Laser distance measuring device and method

Abstract

To provide a laser distance measuring device that makes it possible to measure an absolute distance to a measurement target with high accuracy and a method.SOLUTION: A laser distance measuring device is for measuring a distance to a measurement target from phase information included in an interference light beam on the basis of an interference signal indicating the interference light beam between a reflected signal light beam from the measurement target and a reference light beam. The laser distance measuring device includes a laser oscillator 10 that oscillates a two-mode laser beam of first and second frequencies, a diffraction grating 20 that spectrally splits the two-mode laser beam into a two-mode laser beam, a first frequency laser beam, and a second frequency laser beam, a moving diffraction grating 30 that frequency modulates the first frequency laser beam, and a beam splitter 50 for combining the frequency-modulated first frequency laser beam and the frequency-modulated second frequency laser beam. The two-mode laser beam is used as a signal light beam. The laser beam combined by the beam splitter 50 is used as a reference light beam.SELECTED DRAWING: Figure 7

Description

[Technical Field]

[0001] The present invention relates to a laser distance measuring device and method, and more particularly to a technique for measuring the absolute distance to an object with high accuracy. [Background technology]

[0002] Conventionally, a laser distance measurement device is known that irradiates signal light onto a reflecting surface of an object to be measured, causes the reflected signal light of the signal light reflected by the reflecting surface to interfere with reference light having a different frequency from the signal light (optical heterodyne interference), detects an interference signal representing this interference light with a photodetector, and measures the distance to the object to be measured from the phase information contained in the interference signal (for example, Patent Document 1).

[0003] The round-trip time of the signal light changes depending on the distance (optical path) to the measurement target. In other words, the phase between the reference light and the reflected signal light changes. Therefore, the distance to the measurement target is determined by detecting the phase difference between the reference light and the reflected signal light. Because laser light is a periodic wave, the distance determined from the phase difference is within one wavelength, which leads to a narrow measurement range when using a laser with a wavelength of several hundred nanometers. For this reason, in distance measurement, a modulation with a longer wavelength (lower frequency) is applied to the laser, and the phase difference between the modulated signal light and the reference light is determined (Patent Document 1).

[0004] A known method for modulating laser light is to use a Zeeman laser. The Zeeman laser utilizes the phenomenon in which the oscillation mode splits into two modes when a magnetic field is applied to the laser oscillator itself, and by superimposing the frequencies of the two modes to generate a beat, a modulation signal is obtained that is the frequency difference between the modes (about 10 MHz).

[0005] Another method for modulating laser light is to use an acousto-optic modulator (AOM) to modulate a laser diode (LD). The frequency modulation by an AOM is approximately 100 MHz. The distance measuring sensor described in Patent Document 1 uses an AOM to modulate the laser light from a semiconductor laser. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 06-221808 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the modulation frequency of laser light using a Zeeman laser is about 10 MHz, and that of frequency modulation using an AOM is about 100 MHz. If we consider a typical phase resolution of 0.1°, the distance resolution will be 0.4 mm for frequency modulation using a Zeeman laser and about 40 μm for AOM modulation, which poses a problem of insufficient resolution for precise measurements.

[0008] The present invention has been made in view of the above circumstances, and has as its object to provide a laser distance measuring device and method that can measure the absolute distance to an object to be measured with high accuracy. [Means for solving the problem]

[0009] In order to achieve the above object, a first aspect of the present invention provides a laser distance measurement device that uses relatively frequency-modulated signal light and reference light, makes the signal light incident on a reflecting surface of a measurement object, causes the reflected signal light of the signal light reflected on the reflecting surface to interfere with the reference light, detects an interference signal indicating the interfered interference light with a photodetector, and measures the distance to the measurement object from phase information contained in the interference signal, the device comprising: a laser oscillator that oscillates two-mode laser light of a first frequency and a second frequency in response to an applied current; and modulates the two-mode laser light incident from the laser oscillator into two-mode laser light in a first direction and two-mode laser light of the first frequency in a second direction. The optical fiber communication system includes a spectrometer that splits the laser light into three directions, i.e., a laser light of a first frequency in a third direction, and a laser light of a second frequency in a third direction; a frequency shifter that frequency-modulates the laser light of the first frequency split by the spectrometer; and a combiner that combines the laser light of the first frequency that has been frequency-modulated by the frequency shifter and the laser light of the second frequency split by the spectrometer, wherein the signal light is one of the two-mode laser light split by the spectrometer and the laser light combined by the combiner, and the reference light is the other of the two-mode laser light split by the spectrometer and the laser light combined by the combiner.

[0010] A laser distance measurement device according to a second aspect is the first aspect, wherein the laser oscillator is a laser diode or a solid-state laser that oscillates two modes of laser light in response to an applied current.

[0011] A laser distance measurement device according to a third aspect is the laser distance measurement device of the first or second aspect, wherein the spectroscope includes a diffraction grating.

[0012] A laser distance measurement device according to a fourth aspect is the laser distance measurement device according to any one of the first to third aspects, wherein the frequency shifter is a moving diffraction grating onto which the laser light of the first frequency is incident, or an acousto-optic modulator.

[0013] A laser distance measuring device according to a fifth aspect is the laser distance measuring device according to any one of the first to fourth aspects, wherein the combiner is a beam splitter.

[0014] A sixth aspect of the present invention is a laser distance measurement method that uses relatively frequency-modulated signal light and reference light, makes the signal light incident on a reflecting surface of a measurement object, causes the reflected signal light of the signal light reflected on the reflecting surface to interfere with the reference light, detects an interference signal indicating the interfered interference light with a photodetector, and measures the distance to the measurement object from phase information included in the interference signal, the method comprising the steps of: adjusting a current applied to a laser oscillator; causing the laser oscillator to oscillate two-mode laser light of a first frequency and a second frequency; and a spectrometer that divides the two-mode laser light incident from the laser oscillator into two-mode laser light in a first direction and a laser light of a first frequency in a second direction; The method includes the steps of: splitting the laser light into three directions, a laser light of a second frequency in a third direction, and a laser light of a first frequency in a third direction; a step of frequency-modulating the laser light of the first frequency split by the spectrometer using a frequency shifter; and a step of combining the laser light of the first frequency frequency-modulated by the frequency shifter and the laser light of the second frequency split by the spectrometer using a combiner; wherein one of the two-mode laser light split by the spectrometer and the laser light combined by the combiner is used as the signal light, and the other of the two-mode laser light split by the spectrometer and the laser light combined by the combiner is used as the reference light. [Effects of the Invention]

[0015] According to the present invention, the frequency difference between the two modes of laser light, the first frequency and the second frequency, emitted from the laser oscillator can be made larger than the modulation frequency of a Zeeman laser or an AOM, and the absolute distance to the object to be measured can be measured with high accuracy. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a schematic diagram of two laser beams used for distance measurement. [Figure 2] FIG. 2 is a conceptual diagram showing the spectrum of laser light oscillating in two modes. [Figure 3] FIG. 3 is a diagram used to explain the diffraction conditions caused by the diffraction grating. [Figure 4]FIG. 4 is a diagram showing a configuration for separating two modes of laser light. [Figure 5] FIG. 5 is a diagram showing the observation results of diffracted two-mode and multi-mode laser light. [Figure 6] FIG. 6 is a diagram showing a configuration for generating signal light and reference light used in a laser distance measuring device according to the present invention. [Figure 7] FIG. 7 is a diagram showing a first embodiment of a laser distance measuring device according to the present invention. [Figure 8] FIG. 8 is a diagram showing a second embodiment of the laser distance measuring device according to the present invention. [Figure 9] FIG. 9 is a flowchart showing an embodiment of the laser distance measuring method according to the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0017] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of a laser distance measuring device and method according to the present invention will be described with reference to the accompanying drawings.

[0018] [Summary of the Invention] FIG. 1 is a schematic diagram of two laser beams used for distance measurement.

[0019] The two laser beams are a reference beam and a reflected signal beam that is incident on the reflecting surface of the object to be measured and reflected by the reflecting surface. In Figure 1, the reference beam is represented by a solid line, and the reflected signal beam is represented by a dashed line.

[0020] The reflected signal light is delayed by the time it takes to travel to and from the measurement target, which causes a phase difference between the reference light and the reflected signal light. If the phase difference is Δφ, the modulation frequency of the laser light is f, and the speed of light in the measurement environment is c', the distance L can be expressed by the following equation:

[0021] [Number 1] L=c' / 2f(N+Δφ / 2π) N=integer As is clear from the above equation (1), the distance measurement resolution of the distance L is determined by the resolution of the phase difference Δφ and the modulation frequency f.

[0022] In the conventional modulation method using a Zeeman laser, the frequency difference (modulation frequency) between the two modes is about 10 MHz, while the modulation frequency using an AOM is about 100 MHz. In both cases, the modulation frequency f is low and the ranging resolution is insufficient for precise measurements.

[0023] An object of the present invention is to provide a laser oscillator that emits laser light capable of measuring distances with high resolution.

[0024] In laser diodes (LDs) and solid-state lasers (for example, YAG lasers, which are solid-state lasers that use yttrium aluminum garnet), multiple mode oscillations can be obtained by changing the applied current. This type of laser oscillator has the characteristic that the longitudinal modes generated change from multimode to three-mode to two-mode to single-mode as the applied current is increased. The frequency difference between each mode is, for example, about 100 to 150 GHz.

[0025] FIG. 2 is a conceptual diagram showing the spectrum of laser light oscillating in two modes.

[0026] The dashed line in FIG. 2 is the gain curve of the laser medium, with the horizontal axis representing the frequency and the vertical axis representing the gain, which corresponds to the light intensity.

[0027] 2 oscillates in two longitudinal modes, a longitudinal mode with a first frequency f1 and a longitudinal mode with a second frequency f2, and the frequency difference (f1-f2) between the two modes is 100 to 150 GHz. In this example, for convenience, the higher frequency of the two longitudinal modes is designated as the first frequency f1 and the lower frequency is designated as the second frequency f2, but the reverse is also possible, and in this case the frequency difference |f1-f2| between the two modes is also 100 to 150 GHz.

[0028] In the present invention, these two modes of laser light are separated and used by using a mechanism that utilizes a diffraction grating that functions as a spectroscope.

[0029] FIG. 3 is a diagram used to explain the diffraction conditions caused by the diffraction grating.

[0030] The diffraction grating 2 shown in FIG. 3 is a reflective diffraction grating, and can be produced, for example, by applying a metal coating to an optical element having parallel grooves periodically engraved on its surface.

[0031] In FIG. 3, d is the period of the diffraction grating 2 (the distance between adjacent grooves).

[0032] The direction in which the diffracted light is diffracted by the diffraction grating 2 is determined by the wavelength of the incident light and the period of the diffraction grating 2 .

[0033] Now, assuming that the period of the diffraction grating 2 is d, the wavelength of the incident light is λ, and the angle of incidence and the angle of diffraction are θ and Φ, respectively, the following equation is established.

[0034] [Number 2] λ=(sinθ+sinΦ)×d / m m=order When two-mode laser light is incident on the diffraction grating 2, the light in which the two modes coexist travels in the direction of specular reflection.

[0035] Furthermore, two modes of light with two frequencies having a frequency difference are diffracted at different diffraction angles because they have different frequencies (wavelengths).

[0036] FIG. 4 is a diagram showing a configuration for separating two modes of laser light.

[0037] In FIG. 4, the laser oscillator 10 oscillates two-mode laser light in which the frequency difference between the first frequency f1 and the second frequency f2 is 100 to 150 GHz as shown in FIG.

[0038] The diffraction grating 20 receives two-mode laser light emitted from the laser oscillator 10 as incident light, reflects the zeroth-order component of the laser light in which the two modes are mixed in a specular reflection direction (first direction) (not shown), separates the laser light of the first frequency f1 as diffracted light in a second direction, and separates the laser light of the first frequency f1 as diffracted light in a third direction.

[0039] The diffracted light of the first frequency f1 in the second direction separated by the diffraction grating 20 is reflected by the mirror 22, enters the diffraction grating 20 again, is reflected (specularly reflected) by the diffraction grating 20, and proceeds to the observation surface 23. Similarly, the diffracted light of the second frequency f2 in the third direction separated by the diffraction grating 20 is reflected by the mirror 22, enters the diffraction grating 20 again, is reflected by the diffraction grating 20, and proceeds to the observation surface 23.

[0040] Therefore, the laser light of the first frequency f1 and the laser light of the second frequency f2 are incident on the observation surface 23 at different positions.

[0041] FIG. 5 is a diagram showing the observation results of diffracted two-mode and multi-mode laser light.

[0042] Fig. 5(A) is a diagram showing the observation results of two modes of laser light, and is a photograph obtained by photographing the observation surface 23 shown in Fig. 4. The two high-brightness areas in the photograph shown in Fig. 5(A) are the areas where the laser light of the first frequency f1 and the laser light of the second frequency f2, which have been separated by diffraction, are incident.

[0043] On the other hand, Figure 5(B) shows the observation results of multimode laser light, and is a photograph obtained by photographing the observation surface 23 shown in Figure 4. The five high-brightness areas in the photograph shown in Figure 5(B) are areas where laser light of different frequencies was incident, separated by diffraction.

[0044] Note that Figure 5(A) shows the observation results obtained when a YAG laser was used as the laser oscillator and the YAG laser was oscillated in two modes, and Figure 5(B) shows the observation results obtained when a red laser diode was used as the laser oscillator and the red laser diode was oscillated in multiple modes.

[0045] Fig. 6 is a diagram showing a configuration for generating signal light and reference light used in a laser distance measuring device according to the present invention. In Fig. 6, parts that are common to the configuration for separating two modes of laser light shown in Fig. 4 are given the same reference numerals, and detailed descriptions thereof will be omitted.

[0046] In FIG. 6, two-mode laser light, which is a mixture of laser light of a first frequency f1 and laser light of a second frequency f2 and is emitted from a laser oscillator 10, is incident on a diffraction grating 20, where it is spectrally separated (split) into three-way laser light: two-mode laser light in a first direction, laser light of the first frequency f1 in a second direction, and laser light of the second frequency f2 in a third direction.

[0047] In FIG. 6, the two-mode laser light of the zeroth-order component that is specularly reflected in the first direction on the diffraction grating 20 is shown by a solid line, the laser light of the first frequency f1 that is diffracted in the second direction is shown by a dotted line, and the laser light of the second frequency f2 that is diffracted in the third direction is shown by a dashed-dotted line.

[0048] The two-mode laser light of the zeroth-order component specularly reflected by the diffraction grating 20 is incident on the mirror 22, reflected therefrom, and then incident on the beam splitter 60. The two-mode laser light incident on the beam splitter 60 is a mixture of laser light of a first frequency f1 and laser light of a second frequency f2, and in this example is used as signal light ES traveling toward the measurement object 70. Furthermore, the frequency difference (f1-f2) between the first frequency f1 and the second frequency f2 is, for example, 150 GHz, and the signal light ES has a beat frequency of 150 GHz.

[0049] One of the laser beams having the first frequency f1 separated by the diffraction grating 20 is reflected by the mirror 22, enters the diffraction grating 20 again, is reflected by the diffraction grating 20 and enters the moving diffraction grating 30.

[0050] Moving diffraction grating 30 is a type of frequency shifter that frequency-modulates laser light of first frequency f1, and by moving (rotating) moving diffraction grating 30, modulation of modulation frequency δ is imparted to laser light of first frequency f1.

[0051] The modulation frequency δ is a frequency that is sufficiently low compared to 150 GHz, which is the frequency difference (f1-f2) between the first frequency f1 and the second frequency f2, and can be set to, for example, an easy-to-handle 5 kHz.

[0052] The means for modulating the laser light of the first frequency f1 with the modulation frequency δ is not limited to the moving diffraction grating 30 of this example, and other frequency shifters such as an acousto-optic modulator (AOM) may also be used. Moreover, instead of frequency-modulating the laser light of the first frequency f1, the laser light of the second frequency f2 may be frequency-modulated.

[0053] The laser light having a frequency (f1+δ) that has been frequency-modulated by the moving diffraction grating 30 is reflected by a mirror 40 and enters a beam splitter 50 that functions as a combiner.

[0054] The other laser light of the second frequency f2 separated by the diffraction grating 20 is incident on the beam splitter 50 via the mirror 24, and the beam splitter 50 combines the frequency-modulated laser light of the frequency (f1+δ) with the other laser light of the second frequency f2 separated by the diffraction grating 20.

[0055] The laser light combined by the beam splitter 50 enters the beam splitter 60. In this example, the combined laser light is used as reference light ER. This reference light ER has a beat frequency ((f1-f2)+δ) obtained by adding a modulation frequency δ to the frequency difference (f1-f2) between the first frequency f1 and the second frequency f2.

[0056] [First embodiment of laser distance measuring device] FIG. 7 is a diagram showing a first embodiment of a laser distance measuring device according to the present invention.

[0057] In FIG. 7, the same components as those for generating the signal light ES and the reference light ER shown in FIG. 6 are denoted by the same reference numerals, and detailed description thereof will be omitted.

[0058] In FIG. 7, an oscillator 62 outputs a sine wave signal Sδ having a frequency δ (5 kHz in this example) to a driver 32 and a phase detector 90.

[0059] The driver 32 drives the moving diffraction grating 30 to rotate in synchronization with the frequency δ of the sine wave signal Sδ, and the rotating moving diffraction grating 30 applies modulation at the modulation frequency δ to the laser light of the first frequency f1 that is separated by the diffraction grating 20 and incident thereon.

[0060] The zero-order component signal light ES separated by the diffraction grating 20 passes through the beam splitter 60 and is incident on the reflecting surface 72 of the object to be measured 70, and the reflected signal light ES reflected by the reflecting surface 72 is incident on the beam splitter 60 again.

[0061] On the other hand, the reference light ER, which is the laser light combined by the beam splitter 50, enters the beam splitter 60. Then, the reflected z signal light ES and the reference light ER undergo optical heterodyne interference by the beam splitter 60, and this interference light enters the photodetector 80.

[0062] The photodetector 80 detects an interference signal indicative of the incident interference light. That is, the photodetector 80 converts the incident interference light into a current signal corresponding to the intensity of the interference light, and converts the current signal into a voltage signal, thereby detecting a voltage signal (interference signal) corresponding to the intensity of the interference light.

[0063] Here, the reflected signal light ES and the reference light ER can be expressed by the following equations.

[0064]

number

[0065] In equation [3], uS and uR are the light intensities of the reflected signal light ES and the reference light ER, respectively, f is the frequency difference (f1-f2) between the first frequency f1 and the second frequency f2, φ is the phase difference between the reflected signal light ES and the reference light ER (the phase of the reflected signal light ES when the phase of the reference light ER is set to zero), and δ is the modulation frequency given to the reference light ER.

[0066] Since what is detected as interference light is a change in the intensity of the electric field, the detection signal detected by the photodetector 80 can be expressed by the following equation.

[0067]

number

[0068] As shown in equation (4), the interference signal detected by the photodetector 80 has a period of the modulation frequency δ and includes the phase φ of the reflected signal light ES. This phase φ changes depending on the distance between the beam splitter 60 and the reflecting surface 72 of the object 70.

[0069] The interference signal detected by the photodetector 80 is applied to a phase detector 90. A sine wave signal Sδ of frequency δ is applied to the other input of the phase detector 90 from the oscillator 62, and the phase detector 90 detects the phase φ of the reflected signal light ES included in the interference signal using the sine wave signal Sδ as a reference signal.

[0070] The phase detector 90 outputs phase information indicating the detected phase φ to the distance measurement unit 92. The distance measurement unit 92 calculates the distance to the measurement object 70 based on the phase information indicating the phase φ.

[0071] The distance measurement unit 92 can calculate the absolute distance to the measurement target 70 within a range of one wavelength of the signal light ES from the phase information indicating the phase φ. Here, since the signal light ES in this example has a frequency (f1-f2) of 150 GHz, if the detection resolution of the phase φ by the phase detector 90 is a typical 0.1°, the distance to the measurement target 70 can be measured with a resolution of approximately 0.3 μm.

[0072] [Second embodiment of laser distance measuring device] FIG. 8 is a diagram showing a second embodiment of the laser distance measuring device according to the present invention.

[0073] In FIG. 8, parts common to those in the laser distance measuring device of the first embodiment shown in FIG. 7 are given the same reference numerals, and detailed description thereof will be omitted.

[0074] The laser distance measuring device of the second embodiment shown in Figure 8 differs from the laser distance measuring device of the first embodiment shown in Figure 7 in that frequency-modulated laser light is used as signal light and non-frequency-modulated laser light is used as reference light.

[0075] 8 uses, as signal light ES, frequency-modulated laser light obtained by combining, by beam splitter 50, laser light with a frequency (f1+δ) that has been frequency-modulated by moving diffraction grating 30 and laser light with a second frequency f2 that has been separated by diffraction grating 20. This signal light ES is laser light with a beat frequency ((f1-f2)+δ).

[0076] The laser distance measurement device of the second embodiment uses, as reference light ER, the unmodulated two-mode laser light of the zeroth-order component that is specularly reflected by the diffraction grating 20. This reference light ER has a beat frequency that corresponds to the frequency difference (f1-f2) between the first frequency f1 and the second frequency f2.

[0077] When the laser light (signal light ES) combined by the beam splitter 50 is incident on the beam splitter 61 that constitutes the interference optical system, the beam splitter 61 transmits the incident signal light ES and guides it to the object to be measured 70, and the reflected signal light ES reflected by the reflecting surface 72 of the object to be measured 70 is incident on it again.

[0078] In addition, two-mode laser light (reference light ER) of the zeroth-order component that is specularly reflected by the diffraction grating 20 is incident on the beam splitter 61, and the beam splitter 61 superimposes (causes interference between) the reference light ER and the reflected signal light ES, and causes the interfered light to be incident on the photodetector 80.

[0079] The photodetector 80 detects an interference signal corresponding to the intensity of the interfering light.

[0080] Here, the reflected signal light ES and the reference light ER can be expressed by the following equations.

[0081]

number

[0082] In equation [5], uS and uR are the light intensities of the reflected signal light ES and the reference light ER, respectively, f is the frequency difference (f1-f2) between the first frequency f1 and the second frequency f2, φ is the phase difference between the reflected signal light ES and the reference light ER (the phase of the reflected signal light ES when the phase of the reference light ER is set to zero), and δ is the modulation frequency given to the signal light ES.

[0083] Since what is detected as interference light by the photodetector 80 is a change in the intensity of the electric field, the detection signal detected by the photodetector 80 can be expressed by the following equation.

[0084]

number

[0085] The photodetector 80 of the laser distance measuring device of the first embodiment detects the interference signal (change in the intensity of the electric field) shown in equation [4], whereas the photodetector 80 of the laser distance measuring device of the second embodiment detects the interference signal shown in equation [6].

[0086] Compared to Equation 4, Equation 6 has a different sign for the phase φ, but otherwise provides the same detection results, so that either one of the two-mode laser light having the beat frequency (f1-f2) or the laser light having the beat frequency ((f1-f2)+δ) modulated by the modulation frequency δ can be used as the signal light and the other can be used as the reference light. In other words, the phase-modulated laser light is not limited to being used as the reference light, and may also be used as the signal light, so long as the signal light and the reference light are frequency-modulated relative to each other.

[0087] [Embodiment of laser distance measurement method] FIG. 9 is a flowchart showing an embodiment of the laser distance measuring method according to the present invention.

[0088] The laser distance measuring method shown in FIG. 9 is a measuring method performed by the laser distance measuring device of the first embodiment shown in FIG.

[0089] 9, the current applied to the laser oscillator 10 is adjusted to oscillate laser light in two modes of a first frequency f1 and a second frequency f2 (step S10). In this example, the frequency difference between the two modes is about 150 GHz.

[0090] This two-mode laser light is made incident on the diffraction grating 20, which separates it into three-way laser light: two-mode laser light of the regular reflection zeroth-order component, laser light of the first frequency f1, and laser light of the second frequency f2 (step S12).

[0091] The separated laser light of the first frequency f1 is modulated at a modulation frequency δ by the moving diffraction grating 30, which functions as a frequency shifter (step S14). The modulation frequency δ can be set to, for example, an easy-to-handle frequency of 5 kHz.

[0092] The frequency-modulated laser light with the frequency (f1+δ) and the separated laser light with the second frequency f2 are combined by the beam splitter 50 (step S16).

[0093] The two-mode laser light of the zeroth-order component (laser light having a beat frequency (f1-f2)) dispersed by the diffraction grating 20 is incident on the measurement object 70 via the beam splitter 60 as signal light ES, and the laser light (laser light having a beat frequency ((f1-f2)+δ)) synthesized by the beam splitter 50 is incident on the beam splitter 60 as reference light ER (step S18).

[0094] The reflected signal light ES and the reference light ER reflected by the reflecting surface 72 of the measurement object 70 are superimposed by the beam splitter 60 and enter the photodetector 80 as interference light, where they are detected as an interference signal indicative of the interference light. The phase detector 90 detects the phase φ of the reflected signal light ES contained in the interference signal from the interference signal using a sine wave signal Sδ of frequency δ as a reference signal (step S20).

[0095] The distance measurement unit 92 calculates the absolute distance to the object 70 within a range of one wavelength of the signal light ES based on the phase φ detected by the phase detector 90 (step S22). In this example, since the signal light ES has a frequency of 150 GHz, if the detection resolution of a typical phase φ detected by the phase detector 90 is 0.1°, the distance to the object 70 can be measured with a resolution of approximately 0.3 μm.

[0096] [others] The laser oscillator of this embodiment oscillates two modes of laser light with a frequency difference between the modes of 150 GHz, but is not limited to this and can oscillate two modes of laser light with a frequency difference between the modes of a desired frequency depending on the purpose of distance measurement.

[0097] The greater the frequency difference between the two modes of laser light emitted from the laser oscillator, the higher the resolution of distance measurement of the object to be measured, but the narrower (smaller) the range that can be measured becomes, so there is a trade-off between the resolution of distance measurement and the range that can be measured.The frequency difference between the two modes of laser light can be determined by the resonator length of the laser oscillator.

[0098] Furthermore, in this embodiment, two-mode laser light is incident from a laser oscillator onto one diffraction grating that functions as a spectrometer, and three laser lights are separated into two modes of laser light of zero-order components that are specularly reflected by the diffraction grating, and laser light of a first frequency f1 and a second frequency f2 that have different diffraction angles, but this is not limiting, and three laser lights may be separated using two or more spectrometers.

[0099] It goes without saying that the present invention is not limited to the above-described embodiment, and various modifications are possible without departing from the spirit of the present invention. [Explanation of symbols]

[0100] 10...laser oscillator, 20...diffraction grating, 22, 24, 40...mirror, 30...moving diffraction grating, 32...driver, 50, 60, 61...beam splitter, 62...oscillator, 70...measurement object, 72...reflecting surface, 80...photodetector, 90...phase detector, 92...distance measuring unit

Claims

1. A laser distance measurement device uses relatively frequency-modulated signal light and reference light, causes the signal light to be incident on a reflecting surface of a measurement target, causes reflected signal light of the signal light reflected on the reflecting surface to interfere with the reference light, detects an interference signal indicative of the interfered interference light with a photodetector, and measures the distance to the measurement target from phase information included in the interference signal, a laser oscillator that oscillates laser light in two modes of a first frequency and a second frequency in response to an applied current; a spectrometer that splits the two-mode laser light incident from the laser oscillator into three-way laser light, i.e., the two-mode laser light in a first direction, the laser light of the first frequency in a second direction, and the laser light of the second frequency in a third direction; a frequency shifter that frequency-modulates the laser light of the first frequency split by the spectrometer; a combiner that combines the laser light of the first frequency that has been frequency-modulated by the frequency shifter and the laser light of the second frequency that has been split by the spectrometer, the signal light is one of the two-mode laser light separated by the spectrometer and the laser light combined by the combiner, the reference light is the other of the two-mode laser light separated by the spectroscope and the laser light combined by the combiner; Laser distance measuring device.

2. the laser oscillator is a laser diode or a solid-state laser that oscillates the two-mode laser light in response to an applied current; 2. A laser distance measuring device according to claim 1.

3. the spectrometer includes a diffraction grating; 3. The laser distance measuring device according to claim 1 or 2.

4. the frequency shifter is a moving diffraction grating or an acousto-optic modulator onto which the laser light of the first frequency is incident; The laser distance measuring device according to any one of claims 1 to 3.

5. the combiner is a beam splitter; The laser distance measuring device according to any one of claims 1 to 4.

6. A laser distance measurement method using relatively frequency-modulated signal light and reference light, causing the signal light to be incident on a reflecting surface of a measurement object, causing reflected signal light of the signal light reflected on the reflecting surface to interfere with the reference light, detecting an interference signal indicative of the interfered interference light with a photodetector, and measuring a distance to the measurement object from phase information included in the interference signal, comprising: adjusting a current applied to a laser oscillator to oscillate laser light in two modes of a first frequency and a second frequency from the laser oscillator; a spectrometer splitting the two-mode laser light incident from the laser oscillator into three-way laser light, i.e., the two-mode laser light in a first direction, the laser light of the first frequency in a second direction, and the laser light of the second frequency in a third direction; a step of frequency-modulating the laser light of the first frequency split by the spectrometer using a frequency shifter; a combiner combining the laser light of the first frequency frequency modulated by the frequency shifter and the laser light of the second frequency spectrally separated by the spectrometer; one of the two-mode laser light separated by the spectroscope and the laser light combined by the combiner is used as the signal light, the other of the two-mode laser light separated by the spectroscope and the laser light combined by the combiner is used as the reference light; Laser distance measurement method.

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