JPH1082858A - Optical range finder - Google Patents

Abstract

PROBLEM TO BE SOLVED: To keep the frequency change speed at a constant value with high accuracy by using a frequency shift feedback laser oscillator as an FM laser light source. SOLUTION: A part of the FM laser beam generated by a frequency shift feedback laser oscillator 10 is fed to a measured object A, and the remaining is reflected 21 and fed to a local mirror 22. The FM laser beam reflected on the object A and the local FM laser beam reflected on the local mirror 22 are fed to a light mixer 30 and mixed, and the beat signal having the freuqency difference is generated. The beat signal is detected at 42, 43 and fed to a data processor 41, and the distance to the measured object A is calculated based on the beat frequency. An acousto-optical modulator(AOC) 14 in the oscillator 10 is shifted in one direction by the value equal to the ultrasonic frequency f propagated in the AOC 14 as the Doppler shift quantity each time the laser beam passes, and the frequency is increased by 2f s for each reciprocation. The frequency change speed is kept at a constant value with high accuracy.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical distance meter used in the field of precision measurement and the like, and more particularly to an FM distance meter.
The present invention relates to a laser radar type optical distance meter.

[0002]

2. Description of the Related Art Conventionally, an FM laser radar system has been known as a system for measuring the distance of an object using a laser beam. This FM laser radar method generates an FM signal whose frequency changes with time and irradiates an object to be measured, thereby receiving a reflected FM laser beam having a propagation delay time corresponding to the distance to the object. This is mixed with a local FM laser beam having a known propagation delay time (reference FM laser beam) to obtain a beat signal, and the frequency of the beat signal (beat frequency) is used to generate a reflected signal of the measurement target. It is configured to calculate a distance.

[0003] Local FM with known propagation delay time
As the laser light, a reflected light obtained by irradiating a reflector at a known distance by branching from an FM laser light irradiating an object was used, or the laser light was branched and then passed through a propagation path of a known length. FM laser light or the like is used.

That is, the distance to the object to be measured (if the reflected light from a reflector at a known distance is used as local FM laser light, the difference between the distance to the known reflector)
Let z be z, the beat frequency fb is: fb = τ (Δf / T) = (2z / c) (Δf / T) (1) τ: time delay according to distance Δf: change in oscillation frequency Width T: Change period of oscillation frequency c: Speed of light

From equation (1), the distance z of the object to be measured is given by the following equation. z = (c / 2) (T / Δf) fb (2)

As a method of changing the oscillation frequency of laser light, a method of sweeping in a saw-tooth or triangular shape is generally used. FIGS. 8A and 8B show examples of how the oscillation frequency changes and how the frequency of the beat signal changes when the oscillation frequency is changed in a sawtooth shape. As a method of changing the frequency of the laser light, a method of changing the temperature or the injection current of a semiconductor laser is known, and a method of changing the length of a resonator of a solid-state laser is known. Is difficult to keep constant.

[0007]

In the conventional distance measuring apparatus of the FM laser radar system, it is difficult to keep the rate of change of the frequency constant, and as a result, the beat frequency changes with time. There is. That is, as illustrated in FIGS. 9A and 9B, when the change rate of the oscillation frequency of the FM laser light source changes with time, the beat signal obtained by mixing the local FM light and the reflected FM light is changed. The frequency fb changes with time. In order to prevent the accuracy from deteriorating in this case, additional processing such as time averaging of the frequency fb of the beat signal is required, and there is a problem that the time required for measurement is prolonged.

Further, the conventional FM laser radar type distance measuring apparatus has a problem that it is difficult to increase the rate of change in frequency, and thus it is difficult to increase the distance resolution. That is, from (2), the distance resolution Δz becomes Δz = (c / 2) (T / Δf) Δfb (3), and Δz decreases as the frequency change speed (Δf / T) increases and Δfb decreases. The value, that is, the distance resolution increases. However, in the conventional frequency modulation method,
Since it is difficult to realize a large frequency change width and a short change cycle at the same time, there is a problem that a large frequency change speed cannot be realized.

[0009]

An optical distance meter according to the present invention is configured to use a frequency shift feedback laser oscillator as an FM laser light source.

[0010]

The frequency shift feedback type laser oscillator has an acousto-optic modulator (AOM: Acousti) in a resonator of the laser oscillator.
c Optical Modulator).
Each time the laser beam reciprocates in the resonator through the AOM, the laser beam is deflected by Bragg diffraction by the ultrasonic waves propagating in the AOM, and also undergoes a frequency shift by Doppler shift.

The Doppler shift causes the frequency of the laser beam to change every time the laser beam passes through the AOM, the frequency fs of the ultrasonic wave propagating in the AOM.
(Hereinafter, also referred to as “modulation frequency”), and is shifted in one direction as a Doppler shift amount by a value equal to the value.
Assuming that the time required for the laser beam to reciprocate in the resonator (Round Trip) is τ _RT , the laser beam passes through the AOM twice during one reciprocation, so that the frequency increases by 2 fs for each reciprocation. Therefore, the rate of change of the frequency of the laser beam df
/ dt becomes a constant value with high accuracy, such as 2 fs / τ _RT .

Therefore, the basic formula in the FM laser radar of the present invention using the frequency shift feedback laser oscillator is as follows:
Frequency sweep rate Δf / T in equations (1), (2) and (3)
With the frequency change speed df / dt (= 2fs /
By rewriting to τ _RT ), they can be expressed as follows. fb = (2z / c) (df / dt) (4) z = (c / 2) [1 / (df / dt)] fb (5) Δz = (c / 2) [1 / (Df / dt)] Δfb (6) df / dt = 2fs / τ _RT (7) Hereinafter, the present invention will be described in further detail with examples.

[0013]

FIG. 1 shows an optical distance meter using an FM laser radar system according to an embodiment of the present invention.
10 is a block diagram shown together with FIG. 10, 10 is a frequency shift feedback laser oscillator, 20 is an optical distribution unit, 30 is an optical mixing unit,
0 is a data processing unit.

The frequency shift feedback type laser oscillator 10 comprises:
Excitation section 11 mainly composed of a semiconductor laser, laser medium Nd: YVO ₄ 12 having an excitation-side mirror integrated by coating, output mirror 13 serving as the other mirror of the resonator, acousto-optic conversion element (AOM) 14, a drive signal generating circuit 15 and a collimating lens 16.

The light distribution unit 20 includes a half mirror 21 and a local mirror 22. Light mixing unit 30
Is an avalanche photodiode (APD) or P
It mainly includes a light receiving element such as an IN diode. The data processing unit 40 includes a data processor 41, a frequency counter 42, a spectrum analyzer 43, and a display unit 4.
4 and a keyboard 45.

First, the outline of the operation of the optical distance meter will be described. The FM laser beam generated by the frequency shift feedback laser oscillator 10 is incident on a half mirror 21 in the light distribution unit 20, a part of which passes through the half mirror 21 and is incident on the measurement object A, and the remaining Part of the light is reflected by the half mirror 21 and enters the local mirror 22. Part of the laser light reflected by the measurement target A is reflected by the half mirror 21 and enters the light mixing unit 30.

Similarly, a part of the FM laser beam reflected by the local mirror 22 passes through the half mirror 21 and enters the light mixing section 30. In a light receiving element such as an APD in the light mixing unit 30, the local FM laser beam reflected by the local mirror 22 and the reflected FM laser beam reflected by the measurement target A are mixed, and a beat signal of a difference frequency is received. Occurs.

The beat signal generated by the light mixing unit 30 is supplied to a frequency counter 42 and a spectrum analyzer 43 in the data processing unit 40, and the detected frequency (beat frequency) is input to a data processor 41. The data processor 41 calculates the distance to the measurement target A based on the beat frequency received from the frequency counter 42 or the spectrum analyzer 43. To be more precise, the data processor 41 calculates the path difference between the two reflected waves mixed from the beat frequency, that is, the difference between the distance from the half mirror 21 to the measurement target A and the distance from the half mirror 21 to the local mirror 22. Is calculated.

Next, the operation of the frequency shift feedback type laser oscillator 10 shown in FIG. 1 will be described more specifically. Excitation light output from the excitation unit 11 mainly composed of a semiconductor laser
(810 nm) passes through the excitation side mirror of the resonator and enters the Nd: YVO ₄ crystal 12 functioning as a gain medium. Nd: YVO ₄
A resonator having a length of about 10 cm (resonance wavelength 1,064 nm) is formed by a mirror coating (high reflection 1,064 nm, high transmission 810 nm) on the end face of the crystal and an output mirror 13 having a light transmittance of 23%. AOM 14 is arranged inside the resonator. The 0th-order diffracted light by the AOM 14 is emitted in an oblique direction indicated by a dotted line and is dissipated from inside the resonator. Only the first-order diffracted light indicated by a solid line reciprocates in the resonator, and laser oscillation is performed.

The first-order diffracted light passing through the AOM 14 undergoes a Doppler shift due to the ultrasonic wave propagating in the AOM, and its frequency shifts in the same direction by an amount equal to the frequency of the ultrasonic wave each time the light passes through the AOM 14. I will do it. Accordingly, the frequency of the laser beam output from the output mirror 13 changes over time as shown in FIG. Where fo is the initial oscillation frequency at t = 0, τ
_RT is a propagation time required for the laser oscillation light to make one round trip within the resonance, and fs is a Doppler shift amount.

The laser beam having the oscillation frequency fo output from the pumping system 11 at time to travels halfway in the resonator formed by the mirrors facing each other, and at time τ _RT / 2, the Doppler shift caused by the ultrasonic wave becomes 1 The frequency is fo
The laser beam changes to + fs and reaches the output mirror 13, and a part of the laser beam is output to the outside. Output mirror 13
The remaining part reflected by the optical mirror 13 becomes a laser beam whose frequency has changed to fo + 3fs after receiving a Doppler shift by ultrasonic waves three times at time to + 3τ _RT / 2 after further reciprocating in the resonator one more time. And a part thereof is output to the outside. The remaining part reflected by the output mirror 13 makes one round trip in the resonator, and the time to + 5τ _RT / 2
Then, the laser beam is subjected to the Doppler shift by the ultrasonic wave five times in total, becomes a laser beam whose frequency has changed to fo + 5fs, reaches the output mirror 13, and a part thereof is output to the outside.

Similarly, the frequency of the laser beam which reaches the output mirror 13 and is partially output therefrom increases by 2 fs every time the required propagation time τ _RT for reciprocating in the resonator elapses. Go on. That is, the frequency of the laser beam output from the frequency shifted feedback laser oscillator 10 is linearly increased at a rate constant value df / dt = 2fs / τ _RT determined by the frequency of the ultrasonic wave in length and AOM resonator Go on.

In this embodiment, the frequency fs of the ultrasonic wave propagating in the AOM is set to 78.5 MHz. Therefore, the frequency of the laser beam is increased by 157 each time the time required for the laser beam to make one reciprocation in the 100 mm resonator τ _RT = 0.67 nsec elapses.
It increases by MHz. In this case, the frequency change rate df /
dt = 2.4 × 10 ¹⁷ Hz / sec, which is about three orders of magnitude larger than the conventional value. Note that both fs and τ _RT fluctuate in the long term in accordance with changes in the environment such as the room temperature, but in the short term, the ratio fs / τ _RT between them is kept at a constant value with extremely high accuracy.

The distance resolution Δz is estimated by substituting the conditions of the above embodiment into the equation (3). If the beat frequency resolution is 1 MHz, the distance resolution is 60 μm, and if the beat frequency resolution is 10 KHz, the distance resolution is 0.6 μm. As a result of preliminary experiments by the present inventors, a distance resolution of 100 μm or less was obtained.

As described above, the frequency shift feedback laser oscillator 1
The case has been described where it is assumed that the frequency to be shifted of the laser beam output from 0 is single. In fact, it has been observed that the oscillation frequency spectrum of a frequency-shifted feedback laser becomes a frequency spectrum of multimode oscillation light having a certain spread, and it is thought that a longitudinal mode structure exists in the spread. . The vertical mode interval frequency is ΔF = c / 2L, where ΔF is ΔF.

FIG. 3 shows how the frequency of the multimode laser oscillation light output from the frequency shift feedback laser oscillator 10 shown in FIG. 1 changes with time. The solid lines a, b, c, d,... Indicate the frequencies of the laser beams in the adjacent oscillation modes whose oscillation frequencies differ by the longitudinal mode interval frequency ΔF. As described above with reference to FIG. 2, the frequency of the laser light in each mode linearly increases with time due to the frequency shift effect of the acousto-optic device. For convenience of explanation, the time required for the frequency of the oscillation light in each mode to increase by a value equal to the vertical mode interval frequency ΔF is referred to as a vertical mode time difference ΔT.

More specifically, the level of the oscillated light in each mode begins to increase with the passage of time when the frequency exceeds a certain lower limit, and the level starts to decrease when the frequency increases to some extent, and the upper limit It disappears when it reaches.
FIG. 4 schematically illustrates the relationship between the frequency and the level at a certain point after the start of the oscillation. The mode number of the oscillation light of the maximum level is q, and the mode numbers of the oscillation light adjacent to the high frequency side are q + 1, q + 2, q + 3,.
, And the mode numbers of the oscillating lights adjacent to the low frequency side are q-1, q-2, q-3,.

In the following, for the sake of convenience, the solid lines a, b, c,... In FIG. 30 local FM laser light (reference light)
And The dotted lines α, β, γ, δ, ε... In FIG.
The frequency of the reflected FM laser light that is reflected by the measurement target A and then reflected by the half mirror 21 and returns to the light mixing unit 30 in FIG. 1 is shown. The difference τ in the required propagation time based on the difference in the propagation optical paths of the two reflected laser beams is the time difference Δ between the longitudinal modes.
If it is less than T, dotted lines α, β, γ, δ in FIG.
Are the reflected FM laser beams from the measurement target A corresponding to the solid lines a, b, c, d, respectively.

The difference τ in the required propagation time based on the difference in the propagation optical paths of the two reflected laser beams increases, and this is the time difference Δ between the longitudinal modes.
., The dotted lines β, γ, δ... In FIG. 3 correspond to the measurement objects A corresponding to the solid lines a, b, c.
It becomes reflected light from. The beat frequency in this case is ΔF +
fb. Where fb is the nearest solid line a, b, c
The frequency difference from If the difference τ in the required propagation time based on the difference in the propagation optical paths of the two reflected laser beams further increases and exceeds twice the time difference ΔT between the longitudinal modes, the dotted lines γ, δ, ε. Are solid lines a, b, c, respectively.
A corresponding reflected FM laser beam from the measurement target A is obtained. The beat frequency in this case is 2ΔF + fb. f
Similarly, b is the frequency difference between the nearest solid lines a, b, c,.

The time interval ΔT between the longitudinal modes is given by the following equation (7) and ΔF = c / 2L = 1 / τ _RT . ΔT = ΔF / (df / dt) = ΔF / (2fs / τ _RT ) = 1/2 fs (12) is obtained.

Regarding the order of the beat signal, first, when the mode numbers of the reflected FM laser light from the measuring object A and the local FM laser light are the same, the order of the beat signal generated by mixing the two is set to 0. The frequency of the next beat signal (beat frequency) is denoted by fb (0). Next, when the mode number of the reflected FM laser light is larger by one than the mode number of the local FM laser light, the order of the beat signal generated by mixing the two is set to 1, and the beat frequency is expressed as fb (1).

Similarly, the mode number of the reflected FM laser light is 2, 3...
m, the order of the beat signal generated by mixing the two is set to 2, 3,... m, and the beat frequency is set to fb
(2), fb (3)... Fb (m).

The m-th beat frequency fb (m) is equal to the frequency difference between the local FM laser light having an arbitrary mode number and the local FM laser light having a mode number larger by m than this.
Since it is equal to the sum of mΔF and the zero-order beat frequency fb (0), fb (m) = − mΔF + fb (0) = − mΔF + τ (ΔF / ΔT) = − mΔF + (2z / c) 2fs ΔF・ (13) However, 2z is the difference between the reciprocating propagation optical path of the two reflected laser beams, that is, the distance from the half mirror 21 to the measurement object A, and the distance from the half mirror 21 to the local mirror 22.
This is a value obtained by doubling the value obtained by subtracting the distance to. Normal,
z is a positive value, that is, the measurement target A is farther than the local mirror.

The beat frequency actually observed is ABS [fb
(m)] (where ABS [X] means the absolute value of X), since the negative frequency appears on the positive side as an image, ABS [fb (m)] = ABS [−mΔF + (2z / c) 2fs ΔF] (14) FIG. 5 shows the relationship between the beat frequency fb (GHz) and the beat order m.

From equation (14), the distance z is as follows: z = (c / 4fs) [m + ABS [fb (m)] / ΔF) (15) In the case of the image beat frequency, the distance z is as follows: z = (c / 4fs) [m-ABS [fb (m)] / ΔF) (16)

As is clear from FIG. 5, at any distance, the beat frequency ABS [fb (m)] appears in the band of half the vertical mode interval frequency ΔF / 2. This means that if the beat order m can be determined, the beat frequency should be observed for a frequency band that is half the longitudinal mode interval frequency. Further, the frequency band to be observed can be set to any convenient frequency range for the optical rangefinder of this embodiment.

The beat frequency is determined by the modulation frequency fs
This is realized by detecting a change in the beat frequency ABS [fb (m)] when. As an example, a case where a beat frequency is detected in a frequency band from zero to ΔF / 2 will be described.

FIG. 6 shows that the modulation frequency fs is set to three different values fsa, fsb, fsc (fsa <fsb <fsc), and in each case, the relationship between the distance z and the beat frequency fb is represented by a line segment. It is represented by A, B, and C. The dotted lines of the line segments A, B, and C represent the image beat frequencies. If the beat order of line segment A is m,
The beat frequencies obtained in the six regions from to are
The cases are classified as shown in Table 1 on the next page.

[0039] For example, in the region (15) from the equation, z = (c / 4fsa) (m + f A / ΔF) = (c / 4fsb) (m + f B / ΔF) = (c / 4fsc) (m + f C / ΔF・ ・ ・ ・ ・ Simultaneous equations (17) are obtained. Here, fA, fB and fC are the detected beat frequencies.

[0040] That is, in the region, by solving m from _{(17), m = (fsaf B -fsbf A} ) / [[Delta] F (fsb-fsa)] = (fsbf _C -fscf _B) / [[Delta] F (FSC fsb)] = is (fsaf _C -fscf _A) / [[Delta] F (fsc-fsa)] ... (18) obtained.

[0041]

[0042] Similarly, in the area from the (15) and (16), z = (c / 4fsa) (m + f A / ΔF) = (c / 4fsb) (m + f B / ΔF) = (c / 4fsc ) ((M + 1) −f _C / ΔF)... (19)

[0043] That is, in region (19) by solving m from the _{equation, m = (fsaf B -fsbf A} ) / [[Delta] F (fsb-fsa)] _{= (- fsbf C -fscf B +} fsbΔF) / [[Delta] F (fsc −fsb)] = (− fsaf _C− fscf _A + fsaΔF) / [ΔF (fsc−fsa)] (20)

Hereinafter, similarly, fsa, fsb, fsc, f
_A, f _B, from _{f C, m1 = (fsaf B} -fsbf A) / [ΔF (fsb-fsa)] _{m2 = (fsbf C -fscf B)} / [ΔF (fsc-fsb)] m3 = (fsaf _C -fscf _A) / [ΔF (fsc-fsa)] _{m4 = (- fsaf B -fsbf A} + fsaΔF) / [ΔF (fsb-fsa)] _{m5 = (- fsbf C -fscf B} + fsbΔF) / [[Delta] F (fSC fsb)] _{m6 = (- fsaf C -fscf A} + fsaΔF) / [ΔF (fsc-fsa)] _{m7 = (fsaf B + fsbf A} ) / [ΔF (fsb-fsa)] _{m8 = (fsbf C + fscf B} ) / [ [Delta] F (fsc-fsb)] m9 = get (fsaf _C + fscf _a) / [[Delta] F (fsb-fsa)] ... (21).

From the equation (21), when the determination conditions shown in Table 2 on the next page are satisfied, the region from to and m are determined. Since m is an integer, determination is easy even if there is some measurement error.

From the obtained m, the distance z becomes z = (c / 4fsa) (m + f _A / ΔF) (22) in the case of the region 領域, and z = (c / 4fsa) become _{(m-f a / ΔF)} ···· (23).

[0047]

Data processor 4 in data processing unit 40
1 outputs a control signal to a drive signal generation circuit 15 in the frequency shift feedback laser oscillator 10, and sets a modulation frequency fs to be supplied to an acousto-optic conversion element (AOM) 14 to fsa.
Set to. Next, the data processor 41 receives and stores the beat frequency f _A detected from the frequency counter 42 and the spectrum analyzer 43. Similarly, the data processor 41 sets the modulation frequency fs fsb, to fsc, stores receive beat frequency f _B of this time, the f _C from the frequency counter 42 and the spectrum analyzer 43.

The data processor 41 substitutes the set modulation frequencies fsa, fsb, and fsc and the stored beat frequencies f _A , f _B , and f _C into equation (21) to obtain m1
Ｍm9 is calculated, a combination that satisfies the determination condition is searched, and the area と and the mode order m are determined. The data processor 41 determines the area thus determined and the mode order m
By calculating the distance difference z according to the formula (22) or (23) from the beat frequency and the beat frequency, and adding the known distance to the local mirror 22, the distance from the half mirror 21 to the measurement target A is calculated. Calculate.

In a situation where the longitudinal mode frequency fluctuates due to a change in ambient temperature, mechanical vibration, or the like, the longitudinal mode frequency is measured at the same time as the beat frequency.
This can be achieved by improving the measurement accuracy.

The case where the change of the beat order accompanying the change of the modulation frequency is 1 or less has been described above. However, the addition of simultaneous equations and discriminants can cope with the case where the change in the beat order is larger than one.

By changing the modulation frequency, the beat frequency can be changed over an integral multiple of the longitudinal mode frequency. In this case, the distance can be calculated from a simple electric circuit and the simultaneous equations. This electric circuit is a band-pass filtering circuit having a predetermined center frequency and a narrow pass band above and below the center frequency, and the number of beat frequencies is changed several times while the modulation frequency is changed over a predetermined range. The change amount of the beat order is detected based on whether or not the beat order appears.

That is, referring to FIG. 7, the modulation frequency is changed, the modulation frequency when the beat frequency appears at the center frequency fb of the band-pass circuit is fsa, and this modulation frequency is further changed. Assume that the modulation frequency when the beat frequency appears at the center frequency fb of the bandpass circuit 2n times (n = 1, 2, 3,...) Is fsb. Since the appearing beat frequency includes both the real beat signal and the image beat signal, the change amount of the beat order during this time is n which is half of the appearance number 2n.

At this time, from equation (14), from the real beat frequency and the image beat frequency, Fb = −mΔF + (2z / c) 2fsaΔF = − (m + n) ΔF + (2z / c) 2fsbΔF (24) − Fb = −mΔF + (2z / c) 2fsaΔF = − (m + n) ΔF + (2z / c) 2fsbΔF (25) By solving the equations (24) and (25), z = nc / [4 (fsb-fsa)] (26) is obtained, and the distance z is calculated.

The above series of processing includes a data processor 41 in the data processing unit 40, a drive signal generation circuit 15 in the frequency shift feedback laser oscillator 10, a frequency counter 42 and a spectrum analyzer 43 in the data processing unit 40, Alternatively, it can be easily realized by a combination with a band-pass filtering circuit or the like added here.

As described above, in a situation where the longitudinal mode frequency fluctuates due to a change in ambient temperature, mechanical vibration, or the like, the measurement accuracy is improved by measuring the longitudinal mode frequency simultaneously with the beat frequency. Can be achieved.
For example, equation (24) gives: Fb = −mΔFa + (2z / c) 2fsaΔFa = − (m + n) ΔFb + (2z / c) 2fsbΔFb (27) −Fb = −mΔFa + (2z / c) 2fsaΔFa = − ( m + n) ΔFb + (2z / c) 2fsbΔFb (28) Where ΔFa and ΔFb are modulation frequencies fs
The vertical mode frequency measured simultaneously with a and fsb, and n is the amount of change in the beat order when the modulation frequency fsa is changed from fsa to fsb.

By solving the equations (27) and (28), z = c (ΔFa · Fb−ΔFb · Fb + ΔFa · ΔFb · n) / 4ΔFa · ΔFb (fsb−fsa) (29) z = c (−ΔFa) Fb + .DELTA.Fb.Fb + .DELTA.Fa..DELTA.Fb.n) /4.DELTA.Fa.DELTA.Fb (fsb-fsa) (30) is obtained, and the distance z is calculated. The distinction between a real beat and an image beat is made based on whether the beat frequency increases or decreases with respect to the modulation frequency.

When the specific beat frequency fb is observed (2n + 1) times while changing the modulation frequency, one of the first and last beat signals is a real beat signal and the other is an image beat signal. However, the distance can be calculated from a similar simultaneous equation.

Although the present invention has been described with reference to the embodiment, various modifications similar to the above can be made. For example, a configuration in which the 0th-order diffracted light is used as an output instead of the 1st-order diffracted light of the AOM can be adopted.
Alternatively, a configuration may be used in which a corner cube that returns the reflected light beam to the original optical path accurately is used as the fixed mirror.

Further, in order to obtain a local FM laser beam having a known propagation delay time, an example of a configuration in which the reflected laser beam is generated by branching from the FM laser beam applied to an object and irradiating a reflector at a known position. did. However,
It is also possible to adopt a configuration in which a local FM laser beam is obtained by branching from an FM laser beam to be irradiated on an object and then passing through a propagation path of a known length.

[0061]

As described above in detail, the optical distance meter according to the present invention has a configuration in which the frequency shift feedback laser oscillator is used as the light source of the FM laser light.
It becomes possible to keep the frequency change speed at a constant value with extremely high accuracy. As a result, additional processing such as averaging of beat frequencies becomes unnecessary, and the time required for measurement can be reduced.

Further, according to the optical distance meter of the present invention, a frequency change speed which is about three orders of magnitude higher than that of a conventional FM laser can be realized, and the distance resolution can be dramatically improved.

Further, the optical distance meter of the present invention detects the dependence of the beat frequency on the modulation frequency and detects the beat frequency from the detection result. There is an advantage that it can sufficiently cope with the multi-mode operation occurring in the above.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a configuration of an FM laser radar type distance meter according to an embodiment of the present invention.

FIG. 2 is a timing chart for explaining the operation of the frequency shift feedback laser oscillator shown in FIG. 2;

FIG. 3 is a conceptual diagram for explaining a multi-mode operation of the frequency shift feedback laser oscillator of FIG.

FIG. 4 is a conceptual diagram for explaining a multi-mode operation of the frequency shift feedback laser oscillator of FIG.

5 is a conceptual diagram for explaining a distance, a beat frequency, and a beat order detected by a multi-mode operation of the frequency shift feedback laser oscillator of FIG. 1;

FIG. 6 is a conceptual diagram for explaining a method of determining a beat order by detecting a modulation frequency dependence of a beat frequency in a multi-mode operation of the frequency shift feedback laser oscillator of FIG. 1;

FIG. 7 is a conceptual diagram illustrating a method of measuring an order and a distance from the number of times that a beat frequency matches a specific frequency by changing a modulation frequency.

FIG. 8 is a waveform diagram for explaining the operation principle of an ideal FM laser radar type distance measuring device.

FIG. 9 is a waveform chart for explaining a problem in operation of an actual FM laser radar type distance measuring device.

[Explanation of symbols]

10 Frequency shift feedback laser oscillator 11 Excitation section 12 Nd: YVO ₄ crystal 13 Output mirror 14 AOM (acousto-optic conversion element) 15 Drive signal generation circuit 20 Optical distribution section 21 Half mirror 22 Local mirror 30 Optical mixing section 40 Data processing section 41 Data processor A Measurement target

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takefumi Hara 504-1 Nomax Aoki A 202, Miyagawa-ku, Kawasaki City, Kanagawa Prefecture

Claims (5)

[Claims] 1. A frequency shift feedback laser oscillator for generating an FM laser light whose frequency changes with time, and an FM generator for generating the frequency shift feedback laser oscillator.
Beat signal generating means for irradiating the object to be measured with laser light and mixing the reflected light with the local FM laser light to generate a beat signal; and the frequency of the generated beat signal (hereinafter referred to as "beat frequency") An optical distance meter, comprising: a propagation distance difference detecting means for detecting a difference between a propagation distance between the FM laser light irradiated and reflected on the object to be measured and the local FM laser light. 2. The frequency-shift-feedback laser oscillator according to claim 1, wherein the frequency-shift feedback laser oscillator includes an acousto-optical element as a frequency shift element through which an ultrasonic wave of a modulation frequency is propagated, and receives a Doppler shift. An optical distance meter configured to return the first-order diffracted light to a gain medium. 3. The FM laser beam according to claim 2, wherein the FM laser beam is a plurality of modes of F mode having different frequencies.
An optical distance meter including M laser light, wherein the propagation distance difference detecting means detects dependency of the beat frequency on the modulation frequency, and detects the beat frequency based on a result of the detection. 4. The method according to claim 3, wherein the detection of the dependency of the beat frequency on the modulation frequency includes determining how many times the beat frequency appears in a predetermined band while changing the modulation frequency over a predetermined range. An optical distance meter, which is performed by: 5. The local FM laser light according to claim 1, wherein the local FM laser light is a reflected FM laser light that is separated from an FM laser light applied to the measurement object and is reflected by a reflector whose position is known. An optical distance meter, characterized in that: