JP7575119B2 - Optical comb distance measurement device - Google Patents

Description

The present invention relates to an optical comb distance measurement device that measures the distances to multiple reflecting surfaces that exist in the depth direction of a measurement object.

Conventionally, in devices for optically and non-contactly measuring the thickness and surface shape of transparent or semi-transparent objects such as glass plates and lenses, a light beam is irradiated onto the object, and the surface reflected light and back reflected light reflected by the front and back surfaces of the object are detected by separate photodetectors, and the glass thickness is measured by measuring the time difference between the detected light at each photodetector (see, for example, Patent Documents 1 and 2).

The technology disclosed in Patent Document 1 includes a means for projecting a light beam onto an object to be measured and detecting the optical path of the surface reflected light reflected from the surface of the object to be measured, and a means for detecting the optical path of the back surface reflected light reflected from the back surface of the object to be measured.

In addition, in the technology disclosed in 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 glass plate to be measured, and the surface reflected light and the back reflected light are detected as the time difference of the detected light in a receiver installed at at least two different positions on the receiving optical axis, and the thickness of the glass plate is calculated from the time difference and the distance between the measurement optical system and the glass plate.

The inventors of the present invention have previously proposed an optical comb rangefinder that is capable of performing distance measurements with high accuracy and in a short time by having two optical comb generators that emit pulses of coherent reference light and measurement light, each of which is periodically modulated in intensity or phase and has a different modulation period, detecting the interference light between the reference light pulse irradiated to the reference surface and the measurement light pulse irradiated to the measurement surface with a reference light detector, and detecting the interference light between the reference light pulse reflected by the reference surface and the measurement light pulse reflected by the measurement surface with a measurement light detector, and determining the difference between the distance to the reference surface and the distance to the measurement surface from the time difference between the two interference signals obtained by the reference light detector and the measurement light detector (see, for example, Patent Document 3).

We have also previously proposed an optical comb distance meter that determines the reference point position of the distance to the measurement surface using a reference optical path, enabling long distance measurements to be performed with high accuracy and in a short time (see, for example, Patent Document 4).

Japanese Unexamined Patent Publication No. 59-54910 Japanese Unexamined Patent Publication No. 6-174432 Patent No. 5231883 JP 2020-12641 A

According to the above-mentioned Patent Documents 1 and 2, the thickness of a transparent or semi-transparent object can be optically measured without contact, but it is necessary to provide a measurement optical system that detects the surface reflected light and back reflected light reflected by the front and back surfaces of the object using separate photodetectors.

In addition, the optical comb distance meter previously proposed by the inventors of this invention can measure the distance to the measurement surface with high accuracy and in a short time, but it performs signal processing under the assumption that one measurement point is a reflection from one reflecting surface, and calculates the distance to the measurement surface based on the surface reflected light reflected by the measurement object. Therefore, if there are multiple reflections, the distance cannot be calculated correctly, and it is not possible to measure the thickness of a transparent or translucent object.

In view of the above-mentioned conventional situation, the object of the present invention is to provide a comb distance measurement device that improves the optical comb distance meter and is capable of measuring the distance to multiple reflecting surfaces present in the depth direction of the measurement object with high accuracy and in a short time.

Other objects of the present invention and specific advantages obtained by the present invention will become more apparent from the description of the embodiments described below.

In the present invention, the signal processing algorithm of the optical comb interferometer is expanded to accommodate multiple reflecting surfaces present in the depth direction, thereby performing signal processing to calculate the distances to multiple reflecting surfaces.

That is, the present invention is an optical comb distance measurement device, comprising: a light source unit that emits measurement light and reference light, each of which is periodically modulated in intensity or phase and has a different modulation period from each other; a reference interferometer that superimposes the measurement light and reference light emitted from the light source unit; a measurement interferometer that irradiates a measurement object with the measurement light and superimposes the measurement light and the reference light that are reflected and returned by the measurement object; a reference photodetector that receives interference light obtained by superimposing the measurement light and the reference light by the reference interferometer; a measurement photodetector that receives interference light obtained by superimposing the measurement light and the reference light by the measurement interferometer; and a signal processing unit that calculates distances to the multiple reflecting surfaces from delay times of each reflected signal by a multiple reflecting surfaces present in the depth direction of the object to be measured, the delay times being included in a measurement interference signal obtained as a detection output of interference light by the reference photodetector, relative to a reference interference signal obtained as a detection output of interference light by the reference photodetector, wherein the signal processing unit performs a reflection signal extraction process that extracts a signal portion derived from reflection as a reflection signal from the measurement interference signal, and a phase calculation process that performs a fast Fourier transform on each extracted reflection signal and the reference interference signal to obtain a phase difference of the same order frequency component between each reflection signal and the reference interference signal .

In addition, in the optical comb distance measurement device according to the present invention, the signal processing unit can extract each reflected signal by detecting the peak of a correlation signal envelope obtained by calculating the cross-correlation function between the reference interference signal and the measurement interference signal in the reflected signal extraction process.

In addition, in the optical comb distance measurement device according to the present invention, the signal processing unit can be configured to move a time window function for extracting a predetermined reflected signal waveform in the reflected signal extraction process, extract the reflected signal waveform from the measured interference signal waveform, and extract each reflected signal by detecting the peak of the extracted reflected signal waveform.

Furthermore, in the optical comb distance measurement device according to the present invention, the signal processing unit, in the reflected signal extraction process, can detect the rough position of the 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, extract the reflected signal waveform from the measured interference signal waveform near the detected position using a time window function for extracting a predetermined reflected signal waveform, and extract each reflected signal by detecting the peak of the extracted reflected signal waveform.

In the optical comb distance measurement device of the present invention, the signal processing unit extracts each reflection signal caused by multiple reflection surfaces present in the depth direction of the object to be measured, which is included in the measurement interference signal, and can calculate the distance to the multiple reflection surfaces from the delay time of each extracted reflection signal relative to a reference interference signal .

Therefore, the present invention provides an optical comb distance measurement device that can measure the distance to multiple reflecting surfaces present in the depth direction of the measurement object with high accuracy and in a short time.

FIG. 1 is a block diagram showing an example of the configuration of an optical comb distance measurement device to which the present invention is applied. FIG. 2 is a flowchart showing the procedure of the distance calculation process executed by the signal processing unit in the optical comb distance measurement device. FIG. 3 is a waveform diagram showing a surface reflected light signal waveform and a back surface reflected light signal waveform from the measurement object, and a reference signal waveform from the reference light, which are included in a measurement signal waveform due to measurement light reflected and returned by the measurement object when the measurement object is a translucent substrate. FIG. 4 is a flowchart showing the procedure of the reflected signal extraction process using a cross-correlation function in the distance calculation process executed by the signal processing unit. FIG. 5 is a waveform diagram showing a front surface reflected light signal waveform, a rear surface reflected light signal waveform, and a reference signal waveform, which are provided for explaining the reflected signal extraction process using the cross-correlation function. FIG. 6 is a flowchart showing the procedure of the reflected signal extraction process using a time window function in the distance calculation process executed by the signal processing unit. FIG. 7 is a waveform diagram showing a front surface reflected light signal waveform, a rear surface reflected light signal waveform, and a reference signal waveform, which are provided for explaining the reflected signal extraction process using the above-mentioned time window function. FIG. 8 is a flowchart showing a procedure of the phase calculation process in the distance calculation process executed by the signal processing unit. FIG. 9 is a waveform diagram explaining the reflected light from the object to be measured, in which (A) shows the signal waveform of the surface reflected light from an opaque object to be measured, and (B) shows the signal waveform of the reflected light including the surface reflected light and the back surface reflected light components from a translucent object to be measured. FIG. 10 is a state transition diagram showing state transitions of the drive signals supplied to the two optical comb generators of the light source unit in the optical comb distance measurement device. FIG. 11 is a flowchart showing a procedure of the absolute distance calculation process in the distance calculation process executed by the signal processing unit.

The following describes in detail the embodiments of the present invention with reference to the drawings. Common components are described with common reference numerals in the drawings. Furthermore, the present invention is not limited to the following examples, and can be modified as desired without departing from the spirit of the present invention.

Figure 1 is a block diagram showing an example configuration of an optical comb distance measurement device 100 to which the present invention is applied.

This optical comb distance measurement device 100 includes a light source unit 10 that emits measurement light S1 and reference light S2, each of which is periodically modulated in intensity or phase and has a different modulation period, a reference interferometer 21 that superimposes the measurement light S1 and reference light S2 emitted from the light source unit 10, a measurement interferometer 22 that irradiates the measurement light S1 to the measurement object 1 and superimposes the measurement light S1' reflected and returned by the measurement object 1 and the reference light S2, a reference light detector 31 that receives interference light obtained by superimposing the measurement light S1 and the reference light S2 by the reference interferometer 21, a measurement light detector 32 that receives interference light obtained by superimposing the measurement light S1' and the reference light S2 by the measurement quasi-interferometer 22, and a signal processing unit 40 that calculates the distance to the measurement object 1 from the time difference between a reference interference signal obtained as the detection output of the interference light by the reference light detector 31 and a measurement interference signal obtained as the detection output of the interference light by the measurement light detector 32.

The optical comb generator 10 in this optical comb distance measurement device 100 consists of one laser light source 11, the laser light emitted from this laser light source 11 being split into two laser lights by a beam splitter 12, one of the laser lights being incident on a first optical comb generator 13, and the other laser light being incident via a frequency shifter 14 on a second optical comb generator 15.

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, and emit measurement light S1 and reference light S2 whose intensities or phases are periodically modulated and whose modulation periods are different from each other.

When the laser light to which a predetermined optical frequency shift has been given by the frequency shifter 14 is input to the second optical comb generator 15, the measurement light S1 and reference light S2 output 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 signal of the high-frequency sideband of the carrier frequency and the beat signal of the low-frequency sideband of the carrier frequency are generated in opposite frequency domains across the beat frequency between the carrier frequencies of the beat signals, which is convenient for phase comparison.

The reference interferometer 21 includes a first beam splitter 21A on which the measurement light S1 emitted from the first optical comb generator 13 is incident, and a second beam splitter 21B on 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, and the interference light obtained by superimposing the measurement light S1 and the reference light S2 is incident on the reference photodetector 31.

The measurement interferometer 22 includes a first beam splitter 22A through which the 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 thereon, and a second beam splitter 22B through which the 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 thereon. The measurement light S1 that passes through the first beam splitter 22A is irradiated onto the measurement object 1, and the measurement light S1' reflected and returned by the measurement object 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 light detector 32.

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

The measurement light detector 32 receives interference light obtained by superimposing the measurement light S1' and the reference light S2 incident via the measurement interferometer 22, and supplies a measurement interference signal obtained as a detection output of the interference light to the signal processing unit 40.

Then, the signal processing unit 40 performs a process of calculating the distance to the measurement object 1 according to the procedure shown in the flowchart of FIG. 2, using the reference interference signal obtained as the detection output of the interference light by the reference light detector 31 and the measurement interference signal obtained as the detection output of the interference light by the measurement light detector 32.

That is, in the first input processing step ST1, the signal processing unit 40 first acquires measurement signal waveform data by the reference light S2, measurement signal waveform data by the measurement light S1' reflected and returned by the measurement object 1, and modulation frequency Fm data of the laser light in the first optical comb generator 13 and the second optical comb generator 15 from the reference interference signal obtained as the detection output of the interference light by the reference light detector 31 and the measurement interference signal obtained as the detection output of the interference light by the measurement light detector 32.

In the next reflected signal extraction process ST2, the reflected signal from the object to be measured 1 is extracted based on the waveform data captured in the input process ST1, following the procedures shown in the flowcharts of Figures 4 and 6.

In the next phase calculation process ST3, phase calculation is performed on the reflected signal captured in the reflected signal extraction process ST2 according to the procedure shown in the flowchart in Figure 8.

In the next determination step ST4, the process returns to the reflected signal extraction processing step ST2 to determine whether or not to extract the next reflected signal. If the next reflected signal is to be extracted, i.e., if the determination result is "YES", the process returns to the reflected signal extraction processing step ST2. If the next reflected signal is not to be extracted, i.e., if the determination result is "NO", the process moves to the next determination step ST5, where the output mode set in the optical comb distance measurement device 100 is determined.

If the output mode set in this optical comb distance measurement device 100 is the relative distance mode, the process proceeds to the next relative distance output step ST6, in which relative distance data to multiple reflecting surfaces present in the depth direction of the measurement object 1 is output based on the phase information calculated in the phase calculation processing step ST3.

If the output mode set in this optical comb distance measurement device 100 is the absolute distance mode, the process proceeds to the next absolute distance calculation process ST7, where the absolute distances to the multiple reflecting surfaces present in the depth direction of the measurement object 1 are calculated.

In the next absolute distance output step ST8, absolute distance data indicating the absolute distance to each reflecting surface of the measurement object 1 obtained as a result of the calculation in the absolute distance calculation processing step ST7 is output.

Here, if the measurement object 1 is, for example, a translucent substrate, the measurement signal waveform due to the measurement light S1' reflected and returned by the measurement object 1 includes a surface reflected light signal waveform A and a back surface reflected light signal waveform B due to the measurement object 1, and as shown in Figure 3, the surface reflected light signal waveform and the back surface reflected light signal waveform have time delays T1 and T2 with respect to the reference signal waveform C due to the reference light S2.

The flowchart in Figure 4 shows the steps of the reflected signal extraction process using the cross-correlation function, which is executed in the reflected signal extraction process step S2.

In the reflected signal extraction processing step S2, 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.

That is, in this reflected signal extraction process using the cross-correlation function, the cross-correlation function between the reference signal and the measurement signal is first calculated (processing step ST21).

Next, to confirm the presence of a reflected signal, the peak of the envelope of the correlation signal represented by the cross-correlation function calculated in the above process step ST21 is detected (process step ST22).

If the peak of the envelope of the correlation signal detected by the processing step ST22 exceeds a specified value, it is assumed that a reflected signal is present (processing step ST23), and the reflected signal is detected as shown in FIG. 5 by calculating the occurrence time of the reflected signal from the peak of the correlation signal (processing step ST24). The specified value is determined from the noise level, the signal level of the specular reflection, etc.

In other words, when the reference signal and the measurement signal are reflected at one point, they have the same envelope waveform, and the waveform is delayed by a time corresponding to the distance to be measured. When the reference signal and the measurement signal are AC signals, the cross-correlation waveform contains a vibration waveform derived from the AC component. Here, to accurately determine the delay time, it is necessary to determine the peak and center of gravity of the interference signal envelope waveform without being affected by the vibration derived from the AC component. In the case of an AC signal, a method similar to the AM signal demodulation method can be used, such as using the Hilbert transform to determine a waveform in which the phase of the carrier wave is shifted by π/2, and then calculating the square sum with the original waveform to determine the envelope amplitude. In addition, the time when the absolute value of the correlation magnitude has an extreme value from the cross-correlation function can be calculated using a method similar to the AM signal demodulation method.

If there are multiple reflection points, multiple peaks will occur. It is sufficient if a time can be obtained with sufficient accuracy from each of them. If the accuracy is insufficient, a window function (described later) centered on the signal generation time can be applied to analyze a narrower range.

The flowchart in Figure 6 shows the steps of the reflected signal extraction process using a time window function, which is executed in the reflected signal extraction process step S2.

In the reflected signal extraction process S2, the signal processing unit 40 extracts the reflected signal waveform from the measured interference signal waveform by moving a time window function for extracting a predetermined reflected signal waveform, and extracts each reflected signal by detecting the peak of the extracted reflected signal waveform.

In other words, in the reflected signal extraction process using this time window function, a window function is applied that extracts only a waveform that is short compared to the overall length of the waveform, and the time window function is moved to extract the reflected signal waveform from the measured interference signal waveform (processing step ST21A).

Time window functions include rectangular window functions that are 100% transparent only during a specific time period and have an amplitude of 0 during other times, as well as Gaussian, Hann, and Hamming windows, and other window functions can be selected according to the shape of the signal. Rectangular window functions can cause the waveform to be cut off discontinuously at the ends of the calculation interval, which can cause side lobes in the frequency components and lead to errors in the distance calculation.

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

If the peak of the envelope of the measurement signal waveform detected by the above process ST22A exceeds a specified value, it is assumed that a reflected signal is present (process ST23A), and the reflected signal is detected as shown in FIG. 7 by calculating the occurrence time of the reflected signal from the peak of the envelope of the measurement signal waveform (process ST24A). The above specified value is determined from the noise level, the signal level of the specular reflection, etc.

Here, the signal processing unit 40 may combine the reflected signal extraction process using the cross-correlation function and the reflected signal extraction process using a time window function, and in the reflected signal extraction process step ST2, detect the approximate position of the 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, extract the reflected signal waveform from the measured interference signal waveform near the detected position using a time window function for extracting a predetermined reflected signal waveform, and extract each reflected signal by detecting the peak of the extracted reflected signal waveform.

Figure 8 is a flowchart showing the steps of the phase calculation process executed in phase calculation process ST3.

In the phase calculation process ST3, the signal processing unit 40 performs a fast Fourier transform on each of the reflected signals extracted in the reflected signal extraction process ST2 and the reference interference signal, and performs a phase calculation to determine the phase difference between the frequency components of the same order between each reflected signal and the reference interference signal.

That is, in the phase calculation process, first, each reflected signal and the reference interference signal are subjected to a fast Fourier transform (processing step ST31).

When the number of data points is not a power of two, it is generally called a discrete Fourier transform (DFT). Here, FFT refers to signal processing that converts sampled data of a finite time from the time domain to the frequency domain, regardless of the number of data points. Signal peak shifting may be performed as pre-processing for FFT. The calculation interval is shifted slightly to bring the center of the pulse-like signal to the center of the calculation interval. When peak shifting is performed, the waveform is discontinuously cut off at the ends of the calculation interval, which creates side lobes in the frequency components and minimizes errors in distance calculation.

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).

Then, for each of the reference signal and the measurement signal, the phase of the FFT-processed frequency components is calculated using the waveform data that is determined to be valid. The phase difference of frequency components of the same order between the reference signal and the measurement signal is found. The difference in phase difference is calculated between adjacent modes. The average value of the difference between adjacent modes corresponds to the phase difference between the envelope waveforms of the measurement signal and the reference signal. The delay time is found by dividing the phase by the repetition angular frequency of the modulated wave. The optical distance is found by multiplying the delay time by the speed of light (299,792,458 m/s). The distance to the measurement object is found by further dividing by the refractive index of air (processing step ST33).

In this optical comb distance measurement device 100, the signal processing unit 40 repeatedly performs each process from the reflected signal extraction process ST2 to the determination process ST4 to detect each reflected signal from multiple reflecting surfaces present in the depth direction of the measurement object 1, calculate the distance to the multiple reflecting surfaces, and output relative distance data to the multiple reflecting surfaces present in the depth direction of the measurement object 1 in the relative distance output process ST6.

In this optical comb distance measurement device 100, as shown in FIG. 9(A), the reflected light from an opaque measurement object 1 such as a metal or mirror does not contain multiple reflected light components, so the surface reflected light of the measurement object 1 can be detected to measure the absolute distance to the surface reflection surface of the measurement object 1. Also, as shown in FIG. 9(B), the surface reflected light and back reflection light components contained in the reflected light from a translucent measurement object 1 such as a glass plate or semiconductor substrate can be detected to measure the distance to the surface reflection surface and back reflection surface of the measurement object 1 separately.

When the output mode set in this optical comb distance measurement device 100 is the relative distance mode, the device outputs relative distance data to multiple reflecting surfaces present in the depth direction of the measurement object 1 based on the phase information calculated in the phase calculation process ST3.

Here, the optical comb distance measurement device 100 can in principle measure the distance to the measurement object 1 by emitting measurement light S1 and reference light S2, which are modulated periodically in intensity or phase, from the first optical comb generator 13 and the second optical comb generator 15 provided in the optical comb generating unit 10. Here, the fundamental frequency is f _m , the deviation of the fundamental frequency required for distance determination is Δf _m , and the frequency difference for generating optical comb interference is Δf. As shown in Table 1 below, the first modulation frequency F _m1 =f _m =25000 MHz, the second modulation frequency F _m2 =f+Δf _m =25010 MHz, the third modulation frequency F _m3 =f _m +Δf=25000.5 MHz, and the fourth modulation frequency F _m4 =f _m +Δf _m +Δf=25010.5 MHz _, By outputting the measurement light S1 and the reference light S2 of two types of modulation frequencies _FmA and _FmB , which are cyclically switched between FmA to _Fm4 and have mutually different modulation states, the signal processing unit 40 is able to calculate the absolute distance to the object to be measured 1.

Table 1 shows the modulation frequencies F _mA and F _mB and phase differences of two types of optical combs having mutually different modulation states in each of the selection states #1 to #4.

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

Here, in the optical comb distance measurement 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, and calculates the phase difference between the Nth mode of the reference signal and the measurement signal, where N is the mode number counted from the center frequency of the optical comb, to cancel out the optical phase difference during the optical comb generation and transmission process from the optical comb generator to the reference point. After that, the increment of the phase difference per order on the frequency axis is calculated to determine the phase difference of the signal pulse, thereby calculating the distance from the reference point to the measurement surface.

If the measured distance exceeds half the wavelength of the modulation frequency _fm , the distance of an integer multiple of the half wavelength becomes unclear due to the periodicity of the object light, and the distance cannot be determined uniquely. Therefore, measurements are taken four times using reference light pulses and measurement light pulses set to the four modulation frequencies shown in Table 1, and the same processing is performed in the signal processing unit 40, and the phase differences obtained are used to calculate a distance that exceeds the ambiguity distance equivalent to half the wavelength (L _a = c/2f _m c: speed of light).

In other words, the phase difference between the measurement signal and the reference signal obtained by performing distance measurement using the measurement light S1 and reference light S2 in which the four modulation frequencies shown in Table 1 are cyclically selected and set is _-2πfmT when setting #1 is selected and the modulation frequencies are _fm and _fm + _Δf , -2π( _fm + Δfm)T when setting #2 is selected and the modulation frequencies are fm + _Δfm and _fm + _Δfm + _Δf , -2π( _fm + _Δfm )T when setting # ₃ is selected and the modulation frequencies are _fm + _fm and _fm , and -2π( _fm + Δfm + _Δf )T when setting #4 is selected and the modulation frequencies are _fm + Δfm + _Δf and fm + Δfm.

When the distance (L _a =c/2f _m c: speed of light) is long, the phase difference (−2πf _m T) between the reference signal and the measurement signal is in the form of φ+2mπ, where m is an integer, and only the φ part can be calculated, but the integer value m is unknown.

On the other hand, the difference between the phase difference between the reference signal and the measurement signal in the selected state of setting #1, -2πf _m T, and the phase difference between the reference signal and the measurement signal in the selected state of setting #2, -2π(f _m + Δf _m )T, is 2πΔf _m T; and the difference between the phase difference between the reference signal and the measurement signal in the selected state of setting #3, -2π(f _m + Δf _m + Δf )T, and the phase difference between the reference signal and the measurement signal in the selected state of setting #4, is _{2πΔf m} T, and the phase is uniquely determined up to a distance equivalent to the wavelength of 1/Δf _m (if Δf _m = 10 MHz, then _La is 15 m).

Then, this phase is multiplied by f _m /Δf _m and compared with the phase difference in the selected state of setting #1 to determine the integer m.

Furthermore, 2πΔf is obtained from the difference between the phase difference −2πf _m T in the selected state of setting #1 in Table 1 and the phase difference −2π(f _m +Δf)T in the selected state of setting #3.

Here, if f _m =25 GHz, Δf=500 kHz, and Δf _m =10 MHz, then since Δf=500 kHz, distance measurement can be performed up to L _a =300 m.

Here, if f _m =2.5 GHz, Δf=500 kHz, and Δf _m =10 MHz, then since Δf=500 kHz, distance measurement can be performed up to L _a =300 m.

In this optical comb distance measurement device 100, absolute distance measurement is performed using the measurement signal and reference signal obtained by performing distance measurement using the measurement light S1 and reference light S2, which are cyclically selected and set to the four modulation frequencies shown in Table 1: setting #1, setting #2, setting #3, and setting #4.

When the output mode set in this optical comb distance measurement device 100 is the absolute distance mode, in absolute distance calculation processing step ST7, the signal processing unit 40 calculates the absolute distance to multiple reflecting surfaces present in the depth direction of the measurement object 1 according to the procedure shown in the flowchart of Figure 11.

That is, in the absolute distance calculation process, data showing each signal waveform is selected for the measurement signal and reference signal obtained by performing distance measurement using the measurement light S1 and reference light S2 in the above settings #1, #2, #3, and #4 (processing step ST41), the selected waveform data is sorted to determine which of the above settings #1, #2, #3, and #4 the signal waveform represents (processing step ST42), the order of the sorted waveform data is calculated (processing step ST43), and a process is performed to determine whether the waveform data is valid or invalid (processing step ST44). Using the valid waveform data, the phase difference of the frequency components of the same order between the reference signal and the measurement signal is calculated and averaged (processing step ST45), and the absolute distance to the surface reflection surface of the measurement object 1 is calculated from the phase difference between the measurement signal and the reference signal obtained by performing distance measurement using the measurement light S1 and reference light S2 in the above settings #1, #2, #3, and #4.

Then, in this optical comb distance measurement device 100, in the absolute distance output step ST8, absolute distance data indicating the absolute distance to the surface reflection surface of the measurement object 1 obtained as a result of the calculation in the absolute distance calculation processing step ST7 is output.

Here, the absolute distance calculation process calculates the absolute distance to the surface reflection surface of the object 1 from the phase difference between the measurement signal and the reference signal obtained by performing distance measurement using the measurement light S1 and reference light S2, which are cyclically selected and set with the four modulation frequencies shown in Table 1. By performing signal processing assuming that one measurement point is reflected from one reflection surface, the order of an integer multiple of a half wavelength (about 6 mm for 25 GHz) and distance information are obtained from the phase change due to the cyclic selection and setting of the modulation frequency, and the absolute distance to the measurement surface is calculated based on the surface reflection light reflected by the object 1. By using a similar signal processing, the absolute distance to the back reflection surface can be calculated based on the back reflection light. Even if there are multiple reflection surfaces in the depth direction of the object 1, that is, if there are many reflection points, in principle, the phases of the multiple reflection signals can be determined in a separated state by repeating the process from extraction of the reflection signal to phase calculation at each frequency setting.

In other words, the signal processing unit 40 in this optical comb distance measurement device 100 can independently obtain the order of each of the multiple reflected signals and simultaneously calculate the absolute distances of multiple points by using the absolute distance output process ST8.

In addition, the optical comb distance measurement device 100 functions as a shape measurement device by being equipped with an optical scanning means that scans the measurement object 1 with the measurement light irradiated onto the measurement object 1, and can detect the surface shape and thickness of a semi-transparent substrate separately.

1 Light source unit, 11 Laser light source, 12 Beam splitter, 13, 15 Optical comb generator, 14 Frequency shifter, 22 Reference interferometer, 23 Measurement interferometer, 31 Reference interferometer, 32 Measurement interferometer, 40 Signal processing unit, 100 Optical comb distance measurement device

Claims (4)

a light source unit for emitting a measurement light and a reference light, each of which has a periodically modulated intensity or phase and has a different modulation period;
a reference interferometer for superimposing the measurement light and the reference light emitted from the light source unit;
a measurement interferometer that irradiates a measurement object with the measurement light and superimposes the measurement light reflected by the measurement object and the reference light;
a reference photodetector that receives interference light obtained by superimposing the measurement light and the reference light by the reference interferometer;
a measurement light detector that receives interference light obtained by superimposing the measurement light and the reference light by the measurement interferometer;
a signal processing unit that calculates distances to the plurality of reflecting surfaces from delay times of reflected signals by the plurality of reflecting surfaces present in the depth direction of the measurement object, the delay times being included in a measurement interference signal obtained as a detection output of the interference light by the measurement light detector, relative to a reference interference signal obtained as a detection output of the interference light by the reference light detector;
The optical comb distance measurement device is characterized in that the signal processing unit performs a reflected signal extraction process to extract a signal portion due to reflection from the measurement interference signal as a reflected signal, and a phase calculation process to perform a fast Fourier transform on each extracted reflected signal and the reference interference signal to determine the phase difference of frequency components of the same order between each reflected signal and the reference interference signal. 2. The optical comb distance measurement device according to claim 1, wherein the signal processing unit extracts each reflected signal by detecting a peak of a correlation signal envelope obtained by calculating a cross-correlation function between the reference interference signal and the measurement interference signal in the reflected signal extraction process. The optical comb distance measurement device according to claim 1, characterized in that, in the reflected signal extraction process, the signal processing unit moves a time window function for extracting a predetermined reflected signal waveform, extracts the reflected signal waveform from the measured interference signal waveform, and extracts each reflected signal by detecting peaks of the extracted reflected signal waveform. 2. The optical comb distance measurement device according to claim 1, wherein, in the reflected signal extraction process, the signal processing unit detects a rough position of the reflected signal by detecting a peak of a correlation signal envelope obtained by calculating a cross-correlation function between the reference interference signal and the measurement interference signal, extracts a reflected signal waveform from the measured interference signal waveform near the detected position using a time window function for extracting a predetermined reflected signal waveform, and extracts each reflected signal by detecting a peak of the extracted reflected signal waveform.