JPH0712509A - High-accuracy interference distance meter - Google Patents

Abstract

PURPOSE:To measure an absolute distance accurately without moving an object to be measured by referring to the oscillation frequency of a semiconductor excitation solid laser and using the absorption spectrum of iodine molecule. CONSTITUTION:Laser beams discharged from a semiconductor excitation solid laser 1 are branched by a beam splitter 2 and are fed to an iodine cell 3 and a beam collimater 8. Light input to the collimater 8 is enlarged, branched in two directions by a beam splitter 9, and then input to mirrors 10, 11, and 12. Light reflected by the mirrors 10-12 is superposed again by the splitter 9 and interference fringes are formed and then fringe signals are detected by photodiodes 14 and 15 and are output to a phase meter 16. Since a high-frequency oscillation frequency is locked to a plurality of absorption spectra of iodine molecule, frequency change can be accurately determined and further the phase of interference fringes can be measured by setting frequency within a specific frequency range variably, thus accurately measuring the absolute distance between two points.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high precision interferometric rangefinder, and more particularly to a high precision interferometric rangefinder suitable for measuring a distance by a light wave interference method with a wavelength of laser light as a measurement reference.

[0002]

2. Description of the Related Art Conventionally, in the case of performing highly accurate measurement such as distance measurement or shape measurement, which requires extremely minute resolution (for example, resolution in the unit of nanometer), a known optical wave interferometer is used. Is widely used. The light wave interference type length measuring device measures the distance and the shape based on the phase change of the interference fringes of the light accompanying the movement of the measurement object, with the wavelength of the laser light emitted from the laser light source as the reference of the length, It is characterized by high measurement accuracy and resolution and a non-contact measurement method.

[0003]

However, in the above-mentioned conventional optical wave interferometer, the phase change of the interference fringes of light accompanying the movement of the object to be measured is integrated, and the displacement of the distance is calculated based on the integration result. Since the principle of calculation is used, the displacement of the relative distance can be measured, but the absolute distance cannot be measured. For this reason,
For example, when the laser beam emitted from the laser light source is interrupted for some reason while the measurement object is moving, there is a problem that measurement becomes impossible.

Further, when measuring the distance (long distance) of a section which is considerably separated by the conventional light wave interference type length measuring device, one mirror of the light wave interference type length measuring device is continuously operated from the starting point to the end point of the measuring section. Since it is not realistic to move it to, there is a problem that it is practically impossible to measure a long distance with the conventional optical wave interferometer.

Further, in the conventional light wave interferometer, if the frequency f of the laser light source is changed by Δf (oscillation frequency change) and the phase change of the interference fringes of the light is measured, the absolute distance is measured in principle. However, in order to measure the absolute distance with high accuracy, the condition that Δf is large and Δf is measured with high accuracy is indispensable. In the conventional technology, the frequency of the semiconductor laser is tens to several tens. The scanning was performed over Δf of 100 GHz, and Δf was measured by the Fabry-Perot etalon used when measuring the length using the wavelength of the spectral line. However, even if the Fabry-Perot etalon is used, it is difficult to measure Δf with high accuracy.

[0006]

It is an object of the present invention to improve the disadvantages of the above-mentioned conventional examples, and in particular, by using the absorption spectrum of iodine molecule as a reference of the oscillation frequency of a semiconductor-pumped solid-state laser, it is possible to measure without moving the measurement object. It is an object of the present invention to provide a high-precision interferometric range finder capable of obtaining an absolute value of a distance and measuring the absolute distance with high accuracy.

[0007]

The present invention is provided with a starting point side reflecting mirror and an ending point side reflecting mirror, and based on a laser beam emitted to each of the reflecting mirrors, a reflected wave from each of the reflecting mirrors is used. An interference fringe signal output unit that forms an interference fringe and outputs an interference fringe signal, and a phase measurement unit that measures the phase of the interference fringe based on the interference fringe signal output from the interference fringe signal output unit,
In a high-precision interference distance meter including a distance calculation unit that calculates a distance between the start-point side reflecting mirror and the end-point side reflecting mirror based on the phase of the interference fringes measured by the phase measuring unit, a semiconductor laser is used. And oscillates a fundamental wave based on the excitation light emitted from the semiconductor laser, converts the fundamental wave into a second harmonic wave, and emits the second harmonic wave, and continuously oscillates the second harmonic wave within a predetermined frequency range. A semiconductor pumped solid-state laser generator that is variably set to two, a branching unit that branches the second harmonic wave emitted from the semiconductor pumped solid-state laser generator in two directions, and one second harmonic wave that is branched by the branching unit. And an iodine cell filled with iodine gas, the oscillation frequency of the other second harmonic wave branched by the branching portion is set to an absorption spectrum of iodine molecules in the iodine cell within the predetermined frequency range. Tsu said has a configuration having a semiconductor excitation solid frequency control unit for controlling the laser generator to cause click. This is intended to achieve the above-mentioned object.

[0008]

In the present invention, when the second harmonic wave is emitted from the semiconductor pumped solid-state laser generation section, it is branched into two directions by the branching section, and the frequency control section controls the semiconductor pumped solid-state laser generation section accordingly. By doing so, the oscillation frequency of one of the second harmonics is controlled to be locked to the absorption spectrum of iodine molecules in a predetermined frequency range, and the interference fringe signal output unit causes the other second harmonic to reflect the starting side reflector, It radiates each to the end point side reflecting mirror, forms interference fringes based on the reflected waves from the starting point side reflecting mirror and the end point side reflecting mirror, and outputs an interference fringe signal. When the phase measurement unit measures the phase of the interference fringes along with the output of the interference fringes signal from the interference fringes signal output unit, the distance calculation unit determines the distance between the start-side reflecting mirror and the end-side reflecting mirror based on the phase of the interference fringes. Calculate the distance at. That is, according to the present invention, by continuously variably setting the oscillation frequency of the second harmonic of the semiconductor-pumped solid-state laser within a predetermined frequency range,
It is possible to obtain the same oscillation frequency change as that of the conventional semiconductor laser. Further, according to the present invention, the conventional Fabry-Perot etalon is used by accurately locking the oscillation frequency of the second harmonic of the semiconductor-pumped solid-state laser to a plurality of absorption spectra of iodine molecules in the iodine cell. Compared with the case, the oscillation frequency change of the semiconductor pumped solid-state laser can be determined with high accuracy. Furthermore, according to the present invention, the oscillation frequency of the second harmonic of the semiconductor-pumped solid-state laser is continuously variably set within a predetermined frequency range and the phase of the interference fringe is measured to determine the absolute distance between the two points. It can be measured with high accuracy.

[0009]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment to which the high precision interferometer of the present invention is applied will be described below with reference to the drawings.

The overall structure of the high-precision interferometer according to the present embodiment will be described with reference to FIG. 1. The high-precision interferometer includes a laser light source section A, an interferometer B, and a control section C. ing.

The laser light source unit A includes a semiconductor-excited solid-state laser generation unit 1, a beam splitter 2 that reflects a portion of laser light and transmits another portion, an iodine cell 3 filled with iodine gas, and a photodiode. 4, a lock-in amplifier 5 for automatic synchronization with the waveform signal, a waveform generator 6 for generating the waveform signal, and an adder 7 for adding the differential signal and the waveform signal. In this case, the lock-in amplifier 5, the waveform generator 6, and the adder 7 constitute the frequency controller A1.

The interferometer B includes a beam collimator 8 that splits laser beams and superimposes the laser beams, a beam splitter 9 that reflects a certain portion of the laser beam and transmits the other portion, and mirrors 10, 11, 12 ,. 13, a photodiode 14, 15, and a phase meter 16 for measuring the variation Δm of the interference fringe order. In this case, the beam collimator 8, the beam splitter 9, the mirrors 10, 11,
12, 13 and the photodiodes 14, 15 constitute an interference fringe signal output section B1, and the phase meter 16 constitutes a phase measuring section B2.

The control unit C calculates the distance D between the mirror 11 and the mirror 12 based on the variation Δm of the interference fringe order measured by the phase meter 16, and the Peltier of the semiconductor excitation solid-state laser generation unit 1. It is composed of a controller 18 for controlling the module temperature.

First, the structure of the interferometer B will be described in detail. The semiconductor-excited solid-state laser generator 1 of the laser light source unit emits laser light (second harmonic: visible light).
The beam splitter 2 splits a laser beam into two directions, inputs one laser beam into the iodine cell 3, and inputs the other laser beam into the beam collimator 8. The beam collimator 8 expands the laser beam and inputs it to the beam splitter 9. The beam splitter 9 splits the laser beam into two directions, inputs one laser beam to the mirror 10, and outputs the other laser beam. Mirror the laser light 11 and 12
It is designed to be input to.

The beam splitter 9 forms an interference fringe by overlapping the laser light reflected by the mirror 10 and returned and the laser light reflected by the mirrors 11 and 12 and returned, and one of the interference fringes is formed. The signal is output to the photodiode 14, and the other interference fringe signal is output to the photodiode 15 via the mirror 13. The photodiodes 14 and 15 detect each interference fringe signal and detect the phase meter 1
It is designed to output to 6. The phase meter 16 measures the change amount Δm of the interference fringe order based on the outputs of the photodiodes 14 and 15.

On the other hand, when one of the laser beams emitted from the semiconductor-excited solid-state laser generator 1 is input to the iodine cell 3 via the beam splitter 2, the iodine cell 3 passes the laser beam. Then, the photodiode 4 outputs the laser light to the lock-in amplifier 5. In this case, the iodine molecule in the iodine cell 3 is 500 including the oscillation wavelength range of the second harmonic (visible light).
The photodiode 4 has a plurality of absorption spectra in the range of up to 600 [nm], and by modulating the oscillation frequency of the second harmonic on the absorption spectra, the photodiode 4
The output signal is obtained from.

The lock-in amplifier 5 is adapted to generate a differential signal based on the output signal of the photodiode 4 and output it to the adder 7. The waveform generator 6 outputs the reference waveform signal to the lock-in amplifier 5 and the reference waveform signal. It is designed to output to the adder 7. The adder 7 adds the differential signal and the reference waveform signal, and supplies the added signal to the semiconductor excitation solid-state laser generator 1. The added signal is used to control the electrostrictive element 29 (described later) of the semiconductor-excited solid-state laser generator 1, whereby the oscillation frequency of the laser light is locked to the absorption spectrum of iodine molecules with high accuracy. It is like this.

In this case, since the frequencies of the plurality of absorption spectra of the iodine molecule are set to known values in advance, the oscillation frequency of the second harmonic (visible light) of the laser light is, for example, a frequency of 400 GHz. By locking the absorption spectrum of any iodine molecule within a range with high accuracy, it is possible to determine a plurality of oscillation frequency changes Δf with high accuracy.

Next, the structure of the laser light source section A will be described in detail with reference to FIG.
, The lenses 21 and 22, the first reflection mirror 23, the laser medium 24 (Nd: YVO ₄ ), and the nonlinear optical crystal 25.
(KTP crystal), Peltier module 26 (heat conversion element) attached to the nonlinear optical crystal 25, and polarization element 27
And a second reflection mirror 28, and an electrostrictive element 29 attached to the second reflection mirror 28 and changing the frequency by changing the distance between the first and second reflection mirrors 23 and 28. In this case, a pair of the first reflecting mirror 23 and the second reflecting mirror 28 that are arranged to face each other constitutes a resonator.

The semiconductor laser 20 emits excitation light, and the lenses 21 and 22 refract the excitation light and irradiate it onto the laser medium 24. As a result, the laser medium 24 is excited by the irradiation of the excitation light and oscillates the fundamental wave inside the resonator. The nonlinear optical crystal 25 converts the fundamental wave generated inside the resonator into the second harmonic (visible light), and then passes the beam outside the resonator through the polarization element 27, the second reflection mirror 28, and the electrostrictive element 29. It is designed to fire on the splitter 2. Further, the Peltier module 26 changes the temperature of the nonlinear optical crystal 25 under the control of the controller 18, and the electrostrictive element 29 operates by applying a voltage to expand and contract the resonator length to modulate the oscillation frequency. Has become.

In this case, the semiconductor-excited solid-state laser generating section 1 of the laser light source section A selects the frequency of the oscillation light by the birefringence effect of the nonlinear optical crystal 25 and the polarization effect of the polarization element 27 inside the resonator, and By changing the temperature of the nonlinear optical crystal 25 by the Peltier module 26, the frequency of the second harmonic can be continuously adjusted in the range of 400 GHz, for example, and a voltage is applied to the electrostrictive element 29 to drive it. The feature is that the oscillation frequency can be modulated by expanding and contracting the resonator length.

Next, the configuration of the control unit C will be described in detail with reference to FIG. 2. The computer 17 drives the controller 18 to make the oscillation frequency change Δf in the semiconductor excitation solid-state laser generation unit 1, and causes the phase meter 16 to make a change. The distance D between the mirror 11 and the mirror 12 is calculated based on the measured interference fringe order change Δm.

That is, in the high-precision interference distance meter of this embodiment,
By using a semiconductor-pumped solid-state laser whose oscillation frequency can be continuously changed by about 400 GHz, it is possible to obtain the same frequency change Δf as that of a conventional semiconductor laser. In addition, in this device, the oscillation frequency is locked with high accuracy by using a plurality of absorption spectra of iodine molecules in the iodine cell 3, so that a semiconductor-pumped solid-state laser is compared with the case of using the conventional Fabry-Perot etalon. The oscillation frequency change Δf can be determined with high accuracy. Further, in the present apparatus, as described above, the absolute distance can be measured with high accuracy by changing the frequency f of the semiconductor pumped solid-state laser by Δf and measuring the phase of the interference fringe.

Next, description will be made on a case where distance measurement is performed by the high precision interferometer according to the present embodiment having the above-mentioned structure.

When excitation light is emitted from the semiconductor laser 20 of the semiconductor excitation solid-state laser generator 1, it is irradiated onto the laser medium 24 through the lenses 21 and 22. As a result, the laser medium 24 is excited, and the laser medium 24 is basically excited inside the resonator. The waves oscillate,
The fundamental wave is converted into the second harmonic (visible light) by the nonlinear optical crystal 25, and the polarization element 27 and the second reflection mirror 2
8. It is emitted to the outside of the resonator through the electrostrictive element 29.

The laser light emitted from the semiconductor-excited solid-state laser 1 is branched into two directions by the beam splitter 2 and input to the iodine cell 3 and the beam collimator 8. The laser light input to the beam collimator 8 is expanded, and after being branched in two directions by the beam splitter 9,
It is input to the mirrors 10, 11 and 12. The laser light reflected by the mirror 10 and returned and the laser light reflected by the mirrors 11 and 12 and returned are again overlapped by the beam splitter 9 to form interference fringes. The signals of the interference fringes are detected by the photodiodes 14 and 15 and output to the phase meter 16.

Here, the computer 17 uses the mirror 11
When measuring the distance D between the mirror 12 and the mirror 12, first,
The optical path difference d1 between the mirror 10 and the mirror 11 with respect to the beam splitter 9 is calculated based on the formula: 2d1 = mλ (where m: interference fringe order, λ: wavelength of laser light). In this case, the interference fringe order m is composed of an integer term and a fractional term, but the phase measurable by the phase meter 16 is only the fractional term and the integer term is not known. Therefore, the optical path difference d1 can be calculated only by the above equation. I can't decide.

Therefore, when the wavelength λ of the laser light emitted from the semiconductor-excited solid-state laser generator 1 is changed to λ + Δλ with the optical system of the high-precision interferometric rangefinder shown in FIG. 1 fixed, the laser light As the wavelength changes, the interference fringe order m changes by Δm represented by the formula Δm = 2d1 {1 / λ-1 / (λ + Δλ)}. When the wavelength λ is converted into the frequency f, the interference fringe order change Δm is Δm = 2d1 · Δf / c (where f: optical frequency, c:
Speed of light)

On the other hand, when one of the laser beams emitted from the semiconductor pumped solid-state laser generator 1 is supplied to the lock-in amplifier 5 via the beam splitter 2, the iodine cell 3 and the photodiode 4, The lock-in amplifier 5 creates a differential signal and outputs it to the adder 7, and the waveform generator 6 outputs the waveform signal to the lock-in amplifier 5 and the adder 7.
Output to. The adder 7 adds the differential signal and the waveform signal, and supplies the added signal to the semiconductor excitation solid-state laser generation unit 1. As a result, the oscillation frequency of the laser light is locked to the absorption spectrum of the iodine molecule with high accuracy, so that the plurality of oscillation frequency changes Δf are determined with high accuracy.

That is, since the oscillation frequency change Δf is determined with high accuracy by the semiconductor-pumped solid-state laser generator 1, if the phase fringe 16 measures the interference fringe order change Δm based on each interference fringe signal, the beam splitter The optical path difference d1 between the mirror 10 and the mirror 11 with respect to 9 can be calculated based on the equation: d1 = Δm · c / 2Δf.

Further, the computer 17 controls the optical path difference d2 between the mirror 10 and the mirror 12 with respect to the beam splitter 9.
Is calculated by a procedure similar to the procedure for calculating the optical path difference d1 between the mirror 10 and the mirror 11 for the beam splitter 9 as described above. Then, the computer 17 uses the mirror 1
The distance D between 1 and the mirror 12 is calculated based on the equation D = d2-d1. After this, a display unit (display) attached to the computer 17 and a printing unit (printer)
For example, by outputting the calculated measurement distance D, the measurer can grasp the measurement distance D. With the above distance measurement processing, the distance between two points (between the mirror 11 and the mirror 12) is measured.

As described above, according to the high precision interferometer of this embodiment, the oscillation frequency of the second harmonic is continuously about 4 times.
Since the semiconductor-pumped solid-state laser that can be tuned to 00 GHz is used, the same oscillation frequency change Δf as that of the conventional semiconductor laser can be obtained. In addition, since the oscillation frequency of the second harmonic is locked with high accuracy by using multiple absorption spectra of iodine molecules in the iodine cell 3, semiconductor excitation is performed as compared with the case of using the conventional Fabry-Perot etalon. The oscillation frequency change Δf of the solid-state laser can be determined with high accuracy. Furthermore, since the phase of the interference fringe is measured by changing the oscillation frequency f of the semiconductor-pumped solid-state laser by Δf, the absolute distance can be measured with high accuracy.

[0033]

As described above, according to the high-precision interferometer of the present invention, the oscillation frequency of the second harmonic of the semiconductor-pumped solid-state laser is continuously variably set within a predetermined frequency range. It is possible to obtain the same oscillation frequency change as that of a conventional semiconductor laser, and to lock the oscillation frequency of the second harmonic of a semiconductor-pumped solid-state laser to a plurality of absorption spectra of iodine molecules in an iodine cell with high accuracy. Therefore, it is possible to determine the oscillation frequency change of the semiconductor-pumped solid-state laser with higher accuracy compared to the case where the conventional Fabry-Perot etalon is used, and further, the oscillation frequency of the second harmonic of the semiconductor-pumped solid-state laser is set to a predetermined value. Since the phase of the interference fringe is measured by continuously variably setting it within the frequency range, it is possible to obtain an extremely remarkable effect that the absolute distance between two points can be measured with high accuracy. Kill.

[Brief description of drawings]

FIG. 1 is a block diagram showing a configuration of a high-precision interferometric distance meter of the present embodiment to which the present invention is applied.

FIG. 2 is a block diagram showing a detailed configuration of a semiconductor-pumped solid-state laser generator disclosed in FIG.

[Explanation of symbols]

 1 Semiconductor-Pumped Solid-State Laser Generator 2 Beam Splitter as Branch 3 Iodine Cell 5 Lock-in Amplifier as Frequency Control 6 Waveform Generator as Frequency Control 7 Adder 10 as Frequency Control 10 Mirror as Auxiliary Reflector 11 Mirror as Start Point Side Reflector 12 Mirror as End Point Side Reflector 16 Phase Meter 17 Controller 18 Computer as Distance Calculation Section 20 Semiconductor Laser A Laser Light Source Section A1 Frequency Control Section B Interferometer B1 Interference Fringer Signal Output Section B2 Phase Measuring unit C control unit

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Koichi Matsumoto 1-4-1 Umezono, Tsukuba-shi, Ibaraki Industrial Technology Institute, Institute of Metrology (72) Hiroyuki Morimoto 2-1-1 Sakuranamiki, Midori-ku, Yokohama, Kanagawa Suzuki Research Institute

Claims (2)

[Claims] 1. An interference fringe comprising a starting-point side reflecting mirror and an ending-point side reflecting mirror, wherein interference fringes are formed by reflected waves from the respective reflecting mirrors on the basis of laser light emitted toward the respective reflecting mirrors. An interference fringe signal output unit that outputs a signal, a phase measurement unit that measures the phase of the interference fringes based on the interference fringe signal output from the interference fringe signal output unit, and a phase of the interference fringes measured by the phase measurement unit A high-precision interferometric rangefinder having a distance calculation unit for calculating the distance between the start-point side reflecting mirror and the end-point side reflecting mirror, which has a semiconductor laser and excitation light emitted from the semiconductor laser. A semiconductor pumped solid-state laser generator that oscillates a fundamental wave based on the above, converts the fundamental wave into a second harmonic wave, and emits it, and continuously variably sets the oscillation frequency of the second harmonic wave within a predetermined frequency range. , The semiconductor-excited solid And a iodine cell filled with iodine gas into which one of the second harmonics branched by the branching portion is inputted, A frequency control unit that controls the semiconductor pumped solid-state laser generation unit so that the oscillation frequency of the other second harmonic branched by the branch unit is locked to the absorption spectrum of iodine molecules in the iodine cell within the predetermined frequency range. A high-precision interferometric rangefinder, characterized by comprising: 2. The lock-in amplifier, wherein the frequency control unit generates a differential signal based on the output of the iodine cell,
A waveform generator that generates a reference waveform signal, and a frequency control function that adds the differential signal and the reference waveform signal to generate a frequency control signal and outputs the frequency control signal to the semiconductor pumped solid-state laser generator The high-precision interferometric rangefinder according to claim 1, further comprising an adder.