Optical characteristic inspection circuit, device and method

Abstract

This optical characteristic inspection circuit (10) comprises, in the stated order, an optical input element (11), an optical branching circuit (12) having a resistor (15), a first optical circuit (13_1) to be inspected that is connected to one output of the optical branching circuit, a second optical circuit (13_2) to be inspected that is connected to the other output of the optical branching circuit, and a light detector (14) that detects the intensity of light transmitted through the first optical circuit (13_1) to be inspected and light transmitted through the second optical circuit (13_2) to be inspected.　Through this configuration, the present invention can provide an optical characteristic inspection circuit whereby the number of steps necessary to inspect an optical characteristic can be reduced.

Description

[Technical field]

[0001] The present invention relates to an optical characteristic inspection circuit, an apparatus, and a method for inspecting an optical waveguide. [Background technology]

[0002] Inspection of optical circuits requires time to align the optical fiber with the optical circuit in order to input light into the optical circuit to be inspected and obtain optical output. For example, when using a normal single-mode fiber, the position of the optical fiber needs to be adjusted with a spatial resolution of 1 μm or less, making it difficult to reduce the number of steps.

[0003] In order to reduce the man-hours required for inspection, it is effective to eliminate at least either the optical input or the optical output. Although it is difficult to eliminate the optical input, it is possible to use the electrical output from a photodetector directly connected to the optical circuit instead of the optical output. Electrical input and output only need to be in electrical contact, and usually alignment with a spatial resolution of 10 μm or better is sufficient, so alignment does not require much time, and by utilizing this, the number of inspection man-hours can be significantly reduced.

[0004] However, photodetectors that are directly connected to optical circuits generally have different characteristics. In particular, germanium photodiodes used in silicon photonics have a large variation in sensitivity between individual photodetectors, so in order to test the characteristics of an optical circuit from the absolute value of the photocurrent, it is necessary to individually calibrate the sensitivity of each germanium photodiode.

[0005] 5, in an optical characteristic inspection circuit 30, a wafer surface optical input element 31_1 such as a grating coupler and an optical detector 34_1 are provided in an optical circuit 33_1 formed on a substrate, and a wafer surface optical input element 31_2 and an optical detector 34_2 are provided in an optical circuit 33_2. In this optical characteristic inspection circuit 30, the difference between the electrical output of the optical detector 34_1 and the electrical output of the optical detector 34_2 is measured to evaluate the waveguide loss of the optical circuits 33_1 and 33_2. However, as described above, if there is variation in characteristics between the optical detectors, the waveguide loss cannot be accurately evaluated.

[0006] Furthermore, if there are variations in characteristics between individual wafer surface light input elements 31_1, 31_2, these characteristic variations are also superimposed on the evaluation results based on the electrical outputs of photodetectors 34_1, 34_2, making it impossible to accurately evaluate the waveguide loss.

[0007] In order to reduce the number of inspection steps for optical circuits, an optical characteristic inspection circuit 40 has been disclosed (Patent Document 1) as shown in Fig. 6. The optical characteristic inspection circuit 40 includes a wafer-surface optical input element 41 and an optical detector 44 that are common to two optical circuits 43_1 and 43_2 formed on a substrate. The wavelength of the light input to this optical characteristic inspection circuit 40 is swept to evaluate the waveguide loss, thereby making it possible to avoid the influence of variations in characteristics between the wafer-surface optical input elements and the optical detectors. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2019-96763 A [Non-patent literature]

[0009] [Non-Patent Document 1] Jens Bolten, Jens Hofrichter, Nikolaj Moll, Sophie Schonenberger, Folkert Horst, Bert J. Offrein, Thorsten Wahlbrink, Thomas Mollenhauer, Heinrich Kurz, ``CMOS compatible cost-efficient fabrication of SOI grating couplers,'' Microelectronic Engineering, Volume 86, Issues 4-6, 2009, Pages 1114-1116, ISSN 0167-9317, https: / / doi.org / 10.1016 / j.mee.2008.11.038. Summary of the Invention [Problem to be solved by the invention]

[0010] However, a grating coupler (GC) used as an optical input port in the optical characteristic inspection circuit 40 has a steep wavelength dependency (Non-Patent Document 1). As a result, as disclosed in Patent Document 1, in the method of sweeping the input optical wavelength, the GC wavelength characteristic, which is the input port, is superimposed on the output waveform from the waveguide, which is the device under test (DUT), and therefore the optical characteristic (waveguide loss) of the waveguide cannot be accurately evaluated.

[0011] As described above, it is difficult to accurately evaluate the optical characteristics (waveguide loss) of the waveguide by directly applying the optical characteristic inspection circuit 40 to on-wafer optical characteristic inspection. [Means for solving the problem]

[0012] In order to solve the above-mentioned problems, the optical characteristic inspection circuit according to the present invention comprises: The optical branching circuit includes, in order, an optical input element, an optical branching circuit having a resistor, a first optical circuit to be inspected connected to one output of the optical branching circuit, a second optical circuit to be inspected connected to the other output of the optical branching circuit, and a photodetector that detects the intensity of light transmitted through the first optical circuit to be inspected and the intensity of light transmitted through the second optical circuit to be inspected, the optical branching circuit having two waveguides, and the resistor is disposed in at least one of the two waveguides. A circuit for optical characteristic inspection and a control unit are provided, and the control unit supplies a voltage to the resistor. Changes in Photocurrent input from the photodetector It is a change of Acquire the photocurrent spectrum and calculating the waveguide losses of the first and second optical circuits to be inspected from the photocurrent spectrum. The present invention is characterized by the above. Furthermore, an optical characteristic inspection method according to the present invention is a method for inspecting optical characteristics using an optical characteristic inspection circuit comprising, in order, an optical input element, an optical branching circuit having a resistor, a first optical circuit to be inspected connected to one output of the optical branching circuit, a second optical circuit to be inspected connected to the other output of the optical branching circuit, and a photodetector for detecting intensities of light transmitted through the first optical circuit to be inspected and the light transmitted through the second optical circuit to be inspected, the optical branching circuit having two waveguides, and the resistor disposed in at least one of the two waveguides, the method comprising the steps of: changing a voltage supplied to the resistor; acquiring a photocurrent due to the light transmitted through the first optical circuit to be inspected and the light transmitted through the second optical circuit to be inspected; acquiring a photocurrent spectrum which is a change in the photocurrent with respect to the change in the voltage; and calculating a waveguide loss per unit length from a difference between a maximum value and a minimum value in the photocurrent spectrum and a difference between the first optical circuit to be inspected and the second optical circuit to be inspected. Effect of the Invention

[0013] According to the present invention, it is possible to provide an optical characteristics inspection circuit, device, and method that can reduce the number of steps required for optical characteristics inspection. [Brief description of the drawings]

[0014] [Figure 1] FIG. 1 is a block diagram showing the configuration of an optical characteristic inspection apparatus according to a first embodiment of the present invention. [Diagram 2] FIG. 2 is a schematic diagram showing a configuration example of an optical characteristic inspection circuit according to a first embodiment of the present invention. [Diagram 3] FIG. 3 is a diagram showing an example of an output of the optical characteristic inspection circuit according to the first embodiment of the present invention. [Figure 4] FIG. 4 is a schematic diagram showing a configuration example of an optical characteristic inspection circuit according to a second embodiment of the present invention. [Diagram 5] FIG. 5 is a schematic diagram showing a configuration example of a conventional optical characteristic inspection circuit. [Figure 6] FIG. 6 is a schematic diagram showing a configuration example of a conventional optical characteristic inspection circuit. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] <First embodiment> An optical characteristic inspection circuit and an optical characteristic inspection device according to a first embodiment of the present invention will be described with reference to FIG.

[0016] <Configuration of optical characteristic inspection circuit and optical characteristic inspection device> The optical characteristic inspection device 1 according to this embodiment includes an optical characteristic inspection circuit 10, an optical fiber 21, and a control unit 31, as shown in FIG.

[0017] In addition, the optical characteristic inspection circuit 10 is an on-wafer optical characteristic inspection circuit fabricated on the same wafer, and includes an optical input element 11, an optical branching circuit 12, a first optical circuit to be inspected (hereinafter referred to as the “first optical circuit”) 13_1, a second optical circuit to be inspected (hereinafter referred to as the “second optical circuit”) 13_2, and a photodetector 14.

[0018] Furthermore, in the optical characteristic inspection circuit 10, a resistor 15 is disposed in the waveguide of the optical branching circuit 12. Also, an electrode 16 is disposed on the resistor 15 via an electrical wiring.

[0019] Here, the control unit 31 is electrically connected to both the photodetector 14 and the electrode 16. The control unit 31 has a power source that supplies a voltage to the resistor 15 via the electrode 16. In addition, a photocurrent is input from the photodetector 14. The characteristics (waveguide loss) of the optical circuit are evaluated from a photocurrent spectrum obtained from this photocurrent and the voltage supplied to the resistor 15 (described later).

[0020] Light from a light source propagates through an optical fiber 21, enters an optical input element 11 in an optical characteristic inspection circuit, and is branched by an optical branching circuit 12. One of the branched lights is input to a first optical circuit 13_1, and the other is input to a second optical circuit 13_2. The output lights of the first optical circuit 13_1 and the second optical circuit 13_2 are each input to a photodetector 14.

[0021] In the photodetector 14, the input light is converted into electricity, output as a photocurrent, and input to the control unit 31. Here, the electrical output (photocurrent) from the photodetector 14 corresponds to the sum of the intensities of the lights transmitted through the first optical circuit 13_1 and the second optical circuit 13_2.

[0022] <Operation of the optical characteristic inspection circuit> Next, there will be described a basic operation of the optical characteristic inspection circuit 10. In the optical characteristic inspection circuit 10, the transmission characteristic and the branching ratio of the optical branching circuit 12 change depending on the phase of the input light.

[0023] For input light having an arbitrary single wavelength and an arbitrary phase, one of the branched lights reaches the photodetector 14 after passing through the first optical circuit 13_1, and the other light reaches the photodetector 14 after passing through the second optical circuit 13_2.

[0024] As a result, the photocurrent spectrum detected and output by the photodetector 14 is the sum of the current due to the transmitted light through the first optical circuit 13_1 and the current due to the transmitted light through the second optical circuit 13_2.

[0025] Here, the optical branching circuit 12 is provided with a resistor (heater) 15 that generates heat when supplied with an electrical input from the outside, and an electrode 16 that is connected to the resistor 15 via an electrical wiring. By applying a voltage to the resistor 15 via the electrode 16 and the electrical wiring to heat the resistor 15, the phase of the input light can be changed in accordance with a change in the refractive index due to the thermo-optic effect of the waveguide of the optical branching circuit 12.

[0026] In this way, in the optical characteristic inspection circuit 10, the phase of the input light is changed to change the transmission characteristic and the branching ratio of the optical branching circuit 12. The optical characteristics (waveguide loss) of the first optical circuit 13_1 and the second optical circuit 13_2 are inspected from the change in the photocurrent (photocurrent spectrum) accompanying the change in the branching ratio. Details will be described later.

[0027] <First Example> An optical characteristic inspection circuit and an optical characteristic inspection method according to a first embodiment of the present invention will be described with reference to FIGS.

[0028] <Configuration of the optical characteristic inspection circuit> The optical characteristic inspection circuit 10 according to this embodiment includes a grating coupler 11, an optical branching circuit 12, a first optical circuit 13_1, a second optical circuit 13_2, and a photodetector 14. The grating coupler 11 may be another wafer surface optical input element.

[0029] Furthermore, in the optical characteristic inspection circuit 10, a resistor 15 is disposed in the waveguide of the optical branching circuit 12. Also, an electrode 16 is disposed on the resistor 15 via an electrical wiring.

[0030] <Operation of the optical characteristic inspection circuit> Light having an arbitrary single wavelength and an arbitrary phase is input to the grating coupler 11 from an optical fiber.

[0031] Next, the input light is input to the optical branching circuit 12. The optical branching circuit 12 is a directional coupler consisting of two adjacent waveguides, two input ports, and two output ports. Here, one of the input ports is connected to one of the two waveguides and to one of the output ports. In addition, a grating coupler 11 is connected to one of the input ports.

[0032] The other input port is connected to the other waveguide and to the other output port. Since the other input port is not actually used, the optical branching circuit 12 only needs to have at least one input port.

[0033] A part of the light input to one input port of the optical branching circuit 12 passes through one waveguide and is output from one output port, while another part of the input light is coupled to and passes through the other waveguide in the region where the two waveguides are adjacent to each other, and is output from the other output port.

[0034] Alternatively, all of the light input to one input port of the optical branching circuit 12 may pass through one waveguide and be output from one output port, or all of the input light may be coupled to and pass through the other waveguide in a region where the two waveguides are adjacent to each other, and be output from the other output port.

[0035] Light output from one output port is input to the first optical circuit 13_1. Light output from the other output port is input to the second optical circuit 13_2. Here, the quality of the optical waveguides of the first optical circuit 13_1 and the second optical circuit 13_2 (hereinafter, the waveguides of the first optical circuit 13_1 and the second optical circuit 13_2 are referred to as "optical waveguides") is approximately equal, but the lengths of the optical waveguides are different. Here, the quality corresponds to the waveguide loss per unit length, and depends on the layer structure, cross-sectional shape, etc.

[0036] The light output from each of the first optical circuit 13_1 and the second optical circuit 13_2 is input to the photodetector 14. Although an example in which the output light from the first optical circuit 13_1 and the second optical circuit 13_2 is input to different end faces of the photodetector 14 is shown in Fig. 2, the output light may be input to the same end face of the photodetector 14.

[0037] The input light is converted to electricity in the photodetector 14 and output as a photocurrent, where the photocurrent corresponds to the sum of the intensities of the lights transmitted through the first optical circuit 13_1 and the second optical circuit 13_2.

[0038] In the optical characteristic inspection circuit 10, resistors 15 are arranged in each of two adjacent waveguides in the optical branching circuit 12. These resistors 15 generate heat when a voltage is applied via the electrodes 16 and electrical wiring. As a result, the refractive index changes due to the thermo-optic effect of the waveguide as the temperature of the waveguide changes, so that the phase of the input light can be changed. This change in the phase of the input light changes the transmission characteristics of the waveguides in the optical branching circuit 12, so that the branching ratio of the light branched and output by each waveguide can be changed.

[0039] Here, resistor 15 may be disposed in either of the two waveguides. If a resistor such as a metal is disposed on the surface of the waveguide, the light propagating through the waveguide is scattered by the metal, causing optical loss. In order to make the effect of this optical loss equal in both waveguides, it is desirable to dispose resistors in both waveguides.

[0040] For example, by applying a voltage to resistor 15 arranged in one of the two waveguides, it is possible to change the branching ratio of the output light, in other words, the intensity of the light input to each of first optical circuit 13_1 and second optical circuit 13_2. Alternatively, a voltage may be applied to resistor 15 arranged in both of the two waveguides.

[0041] In this way, in the optical characteristic inspection circuit 10, by changing the voltage applied to the resistor 15, the intensity of the light transmitted through the first optical circuit 13_1 and the second optical circuit 13_2 changes, and the photocurrent in the photodetector 14 changes. From this change in photocurrent (photocurrent spectrum), the waveguide loss of the optical waveguide of the optical circuit can be evaluated. Details will be described below.

[0042] <Optical characteristic inspection method> 3 shows a schematic diagram of a photocurrent spectrum obtained in the photodetector 14. The photocurrent spectrum is obtained by changing the voltage applied to the resistor 15, i.e., the power supplied to the resistor, and measuring the photocurrent.

[0043] 3, the dotted line indicates the photocurrent contribution component of the first optical circuit 13_1, and the dashed line indicates the photocurrent contribution component of the second optical circuit 13_2. The solid line is the photocurrent spectrum measured by the photodetector 14, and corresponds to the sum of the photocurrent contribution components of the first optical circuit 13_1 and the second optical circuit 13_2. The amplitude of each of these photocurrent spectra corresponds to the waveguide loss of the optical circuit.

[0044] In the optical characteristic inspection circuit 10, the first optical circuit 13_1 is longer than the second optical circuit 13_2, and therefore has a larger optical loss. Therefore, the photocurrent contribution component of the first optical circuit 13_1 is smaller than the photocurrent contribution component of the second optical circuit 13_2.

[0045] Therefore, at the minimum value in the photocurrent spectrum (solid line), the optical intensity is minimum, i.e., the optical loss is maximum, and therefore, it corresponds to the case where all of the input light passes through the first optical circuit 13_1. In other words, the photocurrent contribution component of the first optical circuit 13_1 is 100%. Thus, the minimum value in the photocurrent spectrum (solid line) is due to the waveguide loss of the first optical circuit 13_1.

[0046] Moreover, at the maximum value in the photocurrent spectrum (solid line), the optical intensity is maximum, i.e., the optical loss is minimum, so this corresponds to the case where all of the input light passes through the second optical circuit 13_2. In other words, the photocurrent contribution component of the second optical circuit 13_2 is 100%. Thus, the maximum value in the photocurrent spectrum (solid line) is due to the waveguide loss of the second optical circuit 13_2.

[0047] Therefore, the difference (ΔS) between the maximum and minimum values ​​of the photocurrent is due to the difference in waveguide loss caused by the difference in length (ΔL) between the first optical circuit 13_1 and the second optical circuit 13_2.

[0048] Therefore, by calculating ΔS / ΔL (dB / m) as an inspection result of an optical waveguide in an optical circuit, the waveguide loss per unit length can be evaluated.

[0049] Here, when the photocurrent spectrum has multiple maximum values ​​(or minimum values), the average value of the multiple maximum values ​​(or minimum values) may be used to calculate ΔS / ΔL. Alternatively, one value may be selected from the multiple maximum values ​​(or minimum values) and used. The maximum value or minimum value may also be used.

[0050] Here, because the optical characteristic inspection circuit 10 uses a common photodetector 14, even if the sensitivity of each manufactured photodetector varies, the entire photocurrent spectrum only increases or decreases, and its amplitude is not affected. Therefore, by acquiring and evaluating the photocurrent spectrum using optical characteristic inspection circuits having optical circuits of different lengths, it is possible to similarly evaluate the waveguide loss of the optical waveguide of the optical circuit formed on the substrate.

[0051] According to the optical characteristic inspection circuit and the optical characteristic inspection method of the present embodiment, since there is no need to sweep the wavelength of the input light, even if an optical input element having wavelength dependency is used, the optical characteristics (waveguide loss) of the optical waveguide can be accurately evaluated by on-wafer inspection.

[0052] Furthermore, it is only necessary to align the optical fiber on the input side, and after aligning the optical fiber, the photocurrent spectrum can be easily obtained, so that the steps required for inspection can be simplified.

[0053] Furthermore, since a common photodetector is used in the optical characteristic inspection circuit, the influence of variations in the characteristics of the photodetector can be suppressed.

[0054] As described above, the optical characteristics inspection device according to the first embodiment can be configured using the optical characteristics inspection circuit according to this embodiment, and the optical characteristics of an optical circuit can be inspected.

[0055] <Second Example> An optical characteristic inspection circuit according to a second embodiment of the present invention will be described with reference to FIG.

[0056] <Configuration of the optical characteristic inspection circuit> The optical characteristic inspection circuit 20 according to this embodiment includes a grating coupler 11, an optical branching circuit 22, a first optical circuit 13_1, a second optical circuit 13_2, and a photodetector .

[0057] Furthermore, in the optical characteristic inspection circuit 20, a resistor 25 is disposed in the waveguide of the optical branching circuit 22. Also, an electrode 16 is disposed on the resistor 25 via an electrical wiring.

[0058] In the optical characteristic inspection circuit 20, the optical branching circuit 22 is a so-called asymmetric Mach-Zehnder interferometer, which includes a multimode interference (MMI) on both input and output ends and includes two arm waveguides with different lengths.

[0059] <Operation of the optical characteristic inspection circuit> The asymmetric Mach-Zehnder interferometer type optical branching circuit 22 branches light and inputs it to each of the two arm waveguides, merges the light from each arm waveguide, branches it, and outputs it to each of the first optical circuit 13_1 and the second optical circuit 13_2.

[0060] Here, at least one of the resistors 25 arranged in the two arm waveguides is heated by applying a voltage to it. As a result, the refractive index of the arm waveguide changes due to the thermo-optic effect of the arm waveguide as the temperature of the arm waveguide changes, so that the phase of the input light can be changed. This change in the phase of the input light changes the transmission characteristics of the arm waveguide, so that the branching ratio of the output light can be changed.

[0061] In the optical characteristic inspection circuit 20, as in the first embodiment, light from a light source is input to a grating coupler 11, branched by an optical branching circuit 22, transmitted through a first optical circuit 13_1 and a second optical circuit 13_2, and input to a photodetector 14. The waveguide loss of the optical waveguide of the optical circuit is evaluated from a photocurrent spectrum acquired by changing a voltage applied to a resistor 25.

[0062] According to the optical characteristic inspection circuit of this embodiment, it is possible to achieve the same effects as those of the first embodiment.

[0063] The optical characteristics inspection device according to the first embodiment can be configured using the optical characteristics inspection circuit according to this embodiment.

[0064] Using the optical characteristics inspection circuit according to this embodiment, the characteristics of an optical circuit can be inspected by the optical characteristics inspection method according to the first embodiment.

[0065] In the embodiments and examples of the present invention, the directional coupler and the asymmetric Mach-Zehnder interferometer can both be fabricated on a commercially available silicon-on-insulator substrate by using known lithography, thin film deposition and dry etching techniques.

[0066] In addition, in the embodiments and examples of the present invention, a germanium photodiode or the like can be used as the photodetector, and the photodetector can be fabricated on a commercially available silicon-on-insulator substrate by combining selective growth using ultra-high vacuum chemical vapor deposition or the like with lithography, thin film deposition, and dry etching.

[0067] Alternatively, an indium gallium arsenide photodiode or the like may be used as the photodetector. The photodetector can be fabricated by bonding a wafer or die including an indium phosphide thin film onto a commercially available silicon-on-insulator substrate using wafer bonding technology, removing unnecessary portions of the substrate, and then combining lithography, crystal regrowth, and dry etching.

[0068] In the embodiment and examples of the present invention, an example is shown in which a silicon optical circuit is used in which the core material of the optical circuit is silicon, but this is not limited to this. The core material only needs to have a refractive index larger than that of the cladding material. For example, when the cladding material is a silicon oxide film, the core material may be a silicon oxide film, a silicon oxynitride film, a silicon nitride film, a silicon carbide film, or the like with a high silicon content, or may be a compound semiconductor such as gallium arsenide or indium phosphide. The cladding material only needs to have a refractive index smaller than that of the core material. For example, when the core material is silicon, the cladding material may be a silicon oxide film, a silicon oxynitride film, a silicon nitride film, or an organic material such as epoxy resin or polyimide.

[0069] In the embodiment of the present invention, the structure, dimensions, materials, etc. of each component in the configuration and manufacturing method of the optical characteristics inspection circuit are shown as examples, but the present invention is not limited to these. Anything that can exert the function and effect of the optical characteristics inspection circuit may be used. [Industrial Applicability]

[0070] The present invention can be applied to an optical characteristic inspection apparatus used for inspecting an optical waveguide that constitutes an optical device. [Explanation of symbols]

[0071] 1. Optical characteristic inspection equipment 10 Optical characteristic inspection circuit 11 Optical input element 12 Optical branch circuit 13_1 First optical circuit to be inspected (first optical circuit) 13_2 Second optical circuit to be inspected (second optical circuit) 14 Photodetector 15 Resistor 16 electrodes 21 Optical Fiber 31 Control Unit

Claims

1. In order, an optical input element; an optical branch circuit having a resistor; a first test target optical circuit connected to one output of the optical branch circuit; a second test target optical circuit connected to the other output of the optical branch circuit; a photodetector for detecting an intensity of light transmitted through the first optical circuit under test and an intensity of light transmitted through the second optical circuit under test; Equipped with the optical branching circuit has two waveguides, an optical characteristic inspection circuit in which the resistor is disposed in at least one of the two waveguides; Control unit and Equipped with The control unit acquires a photocurrent spectrum, which is a change in photocurrent input from the photodetector with respect to a change in voltage supplied to the resistor, and calculates a waveguide loss of the first optical circuit under inspection and the second optical circuit under inspection from the photocurrent spectrum. An optical characteristic inspection apparatus characterized by the above.

2. An optical characteristic inspection method using an optical characteristic inspection circuit comprising, in order, an optical input element, an optical branching circuit having a resistor, a first optical circuit to be inspected connected to one output of the optical branching circuit, a second optical circuit to be inspected connected to the other output of the optical branching circuit, and a photodetector for detecting the intensity of light transmitted through the first optical circuit to be inspected and the light transmitted through the second optical circuit to be inspected, the optical branching circuit having two waveguides, and the resistor disposed in at least one of the two waveguides, Varying a voltage supplied to the resistor; acquiring a photocurrent due to light transmitted through the first optical circuit under test and light transmitted through the second optical circuit under test; obtaining a photocurrent spectrum, which is a change in the photocurrent with respect to a change in the voltage; calculating a waveguide loss per unit length from a difference between a maximum value and a minimum value in the photocurrent spectrum and a difference between the first optical circuit under inspection and the second optical circuit under inspection; An optical characteristic inspection method comprising:

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