JPH03146803A - Method and instrument for measuring distance - Google Patents

Abstract

PURPOSE:To attain accurate measurement even with one measurement by comparing the phase changing rates between a measuring interferometer and a reference interferometer in each of many sample data of an interference fringe changing signal obtained by one measurement, and averaging the compared results. CONSTITUTION:An incident beam to the reference interferometer 10 is divided into two beams by a beam splitter 6, respective beams are reflected by respective fixed mirrors 7, 8 with an optical path difference and the reflected beams are combined again as one interference beam and projected in the direction rectangular to the incident direction. Then the laser beam projected from the interferometer 10 is detected by a photodetector 11 and amplified by an amplifier 12, so that the intensity change signal 13 with interference fringes formed in accordance with the prescribed optical path difference between both the fixed mirrors 7, 8 is obtained. On the other hand, light penetrated into the beam splitter 5 is guided to the measuring interferometer 17, divided into two beams by a beam splitter 14, reflected by a fixed mirror 15 and a movable mirror 16 with an optical path difference, and combined again as one interference light, which is projected in the direction rectangular to the incident direction. Then the laser beam projected from the interferometer 17 is detected by a photodetecting element 18 to obtain an intensity change signal 20 with interference fringes corresponding to the optical path difference.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application 1] The present invention relates to a distance measuring method and apparatus, particularly a distance measuring method and apparatus using optical interference in which length is measured by observing interference fringes obtained using a semiconductor laser as a light source. It is related to the device.

[Prior Art] Semiconductor lasers are inexpensive, small and lightweight compared to gas lasers and the like, and are widely used as light sources for optical communications, audio, and video optical discs. In addition, applications to light sources for optical interferometers have recently been actively researched.

In particular, interferometric length measurement devices that measure the optical path length to two reflecting members through observation of interference fringes are known, which utilize the oscillation wavelength characteristics that depend on the injection current or device temperature of a semiconductor laser device. .

In this type of device, the oscillation light of a semiconductor laser is incident on an interferometer with a certain optical path difference to form interference fringes, and wavelength scanning is performed by controlling the injection current or element temperature. This is a method of detecting a time change signal (hereinafter referred to as an interference fringe change signal) and determining the optical path difference of an interferometer from the amount of phase change of this signal.

In such devices, the parameters necessary to determine distance are the oscillation wavelength and wavelength scanning width of the semiconductor laser, but these vary considerably between individual semiconductor lasers and also change depending on the environmental temperature. Therefore, in order to perform accurate length measurements, these parameters must be measured every time. However, for this purpose, a spectroscope must be incorporated into the device, which makes the device expensive and large, which is impractical.

For this reason, research has recently been conducted on a method of providing another reference interferometer and measuring the length by comparing it with this interferometer. For example, Japanese Patent Application Laid-open No. 62-251686, 64-353
There are publications such as Publication No. 04. In these methods, in addition to the measurement interferometer, a reference interferometer with a known optical path difference is installed, and the light from the same light source is split into two and incident on both interferometers, and the light is detected as the wavelength of the laser light changes. The length is measured by detecting temporal changes in interference fringes by fringe counting or zero-cross period measurement, and comparing the results.

[Problems to be Solved by the Invention] The number of periods of the sinusoidal interference fringe change signal obtained with the conventional structure as described above is too small, and it is difficult to obtain sufficient measurement accuracy when using the fringe counting method. Ta.

In addition, even in the period measurement method using zero crossing, most of the data is wasted because only the data that crosses the zero level is used, and furthermore, the data that crosses the zero level that is used is also a Since there is a difference in the captured time due to the difference, the superimposed noise is different, and this cannot be corrected using only the reference method, so it was corrected by measuring a large number of zero-crossing intervals and averaging them.

An object of the present invention is to solve the above problems and provide a distance measuring method and device that makes it possible to effectively utilize all the data of the interference fringe change signal and perform accurate length measurements even with a small number of measurements. It is.

[Means for Solving the Problems J] In order to solve the above problems, the present invention irradiates a reflective object with a laser beam, causes the reflected light from the reflective object to interfere, and forms interference fringes. In a distance measurement method that measures distance from interference fringes, a laser beam is irradiated onto two fixed reflective objects to form a first interference fringe by the reflected light from each reflective object, and the laser beam is fixed on one side and the other is fixed. is irradiated onto two movable reflective objects to form second interference fringes by the light reflected from each reflective object, and the wavelength of the laser beam is changed to temporally change the first and second interference fringes. The phase or phase change rate of the first and second interference fringes is determined from the time-varying signal of the interference fringes, and the optical path difference between the two reflecting objects that formed the second interference fringes is calculated from the first interference fringe. A configuration was adopted in which the phase or phase change rate of both interference fringes is determined based on the optical path difference between the two reflective objects formed.

Furthermore, the present invention provides a distance measuring device that irradiates a reflective object with a laser beam, causes the reflected light from the reflective object to interfere with each other to form interference fringes, and measures distance from the interference fringes. H. A reference interferometer that irradiates a laser beam onto two fixed reflecting objects to form interference fringes with the reflected light from each reflecting object, and a reference interferometer that irradiates a laser beam onto two fixed reflecting objects, one of which is fixed and the other movable. a measurement interferometer that irradiates the laser beam to form interference fringes by reflected light from each reflecting object; a means for temporally changing the interference fringes by changing the wavelength of the laser beam; and interference from the reference interferometer. means for receiving the fringes and forming an interference fringe change signal; means for receiving the interference fringes from the measurement interferometer and forming an interference fringe change signal; and a means for receiving the interference fringes from the measurement interferometer to form an interference fringe change signal; means for calculating the phase or phase change rate of the interference fringes in the measurement interferometer; Alternatively, a configuration in which the phase change rate is calculated by comparing the rate of change was also adopted.

[Operation 1] According to the above configuration, the phase change rate of all points of the interference fringe change signal data can be determined by the Fourier transform method, so a large amount of data can be compared even in one measurement. Furthermore, since data captured at the same time can be compared, it is possible to cancel out noise that has the same effect on both signals, such as fluctuations in the oscillation wavelength of the light source, making it possible to perform accurate length measurements with fewer measurements. becomes possible.

[Example 1] Hereinafter, the present invention will be described in detail based on an example shown in the drawings.

FIG. 1 shows the configuration of an interference measuring device employing the present invention. In FIG. 1, the laser light source is a single longitudinal mode oscillation semiconductor laser device 3, and an ATM (i degree adjustment circuit) 2.
temperature control. The ATM 2 controls the temperature of the semiconductor laser element 3 to a desired constant value. The control temperature value is determined by computer 21.

Further, the drive current of the semiconductor laser device 3 is controlled by a semiconductor laser drive circuit 1, and the oscillation wavelength of the semiconductor laser device 3 is adjusted by changing this drive current. In the semiconductor laser element 3, the refractive index of the waveguide changes due to a change in the injection current, and the oscillation wavelength changes. The diverging light emitted from the semiconductor laser element 3 is made into parallel light by the collimator lens 4, enters the beam splitter 5, and is split into two lights. The light reflected by beam splitter 5 is guided into reference interferometer IO. In this example, the reference interferometer lO
For example, a Michelson type interferometer is used.

The laser beam incident on the reference interferometer IO is split into two beams by the beam splitter 6. The two beams of light are reflected by fixed mirrors 7 and 8, respectively, with optical path differences.
They become one again by the beam splitter 6 and interfere,
The light is emitted in a direction perpendicular to the direction of incidence.

The laser beam emitted by the reference interferometer is received by the light receiving element 11, the signal is amplified by the amplifier 12, and the intensity change signal (interference A fringe change signal) 13 is obtained.

On the other hand, the light transmitted through the beam splitter 5 is guided into the measuring interferometer 17. In this embodiment, a Michelson type interferometer is similarly used as the measurement interferometer 17. The laser beam incident on the measurement interferometer 17 is split into two beams by the beam splitter 14. The 62 beams are each reflected by a fixed mirror 15 and a movable mirror 16 with optical path differences, and are again combined into one beam by the beam splitter 14. They interfere with each other and are emitted in a direction perpendicular to the direction of incidence.

The laser beam emitted from the measurement interferometer is received by the light receiving element 18, and the signal is amplified by the amplifier 19 to generate an intensity change signal of interference fringes formed according to a predetermined optical path difference between the fixed mirror 15 and the movable mirror 16. (Interference fringe change signal) 20 is obtained.

The two interference fringe change signals thus obtained are input into the computer 21 and the signals are analyzed. The computer 21 is constituted by a computer system including a microprocessor, memory, and the like.

Next, the operation of the above configuration will be explained in detail.

1. The phase of interference fringes obtained by entering the measurement interferometer 17 with a laser beam having a wavelength of zero is expressed as φ 5 = 2xLs/1.0 - (1), where Ls is the optical path difference. Now, when the laser wavelength is changed from Ito to 几0+八应, the phase of the interference fringe obtained is φ °s = 2 π Ls/(Ro + Δn)
...-(2) is expressed.

In the present invention, a single longitudinal mode oscillation semiconductor laser is used as a variable wavelength coherent light source. A typical injection current-oscillation wavelength characteristic of a single longitudinal mode oscillation semiconductor laser is shown in FIG. Although the linear wavelength tuning range is limited by mode hops, there is a linear relationship between the injection current and the oscillation wavelength in the section between mode hops. This straight line portion is assumed to be used in the processing shown below.

FIG. 3 shows the waveform of the injection current injected into the semiconductor laser. The wavelength is scanned at a constant rate by changing the injection current over time at a constant rate. If the wavelength change rate of the semiconductor laser is K [nm/sec], then the amount of wavelength change is Δλ=Kt. In addition, since the wavelength change of a semiconductor laser is small, it is 90)△y.

When formula (2) is transformed, φ゛s=2πLs/(几0+八尤)=(2xLs/90)(1+(Δλ/),,0)). '),,0), so by approximation, φ'5=
(2xLs/尤20)(几0-八え)=(2xLs
s / IO) − (2x L s /λ20) Δe = (2xLs, /LO) (2iKLs/
几20) t...(3) is obtained.

From equation (3), when the wavelength of the laser beam is 'A, 0 + eight-fold, the position of the interference fringe obtained by the measurement interferometer 17 is 3 φ'5=(2π/a, 0) (1-(Δλ/11 .0) ) Ls = (2π/LO) (1-(Kt/λ0)) Ls = i4). Similarly, the phase of the interference fringes obtained by the reference interferometer 10 is as follows: φ'r=(2π/LO) (l-(Δn/E0))Lr=(2T,/circle 0) (1-(Kt /'A, 0) ) Lr =151. However, Lr is the optical path difference in the reference interferometer 17.

Here, dividing equation (4) by equation (5), φ's/φ
'r=Ls/Lr, i.e. 1, s=(φ°s/$'
r) Lr -16), and even if K and λ0 are not known, if L', φ°S, and φ゛r are measured, the measured distance Ls
can be found.

In this embodiment, the computer 21 performs phase analysis using the Fourier transform method.

4. The Fourier transform method is a method of determining the phase distribution of a sinusoidal signal using FFT (fast Fourier transform), and is used for moire topography and analysis of fringe patterns in surface shape measurement using an interferometer.

References include Journal of the 0
pticalSociety of America/
Vo1.72, No. 1/January1982
rFourier-transform method
of fringe pattern analysis
s for computer-based topo
graphy and interferometry
There are J etc.

The phase analysis method using the Fourier transform method will be explained below.

As a first step, the half-cycle (T/2) waveform of the interference fringe change signal as shown in FIGS. 4 and 5 is taken into the computer 21 as sampling data (6th step).
figure). This signal is as follows: I (t) = a (t) + b (t) cos (2ifct+φ0) = a
(t)+ (1/2)b' (t)exp [
i (2xfct+φ0)]+ (1/2)b
(t) exp [-i (2xfct+$0)]...-(
7) However, a(t) is a change in bias, b (t) is a change in amplitude, fc is a signal frequency, φ0 is an initial phase, and * is a complex conjugate.

Next, as a second step, an appropriate window function (rectangular, Hamming, Hamming, Gaussian, etc.) is superimposed on this signal data, and the frequency is analyzed using known FFT calculation processing to obtain the Fourier spectrum (Figure 7). . Although the power spectrum is shown in FIG. 7, this is for ease of explanation, and in reality, the subsequent processing is performed on the Fourier spectrum. There are three spectral components in this figure, and the DC component at the zero frequency in the middle corresponds to the Fourier transform of a (t) in equation (7), and the positive signal component on the right side is the second term. , the negative signal component on the left corresponds to the third term.

As the third step, we perform Fourier analysis using a weighting function (rectangular, Hamming, Hamming, Gaussian, etc.) to leave only the rightmost of these spectral components.
A filter is applied to the spectrum and IFFT (inverse Fourier transform) calculation processing is performed.

As a result, the second term of equation (7) is r'(t) = (1/2) b (t) exp [i (2-ftfct+φ0)1-(8) is obtained.

As the fourth step, we calculate the complex logarithm of I'(t) obtained in this way: log [I'(t)] =1 og [(1/2)b(t)] +1(2xf
ct+$0) = 19), and the phase component of the interference fringe change signal is obtained in its imaginary part.

In this way, the phase of the fringe change signal can be found using the Fourier transform method, but when the imaginary part of the complex logarithm is calculated using a computer, the answer can only be found in the range from -π to π. Therefore, since parts of the phase that are integral multiples of 2π cannot be determined, the absolute value of the phase cannot be determined. Therefore, equation (6) cannot be applied as is. However, at this time, since a relative value is obtained, the rate of change in phase can be known. If we differentiate equations (4) and (5) with respect to time, we get dφ's/d
t= (2xK/λ20) L s - (10)dφ
'r/dt= (2iK/e20) L r - (1
1), so when formula (i o) is divided by formula (11).

(dφ's/dt)/(dφ'r/dt)=L s
/ L r, that is, Ls = [(d * 's/d t) / (d #'r/
d t)] Lr-(121) From these, the measurement distance Ls can be determined from the phase change rate dφ's/8 dt and dφ'r/dj. As an example, the measurement distance Ls is shown in FIG. A diagram of the phase obtained by analyzing the interference fringe change signals obtained from two reference interferometers using the Fourier transform method is shown, and FIG. 11 shows a diagram of the phase change rate of the same signal.

As shown above, the phase or phase change rate of the interference fringe change signal can be obtained using a reference interferometer with a known optical path difference.
Since the measurement distance is determined from the ratio of the comparison with that of the measurement interferometer, accurate length measurement can be performed without measuring parameters such as the oscillation wavelength and wavelength scanning width of the semiconductor laser.

In addition, although the explanation above was made using a Michelson type interferometer,
It can also be applied to other interferometers such as the Fizeau type, Twyman-Green type, and Matsuha-Zehnder type.

In addition, the beam splitter 5.6.14 shown in Figure 1 includes a cube beam splitter, a wedged half mirror
etc. A polarizing beam splitter may be used as the beam splitter 5.6.14, and in that case, a quarter-wave plate is inserted between each reflecting mirror. This eliminates the return light to the light source side, stabilizes the oscillation wavelength of the semiconductor laser element 3, and enables accurate measurement.

Although the structure for measuring the optical path length of the movable mirror and the fixed mirror that form interference fringes has been described above, by replacing these reflecting members with various members to be measured, such as rough surfaces,
Needless to say, it can perform various length measurements. As an example,
In an ophthalmological measuring device, a similar technique can be implemented when measuring the axial length of the eye through observation of interference fringes formed by reflected light having different optical path differences at the cornea and the fundus.

[Effects of the Invention] As explained above, in the present invention, a measurement interferometer is used for each point of a large number of sample data of interference fringe change signals obtained in one measurement by phase analysis using the Fourier transform method. Since it is possible to compare the phase change rate of the reference interferometer with that of the reference interferometer, highly accurate measurement can be performed even with a single measurement by averaging. Furthermore, even if the semiconductor laser oscillation wavelength fluctuates due to changes in temperature or the like when sampling the interference fringe change signal, it can be canceled.

[Brief explanation of the drawing]

Figure 1 is a block diagram showing the configuration of a distance measuring device using optical interference that employs the present invention, Figure 2 is a diagram showing the wavelength characteristics depending on the injection current of a semiconductor laser element, and Figure 3 is a diagram showing the wavelength characteristics depending on the injection current of a semiconductor laser element. Waveform diagram showing the injection current waveform of the semiconductor laser device, No. 4
The figure shows the waveform of the interference fringe change signal obtained from the reference interferometer, Figure 5 shows the waveform of the interference fringe change signal obtained from the measurement interferometer, and Figure 6 shows the time signal of the interference fringe change signal. A diagram showing the waveform. Figure 7 is a diagram of the power spectrum of the interference fringe change signal. Figure 8 is a diagram of the power spectrum after filtering. Figure 9 is a diagram of the phase of the interference fringe change signal. Figure, 1st
0 is a diagram of the phase of the interference fringe change signals obtained from the two interferometers, and FIG. 11 is a diagram of the phase change rate of the interference fringe change signals obtained from the two interferometers. 3...Semiconductor laser element 1 7.8.15...Fixed mirror 10--Reference interferometer 16--Movable mirror 17...Measurement interferometer 21...Computer #l + Female Ihii Atsushi No. nil engraving Figure 6 Rare Anonymization I Divination Number Nogufusupef) + L Polished Nail 2 Figure 7 Figure 8 Procedural Amendment (Voluntary) December 1999 1゜2゜

Claims (1)

[Claims] 1) In a distance measuring method in which a reflective object is irradiated with a laser beam, the reflected light from the reflective object is interfered with to form interference fringes, and a distance is measured from the interference fringes, the laser beam is fixed. I did 2
The laser beam is irradiated onto two reflective objects, one of which is fixed, and the other is movable, to form a first interference fringe with the reflected light from each reflective object. forming a second interference fringe by changing the wavelength of the laser beam, temporally changing the first and second interference fringes, and determining the first and second interference fringes from the time-varying signal of the interference fringe. Find the phase or phase change rate of the two interference fringes, and calculate the optical path difference between the two reflective objects that formed the second interference fringe with reference to the optical path difference between the two reflective objects that formed the first interference fringe. A distance measuring method characterized in that the distance is determined by comparing phases or phase change rates. 2) The distance measuring method according to claim 1, characterized in that the time-varying signals of the interference fringes are subjected to Fourier exchange, and the phase or phase change rate of each interference fringe is determined by phase analysis. 3) The distance measuring method according to claim 1 or 2, wherein the phase analysis method is a Fourier transform method. 4) The laser beam is formed by a semiconductor laser, and the wavelength of the laser beam is changed by supplying an injection current as a periodic signal having a predetermined amplitude and a predetermined frequency. The distance measuring method according to item 2 or 3. 5) In a distance measuring device that irradiates a reflective object with a laser beam, causes interference of the reflected light from the reflective object to form interference fringes, and measures distance from the interference fringes, a laser light source that generates a laser beam; A reference interferometer that irradiates light onto two fixed reflective objects to form interference fringes with the light reflected from each reflective object, and a reference interferometer that irradiates laser light onto two reflective objects, one of which is fixed and the other movable. a measuring interferometer that forms interference fringes by reflected light from each reflecting object; a means for changing the wavelength of a laser beam to temporally change the interference fringes; and receiving interference fringes from the reference interferometer. means for receiving interference fringes from the measurement interferometer to form an interference fringe change signal; and means for receiving interference fringes from the measurement interferometer to form an interference fringe change signal; means for calculating the phase or phase change rate of the interference fringes; A distance measuring device characterized in that the distance is determined by comparing rates.