Optical interference distance measurement device, optical interference distance measurement method, and optical interference distance measurement program

Abstract

An optical interference distance measurement device (100) comprises: an optical member (11) that focuses, onto an object (13), signal light (14) emitted from a light source (1); a window glass (12) provided between the optical member (11) and the object (13) at a predetermined distance from the optical member (11), and having a planar shape with a predetermined thickness; and a distance correction unit (7) that corrects the distance to the object (13) calculated from a spectrum obtained by heterodyning light generated through reflection of the signal light (14) on the object (13), on the basis of the predetermined distance mentioned above and a spectrum obtained by heterodyning reference light (16) generated through reflection of the signal light (14) on a surface (12A) of the window glass (12) facing the object (13).

Description

Optical Interference Distance Measurement Device, Optical Interference Distance Measurement Method, and Optical Interference Distance Measurement Program

[0001] The present disclosure relates to an optical interference distance measurement device, an optical interference distance measurement method, and an optical interference distance measurement program.

[0002] An optical interference distance measurement device using optical coherence tomography (OCT) technology irradiates one of the branched lights in the device onto a measurement object, returns the light generated by reflection from the measurement object into the device, and interferes the light returned into the device with the other branched light to obtain an optical path length and convert it into a distance. Further, by making it a wavelength sweep type in which the wavelength of the light output from the light source changes in time series, it is easy to separate them when there are many lights reflected by the measurement object or scattered outside the measurement object. At this time, if the wavelength sweep of the light source is not an ideal linear change with respect to time, the distance resolution deteriorates.

[0003] In the signal processing device in the optical sensor device described in Patent Document 1, based on the internal reception signal converted into a digital signal by an analog-to-digital converter (ADC), a first frequency fluctuation reference signal data serving as a reference for the frequency fluctuation of the light output from the wavelength sweep light source is further calculated, and a digital-to-analog converter (DAC) converts the first frequency fluctuation reference signal data calculated by the signal processing device into an analog signal to generate a first frequency fluctuation reference signal as a first clock signal. The ADC samples the reception signal acquired by the optical heterodyne receiver in synchronization with the first frequency fluctuation reference signal generated by the DAC.

[0004] International Publication No.: WO / 2022 / 264393

[0005] However, the optical sensor device described in Patent Document 1 is an optical interference distance measurement device based on OCT, and like known conventional technologies, it is necessary to correct the change in refractive index due to temperature, atmospheric pressure, or aerosol between the point where the light is branched in the device and the distance measurement target object.

[0006] This disclosure aims to provide an optical interference distance measuring device, an optical interference distance measuring method, and an optical interference distance measuring program that can correct for refractive index changes due to temperature, atmospheric pressure, or aerosols.

[0007] The optical interference distance measuring device of the present disclosure comprises: an optical member that focuses signal light emitted from a light source onto a target to be measured; a flat window glass of a predetermined thickness provided between the optical member and the target to be measured at a predetermined distance from the optical member; and a distance correction unit that corrects the distance to the target to be measured calculated from the distance measuring spectrum obtained by performing the heterodyne process on the light generated when the signal light is reflected by the target to be measured, based on a reference light spectrum obtained by performing the heterodyne process on the light generated when the signal light is reflected by the target to be measured, and the predetermined distance.

[0008] The optical interference distance measurement method of the present disclosure is an optical interference distance measurement method performed by a computer, comprising the steps of: obtaining a reference light spectrum by performing heterodyne processing on a reference light generated when signal light emitted from a light source and focused onto a target to be measured by an optical member is reflected by a first surface of a planar window glass, which is provided between the optical member and the target to be measured at a predetermined distance from the optical member and has a predetermined thickness, and facing the target to be measured; obtaining a distance measurement spectrum by performing the heterodyne processing on the light generated when the signal light is reflected by the target to be measured; and correcting the distance to the target to be measured calculated from the distance measurement spectrum based on the reference light spectrum and the predetermined distance.

[0009] The optical interference distance measurement program of this disclosure causes a computer to perform the following steps: obtain a reference light spectrum by performing heterodyne processing on a reference light generated when signal light emitted from a light source and focused onto a target to be measured by an optical member is reflected by a first surface of a flat window glass, which is provided between the optical member and the target to be measured at a predetermined distance from the optical member and has a predetermined thickness, and which is planar; obtain a distance measurement spectrum by performing the heterodyne processing on the light generated when the signal light is reflected by the target to be measured; and correct the distance to the target to be measured calculated from the distance measurement spectrum based on the reference light spectrum and the predetermined distance.

[0010] This disclosure makes it possible to provide an optical interference distance measuring device, an optical interference distance measuring method, and an optical interference distance measuring program that can correct for refractive index changes due to temperature, atmospheric pressure, or aerosols.

[0011] This is a schematic diagram showing an example configuration of an optical interference distance measuring device according to Embodiment 1. This is a block diagram showing an example of the hardware configuration of the calculation unit according to Embodiment 1. This is a schematic diagram showing an example configuration of an optical interference distance measuring device according to Embodiment 3. This is an explanatory diagram showing the interference light spectrum corresponding to reflected light.

[0012] The optical interference distance measuring device according to an embodiment will be described below with reference to the drawings. The following embodiment is merely an example, and it is possible to combine the embodiments as appropriate and modify each embodiment as appropriate.

[0013] Embodiment 1 Figure 1 is a schematic diagram showing an example of the configuration of an optical interference distance measuring device 100 according to Embodiment 1. The optical interference distance measuring device 100 comprises a light source 1, an optical splitter 2, an optical circulator 3, an interference calculation unit 4, an optical transmission component 5, a distance correction unit 7, a distance conversion unit 8, an output unit 9, an optical member 11, and a window glass 12.

[0014] Light source 1 outputs light (wavelength-swept light) whose frequency changes over time to optical splitter 2. In other words, light source 1 performs frequency sweeping (wavelength sweeping). Therefore, light source 1 outputs light whose frequency changes over time to optical splitter 2. For example, as light source 1, a laser light source whose wavelength can be controlled by controlling the resonator length, or a laser light source whose wavelength changes depending on the amount of injected current can be used. By performing frequency sweeping, light source 1 may output light that alternately repeats continuous triangular wave-like up-chirp and down-chirp patterns, or light that repeats sawtooth wave-like up-chirp patterns, or light that repeats sawtooth wave-like down-chirp patterns, or it may output a pulsed up-chirp or down-chirp chirp pulse signal.

[0015] The optical splitter 2 splits the light output by the wavelength-swept light source into signal light 14 and reference light. The optical splitter 2 outputs the split signal light 14 to the optical circulator 3 and outputs the split reference light to the interference calculation unit 4.

[0016] The optical circulator 3 outputs the signal light 14 branched by the optical splitter 2 to the optical transmission component 5. The optical transmission component 5 emits the signal light 14 branched by the optical splitter 2 from its output end face 6 towards the object 13 to be measured via the optical member 11 and the window glass 12, and receives the reflected light reflected by the object 13. The optical transmission component 5 outputs the received reflected light to the interference calculation unit 4 via the optical circulator 3. As will be described later, in Embodiment 1, the optical positional relationships and distances of the optical member 11, the window glass 12, etc., are predetermined.

[0017] The interference calculation unit 4 combines the reference light branched by the optical splitter 2 and the reflected light received by the optical transmission component 5, and obtains a received signal (beat signal) as an electrical signal by photoelectric conversion of the combined light. Specifically, the interference calculation unit 4 performs heterodyne processing on the reference light branched by the optical splitter 2 and the reflected light received by the optical transmission component 5. Furthermore, the interference calculation unit 4 converts the optical signal obtained by the combination into an electrical signal by photoelectric conversion, for example, using a photodiode (PD).

[0018] The distance correction unit 7 performs distance correction by correcting for refractive index changes due to temperature, atmospheric pressure, or aerosols. The optical interference distance measuring device 100 includes a light source 1, an optical element 11 which is a lens that focuses light onto the object 13 to be measured, and a window glass 12. The optical positional relationships and distances of the optical element 11, the window glass 12, etc., are predetermined. The optical interference distance measuring device 100 according to Embodiment 1 acquires the measurement spectrum of the interference signal generated by the reflected light from each surface or end face of the optical elements such as the optical element 11, the window glass 12, and the optical transmission component 5, and the reference light branched by the optical splitter 2. It corrects the distance to the object to be measured based on the difference between the measurement spectrum and the known ideal spectrum based on the known optical distances of the optical elements. For example, the distance correction unit 7 can perform distance correction based on the spectrum of the reference light 16 generated when the signal light 14 is reflected by the surface 12A of the window glass 12 facing the object 13 (referred to as the "first surface" in the claims), and a predetermined distance between the optical member 11 and the window glass 12. Specifically, the correction coefficient is the ratio of the frequency of the ideal spectrum, which is an ideal value in terms of the optical positional relationship and distance in an ideal state free from the influence of refractive index changes due to temperature, atmospheric pressure, or aerosols, to the frequency of the reference light spectrum obtained by combining the reference light 16 with the reference light branched by the optical splitter 2. Distance correction is performed by multiplying the frequency of the distance measurement spectrum obtained by combining the light reflected by the object 13 of the signal light 14 with the reference light branched by the optical splitter 2 by this correction coefficient. Each surface or end face of the optical component is a surface that reflects the signal light 14.

[0019] In addition to multiplying the spectral frequency by a correction coefficient, the distance correction unit 7 may also perform distance correction by using the ratio of a predetermined distance between the optical element 11 and the window glass 12 to the distance calculated based on the reference light spectrum obtained by combining the reference light branched by the optical splitter 2 and the reference light 16. Specifically, the correction coefficient is set as the ratio of a predetermined distance to the distance calculated based on the reference light spectrum obtained by combining the reference light branched by the optical splitter 2 with the reference light 16, and distance correction is performed by multiplying the distance calculated based on the distance measurement spectrum obtained by combining the light reflected by the object 13 of the signal light 14 with the reference light branched by the optical splitter 2 by this correction coefficient.

[0020] Furthermore, when performing the distance correction described above, if the distance corresponding to the frequency of the distance measurement spectrum does not have an ideal linear relationship during the frequency sweep of light source 1, the distance correction can be further performed by correcting it in advance to achieve a linear relationship.

[0021] The optical element 11 is a lens whose surfaces 11A and 11B are spherical or aspherical and made of optical glass or the like with a known refractive index. The window glass 12 is planar and has surfaces 12A and 12B, has a predetermined uniform thickness and is made of optical glass or the like with a known refractive index. The signal light 14 emitted from the optical transmission component 5 is focused by the optical element 11, then passes through the window glass 12 and irradiates the object 13. However, as shown in Figure 1, a portion of the signal light 14 is reflected by surface 12A of the window glass 12 to produce a reference light 16, but a portion of the reference light 16 is also reflected by surface 12B of the window glass 12 facing the optical element 11 (the "second surface" in the claims) to produce ghost light 15. The ghost light 15 is reflected by the object 13 and generates an interference spectrum with the reference light in the interference calculation unit 4. The aforementioned interference spectrum is a ghost spectrum of the interference spectrum obtained from the reflected light of the signal light 14 at object 13, and is unnecessary for distance measurement. It can cause false detection when identifying the interference spectrum obtained from the reflected light of the signal light 14 at object 13. The effect of ghost light 15 is suppressed in Embodiments 2 and 3, which will be described later.

[0022] The distance conversion unit 8 calculates measurement data for object 13 based on the corrected received signal output by the distance correction unit 7. The measurement data calculated by the distance conversion unit 8 may include, for example, information indicating the distance from the optical element 11 of the optical interference distance measuring device 100 to object 13, or information indicating the position of object 13. The distance conversion unit 8 outputs the calculated measurement data to the output unit 9. The output unit 9 is a display device such as an LCD (liquid crystal display), or an output device such as a printer or plotter.

[0023] The distance correction unit 7 and the distance conversion unit 8 may be configured as a calculation unit 10. Figure 2 is a block diagram showing an example of the hardware configuration of the calculation unit 10 according to Embodiment 1. As shown in Figure 2, the calculation unit 10 is composed of a computer in which each of the following components, which are calculation elements (processors), a CPU (Central Processing Unit) 21, main memory 22, an input / output interface (I / O interface) 23, and a storage unit 24, are connected to a system bus 25. The calculation unit 10 may be composed of multiple computers connected by a network, or it may be composed of processing circuits.

[0024] The CPU 21 is an IC (Integrated Circuit) that performs arithmetic processing. In addition to the CPU 21, other arithmetic elements such as a DSP (Digital Signal Processor), GPU (Graphics Processing Unit), Network Processor, or FPGA (Field Programmable Gate Array) may also be used. The CPU 21 can realize an optical interference distance measurement method that has the function of performing distance correction by correcting for refractive index changes due to temperature, atmospheric pressure, or aerosols, and the function of calculating measurement data related to the object 13 by executing the optical interference distance measurement program according to Embodiment 1. As a result, the CPU functions as a distance correction unit 7 and a distance conversion unit 8 by executing the optical interference distance measurement program. The optical interference distance measurement program is provided, for example, on a recording medium on which these are recorded.

[0025] The main memory 22 is composed of a volatile storage device such as RAM (Random Access Memory) or a non-volatile storage device such as ROM (Read Only Memory). The storage unit 24 is composed of a non-volatile storage device such as an HDD (Hard Disk Drive) or flash memory.

[0026] The I / O interface 23 is a port to which the interference calculation unit 4 and the output unit 9, etc., are connected.

[0027] As described above, according to Embodiment 1, in an optical interference distance measuring device 100 in which the optical positional relationships and distances of the optical members 11 and the window glass 12 are predetermined, the ratio of the frequency of the ideal spectrum at the predetermined optical positional relationships and distances in an ideal state unaffected by changes in refractive index due to temperature, atmospheric pressure, or aerosols, to the frequency of the reference light spectrum obtained by combining the reference light 16 generated when the signal light 14 is reflected off the surface 12A of the window glass 12 with the reference light branched by the optical splitter 2 is used as a correction coefficient. By multiplying the frequency of the distance measuring spectrum obtained by combining the light reflected by the signal light 14 from the object 13 with the reference light branched by the optical splitter 2 by this correction coefficient, it becomes possible to correct for changes in refractive index due to temperature, atmospheric pressure, or aerosols.

[0028] Embodiment 2 will now be described. Embodiment 2 differs from Embodiment 1 in that an anti-reflective coating (AR coating) is applied to surface 12B of the window glass 12. However, the other components are the same as in Embodiment 1, and the same reference numerals are used for components that are the same as in Embodiment 1, and detailed explanations will be omitted.

[0029] The optical interference distance measuring device according to Embodiment 2 uses the reflected light from the surface 12A of the window glass 12 that is not coated with AR as the reference light 16, and reduces ghost light 15 by applying an AR coating to the surface 12B of the window glass 12, thereby suppressing the reflected light generated on surface 12B.

[0030] Furthermore, since the surface 12A of the window glass 12 is not coated with AR, a portion of the signal light 14 is reflected by surface 12A, similar to Embodiment 1, generating the reference light 16.

[0031] As described above, according to Embodiment 2, it is possible to use the reflected light from the surface 12A of the window glass 12 that is not coated with AR as the reference light 16, and to reduce ghost light 15 by suppressing the reflected light generated on surface 12B of the window glass 12 by applying an AR coating to surface 12B.

[0032] Embodiment 3 will now be described. Figure 3 is a schematic diagram showing an example of the configuration of the optical interference distance measuring device 200 according to Embodiment 3. The optical interference distance measuring device 200 according to Embodiment 3 differs from Embodiment 2 in that it includes a distance correction / ghost removal unit 17 which has a filter processing unit 18 that removes the interference signal between the ghost light 15, which is the reflected light from surface 12B of the window glass, and the reference light, based on the interference position which is determined by the thickness of the window glass 12, etc., from the interference signal generated by the reflected light from surface 12A and surface 12B of the window glass 12 and the reference light. However, the other configurations are the same as in Embodiment 2, and the same reference numerals as in Embodiment 2 are used for the same configurations, and detailed explanations are omitted.

[0033] The measurement spectrum detected by the optical interference distance measuring device 200 is the spectrum of the interference signal generated by the reflected light from all interfaces, such as the optical member 11, the window glass 12, and the exit end face 6 of the optical transmission component 5, and the reference light branched by the optical splitter 2. As shown in Figure 3, when the signal light 14 is reflected by the exit end face 6 of the optical transmission component 5, reflected light R1 is generated; when it is reflected by the surface 11B of the optical member 11, reflected light R2 is generated; when it is reflected by the surface 11A of the optical member 11, reflected light R3 is generated; when it is reflected by the surface 12B of the window glass 12, reflected light R4 is generated; when it is reflected by the surface 12A of the window glass 12, reflected light R5 is generated; and when it is reflected by the object 13, reflected light R6 is generated. Then, due to the ghost light 15, reflected light R7 is generated as if the signal light 14 had been reflected by the ghost surface 13G.

[0034] Figure 4 is an explanatory diagram showing the interference light spectra (hereinafter abbreviated as "spectrums") S1 to S7 corresponding to reflected light R1 to R7. In Figure 4, the horizontal axis represents frequency and the vertical axis represents spectral intensity. Figure 4 shows that spectrum S1 is generated by reflected light R1, spectrum S2 by reflected light R2, spectrum S3 by reflected light R3, spectrum S4 by reflected light R4, spectrum S5 by reflected light R5, spectrum S6 by reflected light R6, and spectrum S7 by reflected light R7.

[0035] In Figure 4, the spectrum S6 is used for measuring the distance of object 13, while the other spectra S1 to S5 and spectrum S7 correspond to noise in the distance measurement. Of the noise, the spectrum S6 generated by ghost light 15 is particularly problematic, as it occurs at a frequency close to spectrum S6 and is therefore likely to cause distance measurement errors. In Embodiment 3, the distance correction / ghost removal unit 17 determines that spectrum S7 is a spectrum to be ghost-removed and excludes it. For example, if the spectrum S7 to be ghost-removed relates to ghost light 15 that was reflected by the window glass 12 surface 12B after the signal light 14 was reflected by surface 12A of the window glass 12, then it can be identified by the appearance of the spectrum S7 relating to the ghost light 15 at a different frequency from the spectrum S6 obtained by the reflection of the signal light 14 by object 13, using a spectral interval that indicates the difference in frequency calculated based on the predetermined thickness and refractive index of the window glass 12. The distance correction / ghost removal unit 17 improves the accuracy of distance measurement by preventing the spectrum S7, which is considered to be the target of ghost removal, from being used for measuring the distance of object 13.

[0036] In Figures 1 and 3, the object 13 being measured is shown as a planar shape, but this shape is not limited to this.

[0037] As described above, the optical interference distance measuring device 200 according to Embodiment 3 can identify and eliminate the spectrum S7 related to ghost light 15 based on the spectral interval calculated based on the predetermined thickness of the window glass 12 and the refractive index, and on the fact that the spectrum S7 related to ghost light 15 appears at a different frequency from the spectrum S6 obtained when the signal light 14 is reflected by the object 13. As a result, the influence of ghost light 15 on the measurement results for the object 13 can be suppressed.

[0038] 1 Light source, 2 Optical splitter, 3 Optical circulator, 4 Interference calculation unit, 5 Optical transmission component, 6 Outlet end face, 7 Distance correction unit, 8 Distance conversion unit, 9 Output unit, 10 Calculation unit, 11 Optical element, 11A, 11B surface, 12 Window glass, 12A, 12B surface, 13 Object, 13G Ghost surface, 14 Signal light, 15 Ghost light, 16 Reference light, 17 Distance correction / ghost removal unit, 18 Filter processing unit, 21 CPU, 22 Main memory, 23 I / O interface, 24 Storage unit, 100, 200 Optical interference distance measuring device, R1, R2, R3, R4, R5, R6, R7 Reflected light, S1, S2, S3, S4, S5, S6, S7 Spectrum.

Claims

1. An optical interference distance measuring device comprising: an optical member that focuses signal light emitted from a light source onto a target to be measured; a flat window glass of a predetermined thickness, provided between the optical member and the target to be measured at a predetermined distance from the optical member; and a distance correction unit that corrects the distance to the target to be measured calculated from the distance measuring spectrum obtained by applying the heterodyne treatment to the light generated when the signal light is reflected by the target to be measured, based on the reference light spectrum obtained by applying the heterodyne treatment to the light generated when the signal light is reflected by the target to be measured, and the predetermined distance.

2. The optical interference distance measuring device according to claim 1, wherein the distance correction unit uses the ratio of the frequency of the ideal spectrum, which is an ideal value detected at the predetermined distance, to the frequency of the reference light spectrum as a correction coefficient, and corrects the distance to the object to be measured by multiplying the frequency of the distance measuring spectrum by the correction coefficient.

3. The optical interference distance measuring device according to claim 1, wherein the distance correction unit uses the ratio of the predetermined distance to the distance calculated based on the reference optical spectrum as a correction coefficient, and corrects the distance to the object to be measured by multiplying the distance calculated based on the distance measurement spectrum by the correction coefficient.

4. The optical interference distance measuring device according to any one of claims 1 to 3, wherein an anti-reflective coating is applied to the second surface of the window glass facing the optical member.

5. The optical interference distance measuring device according to claim 4, further comprising a filter processing unit which determines that spectra appearing at a different frequency from the distance measuring spectrum at a spectral interval calculated based on the predetermined thickness of the window glass and the refractive index of the window glass are spectra to be ghosted due to ghost light reflected from the second surface of the window glass after the signal light has been reflected from the first surface of the window glass, and excludes them from the spectrum used for measuring the distance of the object to be measured.

6. A method for measuring optical interference distance performed by a computer, comprising: a step of obtaining a reference optical spectrum by performing heterodyne processing on a reference light generated when signal light emitted from a light source and focused onto a target to be measured by an optical member is reflected by a first surface of a flat window glass, which is provided between the optical member and the target to be measured at a predetermined distance from the optical member and has a predetermined thickness; a step of obtaining a distance measuring spectrum by performing the heterodyne processing on the light generated when the signal light is reflected by the target to be measured; and a step of correcting the distance to the target to be measured calculated from the distance measuring spectrum based on the reference optical spectrum and the predetermined distance.

7. An optical interference distance measurement program that causes a computer to perform the following steps:

1. Obtain a reference optical spectrum by performing heterodyne processing on a reference light generated when signal light emitted from a light source and focused onto a target to be measured by an optical element is reflected by a first surface of a flat window glass, which is provided between the optical element and the target to be measured at a predetermined distance from the optical element and has a predetermined thickness; 2. Obtain a distance measurement spectrum by performing the heterodyne processing on the light generated when the signal light is reflected by the target to be measured; and 3. Correct the distance to the target to be measured calculated from the distance measurement spectrum based on the reference optical spectrum and the predetermined distance.

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