JPH0785007B2 - Length measuring instrument - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial field of application> The present invention makes it possible to perform length measurement with high accuracy and high resolution in units of wavelength by utilizing interference of laser light.
The present invention relates to a wavelength device that can obtain an absolute measurement output.

<Prior Art> First, as an example of an absolute length measuring instrument to which the present invention is applied, Japanese Patent Application No. 60-277380 previously proposed by the present applicant will be briefly described with reference to FIG. In this proposal, a wavelength device using Michelson's interference optical system switches two or more lights with different wavelengths and sequentially measures the phase delay of the light according to the distance to the measurement target. The distance to the measurement target is obtained from the relationship between the phase delay amount and the phase delay amount. In the figure, 1 is a laser light source that selectively generates a plurality of coherent light beams (λ _{1 to} λ ₄ ) having different wavelengths, HMR is a half mirror, PD1 is a photodetector, and CC1 and CC2 are cube corners. Cube corner CC1 is a cube corner on the wavelength side that moves according to wavelength operation, and cube corner CC2 is a cube corner on the reference side that is fixed at a fixed distance. AOM is an acousto-optic modulator (hereinafter referred to as AO modulator) that modulates the reference measurement light to detect the amount of optical phase delay, 6 is a modulation signal source that drives the AO modulator at a constant frequency fb, 4 Is a phase detector that detects the amount of phase delay included in the output of the photodetector PD1. The length measuring device configured in this way utilizes Michelson interference of a laser.

The laser light source 1 is composed of, for example, a light source having a constant wavelength and a wavelength shifter (not shown) that shifts the wavelength by an arbitrary amount, and sequentially generates light having an arbitrary wavelength. The arithmetic circuit 5 determines the distance between the cube corners CC1 from the relationship between the wavelength of the light used for measurement and the amount of phase delay at that time.

In the length measuring device configured in this way, the angular frequency of light is ω, and the modulation angular frequency of the AO modulator AOM is ωb (= 2πf
b) and the amplitude Vo of the light emitted from the laser light source 1 is Vo = sinωt (1), the amplitude V1 of the light modulated by the AO modulator AOM1 is V1 = sin (ωt + ωb) t (2) Cube corner The amplitude V2 of the light returning via CC1 is V2 = sin (ωt + φ) (3). Note that φ is a phase delay amount that occurs corresponding to the difference in optical path length between the reference side optical path and the length measuring side optical path.

On the photodetector PD1, the two lights as shown in the above equations (2) and (3) are superposed, so that the amplitude of the incident light is V1 + V2 = sin (ω + ωb) t + sin (ωt + φ) = 2sin (ω + ωbt / 2 + φ) cos {(ωbt −φ) / 2} (4) is the sum of V1 and V2. Where the photo detector
Since the output of PD1 is proportional to the square of the amplitude of the incident light,
Theoretically, (V1 + V2) ² = 4sin {(ω + ωb / 2) t + φ} ・ cos {(ωbt −φ) / 2} (5), but the photodetector PD1 cannot respond to the optical frequency and the average value is The output Vp is Vp = 2 + 2cos (ωbt−φ) (6).

Therefore, if the modulation angular frequency ωb in the AO modulator AOM1 is known, the phase delay amount can be calculated from the value of the output Vp of the photodetector PD1.

Similarly, the distance to the measurement target can be obtained by sequentially measuring the amount of phase delay corresponding to different wavelengths and solving these measurement results as simultaneous equations.

<Problems to be Solved by the Invention> By the way, in the above-described length measuring instrument, when the external environment (for example, temperature or vibration) of the wavelength instrument changes, the phase difference distribution of the synthesized laser is inclined, Photo detector
PD1 output changes. As a result, an error occurs in the difference from the phase of the signal from the AO modulator AOM1 as the reference.

5 (a), (b), and (c) show the phase A of the drive signal of AOM1 as a reference (this has no spatial phase distribution because the electric signal is directly input to the phase difference detector). 3A and 3B are schematic diagrams showing the relationship between the slope B of the phase difference of the light incident on the photodetector PD1 and the phase difference C of A and B. In the figure, (a) shows a state in which the measurement distance is zero and the phase of B is not inclined. In this case, the phase difference between the wavelengths a and b shown in C is zero (a and b are superposed).

(B) shows the condition when the cube corner CC1 on the length measuring side is moved by an arbitrary distance, and it shows the case where the above-mentioned displacement or air sway did not occur on the CC1 side. The phase difference φ corresponding to can be detected.

(C) is the case where the phase difference of the composite wave incident on PD1 has a slope θ due to air turbulence, etc. This slope causes the center of rotation of the phase difference to shift, and the output of PD1 changes by Δφ. , The phase difference that should be φ is φ ₁ . There is a problem in that the inclination of the phase difference due to such air fluctuations causes an error in the measured value.

The present invention has been made in view of the above-mentioned problems of the prior art, and a table for calculating a correction value from the relationship between the inclination of the phase difference and the measurement error is provided in the arithmetic device, and the output of a plurality of photodetectors is used. The purpose is to perform accurate measurement by calculating and correcting the slope of the phase difference.

<Means for Solving the Problems> The configuration of the present invention for solving the above problems is such that at least two or more laser lights having different wavelengths are switched and the phase delay of the light according to the distance to the object to be measured. The Michelson interference optics system is used to measure the quantity sequentially using a photodetector and a phase difference detector, and to calculate the distance to the measurement target in the previous period from the relationship between these wavelengths and the amount of phase delay using an arithmetic unit. In the length measuring instrument used, the photodetector divides its light-receiving surface into a plurality of light receiving surfaces, and the phase difference detector detects the phase difference distribution of the laser light incident on the photodetector based on the output signals of the plurality of photodetectors. It is characterized in that the calculator detects the inclination and corrects the wavelength value according to the inclination of the phase difference distribution.

<Embodiment> FIG. 1 is a block diagram showing an embodiment of the length measuring device of the present invention. In FIG. 1, the same components as those in FIG. 4 are designated by the same reference numerals and the description thereof will be omitted.
In 51, for example, a table for calculating a correction value from the relationship between the slope of the phase difference distribution and the phase drift as shown in FIG. 2 is provided in advance. Further, as shown in the figure, the shape of the photodetector PD1 is divided into four E, F, G, and H to be independent, and they are arranged facing each other.

In the above configuration, each photo detector (E to
The signal H) is output to the phase difference detection device 41, the sum of the outputs is calculated, and from these signals, for example, E-
The F, G-H calculations are performed to calculate the slopes ψ ₁ , ψ ₂ of the phase difference.

These calculation results are sent to the calculation device 51, and the distance to be measured is calculated based on the signal of the sum and the signal from AOM1, but before that, the distance calculated using the photo detectors (E to H) is calculated. The signal of the total sum is corrected from the inclinations ψ ₁ and ψ _{2 of the} phase difference using the correction table.

In the present embodiment, an example in which one photodetector divided into four parts is used has been described. For example, the length measurement having the configuration shown in FIG. 3 which the applicant of the present invention has already filed as Japanese Patent Application No. 62-184270. It can also be applied to vessels. According to such a configuration, the light having different wavelengths emitted from the two lasers passes through CC1 and CC2 in opposite directions, and the phase difference of one signal is applied to the reference side (for example, PD1) and the phase difference of the other. The signal is received on the length measurement side (PD2), and the distance is calculated from the phase difference of those phase difference signals, but in such a case, drift due to air sway, etc., cannot be avoided.

Therefore, even in such a case, a correction table for the gradient of the phase distribution and the amount of phase drift is provided in the arithmetic unit, and each photodetector is divided into a plurality (four in the figure), and PD1, PD1, By obtaining the slope of the phase distribution of the composite wave incident on PD2 and compensating for the phase difference based on the relative difference in the slope, it is possible to realize a length-measuring device without error.

Note that the length measuring device is not limited to that of the above-described embodiment, and a Michelson interference optical system is used, and other than that shown in this embodiment as long as the photodetector receives the phase difference of the composite wave. It is also applicable to.

Further, the photodetector is divided into four, but in principle it may be divided into two, and may be divided into six and eight. In such a case, the inclination can be measured with higher accuracy.

<Effects of the Invention> According to the present invention as specifically described above with reference to the embodiments, the inclination of the phase difference distribution is obtained by a plurality of photodetectors, and the relationship between the inclination of the phase difference distribution and the drift amount obtained in advance is obtained. Since the length measurement value is corrected according to, it is possible to realize a length measuring device in which output drift due to temperature and vibration is corrected.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the length measuring device of the present invention, and FIG.
FIG. 3 is a diagram showing the relationship between the slope of the phase difference distribution and the amount of drift, and FIG. 3 is a configuration diagram showing an example in which the present invention is applied to another length measuring device.
FIG. 4 is a diagram showing a conventional example, and FIG. 5 is a schematic diagram for explaining the relationship between the inclination of the wavefront and the drift. 1,2 …… Laser light source, 41 …… Phase difference detector, 51 …… Computer, AOM …… Acousto-optic modulator, CC1, CC2 …… Cube corner, HMR …… Half mirror, PD1, PD2 …… Photo detector .

Claims (1)

[Claims] 1. At least two or more laser beams having different wavelengths are switched to sequentially measure a phase delay amount of light according to a distance to a measurement target by using a photodetector and a phase difference detection device, and these wavelengths are also measured. In the length measuring device using Michelson's interference optical system, the distance to the measurement object is calculated from the relationship between the phase delay amount and the phase delay amount, and the photodetector divides its light-receiving surface into a plurality of parts. At the same time, the phase difference detection device detects the slope of the phase difference distribution of the laser light incident on the photodetectors based on the output signals of the plurality of photodetectors, and the arithmetic unit measures the slope according to the slopes of the phase difference distribution. A length measuring instrument characterized by being configured to correct a long value.