Optical distance measuring device and optical distance measuring method

Abstract

The optical distance measuring device (1) is provided in an optical path that does not pass through a target (TG) and includes two reflecting mirrors (32, 33) or fiber gratings (10-1, 10-2) having different wavelength dependencies for each reference light, and a signal processing unit (7) that calculates the frequency spectrum of the measurement interference signal of the reference light (R1, R2) using a time window corresponding to these wavelength dependencies and calculates the distance to the target (TG) based on the peak of the frequency spectrum of the reference light (R1, R2).

Description

[Technical Field]

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

[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 allows the change in focal position and the change in tube length to be offset, allowing the difference between the focal position and the light source position to be set arbitrarily. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 6-130267 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in conventional optical distance measuring devices, when multiple reference beams are used, multiple peaks appear simultaneously in the frequency spectrum obtained by Fourier transforming the measurement interference signal due to interference between the reference beams, making it easy for erroneous detections to occur. For example, when multiple reference beams are used, the maximum peak is extracted from the multiple peaks that appear 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 the individual measurement interference signals, and the measurement interference signals are mixed together to form a complex pattern, resulting in multiple peaks overlapping in the spectrum after Fourier transform, making it difficult to accurately extract the maximum peak. Even if the conventional technique described in Patent Document 1 is applied, the above problem cannot be solved if 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. [Means for solving the problem]

[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. [Effects of the Invention]

[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. [Brief explanation of the drawings]

[0009] [Figure 1] 1A and 1B are diagrams illustrating a measured interference signal and a frequency spectrum of the measured interference signal. [Figure 2] 1 is a block diagram showing a configuration of an optical distance measuring device according to a first embodiment. [Figure 3] 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. [Figure 4] 3 is a flowchart showing an optical distance measuring method according to the first embodiment. [Figure 5] 5A and 5B are diagrams showing the wavelength dependence of the reference light. [Figure 6] FIG. 4 is a diagram showing a modified example of the optical path section in the first embodiment. [Figure 7] 1 is a block diagram showing a hardware configuration for realizing the functions of the optical distance measuring device according to the first embodiment. DETAILED DESCRIPTION OF THE INVENTION

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

[0011] (Outline of optical distance measurement method) First, a conventional optical distance measuring method and the optical distance measuring method according to the first embodiment will be compared and explained. 1A is a diagram showing a conventional measurement interference signal and a frequency spectrum of the measurement interference signal. The arrows in FIG. 1A indicate a series of processes in optical ranging, and the diagram from the top to the bottom indicates the data used or obtained 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 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 wavelength swept light branched onto an optical path that passes through the object, while reference light B and C is wavelength swept light branched onto an optical path that does not pass through the object. Therefore, measurement light A and reference light B and C are optical signals whose wavelengths are swept over time at the same slope.

[0013] The measurement light A is a reflected light that is emitted toward the object through an optical path passing through the object, and is reflected or scattered by the object and returned to the optical distance measuring device. The reference light B is combined with the measurement light A by the wavelength swept light reflected by the first reflecting mirror on an optical path that does not pass through the object. The reference light C is combined with the measurement light A in an optical path that does not pass through the object, by reflecting the wavelength swept light off a second reflecting mirror different from the first reflecting mirror. The reference light B and the reference light C have different optical path lengths along which they propagate.

[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 Figure 1A shows the measurement interference signal with a beat frequency f b The horizontal axis is time t, and the vertical axis is 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] The third diagram from the top in Figure 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 ranging methods, the first reflecting mirror and the second reflecting mirror have almost the same wavelength-time dependence, so the time waveform of the measurement interference signal is a mixture of measurement interference signal D1 and measurement interference signal D2, which overlap to form a complex waveform. The optical ranging device calculates the frequency spectrum of the measurement interference signal by Fourier transforming the measurement interference signal included in the time window of the full time length in response to a trigger signal synchronized with the wavelength sweep.

[0016] The bottom diagram in Figure 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 the interference signal, which is a mixture of the measurement interference signals D1 and D2, over a time window with the full time length, and therefore has multiple peaks. 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 Figure 1A, the beat frequency f b1 The peak and beat frequency f b2 In addition to the peak at beat frequency f b2 The frequency 2f is the double of b2 Peak, frequency f b2-b1 peak and frequency f b2+b1A peak appears.

[0017] The optical distance measuring device converts the maximum of these peaks into the distance to the target. In the case of Figure 1A, the beat frequency f b2 The peak of 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 target 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, their 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 in an optical path that does not pass through the object and have different wavelength dependencies for each reference light. It 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 its frequency spectrum in the optical distance measuring device according to embodiment 1. The arrows 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 acquired 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. The measurement light A is wavelength swept light that is branched into an optical path that passes through the object, and the reference lights B and C are wavelength swept light that is branched into an optical path that does not pass through the object. For example, the first reflecting mirror in the optical path through which reference light B propagates has a different wavelength time dependency 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 C. The second diagram from the top in FIG. 1B shows a measurement interference signal with a beat frequency f b The horizontal axis is time t, and the vertical axis is the beat frequency f b The 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 obtained by combining the measurement light A and the reference light C, and has a beat frequency f b2 The measurement interference signals D1 and D2 appear at a time interval for each wavelength sweep period.

[0023] The third diagram from the top in FIG. 1B is a waveform diagram showing the time waveforms of the measurement interference signal D1 and the measurement interference signal D2, where the horizontal axis represents time t and the vertical axis represents amplitude. Reference light B appears when the wavelength with a high reflection rate on the first reflecting mirror is swept, and reference light C appears 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 handled separately from each other.

[0024] The bottom diagram in FIG. 1B shows the frequency spectra 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 using 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 with a high reflection rate on the second reflecting mirror is swept, a spectrum having a peak at beat frequency f b2 A spectrum having a peak of

[0025] The optical distance measuring device according to the first embodiment calculates the distance to the object from the maximum of these peaks. b2 The peak at is the maximum peak. For example, the optical path difference ΔL corresponding to the maximum peak frequency is calculated according to the above formula (1). Furthermore, the optical path difference ΔL calculated according to the above formula (1) is substituted into the above formula (2), thereby making it possible to calculate the distance d to the target object.

[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 constructed by assembling cylindrical members made of aluminum or the like, and includes a semi-transparent mirror 31, a reflecting mirror 32, and a reflecting mirror 33 inside the cylindrical members. A part of the wavelength swept light output from the circulator 4 is irradiated onto the target TG via a semi-transparent mirror 31, and the rest is branched to a reflecting mirror 32 and a reflecting mirror 33. The wavelength swept light that is reflected or scattered by the target TG and returns to the lens barrel 3 is branched as measurement light A to the circulator 4 via the semi-transparent mirror 31 . The wavelength swept light branched to the reflecting mirror 32 is reflected by the reflecting mirror 32 and then branched to the circulator 4 as reference light B via the semi-transparent mirror 31 . The wavelength swept light beam split into the reflecting mirror 33 is reflected by the reflecting mirror 33 and split into the reference light beam C via the semi-transparent mirror 31 to the circulator 4 .

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

[0030] The reference light B is a part of the wavelength swept light that propagates through the optical path passing through the semi-transparent mirror 31 and the reflecting mirror 32 and has a wavelength that is strongly reflected by the reflecting mirror 32. Furthermore, the reference light C is light of a wavelength that is propagated through an optical path passing through the semi-transparent mirror 31 and the reflecting mirror 33 and is strongly reflected by the reflecting mirror 33, out of the wavelength swept light. 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 lens barrel 3.

[0031] 3A is a diagram showing an overview of the lens barrel 3. In FIG. 3A, L0 is a reference distance, which indicates the distance between the semi-transmitting mirror 31 and the reflecting mirrors 32 and 33 at a reference temperature of 25°C. R1 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 R2 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-transmitting 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 optical tube 3, and outputs the light returning from the optical tube 3 to the photoelectric conversion unit 5. The light returning from the optical tube 3 includes measurement light A, reference light B, and reference light C. FIG. 3B is a graph showing the wavelength time dependence of measurement light A and reference light B used in optical distance measurement, where the horizontal axis is time t and the vertical axis is wavelength λ. sweep is the wavelength sweep frequency. Δλ is the wavelength sweep width, where Δλ = maximum wavelength λ max -Minimum wavelength λ min λc is the center wavelength of the wavelength sweep. Measurement light A and 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 performs analog-to-digital conversion on 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 the signal processing section) 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 specifying unit 71, FFT units 72-1 and 72-2, peak acquiring units 73-1 and 73-2, a difference acquiring unit 74, a temperature acquiring unit 75, and an offset unit 76.

[0035] (Time window specification section) 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 square wave conversion unit 23 is set in the time window designation unit 71. The trigger signal is a square 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 section) 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 the 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 section) 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 on the target 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 part) The difference acquiring 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 acquiring unit 74 calculates the optical path length difference according to the above formula (1) from the frequencies of the peaks acquired from the peak acquiring units 73-1 and 73-2.

[0039] (Temperature acquisition section) The temperature acquisition unit 75 acquires the temperature. For example, the temperature acquisition unit 75 acquires temperature information from a temperature sensor attached to the lens barrel 3. Furthermore, the temperature acquisition unit 75 may 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 the temperature of the lens barrel 3 measured in the past.

[0040] (Offset part) 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, which changes 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, which changes 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 equation (3). In the following equation (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. ΔL corrected =ΔL+k·Δf (3)

[0042] The offset unit 76 calculates the corrected optical path difference ΔL according to the following equation (4): corrected The distance d to the target TG is calculated from the optical path difference ΔL corrected Therefore, the above formula (2) can be expressed as the following formula (4). d=ΔL corrected / twenty four)

[0043] (Error correction due to temperature changes) 3C is a diagram showing the peaks in the frequency spectrum of the measurement interference signal due to measurement light A and reference light B, and the peaks in the frequency spectrum of the measurement interference signal due to measurement light A and reference light C, where the horizontal axis is frequency f (Hz) and the vertical axis is 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 spectrum peak of the measured 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, there is a 2.7 kHz change from the frequency of the peak in the spectrum of the measurement interference signal corresponding to reference light B to the frequency of the peak in the spectrum of the measurement interference signal corresponding to reference light C. This is shown in Figure 3D, where the full width at half maximum (FWHM) is 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) follows the formula (5) below. For example, the minimum wavelength λ min to the central wavelength λ c FWHM of the beat frequency peak in a wavelength sweep up to f 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 dependence of the frequency difference between reference light B and reference light C can be expressed by the following equation (7): In the following equation (7), f1 is the frequency (beat frequency) of the maximum peak in the spectrum of the measurement interference signal corresponding to reference light B, and f2 is the beat frequency of the maximum peak in the spectrum of the measurement interference signal corresponding to 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). f2-f1=Δf(ΔT)=Kα((R2-R1) / 2)ΔT (7)

[0047] In this case, the offset portion 76 is calculated by subtracting the length L of the lens barrel 3 from the length L of the lens barrel 3 using the following formula (8): measure Correct the length L measure0 In the following formula (8), ΔL0 is the change in L0 when the temperature changes from the reference temperature (25°C) to the measurement temperature (for example, 30°C). L0 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 of the lens barrel 3. measure The distance d to the target TG is calculated using L measure0 =L measure -ΔL0αΔT (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. Using this trigger signal, the time window designation unit 71 determines 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 combining 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 combining the measurement light A and the reference light C (step ST3-2). The frequency spectrum of the measurement interference signal generated by combining 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 combining 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 diagram in Fig. 5A is a graph showing the wavelength dependence E of the reference light, where the horizontal axis is wavelength λ and the vertical axis is intensity I. The reference light is an optical signal generated by branching wavelength swept light, and the intensity I is almost constant at wavelength λ within the sweep range. The second diagram from the top in Figure 5A shows the wavelength dependence of the reflectance R of a conventional reflecting mirror, where the horizontal axis is wavelength λ and the vertical axis is reflectance R. In conventional optical distance measuring methods, when generating two reference beams, two reflecting mirrors having approximately the same wavelength time dependence F are used, as shown in the second diagram. The bottom diagram in Fig. 5A shows the wavelength dependence of two reference beams generated by reflecting the wavelength-swept beam off two reflecting mirrors, with the horizontal axis representing wavelength λ and the vertical axis representing intensity I. As this diagram shows, the two reference beams G returned to the circulator 4 have almost the same wavelength dependence, making it difficult to separate them.

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

[0052] The peak acquiring unit 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 acquiring unit 73-2 acquires peak information of the spectrum of the measurement interference signal corresponding to the reference light C (step ST4-2). The difference obtaining unit 74 obtains 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 acquisition unit 75 acquires the temperature of the lens barrel 3 (step ST6).

[0053] The offset unit 76 performs an offset to correct the optical path length difference corresponding to the frequency difference 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). Then, the offset unit 76 acquires the distance to the target TG corrected based on the corrected optical path length difference (step ST8). By the optical distance measuring device 1 performing the above method, it is possible to 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] (Variation) Although the reflecting mirrors 32 and 33 are shown as optical path sections that have different wavelength dependencies for the reference lights B and C, the optical path sections are not limited to reflecting mirrors and may be fiber gratings. Fig. 6 is a diagram showing a modified example of the optical path section in the first embodiment. 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 a specific wavelength. That is, the fiber gratings 10-1 and 10-2 are provided in an optical path in the optical system 8 that does not pass through the object TG, and are optical path sections in which the fiber gratings 10-1 and 10-2 have different wavelength dependencies. For example, the wavelength dependency of the fiber gratings 10-1 and 10-2 indicates the proportion of light reflected by each wavelength. They may have high reflectance at certain wavelengths and low reflectance at other wavelengths. Even if fiber gratings 10-1 and 10-2 are used instead of the reflecting mirrors 32 and 33, it is possible to separately handle the plurality of reference 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. 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, FFT units 72-1 and 72-2, peak acquiring units 73-1 and 73-2, difference acquiring unit 74, temperature acquiring unit 75, and 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, FFT units 72-1, 72-2, peak acquiring units 73-1, 73-2, difference acquiring unit 74, temperature acquiring unit 75, and 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 programs stored in the memory 103 to implement 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 the 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 storing a program for causing the 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] Memory 103 may be, for example, a non-volatile or volatile semiconductor memory such as RAM (Random Access Memory), ROM (Read Only Memory), flash memory, EPROM (Erasable Programmable Read Only Memory), EEPROM (Electrically-EPROM) (registered trademark), a magnetic disk, a flexible disk, an optical disk, a compact disk, a mini disk, a DVD, etc.

[0061] Some of the functions of the time window specifying unit 71, FFT unit 72-1, FFT unit 72-2, peak acquiring unit 73-1, peak acquiring unit 73-2, difference acquiring unit 74, temperature acquiring unit 75, and offset unit 76 included in the signal processing unit 7 may be implemented by dedicated hardware, and the other functions may be implemented by software or firmware. For example, the function of the time window specifying unit 71 may be implemented by a processing circuit that is dedicated hardware, and the functions of the FFT unit 72-1, FFT unit 72-2, peak acquiring unit 73-1, peak acquiring unit 73-2, difference acquiring unit 74, temperature acquiring unit 75, and offset unit 76 may be implemented by the processor 102 reading and executing a program stored in memory 103. In this way, the processing circuit can implement 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. Even in this case, 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 and 33 or the fiber gratings 10-1 and 10-2, and calculates the distance to the object TG based on the peaks of the frequency spectrum of the reference lights B and C. Therefore, 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.

[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 corrected optical path length difference. 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 of 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, 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 the 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 the peak acquisition units 73-1 and 73-2 acquire peak information of the frequency spectrum for each reference light; step ST5 in which the difference acquisition unit 74 acquires the difference in peak frequency between the measurement interference signals corresponding to the reference lights B and C based on the peak information; step ST6 in which the temperature acquisition unit 75 acquires the temperature; and steps ST7 and ST8 in which 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. 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 appear 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, the error 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 according to the timing at which the wavelength of reference light B, C, which is wavelength swept light, corresponding to each of the reflecting mirrors 32, 33 or 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] (Appendix 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, wherein a measurement interference signal is generated 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 a plurality of reference lights that are branched onto the optical paths that do not pass through the object, a plurality of optical path sections provided in an optical path that does not pass through the object, the optical path sections having different wavelength dependencies for the reference beams; a signal processing unit that calculates a frequency spectrum of the measurement interference signal for each of the reference beams using a time window according to the wavelength dependency of the optical path unit, and calculates a distance to the object based on a peak of the frequency spectrum for each of the reference beams. An optical distance measuring device characterized by: (Appendix 2) The plurality of optical path sections are provided in a lens barrel and are a plurality of reflecting mirrors having different wavelength dependencies for the respective reference beams. 2. The optical distance measuring device according to claim 1, (Appendix 3) The plurality of optical path sections are a plurality of fiber gratings having different wavelength dependencies for the respective reference lights. 2. The optical distance measuring device according to claim 1, (Appendix 4) The signal processing unit a time window designation unit that determines the time window for each of the reference beams 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 beams into the frequency spectrum for each of the reference beams using the time window specified by the time window specifying unit; a peak acquisition unit that acquires peak information of the frequency spectrum for each of the reference beams; a difference acquiring unit that acquires a difference in peak frequency between the measurement interference signals corresponding to each of the reference beams based on the peak information; a temperature acquisition unit that acquires a temperature; an offset unit that corrects the optical path length difference using a difference in frequency between the measurement interference signals according to the temperature difference, and acquires a corrected distance to the object based on the corrected optical path length difference. 4. The optical distance measuring device according to claim 1, wherein: (Appendix 5) The time window designation unit determines the time window in synchronization with a sweep period of wavelengths of the plurality of reference beams, which are the wavelength swept beams, corresponding to each of the plurality of optical path units, and designates the determined time window to the Fourier transform unit. 5. The optical distance measuring device according to claim 4, (Appendix 6) An optical distance measuring method for an optical distance measuring device according to any one of Supplementary Note 1 to Supplementary Note 4, a step in which a time window designation unit designates the time window having a time length for each of the reference beams; a Fourier transform unit converting the measurement interference signal for each of the reference beams into the frequency spectrum for each of the reference beams by using the time window designated by the time window designation unit; a peak acquiring unit acquiring peak information of the frequency spectrum for each of the reference beams; a difference acquiring unit acquiring a difference in peak frequency between the measurement interference signals corresponding to each of the reference beams based on the peak information; a temperature acquisition unit acquiring a temperature; an offset unit correcting the optical path length difference using a difference in frequency between the measurement interference signals according to the temperature difference, and acquiring a corrected distance to the object based on the corrected optical path length difference. An optical distance measuring method characterized by: (Appendix 7) The time window designation unit determines the time window by synchronizing wavelengths of the plurality of reference beams, which are the wavelength swept beams, corresponding to each of the plurality of optical path units with a sweep period, and designates the determined time window to the Fourier transform unit. 7. The optical distance measuring method according to claim 6,

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

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

[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 Square 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 ranging device having an optical path that passes through an object and an optical path that does not pass through the object, which generates a measurement interference signal by photoelectric conversion of measurement interference light obtained by combining measurement light branched from wavelength-swept light into the optical path that passes through the object and a plurality of reference lights branched into the optical path that does not pass through the object, A plurality of optical path sections are provided in an optical path that does not pass through the aforementioned object, and each of the reference light sections has a different wavelength dependence. The system includes a signal processing unit that calculates the frequency spectrum of the measurement interference signal for each reference light using a time window corresponding to the wavelength dependence of the optical path, and calculates the distance to the object based on the peak of the frequency spectrum for each reference light. An optical distance measuring device characterized by the following features.

2. The multiple optical path sections are provided in the lens barrel and include multiple reflective mirrors having different wavelength dependencies for each reference light. The optical distance measuring device according to feature 1.

3. The multiple optical path sections include multiple fiber gratings having different wavelength dependencies for each reference light. The optical distance measuring device according to claim 1 or 2.

4. The signal processing unit, A time window designation unit that determines the time window for each reference light according to the wavelength dependence of the optical path and designates the determined time window, A Fourier transform unit that converts the measurement interference signal for each reference light into the frequency spectrum for each reference light using the time window specified by the time window specification unit, A peak acquisition unit that acquires peak information of the frequency spectrum for each of the reference light sources, A difference acquisition unit acquires the difference in peak frequencies between the corresponding measurement interference signals for each of the reference light sources based on the aforementioned peak information. A temperature acquisition unit that acquires temperature, The system includes an offset unit that corrects the optical path length difference using the frequency difference between the measured interference signals corresponding to the temperature difference, and obtains the corrected distance to the target object based on the corrected optical path length difference. The optical distance measuring device according to claim 1 or 2.

5. The time window designation unit determines the time window in synchronization with the sweep period of the wavelength corresponding to each of the multiple optical path sections among the multiple reference lights which are wavelength-swept light, and designates the determined time window to the Fourier transform unit. The optical distance measuring device according to feature 4.

6. The optical path passing through the object, A light path that does not pass through the aforementioned object, A measurement interference signal is generated by photoelectric conversion of measurement interference light obtained by combining measurement light, which is branched from wavelength-swept light into an optical path passing through the object, and multiple reference lights, which are branched into optical paths not passing through the object. The optical path that does not pass through the aforementioned object is provided with a plurality of optical path sections having different wavelength dependencies for each of the reference light. An optical distance measuring method for an optical distance measuring device, comprising: calculating the frequency spectrum of the measurement interference signal for each reference light using a time window corresponding to the wavelength dependence of the optical path; and calculating the distance to the object based on the peak of the frequency spectrum for each reference light, The time window designation unit includes the step of designating the time window having the time length for each of the reference lights, The Fourier transform unit performs the steps of converting the measurement interference signal for each reference light into the frequency spectrum for each reference light using the time window specified by the time window specification unit, The peak acquisition unit performs the step of acquiring peak information of the frequency spectrum for each of the reference light sources, The difference acquisition unit acquires the difference in peak frequencies between the corresponding measurement interference signals for each of the reference lights based on the peak information, The temperature acquisition unit performs the step of acquiring the temperature, The offset unit includes the steps of correcting the optical path length difference using the difference in frequencies between the measured interference signals corresponding to the temperature difference, and obtaining the corrected distance to the object based on the corrected optical path length difference. A method for measuring optical distances characterized by the following features.

7. The time window designation unit determines the time window by synchronizing the wavelengths corresponding to each of the multiple optical path sections among the multiple reference light sources, which are the wavelength-swept light, with the sweep period, and designates the determined time window to the Fourier transform unit. The optical distance measuring method according to feature 6.

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