Inspection device and inspection method

Abstract

An inspection device 1 comprises: a pulse formation unit 3 that generates a pulse train Pb in which a plurality of light pulses Pb, Pb2 having different center wavelengths are arranged at predetermined time intervals; a correlation optical system 7 that has polarization dependency in a predetermined polarization direction and generates correlation light Pc of the pulse train Pb outputted from a photonic integrated circuit 5; a light detection unit 8 that detects a time waveform of the correlation light Pc; an evaluation unit 9 that evaluates the waveguide of the photonic integrated circuit 5 on the basis of a feature amount of the time waveform; and a first adjustment unit 4 that causes the polarization direction of each of the light pulses Pb1, Pb2 of the pulse train Pb inputted to the photonic integrated circuit 5 to be orthogonal to the in-plane direction of a substrate 50.

Description

Inspection device and inspection method

[0001] The present disclosure relates to an inspection device and an inspection method.

[0002] The dispersion measurement device described in Patent Document 1 includes a pulse forming unit that forms, from a first optical pulse output from an object to be measured, a pulse train including a plurality of second optical pulses that have a time difference from one another and different center wavelengths; a correlation optical system that receives the pulse train output from the pulse forming unit and outputs correlated light that includes the autocorrelation of the pulse train; a light detection unit that detects the time waveform of the correlated light; and a calculation unit that estimates the amount of chromatic dispersion of the object to be measured based on the feature quantities of the time waveform.

[0003] The dispersion measurement device described above is expected to be used as an inspection device for photonic integrated circuits (PICs). An optical integrated circuit is a device that integrates various optical functions and transmits information signals using light. For example, when a pulse train is input into an optical integrated circuit, chromatic dispersion can occur as the pulse train propagates through the waveguide of the optical integrated circuit. Therefore, the chromatic dispersion in the optical integrated circuit is estimated using a dispersion measurement device, and it is checked whether the chromatic dispersion deviates from the design value. If the deviation is large, the characteristics of the waveguide of the optical integrated circuit are adjusted.

[0004] Japanese Patent Application Laid-Open No. 2020-169946

[0005] When the amount of chromatic dispersion is repeatedly measured using a dispersion measurement device, measurement errors can occur. Because the waveguide length of an optical integrated circuit is relatively short, if the measurement error is large, it may affect the accuracy of inspection of the optical integrated circuit.

[0006] The present disclosure provides an inspection device and an inspection method that can reduce measurement errors of the amount of chromatic dispersion and perform inspection of optical integrated circuits with high precision.

[0007] The gist of the present disclosure is as follows.

[0008] [1] An inspection device for inspecting an optical integrated circuit having a waveguide provided on a substrate, the inspection device comprising: a pulse forming unit that generates a pulse train in which a plurality of optical pulses with different center wavelengths are arranged at a predetermined time interval; a correlation optical system that has polarization dependency in a predetermined polarization direction and generates correlated light including cross-correlated light or auto-correlated light of the pulse train that is output from the optical integrated circuit through the waveguide; a light detecting unit that detects the time waveform of the correlated light; an evaluation unit that evaluates the waveguide of the optical integrated circuit based on features of the time waveform; and a first adjustment unit that orthogonally adjusts the polarization direction of each optical pulse of the pulse train that is input to the optical integrated circuit to be orthogonal to the in-plane direction of the substrate.

[0009] In this inspection device, the first adjustment unit inputs a pulse train to the optical integrated circuit so that the polarization direction of each of the multiple optical pulses is orthogonal to the in-plane direction of the substrate. This makes it less likely that polarization mixing will occur in the pulse train output from the optical integrated circuit. As a result, the efficiency of generating correlated light in the correlation optical system is increased, and at the same time, noise components that distort the correlation waveform are reduced, thereby improving the detection accuracy of the time waveform and reducing measurement errors of chromatic dispersion. This suppresses polarization mixing in the pulse train output from the optical integrated circuit, reduces measurement errors of the amount of chromatic dispersion, and enables accurate inspection of the optical integrated circuit.

[0010] [2] The inspection device according to [1], further comprising a first polarization-maintaining fiber that optically connects the pulse forming unit to the optical integrated circuit, and the first adjustment unit is configured with a rotation mechanism that rotates the first polarization-maintaining fiber about its axis. In this case, by rotating the first polarization-maintaining fiber about its axis, the polarization direction of each optical pulse in the pulse train input to the optical integrated circuit can be easily adjusted.

[0011] [3] The inspection device according to [1], further comprising a first polarization-maintaining fiber that optically connects the pulse shaping unit to the optical integrated circuit, and the first adjustment unit is configured by a rotation mechanism that rotates the in-plane direction of the substrate about the axis of the first polarization-maintaining fiber. In this case, by rotating the in-plane direction of the substrate of the optical integrated circuit about the axis of the first polarization-maintaining fiber, the polarization direction of each optical pulse of the pulse train input to the optical integrated circuit can be easily adjusted.

[0012] [4] The inspection device according to any one of [1] to [3], further comprising a second adjustment unit that adjusts the polarization direction of each optical pulse of the pulse train input to the correlation optical system to match the polarization direction of the correlation optical system. In this case, the second adjustment unit adjusts the input pulse train to the correlation optical system so that the polarization direction of the pulse train matches the polarization direction of the correlation optical system, which has polarization dependency. Therefore, correlation light can be generated efficiently in the correlation optical system. By increasing the efficiency of generating correlation light in the correlation optical system and simultaneously reducing noise components that distort the correlation waveform, measurement errors of the amount of chromatic dispersion can be further reduced, enabling more accurate inspection of optical integrated circuits.

[0013] [5] The inspection device according to [4], further comprising a second polarization-maintaining fiber that optically connects the optical integrated circuit to the correlation optical system, and the second adjustment unit is configured with a rotation mechanism that rotates the second polarization-maintaining fiber about its axis. In this case, the polarization direction of each optical pulse in the pulse train input to the second polarization-maintaining fiber can be easily adjusted by rotating the in-plane direction of the substrate of the optical integrated circuit about the axis of the second polarization-maintaining fiber.

[0014] [6] The inspection device according to any one of [1] to [5], wherein the pulse shaping unit includes a spatial light modulator that sets at least one of the wavelength difference, time difference, and wavelength width of the plurality of optical pulses to an arbitrary value. In this case, for example, by increasing the wavelength difference between each optical pulse in the pulse train, the pulse interval (time difference) between each optical pulse can be increased. By increasing the pulse interval between each optical pulse, the error in repeated measurements of the amount of chromatic dispersion becomes relatively small, allowing for more accurate inspection of optical integrated circuits. Furthermore, for example, by the pulse shaping unit increasing the wavelength width of each of the plurality of optical pulses, the pulse width of the optical pulse becomes narrower, allowing for more accurate inspection of optical integrated circuits.

[0015] [7] An inspection device for inspecting an optical integrated circuit having a waveguide provided on a substrate, the inspection device comprising: a pulse forming unit that generates a pulse train in which a plurality of optical pulses with different center wavelengths are arranged at a predetermined time interval; an optical detection unit that detects a time waveform of the pulse train generated by the pulse forming unit and output from the optical integrated circuit through the waveguide; an evaluation unit that evaluates the waveguide of the optical integrated circuit based on features of the time waveform; and a first adjustment unit that adjusts the polarization direction of each optical pulse of the pulse train input to the optical integrated circuit to be orthogonal to the in-plane direction of the substrate.

[0016] In this inspection device, the first adjustment unit inputs a pulse train to the optical integrated circuit so that the polarization direction of each of the multiple optical pulses is orthogonal to the in-plane direction of the substrate. This makes it less likely that polarization mixing will occur in the pulse train output from the optical integrated circuit. As a result, the detection accuracy of the time waveform is improved and measurement errors in chromatic dispersion are reduced. This suppresses polarization mixing in the pulse train output from the optical integrated circuit, reduces measurement errors in the amount of chromatic dispersion, and enables accurate inspection of the optical integrated circuit.

[0017] [8] An inspection method for inspecting an optical integrated circuit having a waveguide provided on a substrate, the inspection method comprising: a pulse forming step of generating a pulse train in which a plurality of optical pulses having different center wavelengths are arranged at a predetermined time interval; a correlated light generating step of generating correlated light including cross-correlated light or autocorrelated light of the pulse train output from the optical integrated circuit using a correlation optical system having polarization dependency in a predetermined polarization direction; a light detection step of detecting a time waveform of the correlated light; an evaluation step of evaluating the waveguide of the optical integrated circuit based on features of the time waveform; and a first adjustment step of orthogonalizing the polarization direction of each optical pulse of the pulse train input to the optical integrated circuit to be orthogonal to an in-plane direction of the substrate.

[0018] In this inspection method, in the first adjustment step, a pulse train is input to the optical integrated circuit so that the polarization direction of each of the multiple optical pulses is orthogonal to the in-plane direction of the substrate. This makes it less likely that polarization mixing will occur in the pulse train output from the optical integrated circuit. As a result, the efficiency of generating correlated light in the correlation optical system is increased, and at the same time, noise components that distort the correlation waveform are reduced, thereby improving the detection accuracy of the time waveform and reducing measurement errors of chromatic dispersion. This suppresses polarization mixing in the pulse train output from the optical integrated circuit, reduces measurement errors of the amount of chromatic dispersion, and enables accurate inspection of the optical integrated circuit.

[0019] [9] The inspection method according to [8], further comprising a second adjustment step of matching the polarization direction of each optical pulse of the pulse train input to the correlation optical system with the polarization direction of the correlation optical system. In this case, in the second adjustment step, the pulse train is input to the correlation optical system so that the polarization direction of the pulse train matches the polarization direction in which the correlation optical system has polarization dependency. Therefore, correlated light can be generated efficiently in the correlation optical system. By increasing the efficiency of generating correlated light in the correlation optical system and simultaneously reducing noise components that distort the correlation waveform, measurement errors of the amount of chromatic dispersion can be further reduced, enabling more accurate inspection of optical integrated circuits.

[0020]

[10] The inspection method according to [8] or [9], wherein the determining step determines the amount of chromatic dispersion of the optical integrated circuit based on a feature amount of the time waveform of the correlated light detected via the optical integrated circuit and a feature amount of the time waveform of the correlated light detected without via the optical integrated circuit. In this case, the feature amount of the time waveform of the correlated light detected without via the optical integrated circuit is used as a reference point, and a relative dispersion amount with respect to the feature amount of the time waveform of the correlated light detected via the optical integrated circuit is determined, thereby improving measurement robustness of the system (stability against external factors such as environmental changes and laser instability).

[0021]

[11] The inspection method according to [8] or [9], wherein the determining step determines the amount of chromatic dispersion of the optical integrated circuit based on a feature amount of a time waveform of correlated light theoretically calculated in advance assuming that chromatic dispersion is zero and a feature amount of the time waveform of the correlated light detected in the light detecting step. In this case, the amount of chromatic dispersion can be determined efficiently using the feature amount calculated in advance.

[0022] According to the present disclosure, it is possible to reduce measurement errors in the amount of chromatic dispersion and perform inspection of optical integrated circuits with high accuracy.

[0023] 7(a) is a diagram showing an example of the time waveform of correlated light when no chromatic dispersion occurs in the pulse train; FIG. 7(b) is a diagram showing an example of the time waveform of correlated light when chromatic dispersion occurs in the pulse train; FIG. 7(c) is a graph showing an example of the time waveform of correlated light; FIG. 9(a), FIG. 9(b), FIG. 9(c), and FIG. 9(d) are diagrams showing examples of the delay time of a pulse train; FIG. 9(b) is a diagram showing an example of the hardware configuration of an evaluation unit; FIG. 9(c) is a flowchart showing an inspection method using an inspection device; FIG. 9(d) is a diagram showing the relationship between the waveguide length and pulse interval of a waveguide of an optical integrated circuit when an inspection device according to a comparative example is used; FIG. 9(d) is a diagram showing the relationship between the waveguide length and pulse interval of a waveguide of an optical integrated circuit when an inspection device according to an embodiment of the present invention is used. 14(a) is a diagram showing a simplified inspection apparatus according to a second modified example, FIG. 14(b) is a diagram showing a simplified inspection apparatus according to a third modified example, and FIG. 14(c) is a diagram showing a simplified inspection apparatus according to a fourth modified example. FIG. 15(a) is a diagram showing a simplified inspection apparatus according to a fifth modified example, and FIG. 15(b) is a diagram showing a simplified inspection apparatus according to a sixth modified example. FIG. 17(a) is a graph showing a change in average output when a spatial light modulator changes an optical pulse and the wavelength width of each of the optical pulses, and FIG. 17(b) is a graph showing a change in pulse width when a spatial light modulator changes the wavelength width of each of the multiple optical pulses. 18(a) is a graph showing the relationship between the waveguide length of a waveguide in an optical integrated circuit and the pulse interval when the pulse width is 4 nm, and FIG. 18(b) is a graph showing the relationship between the waveguide length of a waveguide in an optical integrated circuit and the pulse interval when the pulse width is 6 nm and 8 nm.FIG. 20( a) is a diagram for explaining the relationship between the wavelength difference and chromatic dispersion before the wavelength difference is widened, and FIG. 20( b) is a diagram for explaining the relationship between the wavelength difference and chromatic dispersion after the wavelength difference is widened.

[0024] Hereinafter, a preferred embodiment of an inspection device according to one aspect of the present disclosure will be described in detail with reference to the drawings.

[0025] [Configuration of Inspection Apparatus] Figure 1 is a diagram schematically showing the configuration of an inspection apparatus according to one embodiment of the present invention. This inspection apparatus 1 is an apparatus for inspecting an optical integrated circuit 5. The optical integrated circuit 5 includes a waveguide 53 (see Figure 4) provided on a substrate 50. In this embodiment, the inspection apparatus 1 is an apparatus for measuring the chromatic dispersion of light to be measured that has propagated through the waveguide 53 of the optical integrated circuit 5. The inspection apparatus 1 includes a pulsed laser light source 2, a pulse forming unit 3, a first adjusting unit 4, a second adjusting unit 6, a correlation optical system 7, a light detecting unit 8, and an evaluating unit 9.

[0026] The optical input end 3a of the pulse forming unit 3 is optically coupled to the pulse laser light source 2 either spatially or via an optical waveguide such as an optical fiber. The optical input end 4a of the first adjusting unit 4 is optically coupled to the optical output end 3b of the pulse forming unit 3 either spatially or via an optical waveguide such as an optical fiber. The optical input end 5a of the optical integrated circuit 5 to be inspected is optically coupled to the optical output end 4b of the first adjusting unit 4 either spatially or via an optical waveguide such as an optical fiber. The optical input end 6a of the second adjusting unit 6 is optically coupled to the optical output end 5b of the optical integrated circuit 5 either spatially or via an optical waveguide such as an optical fiber.

[0027] The optical input end 7a of the correlation optical system 7 is optically coupled to the optical output end 6b of the second adjusting unit 6, either spatially or via an optical waveguide such as an optical fiber. The optical detecting unit 8 is optically coupled to the optical output end 7b of the correlation optical system 7, either spatially or via an optical waveguide such as an optical fiber. The evaluating unit 9 is electrically connected to each of the pulse forming unit 3, the optical integrated circuit 5, and the optical detecting unit 8. The evaluating unit 9 may also be electrically connected to each of the first adjusting unit 4 and the second adjusting unit 6.

[0028] The pulsed laser light source 2 outputs a coherent optical pulse Pa to be measured. The pulsed laser light source 2 is, for example, a femtosecond laser, and in one embodiment, is a solid-state laser light source such as an LD directly pumped Yb:YAG pulsed laser. The time waveform of the optical pulse Pa to be measured is, for example, a Gaussian function. The optical pulse Pa to be measured is an optical pulse having a certain bandwidth and includes multiple continuous wavelength components. The pulsed laser light source 2 may be a light source that outputs an attosecond laser or a picosecond laser, or may be a titanium sapphire laser light source, a semiconductor laser light source, or an F2 laser light source.

[0029] The pulse shaping unit 3 forms a pulse train Pb including a plurality of optical pulses from the measured optical pulse Pa. The pulse train Pb is a group of single pulses generated using each of the wavelength bands that are obtained by dividing the spectrum constituting the measured optical pulse Pa into a plurality of wavelength bands. Note that there may be overlapping portions at the boundaries between the plurality of wavelength bands. In the following example, the pulse train Pb includes two single-pulse optical pulses Pb1 and Pb2. The central wavelength and wavelength band of the optical pulse Pb1 are different from the central wavelength and wavelength band of the optical pulse Pb2.

[0030] The pulse train Pb comprises a plurality of optical pulses Pb1 and Pb2 with different center wavelengths arranged at a predetermined time interval. The peaks of the two optical pulses Pb1 and Pb2 are separated in time, and the propagation timings of the two optical pulses Pb1 and Pb2 are shifted from each other. In other words, one optical pulse Pb2 is delayed in time with respect to the other optical pulse Pb1. However, the tail portions of adjacent optical pulses Pb1 and Pb2 may overlap each other.

[0031] FIG. 2 is a diagram showing an example configuration of the pulse shaping unit 3. The pulse shaping unit 3 includes a diffraction grating 31, a lens 32, a spatial light modulator (SLM) 33, a lens 34, and a diffraction grating 35. The diffraction grating 31 is an example of a spectroscopic element. The diffraction grating 31 is optically coupled to the pulse laser light source 2. The spatial light modulator 33 is optically coupled to the diffraction grating 31 via a lens 32. The diffraction grating 31 spatially separates multiple wavelength components contained in the measured optical pulse Pa into individual wavelengths. Note that other optical components such as a prism may be used as the spectroscopic element instead of the diffraction grating 31. The measured optical pulse Pa is incident obliquely on the diffraction grating 31 and is split into multiple wavelength components. The light P1 containing the multiple wavelength components is focused by the lens 32 into each wavelength component and formed into an image on the modulation surface of the spatial light modulator 33. The lens 32 may be a convex lens made of a light-transmitting member or a concave mirror having a concave light-reflecting surface.

[0032] The spatial light modulator 33 converts the light P1 into modulated light P2. The spatial light modulator 33 may be a reflective spatial light modulator or a transmissive spatial light modulator. Furthermore, the spatial light modulator 33 may be a liquid crystal spatial light modulator or a digital mirror device. The modulated light P2 includes light pulses Pb1 and Pb2. The spatial light modulator 33 selects any two wavelength components from the light P1, which includes multiple wavelength components, as the light pulses Pb1 and Pb2. For example, the spatial light modulator 33 may intensity-modulate the wavelength components of the light pulses Pb1 and Pb2 to a level that does not affect the other wavelength components. The wavelength difference, which is the difference between the wavelength of the light pulse Pb1 and the wavelength of the light pulse Pb2, changes depending on the value of the selected wavelength component. This allows the spatial light modulator 33 to set the wavelength difference between the two light pulses Pb1 and Pb2 to any desired value. The wavelength difference that can be set is, for example, 1 nm to 50 nm.

[0033] The spatial light modulator 33 shifts the phases of the two light pulses Pb1 and Pb2 from each other. To do this, the spatial light modulator 33 performs phase modulation on the light P1. As a result, the light pulse Pb2 has a time delay with respect to the light pulse Pb1. In other words, the spatial light modulator 33 sets the time difference between the two light pulses Pb1 and Pb2 to an arbitrary magnitude. The spatial light modulator 33 performs intensity modulation and phase modulation on the light P1 to generate modulated light P2.

[0034] The modulation of light P1 by the spatial light modulator 33 will be described in more detail. Fig. 3 is a diagram showing the modulation surface 33a of the spatial light modulator 33. As shown in Fig. 3, on the modulation surface 33a, a plurality of modulation regions 33b are arranged along a certain direction A, and each modulation region 33b extends in a direction B intersecting with direction A. Direction A is the direction of light dispersion by the diffraction grating 31. This modulation surface 33a functions as a Fourier transform plane, and each corresponding wavelength component after dispersion is incident on each of the plurality of modulation regions 33b.

[0035] The spatial light modulator 33 modulates the intensity and phase of each incident wavelength component in each modulation region 33b independently of other wavelength components. Each modulation region 33b includes, for example, an intensity modulation pattern and a phase modulation pattern. The intensity modulation pattern and phase modulation pattern are, for example, different for each modulation region 33b. The intensity modulation pattern and phase modulation pattern in each modulation region 33b vary, for example, along direction A and are constant in direction B. The modulation amount of the intensity pattern of the modulation region 33b onto which the wavelength components of the light pulses Pb1 and Pb2 are incident may be greater than the modulation amount of the intensity pattern of the modulation region 33b onto which the other wavelength components are incident. The modulation amount of the phase pattern of the modulation region 33b onto which the wavelength component of the light pulse Pb1 is incident may be greater than the modulation amount of the phase pattern of the modulation region 33b onto which the wavelength component of the light pulse Pb2 is incident.

[0036] The optical pulses Pb1 and Pb2 contained in the modulated light P2 modulated by the spatial light modulator 33 are focused by the lens 34 to a single point on the diffraction grating 35. In this case, the lens 34 functions as a focusing optical system that focuses the modulated light P2. The lens 34 may be a convex lens made of a light-transmitting member, or a concave mirror having a concave light-reflecting surface. The diffraction grating 35 also functions as a combining optical system that combines the optical pulses Pb1 and Pb2. That is, the optical pulses Pb1 and Pb2 of the modulated light P2 are focused and combined by the lens 34 and the diffraction grating 35 to form a bandwidth-controlled multi-pulse (pulse train Pb).

[0037] Referring again to FIG. 1 , the first adjustment unit 4 adjusts the polarization direction of the pulse train Pb output from the pulse shaping unit 3. The first adjustment unit 4 orthogonally adjusts the polarization direction of each optical pulse Pb1, Pb2 of the pulse train Pb input to the optical integrated circuit 5 to be perpendicular to the in-plane direction of the substrate 50. In the example of FIG. 1 , for convenience of explanation, the direction in which the pulse train Pb travels is the X-axis direction. The in-plane direction of the substrate 50 is defined by the Y-axis direction, which is perpendicular to the X-axis direction, and the X-axis direction. The thickness direction of the substrate 50 is defined by the Z-axis direction, which is perpendicular to the X-axis direction and the Y-axis direction.

[0038] The in-plane direction of the substrate 50 coincides with the polarization direction in the TE (Transverse Electric) mode (horizontal polarization mode). In the TE mode, the direction of the main component of the electric field that constitutes the optical pulses Pb1 and Pb2 of the pulse train Pb is along the Y-axis direction, which coincides with the in-plane direction of the substrate 50.

[0039] The direction perpendicular to the in-plane direction of the substrate 50 coincides with the polarization direction in the TM (Transverse Magnetic) mode (longitudinal polarization mode). In the TM mode, the direction of the main component of the electric field that constitutes the optical pulses Pb1 and Pb2 is along the Z-axis direction, which is perpendicular to the in-plane direction of the substrate 50.

[0040] In one example, the inspection device 1 further includes a first polarization-maintaining fiber 11 optically connecting the optical output end 3b of the pulse forming unit 3 and the optical input end 5a of the optical integrated circuit 5. The first polarization-maintaining fiber 11 is, for example, a fiber that maintains the polarization direction of the pulse train Pb. The polarization direction of the pulse train Pb propagating through the first polarization-maintaining fiber 11 is maintained in a constant direction.

[0041] The first adjustment unit 4 is configured with a rotation mechanism that rotates the first polarization-maintaining fiber 11 around its axis. The axis of the first polarization-maintaining fiber 11 coincides with the propagation direction (X-axis direction) of the pulse train Pb. The first adjustment unit 4 is configured with, for example, a fiber rotator. The first adjustment unit 4 receives, for example, from the evaluation unit 9, an instruction regarding the angle by which to rotate the first polarization-maintaining fiber 11. Based on this instruction, the first adjustment unit 4 may rotate the first polarization-maintaining fiber 11 around its axis. For example, the first adjustment unit 4 rotates the first polarization-maintaining fiber 11 90 degrees around its axis. In this case, the polarization directions of the optical pulses Pb1 and Pb2 rotate by 90 degrees.

[0042] 1 , the polarization mode of the optical pulses Pb1 and Pb2 is the TE mode when they are output from the pulse shaping unit 3. That is, the polarization direction of the optical pulses Pb1 and Pb2 coincides with the in-plane direction of the substrate 50. The first adjusting unit 4 converts the polarization mode of the optical pulses Pb1 and Pb2 to the TM mode. As a result, the polarization direction of the optical pulses Pb1 and Pb2 input to the optical integrated circuit 5 is perpendicular to the in-plane direction of the substrate 50.

[0043] FIG. 4 is a schematic diagram showing an optical integrated circuit 5. The optical integrated circuit 5 is a device that integrates various optical functions and transmits information signals by light. The optical integrated circuit 5 includes a substrate 50. As shown in FIG. 4, the optical integrated circuit 5 is configured to include various elements, and these elements are mounted on the substrate 50. In the example shown in FIG. 4, the optical integrated circuit 5 includes an optical input section 51, an optical output section 52, a waveguide 53, photodiodes 54 and 57, an optical ring resonator circuit 55, a photonic crystal 56, a microheater 58, and the like.

[0044] The substrate 50 has a first surface 50a and a second surface 50b opposite to the first surface 50a. In the example of Fig. 4, the optical input section 51, the optical output section 52, the waveguide 53, the photodiodes 54 and 57, the optical ring resonator circuit 55, and the photonic crystal 56 are mounted on the second surface 50b. The microheater 58 is mounted on the first surface 50a.

[0045] The optical input section 51 is a section where light is input from the first adjustment section 4 through the first polarization-maintaining fiber 11 into the optical integrated circuit 5. The optical input section 51 is formed at the optical input end 5a, which is one side of the optical integrated circuit 5. Although only one optical input section 51 is shown in Fig. 4, a plurality of optical input sections 51 are formed at the optical input end 5a. For example, in a practical state after testing, an optical cable is connected to the optical input section 51.

[0046] The optical output unit 52 is a part that outputs the pulse train Pb from the optical integrated circuit 5 toward the second adjustment unit 6. The optical output unit 52 is formed at the optical output end 5b, which is the other end face of the optical integrated circuit 5. Although a single optical output unit 52 is shown in FIG. 4, a plurality of optical output units 52 may be formed at the optical output end 5b. For example, in a practical state after inspection, an optical cable may be connected to each of the plurality of optical output units 52. The optical output unit 52 may output light guided by the waveguide 53 to the outside of the optical integrated circuit 5.

[0047] The photodiodes 54 and 57 are photodetectors provided inside the optical integrated circuit 5. The optical ring resonator circuit 55 is a circuit that sets the resonant wavelength. The photonic crystal 56 has a structure in which the refractive index changes periodically, and is a structure that confines light in a small region and manipulates the light. Note that the optical integrated circuit 5 may include various elements in addition to the above-mentioned components (elements).

[0048] The microheater 58 may be formed on each element such as the waveguide 53, the optical ring resonator circuit 55, or the optical modulator. In the example of Fig. 4, the microheater 58 is formed so as to straddle a part of the waveguide 53 and overlap the optical ring resonator circuit 55.

[0049] The microheater 58 receives an instruction for a set temperature from, for example, the evaluation unit 9. The microheater 58 may then increase its temperature until it reaches the set temperature and heat a portion of the waveguide 53 at the set temperature. The microheater 58 can change the refractive index of a portion of the waveguide 53 by the thermo-optic effect, thereby adjusting the amount of chromatic dispersion of the waveguide. For example, a change in the amount of chromatic dispersion appears as a change in the time difference between the optical pulses Pb1 and Pb2 guided through the waveguide 53, and the microheater 58 may be adjusted using this as an index.

[0050] 4, the waveguide 53 is provided linearly from the optical input portion 51 to the optical output portion 52, but the waveguide 53 may be formed, for example, in a curved or spiral shape. When the waveguide 53 is curved or spiral, the direction of curvature of the waveguide 53 coincides with the in-plane direction of the substrate 50. The direction of curvature of the waveguide 53 refers, for example, to the direction in which the waveguide 53 is curved, that is, the direction in which the curve of the waveguide 53 is described.

[0051] Chromatic dispersion occurs in the pulse train Pb input to the optical integrated circuit 5 as it propagates through the waveguide 53. Since the optical pulses Pb1 and Pb2 have different wavelengths, they propagate at different speeds through the waveguide 53. This causes a difference in the propagation time between the optical pulse Pb1 and the optical pulse Pb2, and the time interval between the optical pulse Pb1 and the optical pulse Pb2 varies.

[0052] In this embodiment, the optical integrated circuit 5 is optically designed so that the polarization mode of the input optical pulses is maintained when they are output. In this case, the optical pulses Pb1 and Pb2 input to the optical integrated circuit 5 in the TM mode are output while maintaining the TM mode as the pulse train Pb propagates through the waveguide 53.

[0053] The likelihood of polarization mixing occurring in the optical pulses Pb1 and Pb2 output from the optical integrated circuit 5 varies depending on the polarization direction of the optical pulses Pb1 and Pb2 when they are input to the optical integrated circuit 5. Polarization mixing is a phenomenon in which the TE mode and TM mode are mixed in the polarization modes of the optical pulses Pb1 and Pb2. In this case, when the ratio of the TE mode component to the TM mode component is expressed as a percentage, the closer the ratio is to 50%, the greater the influence of polarization mixing. If the influence of polarization mixing is large, an error in measuring the amount of chromatic dispersion will occur in the evaluation unit described below, which may make it difficult to accurately inspect the optical integrated circuit 5.

[0054] The magnitude of the effect of polarization mixing varies depending on the polarization direction of the optical pulses Pb1 and Pb2 input to the optical integrated circuit 5. When the polarization direction of the optical pulses Pb1 and Pb2 input to the optical integrated circuit 5 is perpendicular to the in-plane direction of the substrate 50 (when input in TM mode), the ratio of TE mode mixed into TM mode in the optical pulses Pb1 and Pb2 output from the optical integrated circuit 5 is about several percent. In contrast, when the polarization direction of the optical pulses Pb1 and Pb2 input to the optical integrated circuit 5 coincides with the in-plane direction of the substrate 50 (when input in TE mode), the ratio of TM mode mixed into TE mode in the optical pulses Pb1 and Pb2 output from the optical integrated circuit 5 increases to about 20%. In other words, when input in TM mode, the effect of polarization mixing is smaller than when input in TE mode.

[0055] In particular, when the optical integrated circuit 5 includes a waveguide that is curved in the in-plane direction, such as a spiral waveguide, the influence of polarization mixing is smaller when the optical pulses Pb1 and Pb2 are input in TM mode than when they are input in TE mode. Furthermore, when the optical integrated circuit 5 includes waveguides that are close to each other in the in-plane direction, if the optical pulses Pb1 and Pb2 are input in TE mode, the evanescent field is likely to leak in the in-plane direction of the substrate 50, and the optical pulses Pb1 and Pb2 guided through the waveguides may be transferred to a nearby waveguide. On the other hand, if the optical pulses Pb1 and Pb2 are input in TM mode, the evanescent field is likely to leak in the direction perpendicular to the in-plane direction of the substrate 50, which can prevent the optical pulses Pb1 and Pb2 from being transferred between the waveguides.

[0056] Referring again to Figure 1, the second adjustment unit 6 adjusts the polarization direction of the pulse train Pb that is output from the optical integrated circuit 5 and that has undergone chromatic dispersion. The second adjustment unit 6 matches the polarization direction of each of the optical pulses Pb1 and Pb2 of the pulse train Pb that is input to the correlation optical system 7 at the subsequent stage with the polarization direction of the correlation optical system 7. Details will be described later, but in this embodiment, the polarization direction of the correlation optical system 7 matches the polarization direction in the TE mode.

[0057] In one example, the inspection device 1 further includes a second polarization-maintaining fiber 12 that optically connects the optical output end 5b of the optical integrated circuit 5 and the optical input end 7a of the correlation optical system 7. The second polarization-maintaining fiber 12 is, for example, a polarization-maintaining fiber that maintains the polarization direction of the pulse train Pb, similar to the first polarization-maintaining fiber 11.

[0058] The second adjustment unit 6 is configured with a rotation mechanism that rotates the second polarization-maintaining fiber 12 around its axis. The axis of the second polarization-maintaining fiber 12 coincides with the propagation direction (X-axis direction) of the pulse train Pb. The second adjustment unit 6 is configured with, for example, a fiber rotator. The second adjustment unit 6 receives, for example, from the evaluation unit 9, an instruction regarding the angle by which to rotate the second polarization-maintaining fiber 12. Based on this instruction, the second adjustment unit 6 may rotate the second polarization-maintaining fiber 12 around its axis. For example, the second adjustment unit 6 rotates the second polarization-maintaining fiber 12 90 degrees around its axis. In this case, the polarization directions of the optical pulses Pb1 and Pb2 rotate by 90 degrees.

[0059] In the example of FIG. 1 , the polarization mode of the optical pulses Pb1 and Pb2 is the TM mode when they are output from the optical integrated circuit 5. That is, the polarization direction of the optical pulses Pb1 and Pb2 is perpendicular to the in-plane direction of the substrate 50. The second adjustment unit 6 converts the polarization mode of the optical pulses Pb1 and Pb2 to the TE mode. As a result, the polarization direction of the optical pulses Pb1 and Pb2 coincides with the in-plane direction of the substrate 50. Note that even if the polarization mode of the optical pulses Pb1 and Pb2 output from the optical integrated circuit 5 is the TM mode, if the polarization direction of the correlation optical system 7 coincides with the polarization direction in the TM mode, it is not necessary to convert the polarization mode of the optical pulses Pb1 and Pb2 by the second adjustment unit.

[0060] The correlation optical system 7 generates correlation light including cross-correlated light or auto-correlated light of the pulse train Pb output from the optical integrated circuit 5 through the waveguide 53. In this embodiment, the correlation optical system 7 includes a lens 71, an optical element 72, and a lens 73. The lens 71 is provided on the optical path between the pulse forming unit 3 and the optical element 72, and focuses the pulse train Pb output from the second adjustment unit 6 onto the optical element 72.

[0061] The optical element 72 is a light emitter that includes at least one of a nonlinear optical crystal that generates second harmonic waves (SHG) and a phosphor. Examples of nonlinear optical crystals include KTP (KTiOPO4) crystal, LBO (LiB3O5) crystal, and BBO (β-BaB2O4) crystal. Examples of phosphors include coumarin, stilbene, and rhodamine. The optical element 72 receives a pulse train Pb and generates correlation light Pc that includes cross-correlation or auto-correlation of the pulse train Pb.

[0062] The optical element 72 has polarization dependence in a predetermined direction. For example, the optical element 72 is an element that has polarization dependence on the polarization direction of the optical pulses Pb1 and Pb2 of the pulse train Pb. In this embodiment, the polarization direction in which the optical element 72 has polarization dependence matches the polarization direction in TE mode. The second adjustment unit 6 inputs the pulse train Pb to the correlation optical system 7 so that the polarization direction of the optical pulses Pb1 and Pb2 matches the polarization direction in which the optical element 72 has polarization dependence, allowing the correlation optical system 7 to efficiently generate the correlated light Pc. The lens 71 collimates or focuses the correlated light Pc output from the optical element 72.

[0063] An example of the configuration of the correlation optical system 7 will now be described in detail. Fig. 5 is a diagram schematically showing an example of the configuration of the correlation optical system 7. This correlation optical system 7A has a beam splitter 74 as an optical branching component that branches the pulse train Pb into two. The beam splitter 74 is optically coupled to the second adjustment unit 6 shown in Fig. 1, and transmits a portion of the pulse train Pb input from the second adjustment unit 6 and reflects the remainder. The branching ratio of the beam splitter 74 is, for example, 1:1.

[0064] One pulse train Pba split by the beam splitter 74 passes through an optical path 7c including a plurality of mirrors 75 and reaches the lens 71. The other pulse train Pbb split by the beam splitter 74 passes through an optical path 7d including a plurality of mirrors 76 and reaches the lens 71. The optical lengths of the optical paths 7c and 7d are different. Therefore, the plurality of mirrors 75 and the plurality of mirrors 76 form a delay optical system that imparts a time difference between the one pulse train Pba and the other pulse train Pbb split by the beam splitter 74. Furthermore, at least a portion of the plurality of mirrors 76 are mounted on a moving stage 77, making the optical length of the optical path 7d variable. Therefore, with this configuration, the time difference between the pulse trains Pba and Pbb can be varied.

[0065] In this example, the optical element 72 includes a nonlinear optical crystal. The lens 71 focuses each of the pulse trains Pba and Pbb toward the optical element 72, and causes the optical axes of the pulse trains Pba and Pbb to intersect with each other at a predetermined angle in the optical element 72. As a result, in the optical element 72, which is a nonlinear optical crystal, a second harmonic is generated starting from the intersection of the pulse trains Pba and Pbb. This second harmonic is correlated light Pc, which includes the autocorrelation of the pulse train Pb. This correlated light Pc is collimated or focused by the lens 73, and then input to the photodetector 8.

[0066] 6 is a diagram schematically illustrating another exemplary configuration of the correlation optical system 7B. In this correlation optical system 7B, the pulse train Pb passes through an optical path 7e to reach the lens 71, and the reference light pulse Pr, which is a single pulse, passes through an optical path 7f to reach the lens 71. The optical path 7f includes multiple mirrors 78 and is bent in a U-shape. Furthermore, at least some of the multiple mirrors 78 are mounted on a moving stage 79, making the optical length of the optical path 7f variable. Therefore, with this configuration, the time difference between the pulse train Pb and the reference light pulse Pr (the difference in timing at which they reach the lens 71) can be varied.

[0067] In this example, the optical element 72 also includes a nonlinear optical crystal. The lens 71 focuses the pulse train Pb and the reference optical pulse Pr toward the optical element 72, and causes the optical axis of the pulse train Pb and the optical axis of the reference optical pulse Pr to intersect with each other at a predetermined angle in the optical element 72. As a result, in the optical element 72, which is a nonlinear optical crystal, a second harmonic is generated starting from the intersection of the pulse train Pb and the reference optical pulse Pr. This second harmonic is correlated light Pc, which includes the cross-correlation of the pulse train Pb. This correlated light Pc is collimated or focused by the lens 73, and then input to the photodetector 8.

[0068] 7A and 7B are diagrams for conceptually explaining the characteristic quantities of the correlated light Pc. FIG. 7A shows an example of the time waveform of the correlated light Pc when no chromatic dispersion occurs in the pulse train Pb in the waveguide 53 of the optical integrated circuit 5 (the chromatic dispersion is zero). FIG. 7B shows an example of the time waveform of the correlated light Pc when chromatic dispersion occurs in the pulse train Pb in the waveguide 53 of the optical integrated circuit 5 (the chromatic dispersion is not zero). In the example of FIG. 7 , the correlated light Pc includes two optical pulses Pc1 and Pc2 corresponding to the optical pulses Pb1 and Pb2, respectively. Here, the peak intensity of the optical pulse Pc1 is denoted as PE1, and the peak intensity of the optical pulse Pc2 is denoted as PE2. The full width at half maximum (FWHM) of the optical pulse Pc1 is denoted as W1, and the full width at half maximum (FWHM) of the optical pulse Pc2 is denoted as W2. The peak time interval (pulse interval) between the peaks of the optical pulses Pc1 and Pc2 is denoted as G.

[0069] 7(a) and 7(b), when chromatic dispersion occurs in the pulse train Pb, the peak intensities PE1 and PE2 of the optical pulses Pc1 and Pc2 are lower, and the full widths at half maximum W1 and W2 of the optical pulses Pc1 and Pc2 are larger, compared to when chromatic dispersion does not occur. Furthermore, the peak time interval G is longer.

[0070] In this way, when chromatic dispersion occurs in the pulse train Pb, the feature quantities (peak intensities PE1, PE2, full widths at half maximum W1, W2, and peak time interval G) of the time waveform of the correlated light Pc change significantly compared to when chromatic dispersion does not occur. The amount of change depends on the amount of chromatic dispersion of the pulse train Pb in the waveguide 53. Therefore, by observing the change in the feature quantities of the time waveform of the correlated light Pc, the amount of chromatic dispersion of the pulse train Pb in the waveguide 53 can be determined accurately and easily.

[0071] Referring again to FIG. 1 , the photodetector 8 receives the correlated light Pc output from the correlation optical system 7 and detects the time waveform of the correlated light Pc. The photodetector 8 includes a photodetector such as a photodiode. The photodetector 8 detects the time waveform of the correlated light Pc by converting the intensity of the correlated light Pc into an electrical signal. The electrical signal that is the detection result is provided to the evaluation unit 9.

[0072] The evaluation unit 9 estimates the amount of chromatic dispersion based on the feature quantities of the time waveform of the correlated light Pc provided by the optical detection unit 8, and evaluates the waveguide 53 of the optical integrated circuit 5. As described above, when the correlated light Pc including the cross-correlation or autocorrelation of the pulse train Pb is generated, various feature quantities (e.g., pulse interval, peak intensity, pulse width, etc.) in the time waveform of the correlated light Pc have a significant correlation with the amount of chromatic dispersion of the measurement target. Therefore, the evaluation unit 9 can accurately estimate the amount of chromatic dispersion of the pulse train Pb in the optical integrated circuit 5, which is the measurement target, by evaluating the feature quantities of the time waveform of the correlated light Pc. The evaluation unit 9 may, for example, evaluate whether or not adjustment of the waveguide 53 is necessary based on the estimated amount of chromatic dispersion. If it is determined that adjustment of the waveguide 53 is necessary, the evaluation unit 9 may adjust the temperature of the waveguide 53 by heating the microheater 58, thereby adjusting the amount of chromatic dispersion of the optical pulses Pb1 and Pb2.

[0073] The time waveform of the correlated light Pc will be described in more detail. Fig. 8 is a graph showing an example of the time waveform of the correlated light Pc. The horizontal axis of the graph of the time waveform of the correlated light Pc represents the delay time, and the vertical axis of the graph of the time waveform of the correlated light Pc represents the intensity of the correlated light Pc.

[0074] In the correlation optical system 7A shown in FIG. 5, the pulse train Pb is separated into pulse trains Pba and Pbb by a beam splitter 74. The pulse trains Pba and Pbb are then input to an optical element 72. The optical element 72 generates correlated light Pc, which is a second harmonic, and the correlated light Pc is input to a photodetector 8. The photodetector 8 detects the intensity of the correlated light Pc. The intensity of the correlated light Pc varies depending on the time difference between the pulse trains Pba and Pbb—in other words, the delay time of the pulse train Pba relative to the pulse train Pbb, or the delay time of the pulse train Pbb relative to the pulse train Pba. The delay time is adjusted by a moving stage 77. As shown in FIGS. 9( a) to 9(d), delay times T1, T2, and T3 are the times from the midpoint of two pulses included in the pulse train Pba to the midpoint of two pulses included in the pulse train Pbb.

[0075] The light detection unit 8 may include, for example, a control unit in addition to the photodetector, and the delay time may be changed by gradually changing the position of the moving stage 77 using the control unit. The control unit may also include, for example, a counter, and measure the delay time each time the pulse train Pb is input. The control unit may then generate a time waveform of the correlated light Pc by plotting the intensity of the electrical signal of the correlated light Pc detected by the photodetector for each measured delay time.

[0076] The example of Figure 8 illustrates the intensity of the correlated light Pc when the delay times are T1, T2, 0, and T3. As shown in Figure 9(a), delay time T1 is the delay time of pulse train Pba relative to pulse train Pbb. At delay time T1, pulse train Pba lags pulse train Pbb, and pulse train Pbb and pulse train Pba do not overlap at all. As shown in Figure 9(b), delay time T2 is the delay time of pulse train Pba relative to pulse train Pbb. At delay time T2, pulse train Pba lags pulse train Pbb, and part of pulse train Pbb and part of pulse train Pba overlap. When delay time is 0, as shown in Figure 9(c), there is no time difference between pulse train Pba and pulse train Pbb, and in this case, pulse train Pbb and pulse train Pba completely overlap. As shown in FIG. 9(d), delay time T3 is the delay time of pulse train Pbb relative to pulse train Pba. During delay time T3, pulse train Pbb lags behind pulse train Pba, and part of pulse train Pbb overlaps part of pulse train Pba.

[0077] 8 and 9, the smaller the time difference between the pulse trains Pba and Pbb and the larger the overlapping area between the pulse trains Pbb and Pba, the stronger the optical intensity of the correlated light Pc. The time waveform of the correlated light Pc includes a first peak PK1, a second peak PK2, and a third peak PK3. The first peak PK1 occurs when the delay time is zero. The second peak PK2 occurs when the pulse train Pba lags behind the pulse train Pbb, and one of the two pulses in the pulse train Pbb completely overlaps with one of the two pulses in the pulse train Pba. The third peak PK3 occurs when the pulse train Pbb lags behind the pulse train Pba, and the other of the two pulses in the pulse train Pbb completely overlaps with the other of the two pulses in the pulse train Pba.

[0078] The light detection unit 8 detects, for example, the interval between the first peak PK1 and the second peak PK2 or the interval between the first peak PK1 and the third peak PK3 as a peak time interval G, which is a type of feature quantity of the time waveform. The light detection unit 8 may provide the detected peak time interval G to the evaluation unit 9.

[0079] In the above example, the control unit included in the photodetector 8 generates the time waveform of the correlated light Pc. However, the evaluation unit 9 may generate the time waveform of the correlated light Pc. That is, the evaluation unit 9 may function as the photodetector. In this case, the photodetector 8 may include a photodetector and output an electrical signal of the detected correlated light Pc to the evaluation unit 9. Alternatively, the evaluation unit 9 may include a photodetector instead of the photodetector 8. In this case, the correlated light Pc may be input to the evaluation unit 9 from the correlation optical system 7A, and the photodetector of the evaluation unit 9 may detect the time waveform of the correlated light Pc and convert the intensity of the correlated light Pc into an electrical signal. The evaluation unit 9 may generate the time waveform of the correlated light Pc by plotting the intensity of the electrical signal of the correlated light Pc output from the photodetector 8 or the electrical signal converted by the photodetector of the evaluation unit 9 for each measured delay time. The evaluation unit 9 may change the delay time by gradually changing the position of the moving stage 77.

[0080] In the above example, the correlation optical system 7A has been described as an example, but the above example may also be applied to the correlation optical system 7B. In this case, the light detection unit 8 or the evaluation unit 9 may change the delay time of the pulse train Pb with respect to the reference light pulse Pr, or the delay time of the reference light pulse Pr with respect to the pulse train Pb, by gradually changing the position of the moving stage 79. The light detection unit 8 or the evaluation unit 9 may then generate the time waveform of the correlation light Pc by plotting the intensity of the electrical signal of the correlation light Pc for each measured delay time. When applied to the correlation optical system 7B, the evaluation unit 9 may also function as a light detection unit, as in the case of application to the correlation optical system 7A.

[0081] Fig. 10 is a diagram schematically illustrating an example of the hardware configuration of the evaluation unit 9. As shown in Fig. 10, the evaluation unit 9 can be physically configured as a typical computer including a processor (CPU) 91, main storage devices such as a ROM 92 and a RAM 93, input devices 94 such as a keyboard, mouse, and touch screen, output devices 95 such as a display (including a touch screen), a communication module 96 such as a network card for transmitting and receiving data to and from other devices, an auxiliary storage device 97 such as a hard disk, etc.

[0082] The processor 91 of the computer can realize the functions of the evaluation unit 9 by means of a chromatic dispersion amount calculation program. In other words, the chromatic dispersion amount calculation program causes the processor 91 of the computer to operate as the evaluation unit 9. The chromatic dispersion amount calculation program is stored in a storage device (storage medium) inside or outside the computer, such as the auxiliary storage device 97. The storage device may be a non-transitory recording medium. Examples of the recording medium include recording media such as a flexible disk, CD, DVD, recording media such as ROM, semiconductor memory, cloud server, etc.

[0083] The auxiliary storage device 97 stores feature quantities of the time waveform of the correlated light Pc that are theoretically calculated in advance assuming that chromatic dispersion is zero. Therefore, the evaluation unit 9 can determine the amount of change in the feature quantities of the correlated light Pc that is caused by chromatic dispersion by comparing the feature quantities stored in the auxiliary storage device 97 with the feature quantities of the time waveform detected by the light detection unit 8. Therefore, the evaluation unit 9 can estimate the amount of chromatic dispersion of the object to be measured by comparing the feature quantities stored in the auxiliary storage device 97 with the feature quantities of the time waveform detected by the evaluation unit 9.

[0084] Alternatively, the evaluation unit 9 may estimate the amount of chromatic dispersion of the measurement object by comparing a feature amount of the correlated light Pc in a state where the measurement object is present with a feature amount of the correlated light Pc in a state where the measurement object is not present. Specifically, the evaluation unit 9 can estimate the amount of chromatic dispersion of the optical integrated circuit 5 by comparing a feature amount of the time waveform of the correlated light Pc detected via the optical integrated circuit 5 with a feature amount of the time waveform of the correlated light Pc detected without via the optical integrated circuit 5.

[0085] [Inspection Method Using Inspection Apparatus] Fig. 11 is a flowchart showing an inspection method using the inspection apparatus 1 having the above configuration. In this method, first, in a pulse forming step S1, a pulse train Pb is generated in which multiple optical pulses Pb1, Pb2 having different center wavelengths are arranged at a predetermined time interval. For example, multiple wavelength components contained in the measured optical pulse Pa are spatially separated by wavelength, and the multiple wavelength components are intensity-modulated and phase-modulated using a spatial light modulator 33, and then the multiple wavelength components are focused. This makes it possible to easily generate the pulse train Pb.

[0086] Subsequently, in a first adjustment step S2, the polarization directions of the optical pulses Pb1, Pb2 of the pulse train Pb input to the optical integrated circuit 5 are made orthogonal to the in-plane direction of the substrate 50. For example, a rotation mechanism is used to rotate the first polarization-maintaining fiber 11, which optically connects the pulse forming unit 3 and the optical integrated circuit 5, around its axis. The first polarization-maintaining fiber 11 is rotated so that the polarization directions of the optical pulses Pb1, Pb2 are orthogonal to the in-plane direction of the substrate 50.

[0087] Subsequently, in a propagation step S3, the pulse train Pb is propagated in the waveguide 53 of the optical integrated circuit 5. The optical pulses Pb1 and Pb2, whose polarization directions have been rotated in the first adjustment step S2, are input from the optical input unit 51, guided by the waveguide 53, and then output from the optical output unit 52. In the propagation step S3, chromatic dispersion occurs in the pulse train Pb as it propagates through the waveguide 53. For example, a difference occurs between the propagation time of the optical pulse Pb1 and the propagation time of the optical pulse Pb2, and the time interval between the optical pulse Pb1 and the optical pulse Pb2 varies.

[0088] Subsequently, in a second adjustment step S4, the polarization direction of each of the optical pulses Pb1, Pb2 of the pulse train Pb input to the correlation optical system 7 is made to coincide with the polarization direction of the correlation optical system 7. For example, a rotation mechanism is used to rotate the second polarization-maintaining fiber 12, which optically connects the optical integrated circuit 5 and the correlation optical system 7, around its axis. The second polarization-maintaining fiber 12 is rotated so that the polarization directions of the optical pulses Pb1, Pb2 coincide with the polarization direction of the correlation optical system 7.

[0089] Subsequently, in correlation light generating step S5, correlation light Pc including cross-correlation or auto-correlation of the pulse train Pb is generated using an optical element 72 including at least one of a nonlinear optical crystal and a phosphor. For example, as shown in Fig. 5, the pulse train Pb is branched into two, one branched pulse train Pbb is time-delayed relative to the other pulse train Pba, and correlation light Pc including auto-correlation of the pulse train Pb is generated based on the time-delayed one pulse train Pbb and the other pulse train Pba.

[0090] Subsequently, the time waveform of the correlated light Pc is detected in the photodetection step S6, and then the waveguide 53 of the optical integrated circuit 5 is evaluated in the evaluation step S7 based on the feature quantities of the time waveform. For example, the amount of chromatic dispersion of the pulsed laser light source 2 is estimated based on at least one of the peak intensities E1, E2, full widths at half maximum W1, W2, and peak time interval G of the correlated light Pc. In this case, the evaluation step S7 may estimate the amount of chromatic dispersion of the measurement object by comparing the feature quantities of the correlated light Pc in a state where the measurement object is present with the feature quantities of the correlated light Pc in a state where the measurement object is not present. In this case, the time waveform of the correlated light Pc detected without passing through the optical integrated circuit 5 is also detected in the photodetection step S6.

[0091] Then, in the evaluation step S7, for example, the amount of chromatic dispersion estimated by the evaluation unit 9 may be compared with a predetermined threshold value. Then, in the evaluation step S7, if the estimated amount of chromatic dispersion exceeds the threshold value, the evaluation unit 9 may evaluate that adjustment of the waveguide 53 of the optical integrated circuit 5 is necessary (evaluation step S7: YES), and if the estimated amount of chromatic dispersion is below the threshold value, the evaluation unit 9 may determine that adjustment of the waveguide 53 of the optical integrated circuit 5 is not necessary (evaluation step S7: NO).

[0092] If the evaluation unit 9 evaluates that the waveguide 53 of the optical integrated circuit 5 needs adjustment (evaluation step S7: YES), the temperature of the waveguide 53 is adjusted in step S8. For example, the evaluation unit 9 instructs the microheater 58 to set a temperature, and the microheater 58 increases the temperature until the temperature of the microheater 58 reaches the set temperature. After the temperature of the microheater 58 reaches the set temperature, the microheater 58 may maintain the set temperature. This causes a portion of the waveguide 53 to be heated at the set temperature. The microheater 58 may heat a portion of the waveguide 53 at the set temperature for a predetermined time, or may receive an instruction regarding the heating time from the evaluation unit 9 and heat a portion of the waveguide 53 at the set temperature for the instructed heating time.

[0093] In step S8, after the heating time of a portion of the waveguide 53 by the microheater 58 has elapsed, the pulse forming step S1 to the evaluation step S7 are repeated again. If, in the evaluation step S7, the evaluation unit 9 again evaluates that the waveguide 53 of the optical integrated circuit 5 needs adjustment (evaluation step S7: YES), the temperature of the waveguide 53 is adjusted again in step S8. For example, the evaluation unit 9 instructs the microheater 58 to set a new temperature, and the microheater 58 heats a portion of the waveguide 53 at the new temperature. The new temperature may be higher than the previous temperature. In the inspection method of FIG. 4, in the evaluation step S7, the evaluation unit 9 repeatedly adjusts the temperature of the waveguide 53 until it determines that the waveguide 53 of the optical integrated circuit 5 does not need adjustment. If the evaluation unit 9 determines that the waveguide 53 of the optical integrated circuit 5 does not need adjustment (evaluation step S7: NO), the inspection method flow is completed.

[0094] The effects of the inspection device 1 will be described with reference to FIGS. 12 and 13 . FIGS. 12 and 13 are graphs showing the relationship between the waveguide length of the waveguide 53 of the optical integrated circuit 5 and the pulse interval. Hereinafter, the pulse interval corresponds to the peak time interval G. FIG. 12 shows the relationship between the waveguide length of the waveguide 53 and the pulse interval when an inspection device according to a comparative example is used, and FIG. 13 shows the relationship between the waveguide length of the waveguide 53 and the pulse interval when the inspection device 1 is used. The inspection device 1 differs from the inspection device according to the comparative example mainly as follows. In the inspection device 1, the pulse train Pb is input to the optical integrated circuit 5 in a direction in which the polarization direction of the optical pulses Pb1 and Pb2 is perpendicular to the in-plane direction of the substrate 50. In contrast, in the inspection device according to the comparative example, the pulse train Pb is input to the optical integrated circuit 5 in a direction in which the polarization direction of the optical pulses Pb1 and Pb2 is aligned with the in-plane direction of the substrate 50. In the examples of FIGS. 12 and 13, the wavelength difference between the center wavelength of the light pulse Pb1 and the center wavelength of the light pulse Pb2 is 25 nm.

[0095] In the inspection device 1, the wavelength difference set in the spatial light modulator 33 is larger than in the inspection device according to the comparative example. In the inspection device 1, the connection terminal connecting the optical input end 5a of the optical integrated circuit 5 to the first polarization-maintaining fiber 11 and the connection terminal connecting the optical output end 5b of the optical integrated circuit 5 to the second polarization-maintaining fiber 12 are made of materials that are less susceptible to the effects of chromatic dispersion. The inspection device 1 does not include any materials that may disturb the polarization direction of the optical pulses Pb1 and Pb2.

[0096] As shown in FIGS. 12 and 13 , the pulse interval tends to decrease as the length of the waveguide 53 increases. The length of the waveguide 53 and the pulse interval exhibit a linear relationship. FIGS. 12 and 13 show results for each waveguide length, in which the optical detection unit 8 detects the pulse interval five times in succession, and the evaluation unit 9 calculates the average value and standard deviation based on the data provided by the optical detection unit 8. The standard deviation at each plot point in the inspection device according to the comparative example is larger than that in the inspection device 1. If the standard deviation is considered to be the measurement error, the maximum measurement error in the inspection device according to the comparative example is 10 fs (femtoseconds). In contrast, the maximum measurement error in the inspection device 1 is 1 fs.

[0097] 12 and 13, for example, the function and the coefficient of determination R ２ The coefficient of determination R ２ is 0.7766, whereas the coefficient of determination R ２ is 0.9596. As described above, the inspection device 1 can significantly reduce measurement errors compared to the inspection device according to the comparative example, and can inspect the optical integrated circuit 5 with high precision.

[0098] As described above, in the inspection device 1 according to one aspect of the present disclosure, the first adjustment unit 4 inputs the pulse train Pb to the optical integrated circuit 5 so that the polarization directions of the multiple optical pulses Pb1 and Pb2 are orthogonal to the in-plane direction of the substrate 50. This makes it less likely that polarization mixing will occur in the pulse train Pb output from the optical integrated circuit 5. As a result, the generation efficiency of the correlated light Pc in the correlation optical system 7 is increased, and at the same time, noise components that distort the correlation waveform are reduced, thereby improving the detection accuracy of the time waveform and reducing measurement errors of chromatic dispersion. This suppresses polarization mixing in the pulse train Pb output from the optical integrated circuit 5, reduces measurement errors of the amount of chromatic dispersion, and enables accurate inspection of the optical integrated circuit 5.

[0099] The inspection device 1 further includes a first polarization-maintaining fiber 11 that optically connects the pulse forming unit 3 to the optical integrated circuit 5, and the first adjustment unit 4 is configured with a rotation mechanism that rotates the first polarization-maintaining fiber 11 about its axis. In this case, by rotating the first polarization-maintaining fiber 11 about its axis, the polarization directions of the optical pulses Pb1, Pb2 of the pulse train Pb input to the optical integrated circuit 5 can be easily adjusted.

[0100] The inspection device 1 further includes a second polarization-maintaining fiber 12 that optically connects the optical integrated circuit 5 to the correlation optical system 7, and the second adjustment unit 6 is configured with a rotation mechanism that rotates the second polarization-maintaining fiber 12 about its axis. In this case, by rotating the in-plane direction of the substrate 50 of the optical integrated circuit 5 about the axis of the second polarization-maintaining fiber 12, the polarization direction of each of the optical pulses Pb1, Pb2 of the pulse train Pb input to the second polarization-maintaining fiber 12 can be easily adjusted.

[0101] The inspection device 1 further includes a second adjustment unit 6 that matches the polarization direction of each optical pulse Pb1, Pb2 of the pulse train Pb input to the correlation optical system 7 with the polarization direction of the correlation optical system 7. In this case, the second adjustment unit 6 inputs the pulse train Pb to the correlation optical system 7 so that the polarization direction of the pulse train Pb matches the polarization direction of the correlation optical system 7, which has polarization dependency. Therefore, the correlation light Pc can be efficiently generated in the correlation optical system 7. By increasing the generation efficiency of the correlation light Pc in the correlation optical system 7 and simultaneously reducing noise components that distort the correlation waveform, measurement errors of the amount of chromatic dispersion can be further reduced, and inspection of the optical integrated circuit 5 can be performed with even greater accuracy.

[0102] The inspection device 1 further includes a second polarization-maintaining fiber 12 that optically connects the optical integrated circuit 5 to the correlation optical system 7, and the second adjustment unit 6 is configured with a rotation mechanism that rotates the second polarization-maintaining fiber 12 about its axis. In this case, by rotating the in-plane direction of the substrate 50 of the optical integrated circuit 5 about the axis of the second polarization-maintaining fiber 12, the polarization direction of each of the optical pulses Pb1, Pb2 of the pulse train Pb input to the second polarization-maintaining fiber 12 can be easily adjusted.

[0103] The pulse shaping unit 3 includes a spatial light modulator 33 that sets at least one of the wavelength difference, time difference, and wavelength width of the multiple optical pulses Pb1 and Pb2 to an arbitrary value. In this case, for example, by increasing the wavelength difference between the optical pulses Pb1 and Pb2 in the pulse train Pb, the pulse interval (time difference) between the optical pulses Pb1 and Pb2 can be increased. By increasing the pulse interval between the optical pulses Pb1 and Pb2, the measurement error of the chromatic dispersion amount becomes relatively small, allowing for more accurate testing of the optical integrated circuit 5. Furthermore, for example, by increasing the wavelength width of each of the multiple optical pulses Pb1 and Pb2, the pulse width of the optical pulses Pb1 and Pb2 becomes narrower, allowing for more accurate testing of the optical integrated circuit 5.

[0104] In the inspection method described above, in the first adjustment step, the pulse train Pb is input to the optical integrated circuit 5 so that the polarization directions of the multiple optical pulses Pb1 and Pb2 are orthogonal to the in-plane direction of the substrate 50. This makes it difficult for polarization mixing to occur in the pulse train Pb output from the optical integrated circuit 5. As a result, the generation efficiency of the correlated light Pc in the correlation optical system 7 is increased, and at the same time, noise components that distort the correlation waveform are reduced, thereby improving the detection accuracy of the time waveform and reducing measurement errors of chromatic dispersion. This suppresses polarization mixing in the pulse train Pb output from the optical integrated circuit 5, reduces measurement errors of the amount of chromatic dispersion, and enables accurate inspection of the optical integrated circuit 5.

[0105] The inspection method further includes a second adjustment step of matching the polarization direction of each optical pulse Pb1, Pb2 of the pulse train Pb input to the correlation optical system 7 with the polarization direction of the correlation optical system 7. In this case, in the second adjustment step S4, the pulse train Pb is input to the correlation optical system 7 so that the polarization direction of the pulse train Pb matches the polarization direction in which the correlation optical system 7 has polarization dependency. Therefore, the correlation light Pc can be efficiently generated in the correlation optical system 7. By increasing the efficiency of generating the correlation light Pc in the correlation optical system 7 and reducing noise components that distort the correlation waveform, measurement errors of the amount of chromatic dispersion can be further reduced, and inspection of the optical integrated circuit 5 can be performed with even greater accuracy.

[0106] In evaluation step S7, the amount of chromatic dispersion of the optical integrated circuit 5 is found based on the feature amount of the time waveform of the correlated light Pc detected via the optical integrated circuit 5 and the feature amount of the time waveform of the correlated light Pc detected without via the optical integrated circuit 5. In this case, the feature amount of the time waveform of the correlated light detected without via the optical integrated circuit is used as a reference point, and the relative amount of dispersion with the feature amount of the time waveform of the correlated light detected via the optical integrated circuit is found, thereby making it possible to improve the measurement robustness of the system (stability against external factors such as changes in the environment and laser instability).

[0107] In evaluation step S7, the amount of chromatic dispersion of the optical integrated circuit 5 is found based on the feature quantities of the time waveform of the correlated light Pc that have been theoretically calculated in advance assuming that the chromatic dispersion is zero, and the feature quantities of the time waveform of the correlated light Pc detected in light detection step S6. In this case, the amount of chromatic dispersion can be found efficiently using the feature quantities that have been calculated in advance.

[0108] [Modifications] Although the embodiments of the present disclosure have been described above, the present disclosure is not necessarily limited to the above-described embodiments, and various modifications are possible without departing from the spirit of the present disclosure.

[0109] In the inspection devices according to the first to sixth modifications, the first adjustment unit 4 and the second adjustment unit 6 adjust the polarization direction of each optical pulse Pb1, Pb2 of the pulse train Pb by means other than rotating the first polarization-maintaining fiber 11 and the second polarization-maintaining fiber 12 using a fiber rotator.

[0110] [First Modification] In the inspection device according to the first modification, the first adjustment unit 4 is configured with a rotation mechanism that rotates the in-plane direction of the substrate 50 around the axis of the first polarization-maintaining fiber 11, instead of rotating the first polarization-maintaining fiber 11. For example, the rotation mechanism rotates the substrate 50 so that the in-plane direction of the substrate 50 is perpendicular to the polarization direction of the optical pulses Pb1 and Pb2. In this case, the rotation mechanism is configured with a stage mechanism such as a goniostage. This allows the first adjustment unit 4 to easily adjust the polarization direction of each optical pulse Pb1 and Pb2 of the pulse train Pb input to the optical integrated circuit 5. Note that the second adjustment unit 6 may also be configured with a rotation mechanism that rotates the in-plane direction of the substrate 50 around the axis of the second polarization-maintaining fiber 12, instead of rotating the second polarization-maintaining fiber 12.

[0111] [Second to Fourth Modifications] The rotation mechanisms constituting the first adjustment unit 4 and the second adjustment unit 6 are not limited to fiber rotators and can be replaced with various modifications. In the following examples, several examples of the rotation mechanisms will be described.

[0112] 14(a) is a simplified diagram showing an inspection apparatus 1A according to a second modified example. In the inspection apparatus 1A, the rotation mechanism constituting the first adjustment unit 4A includes a lens 41, a half-wave plate 42, a polarizing plate 43, and a lens 44. The first polarization-maintaining fiber 11 is spatially separated between the pulse forming unit 3 and the optical integrated circuit 5. As a result, the first polarization-maintaining fiber 11 includes, as end portions in contact with the space, an end portion 11a on the pulse forming unit 3 side and an end portion 11b on the optical integrated circuit 5 side. The rotation mechanism is disposed along the axis of the first polarization-maintaining fiber 11 between the end portions 11a and 11b.

[0113] The rotation mechanism constituting the second adjustment unit 6A includes a lens 61, a half-wave plate 62, a polarizing plate 63, and a lens 64. The second polarization-maintaining fiber 12 is spatially separated between the optical integrated circuit 5 and the correlation optical system 7. As a result, the second polarization-maintaining fiber 12 includes, as ends that contact the space, an end 12a on the optical integrated circuit 5 side and an end 12b on the correlation optical system 7 side. The rotation mechanism is disposed along the axis of the second polarization-maintaining fiber 12 between the end 12a and the end 12b. The following description will be given using the first adjustment unit 4A as an example, but the same description applies to the second adjustment unit 6A.

[0114] In the inspection device 1A, the pulse train Pb output from the pulse shaping unit 3 propagates through the first polarization-maintaining fiber 11 and is output from the end 11a to the lens 41. The lens 41 converts the pulse train Pb into a plane wave and guides it as a plane wave to the half-wave plate 42. A plane wave is, for example, a parallel light beam whose wavefronts form parallel planes. The half-wave plate 42 includes two optical axes with different refractive indices relative to the vibration direction of the light. When the pulse train Pb passes through the half-wave plate, the light components along these two axes travel at different speeds. By utilizing the characteristics of the half-wave plate and appropriately determining the optical axes, the vibration direction of the pulse train Pb can be rotated by 90 degrees. As a result, for example, the polarization mode of the optical pulses Pb1 and Pb2 of the pulse train Pb is converted from TE mode to TM mode.

[0115] The pulse train Pb output from the half-wave plate 42 is input to the polarizing plate 43. The polarizing plate 43 passes light that vibrates in a specific direction and blocks light that vibrates in other directions. For example, if the polarizing plate 43 passes only the TM mode optical pulses Pb1 and Pb2, it blocks the TE mode components that are mixed in with the optical pulses Pb1 and Pb2 of the pulse train Pb. The lens 44 focuses the pulse train Pb output from the polarizing plate 43 and guides it to the end 11b of the first polarization-maintaining fiber 11.

[0116] 14(b) is a simplified diagram showing an inspection apparatus 1B according to a third modified example. In the inspection apparatus 1B, the rotation mechanisms constituting the first adjustment unit 4B and the second adjustment unit 6B include a paddle-type polarization controller. The paddle-type polarization controller includes a paddle-type wave plate around which the first polarization-maintaining fiber 11 and the second polarization-maintaining fiber 12 are wound in a coil shape. By rotating this wave plate, the paddle-type polarization controller rotates the polarization of the optical pulses Pb1 and Pb2 propagating through the first polarization-maintaining fiber 11 and the second polarization-maintaining fiber 12.

[0117] FIG. 14( c) is a simplified diagram showing an inspection apparatus 1C according to a fourth modified example. In the inspection apparatus 1C, the rotation mechanism constituting the first adjustment unit 4C includes a fiber polarization splitter. The rotation mechanism constituting the second adjustment unit 6C includes a lens 65, a polarization beam splitter 66, and a lens 67. The second polarization-maintaining fiber 12 is spatially separated between the optical integrated circuit 5 and the correlation optical system 7. As a result, the second polarization-maintaining fiber 12 includes an end 12a on the optical integrated circuit 5 side and an end 12b on the correlation optical system 7 side as ends that are in contact with the space. The rotation mechanism is disposed along the axis of the second polarization-maintaining fiber 12 between the end 12a and the end 12b.

[0118] The fiber polarization splitter includes a waveguide divided into two. One waveguide is continuous with the first polarization-maintaining fiber 11, and the other waveguide branches off from the first polarization-maintaining fiber 11. The fiber polarization splitter propagates the optical pulses Pb1 and Pb2 into the two waveguides according to the polarization directions of the optical pulses Pb1 and Pb2. For example, the fiber polarization splitter propagates the TM mode component of the optical pulses Pb1 and Pb2 into one waveguide and the TE mode component into the other waveguide. As a result, the optical pulses Pb1 and Pb2 are input to the optical integrated circuit 5 in the TM polarization mode.

[0119] The lens 65 converts the pulse train Pb output from the end 12a into a plane wave and guides the plane wave to the polarizing beam splitter 66. The polarizing beam splitter 66 reflects or transmits the optical pulses Pb1 and Pb2 depending on the polarization direction of the optical pulses Pb1 and Pb2. For example, the polarizing beam splitter 66 reflects the TM mode component of the optical pulses Pb1 and Pb2 and transmits the TE mode component. As a result, the optical pulses Pb1 and Pb2 enter the correlation optical system 7 in the TE mode polarization mode.

[0120] The rotation mechanisms in the inspection apparatus 1A according to the second modification to the inspection apparatus 1C according to the fourth modification may be combined as appropriate. For example, the rotation mechanism constituting the first adjustment unit 4 may include a lens 61, a half-wave plate 62, a polarizing plate 63, and a lens 64, and the rotation mechanism constituting the second adjustment unit 6 may include a paddle-type polarization controller. Alternatively, the rotation mechanism constituting the first adjustment unit 4 may include a fiber polarization splitter, and the rotation mechanism constituting the second adjustment unit 6 may include a lens 61, a half-wave plate 62, a polarizing plate 63, and a lens 64. In either case, the first adjustment unit 4 orthogonally adjusts the polarization direction of each optical pulse Pb1, Pb2 of the pulse train Pb input to the optical integrated circuit 5 to the in-plane direction of the substrate 50. The second adjustment unit 6 aligns the polarization direction of each optical pulse Pb1, Pb2 of the pulse train Pb input to the correlation optical system 7 with the polarization direction of the correlation optical system 7.

[0121] [Fifth to Sixth Modifications] In the inspection device of the present disclosure, the pulse shaping unit 3 and the optical integrated circuit 5 do not necessarily have to be optically connected by the first polarization-maintaining fiber 11. In this case, the pulse shaping unit 3 and the optical integrated circuit 5 may be spatially and optically coupled. In the inspection device 1D according to the fifth modification and the inspection device 1E according to the sixth modification, the first adjustment unit 4 adjusts the polarization direction of each optical pulse Pb1, Pb2 of the pulse train Pb without using the first polarization-maintaining fiber 11. Similarly, in the inspection device of the present disclosure, the optical integrated circuit 5 and the correlation optical system 7 do not necessarily have to be optically connected by the second polarization-maintaining fiber 12. In this case, the optical integrated circuit 5 and the correlation optical system 7 may be spatially and optically coupled. The second adjustment unit 6 adjusts the polarization direction of each optical pulse Pb1, Pb2 of the pulse train Pb without using the second polarization-maintaining fiber 12.

[0122] 15( a) is a simplified diagram showing an inspection apparatus 1D according to a fifth modified example. The first adjustment unit 4D includes a half-wave plate 42, a polarizing plate 43, and a lens 44. The pulse train Pb output from the pulse forming unit 3 is input to the half-wave plate 42. The half-wave plate 42 rotates the polarization direction of the optical pulses Pb1 and Pb2 and outputs the pulse train Pb to the polarizing plate 43. The polarizing plate 43 blocks the TE mode components of the optical pulses Pb1 and Pb2 and outputs the TM mode components to the lens 44. The lens 44 focuses the pulse train Pb onto the optical integrated circuit 5.

[0123] The second adjustment unit 6D includes a lens 61, a half-wave plate 62, a polarizing plate 63, and a lens 64. An optical fiber 13 for guiding the optical pulses Pb1 and Pb2 to the correlation optical system 7 is connected to the optical input end 7a of the correlation optical system 7. The pulse train Pb output from the optical integrated circuit 5 is converted into a plane wave by the lens 61, and is guided as a plane wave to the half-wave plate 62. The TE mode components of the optical pulses Pb1 and Pb2 pass through the half-wave plate 62 and the polarizing plate 63 and are focused onto the optical fiber 13 by the lens 64. The TE mode components of the optical pulses Pb1 and Pb2 are guided to the correlation optical system 7 by the optical fiber 13.

[0124] 15(b) is a simplified diagram showing an inspection apparatus 1E according to a fifth modified example. The first adjustment unit 4E includes a polarizing beam splitter 45, a plurality of mirrors 46, a flipper mirror 47, and a lens 48. The pulse train Pb output from the pulse forming unit 3 is input to the polarizing beam splitter 45. The polarizing beam splitter 45 reflects or transmits the optical pulses Pb1 and Pb2 depending on the polarization directions of the optical pulses Pb1 and Pb2. For example, the polarizing beam splitter 45 reflects the TE mode component of the optical pulses Pb1 and Pb2 and transmits the TM mode component.

[0125] The TM mode components of the optical pulses Pb1 and Pb2 that have passed through the polarizing beam splitter 45 pass through a flipper mirror 47 and enter a lens 48. The lens 48 focuses the TM mode components of the optical pulses Pb1 and Pb2 onto the optical integrated circuit 5. On the other hand, the TE mode components of the optical pulses Pb1 and Pb2 that have been reflected by the polarizing beam splitter 45 travel along an optical path formed by multiple mirrors 46, are reflected by the flipper mirror 47, and enter the lens 48. The lens 48 focuses the TE mode components of the optical pulses Pb1 and Pb2 onto the optical integrated circuit 5.

[0126] In the first adjustment unit 4E, the TE mode components of the optical pulses Pb1 and Pb2 are input to the flipper mirror 47 via an optical path formed by a plurality of mirrors 46. As a result, the TE mode components of the optical pulses Pb1 and Pb2 are input to the optical integrated circuit 5 later than the TM mode components. The optical integrated circuit 5 may, for example, propagate only the TM mode component that is input first, and block the TE mode component that is input later.

[0127] The second adjustment unit 6E includes a lens 68, a polarizing beam splitter 69, a plurality of mirrors 81, a flipper mirror 82, and a lens 83. As with the second adjustment unit 6D, an optical fiber 13 for guiding the pulse train Pb to the correlation optical system 7 is connected to the optical input end 7a of the correlation optical system 7. The pulse train Pb output from the optical integrated circuit 5 is input to the polarizing beam splitter 69 by the lens 68. The polarizing beam splitter 69 reflects, for example, the TM mode component of the optical pulses Pb1 and Pb2 and transmits the TE mode component.

[0128] The TE mode components of the optical pulses Pb1 and Pb2 that have passed through the polarizing beam splitter 45 pass through a flipper mirror 82 and enter a lens 83. The lens 83 focuses the TE mode components of the optical pulses Pb1 and Pb2 into the optical fiber 13. On the other hand, the TM mode components of the optical pulses Pb1 and Pb2 that have been reflected by the polarizing beam splitter 69 travel along an optical path formed by multiple mirrors 81, are reflected by the flipper mirror 82, and enter the lens 83. The lens 83 focuses the TE mode components of the optical pulses Pb1 and Pb2 into the optical fiber 13.

[0129] In the second adjustment unit 6E, the TM mode components of the optical pulses Pb1 and Pb2 are input to the flipper mirror 82 via an optical path formed by a plurality of mirrors 81. As a result, the TM mode components of the optical pulses Pb1 and Pb2 are input to the correlation optical system 7 later than the TE mode components. The correlation optical system 7 may, for example, generate correlated light of only the TE mode component that inputs first, and block the TM mode component that inputs later.

[0130] [Seventh Modification] The spatial light modulator 33 included in the pulse shaping unit 3 may set the wavelength width of each of the optical pulses Pb1 and Pb2 to any desired size. The wavelength width is, for example, the full width at half maximum (FWHM). The spatial light modulator 33 may include a filter pattern in each modulation region 33b that limits the pass wavelength band of the optical pulses Pb1 and Pb2. As a result, the spatial light modulator 33 may set the wavelength width of each of the optical pulses Pb1 and Pb2 to any desired size in accordance with the pass wavelength band of the filter pattern. The wavelength width setting range is, for example, 2 nm to 15 nm.

[0131] 16 is a graph showing an example of a case where a filter pattern is applied to each of the optical pulses Pb1 and Pb2. The example of FIG. 16 shows an example where an 8 nm filter pattern and a 4 nm filter pattern are applied to each of the optical pulses Pb1 and Pb2. 4 nm and 8 nm represent the wavelength widths of the pass wavelength bands of the optical pulses Pb1 and Pb2. It can be seen that the wavelength widths of the optical pulses Pb1 and Pb2 to which the 4 nm filter pattern is applied are narrower than the wavelength widths of the optical pulses Pb1 and Pb2 to which the 8 nm filter pattern is applied.

[0132] 17A is a graph showing the change in average output when the spatial light modulator 33 changes the wavelength width of each of the light pulses Pb1 and Pb2. The average output can be measured, for example, by measuring the light pulses Pb1 and Pb2 with a power meter or the like. As shown in FIG. 17A, it can be seen that the average output increases as the wavelength width increases.

[0133] 17(b) is a graph showing the change in pulse width when the spatial light modulator 33 changes the wavelength width of each of the light pulses Pb1 and Pb2. The pulse width here represents the time extension of the light pulses Pb1 and Pb2. As shown in FIG. 17(b), it can be seen that the pulse width decreases as the wavelength width increases.

[0134] 18 and 19 show the relationship between the waveguide length of the waveguide 53 of the optical integrated circuit 5 and the pulse interval when the wavelength width is changed. FIG. 18(a) is a graph showing the relationship when the wavelength width is 4 nm. FIG. 18(b) is a graph showing the relationship when the wavelength width is 6 nm. FIG. 19 is a graph showing the relationship when the wavelength width is 8 nm. As shown in FIGS. 18 and 19, as the wavelength width increases, the standard deviation at each plot point decreases. In FIG. 18(a), the maximum measurement error is 4.2 fs. In FIG. 18(b), the maximum measurement error is 1.7 fs. In FIG. 19, the maximum measurement error is 1.2 fs. Also, as shown in FIGS. 18 and 19, as the wavelength width increases, the coefficient of determination R ２ becomes larger.

[0135] From the above, it can be seen that as the wavelength width increases, the pulse width decreases, and accordingly, the repeat measurement error and linearity improve. In this case, for example, by increasing the wavelength width of each of the multiple optical pulses Pb1 and Pb2 by the pulse forming unit 3, the pulse width of the optical pulses Pb1 and Pb2 becomes narrower, and the repeat measurement error of the chromatic dispersion amount can be relatively reduced. This allows for more accurate inspection of the optical integrated circuit 5.

[0136] 12 and 13, it can be seen that the measurement error is improved when TM mode optical pulses Pb1 and Pb2 are input to the optical integrated circuit 5. Furthermore, it can be seen from the results of Fig. 18 and 19 that the measurement error is improved and the linearity of the measurement is improved when the wavelength width is increased.

[0137] 20 is a diagram for explaining the relationship between the wavelength difference and the wavelength resolution of the measured chromatic dispersion. The unit of chromatic dispersion D in this case is expressed in [ps / nm / km]. Chromatic dispersion D is the wavelength resolution of two light λ １ , λ ２ is expressed in picoseconds as the time difference that occurs when light λ propagates through the waveguide for 1 km. １ and light λ ２The larger the wavelength difference between the two wavelengths (λ ) used for measurement, the larger the time difference that occurs, making it easier to detect the time difference and increasing the detection sensitivity. However, on the other hand, an increase in the wavelength difference causes a decrease in wavelength resolution. １ and λ ２ ) and the wavelength λ of the chromatic dispersion D actually measured Ｍ λ Ｍ is λ Ｍ = λ １ + (λ １ -λ ２ ) / 2), so λ １ and λ ２ The closer the values ​​of λ are, the better the wavelength resolution of the measured chromatic dispersion D. However, as shown in FIG. １ and λ ２ If the wavelength difference between the measured and the wavelength dispersion D is large, the measured value of the wavelength dispersion D will be D1, which may result in a large deviation from the true value D2. Therefore, when actually inspecting an optical integrated circuit, the wavelength difference should be set taking into consideration the trade-off between the detection sensitivity and the wavelength resolution.

[0138] The inspection device and inspection method are not limited to the above-described embodiment and configuration example, and various modifications are possible. For example, in the above-described embodiment, the correlation optical system 7 is used to determine the amount of chromatic dispersion of the optical integrated circuit 5, but instead of the correlation optical system 7, the amount of chromatic dispersion of the optical integrated circuit 5 may be determined using a pulse shift method, a phase modulation method, or spectral interference.

[0139] The pulse shift method is a method for directly measuring the group delay time of two optical pulses Pb1 and Pb2 for each wavelength in the time domain. In the pulse shift method, optical pulses Pb1 and Pb2 having a pulse width on the order of nanoseconds are propagated through an optical fiber, and the group delay time of each optical pulse is measured. This makes the measurement easy. However, to obtain a measurable delay time, the optical pulses Pb1 and Pb2 must be propagated through an optical fiber with a sufficiently long fiber length (e.g., several kilometers).

[0140] Spectral interferometry is a method of estimating chromatic dispersion from changes in the spectral shape of two optical pulses Pb1 and Pb2 used as sample pulse light by observing the interference between the sample pulse light and a reference pulse light in the spectral domain. For example, a Mach-Zehnder optical system is used as the optical system. Spectral interferometry can accurately measure relatively small changes in dispersion, making it applicable to dispersion measurements using relatively short optical fibers. However, compared to pulse shifting, this method requires more precise manipulation to synchronize the timing (delay) of the interfering pulses, and therefore requires advanced adjustments each time the sample being observed changes.

[0141] When the inspection device does not include a correlation optical system 7, the first adjustment unit 4 inputs the pulse train Pb to the optical integrated circuit 5 so that the polarization directions of the multiple optical pulses Pb1, Pb2 are orthogonal to the in-plane direction of the substrate 50. This makes it difficult for polarization mixing to occur in the pulse train Pb output from the optical integrated circuit 5. As a result, the detection accuracy of the time waveform is improved and measurement errors in chromatic dispersion are reduced. This suppresses polarization mixing in the pulse train Pb output from the optical integrated circuit 5, allowing for accurate inspection of the optical integrated circuit 5.

[0142] The evaluation unit 9 may determine the acceptability of the optical integrated circuit 5 instead of evaluating the waveguide 53 of the optical integrated circuit 5. For example, in the inspection flow shown in Fig. 12, in the evaluation step S7, if the estimated chromatic dispersion amount exceeds a threshold, the evaluation unit 9 may determine that the chromatic dispersion amount of the pulse train Pb in the optical integrated circuit 5 is abnormal, and if it is below the threshold, the evaluation unit 9 may determine that the chromatic dispersion amount of the pulse train Pb in the optical integrated circuit 5 is normal. In this case, heating of a part of the waveguide 53 by the microheater 58 may not be performed.

[0143] 1, 1A, 1B, 1C, 1D, 1E... inspection device, 3... pulse forming unit, 7, 7A, 7B... correlation optical system, 4, 4A, 4B, 4C, 4D, 4E... first adjustment unit, 6, 6A, 6B, 6C, 6D, 6E... second adjustment unit, 5... optical integrated circuit, 8... photodetection unit, 9... evaluation unit, 11... first polarization-maintaining fiber, 12... second polarization-maintaining fiber, 33... spatial light modulator, 50... substrate, 53... waveguide, Pb, Pba, Pbb... pulse train, Pb1, Pb2, Pc1, Pc2... optical pulse, Pc... correlated light, S1... pulse forming step, S2... first adjustment step, S4... second adjustment step, S5... correlated light generation step, S7... evaluation step.

Claims

1. An inspection device for inspecting an optical integrated circuit having a waveguide provided on a substrate, comprising: a pulse forming unit that generates a pulse train in which a plurality of optical pulses having different center wavelengths are arranged at a predetermined time interval; a correlation optical system that has polarization dependency in a predetermined polarization direction and generates correlated light including cross-correlated light or auto-correlated light of the pulse train that is output from the optical integrated circuit through the waveguide; a light detecting unit that detects the time waveform of the correlated light; an evaluation unit that evaluates the waveguide of the optical integrated circuit based on features of the time waveform; and a first adjustment unit that orthogonally adjusts the polarization direction of each optical pulse of the pulse train that is input to the optical integrated circuit to be perpendicular to the in-plane direction of the substrate.

2. An inspection device according to claim 1, further comprising a first polarization-maintaining fiber that optically connects said pulse forming section to said optical integrated circuit, and said first adjustment section is configured by a rotation mechanism that rotates said first polarization-maintaining fiber around its axis.

3. An inspection device according to claim 1, further comprising a first polarization-maintaining fiber that optically connects said pulse forming section to said optical integrated circuit, and said first adjustment section is configured by a rotation mechanism that rotates said in-plane direction of said substrate around the axis of said first polarization-maintaining fiber.

4. An inspection device as described in any one of claims 1 to 3, further comprising a second adjustment unit that aligns the polarization direction of each optical pulse of the pulse train input to the correlation optical system with the polarization direction of the correlation optical system.

5. An inspection device according to claim 4, further comprising a second polarization-maintaining fiber that optically connects the optical integrated circuit to the correlation optical system, and wherein the second adjustment unit is configured by a rotation mechanism that rotates the second polarization-maintaining fiber around its axis.

6. An inspection device according to any one of claims 1 to 5, wherein the pulse forming section includes a spatial light modulator that sets at least one of the wavelength difference, time difference, and wavelength width of the plurality of optical pulses to an arbitrary value.

7. An inspection device for inspecting an optical integrated circuit having a waveguide provided on a substrate, comprising: a pulse forming unit that generates a pulse train in which a plurality of optical pulses having different center wavelengths are arranged at predetermined time intervals; an optical detection unit that detects the time waveform of the pulse train generated by the pulse forming unit and output from the optical integrated circuit after passing through the waveguide; an evaluation unit that evaluates the waveguide of the optical integrated circuit based on features of the time waveform; and a first adjustment unit that orthogonally adjusts the polarization direction of each optical pulse of the pulse train input to the optical integrated circuit to be perpendicular to the in-plane direction of the substrate.

8. An inspection method for inspecting an optical integrated circuit having a waveguide provided on a substrate, comprising: a pulse forming step of generating a pulse train in which a plurality of optical pulses having different center wavelengths are arranged at a predetermined time interval; a correlated light generating step of generating correlated light including cross-correlated light or auto-correlated light of the pulse train output from the optical integrated circuit using a correlation optical system having polarization dependency in a predetermined polarization direction; a light detection step of detecting the time waveform of the correlated light; an evaluation step of evaluating the waveguide of the optical integrated circuit based on features of the time waveform; and a first adjustment step of orthogonalizing the polarization direction of each optical pulse of the pulse train input to the optical integrated circuit to be perpendicular to the in-plane direction of the substrate.

9. An inspection method according to claim 8, further comprising a second adjustment step of matching the polarization direction of each optical pulse of said pulse train input to said correlation optical system with said polarization direction of said correlation optical system.

10. An inspection method according to claim 8 or 9, wherein the evaluation step determines the amount of chromatic dispersion of the optical integrated circuit based on the feature amount of the time waveform of the correlated light detected via the optical integrated circuit and the feature amount of the time waveform of the correlated light detected without passing through the optical integrated circuit.

11. An inspection method according to claim 8 or 9, wherein in the evaluation step, the amount of chromatic dispersion of the optical integrated circuit is determined based on a feature of the time waveform of the correlated light theoretically calculated in advance on the assumption that chromatic dispersion is zero, and a feature of the time waveform of the correlated light detected in the light detection step.

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