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WO2010001809A1 - Télémètre, procédé de télémétrie et dispositif optique de détermination de forme tridimensionnelle - Google Patents

Télémètre, procédé de télémétrie et dispositif optique de détermination de forme tridimensionnelle Download PDF

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Publication number
WO2010001809A1
WO2010001809A1 PCT/JP2009/061634 JP2009061634W WO2010001809A1 WO 2010001809 A1 WO2010001809 A1 WO 2010001809A1 JP 2009061634 W JP2009061634 W JP 2009061634W WO 2010001809 A1 WO2010001809 A1 WO 2010001809A1
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Prior art keywords
light
measurement
distance
frequency
phase
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English (en)
Japanese (ja)
Inventor
元伸 興梠
一宏 今井
和哉 太田
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Optical Comb Inc
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Optical Comb Inc
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02055—Reduction or prevention of errors; Testing; Calibration
    • G01B9/0207—Error reduction by correction of the measurement signal based on independently determined error sources, e.g. using a reference interferometer
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02001—Interferometers characterised by controlling or generating intrinsic radiation properties
    • G01B9/02007—Two or more frequencies or sources used for interferometric measurement
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B9/00—Measuring instruments characterised by the use of optical techniques
    • G01B9/02—Interferometers
    • G01B9/02001—Interferometers characterised by controlling or generating intrinsic radiation properties
    • G01B9/02007—Two or more frequencies or sources used for interferometric measurement
    • G01B9/02008—Two or more frequencies or sources used for interferometric measurement by using a frequency comb
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S17/00—Systems using the reflection or reradiation of electromagnetic waves other than radio waves, e.g. lidar systems
    • G01S17/02—Systems using the reflection of electromagnetic waves other than radio waves
    • G01S17/06—Systems determining position data of a target
    • G01S17/08—Systems determining position data of a target for measuring distance only
    • G01S17/32—Systems determining position data of a target for measuring distance only using transmission of continuous waves, whether amplitude-, frequency-, or phase-modulated, or unmodulated
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01S—RADIO DIRECTION-FINDING; RADIO NAVIGATION; DETERMINING DISTANCE OR VELOCITY BY USE OF RADIO WAVES; LOCATING OR PRESENCE-DETECTING BY USE OF THE REFLECTION OR RERADIATION OF RADIO WAVES; ANALOGOUS ARRANGEMENTS USING OTHER WAVES
    • G01S7/00—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00
    • G01S7/48—Details of systems according to groups G01S13/00, G01S15/00, G01S17/00 of systems according to group G01S17/00
    • G01S7/481—Constructional features, e.g. arrangements of optical elements
    • G01S7/4811—Constructional features, e.g. arrangements of optical elements common to transmitter and receiver

Definitions

  • the present invention detects interference light between the reflected light of the reference light irradiated to the reference surface and the reflected light of the measurement light irradiated to the measurement surface from the measurement surface, and detects the distance to the reference surface and the
  • the present invention relates to a distance meter, a distance measurement method, and an optical three-dimensional shape measuring machine for obtaining a difference in distance to a measurement surface.
  • distance measurement based on an optical principle using laser light is known as an active distance measurement method capable of measuring a precise point distance.
  • the object to be measured is based on the difference between the time when the laser light is emitted and the time when the laser light reflected by the measurement object is detected by the light receiving element. Is calculated (see, for example, JP-A-2001-343234).
  • the driving current of the semiconductor laser is modulated with a triangular wave or the like
  • the reflected light from the object is received using a photodiode embedded in the semiconductor laser element, and sawtooth appearing in the photodiode output current
  • the distance information is obtained from the main wave number of the wave.
  • OCT optical tomography method using optically low-coherence light as a probe (probe)
  • OCT optical Coherence Tomography
  • the Fourier domain OCT uses a broadband light source, and the output light is split by a diffraction grating in a spectroscope and a spectral interference signal is obtained by a light receiving element, but a white broadband light source such as a light emitting diode is used. Therefore, the value of the optical frequency that is split by the diffraction grating and incident on the light receiving element depends on the performance of the spectrometer and the arrangement of the light receiving elements.
  • the present applicant uses the optical frequency comb generator, which is a light source having an equidistant spectral distribution, instead of the broadband light source in the Fourier domain OCT, thereby increasing the information on the frequency axis or the absolute distance information in the real space.
  • An optical interference observation apparatus capable of detecting with high accuracy has been proposed (see, for example, Japanese Patent Application Laid-Open No. 2009-025245).
  • a general laser rangefinder performs distance measurement by measuring the delay time of the light which gave intensity
  • the phase of the electrical modulation signal applied to the laser and the output signal of the photodetector is compared, and the delay time is measured based on the modulation signal applied to the laser.
  • a distance resolution of 1 ⁇ m it is necessary to have a time resolution (about 7 femtoseconds) that is equivalent to the time for which light travels a distance of 2 ⁇ m in a round-trip manner. Since it is necessary to increase from several hundred GHz to several THz, it is impossible with the current technology.
  • the amount of displacement when a displacement meter using laser interference is used, it is possible to measure the amount of displacement from a reference plane with a resolution or accuracy on the order of nanometers. Since the wavelength of the laser light is in the range of several hundred nanometers to several micrometers, it is much shorter than the wavelength of the electrical signal. For example, when the highly coherent light generated from the laser is separated by a beam splitter, irradiated onto the reference surface and the measurement surface, superimposed again, and input to the photodetector, the distance to the reference surface and the distance to the measurement surface An interference signal corresponding to the above is obtained. Since the interference signal changes for one period when the measurement surface moves by a half wavelength of light, it is possible to obtain a displacement amount with a resolution higher than the wavelength of light.
  • the displacement meter is suitable for applications in which the amount of change from a reference point is measured with high resolution, but is not suitable for measuring the distance from the hand to the measurement surface with high accuracy.
  • the object of the present invention is to provide a distance meter, a distance measuring method, and an optical three-dimensional shape measuring machine capable of performing long distance measurement with high accuracy and in a short time in view of the conventional situation as described above. There is to do.
  • the distance meter according to the present invention periodically emits coherent reference light and measurement light whose intensity or phase is periodically modulated and whose modulation periods are different from each other or which can relatively change the phase of a modulation signal.
  • a reference light detector for detecting interference light between the reference light emitted from the first light source and the measurement light emitted from the second light source, and the first light source
  • a reference surface irradiated with the reference light emitted from the second light source
  • a measurement surface irradiated with the measurement light emitted from the second light source
  • the reference light reflected by the reference surface and reflected by the measurement surface
  • a measurement light detector for detecting interference light with the measurement light, and a time difference or phase difference between the interference signal detected by the reference light detector and the interference signal detected by the measurement light detector, at the light speed and the measurement wavelength.
  • the first and second light sources may be, for example, two optical frequency comb generators that have different mode frequency intervals or can change the relative phase of the mode interval frequency. it can.
  • the signal processing unit for example, frequency-analyze the interference signal detected by the reference photodetector, and collectively obtain phase information of a number of optical frequency combs, Interfering signals detected by the measurement photodetector are analyzed in frequency to obtain the phase information of a number of optical frequency combs at once, and the rate of change of each phase characteristic with respect to the frequency is obtained. The difference between the distance to the surface and the distance to the measurement surface is calculated.
  • the first and second light sources two light sources whose intensity or phase is periodically modulated and the frequency is stabilized are used, and the signal processing is performed.
  • the unit calculates an absolute distance due to a time difference or a phase difference between interference signals between the reference photodetector and the measurement photodetector and a phase displacement of an arbitrary frequency component.
  • the first and second light sources are, for example, two pairs of oscillators that perform up to a frequency band in which the relative phase synchronization is high and have short-term relative phase fluctuations.
  • the signal processing unit performs a calibration process of the measurement distance based on a distance measurement result with a plurality of values having the same mode frequency difference, for example.
  • the signal processing unit is capable of performing absolute measurement at a distance longer than a value obtained by dividing the speed of light by the mode frequency difference, for example, based on a distance measurement result at a plurality of values having the same modulation frequency. Calculate the distance measurement.
  • the intensity or phase is modulated periodically, and the coherent reference light and the measuring light that can change the modulation signal phase or the phase of the modulation signal are different from each other.
  • the first interference light between the reference surface and the reference light irradiated to the measurement surface and the measurement light is detected, and the reference light reflected by the reference surface and the measurement surface are reflected.
  • the optical three-dimensional shape measuring instrument is a coherent reference light whose intensity or phase is modulated periodically and whose modulation period is different from each other or the phase of the modulation signal can be relatively changed.
  • first and second light sources that emit measurement light
  • a reference light detector that detects interference light between the reference light emitted from the first light source and the measurement light emitted from the second light source
  • Measurement light detection for detecting interference light between the reference surface irradiated with the reference light emitted from the first light source, the reference light reflected by the reference surface and the measurement light reflected by the target object From the time difference or phase difference between the interference signal detected by the reference light detector and the interference signal detected by the measurement light detector, the distance from the refractive index at the light speed and measurement wavelength to the reference surface, and the target
  • the difference in distance to the object A distance meter including a signal processing unit, an optical scanning device that scans a target object with measurement light emitted from the distance meter, and returns the measurement light reflected by the target object to the distance meter;
  • the optical scanning device is controlled to scan the laser beam and simultaneously acquire the absolute distance information measured by the distance meter.
  • the present invention can provide a laser distance meter, a laser distance measuring method, and an optical three-dimensional shape measuring machine that can measure a long distance with high accuracy and in a short time.
  • FIG. 1 is a block diagram showing a basic configuration of a laser distance meter according to the present invention.
  • FIG. 2 shows the light received by the reference light detector when measuring the time of the reference light pulse S 1 and the measurement light pulse S 2 by the interference of two coherent pulse light sources having different modulation periods. It is a schematic diagram showing a pulse train 3, the reference light pulses S 1 measured in the time measuring light pulse S 2, when performed by the interference of the two pulsed light source with a different interference resistant modulation period each other, the pulse train measuring light detector receives It is a schematic diagram showing.
  • FIG. 4 is a schematic diagram of an optical spectrum.
  • FIG. 5 is a schematic diagram of a beat signal spectrum.
  • FIG. 6 is a cross-sectional view schematically showing the structure of an optical frequency comb generator used as a light source in the laser distance meter.
  • FIG. 7 is a diagram schematically showing the output of the optical frequency comb generator.
  • FIG. 8 is a block diagram schematically showing the configuration of a light source using two optical frequency comb generators in the laser distance meter.
  • FIG. 9A is a diagram schematically showing the output of one of the two optical frequency comb generators constituting the light source
  • FIG. 9B is the generation of the two optical frequency combs constituting the light source. It is a figure which shows typically the other output of a container.
  • FIG. 10 is a diagram illustrating an example of an interference waveform in the vicinity of the measurement distance in the laser rangefinder.
  • FIG. 10 is a diagram illustrating an example of an interference waveform in the vicinity of the measurement distance in the laser rangefinder.
  • FIG. 11 is a diagram schematically showing the relationship between the measured distance and the detected interference signal in the laser rangefinder.
  • FIG. 12 is a diagram schematically showing the relationship between the measured distance and the detected interference signal in the laser rangefinder.
  • FIG. 13 is a diagram schematically showing the relationship between the measured distance and the detected interference signal in the laser rangefinder.
  • FIG. 14 is a diagram schematically showing the relationship between the measured distance and the detected interference signal in the laser rangefinder.
  • FIG. 15 is a diagram schematically showing the relationship between the measured distance and the detected interference signal in the laser rangefinder.
  • FIG. 16 is a diagram schematically showing the relationship between the measured distance and the detected interference signal in the laser rangefinder.
  • FIG. 17 is a diagram schematically showing the relationship between the measured distance and the detected interference signal in the laser rangefinder.
  • FIG. 18 is a block diagram illustrating a configuration example of a signal processing unit of the laser distance meter.
  • FIG. 19 is a block diagram showing another configuration example of the signal processing unit of the laser distance meter.
  • FIG. 20 is a block diagram showing another configuration example of the laser distance meter.
  • FIG. 21 is a block diagram showing a configuration of an optical three-dimensional shape measuring machine using the laser distance meter according to the present invention.
  • Laser rangefinder 10 according to the present invention, for example as shown in FIG.
  • the first light source 1 emits a reference beam S 1
  • a second light source 2 emits a measuring beam S 2
  • the reference beam S 1 and the reference light detector 3 for detecting the interference light S 3 between the measuring beam S 2
  • the reference plane 4 that the reference light S 1 is being irradiated
  • the measurement surface 5 of the measuring light is irradiated
  • the The measurement light detector 6 that detects the interference light S 4 between the reference light S 1 ′ reflected by the reference surface 4 and the measurement light S 2 ′ reflected by the measurement surface 5, and the reference light detector 3 the interference signal and the measuring light detector 6 obtained by detecting interference light S 3 comprises a signal processing unit 7 for interference signal obtained by detecting the interference light S 4 is supplied.
  • the first and second light sources 1 and 2 each have a coherent reference light S 1 whose intensity or phase is periodically modulated and whose modulation period is different or the phase of the modulation signal can be relatively changed.
  • measuring light S 2 each of which is periodically modulated in intensity or phase, and has a coherent reference light S 1 capable of changing the modulation signal phase relative to each other or having different modulation periods.
  • two light source comprising an optical modulator for emitting the measuring light S2, two optical frequency comb generators having different optical frequency comb mode intervals, or two pulse light sources having different optical pulse repetition frequencies Become.
  • the reference light S 1 and measurement light S 2 emitted from the first and second light sources 1 and 2 are mixed and overlapped by a light mixing element 11 made of a semi-transparent mirror or a polarizing beam splitter, and light made of a semi-transparent mirror.
  • the separation element 12 separates the light toward the reference light detector 3 and the light toward the measurement target.
  • the reference light S1 and measurement light S2 emitted from the first and second light sources 1 and 2 are mixed by the light mixing element 11 made of a semi-transparent mirror, assuming that the polarization planes are orthogonal to each other.
  • the mixed light is reflected by the light separating element 12 and enters the reference photodetector 3 through the polarizer 13, and the mixed light that has passed through the light separating element 12 is polarized by the polarizing beam splitter 14.
  • the reference light S 1 and the measurement light S 2 are separated, and the reference light S 1 is incident on the reference surface 4, and the measurement light S 2 is incident on the measurement surface 5.
  • the reference light S 1 and the measurement light S 2 emitted from the first and second light sources 1 and 2 are assumed to have polarization planes orthogonal to each other.
  • a splitter may be used to mix components of the reference light S 1 and the measurement light S 2 whose polarization planes are orthogonal to each other.
  • the reference light S 1 ′ reflected by the reference surface 4 and the measurement light S 2 ′ reflected by the measurement surface 5 are mixed by the polarization beam splitter 14, and the mixed light is mixed with the light separation element 12. And is incident on the measurement light detector 6 through the polarizer 15.
  • the reference light detector 3 receives the mixed light of the reference light S 1 and the measurement light S 2 incident through the polarizer 13, thereby the first and second light sources 1 and 2.
  • the reference light S 1 emitted from 2 and the interference light S 3 of the measurement light S 2 are detected.
  • the measurement light detector 6 receives the mixed light of the reference light S 1 ′ and the measurement light S 2 ′ incident through the polarizer 15 and is reflected by the reference surface 4.
  • the interference light S 4 between the reference light S 1 ′ and the measurement light S 2 ′ reflected by the measurement surface 5 is detected.
  • the optical path from the optical mixing element 11 shown by a bold line to the polarization beam splitter 14 in FIG. 1, the reference light S 1 and the measuring light S 2 is Yes by orthogonally polarized so as not to interfere,
  • the polarization beam splitter 14 separates the reference light S 1 and the measurement light S 2 in accordance with the polarization, and enters the reference surface 4 and the measurement surface 5.
  • the reference light S 1 ′ and the measurement light S 2 ′ reflected by the reference surface 4 and the measurement surface 5 are mixed by the polarization beam splitter 14, and the mixed light is reflected by the light separation element 12.
  • the measurement light detector 6 causes the reference light S 1 ′ incident on the measurement light detector 6 and reflected by the reference surface 4 and the interference light S 4 of the measurement light S 2 ′ reflected by the measurement surface 5 by the measurement light detector 6. To detect.
  • the interference light S 3 in which the components of the reference light S 1 and the measurement light S 2 are mixed as a transmission component of the polarizer 13 is used as a reference.
  • An interference signal is obtained by the reference photodetector 3 so as to be incident on the photodetector 3.
  • the measurement light detector is used as it is.
  • the polarizer 15 Since an interference signal cannot be obtained even if the light enters the light beam 6, the polarizer 15 is inserted, and the direction of the polarizer 15 is set so as to be inclined with respect to the polarization of the reference light S 1 ′ and the measurement light S 2 ′. By adjusting, the interference light S 4 in which the components of the reference light S 1 ′ and the measurement light S 2 ′ are mixed as the transmission component of the polarizer 15 is incident on the measurement light detector 6. The measurement light detector 6 obtains an interference signal. Note that a half-wave plate and a polarizing beam splitter may be used instead of the polarizer.
  • the interference signal obtained by the reference light detector 3 has a carrier frequency that is a difference between the carrier light frequencies of the reference light S 1 and the measurement light S 2 emitted from the first and second light sources 1 and 2.
  • the amplitude changes at the moment when the phase difference between the optical pulse repetition frequencies of the reference light S 1 and the measuring light S 2 matches an integer multiple of 2 ⁇ . Same waveform at a frequency of the difference between the optical pulse repetition frequency of the reference light S 1 and the measuring light S 2 is repeated when the reference light S 1 and the optical pulse repetition frequency of the measuring beam S 2 are different.
  • the role of the reference photodetector 3 is to generate a reference for delay time measurement.
  • the reference light S 1 and measurement light S 2 emitted from the first and second light sources 1 and 2 are not equal in repetition frequency, even if the timing is shifted when the light source starts operation, the timing is gradually changed. is going to shift, always somewhere in the moment appears the light pulse of the measuring light S 2 and the light pulse reference light S 1 overlap in. Further, the overlapping moment appears periodically at a repetition frequency of the difference between the repetition frequency of the reference light S 1 and the measurement light S 2. The instant at which this light pulse and the light pulse overlap is the reference for delay time measurement.
  • the reference light S 1 and the measurement light S 2 can relatively change the phase of the repetition frequency even if the repetition frequencies are equal, the optical pulse and the optical pulse can be obtained by matching the relative phase to an integer multiple of 2 ⁇ .
  • This phase is a reference for phase difference measurement.
  • the interference signal obtained by the measurement light detector 6 has a carrier frequency that is the difference between the carrier light frequencies of the reference light S 1 ′ and the measurement light S 2 ′, similar to the interference signal obtained by the reference light detector 3. having the same repetition frequency as the difference between the reference light S 1 and the optical pulse repetition frequency of the measuring beam S 2.
  • the amplitude of the interference signal obtained by the measurement photodetector 6 changes at the moment when the relative phase coincides with an integral multiple of 2 ⁇ .
  • the light pulse input to the measuring light detector 6 the absolute value of the distance difference between the distance L 2 between the distance L 1 to the reference reflecting mirror 4 to the measurement reflector 5 (L 2 -L 1) minute
  • the instant or phase at which the optical pulse overlaps is delayed as compared with the interference signal obtained by the reference photodetector 3.
  • This delay time or phase is a delay time or phase due to propagation of a light pulse over a distance twice the absolute value (L 2 ⁇ L 1 ) of the distance difference, and the refractive index n is multiplied by the speed of light C in vacuum.
  • the distance can be obtained by dividing by g or by dividing the delayed phase by the wave number of the repetition frequency of the optical pulse and further by the group refractive index ng .
  • a reference of an interference signal that gives a time reference or a phase reference.
  • the photodetector 3 is indispensable, and distance measurement is possible only by comparing the time difference or phase difference between the interference signals obtained by the reference photodetector 3 and the measurement photodetector 6.
  • the laser rangefinder 10, the signal processing unit 7, detects the interference light S 3 by the reference photodetector 3 by the interference signal and the measuring light detector 6 obtained by detecting the interference light S 3
  • the signal processing unit 7, the interference signal by the reference photodetector 3 by the interference signal and the measuring light detector 6 obtained by detecting the interference light S 3 obtained by detecting the interference light S 3 from the phase difference it performs the process of determining the speed of light and the absolute value of the distance difference between the distance L 2 from the refractive index to the distance L 1 and the measurement surface to the reference plane at a measurement wavelength (L 2 -L 1). That is, in the laser distance meter 10, the intensity or phase emitted from the first and second light sources 1 and 2 is periodically modulated, and the modulation period is different from each other or the phase of the modulation signal is relatively changed.
  • the light S 3 is detected by the reference light detector 3, and the interference light S 4 between the reference light S 1 ′ reflected by the reference surface 4 and the measurement light S 2 ′ reflected by the measurement surface 5 is measured light.
  • the signal processing unit 7 detects the interference light S 3 or by From the phase difference, the light velocity and the refractive index at the measurement wavelength Obtaining a difference between distances to the distance and the measuring surface to Luo said reference plane.
  • the principle of distance measurement in the laser distance meter 10 will be described. The principle of distance measurement is based on a distance meter that obtains the distance from the time delay of the optical pulse.
  • the Fourier transform operation is represented by FFT []. If the reference pulse is affected by the delay due to the propagation of the measurement distance, the waveform of the delayed pulse and its Fourier transform are represented by the following equation (2).
  • time ⁇ T is a delay time.
  • ⁇ T is obtained from the time waveform f (t ⁇ T) of the envelope of the time axis, or the phase characteristic e ⁇ jB of the frequency axis indicated by the B term on the right side of the equation (2) is obtained.
  • the jA term on the left side of equation (2) represents the phase shift of the carrier component. This term is a highly sensitive component that changes by 2 ⁇ radians at a half-wave distance of light, and is used for displacement measurement.
  • the time resolution for obtaining the delay time ⁇ T from the envelope time waveform f (t ⁇ T) or the phase characteristic e ⁇ jB of the frequency axis must be increased to the order of femtoseconds. I must. It is difficult to consider that the upper limit of the frequency band of the electric circuit is several tens of GHz. Therefore, two light sources that generate coherent reference light S 1 and measurement light S 2 that have different modulation periods or can relatively change the phase of the modulation signal are prepared and caused to interfere with each other and processed electrically.
  • the distance measurement method using the laser rangefinder 10 measures the delay time ⁇ T by reducing the frequency to a frequency where the laser distance meter 10 can be used.
  • FIGS. FIG. 2 shows an optical pulse train received by the reference photodetector 3.
  • S 1 and S 2 are time waveforms of envelopes of the reference light pulse and the measurement light pulse, respectively.
  • Repetition frequency is assumed to reference light pulse S 1 is f m + ⁇ f m
  • the measuring light pulse S 2 is f m.
  • the pulse S 1 and S 2 each N is N-th pulse at the time of integral reaches the detector It will be.
  • the number N of pulses until the timings of S 1 and S 2 coincide is obtained by the following equation (3).
  • Interference signal S 1 and S 2 is generated when the mutual pulse overlap. Therefore, the period T b of the interference signal is expressed by the following equation (4) is equal to the repetition reciprocal of the frequency difference Delta] f m of the two pulse trains.
  • FIG. 3 shows a pulse train received by the measurement light detector 6.
  • the measurement light pulse S 2 arrives with a delay of time ⁇ T due to the round trip of the optical path length (L 2 -L 1 ).
  • the number N ′ where the pulses of S 1 and S 2 overlap is the moment when the sum of the deviation of the period and ⁇ T, which increases in proportion to N ′, coincides with the period T of the measurement light pulse.
  • N ′ is given by the following equation (6).
  • N′T ′ changes linearly from T b to 0. Even if there is a delay time [Delta] T, for sure one position S 1 pulse until 0 ⁇ T b is present time going overtake S 2 pulse, it is always an interference signal between 0 ⁇ T b obtained .
  • the repetition frequency of the reference light pulse is 25 GHz + 100 kHz and the repetition frequency of the measurement light pulse is 25 GHz
  • the generation time of the interference signal changes between 10 ⁇ s and 0. Changes occurring within a time of 40 ps can be measured by extending the time width to 10 ⁇ s. Even a time difference of 1 femtosecond can be observed as 250 ps, so that it can be handled by an electric circuit in a much lower frequency band than when time measurement is performed directly with femtosecond resolution.
  • the relationship between the time delay code given to the measurement optical pulse and the time delay code of the beat signal depends on the relationship between the repetition frequency of S 1 and S 2 and the carrier frequency.
  • S 1 represents the spectrum of the reference light
  • S 2 represents the spectrum of the measurement light
  • S 1 S 2 is has a comb-like mode that matches the repetition frequency of the optical pulses
  • each mode spacing S 1 is a f m + ⁇ f m
  • S 2 is f m.
  • the interference waveforms of S 1 and S 2 include difference frequencies between various modes. However, since the difference frequency between the same mode numbers appears in the lowest frequency band, it is high when a photodetector having an appropriate frequency band is used. The difference frequency component is excluded from the detection signal.
  • FIG. 5 is a schematic diagram of a beat signal spectrum.
  • Comb-shaped electrical signal spectrum of Delta] f m spacing is obtained around the frequency f a.
  • the time waveform of the beat signal is obtained by superimposing the frequency components.
  • the reference phase characteristic is obtained from the spectrum of the output beat signal of the reference photodetector, and at the same time, the phase characteristic obtained from the output beat signal spectrum of the measurement photodetector is obtained. Compare them.
  • the difference in the phase characteristics obtained by the comparison is due to the propagation of the measurement distance (L 2 ⁇ L 1 ).
  • the phase difference information of each mode of the measurement light spectrum and the reference light spectrum is reflected in the phase of each mode number of the beat signal spectrum.
  • the relationship between the mode number and phase difference of the beat signal spectrum is replaced with the relationship between the mode number and phase difference of the measurement optical spectrum to obtain the relationship ⁇ T between the optical frequency and the phase difference, and ⁇ T is obtained from the coefficient obtained by differentiating the straight line with ⁇ . Ask.
  • Doing distance measurement by the optical comb interference by frequency analysis of the beat signal it is possible to electrically analyze a broad band with the optical spectrum is compressed to Delta] f m / f m, a distance meter for measuring a round trip time of the optical pulse High resolution can be obtained.
  • the time required for measurement, the period T b When 100kHz to ⁇ f one period T b of the interference signal is 10 [mu] s, it is possible to measure the distance in a short time. Therefore, in the laser range finder 10 having such a configuration, the intensity or phase emitted from the first and second light sources 1 and 2 is periodically modulated, and the modulation periods are different from each other or the phases of the modulation signals are relative to each other.
  • the reference light S 1 and the measurement light S 2 having a coherent property that can be changed in a normal manner are irradiated onto the reference surface 4 and the measurement surface 5, and the reference light S 1 and the measurement light S irradiated onto the reference surface 4 and the measurement surface 5. 2 is detected by the reference light detector 3 , and the interference light S between the reference light S 1 ′ reflected by the reference surface 4 and the measurement light S 2 ′ reflected by the measurement surface 5 is detected.
  • the first and second light sources 1 and 2 in the laser rangefinder 10 for example, two optical frequency comb generators having different mode frequency intervals, or the intensity or phase thereof is periodically modulated, respectively. Two light sources having a stabilized frequency can be used.
  • the performance as a distance meter is determined by the performance of the first and second light sources 1 and 2 that emit substantially the reference light S 1 and the measurement light S 2 .
  • the resolution of the distance measurement depends on the optical spectrum width or the optical pulse width, and the wider the optical spectrum width or the narrower the optical pulse width, the higher the distance measurement resolution.
  • the accuracy of absolute distance measurement depends on the frequency interval of the optical comb mode or the accuracy of the repetition frequency of the optical pulse. The higher the absolute frequency accuracy of the microwave, the higher the accuracy of absolute distance measurement. Further variations of the measured values depends on the stability of the f m and f m + Delta] f m.
  • the distance is measured using interference of light emitted from the two light sources 1 and 2, so that the first and second light sources 1 and 2 have an optical comb mode interval.
  • the optical pulse repetition frequency or modulation cycle must be different and have good coherence.
  • Independently oscillating pulse lasers usually vary in the center frequency and repetition frequency of laser oscillation, and there is no correlation in their fluctuations. Therefore, when distance measurement is performed using two independent pulse lasers, it is important to relatively fix the oscillation wavelength, optical phase, and pulse repetition frequency in order to improve accuracy. If two externally modulated light sources or two optical frequency comb generators are used, a light source that satisfies the requirements of a distance meter can be realized comparatively easily.
  • an optical frequency comb generator synchronized with two oscillators has characteristics such as good coherence, stable repetition frequency, large spectrum spread and short pulse width. It is the best light source.
  • the optical frequency comb generator 20 is formed, for example, by inserting an optical phase modulator 22 inside an optical resonator 21 composed of a pair of reflecting mirrors 21A and 21B, as shown in FIG.
  • an optical phase modulator 22 is driven at a frequency that matches an integer multiple of the free spectral range (FSR) of the optical resonator 21, the inside of the optical resonator 21.
  • the modulation signal period is synchronized with the modulation signal period, so that the modulation is extremely efficient compared to an optical phase modulator without a resonator.
  • the number of sidebands reaches several hundred to several thousand, and several terahertz.
  • An optical frequency comb having a spectral spread of The optical frequency comb generator 20 can generate a pulse that is short in time, and can generate an optical pulse having a time width of 1 picosecond or less.
  • the output of the optical frequency comb generator 20 is comb-shaped light whose center frequency is equal to the input frequency and whose frequency interval is equal to the modulation frequency. As shown in FIG. 7, the repetition frequency is f m on the time axis. Is a pulse train.
  • the light source 100 is configured as shown in FIG. That is, in this light source 100, laser light emitted from one single-frequency oscillation laser light source 101 is split by a beam splitter 102 and input to two optical frequency comb generators (OFCG1, OFCG2) 20A, 20B. It has come to be. Two optical frequency comb generator 20A, 20B are driven by the oscillator 103A, 103B to oscillate at different frequencies f m + Delta] f m and the frequency f m from each other.
  • Each oscillator 103A, 103B by being phase-synchronized by a common reference oscillator 104, the relative frequency of f m + Delta] f m and f m is stabilized.
  • a frequency shifter 105 such as an acousto-optic frequency shifter (Aofs)
  • the optical frequency shift of the frequency f a by the frequency shifter 105 to the input laser beam To give. This makes it an AC signal of frequency f a not a beat frequency between the carrier frequency is a DC signal.
  • the two optical frequency comb generators (OFCG1, OFCG2) 20A and 20B constituting the light source 100 output optical frequency combs having frequencies as shown in FIGS. 9A and 9B.
  • the optical frequency comb generator (OFCG2) output 20B, as shown in FIG. 9A, comb-like mode are arranged at a frequency interval f m to the center.
  • E Rn represents the electric field of the reference light S 1 emitted from the optical frequency comb generator (OFCG1) 20A
  • E Tn is the measuring light S 2 emitted from the optical frequency comb generator (OFCG2) 20B
  • Interference signal between the following numbers of different modes appear in and around the modulation frequency f m. Therefore, if the band of the photodetector is sufficiently wider than f a and ⁇ f but smaller than f m , or if high frequency components are removed using a filter, only the beat frequency between modes of the same order remains.
  • ⁇ n is the phase difference of the n-th mode.
  • the output current i n (t) of the photodetector can be expressed by the following equation (9), where a is a coefficient.
  • the time delay giving ⁇ n in the equation (9) depends on the optical path difference from the separation of the light by the beam splitter 102A to the superposition by the light separation element 11 and the length of the signal cable. Dependent. This time delay, since the reference photodetector 3 is common to the measuring light detector 6, is removed by subtracting the theta n of the output of the reference photodetector 3 from theta n of the output of the measuring photodetector 6.
  • the time waveform of the output current from the photodetector is the result of superimposing all the nth-order currents and can be represented by ⁇ i n (t).
  • Waveform of the output current is a waveform signal of a carrier frequency f a is modulated with a period of Delta] f
  • theta n determines the time of the envelope.
  • the influence of ⁇ n can be removed by comparing the generation time of the beat signal output from the reference photodetector 3 with the generation time of the beat signal output from the measurement photodetector 6.
  • ⁇ n of the output of the measurement light detector 6 is ⁇ n ′
  • the time difference between the interference signals obtained as detection by the reference light detector 3 and the measurement light detector 6 is ( ⁇ n ′ ⁇ n on the frequency axis).
  • 11 to 17 show changes in the interference waveform when the distance (L 2 ⁇ L 1 ) is changed from about ⁇ 3 mm to about +3 mm in about 1 mm increments.
  • Ch1 shows an example of the waveform of the interference signal obtained as the detection output by the reference photodetector 3
  • Ch2 shows an example of the waveform of the interference signal obtained as the detection output by the measurement photodetector 6.
  • ⁇ f 100 kHz
  • the signal processing unit 7 of the laser distance meter 10 may obtain a time difference between signal peaks using a peak detection circuit, or may obtain a relationship between frequency and phase by performing fast Fourier transform on the signal. Since signal repetition is fast, distance measurement can be performed in a short time. That is, each interference signal obtained as a detection by the reference light detector 3 and the measurement light detector 6 oscillates due to the difference in carrier frequency between the reference light S 1 and the measurement light S 2 as shown in FIGS. The signal is modulated in a pulse shape.
  • the signal processing unit 7 can convert an envelope signal having no carrier component. Envelope detection is performed on the interference signal obtained as a detection by the reference light detector 3 and the interference signal obtained as a detection by the measurement light detector 6, respectively, and the time difference measurement unit 73 uses a peak detection circuit. The delay time can be obtained by detecting the time when the envelope becomes a peak and obtaining the time difference, and the distance calculation unit 74 can calculate the distance from the time difference.
  • the signal processing unit 7 can perform higher-level analysis by performing analysis on the frequency axis using the frequency stability of the optical frequency comb.
  • the interference light input to the reference light detector 3 and the interference light input to the measurement light detector 6 are synchronized, and each of the relative phases ⁇ n of the n-th mode is collectively obtained, and the relationship between the frequency and ⁇ n is used as a reference.
  • the delay time can be obtained by comparing the interference signal obtained as the detection by the photodetector 3 with the interference signal obtained as the detection by the measurement photodetector 6. That is, for example, as shown in FIG.
  • the signal processing unit 7 performs frequency analysis on the interference signal detected by the reference photodetector 3 by the Fourier transform unit 75 and collects phase information of a number of optical frequency combs at once.
  • the interference signal detected by the measurement light detector 6 is frequency-analyzed by the Fourier transform unit 76 to acquire the phase information of a number of optical frequency combs at once, and the phase difference measurement unit 77 respectively obtains the phase information.
  • the change rate of the phase characteristic with respect to the frequency is obtained, and the distance calculation unit 78 calculates the difference between the distance to the reference surface and the distance to the measurement surface from the difference in inclination.
  • the optical frequency comb generator (OFCG1) ⁇ 1 the angular frequency of the reference of the light S 1 mode generated by 20B
  • the optical frequency comb generator (OFCG2) mode of the measuring light S 2 generated by 20A Is represented by the following equation (10), where ⁇ 2 is the angular frequency of the frequency shifter, ⁇ a is the angular frequency of the frequency shifter, and ⁇ is the angular frequency difference between the modulation signals.
  • the phases ⁇ 1 and ⁇ 2 of the reference light S 1 and the measurement light S 2 that are directed to the reference light detector 3 via the light separation element 12 are respectively expressed by the following formula (11) at time t.
  • phase term that does not depend on time common to the reference photodetector and the measurement photodetector is omitted. Then, through the light splitting element 12, reference light S 1 towards the measuring light detector 6 ', measuring light S 2' each phase theta 1 of ', theta 2', as in the following equation (12) Can be represented.
  • L 0 is the length of the medium from the separation element 12 to the polarization beam splitter 14.
  • N 01 and N 02 are refractive indexes at the angular frequency ⁇ 1 and the angular frequency ⁇ 2 of the medium from the separating element 12 to the polarizing beam splitter 14.
  • N g is a group refractive index of the medium from the separation element 12 to the polarization beam splitter 14.
  • the first term in the equation (13) represents a phase that changes in proportion to the distance (L 2 ⁇ L 1 ) of the measurement target. When the distance to be measured changes by half wavelength of light, it changes by 2 ⁇ radians.
  • the second term depends on the length L 0 from the separation element 12 to the polarizing beam splitter 14, the group refractive index N g, and the fixed distance L 1 from the polarizing beam splitter 14 to the reference reflecting mirror 4. This is the phase offset.
  • the second term changes by 2 ⁇ radians at half the wavelength of the beat signal depending on the mode order, so the phase change is slower than the first term.
  • the distance L 2 -L 1 to be measured is obtained from the rate of change with respect to the mode order of the phase. ⁇ n + 1 ⁇ n can be calculated by the following equation (14).
  • ⁇ 0.
  • averaging may be performed by measuring the phase change rate when the frequency difference is ⁇ and ⁇ .
  • the same interference signal is repeated when the time delay when reciprocating the measurement distance (L 2 -L 1 ) exceeds the optical pulse repetition period.
  • the number of pulses cannot be determined only by distance measurement using a set of microwave frequencies.
  • measurements are performed with two or more different sets of microwaves ⁇ m and ( ⁇ n + 1 ⁇ n ) are compared. Assuming that the same distance measurement is performed at two frequencies ⁇ m and ⁇ m + ⁇ m , the phase difference ( ⁇ n + 1 ⁇ n ) at the frequency ⁇ m can be expressed by the following equation (15).
  • phase difference ( ⁇ n + 1 ′ ⁇ n ′) at the frequency ⁇ m + ⁇ m can be expressed by the following equation (16).
  • the absolute distance can be obtained in a range where (L 2 ⁇ L 1 ) is approximately 150 m.
  • the distance L 2 -L 1 can be measured with the same measurement accuracy as the frequency stability of the microwave oscillator.
  • Long-term stability is determined by the frequency stability of the reference oscillator input to the phase locked loop of the microwave oscillator. Stability of the relative phase of the short-term f m and f m + Delta] f m is required in order to shorten the measurement time.
  • phase jitter may accumulate when the frequency of a low-frequency reference signal is increased to the driving frequency of the microwave band.
  • the measurement accuracy when measuring in a short time decreases. Therefore, in order to shorten the measurement time, a pair of oscillators having a wide band of the phase locked loop is required. Since the distance meter so far measures the distance based on the frequency of the microwave oscillator, it is irrelevant to the stability of the optical frequency. Since the wavelength of the microwave is long, in order to measure with a resolution higher than nanometers, a comb mode band of several tens of terahertz to several hundred terahertz is necessary, but it is not easy to widen.
  • the displacement can be obtained with high resolution from the phase of the light that changes at the nanometer level.
  • the distance can be obtained with accuracy less than the light wavelength by the frequency stability of the microwave by the relative frequency of the comb mode, and the distance can be determined based on the wavelength of light with higher accuracy.
  • a displacement meter that obtains the distance based on the wavelength of light needs to return to the origin in order to obtain the absolute distance.
  • the measurement based on the microwave frequency and the measurement based on the optical frequency are used together, the origin is restored.
  • the signal processing unit 7 can calculate the absolute speed at a distance longer than a value obtained by dividing the speed of light by the mode frequency difference based on distance measurement results at a plurality of values where the modulation frequencies are not the same. A distance measurement can be calculated. Further, in the laser distance meter 10, the signal processing unit 7 may perform the measurement distance calibration process based on, for example, distance measurement results with a plurality of values having different mode frequency differences. In the laser rangefinder 10, but so as to irradiate the measurement surface 5 and the reference surface 4 by separating the measurement light S 2 with the reference light S 1 by the polarizing beam splitter 14, the laser range finder 110 shown in FIG.
  • the semi-transparent mirror 111 that partially reflects light regardless of the polarization and the polarizers 112 and 113 that transmit only a specific polarization component.
  • This laser is adjusted by adjusting the orientation of the polarizer 112 so that only the reference light S 1 is directed toward the reference surface 4 and by adjusting the orientation of the polarizer 113 so that only the measurement light S 2 is directed toward the measurement surface 5.
  • the distance meter 110 operates in the same manner as the laser distance meter 10 described above.
  • an optical three-dimensional shape measuring machine 200 as shown in FIG. 21 can be configured.
  • the optical three-dimensional shape measuring machine 200 includes an optical scanning device 220 that scans a target object with the measurement light S 2 in the laser distance meter 10, a reference light detector 3 and a measurement light detector 6 in the laser distance meter 10. And a signal processing device 230 that obtains a stereoscopic image by measuring absolute distances to a plurality of points of the target object 250 based on the respective detection outputs.
  • the measurement light S 2 from the laser rangefinder 10 is radiated toward the target object 250 from the optical scanning device 220, the reflected light from the object 250 is returned to the laser rangefinder 10
  • the absolute distance to the object surface is measured by the signal processing device 230.
  • the signal processing device 230 controls the optical scanning device 220 to scan the laser beam and simultaneously acquire absolute distance information measured by the laser distance meter 10 and accumulates the absolute distance to the beam irradiation position and the location for a plurality of points. By doing so, the three-dimensional shape of the object is measured without contact. Note that the target object 250 may be moved instead of scanning the light beam by the optical scanning device 220.

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  • General Physics & Mathematics (AREA)
  • Engineering & Computer Science (AREA)
  • Electromagnetism (AREA)
  • Computer Networks & Wireless Communication (AREA)
  • Radar, Positioning & Navigation (AREA)
  • Remote Sensing (AREA)
  • Length Measuring Devices By Optical Means (AREA)
  • Optical Radar Systems And Details Thereof (AREA)
  • Instruments For Measurement Of Length By Optical Means (AREA)
  • Measurement Of Optical Distance (AREA)

Abstract

Selon l'invention, un signal d'interférence est obtenu par la détection, au moyen d'un détecteur de lumière de référence (3), d'une lumière d'interférence S3 produite lorsqu'une lumière de référence S1 émise par une première source de lumière (1) et dirigée vers une surface de référence (4) et une lumière de mesure S2 émise par une seconde source de lumière (2) et dirigée vers une surface de mesure (6) interfèrent l'une avec l'autre. Un signal d'interférence est obtenu par la détection, au moyen d'un détecteur de lumière de mesure (6), d'une lumière d'interférence S4 produite lorsque la lumière de référence S1' réfléchie par la surface de référence (4) et une lumière de mesure S2' réfléchie par la surface de mesure (5) interfèrent l'une avec l'autre. Le signal d'interférence obtenu par la détection de la lumière d'interférence S3 et le signal d'interférence obtenu par la détection de la lumière d'interférence S4 sont adressés à une unité de traitement de signal (7). A l'aide de la différence de temps ou de phase entre les deux signaux d'interférence, la différence entre la distance à la surface de référence et celle à la surface de mesure est déterminée à partir de la vitesse de la lumière et de l'indice de réfraction à la longueur d'onde de mesure au moyen de l'unité de traitement de signal (7).
PCT/JP2009/061634 2008-07-03 2009-06-25 Télémètre, procédé de télémétrie et dispositif optique de détermination de forme tridimensionnelle Ceased WO2010001809A1 (fr)

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