JP2014202716A - Distance measuring device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a distance measuring device capable of highly accurate measurement.SOLUTION: A distance measuring device includes: a laser beam generator which generates a laser beam whose frequency changes into a triangular shape with time; and an optical distributor which distributes the laser beam. The distributed laser beam is supplied to a measurement interference device and an auxiliary interference device. The measurement interference device outputs a beat signal having frequency corresponding to a distance to an object to be measured. On the other hand, the auxiliary interference device outputs a sampling clock for sampling the beat signal by receiving the laser beam. Further, the distance measuring device includes a photodetector which receives the laser beam and outputs a monitor signal, and timing for sampling the beat signal is determined by the monitor signal. An average value of the beat signal in 1 cycle of the laser beam that changes into the triangular shape is obtained by a digital signal obtained by sampling, and a distance is calculated based on the obtained average value.

Description

  The present invention relates to a distance measuring device, and more particularly to a distance measuring device that measures a distance to an object to be measured using a frequency-modulated laser beam.

  When the distance to the measurement object is obtained along the surface of the measurement object, the shape of the measurement object can be measured. For example, there is an FMCW (Frequency Modulated Continuous Waves: hereinafter referred to as FMCW) system as a measurement system used for measuring the shape of medium-sized or large-sized processed parts such as turbines, vehicles, and aircraft. This FMCW system has a relatively simple configuration for realizing it, and can measure the distance to the object to be measured with relatively high accuracy.

  In this FMCW system, a laser beam that is a continuous wave signal (Continuous Wave: hereinafter also referred to as CW) that is frequency-modulated (Frequency Modulation: hereinafter also referred to as FM) is used. This laser light is radiated into the atmosphere as a transmission wave, reflected by the object to be measured existing in the atmosphere, and returned as a reception wave. In order to measure the distance to the device under test, a beat signal having a frequency difference between the received received wave and the transmitted transmitted wave is generated, and the frequency spectrum of the generated beat signal is obtained. The distance to the object to be measured is calculated.

  In order to measure the distance to the object to be measured with high accuracy using the FMCW method, it is important to solve the problem of Doppler shift that occurs when the object to be measured or the measuring apparatus vibrates. In addition, a laser diode (hereinafter also referred to as LD) is used to generate laser light. However, there is a problem that an error occurs in frequency modulation due to nonlinearity in optical frequency modulation of the LD. It is also important to solve this problem.

  As a technique for solving the problem of Doppler shift, for example, there is a technique disclosed in Patent Document 1. Further, as a technique for solving a problem related to an error in frequency modulation caused by nonlinearity of an LD, there are techniques disclosed in Patent Document 2 and Non-Patent Document 1, for example.

JP 09-257415 A JP 2000-1111312 A

K. Iiyama, M .; Yasuda, and S.A. Takamiya, “Extended-Range High-Resolution FMCW Reflectometric by Means of Electrically-Frequency Sampling Signals”. Electron. , Vol. E89-C, No. 6, pp. 823-829, June 2006.

  Regarding the problem of Doppler shift caused by the vibration of the object to be measured or the measuring apparatus, in Patent Document 1, the frequency of the beat signal may be Doppler shifted in the reverse direction between the rising portion and the falling portion in the triangular wave modulation due to the vibration. It is shown. By utilizing this, Patent Document 1 shows that an error caused by a Doppler shift is canceled by preparing two systems of optical paths in which triangular wave modulation is shifted by a half cycle. In this case, there is a concern that the cost may increase due to the provision of two optical paths.

  The problem regarding the error in the frequency modulation caused by the nonlinearity of the LD is that, even if the drive current that drives the LD is linearly swept, the optical frequency is affected by the heat generated by the LD solid and / or its aging. Occurs by not being linear over time. That is, even if the LD drive current is linearly swept along the time, the optical frequency modulation becomes nonlinear with respect to time.

  To deal with this problem, Patent Document 2 discloses that a separate reference interferometer is provided to determine the instantaneous beat frequency and generate a correction signal waveform. However, since an optical path difference occurs between the interferometer used for actual measurement and the reference interferometer provided as a reference, there is a concern that a slight error may occur.

  In Non-Patent Document 1, an auxiliary interferometer is provided in FIG. 3 in addition to the measurement interferometer, the output of the auxiliary interferometer is electrically multiplied and used as a sampling clock, and the optical frequency modulation of the LD is performed. A measuring device is shown which reduces errors due to non-linearity. Even in this case, since an optical path difference is generated between the measurement interferometer and the auxiliary interferometer, there is a concern that an error may occur although it is very small.

  An object of the present invention is to provide a distance measuring device capable of highly accurate measurement.

  The above and other objects and novel features of the present invention will be apparent from the description of this specification and the accompanying drawings.

  Of the inventions disclosed in the present application, the outline of typical ones will be briefly described as follows.

  That is, the distance measuring device has a laser light generator that generates laser light whose frequency changes with time, and an optical distributor that distributes the laser light. The laser light distributed by the optical distributor is supplied to the measurement interferometer and the auxiliary interferometer. The measurement interferometer receives the laser beam and outputs a beat signal having a frequency corresponding to the distance to the object to be measured. On the other hand, the auxiliary interferor outputs a sampling clock for sampling the beat signal by receiving the laser beam. The distance measuring device includes a photodetector that receives the laser light and outputs a monitor signal, and the timing for sampling the beat signal is determined by the monitor signal.

  Since the timing for sampling the beat signal is determined by the monitor signal according to the laser light, the sampling timing can be determined in accordance with the characteristics of the laser light generator that generates the laser light. Therefore, a beat signal can be captured (sampling) in a region with little error in accordance with the characteristics of the laser light generator, and a distance measuring device capable of highly accurate measurement can be provided.

  The laser light generator is driven so that its frequency is modulated by a triangular wave-like modulation signal. The sampled value during the period (part) in which the frequency of the laser beam is rising due to the triangular wave-like modulation signal and the sampled period during which the frequency of the laser beam is being lowered due to the triangular wave-like modulation signal An average value with the value is obtained, and the distance is calculated from the average value. That is, the average value (average frequency) of the frequency of the beat signal during the period when the frequency of the laser light is increasing and the frequency of the beat signal during the period when it is decreasing is obtained. The distance is calculated from the obtained average value. As a result, it is possible to reduce errors caused by the Doppler shift while suppressing an increase in cost.

  In one embodiment disclosed in this specification, sampling is performed in a period in which the frequency of the laser beam is increasing, and in an interval in which the instantaneous beat frequency of the value to be sampled and the frequency of the laser beam are decreasing. The laser light generator is driven so that each of the certain instantaneous beat frequencies approaches a constant value. That is, the laser light generator is driven so that the slope of the measured beat frequency of the sampled value approaches zero. As a result, the laser light generator is driven so as to cancel out the nonlinear characteristics of the laser light generator due to temperature and / or secular change, and it is possible to provide a distance measuring device with less errors. .

  Of the inventions disclosed in the present application, effects obtained by typical ones will be briefly described as follows.

  A distance measuring device capable of highly accurate measurement can be provided.

It is a block diagram which shows the structure of the distance measuring device concerning one embodiment of this invention. (A) And (b) is a wave form diagram which shows the waveform of a FMCW system. (A) And (b) is a wave form diagram which shows the waveform of a FMCW system. (A) And (b) is a wave form diagram which shows the waveform of a FMCW system. (A)-(d) is a wave form diagram for demonstrating the distance measuring device concerning this invention. (A)-(e) is a wave form diagram for demonstrating the distance measuring device concerning this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that components having the same function are denoted by the same reference symbols throughout the drawings for describing the embodiment, and the repetitive description thereof will be omitted.

  FIG. 1 is a block diagram showing a configuration of a distance measuring device according to an embodiment of the present invention. The distance measuring apparatus shown in the figure can measure the distance to the object to be measured 100 along the surface of the object to be measured. Therefore, the distance measuring device shown in FIG. 1 corresponds to a three-dimensional shape measuring device.

  The distance measuring device of the figure is roughly classified into a laser generator that generates laser light, an optical distributor that distributes the generated laser light, a measurement interferor and an auxiliary interferor that receive the distributed laser light, and a measurement. A converter and a calculator for receiving a beat signal from the interferometer are provided.

  In the figure, reference numeral 3 denotes a laser light generator for generating laser light. In the present embodiment, the laser light generator 3 has a laser diode 23 and a photo detector 95. The laser diode 23 receives the drive current output from the adder 51 and generates laser light having a frequency according to the drive current. The photo detector 95 receives the laser beam generated by the laser diode 23 and forms a monitor signal according to the frequency of the laser beam. The monitor signal is not particularly limited, but in this embodiment, the monitor signal is an analog signal whose voltage value changes according to the frequency of the received laser beam.

  The adder 51 receives the predetermined bias current 2 and the current from the waveform generator 1 that generates a waveform having an arbitrary shape, and adds the waveform current output from the waveform generator 1 to the predetermined bias current 2. As a drive current, the laser diode 23 is supplied. In this embodiment, the waveform generator 1 generates a current waveform whose current value changes in a triangular shape with time. As will be described later, the waveform generator 1 receives a control signal from the computing unit 15, and the shape of the generated triangular current waveform is changed by the control by the control signal. The laser light emitted by the laser light generator 3 is applied to a fiber coupler 41 that forms an optical distributor. The fiber coupler 41 distributes the received laser beam to at least two laser beams. One laser beam distributed by the fiber coupler 41 is supplied to the measurement interferometer 20, and the other laser beam distributed is supplied to the auxiliary interferometer 21.

  The measurement interferometer 20 includes fiber couplers 42 and 43, a circulator 5, and a collimator 6. The measurement interferometer 20 further includes a focus lens 7 and a galvanometer scanner 8. Although not particularly limited, the focus lens 7 and the galvanometer scanner 8 are installed in the atmosphere together with the object 100 to be measured.

  Laser light (measurement light) output from the measurement interferometer 20 is received by the photo detectors 91 and 92 and converted into electric signals by the photo detectors 91 and 92. The output of the measurement interferometer 20 converted into an electric signal is added by an adder 52. The analog signal obtained by the addition is amplified by the amplifier 10 and supplied to the converter 14 as a measurement beat signal. Since the measurement beat signal is output from the amplifier 10, the measurement interferometer 20, the photo detectors 91 and 92, the adder 52, and the amplifier 10 can be regarded as constituting a measurement interferometer.

  In this embodiment, the measurement interferometer 20 is provided with a galvanometer scanner 8. The irradiation angle of the laser light is controlled by the galvanometer scanner 8 so as to scan the surface of the object 100 to be measured. As a result, the measurement position on the surface of the measurement object 100 is changed, the distance at each measurement position is measured, and the three-dimensional shape of the measurement object 100 can be measured. The galvanometer scanner 8 functions as a scanner (laser scanner) and is not shown in the figure, but information on the angle at which the laser beam is irradiated during scanning (angle information) will be described later. This is supplied to the calculator 15.

Auxiliary interferometer 21 is provided with an optical fiber L _R for providing the fiber couplers 44 and 45, and the optical path difference. The laser light output from the auxiliary interferometer 21 is received by the photo detectors 93 and 94 and converted into electric signals by the photo detectors 93 and 94. The output of the auxiliary interferometer 21 converted into the electrical signal is added by the adder 53. The analog signal (addition result) output from the adder 53 is supplied to the clock multiplier 22. The clock multiplier 22 includes a comparator 11, a 1 / n counter 13, and a phase synchronization circuit PLL12.

  The analog signal supplied to the clock multiplier 22 is supplied to one input terminal of the comparator 11. A predetermined voltage (ground voltage in the embodiment) is applied to the other input terminal of the comparator 11. Thereby, the voltage of the analog signal is compared using the predetermined voltage as the reference voltage, and converted from the analog signal to the clock signal. The converted clock signal is supplied to the phase synchronization circuit PLL12. The phase synchronization circuit PLL12 compares the phase difference between the clock signal output from the 1 / n counter (1 / n counter) 13 and the clock signal from the comparator 11, and the frequency at which the phases of both clocks coincide with each other. The clock signal is formed. The output clock signal from the phase synchronization circuit PLL12 is divided by the 1 / n counter 13 and supplied again to the phase synchronization circuit PLL12.

  In this embodiment, the frequency corresponding to the frequency difference (beat) between the two laser beams output from the auxiliary interferometer 21 is multiplied according to the frequency division ratio set in the 1 / n counter 13. The clock signal obtained by multiplication is supplied to the converter 14 as a sampling clock.

  In this embodiment, the sampling clock is formed by the auxiliary interferometer 21, the photo detectors 93 and 94, the adder 53 and the clock multiplier 22. From the viewpoint of forming a sampling clock from the laser beam by the auxiliary interferor, the auxiliary interferor is composed of the auxiliary interferometer 21, the photo detectors 93 and 94, the adder 53 and the clock multiplier 22. Can be considered.

  In the present embodiment, the converter 14 is an analog / digital converter that samples a measurement beat signal, which is an analog signal, based on a sampling clock output from the auxiliary interferometer and converts it into a digital signal. In this embodiment, the sampling start and end timings are determined by the monitor signal output from the laser light generator 3 described above. The measurement beat signal converted into a digital signal by the converter 14 is supplied to the calculator 15. As will be described later, the computing unit 15 calculates and outputs a distance to the device under test 100 based on the measurement beat signal. When performing three-dimensional shape measurement, the angle information related to the angle of the laser beam is received from the galvanometer scanner 8, and the data representing the three-dimensional shape is based on the angle of the laser beam and the distance at the angle. Is calculated. Further, in this embodiment, the calculator 15 controls the waveform generator 1 based on the measurement beat signal. By this control, the waveform generator 1 changes the shape of the generated triangular wave.

  Next, the operation of the distance measuring apparatus shown in FIG. 1 will be described.

  The waveform generator 1 generates a triangular waveform. The adder 51 adds the waveform (triangular wave) generated by the waveform generator 1 to the direct current (predetermined bias current) 2 for driving the laser diode 23 in the laser light generator 3. A drive current for driving is formed. The laser diode 23 is driven by frequency modulation (triangular wave modulation) with a triangular wave drive current, and the laser diode 23 generates laser light whose frequency (optical frequency) changes in a triangular wave shape with time. . The triangular-wave modulated laser light is incident on the fiber coupler 41 through the optical fiber. The fiber coupler 41 functions as an optical distributor and splits the incident laser light into at least two paths. One of the two paths is a path to the measurement interferometer 20, and the other path is a path of the auxiliary interferometer 21.

  The laser light input to the measurement interferometer 20 enters the fiber coupler 42 and is split. That is, the fiber coupler 42 splits the laser light into measurement light and reference light. The measurement light is incident on the collimator 6 from the circulator 5 and is emitted from the collimator 6 as parallel light into the atmosphere. The emitted measurement light is collected by the focus lens 7, scanned in the X coordinate / Y coordinate direction by the galvanometer scanner 8, and irradiated to the measurement position of the object 100 to be measured.

  The measurement light is scattered by the device under test 100, and the scattered light is transmitted through the galvanometer scanner 8, the focus lens 7, and the collimator 6 in this order, and reaches the circulator 5. Since the scattered light is used for measurement, the scattered light is also referred to as measurement light in this specification. The circulator 5 irradiates the fiber coupler 43 with incident scattered light (measurement light). Reference light is incident on the fiber coupler 43 from the fiber coupler 42, and the fiber coupler 43 causes the reference light and the measurement light to interfere with each other. Output light from the fiber coupler 43 enters the photodetectors 91 and 92 and is converted into an electric signal. The converted electrical signal is subtracted by the adder 52 to obtain an electrical signal corresponding to the frequency difference between the reference light and the measurement light. An electric signal corresponding to this frequency difference is amplified by the amplifier 10 and supplied to an analog / digital (hereinafter also referred to as AD) converter 14 as a measurement beat signal.

The laser light obtained by spectroscopy by the fiber coupler 41 and incident on the auxiliary interferometer 21 is incident on the fiber coupler 44 in the auxiliary interferometer 21. The fiber coupler 44 separates the incident laser light and makes each incident on a different optical fiber. In order to generate an optical path difference between the different optical fibers, the physical length of one optical fiber is increased with respect to the other optical fiber. In this embodiment, (shown as L _R is the same figure) in order to generate an optical path difference of L _R, it is a long length of the upper optical fiber in FIG. By transmitting these optical fibers, the two laser beams in such having an optical path difference L _R is incident on the fiber coupler 45. In this fiber coupler 45, two laser beams having an optical path difference are caused to interfere with each other. The laser beams interfered with each other are incident on the photodetectors 93 and 94 and converted into electric signals in the photodetectors 93 and 94.

The electrical signals from the photo detectors 93 and 94 are subtracted in the adder 53. This by the subtraction, an analog signal having a value corresponding to the frequency (optical frequency) difference between the two laser beams generated by the optical path difference L _R is formed. That is, an analog signal having a value corresponding to the beat frequency caused by the optical path difference L _R (auxiliary beat signal) is formed. The analog signal output from the adder 53 is supplied to the clock multiplier 22. In the clock multiplier 22, as described above, the comparator 11 converts the analog signal into a digital signal (clock signal). By setting the frequency division ratio of the 1 / n counter 13 to a value corresponding to a predetermined multiplication factor M, the clock signal (digital) corresponding to the auxiliary beat signal is obtained by the functions of the phase synchronization circuit PLL12 and the 1 / n counter 13. As for (signal), a clock signal (clock signal whose frequency has been multiplied) according to the set multiplication factor M is formed. The formed clock signal is supplied to the converter 14 as a sampling clock.

  In the converter 14, the measurement beat signal output from the measurement interferometer is sampled by the sampling clock generated by the auxiliary interferometer. In addition, the monitor signal formed by the photodetector 95 is supplied to the converter 14 as an analog trigger signal. The photodetector 95 is not particularly limited, but is provided in the laser light generator 3 and functions as a photodetector. That is, the photo detector 95 is arranged close to receive the laser beam from the laser diode 23 that generates the laser beam, converts the received laser beam into an analog electric signal, and outputs it as a monitor signal. To do. Therefore, the measurement light incident on the measurement interferometer 20 and the monitor signal are synchronized, and the measurement beat signal and the triangular wave of the monitor signal are also synchronized. Of course, the triangular wave of the monitor signal is also synchronized with the triangular wave generated by the waveform generator 1.

  Accordingly, by examining the monitor signal, it is possible to determine at which position of the triangular wave (triangular wave generated by the waveform generator 1) the measured beat signal at that time. If further described, it is possible to grasp from the monitor signal which position of the triangular wave the measurement beat signal in an arbitrary time domain is. Furthermore, even when the characteristics of the laser diode 23 change due to temperature and / or aging, the monitor signal changes by following the change in characteristics because it is formed by receiving laser light. Therefore, even if the characteristics of the LD change, it is possible to associate the measured beat signal with the position of the triangular wave at that time.

  In this embodiment, the timing at which the converter 14 samples with the sampling clock is determined by the monitor signal. Sampling of the measurement beat signal by the sampling clock is performed from when the monitor signal exceeds a predetermined value. When the monitor signal falls below the predetermined value, sampling is stopped. Thereby, it becomes possible to sample the measurement beat signal corresponding to the appropriate position of the triangular wave. In this embodiment, as will be described later with reference to FIGS. 5 and 6, the measurement beat signal is sampled in the stable region in the rising region (rising portion) and the falling region (falling portion) of the triangular wave.

  The measurement beat signal acquired by sampling is AD converted in the converter 14 and supplied to the calculator 15. The computing unit 15 performs fast Fourier transform (hereinafter sometimes referred to as FFT) on the digital signal corresponding to the supplied measurement beat signal, performs frequency analysis, and calculates a frequency spectrum.

  Next, the operation principle of measurement will be described below.

  2A and 2B show ideal waveforms of the FMCW system. In FIG. 2A, the horizontal axis indicates time, and the vertical axis indicates the frequency (optical frequency) of the laser beam. In the drawing, a waveform (measurement light) indicated by a broken line indicates reflected light (measurement light) obtained by irradiating the object to be measured 100 with laser light and reflecting it, and also indicated by a solid line. The waveform (reference light) indicates the reference light described in FIG. As described in FIG. 1, the laser light is frequency-modulated by a triangular drive current formed by the waveform generator 1. Therefore, the frequency of the measurement light and the reference light changes in a triangular shape over time.

The distance to the device under test 100 is represented by the optical path length difference between the reference light and the measurement light. That is, an optical path difference occurs between the measurement light and the reference light according to the distance to the object 100 to be measured. As a result, a time difference occurs until the reference light and the measurement light reach the same frequency. In the example of FIG. 2A, the triangular waveform of the measurement light is delayed by Δt to reach the same frequency as the triangular waveform of the reference light, depending on the optical path length difference. Therefore, when viewed at the same time, a frequency difference of fb occurs between the measurement light and the reference light. Therefore, when the reference wave and the measurement wave are caused to interfere, a beat signal having this frequency difference is generated. Here, when the repetition frequency of the triangular wave modulation is f _m and the modulation frequency width is ΔF, the frequency (beat frequency) fb of the beat signal is expressed by Expression (1).

  At this time, if the triangular wave modulation is linearly modulated, as shown in FIG. 2 (b), the frequency of the beat signal is equal to the frequency of the triangular wave except for the time around the upper and lower vertices. In the stable region, the beat frequency fb has a constant value. In practice, however, the frequency modulation (optical frequency modulation) of the laser diode does not linearize the frequency of the laser that emits light with respect to changes in drive current due to its own heat and / or aging. For example, the modulation of the frequency of the generated laser reacts with delay with respect to the modulation of the drive current, as shown in FIG. That is, it has nonlinearity.

  3A and 3B, as in FIGS. 2A and 2B, the horizontal axis represents time, and the vertical axis represents frequency. However, FIGS. 3A and 3B show examples of actual waveforms in the FMCW system. Since the frequency of the laser beam becomes non-linear with time, the frequency difference between the measurement beam (dashed waveform) and the reference beam (solid waveform) as shown in FIG. (Beat frequency) increases or decreases with time. Therefore, as shown in FIG. 3B, the beat signal frequency (beat frequency) does not become a constant value even in the stable region except for the time around the triangular wave turning back at the upper and lower vertices. It will fluctuate.

In the distance measuring device shown in FIG. 1, an auxiliary interferometer 21 and the like are provided in order to reduce errors due to nonlinearity in optical frequency modulation of the laser diode. Laser light from the same laser diode 23 as the measurement interferometer 20 is incident on the auxiliary interferometer 21. In the auxiliary interferometer comprising an auxiliary interferometer 21, as described with reference to FIG. 1, the auxiliary beat signal corresponding to the optical path difference L _R by the adder 53 is formed. Since the same laser diode 23 is used, the auxiliary beat signal also changes in accordance with the characteristics of the laser diode 23, similarly to the beat signal (measurement beat signal) obtained from the measurement light and the reference light.

  That is, the influence of the characteristic change of the laser diode 23 hardly appears in the relative relationship between the measurement beat signal and the auxiliary beat signal. In order to utilize this relative relationship, in the distance measuring apparatus shown in FIG. 1, a sampling clock is formed from the auxiliary beat signal, and the measurement beat signal is sampled. That is, in the distance measuring device shown in FIG. 1, the sampling clock has a nonlinear characteristic that matches the nonlinear characteristic of the laser diode 23. By sampling with this sampling clock, an error caused by nonlinearity in the optical frequency modulation of the laser diode 23 is reduced.

By the way, the sampling clock described above is set so as to be at least twice the frequency of the measurement beat signal generated by the measurement interferometer 20 from the sampling theorem. This means that in the auxiliary interferometer 21, is meant to set the optical path difference L _R to be set, at least twice the optical path difference L _S occurring in the measurement interferometer 20. However, in the auxiliary interferometer 21, if the optical path difference _LR is set too long, it is assumed that the apparatus becomes unstable due to changes in the surrounding environment when using the distance measuring apparatus. On the other hand, it is conceivable to isolate the auxiliary interferometer 21 from surrounding environmental fluctuations. In this case, however, there is a concern that the cost for the configuration for the isolation may increase.

In order to solve this, a clock multiplier 22 is provided in the distance measuring apparatus shown in FIG. The auxiliary beat signal is electrically multiplied by the clock multiplier 22. Thus, it is possible to prevent the optical path difference L _R to be set in the auxiliary interferometer 21 is prolonged.

When c is the speed of light and nf is the refractive index of the optical fiber, the delay time Δt _R of the auxiliary interferometer 21 (corresponding to Δt in FIGS. 2 and 3) and the delay time Δt _S of the measurement interferometer 20 (FIGS. 2 and 3). (Corresponding to Δt in FIG. 2) is expressed by the equations (2) and (3).

If the multiplication factor of the clock multiplier 22 is M as described with reference to FIG. 1, the frequency fb _R of the beat signal (auxiliary beat signal) of the auxiliary interferometer 21 and the frequency fb _S of the measurement beat signal of the measurement interferometer 20. Is represented by Formula (4) and Formula (5).

Due to the limitation of the sampling theorem, when fb _R = 2 × fb _S , the maximum measurement distance L _S max is expressed by Equation (6).

When frequency analysis is performed by Fourier transform on the measurement beat signal sampled by the converter 14 that performs AD conversion, if the number of extracted data in the time domain is N, the effective number of data in the Fourier-transformed data is N / 2, which corresponds to the maximum measurement distance L _S max. If the peak position of the Fourier-transformed frequency spectrum is P, the optical path difference L _S of the measurement interferometer 20 can be calculated from the relational expression of Expression (7) by Expression (8).

The optical path difference L _S in the above equation (8) corresponds to the distance to the DUT 100 in FIG. In the embodiment shown in FIG. 1, the focus lens 7 and the galvanometer scanner 8 are installed in the atmosphere. Therefore, since there is a difference between the optical path in the optical fiber and the optical path in the atmosphere, by multiplying the refractive index nf of the optical fiber by the equation (8), the distance L to the object 100 to be measured is the equation (9). Can be calculated.

  In Expression (9), since the parameters excluding the peak position P in the frequency spectrum of the measurement beat signal are known, the distance L to the DUT 100 can be calculated by calculating the peak position P.

In the distance measuring apparatus shown in FIG. 1, an auxiliary interferometer 21 is provided to reduce errors caused by the nonlinearity of the optical frequency modulation characteristic of the laser diode 23. However, between the optical path difference L _S in the optical path difference L _R and the measurement interferometer 20 in the auxiliary interferometer 21, there is a difference in length. Due to the difference between the optical path differences, a time delay occurs between the measurement beat signal and the auxiliary beat signal. 4A and 4B show beat signal waveforms when the auxiliary interferometer 21 is provided. In FIG. 4A, the horizontal axis indicates time and the vertical axis indicates frequency. Moreover, in the figure, the solid line has shown the waveform of the measurement beat signal, and the broken line has shown the waveform of the auxiliary beat signal. Each of the auxiliary beat signal and the measured beat signal shows similar frequency changes, but due to the difference between the optical path differences described above, in this example, the auxiliary beat signal is delayed with respect to the measured beat signal. ing.

  In FIG. 4B, the measurement beat signal is sampled by the sampling clock formed by the auxiliary beat signal, and the change in the frequency of the sampled measurement beat signal (hereinafter also referred to as the instantaneous beat frequency) is shown with respect to time. The plotted waveform is shown. In the figure, a dashed line indicates a change in instantaneous beat frequency in a measurement beat signal when the optical frequency modulation characteristic of the laser diode 23 is linear. That is, an ideal frequency change of the measurement beat signal is indicated by a one-dot broken line. On the other hand, the waveform shown by the solid line in the figure is a change in the instantaneous beat frequency of the measured beat signal when the auxiliary interferometer 21 is provided and there is a difference between the optical path differences ((a) in FIG. 4). It is shown.

  When the laser beam is frequency-modulated with a triangular waveform, the frequency of the beat signal changes near the upper (upper) and lower (lower) vertices of the triangular waveform, as described in FIGS. . In contrast, in the time domain in which the triangular waveform is rising and falling, the frequency of the beat signal is stable, and in this stable time domain, the frequency of the measurement beat signal (instantaneous It is desirable to measure the beat frequency. In FIG. 4B, the above stable time region is indicated by a broken line as the stable region.

  In FIG. 4B, when the frequency change of the ideal measurement beat signal is compared with the frequency change of the measurement beat signal when the auxiliary interferometer 21 is provided, there is a difference between them even in the stable region. Yes. That is, in the stable region that exists during the period when the triangular waveform is rising, the beat frequency of the measurement beat signal when the auxiliary interferometer 21 is provided is higher than the ideal measurement beat signal. On the contrary, in the stable region existing during the period in which the triangular waveform is falling, the beat frequency of the measurement beat signal when the auxiliary interferometer 21 is provided is lower than the ideal measurement beat signal. . In any stable region, the measured beat frequency when the auxiliary interferometer 21 is provided tends to decrease. In this way, even if the auxiliary interferometer 21 is provided, the frequency of the measurement beat signal is different from the ideal measurement beat signal frequency, and there is a concern that an error may occur in the distance measurement. .

  In the present embodiment, the driving current for driving the laser diode 23 is calibrated in order to reduce the frequency difference between the measurement beat signal and the ideal measurement beat signal. In order to perform calibration, the instantaneous beat frequency in the stable region during the rising period of the triangular wave and the slope of the instantaneous beat frequency in the stable region during the falling period of the triangular wave are obtained. The drive current described above is calibrated (changed) so that the obtained slope approaches 0.

  The calculation for obtaining the slope of the instantaneous beat frequency and the calculation for making the above-mentioned slope approach 0 are executed in the calculator 15, and the calculator 15 applies the waveform generator 1 to the waveform generator 1 based on the result of the calculation. To instruct the waveform deformation (calibration). Thereby, the waveform of the triangular wave is changed, and the drive current of the laser diode 23 is calibrated.

  Next, more specific calibration will be described with reference to FIGS. 5 and 6. FIG. FIG. 5 shows a waveform before the calibration is performed, and FIG. 6 shows a waveform after the calibration is performed. 5 (a) and 6 (a) show the waveforms of the drive currents that drive the laser diode 23, respectively, in which the horizontal axis indicates time and the vertical axis indicates the drive current value. Is shown. FIG. 5B and FIG. 6B respectively show the optical frequency of the laser light emitted by the laser diode 23 and used as measurement light, and the waveform of the optical frequency of the laser light used as reference light. Yes. In each of FIG. 5B and FIG. 6B, the horizontal axis indicates time and the vertical axis indicates optical frequency.

  5 (c) and 6 (c) show waveforms obtained by plotting the instantaneous beat frequency obtained by frequency analysis of the measured beat signal obtained by sampling with respect to the passage of time by the calculator 15. FIG. In each figure, the horizontal axis indicates time, and the vertical axis indicates frequency. 5 (d) and 6 (d) show the waveform of the monitor signal formed by the photodetector 95. In each figure, the horizontal axis indicates time, and the vertical axis indicates the monitor voltage. Show.

  FIG. 6E is a waveform diagram of a frequency spectrum obtained by performing frequency analysis on the measurement beat signal obtained by sampling by the calculator 15. In the figure, the horizontal axis represents frequency and the vertical axis represents amplitude.

  First, the description before calculating the distance L to the device under test 100 (before calibration) will be given using FIG. 1 and FIGS. 5A to 5C, and then when calculating the distance L (after calibration). ) Will be described with reference to FIGS. 1 and 6A to 6C.

A triangular wave is generated by the waveform generator 1. The generated triangular waveform current waveform is added to the bias current 2. The current obtained by this addition becomes the drive current for the laser diode 23. The waveform of this drive current is shown in FIG. Frequency of the triangular wave generated by the waveform generator 1 is f _m, and one cycle of triangular wave is 1 / f _m. The laser diode 23 emits light by this drive current. The optical frequency of the emitted laser light changes according to the drive current. That is, the optical frequency of the emitted laser light changes in a triangular shape. For example, in the rising region of the triangular wave generated by the waveform generator 1, the optical frequency of the laser light is sequentially increased, and in the falling region after reaching the apex of the triangular wave, the optical frequency of the laser light is sequentially decreased.

  The laser light emitted by the laser diode 23 passes through the path described with reference to FIG. The laser beam reflected by the DUT 100 is indicated by a broken line as a measurement beam in FIG. 5B. In FIG. 5B, the frequency waveform of the reference light dispersed by the fiber coupler 42 is shown by a solid line. Since the measurement light and the reference light are both based on the laser light emitted by the laser diode 23, the optical frequency changes in a triangular shape as shown in FIG. 5B. However, due to nonlinearity in the optical frequency modulation of the laser diode 23, the optical frequency changes in a shape different from the shape of the drive current that changes in a triangular shape. The symbols used in 5 (b) and FIG. 6 (b) described later are the same as the symbols used in FIG. 2 (a) and FIG. 3 (a).

  As described in FIG. 1, the difference between the optical frequency of the reference light and the optical frequency of the measurement light is supplied to the converter 14 as a measurement beat signal. On the other hand, the auxiliary beat signal formed by the auxiliary interferometer is supplied to the converter 14 as a sampling clock. In the embodiment shown in FIG. 1, the period sampled by the converter 14 is determined by the monitor signal. For ease of explanation, a case where the sampling period is not determined by the monitor signal will be described next.

  By sampling the measurement beat signal with the sampling clock, the beat frequency of the measurement beat signal at the time of sampling can be obtained. That is, the beat frequency (instantaneous beat frequency) at the time of sampling (instantaneous) is obtained. FIG. 5C is a waveform diagram in which the instantaneous beat frequencies are arranged in time series and expressed as a waveform. In FIG. 5C, when the drive current is near the top and bottom vertices of the triangle, the frequency difference between the measurement light and the reference light is large, and the instantaneous beat frequency varies greatly. In other words, the region near the upper and lower vertices of the triangle is a region where the instantaneous beat frequency is unstable. On the other hand, in the period when the drive current increases and decreases according to the triangular shape, the instantaneous beat frequency is constant in the case of an ideal beat signal (FIG. 4B). In other words, the rising and falling periods of the triangle are stable periods in which the instantaneous beat frequency is stable. In FIG. 5C, the stable period in the rising portion (period) of the triangular wave is indicated by a broken line as the stable region A, and the stable period in the falling portion (period) of the triangular wave is indicated by a broken line as the stable region B.

  Even in the stable regions A and B, the instantaneous beat frequency changes due to the nonlinear characteristics of the laser diode. The slope of the instantaneous beat frequency in the stable region A in the rising portion of the triangular wave is au, and the slope of the instantaneous beat frequency in the stable region B in the falling portion of the triangular wave is ad. The slope of the instantaneous beat frequency can be calculated by differentiation. Specifically, the subtraction value of the adjacent instantaneous beat frequencies in the stable region is the slope. The waveform of the drive current that drives the laser diode 23 is calibrated so that the slopes au and ad of the instantaneous beat frequency fall within the preset allowable range ± amax. By this calibration, for example, the triangular wave of the drive current is deformed as shown in FIG. That is, the triangular waveform indicated by the broken line is transformed into a triangular waveform having a bulge as indicated by the solid line. In order to deform the triangular wave, the waveform generator 1 may be instructed from the computing unit 15, or the transformed triangular wave is calculated by the computing unit 15, supplied to the waveform generator 1, and after the transformation. The triangular wave may be supplied from the waveform generator 1 to the adder 51.

  As shown in FIG. 6A, in order to smooth the slope of the drive current, a modification function such as a quadratic function is prepared in the computing unit 15 so that the triangular wave is deformed according to this modification function. The waveform generator 1 may be controlled from the calculator 15.

  An example of a quadratic function used as a deformation function will be described next. The rising portion of the triangular wave is a function fu0 (t), the falling portion is a function fd0 (t), the rising portion after deformation is a quadratic function fu1 (t), and the falling portion after deformation is a quadratic function fd1 (t). In this case, each function is expressed by Expression (10) to Expression (13).

  In the ascending portion fu1 (t) after the deformation of Expression (12), when time t = 0, the inclination becomes a + c, and when time t = 1, the inclination becomes a−c, and the inclination changes smoothly (expression) Differentiation of (12)). Similarly, the slope of the descending portion fd1 (t) becomes smooth. Of the parameters used for the quadratic function, a and b are known from the function before deformation. Therefore, the degree of deformation can be adjusted by finely changing the parameter c in Expression (12) and the parameter d in Expression (13). Of course, the deformation function is not limited to a quadratic function.

  By changing the drive current of the laser diode 23 from a triangle, each of the measurement light and the reference light is calibrated to a waveform closer to a triangular wave as shown in FIG. Along with this, the instantaneous beat frequency of the measurement beat signal also approaches a constant value when the slope becomes almost zero in the stable regions A and B, as shown in FIG. .

  Next, the determination of the sampling period using the monitor signal will be described. The monitor signal is shown in FIG. 5 (d) and FIG. 6 (d). The voltage of the monitor signal changes according to the optical frequency of the laser light emitted by the laser diode 23. Therefore, by looking at the voltage of the monitor signal, it is possible to determine which region (position) of the triangular wave the currently generated measurement beat signal corresponds to. A monitor voltage corresponding to the position on the triangular wave corresponding to the region including the stable regions A and B is obtained in advance, and the obtained monitor voltage is set as a reference voltage Vrefd. The converter 14 compares the reference voltage Vrefd with the voltage of the monitor signal, and samples the measurement beat signal with the sampling clock while the monitor voltage exceeds the reference voltage Vrefd.

The converter 14 AD-converts the measurement beat signal obtained by sampling and supplies it to the calculator 15. In the example shown in FIG. 5D and FIG. 6D, the measurement beat signal is sampled and AD-converted data is supplied to the computing unit 15 in the period from time t ₁ to time t _4. The Thereby, even if the laser diode 23 has non-linear characteristics, the measurement beat signals in the stable regions A and B are sampled, and the error can be reduced.

In the present embodiment, sampling is performed even during the period from time t ₂ to t ₃ , which is an unstable region, and AD converted data is supplied to the calculator 15. Since the position of this unstable region can be grasped in advance, for example, the calculator 15 can be configured not to perform calculation on data in an unstable period. Of course, as a reference voltage, not only Vrefd described above, the reference voltage Vrefu the upper limit corresponding to the time _{t 3} and time _{t 4} may be as provided (not shown). In this case, the converter 14 samples the measurement beat signal only during the period in which the voltage of the monitor signal is between the reference voltages Vrefd and Vreffu.

  The sampling data in each of the stable regions A and B obtained in this way are AD-converted as a first digital signal (sampling data in the stable region A) and a second digital signal (sampling data in the stable region B). , And supplied to the calculator 15. The computing unit 15 performs the above-described computation using these digital signals, thereby forming an LD driving current that changes the triangular wave.

  Although there is a time difference between the waveform of the drive current shown in FIG. 5A and the waveform of the measurement wave and the reference wave shown in FIG. 5B, in order to simplify the drawing, In the figure, they are drawn up and down. The same applies to FIGS. 6A and 6B.

  In FIG. 6 (e), the measurement beat signal is sampled, AD conversion is performed, and the arithmetic unit 15 performs a Fourier transform operation on the data obtained by the AD conversion, and the frequency spectrum obtained thereby is obtained. It is shown. In the figure, the horizontal axis represents frequency and the vertical axis represents amplitude. By performing the frequency analysis in this way, a peak P is generated at a frequency corresponding to the distance to the DUT 100. By substituting the value of the frequency at this peak P into equation (9), the distance to the device under test 100 is obtained. Since the frequency corresponding to the peak position is obtained from the frequency spectrum by the calculation by the calculator 15, the calculator 15 can be regarded as functioning as a frequency discriminator at this time.

  In the present embodiment, the frequency of the measurement beat signal in the two stable regions of the stable region A in the rising region of the triangular wave and the stable region B in the falling region is handled as one unit. That is, the frequency of the measurement beat signal in the two stable regions A and B is averaged, and the averaged frequency of the measurement beat signal is used as data for Fourier transform. Specifically, an average value of the first digital signal (stable region A) and the second digital signal (stable region B) is obtained. The obtained average value is used as data for Fourier transform. The distance is obtained by calculating the peak of the frequency spectrum by Fourier transform. By doing so, it is possible to cancel the frequency fluctuation corresponding to the Doppler shift generated by the vibration of the device under test 100 and the like, and to measure only the measurement optical path difference.

  In this case, since it is not necessary to provide another optical system as a countermeasure against Doppler shift, it is possible to reduce errors in measurement while suppressing an increase in cost. Note that a plurality of times of laser light irradiation are performed for measuring the distance at one place. Therefore, the distance is measured using a plurality of the above-mentioned one unit.

  In the present embodiment, both the rising region and the falling region of the drive waveform for driving the laser diode 23 are deformed as shown in FIG. 6A, but either the rising region or the falling region is used. You may make it deform | transform only.

  Although the invention made by the inventor has been specifically described based on the above embodiment, the present invention is not limited to the above embodiment, and various modifications can be made without departing from the scope of the invention. For example, in the above-described embodiment, the calculation is performed so that the slope approaches 0 so that the instantaneous beat frequency in the stable region becomes a constant value. However, the instantaneous beat frequency in the stable region in the rising and falling portions of the triangular wave is shown. An average value may be obtained and calculated so that an error from the average value is reduced.

  Further, for example, the above-described deformation function may be provided in the waveform generator 1.

  Further, the deformation of the drive waveform for driving the laser diode may be repeatedly executed. In this case, for example, a drive waveform according to the function after deformation is output from the arbitrary waveform generator 1, and the instantaneous beat frequency obtained thereby is calculated. The slopes au and ad are obtained based on the calculated instantaneous beat frequency. The obtained gradient is compared with the allowable range ± amax, and the parameters (coefficients) c and d of the deformation functions fu1 (t) and fd1 (t) are changed and converged until they are within the allowable range. By repeating in this way, the error due to the nonlinearity of the optical frequency modulation of the laser diode can be further reduced, and the distance can be measured with high accuracy.

  Further, the computing unit 15 may be a computer. In this case, the computer may be shared with other systems.

DESCRIPTION OF SYMBOLS 1 Waveform generator 3 Laser light generator 14 Converter 15 Calculator 23 Laser diode 41-45 Fiber coupler 51-53 Adder 91-95 Photo detector

Claims (4)

A laser light generator for generating laser light whose frequency changes to a triangular shape with time;
A light distributor that receives and distributes the laser light generated by the laser light generator;
A measurement interferometer that receives the laser light distributed by the optical distributor and outputs a beat signal having a frequency corresponding to the distance to the object to be measured;
An auxiliary interferor for receiving a laser beam distributed by the optical distributor and sampling the beat signal to form a sampling clock;
A photodetector for receiving a laser beam generated by the laser beam generator and outputting a monitor signal;
A converter that samples the beat signal in accordance with the sampling clock and converts the beat signal into a digital signal corresponding to the beat signal, wherein the sampling period is determined by the monitor signal;
An arithmetic unit that receives a digital signal from the converter and calculates data representing a distance to the object to be measured;
Comprising
The computing unit outputs a first digital signal output from the converter during a period in which the frequency of the laser light increases and a first digital signal output from the converter in a period during which the frequency of the laser light decreases. A distance measuring device that receives two digital signals, calculates an average of a first frequency of the first digital signal and a second frequency of the second digital signal, and calculates data representing a distance. The distance measuring device according to claim 1,
The distance measuring device includes a waveform generator that is controlled by the calculator and supplies a triangular wave-like modulation signal to the laser light generator,
The arithmetic unit receives the digital signal output from the converter, obtains a frequency with respect to the passage of time of the digital signal, and controls the waveform generator so that the frequency with respect to the passage of time is constant. measuring device. The distance measuring device according to claim 1,
The distance measuring device includes a waveform generator that is controlled by the calculator and supplies a triangular wave-like modulation signal to the laser light generator,
The calculator outputs the first digital signal output from the converter during a period in which the frequency of the laser light increases and is output from the converter during a period in which the frequency of the laser light decreases. The second digital signal is received, an average of the first frequency of the first digital signal and the second frequency of the second digital signal is obtained, and the obtained average value, the first frequency, and the second frequency A distance measuring device that controls the waveform generator such that the difference between each of the two is reduced. In the distance measuring device according to any one of claims 1 to 3,
The measurement interferometer includes a scanner that scans the measurement position of the object to be measured.
The distance measuring unit, wherein the computing unit receives angle information from the scanner and calculates a three-dimensional shape of the object to be measured based on the angle information and data representing the distance.

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