Light detection circuit

Abstract

The present invention provides an optical detection circuit that can prevent reflected light from entering the optical waveguide. [Solution] The light detection means detects light incident on the first and second light-receiving surfaces. The light guiding means splits the light incident on the first optical waveguide into first and second branched light and emits them to the first and second light-receiving surfaces, respectively, and prevents the first and second reflected light, generated by the reflection of the first and second branched light by the first and second light-receiving surfaces, respectively, from entering the first optical waveguide.

Description

Technical Field

[0001] The present disclosure relates to a photodetection circuit.

Background Art

[0002] In an integrated optical circuit fabricated using silicon photonics technology or the like, in order to control the operation of the circuit, a large number of waveguide-type photodiodes (hereinafter referred to as PDs) are provided to monitor the light intensity in the circuit. For example, Patent Document 1 proposes an optical circuit that monitors light output from a light source.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] When the refractive index of the medium is different between the PD and the optical waveguide, when light enters the PD from the optical waveguide, reflected light is generated. It is known that when the reflected light from the PD propagates in the opposite direction through the optical waveguide and enters other optical components such as a light source, it affects the operation of the optical integrated circuit. In particular, in a wavelength tunable laser, problems such as destabilization of laser oscillation and inability to perform continuous wavelength tuning occur when the reflected light returns to the light source.

Means for Solving the Problems

[0005] A photodetection circuit according to an embodiment includes a first optical waveguide, a photodetection unit that detects light incident on first and second light receiving surfaces, and a branching unit that branches light incident from the first optical waveguide into first and second branched lights and emits them to the first and second light receiving surfaces, respectively, and an optical waveguide unit that prevents the first and second reflected lights generated by reflection of the first and second branched lights by the first and second light receiving surfaces, respectively, from entering the first optical waveguide. [Effects of the Invention]

[0006] According to one embodiment, it is possible to provide an optical detection circuit that can prevent reflected light from entering the optical waveguide from entering the optical waveguide. [Brief explanation of the drawing]

[0007] [Figure 1] This figure shows an example configuration of a tunable light source. [Figure 2] This figure schematically shows the configuration of a photodetection circuit according to one embodiment. [Figure 3] This figure schematically shows the configuration of a photodetection circuit according to one embodiment. [Figure 4] This figure schematically shows an example of the configuration of a light detection unit according to one embodiment. [Figure 5] This diagram shows the round-trip optical path length of an optical waveguide. [Figure 6] This figure schematically shows the configuration of a photodetection circuit according to one embodiment. [Figure 7] This diagram shows the summing and demultiplexing of light waves in a 2x2 optical coupler. [Figure 8] This figure schematically shows the configuration of a photodetection circuit according to one embodiment. [Figure 9] This figure schematically shows the configuration of a photodetection circuit according to one embodiment. [Modes for carrying out the invention]

[0008] Embodiments of the present invention will be described below with reference to the drawings. In each drawing, the same elements are denoted by the same reference numerals, and redundant explanations are omitted where necessary.

[0009] When we refer to one embodiment below, it means that it is applicable to any of the embodiments described below, or to a combination of two or more embodiments, and that its application is not limited to a specific embodiment.

[0010] Embodiment 1 A photodetection circuit according to Embodiment 1 will be described. The photodetection circuit is used, for example, to monitor the intensity of light in various optical circuits. Below, an example of the use of the photodetection circuit in a tunable light source will be described. Figure 1 is a diagram showing an example of the configuration of a tunable light source. The tunable light source 1000 has a semiconductor optical amplifier (hereinafter referred to as SOA) 1001 for laser oscillation, a tunable filter 1002, a wavelength rocker 1003, a semiconductor amplifier BOA (Booster SOA) 1004 for optical output, and photodetection circuits 1010 and 1020.

[0011] In this configuration, the SOA1001 and the tunable filter 1002 constitute a laser resonator. The optical light LA ​​output from the SOA1001 oscillates back and forth between the SOA1001 and the tunable filter 1002, producing a laser beam LB of a predetermined wavelength. The wavelength rocker 1003 outputs a laser beam LC with a fixed wavelength based on the input laser beam LB to the BOA1004. The BOA1004 amplifies the laser beam LC before outputting it. As a result, the tunable light source 1000 can supply a laser beam LC of a desired wavelength to other optical devices.

[0012] In this case, the tunable light source 1000 is required to monitor the intensity of the propagating light in order to control the operation of, for example, the SOA 1001, the tunable filter 1002, and the wavelength rocker 1003. In this example, the photodetection circuits 1010 and 1020 detect the laser light LB from the tunable filter 1002 and the laser light LC from the wavelength rocker 1003, respectively. Then, for example, the control unit 1005 controls the operation of the SOA 1001, the tunable filter 1002, and the wavelength rocker 1003 by providing control signals CON1 to CON3 according to the detection results D1 and D2 from the photodetection circuits 1010 and 1020.

[0013] The photodetection circuit will now be described. Figure 2 is a schematic diagram showing the configuration of a photodetection circuit according to one embodiment. The photodetection circuit 100 in Figure 2 is used as the photodetection circuits 1010 and 1020 described above. The photodetection circuit 100 has an optical waveguide 1, an optical waveguide section 2, and a photodetection section 3.

[0014] The optical waveguide unit 2 is configured as an optical circuit that guides the light L propagating through the optical waveguide 1 to the photodetector unit 3. Hereinafter, the optical waveguide 1 will also be referred to as the first optical waveguide. The light L input from the optical waveguide 1 to the optical waveguide unit 2 is branched into branched light SL1 and SL2 by the optical waveguide 1. Here, it is assumed that the light L is divided into two equal parts, branched light SL1 and SL2. However, the branching ratio of the light L is not limited to 1:1 and may be any branching ratio. The photodetector unit 3 receives the branched light SL1 and SL2 incident on the light-receiving surfaces 31A and 32A, respectively. The photodetector unit 3 then outputs a detection signal DET indicating the light reception result. Hereinafter, the branched light SL1 and SL2 will also be referred to as the first and second branched light, respectively. The light-receiving surfaces 31A and 32A will also be referred to as the first and second light-receiving surfaces, respectively.

[0015] The branched light beams SL1 and SL2 are received by the light-receiving surfaces 31A and 32A, but some of them are reflected. As a result, the reflected light beams RL1 and RL2 of the branched light beams SL1 and SL2 return to the optical waveguide section 2. If the reflected light beams RL1 and RL2 pass through the optical waveguide section 2 and propagate in the reverse direction along the optical waveguide 1, it may adversely affect the operation of other optical components connected to the optical waveguide 1. In the example in Figure 1, for example, if reflected light returns from the photodetector circuit to the wavelength filter or SOA, problems such as instability in laser oscillation may occur. Therefore, the optical waveguide section 2 is configured so that the reflected light beams RL1 and RL2 do not enter the optical waveguide 1. Hereinafter, the reflected light beams RL1 and RL2 will also be referred to as the first and second reflected light beams, respectively.

[0016] The configuration of the optical detection circuit 100 will be described in more detail. FIG. 3 is a diagram schematically showing the configuration of an optical detection circuit according to an embodiment. The optical waveguide section 2 includes a 1×2 optical coupler 20, optical waveguides 21 and 22. The input port P11 of the 1×2 optical coupler 20 is connected to the optical waveguide 1. Thereby, light L is incident on the input port P11 through the optical waveguide 1. The output port P21 of the 1×2 optical coupler 20 is connected to the optical waveguide 21. The output port P22 of the 1×2 optical coupler 20 is connected to the optical waveguide 22. Hereinafter, the optical waveguides 21 and 22 will also be referred to as the second and third optical waveguides, respectively. Hereinafter, the output ports P21 and P22 will also be referred to as the first and second output ports, respectively.

[0017] In the optical waveguide 21, a tapered portion 21B whose width continuously increases toward the optical detection unit 3 is connected to a uniform-width optical waveguide 21A extending from the output port P21 of the 1×2 optical coupler 20 toward the optical detection unit 3. Similarly, in the optical waveguide 22, a tapered portion 22B whose width continuously increases toward the optical detection unit 3 is connected to a uniform-width optical waveguide 22A extending from the output port P22 of the 1×2 optical coupler 20 toward the optical detection unit 3.

[0018] The 1×2 optical coupler 20 branches the light L into branched light SL1 and branched light SL2. The 1×2 optical coupler 20 emits the branched light SL1 from the output port P21 to the optical waveguide 21. The 1×2 optical coupler 20 emits the branched light SL2 from the output port P22 to the optical waveguide 22. The branched lights SL1 and SL2 propagate through the optical waveguides 21 and 22, respectively, and are incident on the light receiving surfaces 31A and 32A of the optical detection unit 3, respectively.

[0019] The optical detection unit 3 includes a detection circuit 30, photodiodes 31 and 32. Hereinafter, the photodiodes will be denoted as PDs. The PDs 31 and 32 may use various types of PDs such as PIN (P-intrinsic-N), avalanche PD, and waveguide-type PD.

[0020] PD31 is connected to the tapered portion 21B of the optical waveguide 21. Branched light SL1 is incident on the light-receiving surface 31A of PD31 via the optical waveguide 21. PD31 converts the branched light SL1 incident on the light-receiving surface 31A into a current signal. PD32 is connected to the tapered portion 22B of the optical waveguide 22. Branched light SL2 is incident on the light-receiving surface 32A of PD32 via the optical waveguide 22. PD32 converts the branched light SL2 incident on the light-receiving surface 32A into a current signal.

[0021] The detection circuit 30 is provided with an anode pad 30A, which is the positive electrode, and a cathode pad 30B, which is the negative electrode. The anode and cathode of PD31 are connected to wirings W11 and W21, respectively. The anode and cathode of PD32 are connected to wirings W12 and W22, respectively. Wirings W11 and W12 are connected to the anode pad 30A via wiring W1. Wirings W12 and W22 are connected to the cathode pad 30B via wiring W2. As a result, the detection circuit 30 can receive the sum of the current signals output by PD31 and PD32. The detection circuit 30 sums the received current signals to detect the total intensity of the branched light SL1 and SL2, i.e., the intensity of light L. The detection circuit 30 then outputs a detection signal DET indicating the intensity of the incident light.

[0022] As described above, the photodetector circuit 100 uses the optical coupler 20 to split the light L, but it can receive a current signal of the same magnitude as when the light L is received by a general photodetector circuit composed of a single photodiode. This makes it possible to achieve the anti-reflection effect described later while maintaining the same optical circuit loss as a general photodetector circuit using a single photodiode.

[0023] Regarding the wiring in the light detection unit 3, multilayer wiring is used to avoid duplication while providing wiring. Figure 4 is a schematic diagram showing an example of the configuration of a light detection unit according to one embodiment. As shown in Figure 4, the detection circuit 30, PD31 and 32 are mounted on the main surface of the substrate 34. Wirings W1, W11 and W12 are formed on the main surface of the substrate 34. Wiring W21 is led from the main surface of the substrate 34 to the bottom surface of the substrate 34 via via V1. Wiring W22 is led from the main surface of the substrate 34 to the bottom surface of the substrate 34 via via V2. Wirings W21 and W22 on the bottom surface of the substrate 34 are connected to wiring W2 formed on the bottom surface of the substrate 34. Wiring W2 formed on the bottom surface of the substrate 34 is connected to cathode pad 30B formed on the main surface of the substrate 34 via via V3. In Figure 4, the parts of wiring W2, W21 and W22 formed on the bottom surface of the substrate 34 are shown with dashed lines. Hereinafter, the main surface of the substrate 34 will also be referred to as the first surface. The bottom surface, which is the surface opposite the main surface of the substrate 34, will also be referred to as the second surface. Wirings W12 and W21 will also be referred to as the first and second wirings, respectively.

[0024] Thus, even when there are overlapping wirings such as wiring W12 and wiring W21 when the light detection unit 3 is viewed from the normal direction of the substrate 34, the PD31 and 32 and the detection circuit 30 can be efficiently connected without rerouting the wiring on the substrate 34 by forming the two intersecting wirings on separate layers.

[0025] Next, we will explain how the optical waveguide 2 prevents reflected light RL1 and RL2 from entering the optical waveguide 1. In the optical detection circuit 100, the branched light SL1 guided from the optical waveguide 21 to PD31 is reflected by the light-receiving surface 31A of PD31. As a result, the reflected light RL1 enters the output port P21 of the 1x2 optical coupler 20 through the optical waveguide 21. Also, the branched light SL2 guided from the optical waveguide 22 to PD32 is reflected by the light-receiving surface 32A of PD32. As a result, the reflected light RL2 enters the output port P22 of the 1x2 optical coupler 20 through the optical waveguide 22.

[0026] The light detection circuit 100 is configured such that the phases of the reflected light RL1 and RL2 incident on the 1×2 optical coupler 20 are inverted relative to each other. Here, the round-trip optical path lengths when light travels back and forth through optical waveguides 21 and 22 are denoted as L1 and L2, respectively. Figure 5 is a diagram showing the round-trip optical path lengths of the optical waveguides. In the optical waveguide section 2, the lengths of optical waveguides 21 and 22 may be made different such that the difference between the round-trip optical path length L1 of optical waveguide 21 and the round-trip optical path length L2 of optical waveguide 22 corresponds to an optical path length equal to a phase difference of half a period of light L.

[0027] For the sake of simplicity, we will assume here that the difference between the round-trip optical path length L1 of optical waveguide 21 and the round-trip optical path length L2 of optical waveguide 22 corresponds to the optical path length corresponding to the phase difference of half a period of light L. However, the difference between the round-trip optical path lengths L1 and L2 is not limited to this. If n is an integer, the difference between the round-trip optical path lengths L1 and L2 may also correspond to the optical path length obtained by adding the phase difference of 180° of half a period of light L to an optical path length that is an integer multiple of the period (180° + n × 360°).

[0028] As a result, the reflected light RL1 incident on the output port P21 of the 1x2 optical coupler 20 and the reflected light RL2 incident on the output port P22 of the 1x2 optical coupler 20 have their phases inverted relative to each other. Therefore, when the 1x2 optical coupler 20 combines the reflected light RL1 and the reflected light RL2, the reflected light RL1 and the reflected light RL2 cancel each other out. Consequently, it is possible to prevent the incident of reflected light on the input port P11 of the 1x2 optical coupler 20.

[0029] Here, we have explained that the lengths of optical waveguides 21 and 22 are made different to create a difference between the round-trip optical path length L1 and the round-trip optical path length L2, but this is merely an example. Other methods may be used as long as the desired difference between the round-trip optical path length L1 and the round-trip optical path length L2 can be achieved. For example, the difference between the round-trip optical path length L1 and the round-trip optical path length L2 can be created by adjusting the refractive index by making the waveguide widths of optical waveguides 21 and 22 different. In general, the propagation mode of light changes depending on the waveguide width, and the effective refractive index of the optical waveguide changes depending on the propagation mode. Therefore, by making the waveguide widths different, the propagation mode of light traveling back and forth in optical waveguide 21 and the propagation mode of light traveling back and forth in optical waveguide 22 can be made different. As a result, a phase difference can be created between the light traveling back and forth in optical waveguide 21 and the light traveling back and forth in optical waveguide 22. Alternatively, for example, the refractive index may be adjusted by heating the optical waveguides 21 and 22 with heaters, thereby creating a difference between the round-trip optical path length L1 and the round-trip optical path length L2. Alternatively, the refractive index may be adjusted by applying a voltage to electrodes provided in the optical waveguides 21 and 22, thereby creating a difference between the round-trip optical path length L1 and the round-trip optical path length L2.

[0030] Therefore, the light detection circuit 100 can prevent adverse effects caused by reflected light entering other optical components connected to the optical waveguide 1.

[0031] For example, if a laser device or a semiconductor optical amplifier is connected to the optical waveguide 1, reflected light entering these devices may cause instability in the laser oscillation. In contrast, the optical detection circuit 100 can prevent reflected light from entering the laser device or semiconductor optical amplifier through the optical waveguide 1, thereby maintaining a stable laser oscillation.

[0032] Embodiment 2 A photodetection circuit according to Embodiment 2 will now be described. Figure 6 is a schematic diagram showing the configuration of a photodetection circuit according to one embodiment. The photodetection circuit 200 has a configuration in which the optical waveguide section 2 in the photodetection circuit 100 is replaced with an optical waveguide section 4.

[0033] The optical waveguide section 4, like the optical waveguide section 2, is configured as an optical circuit that guides the light L propagating through the optical waveguide 1 to the photodetector section 3. The optical waveguide section 4 includes a 2x2 optical coupler 40, optical waveguides 41-43, and an optical terminator 44.

[0034] The 2x2 optical coupler 40 is an optical coupler having two input ports P11 and P12 and two output ports P21 and P22. Input port P11 is connected to optical waveguide 1. As a result, light L is incident on input port P11 through optical waveguide 1. Input port P12 is connected to optical terminator 44 via optical waveguide 43. Output port P21 and PD31 are connected by optical waveguide 41. Output port P22 and PD32 are connected by optical waveguide 42. Hereinafter, optical waveguides 41 and 42 will also be referred to as the second and third optical waveguides, respectively, as with optical waveguides 21 and 22. Input ports P11 and P12 will also be referred to as the first and second input ports, respectively.

[0035] Optical waveguides 41 and 42 have the same optical path length and, like optical waveguides 21 and 22, are composed of an optical waveguide of uniform width and a tapered section. Specifically, in optical waveguide 41, a tapered section 41B, whose width continuously increases toward PD31, is connected to an optical waveguide 41A of uniform width extending from the output port P21 of the 2x2 optical coupler 40 toward PD31. In optical waveguide 42, a tapered section 42B, whose width continuously increases toward PD32, is connected to an optical waveguide 42A of uniform width extending from the output port P22 of the 2x2 optical coupler 40 toward PD32.

[0036] The 2x2 optical coupler 40 splits the optical light L into branched light SL1 and branched light SL2. The 2x2 optical coupler 40 emits branched light SL1 from its output port P21 into the optical waveguide 41. Branched light SL1 propagates through the optical waveguide 41 and is incident on the light-receiving surface 31A of PD31. The 2x2 optical coupler 40 also emits branched light SL2 from its output port P22 into the optical waveguide 42. Branched light SL2 propagates through the optical waveguide 42 and is incident on the light-receiving surface 32A of PD32.

[0037] In the optical detection circuit 200, the optical waveguide 4 is configured to prevent reflected light from entering the optical waveguide 1, similar to the optical waveguide 2. Figure 7 shows the summing and desaturation of light in a 2x2 optical coupler.

[0038] The 2x2 optical coupler 40 splits the optical fiber L into branched optical fiber SL1 and branched optical fiber SL2. At this time, branched optical fiber SL2, which is output from output port P22 (a cross port), is phase-delayed by 1 / 4 of a period compared to branched optical fiber SL1, which is output from output port P21 (a through port).

[0039] Subsequently, similar to the optical waveguide section 2, the branched light SL1 and SL2 are reflected by the light-receiving surfaces 31A and 32A, respectively, generating reflected light RL1 and RL2. The reflected light RL1 and RL2 return to the output ports P21 and P22 of the 2x2 optical coupler 40 through the optical waveguides 41 and 42, respectively.

[0040] The 2x2 optical coupler 40 combines the reflected light RL1 and the reflected light RL2. At this time, the reflected light RL1 and RL2 are separated and sent to input ports P11 and P12, respectively. The reflected light RL1 going to input port P12 is delayed by 1 / 4 of a phase relative to the reflected light RL1 going to input port P11. Similarly, the reflected light RL2 going to input port P11 is delayed by 1 / 4 of a phase relative to the reflected light RL2 going to input port P12.

[0041] Therefore, the reflected light RL1 and RL2 heading towards input port P11 cancel each other out because their phases are inverted. As a result, reflected light from input port P11 into optical waveguide 1 can be prevented.

[0042] The reflected light RL1 and RL2 heading toward input port P12 are similarly delayed in phase by 1 / 4 period, and therefore reinforce each other through interference. As a result, the interfered light RL of the reflected light RL1 and RL2 is incident from input port P12 into optical waveguide 43. Subsequently, the interfered light RL is incident on optical termination device 44 through optical waveguide 43. Optical termination device 44 is composed of, for example, an optical absorber and terminates the incident interfered light RL. Hereafter, optical waveguides 41 and 42 will also be referred to as the second and third optical waveguides, respectively, as with optical waveguides 21 and 22. Optical waveguide 43 will also be referred to as the fourth optical waveguide.

[0043] As described above, in this configuration, by utilizing the summing and demultiplexing characteristics of the 2x2 optical coupler 40, the reflected light RL1 and RL2 can be canceled out by interference, preventing the reflected light from entering the optical waveguide 1.

[0044] Furthermore, since the interference light RL produced by the reflected light RL1 and RL2 emitted from the input port P12 is terminated by the optical terminator 44, it is possible to prevent interference with surrounding optical components due to leakage of the interference light RL.

[0045] Embodiment 3 In the above-described embodiment, a photodetection circuit using two photodiodes (PDs) was explained, but it is also possible to configure a photodetection circuit with a reduced number of PDs. Figure 8 is a schematic diagram showing the configuration of a photodetection circuit according to one embodiment. In the photodetection circuit 300, the optical waveguide section 2 and the photodetection section 3 in the photodetection circuit 100 are replaced with the optical waveguide section 5 and the photodetection section 6, respectively.

[0046] The 1x2 optical coupler 50, optical waveguides 51 and 52 of optical waveguide section 5 correspond to the 1x2 optical coupler 20, optical waveguides 21 and 22 of optical waveguide section 2, respectively. Optical waveguide 51A and tapered section 51B of optical waveguide 51 correspond to optical waveguide 21A and tapered section 21B, respectively. Optical waveguide 52A and tapered section 52B of optical waveguide 52 correspond to optical waveguide 22A and tapered section 22B, respectively. Hereinafter, optical waveguides 51 and 52 will also be referred to as the second and third optical waveguides, respectively, similar to optical waveguides 21 and 22.

[0047] The optical detection unit 6 has a detection circuit 60 and a waveguide-type PD 61. The detection circuit 60 corresponds to the 1x2 optical coupler 50 of the optical waveguide unit 5.

[0048] The waveguide-type PD61 is configured to detect light incident on the optical waveguide 61A. Branched light SL1 is incident on the light-receiving surface 61B, which is the end face of the optical waveguide 61A, from the tapered portion 51B. Branched light SL2 is incident on the light-receiving surface 61C, which is the end face of the optical waveguide 61A, from the tapered portion 52B. That is, branched light SL1 and SL2 are incident on the optical waveguide 61A from both end faces, respectively. Therefore, the waveguide-type PD61 can be considered to be receiving light L, which is the sum of branched light SL1 and SL2. Hereinafter, the optical waveguide 61A will also be referred to as the fifth optical waveguide. The light-receiving surfaces 61B and 61C will also be referred to as the first and second light-receiving surfaces, respectively, similar to the light-receiving surfaces 31A and 32A.

[0049] The anode 61D of the waveguide-type PD61 is connected to the anode pad 60A of the detection circuit 60. The cathode 61E of the waveguide-type PD61 is connected to the cathode pad 60B of the detection circuit 60.

[0050] As a result, the detection circuit 60 can receive the current signal output by the waveguide-type PD61. Based on the received current signal, the detection circuit 60 detects the intensity of light L. The detection circuit 60 then outputs a detection signal DET indicating the intensity of light L.

[0051] As described above, the light detection circuit 300 can detect light L in the same way as the light detection circuit 100 while reducing the number of PDs.

[0052] Furthermore, in the optical detection circuit 300, the optical waveguide section 5 has the same configuration as the optical waveguide section 2, and therefore, it can similarly prevent reflected light from entering the optical waveguide 1.

[0053] Although the photodetection circuit 300 has been described as a modified version of the photodetection circuit 100, the number of photodiodes (PDs) in the photodetection circuit 200 may be reduced. Figure 9 is a schematic diagram showing the configuration of a photodetection circuit according to one embodiment. In the photodetection circuit 301, the optical waveguide section 4 and the photodetection section 3 in the photodetection circuit 200 are replaced with the optical waveguide section 7 and the photodetection section 6, respectively.

[0054] The 2x2 optical coupler 70, optical waveguide 73, and optical terminator 74 of optical waveguide section 7 correspond to the 2x2 optical coupler 40, optical waveguide 43, and optical terminator 44 of optical waveguide section 4, respectively. The optical waveguides 71 and 72 of optical waveguide section 7 correspond to 51 and 52 of optical waveguide section 5, respectively. The optical waveguide 71A and tapered section 71B of optical waveguide 71 correspond to optical waveguide 71A and tapered section 71B, respectively. The optical waveguide 72A and tapered section 72B of optical waveguide 72 correspond to optical waveguide 52A and tapered section 52B, respectively. Hereafter, optical waveguides 71 and 72 will also be referred to as the second and third optical waveguides, respectively, similar to optical waveguides 21 and 22. Optical waveguide 73 will also be referred to as the fourth optical waveguide, similar to optical waveguide 43.

[0055] Since the light detection unit 6 is the same as the light detection circuit 300, a redundant explanation will be omitted.

[0056] Therefore, in the light detection circuit 301 as well, the detection circuit 60 can output a detection signal DET indicating the intensity of the detected light L based on the received current signal.

[0057] Therefore, the photodetection circuit 301 can detect light L in the same way as the photodetection circuit 200 while reducing the number of PDs. Furthermore, since the optical waveguide section 7 in the photodetection circuit 301 has the same configuration as the optical waveguide section 4, it can similarly prevent reflected light from entering the optical waveguide 1.

[0058] Furthermore, since the photodetection circuit according to this embodiment uses only one photodiode (PD), it is possible to avoid the influence of manufacturing errors in the PD on the detection of the branched light SL1 and SL2, compared to the case where two PDs are used as in Embodiments 1 and 2. As a result, the photodetection circuit according to this embodiment can detect the intensity of light L with greater accuracy.

[0059] Other embodiments Although the present disclosure has been described above with reference to embodiments, the present disclosure is not limited to the embodiments described above. Various modifications to the structure and details of the present disclosure can be made as can be understood by those skilled in the art within the scope of the present disclosure. Furthermore, each embodiment can be combined with other embodiments as appropriate.

[0060] In the above-described embodiment, the means for combining and decomposing in the optical waveguide section was described as an optical coupler. However, various other means for combining and decomposing, such as directional couplers, may be used as long as similar optical combining and decomposing is possible.

[0061] In the embodiments described above, the photodetector circuit was explained as being used in a tunable light source, but this is merely an example. The photodetector circuit according to the embodiments described above may be applied to any optical circuit other than a tunable light source.

[0062] In the above, when it is stated that optical waveguides or optical components are connected, it means that they are optically connected in a way that allows light to be guided between them. Therefore, it does not indicate that the connected components are in physical contact with each other, nor does it preclude them from being installed at a distance from each other, as long as light can be guided.

[0063] Each drawing is merely illustrative to illustrate one or more embodiments. Each drawing may be associated with one or more other embodiments rather than with only one specific embodiment. As those skilled in the art will understand, various features or steps described with reference to any one drawing can be combined with features or steps shown in one or more other drawings, for example, to create embodiments not explicitly shown or described. Not all features or steps shown in any one drawing to illustrate an exemplary embodiment are necessarily required, and some features or steps may be omitted. The order of steps shown in any of the drawings may be changed as appropriate.

[0064] Some or all of the above embodiments may also be described as follows, but are not limited to the following:

[0065] (Note 1) The first optical waveguide and Light detection means for detecting light incident on the first and second light-receiving surfaces, The optical waveguide includes an optical waveguide means that splits the light incident from the first optical waveguide into first and second branched light and emits them to first and second light-receiving surfaces, respectively, and prevents the first and second reflected light, generated by the reflection of the first and second branched light by the first and second light-receiving surfaces, respectively, from entering the first optical waveguide. Light detection circuit.

[0066] (Note 2) The optical wave guiding means is An optical combining and demultiplexing means having an input port and first and second output ports, which splits the light incident on the input port from the first optical waveguide into first and second branched light and outputs them from the first and second output ports, respectively. A second optical waveguide that guides the first branched light from the first output port to the first light receiving surface, The system comprises a third optical waveguide that guides the second branched light from the second output port to the second light-receiving surface, A difference in optical path length is provided between the optical path length of the second optical waveguide and the third optical waveguide such that the phase of the second reflected light incident on the second output port from the second light-receiving surface through the third optical waveguide is inverted with respect to the first reflected light incident on the first output port from the first light-receiving surface through the second optical waveguide. The optical combining and demultiplexing means combines the first reflected light incident on the first output port with the second reflected light, which has a phase inverted relative to the first reflected light incident on the first output port, so that the first reflected light and the second reflected light cancel each other out. The light detection circuit described in Appendix 1.

[0067] (Note 3) The difference between the optical path length of the second optical waveguide and the optical path length of the third optical waveguide is the optical path length corresponding to half the period of the light. The light detection circuit described in Appendix 2.

[0068] (Note 4) The optical wave guiding means is Optical combining and demultiplexing means having first and second input ports and first and second output ports, which splits the light incident on the first input port from the first optical waveguide into first and second branched light, outputs the first branched light from the first output port, and outputs the second branched light from the second output port, which is delayed by 1 / 4 period in phase with respect to the first branched light due to the splitting, A second optical waveguide that guides the first branched light from the first output port to the first light receiving surface, The system comprises a third optical waveguide that guides the second branched light from the second output port to the second light-receiving surface, The aforementioned photomultiplier / demultiplier means is The first reflected light that enters the first output port from the first light-receiving surface through the second optical waveguide is branched toward the first and second input ports. The second reflected light that enters the second output port from the second light-receiving surface through the third optical waveguide is branched toward the first and second input ports. The first reflected light, which is branched toward the second input port, is delayed in phase by 1 / 4 period relative to the first reflected light branched toward the first input port by the branching by the photomultiplier / demultiplier means. The second reflected light, which is branched toward the first input port, is delayed in phase by 1 / 4 period relative to the second reflected light branched toward the second input port by the branching by the photomultiplier / demultiplier means. The first and second reflected lights, which are branched toward the first input port and whose phases are inverted relative to each other, cancel each other out by interference. The light detection circuit described in Appendix 1.

[0069] (Note 5) The optical wave guiding means is A fourth optical waveguide connected to the second input port, The optical termination means is further connected to the second input port via the fourth optical waveguide, The interference light of the first and second reflected light, which has been branched toward the second input port, is incident on the optical termination means through the fourth optical waveguide. The optical termination means terminates the interfering light. The light detection circuit described in Appendix 4.

[0070] (Note 6) The aforementioned light detection means is A first photodetector that outputs a signal indicating the result of receiving the first branched light incident on the first light-receiving surface, A second photodetector that outputs a signal indicating the result of receiving the second branched light incident on the second photodetector, The system includes a detection means that detects the light by summing the signal output by the first light-receiving element and the signal output by the second light-receiving element. A light detection circuit as described in any one of the appendices 1 to 5.

[0071] (Note 7) The first and second light-receiving elements and the detection means are provided on the first surface of the substrate, The plurality of wires connecting the first and second light-receiving elements and the detection means include the first and second wires, A portion or all of the first wiring is provided on the first surface, and a portion or all of the second wiring is provided on the second surface of the substrate opposite to the first surface, so that when the substrate is viewed from the direction normal to the first surface, the overlapping portions of the first wiring and the second wiring are separated in the direction normal to the first surface. The light detection circuit described in Appendix 6.

[0072] (Note 8) The aforementioned light detection means is A waveguide-type photodiode has a fifth optical waveguide whose end faces on both sides are the first and second light-receiving surfaces, respectively, and outputs a signal indicating the result of receiving the first and second branched light incident on the fifth optical waveguide via the first and second light-receiving surfaces, The system includes a detection means for detecting the sum of the first and second branched light based on the signal output by the waveguide-type photodiode. A light detection circuit as described in any one of the appendices 1 to 5. [Explanation of symbols]

[0073] 1, 21, 22, 41~43, 51, 52, 61A, 71~73 Optical waveguide 2, 4, 5, 7 Optical waveguide 3, 6 Light detection unit 20, 50 1x2 Optical Coupler 21A, 22A, 41A, 42A, 51A, 52A, 71A, 72A Optical waveguide 21B, 22B, 41B, 42B, 51B, 52B, 71B, 72B Tapered section 30, 60 detection circuit 30A, 60A Anode Pads 30B, 60B cathode pads 31, 32 PD 31A, 32A, 61B, 61C light receiving surface 40, 70 2x2 Optical Coupler 44, 74 Optical terminator 61 Waveguide PD 61D Anode 61E Cathode 100, 200, 300, 301, 1010, 1020 Photodetection Circuits 1000 wavelength tunable light source 1001 SOA 1004 BOA 1002 Wavelength Tunable Filter 1003 Wavelength Rocker 1005 Control Unit CON1~CON3 Control Signals D1, D2 detection results DET detection signal L, LA Light LB, LC laser light P11, P12 input ports P21, P22 output ports RL Interferometry RL1, RL2 reflected light SL1, SL2 SL2 Split Light V1~V3 Beer Wiring for W1, W2, W11, W12, W21, and W22.

Claims

1. The first optical waveguide and Light detection means for detecting light incident on the first and second light-receiving surfaces, The optical waveguide includes an optical waveguide means that splits light incident from a first optical waveguide into first and second branched light and emits them to the first and second light-receiving surfaces, respectively, and prevents the first and second reflected light, generated by the reflection of the first and second branched light by the first and second light-receiving surfaces, respectively, from entering the first optical waveguide. Light detection circuit.

2. The optical wave guiding means is An optical combining and demultiplexing means having an input port and first and second output ports, which splits the light incident on the input port from the first optical waveguide into first and second branched light and outputs them from the first and second output ports, respectively. A second optical waveguide that guides the first branched light from the first output port to the first light receiving surface, The system comprises a third optical waveguide that guides the second branched light from the second output port to the second light-receiving surface, A difference in optical path length is provided between the optical path length of the second optical waveguide and the third optical waveguide such that the phase of the second reflected light incident on the second output port from the second light-receiving surface through the third optical waveguide is inverted with respect to the first reflected light incident on the first output port from the first light-receiving surface through the second optical waveguide. The optical combining and demultiplexing means combines the first reflected light incident on the first output port with the second reflected light, which has an inverted phase with respect to the first reflected light incident on the first output port, so that the first reflected light and the second reflected light cancel each other out. The light detection circuit according to claim 1.

3. The difference between the optical path length of the second optical waveguide and the optical path length of the third optical waveguide is the optical path length corresponding to half the period of the light. The light detection circuit according to claim 2.

4. The optical wave guiding means is Optical combining and demultiplexing means having first and second input ports and first and second output ports, which splits the light incident on the first input port from the first optical waveguide into first and second branched light, outputs the first branched light from the first output port, and outputs the second branched light from the second output port, which is delayed by 1 / 4 period in phase with respect to the first branched light due to the splitting, A second optical waveguide that guides the first branched light from the first output port to the first light receiving surface, The system comprises a third optical waveguide that guides the second branched light from the second output port to the second light-receiving surface, The aforementioned photomultiplier / demultiplier means is The first reflected light that enters the first output port from the first light-receiving surface through the second optical waveguide is branched toward the first and second input ports. The second reflected light that enters the second output port from the second light-receiving surface through the third optical waveguide is branched toward the first and second input ports. The first reflected light, which is branched toward the second input port, is delayed in phase by 1 / 4 period relative to the first reflected light branched toward the first input port by the branching by the photomultiplier / demultiplier means. The second reflected light, which is branched toward the first input port, is delayed in phase by 1 / 4 period relative to the second reflected light branched toward the second input port by the branching by the photomultiplier / demultiplier means. The first and second reflected lights, which are branched toward the first input port and whose phases are inverted relative to each other, cancel each other out by interference. The light detection circuit according to claim 1.

5. The optical wave guiding means is A fourth optical waveguide connected to the second input port, The optical termination means is further connected to the second input port via the fourth optical waveguide, The interference light of the first and second reflected light, which has been branched toward the second input port, is incident on the optical termination means through the fourth optical waveguide. The optical termination means terminates the interfering light. The light detection circuit according to claim 4.

6. The aforementioned light detection means is A first photodetector that outputs a signal indicating the result of receiving the first branched light incident on the first light-receiving surface, A second photodetector that outputs a signal indicating the result of receiving the second branched light incident on the second photodetector, The system includes a detection means that detects the light by summing the signal output by the first light-receiving element and the signal output by the second light-receiving element. The light detection circuit according to claim 1 or 2.

7. The first and second light-receiving elements and the detection means are provided on the first surface of the substrate, The plurality of wires connecting the first and second light-receiving elements and the detection means include the first and second wires, A portion or all of the first wiring is provided on the first surface, and a portion or all of the second wiring is provided on the second surface of the substrate opposite to the first surface, so that when the substrate is viewed from the direction normal to the first surface, the overlapping portions of the first wiring and the second wiring are separated in the direction normal to the first surface. The light detection circuit according to claim 6.

8. The aforementioned light detection means is A waveguide-type photodiode has a fifth optical waveguide whose end faces on both sides are the first and second light-receiving surfaces, respectively, and outputs a signal indicating the result of receiving the first and second branched light incident on the fifth optical waveguide via the first and second light-receiving surfaces, The system includes a detection means for detecting the sum of the first and second branched light based on the signal output by the waveguide-type photodiode. The light detection circuit according to claim 1 or 2.

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