Optical distance measurement device and optical distance measurement method

Abstract

An optical distance measurement device (1) comprises: two reflection mirrors (32, 33) or fiber gratings (10-1, 10-2) that have different wavelength dependence for each reference light beam and that are provided in an optical path that does not pass through a target (TG); and a signal processing unit (7) that calculates the frequency spectrum of a measurement interference signal of reference light beams (R1, R2) by using a time window corresponding to the wavelength dependence, and calculates the distance to the target (TG) on the basis of the peak of the frequency spectrum of the reference light beams (R1, R2).

Description

Optical distance measuring device and optical distance measuring method

[0001] The present disclosure relates to an optical distance measuring device and an optical distance measuring method.

[0002] Some optical distance measuring devices using a wavelength swept light source split the light from the wavelength swept light source into a measurement light that passes through the object and a reference light that does not pass through the object, obtain a measurement interference signal proportional to the difference in optical path length between these, and use the measurement interference signal to measure the distance to the object. As the wavelength of the wavelength swept light source changes continuously, the measurement interference signal also changes continuously. By Fourier transforming and analyzing the measurement interference signal, the optical path length difference between the optical path length through which the measurement light passes through the object and the optical path length through which the reference light passes through the reference mirror can be determined, and the distance to the object can be calculated based on this optical path length difference.

[0003] If a temperature difference occurs between the path of the measurement light and the path of the reference light, the distance measurement accuracy may decrease. In response to this, for example, the optical device described in Patent Document 1 corrects the change in focal length caused by a change in the tube length of the lens barrel used to obtain the reference light due to a temperature change. The lens held in the lens barrel described in Patent Document 1 is made of a material whose focal length changes in response to temperature changes, and which offsets the change in the focal position and the change in the tube length, allowing the difference between the focal position and the light source position to be set arbitrarily.

[0004] Japanese Patent Application Publication No. 6-130267

[0005] However, in conventional optical distance measuring devices, when multiple reference beams are used, interference between the reference beams simultaneously produces multiple peaks in the frequency spectrum obtained by Fourier transforming the measurement interference signal, which can easily lead to false detection. For example, when multiple reference beams are used, the maximum peak is extracted from the multiple peaks produced in the spectrum obtained by Fourier transforming the measurement interference signal, and the distance to the object is calculated from the frequency of the extracted maximum peak. Here, since the multiple reference beams propagate along paths with the same wavelength dependency, the measurement interference signals generated by these reference beams overlap. As a result, it is not possible to clearly distinguish between the individual measurement interference signals, and the measurement interference signals are mixed together to form a complex pattern, resulting in multiple overlapping peaks in the spectrum after Fourier transform, making it difficult to accurately extract the maximum peak. Note that even if the conventional technology described in Patent Document 1 is applied, the above problem cannot be solved when there is interference between the reference beams.

[0006] The present disclosure is intended to solve the above-mentioned problem, and aims to provide an optical distance measuring device that can avoid a situation in which peaks due to multiple reference beams simultaneously appear in the spectrum of a measurement interference signal.

[0007] The optical distance measuring device according to the present disclosure has an optical path that passes through an object and an optical path that does not pass through the object, and generates a measurement interference signal by photoelectrically converting measurement interference light obtained by combining measurement light branched from wavelength swept light onto the optical path that passes through the object and multiple reference light branched onto optical paths that do not pass through the object. The optical distance measuring device includes multiple optical path sections that are provided on the optical path that does not pass through the object and have different wavelength dependencies for each reference light, and a signal processing section that calculates the frequency spectrum of the measurement interference signal for each reference light using a time window according to the wavelength dependency of the optical path section, and calculates the distance to the object based on the peak of the frequency spectrum for each reference light.

[0008] According to the present disclosure, the optical distance measuring device has a plurality of optical path sections provided in an optical path that does not pass through an object, each of which has a different wavelength dependency for each reference light, calculates the frequency spectrum of the measurement interference signal for each reference light using a time window according to the wavelength dependency of the optical path section, and calculates the distance to the object based on the peak of the frequency spectrum for each reference light. This makes it possible for the optical distance measuring device according to the present disclosure to avoid a situation in which peaks due to multiple reference lights simultaneously appear in the spectrum of the measurement interference signal.

[0009] 1A and 1B are diagrams showing a measurement interference signal and a frequency spectrum of the measurement interference signal. FIG. 3A, 3B, 3C, and 3D are diagrams showing an overview of temperature variation compensation by the optical distance measuring device according to the first embodiment. FIG. 4A is a flowchart showing an optical distance measuring method according to the first embodiment. FIG. 5A and 5B are diagrams showing wavelength dependence of reference light. FIG. 6A is a diagram showing a modification of the optical path section according to the first embodiment. FIG. 7 is a block diagram showing a hardware configuration realizing the functions of the optical distance measuring device according to the first embodiment.

[0010] Embodiment 1. In embodiment 1, an optical distance measuring device is described that can avoid a situation in which peaks due to multiple reference beams simultaneously appear in a spectrum obtained by Fourier transforming a measurement interference signal. The optical distance measuring device according to embodiment 1 has an optical path that passes through an object and an optical path that does not pass through the object, and generates a measurement interference signal by photoelectrically converting measurement interference light obtained by combining measurement light that is branched from wavelength swept light onto the optical path that passes through the object and multiple reference beams that are branched onto optical paths that do not pass through the object.

[0011] (Outline of Optical Distance Measuring Method) First, a conventional optical distance measuring method and the optical distance measuring method according to embodiment 1 will be compared and described. Fig. 1A is a diagram showing a conventional measurement interference signal and the frequency spectrum of the measurement interference signal. The arrows in Fig. 1A indicate a series of processes in optical distance measuring, and the diagram from the top to the bottom indicates the data used or acquired in each process.

[0012] The top diagram in Figure 1A is a graph showing the wavelength time dependence of measurement light A and reference light B, C used in optical distance measurement, with the horizontal axis representing time t and the vertical axis representing wavelength λ. Measurement light A and reference light B, C are obtained by branching wavelength swept light emitted from a wavelength swept light source. Measurement light A is obtained by branching wavelength swept light into an optical path that passes through an object, while reference light B, C is obtained by branching wavelength swept light into an optical path that does not pass through an object. Therefore, measurement light A and reference light B, C are optical signals whose wavelengths are swept over time at the same slope.

[0013] Measurement light A is wavelength swept light irradiated toward an object, reflected or scattered by the object, and returned to the optical distance measuring device in an optical path that passes through the object. Reference light B is wavelength swept light reflected by a first reflecting mirror and combined with measurement light A in an optical path that does not pass through the object. Reference light C is wavelength swept light reflected by a second reflecting mirror different from the first reflecting mirror and combined with measurement light A in an optical path that does not pass through the object. Reference light B and reference light C have different optical path lengths along their propagation paths.

[0014] The optical distance measuring device generates a measurement interference signal by combining measurement light A with reference light B and C. The second diagram from the top in FIG. 1A shows the measurement interference signal at a beat frequency f ｂ 1 is a graph showing the time dependence of the beat frequency f ｂ The measurement interference signal D1 is an interference signal obtained by combining the measurement light A and the reference light B, and the beat frequency f ｂ is the frequency f ｂ１ The measurement interference signal D2 is an interference signal obtained by combining the measurement light A and the reference light C, and the beat frequency f ｂ is the beat frequency f ｂ１ beat frequency f ｂ２ is.

[0015] 1A is a waveform diagram showing the time waveform of the measurement interference signal, with the horizontal axis representing time t and the vertical axis representing amplitude. In conventional optical distance measuring methods, the first reflecting mirror and the second reflecting mirror have approximately the same wavelength-time dependency, so the time waveform of the measurement interference signal is a mixture of the measurement interference signal D1 and the measurement interference signal D2, which overlap to form a complex waveform. The optical distance measuring device calculates the frequency spectrum of the measurement interference signal by Fourier transforming the measurement interference signal included in a time window of the full time length in response to a trigger signal synchronized with the wavelength sweep.

[0016] The bottom diagram in Fig. 1A shows the frequency spectrum of the measurement interference signal, with the horizontal axis representing frequency and the vertical axis representing intensity. This spectrum is obtained by Fourier transforming an interference signal containing both the measurement interference signal D1 and the measurement interference signal D2 using a time window with the full time length, and therefore multiple peaks appear in the spectrum. These peaks represent the frequency components of the measurement interference signal, and the position of each peak corresponds to the optical path length difference. In the example of Fig. 1A, the beat frequency f ｂ１ and the beat frequency f ｂ２ In addition to the peak at beat frequency f ｂ２ The frequency 2f is the double wave of ｂ２ Peak, frequency f ｂ２－ｂ１ and frequency f ｂ２＋ｂ１ A peak appears.

[0017] The optical distance measuring device converts the maximum of these peaks into the distance to the object. In the case of FIG. 1A, the beat frequency f ｂ２ is the maximum peak. For example, the optical path length difference ΔL corresponding to the frequency of the maximum peak is calculated. The optical path length difference ΔL is expressed by the following formula (1). In the following formula (1), c is the speed of light, n is the refractive index of the medium, and Δf is the frequency difference (peak frequency). ΔL=c / (2nΔf) (1)

[0018] The distance d to the object can be calculated by substituting the optical path difference ΔL calculated according to the above formula (1) into the following formula (2). Since the measurement light A travels this distance d back and forth, the actual distance is half the optical path difference: d = ΔL / 2 (2)

[0019] However, when different measurement interference signals are close to each other, the peaks overlap, making accurate peak detection difficult. Therefore, the optical distance measuring device according to the first embodiment has multiple optical path sections that are provided on an optical path that does not pass through an object and have different wavelength dependencies for each reference light, calculates the frequency spectrum of the measurement interference signal for each reference light using a time window according to the wavelength dependency of the optical path section, and calculates the distance to the object based on the peak of the frequency spectrum for each reference light. This makes it possible to avoid a situation in which peaks due to multiple reference lights simultaneously appear in the spectrum of the measurement interference signal.

[0020] 1B is a diagram showing a measurement interference signal and a frequency spectrum of the measurement interference signal in the optical distance measuring device according to embodiment 1. The arrows shown in Fig. 1B indicate a series of processes in optical distance measuring, similar to Fig. 1A, and the diagram from the top to the bottom indicates data used or obtained in each process.

[0021] The top diagram in FIG. 1B is a graph showing the wavelength time dependence of measurement light A and reference light B and C used in optical distance measurement, with the horizontal axis representing time t and the vertical axis representing wavelength λ. Measurement light A and reference light B and C are branched wavelength swept light emitted from a wavelength swept light source. Measurement light A is obtained by branching wavelength swept light into an optical path that passes through an object, while reference light B and C are obtained by branching wavelength swept light into an optical path that does not pass through an object. For example, the first reflecting mirror in the optical path through which reference light B propagates has a different wavelength time dependence from the second reflecting mirror in the optical path through which reference light C propagates. Therefore, reference light B appears in synchronization with the sweep cycle in which a wavelength with a high reflection rate at the first reflecting mirror is swept, and reference light C appears in synchronization with the sweep cycle in which a wavelength with a high reflection rate at the second reflecting mirror is swept.

[0022] The optical distance measuring device according to the first embodiment generates a measurement interference signal by combining measurement light A with reference light B and reference light C. The second diagram from the top in FIG. ｂ 1 is a graph showing the time dependence of the beat frequency f ｂThe measurement interference signal D1 is obtained by combining the measurement light A and the reference light B and has a beat frequency f ｂ１ Furthermore, the measurement interference signal D2 is an interference signal having a beat frequency f ｂ２ The measurement interference signals D1 and D2 appear at a time interval for each wavelength sweep period.

[0023] 1B is a waveform diagram showing the time waveforms of the measurement interference signal D1 and the measurement interference signal D2, with the horizontal axis representing time t and the vertical axis representing amplitude. Reference light B appears at the timing when the wavelength with a high reflection rate on the first reflecting mirror is swept, and reference light C appears at the timing when the wavelength with a high reflection rate on the second reflecting mirror is swept. Therefore, the measurement interference signal D1 and the measurement interference signal D2 can be treated separately from each other.

[0024] 1B is a diagram showing the frequency spectrum of the measurement interference signals D1 and D2, where the horizontal axis represents frequency and the vertical axis represents intensity. In this way, the measurement interference signals D1 and D2 can be Fourier transformed separately. That is, by Fourier transforming the measurement interference signal D1 with a time window synchronized with the sweep period in which the wavelength with a high reflection rate on the first reflecting mirror is swept, the beat frequency f ｂ１ Furthermore, by Fourier transforming the measured interference signal D2 with a time window synchronized with the sweep period in which the wavelength having a high reflection rate on the second reflecting mirror is swept, a spectrum having a peak of beat frequency f ｂ２ A spectrum having a peak of

[0025] The optical distance measuring device according to the first embodiment converts the maximum of these peaks into the distance to the object. ｂ２ is the maximum peak. For example, the optical path length difference ΔL corresponding to the frequency of the maximum peak is calculated according to the above formula (1). Furthermore, by substituting the optical path length difference ΔL calculated according to the above formula (1) into the above formula (2), the distance d to the object can be calculated.

[0026] (Basic Configuration of Optical Distance Measuring Device) Fig. 2 is a block diagram showing the configuration of the optical distance measuring device 1 according to embodiment 1. As shown in Fig. 2, the optical distance measuring device 1 includes a wavelength swept light source 2, a lens barrel 3, a circulator 4, a photoelectric conversion unit 5, an AD conversion unit 6, and a signal processing unit 7, and measures the distance to a target TG.

[0027] The wavelength swept light source 2 outputs wavelength swept light whose wavelength is swept linearly with respect to time based on a sweep signal. The wavelength swept light source 2 includes an electrical unit 21, a light source 22, and a square wave converter 23. The electrical unit 21 generates a sweep signal and outputs the sweep signal to the light source 22. The light source 22 generates wavelength swept light whose wavelength changes in synchronization with the potential of the sweep signal and outputs the wavelength swept light. The wavelength swept light output from the wavelength swept light source 2 is output to the circulator 4 via an optical fiber. The square wave converter 23 converts the sweep signal obtained from the electrical unit 21 into a square wave trigger signal synchronized with the wavelength sweep period and outputs the trigger signal to the signal processor 7.

[0028] The lens barrel 3 is formed by combining cylindrical members made of aluminum or the like, and includes a semi-transmitting mirror 31, a reflecting mirror 32, and a reflecting mirror 33 inside the cylindrical member. A portion of the wavelength swept light output from the circulator 4 is irradiated onto the object TG via the semi-transmitting mirror 31, and the remainder is branched to the reflecting mirror 32 and the reflecting mirror 33. The wavelength swept light reflected or scattered by the object TG and returned to the lens barrel 3 is branched to the circulator 4 as measurement light A via the semi-transmitting mirror 31. The wavelength swept light branched to the reflecting mirror 32 is reflected by the reflecting mirror 32 and branched to the circulator 4 as reference light B via the semi-transmitting mirror 31. The wavelength swept light branched to the reflecting mirror 33 is reflected by the reflecting mirror 33 and branched to the circulator 4 as reference light C via the semi-transmitting mirror 31.

[0029] (Outline of Optical Path Section) The reflecting mirror 32 and the reflecting mirror 33 are provided in an optical path in the lens barrel 3 that does not pass through the target object TG, and are optical path sections in which the reflecting mirror 32 and the reflecting mirror 33 have different wavelength dependencies. For example, the wavelength dependency of the reflecting mirror 32 and the reflecting mirror 33 indicates the proportion of light reflected at each wavelength. There are cases where the reflecting mirror 32 and the reflecting mirror 33 have a high reflectance at certain wavelengths and a low reflectance at other wavelengths. Here, the characteristic of strongly reflecting light of a certain wavelength is referred to as wavelength dependency.

[0030] Reference light B is a part of the wavelength swept light that propagates through an optical path that passes through semi-transparent mirror 31 and reflecting mirror 32 and has a wavelength that is strongly reflected by reflecting mirror 32. Reference light C is a part of the wavelength swept light that propagates through an optical path that passes through semi-transparent mirror 31 and reflecting mirror 33 and has a wavelength that is strongly reflected by reflecting mirror 33. Furthermore, reference light B and reference light C propagate through optical paths with different optical path lengths depending on the positions of reflecting mirror 32 and reflecting mirror 33 in the lens barrel 3.

[0031] FIG. 3A is a diagram showing an outline of the lens barrel 3. In FIG. ０ is a reference distance, which indicates the distance between the semi-transparent mirror 31 and the reflecting mirrors 32 and 33 at a reference temperature of 25°C. １ is the optical path length difference of the reference light B from the wavelength swept light source 2 to the photoelectric conversion unit 5, and R ２ is the optical path length difference of the reference light C from the wavelength swept light source 2 to the photoelectric conversion unit 5. When the temperature changes from the reference temperature of 25°C, the distance between the semi-transparent mirror 31 and the reflecting mirrors 32 and 33 changes from the reference distance due to thermal expansion or thermal contraction of the lens barrel 3. Even if the distance to the target TG changes due to a change in the optical path length caused by a temperature change, the optical distance measuring device 1 corrects this change.

[0032] The circulator 4 outputs a portion of the wavelength swept light output from the wavelength swept light source 2 to the lens barrel 3, and outputs the light returning from the lens barrel 3 to the photoelectric conversion unit 5. The light returning from the lens barrel 3 includes measurement light A, reference light B, and reference light C. Figure 3B is a graph showing the wavelength time dependence of measurement light A and reference light B used in optical distance measurement, with the horizontal axis representing time t and the vertical axis representing wavelength λ. In Figure 3B, f ｓｗｅｅｐ is the wavelength sweep frequency. Δλ is the wavelength sweep width, and Δλ = maximum wavelength λ ｍａｘ -Minimum wavelength λ ｍｉｎ λc is the center wavelength of the wavelength sweep. The measurement light A and the reference light B are 1 / f ｓｗｅｅｐ The same applies to the reference light C.

[0033] The photoelectric conversion unit 5 multiplexes the input wavelength swept light and photoelectrically converts the multiplexed light to generate an electrical measurement interference signal. The AD conversion unit 6 analog-to-digital converts the electrical measurement interference signal generated by the photoelectric conversion unit 5 to generate a digital measurement interference signal. The measurement interference signal converted into a digital signal by the AD conversion unit 6 is output to the signal processing unit 7.

[0034] (Outline of Signal Processing Unit) The signal processing unit 7 calculates the frequency spectrum of the measurement interference signal of the reference lights B and C using a time window according to the wavelength dependence of the reflecting mirror 32 and the reflecting mirror 33, and calculates the distance to the target TG based on the peaks of the frequency spectrum of the reference lights B and C. For example, as shown in FIG. 2 , the signal processing unit 7 includes a time window designation unit 71, FFT units 72-1 and 72-2, peak acquisition units 73-1 and 73-2, a difference acquisition unit 74, a temperature acquisition unit 75, and an offset unit 76.

[0035] (Time Window Designation Unit) The time window designation unit 71 designates time windows of time lengths corresponding to the reference light B and the reference light C to the FFT units 72-1 and 72-2. For example, a trigger signal generated by the rectangular wave conversion unit 23 is set in the time window designation unit 71. The trigger signal is a rectangular wave signal synchronized with the wavelength sweep period of the wavelength transmission light output from the wavelength swept light source 2, and is a signal that indicates the start point or a specific phase of the wavelength sweep. Based on the trigger signal, the time window designation unit 71 designates time windows of the measurement interference signal corresponding to the reference light B and C to the FFT units 72-1 and 72-2. These time windows designate the data range to be used for Fourier transform.

[0036] (Fourier Transform Unit) The FFT units 72-1 and 72-2 are Fourier transform units that convert the measurement interference signals corresponding to the reference light B and the reference light C into frequency spectra of the measurement interference signals corresponding to the reference light B and the reference light C, using a time window specified by the time window specifying unit 71. For example, when the time window of the measurement interference signal corresponding to the reference light B is specified by the time window specifying unit 71, the FFT unit 72-1 calculates a frequency spectrum by performing a fast Fourier transform (FFT) on the measurement interference signal. When the time window of the measurement interference signal corresponding to the reference light C is specified by the time window specifying unit 71, the FFT unit 72-2 calculates a frequency spectrum by performing an FFT on the measurement interference signal.

[0037] (Peak Acquisition Unit) The peak acquisition units 73-1 and 73-2 acquire peak information of the frequency spectrum of the measurement interference signal corresponding to the reference light B and the reference light C, respectively. In the spectrum of the measurement interference signal, a reflection peak appears at a frequency position corresponding to the distance from the reflection point of the object TG. For example, the peak acquisition unit 73-1 selects the maximum peak from the spectrum calculated by the FFT unit 72-1 and acquires peak information indicating the frequency of the selected peak. Similarly, the peak acquisition unit 73-2 selects the maximum peak from the spectrum calculated by the FFT unit 72-2 and acquires peak information indicating the frequency of the selected peak.

[0038] (Difference Acquisition Unit) The difference acquisition unit 74 acquires the difference in peak frequency between the measurement interference signal corresponding to reference light B and the measurement interference signal corresponding to reference light C, based on the peak information. The position (frequency) of the maximum peak acquired from the frequency spectrum of the measurement interference signal corresponding to reference light B and the position of the maximum peak acquired from the frequency spectrum of the measurement interference signal corresponding to reference light C correspond to the respective optical path length differences. The difference acquisition unit 74 calculates the optical path length difference according to the above formula (1) from the frequencies of the peaks acquired from the peak acquisition units 73-1 and 73-2.

[0039] (Temperature Acquisition Unit) The temperature acquisition unit 75 acquires temperature. For example, the temperature acquisition unit 75 acquires temperature information from a temperature sensor attached to the lens barrel 3. The temperature acquisition unit 75 may also acquire temperature information stored in a storage device accessible from the signal processing unit 7. The temperature information includes an optical path length difference associated with a temperature of the lens barrel 3 that was measured in the past.

[0040] (Offset Unit) The offset unit 76 corrects the optical path length difference using the difference in frequency between the measurement interference signals according to the temperature difference, and acquires the corrected distance to the target TG based on the corrected optical path length difference. Temperature changes cause thermal expansion or contraction of the lens barrel 3, changing the optical path length in the lens barrel 3. Alternatively, temperature changes change the refractive index of the medium that makes up the optical fiber, changing the propagation speed of light. The offset unit 76 calculates the difference in frequency peaks in the spectra of the measurement interference signals obtained under different temperature conditions, based on the temperature information acquired by the temperature acquisition unit 75 and the information on the optical path length difference acquired by the difference acquisition unit 74.

[0041] The frequency difference Δf reflects the change in the optical path length difference due to a temperature change. Using the frequency difference Δf, the offset unit 76 corrects the optical path length difference ΔL according to the following formula (3). In the following formula (3), ΔL ｃｏｒｒｅｃｔｅｄ is the corrected optical path difference length, and k is the correction coefficient, which indicates the relationship between the optical path length and the frequency difference. ｃｏｒｒｅｃｔｅｄ =ΔL+k・Δf (3)

[0042] The offset unit 76 calculates the corrected optical path length difference ΔL according to the following equation (4): ｃｏｒｒｅｃｔｅｄ Since the measurement light A travels back and forth through the object TG, the actual distance d is calculated by the optical path difference ΔL ｃｏｒｒｅｃｔｅｄ Therefore, the above formula (2) can be expressed as the following formula (4): d = ΔL ｃｏｒｒｅｃｔｅｄ / 2 (4)

[0043] (Error Correction Due to Temperature Change) Fig. 3C is a diagram showing the peaks in the frequency spectrum of the measurement interference signal produced by the measurement light A and the reference light B, and the peaks in the frequency spectrum of the measurement interference signal produced by the measurement light A and the reference light C, where the horizontal axis represents frequency f (Hz) and the vertical axis represents intensity. For example, suppose that a temperature change of 10°C occurs from a reference temperature of 25°C, causing thermal expansion of the lens barrel 3. Fig. 3D is a diagram showing the temperature dependence of the peaks in the spectrum of the measurement interference signal, where the horizontal axis represents temperature T (°C) and the vertical axis represents frequency (Hz).

[0044] In the example shown in FIG. 3C, the full width at half maximum f ＦＷＨＭ is 1.1 kHz, and in a temperature range of 10° C., the frequency of the peak in the spectrum of the measurement interference signal corresponding to reference light B changes by 2.7 kHz to the frequency of the peak in the spectrum of the measurement interference signal corresponding to reference light C. This is because, as shown in FIG. 3D, the full width at half maximum f ＦＷＨＭ This shows that the frequency fluctuates by 2.7 kHz over a temperature range of 10° C., compared to 1.1 kHz.

[0045] Furthermore, the distance to the target TG and the beat frequency are proportional to each other, and the proportionality coefficient K (Hz / m) is given by the following formula (5): ｍｉｎ to the central wavelength λ ｃ The full width at half maximum of the beat frequency peak in the wavelength sweep up to ＦＷＨＭ is expressed by the following formula (6): K = ((1 / λ ｍｉｎ ) - (1 / λ ｃ ))・2F ｓｗｅｅｐ (5) f ＦＷＨＭ = K(2ln(2) / π)・((λ ｃ -λ ｍｉｎ ) / 2) ２ / (λ ｃ -λ ｍｉｎ ) (6)

[0046] The temperature dependency of the frequency difference between the reference light B and the reference light C can be expressed by the following formula (7): １ is the frequency (beat frequency) of the maximum peak in the spectrum of the measurement interference signal corresponding to the reference light B, and f ２ is the beat frequency of the maximum peak in the spectrum of the measurement interference signal corresponding to the reference light C. Also, α is the thermal expansion coefficient of the lens barrel 3, and ΔT is the temperature difference from the reference temperature (25° C.). ２ -f １ =Δf(ΔT)=Kα((R ２ -R １ ) / 2) ΔT (7)

[0047] In this case, the offset portion 76 is calculated by using the following formula (8) to obtain the length L of the lens barrel 3. ｍｅａｓｕｒｅ is corrected, and the corrected length L ｍｅａｓｕｒｅ０ In the following formula (8), ΔL ０ is the L when the temperature changes from the reference temperature (25°C) to the measurement temperature (for example, 30°C). ０ This is the change in L ０ is the distance between the semi-transparent mirror 31 and the reflecting mirror in the lens barrel 3. The offset portion 76 is the corrected length L ｍｅａｓｕｒｅ The distance d to the target object TG is calculated using L ｍｅａｓｕｒｅ０ =L ｍｅａｓｕｒｅ -ΔL ０ αΔT (8)

[0048] Next, the operation of the optical distance measuring device 1 according to the first embodiment will be described. FIG. 4 is a flowchart showing the operation of the optical distance measuring device 1 according to the first embodiment. The time window designation unit 71 acquires a measurement interference signal from the AD conversion unit 6 (step ST1). At this time, a trigger signal is set in the time window designation unit 71 from the rectangular wave conversion unit 23 of the wavelength swept light source 2. The time window designation unit 71 uses this trigger signal to determine time windows of time lengths corresponding to the reference light B and the reference light C, and designates the time window corresponding to the reference light B to the FFT unit 72-1 and the time window corresponding to the reference light C to the FFT unit 72-2 (step ST2).

[0049] The FFT unit 72-1 performs FFT on the measurement interference signal generated by multiplexing the measurement light A and the reference light B (step ST3-1). Similarly, the FFT unit 72-2 performs FFT on the measurement interference signal generated by multiplexing the measurement light A and the reference light C (step ST3-2). The frequency spectrum of the measurement interference signal generated by multiplexing the measurement light A and the reference light B is output to the peak acquisition unit 73-1, and the frequency spectrum of the measurement interference signal generated by multiplexing the measurement light A and the reference light C is output to the peak acquisition unit 73-2.

[0050] FIG. 5A shows the wavelength dependence of the reference light in a conventional optical distance measuring method. The top graph in FIG. 5A is a graph showing the wavelength dependence E of the reference light, with the horizontal axis representing wavelength λ and the vertical axis representing intensity I. The reference light is an optical signal generated by branching wavelength-swept light, and its intensity I is approximately constant at wavelength λ within the sweep range. The second graph from the top in FIG. 5A shows the wavelength dependence of the reflectivity R of a conventional reflecting mirror, with the horizontal axis representing wavelength λ and the vertical axis representing reflectivity R. In the conventional optical distance measuring method, when generating two reference light beams, two reflecting mirrors with approximately the same wavelength-time dependence F are used, as shown in the second graph. The bottom graph in FIG. 5A shows the wavelength dependence of two reference light beams generated by reflecting wavelength-swept light beams off two reflecting mirrors, with the horizontal axis representing wavelength λ and the vertical axis representing intensity I. As shown in this graph, the two reference light beams G returned to the circulator 4 have approximately the same wavelength dependence, making it difficult to separate them.

[0051] FIG. 5B is a graph showing the wavelength dependence of the reference light in the optical distance measuring method according to the first embodiment. The topmost graph in FIG. 5B is a graph showing the wavelength dependence E of the reference light, with the horizontal axis representing wavelength λ and the vertical axis representing intensity I. The reference light is an optical signal generated by branching wavelength swept light, and its intensity I is approximately constant at wavelength λ within the sweep range. The second graph from the top in FIG. 5B is a graph showing the wavelength dependence of the reflectivity R of the reflecting mirrors 32 and 33, with the horizontal axis representing wavelength λ and the vertical axis representing reflectivity R. As shown in this graph, the reflecting mirrors 32 and 33 held by the lens barrel 3 in the first embodiment have different wavelength dependences. For example, they have a characteristic H1 that strongly reflects low wavelength bands and a characteristic H2 that strongly reflects high wavelength bands. The bottommost graph in FIG. 5B is a graph showing the wavelength dependence of the reference light I1 and I2 generated by the wavelength swept light reflected by the reflecting mirrors 32 and 33, with the horizontal axis representing wavelength λ and the vertical axis representing intensity I. As shown in this figure, the reflecting mirrors 32 and 33, which have different wavelength dependencies, output mutually separated reference beams I1 and I2 to the circulator 4. This allows the optical distance measuring device 1 to easily generate a measurement interference signal corresponding to the reference beam I1 and a measurement interference signal corresponding to the reference beam I2.

[0052] The peak acquirer 73-1 acquires peak information of the spectrum of the measurement interference signal corresponding to the reference light B (step ST4-1). Similarly, the peak acquirer 73-2 acquires peak information of the spectrum of the measurement interference signal corresponding to the reference light C (step ST4-2). The difference acquirer 74 acquires the difference in peak frequency between the measurement interference signal corresponding to the reference light B and the measurement interference signal corresponding to the reference light C based on the peak information (step ST5). The temperature acquirer 75 acquires the temperature of the optical column 3 (step ST6).

[0053] The offset unit 76 performs offsetting, correcting the optical path length difference corresponding to the difference in frequency between the measurement interference signals according to the temperature difference, based on the peak information acquired by the peak acquisition units 73-1 and 73-2 and the temperature information acquired by the temperature acquisition unit 75 (step ST7).The offset unit 76 then acquires the corrected distance to the target TG based on the corrected optical path length difference (step ST8).By performing the above method, the optical distance measuring device 1 can avoid a situation in which peaks due to the reference lights B and C simultaneously appear in the spectrum of the measurement interference signal.

[0054] (Modification) Reflecting mirrors 32 and 33 are shown as optical path sections having different wavelength dependencies for reference beams B and C, but the optical path sections are not limited to reflecting mirrors and may be fiber gratings. Fig. 6 is a diagram showing a modification of the optical path section in embodiment 1. In Fig. 6, optical system 8, in place of lens barrel 3, has the function of irradiating wavelength swept light onto object TG and receiving reflected light, as well as generating two reference beams B and C. Optical system 8 is configured with optical coupler 9 and fiber gratings 10-1 and 10-2.

[0055] The optical system 8 uses an optical coupler 9 to branch or combine wavelength-swept light within an optical fiber, and fiber gratings 10-1 and 10-2 to selectively reflect light of specific wavelengths. That is, fiber gratings 10-1 and 10-2 are provided in an optical path in the optical system 8 that does not pass through the target TG, and are optical path sections in which fiber gratings 10-1 and 10-2 have different wavelength dependencies. For example, the wavelength dependencies of fiber gratings 10-1 and 10-2 indicate the proportion of light reflected by that wavelength. They may have high reflectance at specific wavelengths and low reflectance at other wavelengths. Even if fiber gratings 10-1 and 10-2 are used instead of reflecting mirrors 32 and 33, it is possible to separately handle multiple reference light beams.

[0056] Next, a hardware configuration for realizing the functions of the optical distance measuring device 1 according to the first embodiment will be described. FIG. 7 is a block diagram showing a hardware configuration for realizing the functions of the signal processing unit 7 included in the optical distance measuring device 1 according to the first embodiment. The functions of the time window specifying unit 71, the FFT units 72-1 and 72-2, the peak acquiring units 73-1 and 73-2, the difference acquiring unit 74, the temperature acquiring unit 75, and the offset unit 76 included in the signal processing unit 7 are realized by processing circuits. That is, the signal processing unit 7 includes a processing circuit for executing the processing of each step shown in FIG. 4. The processing circuit may be a CPU (Central Processing Unit) that executes a program stored in a memory.

[0057] The time window specifying unit 71 acquires the measurement interference signal, which is a digital signal output from the AD conversion unit 6, via the input interface 100, for example. If the measurement interference signal is stored in a storage unit included in the optical distance measuring device 1, the time window specifying unit 71 may acquire the measurement interference signal by reading it from the storage unit via the input interface 100. The offset unit 76 outputs the distance d to the target TG, which is the distance measurement result, to an external device via the output interface 101.

[0058] The functions of the time window specifying unit 71, the FFT units 72-1 and 72-2, the peak acquiring units 73-1 and 73-2, the difference acquiring unit 74, the temperature acquiring unit 75, and the offset unit 76 included in the signal processing unit 7 are realized by software, firmware, or a combination of software and firmware. The software or firmware is written as a program and stored in the memory 103.

[0059] The processor 102 reads and executes the programs stored in the memory 103 to realize the functions of the time window designation unit 71, the FFT units 72-1 and 72-2, the peak acquisition units 73-1 and 73-2, the difference acquisition unit 74, the temperature acquisition unit 75, and the offset unit 76 included in the signal processing unit 7. For example, the signal processing unit 7 includes a memory 103 for storing programs that, when executed by the processor 102, result in the processing of each step shown in FIG. 4 being executed. These programs cause a computer to execute the procedures or methods of the processing performed by the time window designation unit 71, the FFT units 72-1 and 72-2, the peak acquisition units 73-1 and 73-2, the difference acquisition unit 74, the temperature acquisition unit 75, and the offset unit 76. The memory 103 may be a computer-readable storage medium that stores a program for causing a computer to function as a time window designation unit 71, an FFT unit 72-1, an FFT unit 72-2, a peak acquisition unit 73-1, a peak acquisition unit 73-2, a difference acquisition unit 74, a temperature acquisition unit 75, and an offset unit 76.

[0060] The memory 103 may be, for example, a non-volatile or volatile semiconductor memory such as a RAM (Random Access Memory), a ROM (Read Only Memory), a flash memory, an EPROM (Erasable Programmable Read Only Memory), or an EEPROM (Electrically-EPROM) (registered trademark), a magnetic disk, a flexible disk, an optical disk, a compact disk, a mini disk, or a DVD.

[0061] Some of the functions of the time window designation unit 71, FFT unit 72-1, FFT unit 72-2, peak acquisition unit 73-1, peak acquisition unit 73-2, difference acquisition unit 74, temperature acquisition unit 75, and offset unit 76 included in the signal processing unit 7 may be realized by dedicated hardware, and the other functions may be realized by software or firmware. For example, the function of the time window designation unit 71 may be realized by a processing circuit that is dedicated hardware, and the functions of the FFT unit 72-1, FFT unit 72-2, peak acquisition unit 73-1, peak acquisition unit 73-2, difference acquisition unit 74, temperature acquisition unit 75, and offset unit 76 may be realized by the processor 102 reading and executing a program stored in memory 103. In this way, the processing circuit can realize the above functions by hardware, software, firmware, or a combination of these.

[0062] Although the case where two reference beams B and C are used has been described, the optical distance measuring device 1 is not limited to this. For example, three or more reference beams may be used. In this case, too, a measurement interference signal is generated for each reference beam, an optical path length difference is calculated for each measurement interference signal, and the calculated optical path length difference can be used to calculate the distance d to the target TG.

[0063] As described above, the optical distance measuring device 1 according to the first embodiment has an optical path that passes through the object TG and an optical path that does not pass through the object TG, and generates a measurement interference signal by photoelectrically converting measurement interference light obtained by combining measurement light A branched from wavelength swept light onto the optical path that passes through the object TG with reference light B and C branched onto the optical path that does not pass through the object TG. The optical distance measuring device 1 further comprises: reflection mirrors 32 and 33 or fiber gratings 10-1 and 10-2 that are provided on the optical path that does not pass through the object TG and have different wavelength dependencies on the reference light B and C; and a signal processing unit 7 that calculates the frequency spectrum of the measurement interference signal of the reference light B and C using time windows according to these wavelength dependencies, and calculates the distance to the object TG based on the peaks of the frequency spectrum of the reference light B and C. The optical distance measuring device 1 calculates the frequency spectrum of the measurement interference signal of the reference lights B and C using a time window corresponding to the wavelength dependence of the reflecting mirrors 32, 33 or the fiber gratings 10-1, 10-2, and calculates the distance to the target TG based on the peaks of the frequency spectrum of the reference lights B and C, thereby avoiding a situation in which peaks due to the reference lights B and C simultaneously appear in the spectrum of the measurement interference signal.

[0064] In the optical distance measuring device 1 according to the first embodiment, the multiple optical path sections are provided in the lens barrel 3 and are reflection mirrors 32 and 33 that have different wavelength dependencies on the reference lights B and C. This allows the optical distance measuring device 1 to handle the multiple reference lights separately.

[0065] In the optical distance measuring device 1 according to the first embodiment, the multiple optical path sections are fiber gratings 10-1 and 10-2 that have different wavelength dependencies for each reference light, which allows the optical distance measuring device 1 to handle the multiple reference light beams separately.

[0066] In the optical distance measuring device 1 according to the first embodiment, the signal processing unit 7 includes a time window designation unit 71 that determines a time window for the reference lights B and C in accordance with the wavelength dependency of the reflecting mirrors 32 and 33 or the fiber gratings 10-1 and 10-2 and designates the determined time window; FFT units 72-1 and 72-2 that convert the measurement interference signals of the reference lights B and C into the frequency spectrum of the reference lights B and C using the time window designated by the time window designation unit 71; peak acquisition units 73-1 and 73-2 that acquire peak information of the frequency spectrum of the reference lights B and C; a difference acquisition unit 74 that acquires the difference in peak frequency between the measurement interference signals corresponding to the reference lights B and C based on the peak information; a temperature acquisition unit 75 that acquires the temperature; and an offset unit 76 that corrects the optical path length difference using the difference in frequency between the measurement interference signals according to the temperature difference and acquires the corrected distance to the target TG based on the optical path length difference after the correction. As a result, even if a temperature difference occurs between the path of the measurement light and the optical path of the reference light, the optical distance measuring device 1 can correct the error caused by the temperature difference.

[0067] In the optical distance measuring device 1 according to the first embodiment, the time window specifying unit 71 determines a time window in synchronization with the sweep period in which the wavelengths of the reference beams B and C, which are wavelength swept beams, corresponding to the reflecting mirrors 32 and 33 or the fiber gratings 10-1 and 10-2, respectively, are swept, and specifies the determined time window to the FFT units 72-1 and 72-2. This enables the optical distance measuring device 1 to accurately specify a time window for each reference beam.

[0068] The optical distance measuring method of the optical distance measuring device 1 according to the first embodiment includes step ST2 in which a time window designation unit 71 designates a time window having the time lengths of the reference lights B and C; steps ST3-1 and ST3-2 in which FFT units 72-1 and 72-2 convert the measurement interference signals of the reference lights B and C into the frequency spectrum of the reference lights B and C using the time window designated by the time window designation unit 71; steps ST4-1 and ST4-2 in which peak acquisition units 73-1 and 73-2 acquire peak information of the frequency spectrum for each reference light; step ST5 in which a difference acquisition unit 74 acquires the difference in peak frequencies between the measurement interference signals corresponding to the reference lights B and C based on the peak information; step ST6 in which a temperature acquisition unit 75 acquires the temperature; and steps ST7 and ST8 in which an offset unit 76 corrects the optical path length difference using the difference in frequency between the measurement interference signals according to the temperature difference, and acquires the corrected distance to the object TG based on the corrected optical path length difference. By executing the above method, the optical distance measuring device 1 can avoid a situation in which peaks due to reference lights B and C simultaneously occur in the spectrum of the measurement interference signal, and even if a temperature difference occurs between the path of the measurement light and the path of the reference light, errors caused by the temperature difference can be corrected.

[0069] In the optical distance measuring method of the optical distance measuring device 1 according to the first embodiment, the time window specifying unit 71 determines a time window in accordance with the timing at which the wavelength of the reference light B, C, which is wavelength swept light, corresponding to each of the reflecting mirrors 32, 33 or the fiber gratings 10-1, 10-2, is swept, and specifies the determined time window to the FFT units 72-1, 72-2. By performing the above method, the optical distance measuring device 1 can accurately specify a time window for each reference light.

[0070] Various aspects of the present disclosure are summarized below as appendices.

[0071] (Supplementary Note 1) An optical distance measuring device having an optical path that passes through an object and an optical path that does not pass through the object, and generating a measurement interference signal by photoelectrically converting measurement interference light obtained by combining measurement light branched from wavelength swept light onto the optical path that passes through the object and multiple reference light branched onto the optical path that does not pass through the object, the optical distance measuring device comprising: multiple optical path sections provided on the optical path that does not pass through the object, the multiple optical path sections having different wavelength dependencies for each of the reference light beams; and a signal processing section that calculates a frequency spectrum of the measurement interference signal for each of the reference light beams using a time window according to the wavelength dependency of the optical path sections, and calculates the distance to the object based on a peak of the frequency spectrum for each of the reference light beams. (Supplementary Note 2) The optical distance measuring device according to Supplementary Note 1, wherein the multiple optical path sections are multiple reflecting mirrors provided on a lens barrel and having different wavelength dependencies for each of the reference light beams. (Supplementary Note 3) The optical distance measuring device according to Supplementary Note 1, wherein the multiple optical path sections are multiple fiber gratings having different wavelength dependencies for each of the reference light beams. (Supplementary Note 4) The optical distance measuring device according to any one of Supplementary Note 1 to Supplementary Note 3, wherein the signal processing unit comprises: a time window designation unit that determines the time window for each of the reference lights in accordance with the wavelength dependency of the optical path unit and designates the determined time window; a Fourier transform unit that converts the measurement interference signal for each of the reference lights into the frequency spectrum for each of the reference lights using the time window designated by the time window designation unit; a peak acquisition unit that acquires peak information of the frequency spectrum for each of the reference lights; a difference acquisition unit that acquires a difference in peak frequency between the measurement interference signals corresponding to each of the reference lights based on the peak information; a temperature acquisition unit that acquires a temperature; and an offset unit that corrects an optical path length difference using a difference in frequency between the measurement interference signals according to a temperature difference, and acquires a corrected distance to the object based on the optical path length difference after the correction. (Supplementary Note 5) The optical distance measuring device described in Supplementary Note 4 is characterized in that the time window designation unit determines the time window in synchronization with the sweep period of the wavelengths of the plurality of reference light beams, which are the wavelength swept light, corresponding to each of the plurality of optical path sections, and designates the determined time window to the Fourier transform unit.(Supplementary Note 6) An optical distance measuring method for an optical distance measuring device according to any one of Supplementary Note 1 to Supplementary Note 4, comprising: a step in which a time window designation unit designates the time window having a time length for each of the reference lights; a step in which a Fourier transform unit converts the measurement interference signal for each of the reference lights into the frequency spectrum for each of the reference lights using the time window designated by the time window designation unit; a step in which a peak acquisition unit acquires peak information of the frequency spectrum for each of the reference lights; a step in which a difference acquisition unit acquires a difference in peak frequency between the measurement interference signals corresponding to each of the reference lights based on the peak information; a step in which a temperature acquisition unit acquires a temperature; and a step in which an offset unit corrects an optical path length difference using a difference in frequency between the measurement interference signals according to a temperature difference, and acquires a corrected distance to the object based on the optical path length difference after the correction. (Supplementary Note 7) The optical distance measuring method described in Supplementary Note 6, characterized in that the time window designation unit determines the time window by synchronizing the wavelengths of the plurality of reference light beams, which are the wavelength swept light beams, corresponding to each of the plurality of optical path sections with a sweep period, and designates the determined time window to the Fourier transform unit.

[0072] Any of the components of the embodiments may be modified or omitted.

[0073] The optical distance measuring device according to the present disclosure can be used, for example, to measure the distance to an object.

[0074] 1 Optical distance measuring device, 2 Wavelength swept light source, 3 Optical tube, 4 Circulator, 5 Photoelectric conversion unit, 6 AD conversion unit, 7 Signal processing unit, 8 Optical system, 9 Optical coupler, 10-1, 10-2 Fiber grating, 21 Electrical unit, 22 Light source, 23 Rectangular wave conversion unit, 31 Semi-transparent mirror, 32, 33 Reflecting mirror, 71 Time window designation unit, 72-1, 72-2 FFT unit, 73-1, 73-2 Peak acquisition unit, 74 Difference acquisition unit, 75 Temperature acquisition unit, 76 Offset unit, 100 Input interface, 101 Output interface, 102 Processor, 103 Memory.

Claims

1. An optical distance measuring device that has an optical path that passes through an object and an optical path that does not pass through the object, and generates a measurement interference signal by photoelectrically converting measurement interference light that is obtained by combining measurement light that is branched from wavelength swept light onto the optical path that passes through the object and multiple reference light beams that are branched onto the optical path that does not pass through the object, characterized in that the optical distance measuring device comprises: multiple optical path sections that are provided on the optical path that does not pass through the object and have different wavelength dependencies for each reference light; and a signal processing section that calculates the frequency spectrum of the measurement interference signal for each reference light beam using a time window according to the wavelength dependency of the optical path sections, and calculates the distance to the object based on the peak of the frequency spectrum for each reference light beam.

2. The optical distance measuring device according to claim 1, characterized in that the plurality of optical path sections are provided in a lens barrel and include a plurality of reflecting mirrors having different wavelength dependencies for each of the reference beams.

3. An optical distance measuring device according to claim 1 or claim 2, characterized in that the plurality of optical path sections include a plurality of fiber gratings having different wavelength dependencies for each of the reference beams.

4. The optical distance measuring device of any one of claims 1 to 3, characterized in that the signal processing unit comprises: a time window designation unit that determines the time window for each of the reference lights in accordance with the wavelength dependency of the optical path unit and designates the determined time window; a Fourier transform unit that converts the measurement interference signal for each of the reference lights into the frequency spectrum for each of the reference lights using the time window designated by the time window designation unit; a peak acquisition unit that acquires peak information of the frequency spectrum for each of the reference lights; a difference acquisition unit that acquires the difference in peak frequency between the measurement interference signals corresponding to each of the reference lights based on the peak information; a temperature acquisition unit that acquires the temperature; and an offset unit that corrects the optical path length difference using the difference in frequency between the measurement interference signals according to the temperature difference and acquires the corrected distance to the object based on the optical path length difference after the correction.

5. The optical distance measuring device according to claim 4, characterized in that the time window designation unit determines the time window in synchronization with the sweep period of the wavelengths of the plurality of reference light beams, which are the wavelength swept light, corresponding to each of the plurality of optical path sections, and designates the determined time window to the Fourier transform unit.

6. An optical distance measuring method for an optical distance measuring device, comprising: an optical path that passes through an object; an optical path that does not pass through the object; generating a measurement interference signal by photoelectrically converting measurement interference light obtained by combining measurement light branched from wavelength swept light onto the optical path that passes through the object and multiple reference light branches onto the optical path that does not pass through the object; the optical path that does not pass through the object is provided with multiple optical path sections having different wavelength dependencies for each of the reference lights; calculating a frequency spectrum of the measurement interference signal for each of the reference lights using a time window according to the wavelength dependency of the optical path section, and calculating a distance to the object based on a peak of the frequency spectrum for each of the reference lights, comprising: a step in which a time window designation section designates the time window having a time length for each of the reference lights; a step in which a Fourier transform section converts the measurement interference signal for each of the reference lights into the frequency spectrum for each of the reference lights using the time window designated by the time window designation section; and a step in which a peak acquisition section acquires peak information of the frequency spectrum for each of the reference lights. an offset unit correcting the optical path length difference using the frequency difference between the measurement interference signals corresponding to the temperature difference, and acquiring the corrected distance to the object based on the corrected optical path length difference.

7. The optical distance measuring method according to claim 6, characterized in that the time window designation unit determines the time window by synchronizing the wavelengths of the plurality of reference light beams, which are the wavelength swept light beams, corresponding to each of the plurality of optical path sections with a sweep period, and designates the determined time window to the Fourier transform unit.

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