines and high-impedance transmission lines described above can be realized by reducing the width of a portion of the transmission line 223a in this way. A high-frequency wiring board 222 in which impedance lines and high-impedance lines are combined may be realized by partially changing the dielectric material constituting the high-frequency wiring board 222, or a high-frequency wiring board 222 in which impedance lines and high-impedance lines are combined may be realized by designing the structure of the high-frequency wiring board 222 and the combination of dielectric materials.

[0022] Figure 2(c) is a bottom view of the high-frequency wiring board 222, and Figure 2(d) is a cross-sectional view of the high-frequency wiring board 222. As shown in Figure 2, the high-frequency wiring board 222 has thin metallic films formed on four surfaces of the dielectric 224, excluding the two main surfaces (ZX surfaces). The thin metallic films formed on the bottom surface of the dielectric 224, i.e., the surface facing the top surface of PD101 and bias circuit 102, are separated by slits. The thin film located in the center of the bottom surface of the dielectric 224 constitutes transmission line 223a. The thin film located on the side of the bottom surface of the dielectric 224 and separated from transmission line 223a by slits is continuous with the thin films formed on the side surfaces (YZ surfaces) and top surface of the dielectric 224, and constitutes transmission line 223b. Transmission line 223a is a given bias potential shared from bias circuit 102, and transmission line 223b is a reference potential such as ground. The lines 223a and 223b of the high-frequency wiring board 222 are positioned in contact with the bumps 221 formed on the upper surfaces of the PD 101 and the bias circuit 102.

[0023] Figure 9 shows the frequency response characteristics of the output section of the TIA104 at 50 Gbaud. Figure 10 shows the frequency response characteristics of the output section of the TIA104 at 100 Gbaud. In Figures 9 and 10, the characteristics of a conventional optical receiver as shown in Figure 1 are shown by dashed lines, and the characteristics of an optical transmitter according to the embodiment of this disclosure are shown by solid lines. The characteristics shown in Figures 9 and 10 show the frequency characteristics calculated while varying the distance (inter-element distance) between the PD and the bias circuit and between the PD and the TIA in the optical receiver.

[0024] First, referring to the frequency response characteristics of a conventional optical receiver in Figure 9, the 3dB bandwidth at an inter-element distance of 300 μm is 51.0 GHz, which is above the Nyquist frequency of a 100 Gbaud signal (50 GHz). However, it can be seen that as the inter-element distance changes, the frequency of the 3dB bandwidth decreases and falls below 50 GHz. On the other hand, in the proposed optical receiver in this disclosure, that is, in an optical receiver using a high-frequency wiring board in which the impedance components of lines 223a and 223b are changed according to the inter-element distance, the decrease in the frequency of the 3dB bandwidth due to changes in the inter-element distance is smaller compared to wire mounting, and the frequency of the 3dB bandwidth is above 50 GHz at any inter-element distance. Thus, it can be seen that the proposed optical receiver in this disclosure can achieve a bandwidth of above 50 GHz, which is the Nyquist frequency of a 100 Gbaud signal.

[0025] Next, referring to the frequency characteristics at 100 Gbaud in Figure 10, in conventional optical receivers, the frequency required to secure a given response such as a 3 dB bandwidth decreases in response to changes in the inter-element distance, but such a frequency decrease is not observed in the proposed optical receiver in this disclosure.

[0026] From the above, it has been shown that the proposed optical receiver in this disclosure can suppress the decrease in frequency required to secure a given frequency response due to increased inter-element distance and variations in inter-element distance that may occur during mounting, compared to conventional optical receivers. Furthermore, although verification was performed with baud rates of 100 Gbaud and 50 Gbaud in this embodiment, similar effects can be obtained by redesigning the high-frequency wiring board according to the applied baud rate.

[0027] (Embodiment 2) An optical receiver according to Embodiment 2 of this disclosure will be described with reference to Figure 3. Figure 3(a) is a top view of the optical receiver 300 of this embodiment, and Figure 3(b) is a cross-sectional view of the optical receiver 300. Figure 3(c) is a bottom view of the high-frequency wiring board 222, and Figure 3(d) is a cross-sectional view of the high-frequency wiring board 222.

[0028] The optical receiver 300 shown in Figure 3 differs from the optical receiver 200 described with reference to Figure 2, which is configured to match the height of the upper surface of the PD 101, which is bridged by a high-frequency wiring board, and the upper surface of the bias circuit 102, by having a step provided on the upper surface of the PD subcarrier 303. In this configuration, the height of the PD 101, which is placed on the upper surface of the PD subcarrier 203 without a step, is matched by polishing. In this embodiment, there is no need to polish the PD, so the operational reliability of the PD is not compromised. The PD subcarrier 303, on which the back-incident type PD 101 is mounted, has an opening 303a formed by a through hole.

[0029] The other configurations of the optical receiver 300 are the same as those of the optical receiver 200. The optical receiver 300 in this embodiment also has the same effects as the optical receiver 200 in Embodiment 1.

[0030] (Embodiment 3) An optical receiver according to Embodiment 3 of this disclosure will be described with reference to Figure 4. Figure 4(a) is a top view of the optical receiver 400 of this embodiment, and Figure 4(b) is a cross-sectional view of the optical receiver 400. Figure 4(c) is a bottom view of the high-frequency wiring board 222, and Figure 4(d) is a cross-sectional view of the high-frequency wiring board 222.

[0031] The optical receiver 400 shown in Figure 4 differs from the optical receiver 300 described with reference to Figure 3 in that the bias circuit 102 (capacitor), PD 101, and TIA 104 are mounted on a single subcarrier 403. This configuration reduces the number of components used compared to the configurations of embodiments 1 and 2. The subcarrier 403 is a dielectric carrier. A step provided on the upper surface of the subcarrier 403 allows the upper surface of the PD 101, which is bridged by the high-frequency wiring board, to be at the same height as the upper surface of the bias circuit 102. The PD subcarrier 403, on which the back-injection type PD 101 is mounted, has an opening 403a provided by a through hole.

[0032] The other configurations of the optical receiver 400 are the same as those of the optical receiver 200. The optical receiver 400 in this embodiment also has the same effects as the optical receiver 200 in Embodiment 1.

[0033] (Embodiment 4) An optical receiver according to Embodiment 4 of this disclosure will be described with reference to Figure 5. Figure 5(a) is a top view of the optical receiver 500 of this embodiment, and Figure 5(b) is a cross-sectional view of the optical receiver 500. Figure 5(c) is a bottom view of the high-frequency wiring board 222, and Figure 5(d) is a cross-sectional view of the high-frequency wiring board 222.

[0034] The optical receiver 500 shown in Figure 5 is configured with a front-incident PD501 instead of the back-incident PD201 in the optical receiver 300 described with reference to Figure 3. As shown in Figure 5, the PD subcarrier 503 of the optical receiver 500 does not require an opening for signal light to pass through via a through-hole, thus reducing the mounting process. The subcarrier 503 is a dielectric carrier. A step provided on the upper surface of the subcarrier 503 is configured to match the height of the upper surface of the PD501, which is bridged by the high-frequency wiring board, with the upper surface of the bias circuit 102.

[0035] The other configurations of the optical receiver 500 are the same as those of the optical receiver 300. The optical receiver 500 in this embodiment also has the same effects as the optical receiver 300 of Embodiment 2, that is, the same effects as the optical receiver 200 of Embodiment 1.

[0036] (Embodiment 5) An optical receiver according to Embodiment 5 of this disclosure will be described with reference to Figure 6. Figure 6(a) is a top view of the optical receiver 600 of this embodiment, and Figure 6(b) is a cross-sectional view of the optical receiver 600. Figure 6(c) is a bottom view of the high-frequency wiring board 222, and Figure 6(d) is a cross-sectional view of the high-frequency wiring board 222.

[0037] The optical receiver 600 shown in Figure 6 is a configuration of the optical receiver 300 described with reference to Figure 3, in which a metal wire 106a is used for the connection between the bias circuit 102 (capacitor) and PD101, and a high-frequency wiring board 222 is used for the connection between PD101 and TIA104. The height of TIA105 is adjusted to align the top surface of PD101 with the top surface of TIA104.

[0038] In the configuration of the optical receiver 600, connection is easy even if the spacing of the bumps 221 on the top surface of the PD101 and the TIA104 are different. For example, by tapering the patterns of lines 223a and 223b formed on the back surface of the high-frequency wiring board 222 according to the spacing of the bumps 221, the PD101 and the TIA104 can be easily connected.

[0039] The other configurations of the optical receiver 600 are the same as those of the optical receiver 300. The optical receiver 600 in this embodiment also has the same effects as the optical receiver 300 of Embodiment 2, that is, the same effects as the optical receiver 200 of Embodiment 1.

[0040] (Embodiment 6) An optical receiver according to Embodiment 6 of this disclosure will be described with reference to Figure 7. Figure 7(a) is a top view of the optical receiver 700 of this embodiment, and Figure 7(b) is a cross-sectional view of the optical receiver 700. Figure 7(c) is a bottom view of a part of the high-frequency wiring board 222, and Figure 7(d) is a cross-sectional view of the high-frequency wiring board 222.

[0041] The optical receiver 700 shown in Figure 7 is a configuration in which the connection between PD101 and TIA104 in the optical receiver 300 described with reference to Figure 3 is made using a high-frequency wiring board 222 instead of a metal wire 106b. The height of TIA105 is adjusted to align the top surface of PD101 with the top surface of TIA104. The Y-direction length of the high-frequency wiring board 222 used for the connection between PD101 and bias circuit 102 (capacitor) in the optical receiver 300 is increased. A single high-frequency wiring board 222 is placed on the top surfaces of the bias circuit 102, PD101, and TIA104. Bumps 221 on the top surface of the bias circuit 102 and bumps 221 on the top surface of the PD101 are connected by lines 223a and 223b, and bumps 221 on the top surface of the PD101 and bumps 221 on the top surface of the TIA104 are connected by lines 223a and 223b. Lines 223a and 223b connecting the bumps 221 on the bias circuit 102 side and the bumps 221 on the TIA104 side are not provided on the back surface of the single high-frequency wiring board 222.

[0042] The optical receiver 700 shown in Figure 7 can reduce the number of components for electrical connection between elements and the number of steps for mounting. In this embodiment, if a surface-incident type PD501 (Figure 5) is used instead of a back-incident type PD201, the signal light may be incident on the light-receiving surface of the APD501 by providing an opening in the center of the high-frequency wiring board 222 through through holes that penetrate the top and bottom surfaces. Since the current distribution in the lines 223a and 223b of the high-frequency wiring board 222 is concentrated at the electrode edges (around the bumps 221), there is almost no degradation of the high-frequency electrical signal due to creating through holes in the center of the high-frequency wiring board 222.

[0043] The other configurations of the optical receiver 700 are the same as those of the optical receiver 300. The optical receiver 700 in this embodiment also has the same effects as the optical receiver 300 of Embodiment 2, that is, the same effects as the optical receiver 200 of Embodiment 1.

[0044] (Embodiment 7) An optical receiver according to Embodiment 7 of this disclosure will be described with reference to Figure 8. Figure 8(a) is a top view of the optical receiver 800 of this embodiment, and Figure 8(b) is a cross-sectional view of the optical receiver 800. Figure 8(c) is a bottom view of the high-frequency wiring boards 222a and 222b, and Figure 8(d) is a cross-sectional view of the high-frequency wiring boards 222a and 222b.

[0045] The optical receiver 800 shown in Figure 8 is configured such that the connection between PD101 and TIA104 in the optical receiver 300 described with reference to Figure 3 is made using a high-frequency wiring board 222b instead of a metal wire 106b. The high-frequency wiring board 222a corresponds to the high-frequency wiring board 222 in the optical receiver 300 described with reference to Figure 3. In the optical receiver 800, the height of TIA105 is adjusted to align the top surface of PD101 and the top surface of TIA104 in the optical receiver 300.

[0046] In the optical receiver 800, the high-frequency wiring board 222b connecting the PD101 and the TIA104 can be constructed using a different material than the material used to construct the high-frequency wiring board 222 connecting the bias circuit 102 and the PD101. Furthermore, if a front-incident type PD501 is used instead of a back-incident type PD101, it is not necessary to drill through holes in the PD subcarrier 303, thus reducing the number of steps required for mounting.

[0047] The other configurations of the optical receiver 800 are the same as those of the optical receiver 300. The optical receiver 800 in this embodiment also has the same effects as the optical receiver 300 of Embodiment 2, that is, the same effects as the optical receiver 200 of Embodiment 1. [Industrial applicability]

[0048] An optical receiver is provided that is less susceptible to degradation of its operating bandwidth even when the distance between the elements constituting the optical receiver changes. [Explanation of Symbols]

[0049] 100, 200, 300, 400, 500, 600, 700, 800 optical receiver 101, 501 Photodiode (PD) 102 Bias circuit (capacitor) 103, 203, 303 PD subcarrier 103a, 303a, 403a opening (through hole) 104 Transimpedance Amplifier (TIA) 105 TIA Subcarrier 106a, 106b wires 221 Bump 222, 222a, 222b High-Frequency Printed Circuit Boards 223a, 223b tracks 224 Dielectrics 403 Subcarrier 503 PD Subcarrier

Claims

1. Photodetector (PD), A bias circuit that applies a bias voltage, Transimpedance amplifier (TIA) and, The PD, the bias circuit, and at least one subcarrier on which the TIA is mounted, At least one high-frequency wiring board connecting the upper surface of the PD and the upper surface of the bias circuit, and the upper surface of the PD and the upper surface of the TIA. Equipped with an optical receiver.

2. The lower surfaces of the bias circuit, the PD, and the TIA face the upper surface of at least one subcarrier. The optical receiver according to claim 1, wherein the upper surface of the bias circuit, the upper surface of the PD, and the upper surface of the TIA are connected by a single high-frequency wiring board.

3. The lower surfaces of the bias circuit, the PD, and the TIA face the upper surface of the at least one subcarrier. The upper surface of the bias circuit and the upper surface of the PD are connected by a first high-frequency wiring board. The optical receiver according to claim 1, wherein the upper surface of the PD and the upper surface of the TIA are connected by a second high-frequency wiring board separate from the first high-frequency wiring board.

4. Photodetector (PD), A bias circuit that applies a bias voltage, Transimpedance amplifier (TIA) and, The PD, the bias circuit, and at least one subcarrier on which the TIA is mounted, A high-frequency wiring board connecting the upper surface of the PD and the upper surface of the bias circuit, A wire connecting the upper surface of the PD and the upper surface of the TIA, Equipped with an optical receiver.

5. The optical receiver according to any one of claims 1 to 4, wherein the PD is a back-surface incidence type PD.

6. The optical receiver according to any one of claims 1 to 4, wherein the PD is a surface-incident type PD.

7. The bias circuit and the PD are installed on the upper surface of the first subcarrier. The TIA is installed on the upper surface of the second subcarrier. The optical receiver according to any one of claims 1 to 4, wherein the first subcarrier and the second subcarrier are separated.

8. The optical receiver according to any one of claims 1 to 4, wherein the bias circuit, the PD, and the TIA are installed on a single subcarrier.

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