JPH077060B2 - Distance measurement method using laser beam - Google Patents

Description

The present invention relates to a distance measuring method using a laser beam.

"Prior Art" Conventionally, various distance measuring methods using a laser beam have already been proposed. Among them, a reflection obtained by irradiating a part of the laser beam to an object to be measured through a beam splitter. The light and the reflected light obtained by irradiating the other part of the laser beam to the reference reflector spaced apart from the beam splitter by a predetermined distance through the beam splitter are overlapped, and the laser beam Modulates the frequency to generate a beat wave in the above reflected light,
It is known that the beat wave is measured to measure the distance to the object to be measured.

The principle of the distance measuring method will be described here. In FIG. 2, reference numeral 1 denotes a direct current from a direct current power source 2, which oscillates a single-mode laser beam and which corresponds to the magnitude of the driving current. A laser beam emitted from the semiconductor laser 1 is transmitted through a convex lens 3, a beam splitter 4, and a lens group 5 composed of a concave lens and a convex lens to irradiate a DUT 6 having a rough surface. To be done. The reflected light reflected by the DUT 6 is reflected by the beam splitter 4, then passes through the convex lens 7, and is received by the light receiving element 8 such as a photodiode.
Furthermore, the signal is introduced into the measuring device 9.

On the other hand, a part of the laser beam oscillated from the semiconductor laser 1 is reflected by the beam splitter 4 and is applied to a reference reflector 10 such as a mirror or a prism provided apart from the beam splitter 4 by a predetermined distance L _{1 in} advance, The reflected light reflected by the reference reflector 10 passes through the beam splitter 4, is superimposed on the reflected light from the DUT 6, passes through the convex lens 7, and is received by the light receiving element 8.

Therefore, since the signals of the two reflected lights introduced into the measuring device 9 are time-delayed due to the optical path difference between them, specifically, the distance L between the beam splitter 4 and the reference reflector 10 is L.
₁ and the distance L ₂ between the beam splitter 4 and the DUT 6
Due to the time delay corresponding to the distance twice the difference between the phase difference and the phase difference.

Then, in this state, the semiconductor laser 1
The magnitude of the drive current to be supplied to the semiconductor laser 1 is controlled by the modulator 11, and as shown in FIG. 3, a modulation current having a linear waveform is applied to the semiconductor laser 1 so that its oscillation frequency becomes linear in the same manner as the modulation current. When modulated, the frequency of the laser beam also changes linearly as shown in FIG. Then, the phase difference changes in accordance with the change in the frequency of the laser beam, whereby the beat wave is generated.

The measuring device 9 measures the beat frequency fb of the beat wave. If the beat frequency fb can be measured, the distance to the DUT 6 can be calculated and measured.

That is, when the time difference of the superimposed reflected light is τ, the difference between the distances L ₂ and L ₁ is R, and the speed of light is c, the time difference τ
Is expressed by the following equation.

τ = 2R / c (1) Further, as shown in FIG. 4, the time difference τ between the two reflected lights introduced into the measuring device 9 and the predetermined half wavelength of the modulation frequency fm given to the semiconductor laser 1 are determined. The time T ₀ , the frequency shift amount δ of the modulation frequency fm, and the beat frequency fb have the following proportional relationship.

τ: fb = T ₀ : δ (2 ″) From this expression (2 ″), the following expression is obtained.

fb = δ · τ / T ₀ (2 ′) Further, since the predetermined time T ₀ is obtained by T ₀ = 1 / 2fm, the following equation is obtained from this equation and the equation (2 ′).

fb = 2fm · δ · τ (2) Therefore, the following formula is obtained from the formulas (1) and (2).

R = c · fb / (4fm · δ) (3) By the way, the frequency shift amount δ is the amplitude of the laser beam which is periodically changed, and is obtained by subtracting the minimum frequency from the maximum frequency. . The frequency is the reciprocal of the wavelength, and since the wavelength of the maximum frequency and the wavelength of the minimum frequency can be measured by the spectrum analyzer, the frequency shift amount δ can be calculated in advance by the measurement. .

As described above, since the frequency shift amount δ can be measured in advance, the beat frequency fb is measured by the measuring device 9.
Can be calculated and the distance R ₂ to the object 6 can be calculated.

As is clear from the above description, it is necessary to measure the beat frequency in order to measure the distance to the DUT 6, and whether or not this frequency can be accurately measured directly affects the distance measurement accuracy. It was

By the way, conventionally, the beat frequency has been measured as follows, for example. That is, since the wave of light appears as the intensity of light, the intensity of light is continuously observed by the optical sensor, and a signal is sent when a certain intensity or more is detected. Then, the beat frequency can be measured by resetting when the intensity falls below this constant intensity, sending a signal when the intensity becomes large again, and repeating this to count the number of signals by the counter.

[Problems to be Solved by the Invention] However, in the conventional beat frequency measuring method as described above, the precision measurement of the phase less than 1 cycle is insufficient, and the phase less than 1 cycle, that is, the decimal point or less. It has been desired to measure the wave number of the with higher accuracy.

“Means for Solving Problems” In view of such circumstances, the present invention measures the number of beat waves instead of measuring the frequency of beat waves.
It enables high-accuracy distance measurement.

That is, the phase shift amount Φ of the beat wave and the wave number N of the beat wave are respectively given by the following equations.

Φ = 2π · δ · τ (4) N = Φ / 2π = δ · τ (5) Therefore, the difference R in the distance is R = c · N / (2δ) (6), and this ( From equation (6), the frequency shift amount δ is determined in advance, the frequency of the laser beam is given the predetermined frequency shift amount δ, and is longer than one cycle of the beat wave generated as described later. If modulation is performed for a predetermined time T _{0 and} the wave number N of the beat wave during that period is measured, the difference R
Therefore, the distance L ₂ to the measured object can be calculated.

More specifically, according to the present invention, the frequency of the laser beam is given a predetermined frequency shift amount and is modulated for a predetermined time longer than one cycle (one wave number) of the generated beat wave. The beat wave is generated, and the number of beat waves within the predetermined time is measured as an integer value, with one cycle of the beat wave as one wave number, and a clock having a frequency higher than the frequency of the beat wave is generated. Is counted by a clock counter, and the clock counter measures the number of counts in one cycle of the beat wave and the number of counts that is less than an integral multiple of one cycle of the beat wave within the predetermined time and counts those counts. From the ratio of the above, the wave number below the decimal point of the beat wave is calculated, and the total wave number including the wave number below the decimal point of the beat wave within the predetermined time It is obtained by allowing calculation measure the distance to the object to be measured.

[Operation] According to such a method, if the frequency of the clock is set sufficiently higher than the beat frequency, the wave number below the decimal point, which is less than one cycle, can be measured extremely accurately, and therefore, highly accurate distance measurement can be performed. Will be able to.

Further, when measuring the frequency of the beat wave, it is necessary to keep the frequency strictly constant because the fluctuation of the frequency directly causes a measurement error of the distance. It is not necessary to keep the frequency of the beat wave strictly constant just by obtaining the frequency deviation amount δ, so it is easier to improve the ranging accuracy than when measuring the frequency of the beat wave. You will be able to plan.

[Embodiment] The present invention will be described below with reference to the illustrated embodiment. In FIG. 1, a measuring apparatus 20 according to the present invention includes a waveform shaping circuit 21 for shaping the beat signal from the light receiving element 8. The signal whose waveform has been shaped by the waveform shaping circuit 21 is input to the integer wave counter 22 that counts the number of integer waves in the beat wave, that is, the number of waves in each cycle. This counter 22
Corresponds to, for example, a counter used in the above-described conventionally known beat frequency measuring method.

Therefore, since the integer wave counter 22 can count only the number of times the beat wave is an integral number of times, a clock generator 23 is provided to measure the number of waves below the decimal point, and this clock generator 23 corresponds to the frequency of the beat wave. A clock is generated at a much higher frequency, and the clock counter 24 counts the clock.

The integer wave counter 22 outputs a reset signal 25 to the clock counter 24 every time it counts the number of integer waves of the beat wave, and resets the clock counter 24. At that time, the arithmetic circuit 26 makes one cycle of the beat wave. Input and store the corresponding count number. Then, the time setter 27 is set with a predetermined time T ₀ for applying the modulation to the laser beam to obtain the predetermined frequency shift amount δ, and the time setter 27 has passed the time. When the above is detected, the clock counter 24
A count stop command signal 28 is output to the clock counter 24, and the clock counter 24 stops counting. In this state, the clock counter 24 counts the number of waves less than one cycle, that is, the number of waves below the decimal point.

Further, the arithmetic circuit 26 inputs a count number less than one cycle from the clock counter 24, and calculates the wave number of the beat signal from the decimal point from the count number for one cycle of the beat wave previously input and stored. . As can be understood from this, the arithmetic circuit 26 needs to input and store the count number corresponding to one cycle of the beat wave in advance. For this purpose, the predetermined time T ₀ is equal to the frequency described above. In addition to giving the shift amount δ, it is necessary that the time is longer than one cycle (one wave number) of the generated beat wave.

Further, the arithmetic circuit 26 calculates the sum of the calculated wave number below the decimal point and the wave number input from the integer wave counter 22,
Furthermore, calculate the distance L ₂ to the DUT 6 and display it.
Display on 29. At this time, the wave number of the beat wave may be displayed on the display unit 29 to the right of the decimal point.

Although the clock counter 24 is reset every cycle of the beat wave in the above embodiment, the clock counter 24 is caused to count the total number of clocks for the predetermined time T ₀ , and the value corresponds to one cycle. The wave number of the beat wave at the predetermined time T ₀ may be obtained from the number of clocks to the right of the decimal point. Further, if the measurement points of the measured object are two-dimensionally swept in the XY directions to obtain the distance, the three-dimensional shape of the measured object can be measured.

[Advantages of the Invention] As described above, according to the present invention, since the wave number of the beat wave can be precisely measured, it is possible to obtain an effect that distance measurement can be performed with higher accuracy.

[Brief description of drawings]

1 is a block diagram for explaining an embodiment of the method of the present invention, FIG. 2 is a system diagram for explaining a conventional distance measuring method, and FIG. 3 is a current waveform applied to the semiconductor laser 1. 4 is an explanatory diagram showing a state in which the frequency of the laser beam linearly changes based on the current waveform applied to the semiconductor laser 1. 1 ... Semiconductor laser, 6 ... Object to be measured 8 ... Light receiving element, 10 ... Reference reflector, 20 ... Measuring device, 23 ... Clock generator, 24 ... Clock counter, 26 ... Arithmetic circuit, 27 ... Time setter

Claims (1)

[Claims] 1. Reflected light obtained by irradiating an object to be measured with a part of a laser beam through a beam splitter, and another part of the laser beam with a predetermined amount from the beam splitter through the beam splitter. Superimpose the reflected light obtained by irradiating a reference reflector separated by a distance, modulate the frequency of the laser beam to generate a beat wave in the superimposed reflected light, and measure the beat wave. In the distance measuring method using a laser beam for measuring the distance to the object to be measured, the frequency of the laser beam is given a predetermined frequency shift amount, and is calculated from one cycle (one wave number) of the generated beat wave. Is generated for a long predetermined time to generate a beat wave, and the number of beat waves within the predetermined time is measured as an integer value, with one cycle of the beat wave as one wave number. A clock having a frequency higher than the frequency of the beat wave is generated, and this clock is counted by a clock counter, and the clock counter counts the count number in one cycle of the beat wave and at least one cycle of the beat wave within the predetermined time. Count less than an integer multiple and calculate the number of decimals of the beat wave from the ratio of those counts, and further from the total number of beats including the number of decimals of the beat wave within the predetermined time A distance measuring method using a laser beam, wherein the distance to the object to be measured is calculated and measured.