Optical distance measuring device

Abstract

When reflected light contains multiple signals due to the influence of multiple reflections from the structure being observed, distance information for each signal is extracted independently, and the distance to each of the multiple reflective surfaces existing in the depth direction of the object being measured is measured with high precision and in a short time. [Solution] In the signal processing unit 40, measurement interference signals from multiple reflective surfaces present in the depth direction of the object to be measured 1 are detected in descending order of amplitude of the reflected signals, the distance to the reflective surface is calculated from the delay time relative to the reference interference signal, a pseudo-signal corresponding to the first processed reflected signal is created from the reference signal based on distance information (phase information) obtained from the first processed reflected signal obtained by the distance calculation process, the created pseudo-signal is subtracted from the measurement interference signal to remove the signal component corresponding to the first processed reflected signal, and the distance to the N reflective surfaces present in the depth direction is sequentially calculated by detecting the next reflected signal with the largest amplitude as the processed reflected signal.

Description

Technical Field

[0001] The present invention relates to an optical distance measurement device that measures the distances to a plurality of N (where N is an integer of 2 or more) reflecting surfaces existing in the depth direction of a measurement object.

Background Art

[0002] Conventionally, for a transparent or translucent measurement object such as a glass plate or a lens, in a device that optically and non-contactedly measures the plate thickness dimension and the surface shape thereof, a light beam is irradiated onto the measurement object, and the surface reflected light and the back surface reflected light reflected by the front and back surfaces of the measurement object are detected by individual photodetectors, and the glass plate thickness is measured by measuring the time difference of the detected light at each photodetector (see, for example, Patent Documents 1 and 2).

[0003] In the disclosed technology of Patent Document 1, there are provided means for projecting a light beam onto a measurement object and detecting the optical path of the surface reflected light reflected from the front surface of the measurement object, and means for detecting the optical path of the back surface reflected light reflected from the back surface of the measurement object.

[0004] Also, in the disclosed technology of Patent Document 2, a laser beam that moves parallel to the optical axis of the irradiation optical system and at a substantially constant speed is irradiated onto the measurement glass plate, and the surface reflected light and the back surface reflected light are detected as the time difference of the detected light at at least two different positions on the light receiving optical axis. The thickness of the glass plate is calculated from the time difference and the distance between the measurement optical system and the glass plate.

[0005] The present inventors have previously proposed an optical comb distance meter that can perform distance measurement with high accuracy and in a short time by equipping two optical comb generators that pulse coherent reference light and measurement light, each periodically modulated in intensity or phase and having different modulation periods. The interference light between the reference light pulse irradiated onto a reference surface and the measurement light pulse irradiated onto a measurement surface is detected by a reference photodetector, and the interference light between the reference light pulse reflected by the reference surface and the measurement light pulse reflected by the measurement surface is detected by a measurement photodetector. The difference between the distance to the reference surface and the distance to the measurement surface is determined from the time difference between the two interference signals obtained by the reference photodetector and the measurement photodetector (see, for example, Patent Document 3).

[0006] Furthermore, we have previously proposed an optical comb distance meter that defines the reference point position for the distance to the measurement surface using a reference optical path, enabling high-precision and rapid long-distance measurements (see, for example, Patent Document 4). [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 59-54910 [Patent Document 2] Japanese Patent Application Publication No. 6-174432 [Patent Document 3] Patent No. 5231883 [Patent Document 4] Japanese Patent Publication No. 2020-12641 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] Incidentally, according to the above-mentioned Patent Documents 1 and 2, the thickness of a transparent or translucent object to be measured can be measured optically and non-contact, but it is necessary to have a measuring optical system that detects the surface reflected light and back reflected light reflected from the surface and back surfaces of the object to be measured using separate photodetectors.

[0009] Furthermore, while the optical comb distance meter previously proposed by the inventors of this case can measure the distance to the measurement surface with high accuracy and in a short time, it calculates the distance to the measurement surface based on surface reflected light reflected by the object being measured, by performing signal processing that assumes that each measurement point is reflected from only one reflective surface. Therefore, if there are multiple reflections, the distance cannot be calculated correctly, and it is not possible to measure the thickness of transparent or translucent objects being measured.

[0010] The object of the present invention is, in view of the conventional circumstances described above, to provide an optical distance measuring device that improves upon the optical comb distance meter so that when reflected light contains multiple signals due to the effect of multiple reflections from a structure such as an object to be observed, the distance information of each signal can be independently extracted, and the distance to each of the multiple reflective surfaces existing in the depth direction of the object to be measured can be measured with high precision and in a short time.

[0011] Other objects of the present invention and specific advantages obtained by the present invention will become even clearer from the description of the embodiments described below. [Means for solving the problem]

[0012] In this invention, the signal processing algorithm of an optical comb interferometer is extended to accommodate multiple reflective surfaces located in the depth direction, thereby performing signal processing to individually calculate the distance to each of the multiple reflective surfaces.

[0013] In other words, the present invention is an optical distance measuring device, each Periodically modulated intensity or phase and mutual Measurement light with different modulation periods and reference light The system comprises a light source unit that emits light, a reference photodetector that generates a reference signal for time reference from a portion of the measurement light, a measurement photodetector that detects the measurement light that has traveled back and forth over the object to be measured and generates a measurement signal, and a signal processing unit that calculates the distance from the difference (phase difference) in arrival times between the reference signal and the measurement signal. The above-mentioned reference photodetector receives interference light obtained by superimposing the measurement light emitted from the above-mentioned light source unit and the reference light using a reference interference system, and uses the reference interference signal obtained as the detection output of the interference light as the reference signal that serves as the time reference.The above-mentioned measuring photodetector irradiates the object to be measured with the above-mentioned measuring light, receives the interference light obtained by superimposing the measuring light reflected back by the object to be measured with the above-mentioned reference light using a measuring interferometer, and uses the measurement interference signal obtained as the detection output of the interference light as the above-mentioned measuring signal. The above-mentioned signal processing unit detects the above-mentioned measuring signal, which includes each reflection signal from a plurality of N (N is an integer of 2 or more) reflective surfaces existing in the depth direction of the object to be measured, as the reflection signals to be processed in order of decreasing amplitude, and performs distance calculation processing to calculate the distance to the reflective surface that gives the reflection signal to be processed from the delay time relative to the above-mentioned reference signal. The reference interference signal obtained by the above-mentioned reference photodetector is used as the reference signal. The method is characterized by sequentially calculating the distances to the N reflective surfaces present in the depth direction of the object being measured by creating a pseudo-signal corresponding to the reflective signal to be processed from a reference signal based on distance information (phase information) from the 1 reflective signal to be processed obtained by the above distance calculation process, subtracting the created pseudo-signal from the measurement signal, thereby removing the signal component corresponding to the reflective signal to be processed from the measurement signal, and then detecting the next reflective signal with the largest amplitude as the reflective signal to be processed.

[0015] In the optical distance measuring device according to the present invention, the signal processing unit detects the first reflection signal with the largest amplitude as the reflection signal to be processed from the measurement interference signal which includes each reflection signal from a plurality of N (N is an integer of 2 or more) reflection surfaces present in the depth direction of the object to be measured, and calculates the distance to the first reflection surface that gives the first reflection signal from the delay time relative to the reference interference signal, and based on the distance information (phase information) obtained, the above A first pseudo-signal corresponding to the first reflected signal is created from the reference signal, and by subtracting the first pseudo-signal from the measurement interference signal, the signal component corresponding to the first reflected signal is removed from the measurement interference signal. The second reflected signal with the largest amplitude is then detected as the reflected signal to be processed, and the distance to the second reflective surface that gives the second reflected signal is calculated from the delay time relative to the reference interference signal.

[0016] In the optical distance measuring device according to the present invention, the signal processing unit calculates the distance to the nth reflective surface that gives the nth reflection signal, and based on the distance information (phase information) obtained by doing so, the above By creating an nth pseudo-signal corresponding to the nth reflected signal from the reference signal, and subtracting the nth pseudo-signal from the measurement interference signal, the measurement interference signal from which the signal component corresponding to the nth reflected signal has been removed is used to detect the (n+1)th reflected signal with the largest amplitude as the reflected signal to be processed. The process of calculating the distance to the (n+1)th reflective surface that gives the (n+1)th reflected signal from the delay time relative to the reference interference signal is repeated from n=1 to n=N-1, thereby sequentially calculating the distances to the N reflective surfaces present in the depth direction of the object to be measured.

[0017] In the optical distance measuring device according to the present invention, the signal processing unit calculates the distance to the nth reflective surface that gives the nth reflection signal, and based on the nth distance information (phase information) obtained by this calculation, the above A pseudo-signal corresponding to the nth reflected signal is created from the reference signal, and the nth pseudo-signal is subtracted from the measured interference signal to remove the signal component corresponding to the nth reflected signal. The (n+1)th reflected signal with the largest amplitude is detected as the reflected signal to be processed, and the distance to the (n+1)th reflective surface that gives the (n+1)th reflected signal is calculated. Based on the (n+1)th distance information (phase information) obtained from this, the above A pseudo-signal corresponding to the (n+1)th reflected signal is created from the reference signal, and the (n+1)th pseudo-signal is subtracted from the measured interference signal to remove the signal component corresponding to the (n+1)th reflected signal. The nth reflected signal with the largest amplitude is detected as the reflected signal to be processed, and the distance to the nth reflective surface that gives the nth reflected signal is recalculated to obtain error-improved distance information (phase information) for the nth order. Based on this error-improved distance information (phase information) for the nth order, the aboveRe-create the n-th pseudo signal corresponding to the n-th reflected signal from the reference signal, and subtract the n-th pseudo signal from the measured interference signal to obtain a measured interference signal with the signal component corresponding to the n-th reflected signal removed. Detect the (n + 1)-th reflected signal with the largest amplitude as the processing target reflected signal, and calculate the distance to the (n + 1)-th reflecting surface that gives the (n + 1)-th reflected signal, thereby obtaining the error improvement (n + 1)-th distance information (phase information). By repeating the process with n = n + 1 from n = 1 to n = N - 1, N - 1 pseudo signals are created, and by subtracting all pseudo signals other than the n-th pseudo signal from the measured interference signal including each reflected signal by the N reflecting surfaces, a measured interference signal with the signal components corresponding to the reflected signals other than the n-th reflected signal removed is detected as the processing target reflected signal, and the process of recalculating the distance to the n-th reflecting surface that gives the n-th reflected signal is repeated from n = 1 to n = N - 1, whereby the distances to the N reflecting surfaces can be calculated.

[0018] In the optical distance measuring device according to the present invention, the signal processing unit extracts a signal portion derived from reflection as a reflected signal from the measured interference signal, performs a fast Fourier transform on each extracted reflected signal and the reference interference signal, obtains the phase difference of the frequency components of the same order between each reflected signal and the reference interference signal, and performs a phase calculation process, whereby the distances to N reflecting surfaces existing in the depth direction of the measurement object can be sequentially calculated.

Effects of the Invention

[0019] In the optical distance measuring device according to the present invention, in the signal processing unit, each reflected signal by a plurality of N (N is an integer of 2 or more) reflecting surfaces existing in the depth direction of the measurement object included in the measurement signal is extracted, and each distance to the plurality of reflecting surfaces can be individually calculated from the delay time of each extracted reflected signal with respect to the reference signal.

[0020] Therefore, the present invention can provide an optical distance measurement device capable of measuring with high precision and in a short time the distances to a plurality of N (where N is an integer of 2 or more) reflecting surfaces existing in the depth direction of the measurement object.

Brief Description of the Drawings

[0021] [Figure 1] FIG. 1 is a block diagram showing a configuration example of an optical distance measurement device to which the present invention is applied. [Figure 2] FIG. 2 is a flowchart showing the procedure of distance calculation processing executed by the signal processing unit in the above optical distance measurement device. [Figure 3] FIG. 3 is a waveform diagram showing the surface reflection light signal waveform, the back surface reflection light signal waveform, and the reference signal waveform included in the measurement signal waveform by the measurement light reflected and returned by the measurement object when the measurement object is a translucent substrate. [Figure 4] FIG. 4 is a flowchart showing the procedure of reflection signal extraction processing by the cross-correlation function in the distance calculation processing executed by the signal processing unit. [Figure 5] FIG. 5 is a waveform diagram showing the surface reflection light signal waveform, the back surface reflection light signal waveform, and the reference signal waveform for explaining the reflection signal extraction processing by the cross-correlation function. [Figure 6] FIG. 6 is a flowchart showing the procedure of reflection signal extraction processing using the time window function in the distance calculation processing executed by the signal processing unit. [Figure 7] FIG. 7 is a waveform diagram showing the surface reflection light signal waveform, the back surface reflection light signal waveform, and the reference signal waveform for explaining the reflection signal extraction processing using the time window function. [Figure 8] FIG. 8 is a flowchart showing the procedure of phase calculation processing in the distance calculation processing executed by the signal processing unit. [Figure 9]Figure 9 shows waveform diagrams illustrating reflected light from the object being measured. (A) shows the signal waveform of surface reflected light from an opaque object, and (B) shows the signal waveform of reflected light including surface reflected light and back reflected light components from a semi-transparent object. [Figure 10] Figure 10 is a schematic diagram showing the signal analysis procedure when measuring the distance to the surface reflective surface of an opaque object. [Figure 11] Figure 11 is a schematic diagram showing the signal analysis procedure for measuring the distance between the front and back reflective surfaces of a translucent substrate. [Figure 12] Figure 12 is a waveform diagram showing the FFT processing frame for obtaining the signal spectrum of the first reflected signal when performing signal analysis on the signal waveform of the first measurement signal, along with the signal waveform and the reference signal waveform. [Figure 13] Figure 13(A) is a waveform diagram of a pseudo-signal corresponding to the first reflected signal, which is created based on the first pulse distance information (phase information) and the reference signal, and Figure 13(B) is a waveform diagram of the second measurement signal from which the signal component corresponding to the reflected signal in 1 above has been removed. [Figure 14] Figure 14 is a schematic diagram showing the signal analysis procedure when there are multiple N (where N is an integer of 3 or more) reflective surfaces in the depth direction of the object being measured. [Figure 15] Figure 15 is a schematic diagram showing the iterations performed in the signal processing unit to reduce errors in the distance calculation result caused by the influence of signals from the second reflected signal onwards on the distance calculation based on the first reflected signal. [Figure 16] Figure 16 is a state transition diagram showing the state transitions of the drive signals supplied to the two optical comb generators in the light source unit of the optical distance measuring device described above. [Figure 17] Figure 17 is a flowchart showing the procedure for the absolute distance calculation process in the distance calculation process performed by the signal processing unit described above. [Figure 18] Figure 18 is a schematic diagram illustrating stray light generated by the surface irregularities of an object being measured when measuring the distance to the object. [Figure 19] Figure 19 is a schematic diagram illustrating a situation where, in a location where stray light is strongly generated, the reflected light from the target reflective surface is obscured by the stray light, making it impossible to accurately measure the surface shape of the object being measured. [Figure 20] Figure 20 is a waveform diagram showing a situation where stray light is strongly generated, and the reflected light from the target reflective surface is obscured by the stray light. [Figure 21] Figure 21 is a schematic diagram showing how the signal processing unit can obtain distance information to the target reflective surface as a measurement result by performing signal analysis on a multiple reflection signal that includes stray light components. [Modes for carrying out the invention]

[0022] Embodiments of the present invention will be described in detail below with reference to the drawings. Common components will be denoted by common reference numerals in the drawings. Furthermore, it goes without saying that the present invention is not limited to the following examples and can be modified as needed without departing from the spirit of the invention.

[0023] Figure 1 is a block diagram showing an example configuration of an optical distance measuring device 100 to which the present invention is applied.

[0024] The optical distance measuring device 100 includes a light source unit 10 that emits measuring light S1 and reference light S2, each having periodically modulated intensity or phase and different modulation periods; a reference interferometer 21 that superimposes the measuring light S1 and reference light S2 emitted from the light source unit 10; a measuring interferometer 22 that irradiates the object to be measured 1 with the measuring light S1 and superimposes the measuring light S1' reflected back by the object to be measured 1 with the reference light S2; a reference photodetector 31 that receives the interference light obtained by superimposing the measuring light S1 and reference light S2 by the reference interferometer 21; a measuring photodetector 32 that receives the interference light obtained by superimposing the measuring light S1' and reference light S2 by the measuring quasi-interferometer 22; and a signal processing unit 40 that calculates the distance to the object to be measured 1 from the time difference between the reference interference signal obtained as the detection output of the interference light by the reference photodetector 31 and the measuring interference signal obtained as the detection output of the interference light by the measuring photodetector 32.

[0025] The optical comb generator 10 in this optical distance measuring device 100 consists of one laser light source 11, a first optical comb generator 13 into which one of the laser beams is incident after the laser light emitted from the laser light source 11 is split into two laser beams by a beam splitter 12, and a second optical comb generator 15 into which the other laser beam is incident via a frequency shifter 14.

[0026] The first optical comb generator 13 and the second optical comb generator 15 are driven by oscillators (not shown) that are phase-synchronized with each other and oscillate at different frequencies fm+Δfm and fm, respectively. Each emits a measurement light S1 and a reference light S2, whose intensity or phase is periodically modulated and whose modulation periods are different from each other.

[0027] Here, when laser light to which a predetermined optical frequency shift has been applied by the frequency shifter 14 is incident on the second optical comb generator 15, the measurement light S1 and reference light S2 emitted from the first optical comb generator 13 and the second optical comb generator 15 have a beat frequency between the carrier frequencies that is not a DC signal but an AC signal of the predetermined optical frequency. As a result, the beat signals of the high-frequency sideband and the low-frequency sideband of the carrier frequencies are generated in opposing frequency domains with the beat frequency between the carrier frequencies of the beat signals in between, which is convenient for phase comparison.

[0028] The above-mentioned reference interferometer 21 comprises a first beam splitter 21A into which the measurement light S1 emitted from the first optical comb generator 13 is incident, and a second beam splitter 21B into which the reference light S2 emitted from the second optical comb generator 15 is incident. The measurement light S1 is reflected by the first beam splitter 21A and incident on the second beam splitter 21B, thereby causing the interference light obtained by superimposing the measurement light S1 and the reference light S2 to be incident on the reference photodetector 31.

[0029] Furthermore, the measurement interferometer 22 includes a first beam splitter 22A into which measurement light S1 emitted from the first optical comb generator 13 passes through the first beam splitter 21A of the reference interferometer 21 and is incident, and a second beam splitter 22B into which reference light S2 emitted from the second optical comb generator 15 passes through the second beam splitter 21B of the reference interferometer 21 and is incident. The measurement light S1 that has passed through the first beam splitter 22A is irradiated onto the object to be measured 1, and the measurement light S1' that has been reflected back by the object to be measured 1 is reflected by the first beam splitter 22A and incident on the second beam splitter 22B, thereby causing the interference light obtained by superimposing the measurement light S1' and the reference light S2 to be incident on the measurement photodetector 32.

[0030] The reference photodetector 31 receives interference light obtained by superimposing the measurement light S1 and the reference light S2 incident via the reference interferometer 21, and supplies the reference interference signal obtained as the detection output of the interference light to the signal processing unit 40.

[0031] Furthermore, the measurement photodetector 32 receives interference light obtained by superimposing the measurement light S1' and the reference light S2 incident via the measurement interferometer 22, and supplies the measurement interference signal obtained as the detection output of the interference light to the signal processing unit 40.

[0032] The signal processing unit 40 then calculates the distance to the object to be measured 1 using the reference interference signal obtained as the detection output of the interference light by the reference photodetector 31 and the measurement interference signal obtained as the detection output of the interference light by the measurement photodetector 32, following the procedure shown in the flowchart of Figure 2.

[0033] In other words, the signal processing unit 40 first acquires, in the initial input processing step ST1, the reference interference signal obtained as the detection output of the interference light by the reference photodetector 31 and the measurement interference signal obtained as the detection output of the interference light by the measurement photodetector 32, the measurement signal waveform data from the reference light S2, the measurement signal waveform data from the measurement light S1' that has been reflected back by the object to be measured 1, and the modulation frequency Fm data of the laser light from the first optical comb generator 13 and the second optical comb generator 15.

[0034] In the next reflection signal extraction process ST2, the reflection signal from the object to be measured 1 is extracted based on the waveform data acquired in the input processing process ST1, following the procedure shown in the flowcharts in Figures 4 and 6.

[0035] In the next phase calculation process ST3, the reflected signal acquired in the reflected signal extraction process ST2 is used to perform a phase calculation according to the procedure shown in the flowchart of Figure 8.

[0036] In the next determination step ST4, the process returns to the reflection signal extraction process ST2 to determine whether or not to extract the next reflection signal. If the result is "YES", the process returns to the reflection signal extraction process ST2. If the result is "NO", the process moves to the next determination step ST5 to determine the output mode set in the optical distance measuring device 100.

[0037] Then, if the output mode set in the optical comb distance measuring device 100 is relative distance mode, the process moves to the next relative distance output step ST6, where relative distance data to multiple reflective surfaces existing in the depth direction of the object to be measured 1 is output based on the phase information calculated in the phase calculation processing step ST3.

[0038] Furthermore, if the output mode set in the optical comb distance measuring device 100 is absolute distance mode, the process moves to the next absolute distance calculation step ST7 to calculate the absolute distance to the multiple reflective surfaces located in the depth direction of the object to be measured 1.

[0039] In the next absolute distance output step ST8, absolute distance data is output that shows the absolute distance to each reflective surface of the object to be measured 1, which is obtained as a result of the calculation in the absolute distance calculation processing step ST7.

[0040] Here, if the object to be measured 1 is, for example, a translucent substrate, the measurement signal waveform produced by the measurement light S1' reflected back by the object to be measured 1 includes the surface reflected light signal waveform A and the back reflected light signal waveform B from the object to be measured 1. As shown in Figure 3, the surface reflected light signal waveform and the back reflected light signal waveform have time delays T1 and T2 with respect to the reference signal waveform C produced by the reference light S2.

[0041] The flowchart in Figure 4 shows the procedure for extracting reflected signals using the cross-correlation function, which is performed in the reflected signal extraction process ST2 described above.

[0042] In the reflected signal extraction process ST2, the signal processing unit 40 extracts each reflected signal by detecting the peak of the correlation signal envelope obtained by calculating the cross-correlation function between the reference interference signal and the measured interference signal.

[0043] In other words, in this reflection signal extraction process using the cross-correlation function, the cross-correlation function between the reference signal and the measured signal is first calculated (processing step ST21).

[0044] Next, in order to confirm the presence of a reflected signal, the peak of the envelope of the correlation signal, which is shown by the cross-correlation function calculated in the above processing step ST21, is detected (processing step ST22).

[0045] Then, if the peak of the envelope of the correlation signal detected in the above processing step ST22 exceeds a specified value, it is assumed that a reflected signal exists (processing step ST23), and the time of occurrence of the reflected signal is calculated from the peak of the correlation signal to detect the reflected signal as shown in Figure 5 (processing step ST24). The above specified value is determined from the noise level, the signal level of specular reflection, etc.

[0046] In other words, when the reflection occurs at a single point, the reference signal and the measured signal have the same envelope waveform, and the waveform is delayed by a time corresponding to the distance being measured. When the reference signal and the measured signal are AC signals, the cross-correlation waveform includes vibrational waveforms originating from the AC component. Here, in order to accurately determine the delay time, it is necessary to determine the peak and centroid of the interference signal envelope waveform without being affected by vibrations originating from the AC component. In the case of an AC signal, a method similar to that used to demodulate AM signals can be used, such as obtaining a waveform with the carrier phase shifted by π / 2 using a Hilbert transform and then calculating the sum of squares of the original waveform and the waveform to determine the amplitude of the envelope. Alternatively, the time at which the absolute value of the magnitude of the correlation reaches an extremum can be calculated from the cross-correlation function using a method similar to that used to demodulate AM signals.

[0047] Note that if there are multiple reflection points, multiple peaks will be generated. It is sufficient if each peak provides a sufficiently accurate time. If the accuracy is insufficient, a window function centered on the signal generation time can be applied to perform analysis over a narrower range.

[0048] Furthermore, the flowchart in Figure 6 shows the procedure for the reflected signal extraction process using a time window function, which is performed in the reflected signal extraction process ST2 described above.

[0049] In the reflection signal extraction process ST2, the signal processing unit 40 moves a preset time window function for extracting reflection signal waveforms to extract reflection signal waveforms from the measured interference signal waveforms, and extracts each reflection signal by detecting the peaks of the extracted reflection signal waveforms.

[0050] In other words, in this reflection signal extraction process using a time window function, a window function is applied that extracts only waveforms with a shorter duration compared to the total length of the waveform. By shifting the time window function, the reflection signal waveform is extracted from the measured interference signal waveform (processing step ST21A).

[0051] Time window functions include rectangular window functions that allow 100% transmission only during specific time periods and have zero amplitude during other time periods, as well as Gaussian windows, Hann windows, and Hamming windows. Depending on the shape of the signal, an appropriate window function can be selected. However, with rectangular window functions, the waveform may be discontinuously truncated at the ends of the calculation interval, potentially causing side lobes in the frequency components and leading to errors in distance calculations.

[0052] Next, to confirm the presence of a reflected signal, the peak of the envelope of the measurement signal waveform by the measurement light S1' is detected within the range of the time window function (processing step ST22A).

[0053] Then, if the peak of the envelope of the measurement signal waveform detected in the above processing step ST22A exceeds a specified value, a reflected signal is considered to exist (processing step ST23A), and the time of occurrence of the reflected signal is calculated from the peak of the envelope of the measurement signal waveform, thereby detecting the reflected signal as shown in Figure 7 (processing step ST24A). The above specified value is determined from the noise level, the signal level of specular reflection, etc.

[0054] Here, the signal processing unit 40 may combine the reflection signal extraction process using the cross-correlation function and the reflection signal extraction process using the time window function, and in the reflection signal extraction process ST2, it may roughly detect the position of the reflection signal by detecting the peak of the correlation signal envelope obtained by calculating the cross-correlation function of the reference interference signal and the measured interference signal, extract the reflection signal waveform from the measured interference signal waveform near the detected position using a pre-set time window function for extracting the reflection signal waveform, and extract each reflection signal by detecting the peak of the extracted reflection signal waveform.

[0055] Figure 8 is a flowchart showing the steps of the phase calculation process performed in the phase calculation process ST3.

[0056] In the phase calculation process ST3, the signal processing unit 40 performs a phase calculation to determine the phase difference of frequency components of the same order between each reflected signal and the reference interference signal by performing a Fast Fourier Transform on each reflected signal extracted in the reflected signal extraction process ST2 and the reference interference signal.

[0057] In other words, in the phase calculation process, first, each reflected signal and the reference interference signal are subjected to a Fast Fourier Transform (processing step ST31).

[0058] Here, when the number of data points is not a power of 2, it is generally called the Discrete Fourier Transform (DFT). In this context, regardless of the number of data points, the FFT is defined as a signal processing method that transforms sampled data over a finite time period from the time domain to the frequency domain. Signal peak shifting is sometimes performed as a preprocessing step for the FFT. This involves slightly shifting the calculation interval to bring the center of the pulsed signal to the center of the calculation interval. Peak shifting minimizes the side lobes that occur in the frequency components due to the discontinuous truncation of the waveform at the ends of the calculation interval, thus minimizing errors in distance calculation.

[0059] Next, a process is performed to determine whether the waveform data is valid or invalid based on the signal waveform and the spectral waveform distribution after FFT (processing step ST32).

[0060] Then, for both the reference signal and the measurement signal, the phase of the FFT-processed frequency components is calculated using the waveform data determined to be valid. The phase difference of frequency components of the same order is determined between the reference signal and the measurement signal. The difference in phase differences is calculated between adjacent modes. The average of the differences between adjacent modes matches the phase difference of the envelope waveforms of the measurement signal and the reference signal. Dividing the phase by the repetition angular frequency of the modulated wave gives the delay time. Multiplying the delay time by the speed of light (299,792,458 m / s) gives the optical distance. Dividing this further by the refractive index of air gives the distance to the object being measured (processing step ST33).

[0061] In this optical comb distance measuring device 100, the signal processing unit 40 repeatedly performs each of the processes from the reflection signal extraction process ST2 to the determination process ST4 to detect each reflection signal from multiple reflection surfaces present in the depth direction of the object to be measured 1, calculate the distance to the multiple reflection surfaces, and output relative distance data to the multiple reflection surfaces present in the depth direction of the object to be measured 1 in the relative distance output process ST6.

[0062] In this optical distance measuring device 100, as shown in Figure 9(A), the reflected light from an opaque object 1 such as a metal or mirror does not contain multiple reflected light components. Therefore, the absolute distance to the surface reflective surface of the object 1 can be measured by detecting the surface reflected light. Furthermore, as shown in Figure 9(B), the surface reflected light and back-reflected light components contained in the reflected light from a translucent object 1 such as a glass plate or semiconductor substrate can be detected, allowing for the separation and measurement of the distance to the surface reflective surface and back-reflected surface of the object 1.

[0063] In other words, if the optical distance measuring device 100 is measuring the distance to the surface reflection of an opaque object 1, the reflected light from the object 1 does not contain multiple reflection components. Therefore, as shown in Figure 10, the signal analysis procedure PR0 involves creating a time window function centered on the peak position detected by peak searching of the signal waveform, performing an FFT within the FFT processing frame to obtain the signal spectrum, and performing an FFT on the reference waveform to obtain a reference spectrum. Distance information can then be obtained from the phase difference between the reference signal and the measured signal by performing a phase calculation using the relative amplitude information of the frequency components of the FFT-processed reference waveform and the frequency components of the FFT-processed signal waveform.

[0064] Furthermore, in this optical distance measuring device 100, if the object to be measured 1 is, for example, a translucent substrate, as described above, the measurement signal waveform from the measurement light S1' reflected back by the object to be measured 1 includes the surface reflected light signal waveform A and the back reflected light signal waveform B from the object to be measured 1. As shown in Figure 3, the surface reflected light signal waveform and the back reflected light signal waveform have time delays T1 and T2 with respect to the reference signal waveform C from the reference light S2. Therefore, the signal processing unit 40 performs its signal analysis procedure as shown in Figure 11, using the signal waveform of the input first measurement signal, that is, the measurement For the measurement signal waveform produced by the measurement light S1' reflected back by the target object 1, the first pulse signal analysis procedure PR1 is used to calculate the distance to the first reflective surface (surface reflective surface) that gives the first reflected signal with the largest amplitude. Using the signal analysis procedure PR0 shown in Figure 10, the first pulse distance information (phase information) is obtained from the phase difference between the reference signal and the measurement signal obtained by performing a phase calculation using the relative amplitude information of the frequency components of the FFT-processed reference waveform (reference signal waveform C produced by the above reference light S2) and the FFT-processed signal waveform (measurement signal waveform produced by the above measurement light S1').

[0065] Here, as shown in Figure 12, a time window function is created centered on the peak position detected by peak searching for the signal waveform of the first measurement signal. By performing an FFT within the FFT processing frame to obtain the signal spectrum and then performing signal analysis, the influence of signals from the second reflected signal onward can be reduced, and the first pulse distance information (phase information) from the first reflected signal can be obtained.

[0066] Based on the obtained first pulse distance information (phase information) and reference signal, a pseudo-signal corresponding to the first reflected signal is created as shown in Figure 13(A), and a second measurement signal is obtained by subtracting the created pseudo-signal from the first measurement signal as shown in Figure 13(B), thereby removing the signal component corresponding to the first reflected signal.

[0067] Then, for the signal waveform of this second measurement signal, a second pulse signal analysis procedure PR2 is used to calculate the distance to the second reflective surface (back surface) that gives the second reflection signal with the next larger amplitude. This procedure uses the signal analysis procedure PR0 shown in Figure 10 to perform a phase calculation using the frequency components of the FFT-processed reference waveform and the relative amplitude information of the frequency components of the FFT-processed signal waveform. The second pulse distance information (phase information) is obtained from the phase difference between the reference signal and the measurement signal.

[0068] Here, the above pseudo-signal can be generated as follows.

[0069] Let Sr(t) be the reference signal and Sr^(f) be its FFT signal, and let Sm^(f) be the reflected signal from the object being measured, with its FFT signal being Sm^(f).

[0070] First, the time delay dt of the reflected signal Ss(t) relative to the reference signal Sr(t) is calculated using the conventional optical comb length measurement method. This information is then used to determine the relative amplitude R=| of the reflected signal Ss(t) and the reference signal Sr(t).<Ss(t) / Sr(t)> Using |, the simulated signal Sm(t) is, Sm^(f)=Sr^(f)exp(-2πfdtj+p) / R This can be obtained as the inverse FFT transform of , where p is the overall phase shift between Sr^(f) and Sm^(f).

[0071] Furthermore, if there are multiple N (where N is an integer of 3 or more) reflective surfaces in the depth direction of the object to be measured 1, the signal analysis procedure is as shown in Figure 14. Using the signal analysis procedure PR0 shown in Figure 10, the second pulse distance information (phase information) is obtained from the phase difference between the reference signal and the measurement signal obtained by performing a phase calculation using the frequency components of the FFT-processed reference waveform and the relative amplitude information of the frequency components of the FFT-processed signal waveform for the second signal waveform. Based on the obtained second pulse distance information (phase information) and the reference signal, a pseudo-signal corresponding to the second reflection signal is created, and a third measurement interference signal is obtained by subtracting the created pseudo-signal from the measurement interference signal to remove the signal component corresponding to the second reflection signal.

[0072] Then, for the signal waveform of this third measurement signal, a third pulse signal analysis procedure PR3 is used to calculate the distance to the third reflective surface that gives the next largest amplitude reflected signal (number 3). This procedure uses the signal analysis procedure PR0 shown in Figure 10 to perform a phase calculation using the frequency components of the FFT-processed reference waveform and the relative amplitude information of the frequency components of the FFT-processed signal waveform. The third pulse distance information (phase information) is obtained from the phase difference between the reference signal and the measurement signal.

[0073] By repeating the operation of the second pulse signal analysis procedure PR2 described above, it is possible to obtain the Nth pulse distance information (phase information) to any number of N (where N is an integer of 2 or more) reflective surfaces.

[0074] In this optical distance measuring device 100, the signal processing unit 40 repeatedly performs each of the processes from the reflection signal extraction process ST2 to the determination process ST4 to detect each reflection signal from a plurality of N (N is an integer of 2 or more) reflection surfaces present in the depth direction of the object to be measured 1, calculates the distance to the plurality of reflection surfaces, and outputs distance data to the plurality of reflection surfaces present in the depth direction of the object to be measured 1 in the multiple distance output process ST4.

[0075] In other words, when the output mode set in the optical distance measuring device 100 is the relative distance mode, the signal processing unit 40 detects the measurement interference signal, which includes each reflection signal from a plurality of N (N is an integer of 2 or more) reflective surfaces present in the depth direction of the object to be measured 1, in order of decreasing amplitude as the reflection signals to be processed, and performs distance calculation processing to calculate the distance to the reflective surface that gives the reflection signal to be processed from the delay time relative to the reference interference signal. Based on the distance information (phase information) from the 1 reflection signal to be processed obtained by the distance calculation processing, it creates a pseudo-signal corresponding to the 1 reflection signal to be processed from the reference signal, and subtracts the created pseudo-signal from the measurement interference signal to remove the signal component corresponding to the 1 reflection signal to be processed. Then, it detects the reflection signal with the next largest amplitude as the reflection signal to be processed, and performs signal analysis processing to sequentially calculate the distance to the plurality of N (N is an integer of 2 or more) reflective surfaces present in the depth direction of the object to be measured.

[0076] In the optical distance measuring device 100 described above, the signal processing unit 40 calculates the distances to the N reflective surfaces present in the depth direction of the object being measured. This is done by calculating the distance information (phase information) obtained by calculating the distance to the nth reflective surface that gives the nth reflection signal, creating an nth pseudo-signal corresponding to the nth reflection signal from the reference signal, subtracting the nth pseudo-signal from the measurement interference signal to remove the signal component corresponding to the nth reflection signal, detecting the (n+1)th reflection signal with the largest amplitude as the reflection signal to be processed, and repeating the process of calculating the distance to the (n+1)th reflective surface that gives the (n+1)th reflection signal from the delay time relative to the reference interference signal from n=1 to n=N-1.

[0077] In the optical distance measuring device 100 described above, when calculating the distance to multiple N (where N is an integer of 2 or more) reflective surfaces located in the depth direction of the object to be measured 1, if the object to be measured 1 has multiple N (where N is an integer of 2 or more) reflective surfaces made of a translucent N-1 layered structure, the peaks of each reflection signal will decrease sequentially. However, for example, if each layer is transparent and the amplitude of the signals from the second reflection signal onward is the same as that of the first reflection signal, or if the time interval between the first and second reflection signals is narrow due to an extremely thin laminated film, the signals from the second reflection signal onward may affect the distance calculation based on the first reflection signal, causing errors in the distance calculation result.

[0078] In this case, the signal processing unit 40 can reduce the error by performing an iteration as shown in Figure 15 to recalculate the distance.

[0079] Specifically, based on the first distance information (phase information) obtained by calculating the distance to the first reflective surface that gives the first reflected signal, a first pseudo-signal corresponding to the first reflected signal is created from the reference signal. By subtracting the first pseudo-signal from the measurement interference signal, the signal component corresponding to the first reflected signal is removed from the measurement interference signal. The second reflected signal with the largest amplitude is detected as the reflected signal to be processed, and based on the second distance information (phase information) obtained by calculating the distance to the second reflective surface that gives the second reflected signal, a second pseudo-signal corresponding to the second reflected signal is created from the reference signal.

[0080] Then, by subtracting the second pseudo-signal from the measurement interference signal, the signal component corresponding to the second reflection signal is removed from the measurement interference signal. The first reflection signal with the largest amplitude is detected as the reflection signal to be processed, and the distance to the first reflective surface that gives the first reflection signal is recalculated. This allows for obtaining error-reduced first distance information (phase information).

[0081] Furthermore, based on this first error-improving distance information (phase information), a second pseudo-signal corresponding to the second reflected signal is created again from the reference signal, and by subtracting the second pseudo-signal from the measurement interference signal, the signal component corresponding to the second reflected signal is removed from the measurement interference signal. The second reflected signal with the largest amplitude is then detected as the reflected signal to be processed, and the distance to the second reflective surface that gives the second reflected signal is recalculated, thereby obtaining a second error-improving distance information (phase information) with reduced error.

[0082] Here, by performing the iteration shown in Figure 15 to recalculate the distance to the first reflective surface and the distance to the second reflective surface, we obtained error-reduced first distance information (phase information) and error-reduced second distance information (phase information). However, even when the number of reflected signals to be calculated is three or more, the error included in the distance calculation result can be reduced by similarly performing a phase calculation once, then subtracting all simulated signals other than the target reflected signal, and recalculating each distance to the N reflective surfaces.

[0083] In other words, the signal processing unit 40 calculates the distance to the nth reflective surface that gives the nth reflected signal, and based on the nth distance information (phase information) obtained by calculating the distance to the nth reflective surface that gives the nth reflected signal, it creates an nth pseudo-signal corresponding to the nth reflected signal from the reference signal, subtracts the nth pseudo-signal from the measurement interference signal to remove the signal component corresponding to the nth reflected signal, detects the (n+1)th reflected signal with the largest amplitude as the reflected signal to be processed, calculates the distance to the (n+1)th reflective surface that gives the (n+1)th reflected signal, and based on the (n+1)th distance information (phase information) obtained by calculating the distance to the (n+1)th reflective surface that gives the (n+1)th reflected signal, it creates an (n+1) pseudo-signal corresponding to the (n+1)th reflected signal from the reference signal, subtracts the (n+1)th pseudo-signal from the measurement interference signal to remove the signal component corresponding to the (n+1)th reflected signal, detects the (n)th reflected signal with the largest amplitude as the reflected signal to be processed, and obtains error-improved nth distance information (phase information) by recalculating the distance to the nth reflective surface that gives the nth reflected signal, and this error Based on the improved nth distance information (phase information), a pseudo-signal corresponding to the nth reflected signal is recreated from the reference signal, and the nth pseudo-signal is subtracted from the measured interference signal to remove the signal component corresponding to the nth reflected signal. The (n+1)th reflected signal with the largest amplitude is detected as the reflected signal to be processed, and the distance to the (n+1)th reflective surface that gives the (n+1)th reflected signal is calculated to obtain the error-improved distance information (phase information) for the (n+1)th. This process is repeated from n=1 to n=n. By repeating this process up to N-1, N-1 pseudo-signals are created. By subtracting all pseudo-signals other than the nth pseudo-signal from the measurement interference signal which includes each reflection signal from the N reflection surfaces, the measurement interference signal from which the signal components corresponding to reflection signals other than the nth reflection signal have been removed is detected as the reflection signal to be processed. The distance to the nth reflection surface that gives the nth reflection signal is recalculated. This process is repeated from n=1 to n=N-1, thereby calculating the distance to each of the N reflection surfaces.

[0084] Furthermore, this optical distance measuring device 100 can, in principle, measure the distance to the object to be measured 1 by emitting a measurement light S1 and a reference light S2 from a first optical comb generator 13 and a second optical comb generator 15, respectively, which are periodically modulated in intensity or phase and have different modulation states. Here, the fundamental frequency is fm, the deviation of the fundamental frequency necessary for distance determination is Δfm, and the frequency difference for generating optical comb interference is Δf. The first modulation frequency is Fm1 = fm = 25000 MHz, and the second As shown in Table 1, the signal processing unit 40 is configured to calculate the absolute distance to the object to be measured 1 by outputting two types of measurement light S1 and reference light S2 with different modulation states for each of the four types of modulation frequencies Fm1 to Fm4, which are cyclically switched between.

[0085] [Table 1]

[0086] Table 1 shows the modulation frequencies FmA, FmB and phase difference of two optical combs with different modulation states for each selection state from #1 to #4.

[0087] Figure 16 is a state transition diagram showing the state transitions of the drive signals supplied to the two optical comb generators 13 and 15 of the light source unit 10 in the optical distance measuring device 100.

[0088] In the optical distance measuring device 100, the signal processing unit 40 performs frequency analysis on the reference interference signal (reference signal) obtained by the reference photodetector 31 and the measurement interference signal (measurement signal) obtained by the measurement photodetector 32. Taking the mode number counted from the center frequency of the optical comb as N, the signal processing unit 40 calculates the phase difference between the Nth-order modes of the reference signal and the measurement signal to cancel out the optical phase difference during the optical comb generation and transmission process from the optical comb generator to the reference point. Then, it calculates the increment of the phase difference per order 1 on the frequency axis to obtain the phase difference of the signal pulses, thereby calculating the distance from the reference point to the measurement surface.

[0089] Furthermore, if the measurement distance exceeds half a wavelength of the modulation frequency fm, the distance cannot be uniquely determined because the periodicity of the object light makes it impossible to determine the distance as an integer multiple of that half wavelength. Therefore, four measurements are taken using a reference light pulse and a measurement light pulse set to the four modulation frequencies shown in Table 1, and the signal processing unit 40 uses the phase difference obtained by performing the same processing to calculate the distance exceeding the ambiguous distance equivalent to half a wavelength (La=c / 2fm c: speed of light).

[0090] In other words, the phase difference between the measurement signal and the reference signal obtained by measuring distance using the measurement light S1 and reference light S2, which are cyclically selected from the four modulation frequencies shown in Table 1, is -2πfmT when setting #1 is selected, where the modulation frequencies are fm and fm+Δf; -2π(fm+Δfm)T when setting #2 is selected, where the modulation frequencies are fm+Δfm and fm+Δfm+Δf; -2π(fm+Δfm+Δf)T when setting #3 is selected, where the modulation frequencies are fm+fm and fm; and -2π(fm+Δfm+Δf)T when setting #4 is selected, where the modulation frequencies are fm+Δfm+Δf and fm+Δfm.

[0091] When the distance (La = c / 2fm, where c is the speed of light) is long, the phase difference between the reference signal and the measured signal (-2πfmT) is in the form φ + 2mπ, where m is an integer. Only the φ part can be calculated, but the integer value m remains unknown.

[0092] On the other hand, the difference between the phase difference between the reference signal and the measured signal in setting #1 (-2πfmT) and the phase difference between the reference signal and the measured signal in setting #2 (-2π(fm+Δfm)T) is 2πΔfmT. Similarly, the difference between the phase difference between the reference signal and the measured signal in setting #3 (-2π(fm+Δf)T) and the phase difference between the reference signal and the measured signal in setting #4 (-2π(fm+Δfm+Δf)T) is 2πΔfmT. Thus, the phase is uniquely determined up to a distance equivalent to the wavelength of 1 / Δfm (if Δfm=10MHz, then La is 15m).

[0093] Then, by multiplying this phase by fm / Δfm and comparing it with the phase difference in the selected state of setting #1, the integer m can be determined.

[0094] Furthermore, 2πΔf can be obtained from the difference between the phase difference of -2πfmT when setting #1 is selected in Table 1 and the phase difference of -2π(fm+Δf)T when setting #3 is selected.

[0095] Here, if we set fm = 25 GHz, Δf = 500 kHz, and Δfm = 10 MHz, then since Δf = 500 kHz, we can measure distances up to La = 300 m.

[0096] In this optical distance measuring device 100, absolute distance measurement is performed using the measurement signal and reference signal obtained by measuring distance using the measurement light S1 and reference light S2, which are set to four modulation frequencies (settings #1, #2, #3, and #4) that are cyclically selected and set to the four modulation frequencies shown in Table 1.

[0097] When the output mode set in the optical distance measuring device 100 is absolute distance mode, the signal processing unit 40 calculates the absolute distance to the surface reflection surface of the object to be measured 1 in the absolute distance calculation processing step ST7 according to the procedure shown in the flowchart of Figure 17.

[0098] In other words, in the absolute distance calculation process, the measurement signal and reference signal obtained by measuring the distance using the measurement light S1 and reference light S2 of the above settings #1, #2, #3, and #4 are first selected (processing step ST41), sorted to determine which of the above settings #1, #2, #3, and #4 the selected waveform data represents (processing step ST42), the order of the sorted waveform data is calculated (processing step ST43), further processing is performed to determine whether the waveform data is valid or invalid (processing step ST44), and using the valid waveform data, the phase difference of frequency components of the same order between the reference signal and the measurement signal is calculated and averaged (processing step ST45), thereby calculating the absolute distance to the surface reflection surface of the object to be measured 1 from the phase difference of the measurement signal and reference signal obtained by measuring the distance using the measurement light S1 and reference light S2 of the above settings #1, #2, #3, and #4.

[0099] In this optical distance measuring device 100, the absolute distance output step ST8 outputs absolute distance data indicating the absolute distance to the surface reflection surface of the object to be measured 1, which is obtained as a result of the calculation in the absolute distance calculation processing step ST7.

[0100] Here, the absolute distance calculation process, which calculates the absolute distance to the surface reflection surface of the object being measured 1 from the phase difference between the measurement signal and the reference signal obtained by measuring distance using a measurement light S1 and a reference light S2, for which four modulation frequencies shown in Table 1 are cyclically selected and set, performs signal processing assuming that one measurement point is reflected from one reflection surface. This process obtains order information that is an integer multiple of half a wavelength (approximately 6 mm for 25 GHz) from the phase change due to the cyclic selection and setting of the modulation frequency, and calculates the absolute distance to the measurement surface based on the surface reflected light reflected by the object being measured 1. In principle, even if there are multiple reflection surfaces in the depth direction of the object being measured 1, i.e., if there are many reflection points, the phases of multiple reflection signals can be determined separately by repeating the process from extracting the reflection signal to calculating the phase for each frequency setting.

[0101] In other words, the signal processing unit 40 in this optical distance measuring device 100 can independently determine the order of multiple reflected signals and simultaneously calculate the absolute distances of multiple points by the absolute distance output step ST8 described above.

[0102] Furthermore, this optical distance measuring device 100 functions as a shape measuring device by being equipped with an optical scanning means that scans the object to be measured 1 with measuring light irradiated onto the object to be measured 1, and can separately detect the surface shape and thickness of a translucent substrate.

[0103] Furthermore, the optical distance measuring device 100 described above uses two optical combs, S1 and S2, which are periodically modulated in intensity or phase and have different modulation periods, to perform Time of Flight (TOF) distance measurement. In cases where the reflected light contains multiple signals due to the effect of multiple reflections from the structure being observed, this technology allows for the independent extraction of distance information for each signal. This technology is not limited to optical combs, but can also be applied to Time of Flight (TOF) distance measurement using other lasers to determine distance from the time difference of pulsed light with a reference signal.

[0104] Furthermore, it is effective not only when you want to obtain distance information from multiple signals that have undergone multiple reflections, but also when you want to remove signals caused by stray light.

[0105] In other words, when the optical distance measuring device 100 measures the distance to an object 1 with an uneven surface, stray light is generated due to the unevenness on the surface of the object 1, as shown in Figure 18. As shown in Figures 19 and 20, in places where stray light is strongly generated, the amplitude of the stray light increases, and the reflected light from the target reflective surface is buried in the stray light, making it impossible to accurately measure the surface shape of the object 1. However, by performing the above-described signal analysis on the multiple reflection signal including the stray light component in the signal processing unit 40, distance information to the target reflective surface can be obtained as a measurement result from the measurement signal of the reflected light from the target reflective surface that is buried in the stray light. As shown in Figure 21, by selecting this as the measurement result of an effective measurement point, the surface shape of the object 1 can be accurately measured.

[0106] Here, in the optical distance measuring device 100, the measurement light S1 and reference light S2, which are emitted from the light source unit 10 and whose intensity or phase is periodically modulated and which have different modulation periods, are superimposed by the reference interferometer 21 to obtain interference light, which is received by the reference photodetector 31, and the reference interference signal obtained as the detection output of the interference light is used as a reference signal that serves as a time reference, the measurement light S1 is irradiated onto the object to be measured 1, and the measurement light S1' that is reflected back from the object to be measured 1 and the reference light S2 are superimposed by the measurement interferometer to obtain interference light, which is received by the measurement photodetector 32. The measurement interference signal obtained from the detection output of the interference light is used as the measurement signal, and the signal processing unit 40 calculates the distance to the object to be measured 1 from the time difference between the reference interference signal and the measurement interference signal. However, the present invention is not limited to measuring the distance of interfered light, and can also be applied to optical distance measuring devices that perform distance and shape measurement of a general laser distance meter type. In this case, the distance can be measured from the difference in arrival time (phase difference) between the reference signal and the measurement signal without requiring a reference interferometer or a measurement interferometer, and periodically emitted from the light source unit In a signal processing unit that generates a time-referenced reference signal using a reference photodetector that receives a portion of the measurement light whose intensity or phase has been modulated, and generates a measurement signal by detecting the measurement light that has traveled back and forth over the object to be measured using the measurement photodetector, and calculates the distance from the difference in arrival time (phase difference) between the reference signal and the measurement signal, the measurement signal, which includes each reflection signal from a plurality of N (N is an integer of 2 or more) reflective surfaces present in the depth direction of the object to be measured, is detected as a target reflection signal in order of decreasing amplitude, and a distance calculation process is performed to calculate the distance to the reflective surface that gives the target reflection signal from the delay time relative to the reference signal. In this process, a pseudo-signal corresponding to the first target reflection signal is created from a reference signal based on distance information (phase information) from the first target reflection signal obtained by the distance calculation process, and the signal component corresponding to the first target reflection signal is removed from the measurement signal. The next reflection signal with the largest amplitude is then detected as a target reflection signal, thereby sequentially calculating the distances to the N reflective surfaces present in the depth direction of the object to be measured.

[0107] 1 Light source unit, 11 Laser light source, 12 Beam splitter, 13,15 Optical comb generator, 14 Frequency shifter, 21 Reference interferometer, 22 Measuring interferometer, 31 Reference interferometer, 32 Measuring interferometer, 40 Signal processing unit, 100 Optical distance measuring device

Claims

1. A light source unit that emits measurement light and reference light, each of which is periodically modulated in intensity or phase and has different modulation periods from one another, A reference photodetector that generates a time reference signal from a portion of the measured light, A measurement photodetector that detects measurement light traveling back and forth over the object to be measured and generates a measurement signal, It includes a signal processing unit that calculates distance from the difference (phase difference) in arrival times between a reference signal and a measurement signal. The above-mentioned reference photodetector receives interference light obtained by superimposing the measurement light emitted from the above-mentioned light source unit and the reference light using a reference interference system, and uses the reference interference signal obtained as the detection output of the interference light as the reference signal that serves as the time reference. The above-mentioned measuring light detector irradiates the object to be measured with the above-mentioned measuring light, and receives the interference light obtained by superimposing the measuring light reflected back by the object to be measured with the above-mentioned reference light using a measuring interferometer, and uses the measurement interference signal obtained as the detection output of the above-mentioned interference light as the above-mentioned measuring signal. The above signal processing unit is, With respect to the measurement signal, which includes each reflection signal from multiple N (where N is an integer of 2 or more) reflective surfaces located in the depth direction of the object to be measured, the reflection signals are detected in order of decreasing amplitude as the reflection signals to be processed, and a distance calculation process is performed to calculate the distance to the reflective surface that gives the reflection signal to be processed from the delay time relative to the reference signal. Using the reference interference signal obtained by the above reference photodetector as a reference signal, a pseudo-signal corresponding to the above-mentioned reflected signal obtained by the above-mentioned distance calculation process is created from the reference signal based on distance information (phase information). By subtracting the created pseudo-signal from the above measurement signal, the signal component corresponding to the reflected signal to be processed in step 1 is removed from the measurement signal. Then, the reflected signal with the next largest amplitude is detected as the reflected signal to be processed, and the distances to the N reflective surfaces present in the depth direction of the object being measured are calculated sequentially. An optical distance measuring device characterized by the following features.

2. The above signal processing unit processes the measurement interference signal, which includes the reflection signals from multiple N (where N is an integer of 2 or more) reflective surfaces located in the depth direction of the object being measured, The first reflected signal with the largest amplitude is detected as the reflected signal to be processed, and based on the distance information (phase information) obtained by calculating the distance to the first reflective surface that gives the first reflected signal from the delay time relative to the reference interference signal, a first pseudo-signal corresponding to the first reflected signal is created from the reference signal. By subtracting the first pseudo-signal from the measured interference signal, the signal component corresponding to the first reflected signal is removed from the measured interference signal. The second reflected signal with the largest amplitude is detected as the reflected signal to be processed, and the distance to the second reflective surface that gives the second reflected signal is calculated from the delay time relative to the reference interference signal. The optical distance measuring device according to feature 1.

3. The above signal processing unit is, Based on the distance information (phase information) obtained by calculating the distance to the nth reflective surface that gives the nth reflected signal, a pseudo-signal corresponding to the nth reflected signal is created from the reference signal. By subtracting the above-mentioned nth pseudo-signal from the above-mentioned measurement interference signal, the signal component corresponding to the above-mentioned nth reflected signal is removed from the measurement interference signal. The (n+1)th reflected signal with the largest amplitude is detected as the reflected signal to be processed, and the distance to the (n+1)th reflective surface that gives the (n+1)th reflected signal is calculated from the delay time relative to the above-mentioned reference interference signal. This process is repeated from n=1 to n=N-1. The distances to the N reflective surfaces located in the depth direction of the object being measured are calculated sequentially. The optical distance measuring device according to feature 1.

4. The above signal processing unit is, Based on the nth distance information (phase information) obtained by calculating the distance to the nth reflective surface that gives the nth reflected signal, a pseudo-signal corresponding to the nth reflected signal is created from the above reference signal. By subtracting the above-mentioned nth pseudo-signal from the above-mentioned measurement interference signal, the measurement interference signal from which the signal component corresponding to the above-mentioned nth reflected signal has been removed is used to detect the (n+1)th reflected signal with the largest amplitude as the reflected signal to be processed. Based on the (n+1)th distance information (phase information) obtained by calculating the distance to the (n+1)th reflective surface that gives the (n+1)th reflected signal, a pseudo-signal corresponding to the (n+1)th reflected signal is created from the above reference signal. By subtracting the above (n+1) pseudo-signal from the above measurement interference signal, the measurement interference signal from which the signal component corresponding to the above (n+1) reflected signal has been removed is used to detect the nth reflected signal with the largest amplitude as the reflected signal to be processed. By recalculating the distance to the nth reflective surface that gives the nth reflected signal, we obtain error-improving distance information (phase information) for the nth step. Based on this error-correcting nth distance information (phase information), a pseudo-signal corresponding to the nth reflected signal is created again from the reference signal. By subtracting the above-mentioned nth pseudo-signal from the above-mentioned measurement interference signal, the measurement interference signal from which the signal component corresponding to the above-mentioned nth reflected signal has been removed is used to detect the (n+1)th reflected signal with the largest amplitude as the reflected signal to be processed. By calculating the distance to the (n+1) reflective surface that gives the (n+1) reflected signal, and obtaining the error-improving (n+1) distance information (phase information), the process is repeated from n=1 to n=N-1, with n=n+1, thereby creating N-1 pseudo-signals. By subtracting all pseudo-signals except the nth pseudo-signal from the measurement interference signal which includes the reflection signals from the N reflection surfaces, the signal components corresponding to reflection signals other than the nth reflection signal are removed, and the measurement interference signal obtained is detected as the reflection signal to be processed. The distance to the nth reflection surface which gives the nth reflection signal is recalculated, and this process is repeated from n=1 to n=N-1 to calculate the distance to each of the N reflection surfaces. The optical distance measuring device according to feature 1.

5. The above signal processing unit is, From the above measured interference signal, the signal portion originating from reflection is extracted as the reflected signal. The optical distance measuring device according to any one of claims 1 to 4, characterized in that it sequentially calculates the distance to N reflective surfaces existing in the depth direction of the object to be measured by performing a fast Fourier transform on each extracted reflected signal and the above reference interference signal to obtain the phase difference of the same order frequency components between each reflected signal and the above reference interference signal, and performing a phase calculation process.

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