JPS62204103A - Length measuring instrument - Google Patents

Abstract

PURPOSE:To obtain a measured value close to a real value by making two light beams which differ in wavelength incident on an interference optical system one over the other, separating return light according to the direction of its plane of polarization, and receiving separated light beams by different measuring means respectively. CONSTITUTION:Laser light sources 11 and 12 emit light beams with wavelengths lambda1 and lambda2. A polarization beams splitter PBS1 and a lambda/2 plate 5 superpose both light beams one over the other so that the planes of polarization cross each other. The superposed light is incident on the interference optical system composed of cube corners CC1 and CC2, etc., through a half-mirror HMR1. The obtained interference light is split by a polarization beam splitter PBS2 according to the direction of its plane of polarization and the light beams with the wavelengths lambda1 and lambda2 are incident on photodetectors PD1 and PD2 respectively. The outputs of the photodetectors are inputted to a phase detecting circuit 3, which outputs a signal corresponding to the difference in phase delay between the light beams with the wavelengths. Thus, the quantities of phase delay are measured on th same conditions at the same time, so the influence of fluctuations of air, etc., is reduced, so that measurement output close to the real value is obtained even if affected.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention makes it possible to perform high-accuracy, high-resolution length measurement using wavelength as a unit by using laser light interference, and also to perform absolute measurement. This invention relates to a length measuring device that can obtain a long output.

[Conventional technology]

Conventionally, high-precision length measuring instruments that use optical interference are of the incremental type, which perform measurements by cumulatively counting pulse signals obtained according to the amount of movement of the surface to be measured (displacement of interference fringes). This method is designed to find the displacement of the target surface. Therefore, if the power is cut off during length measurement operation,
Even if the power is turned on again, the measured quantities up to that point will be reset, and subsequent measured values will be completely meaningless.

In order to solve these problems, the applicant of the present application has already proposed a sub-length instrument capable of obtaining an absolute length measurement output in Japanese Patent Application No. 277380/1983. In a length measuring device that uses Michelson's interference optical system, this method switches between at least two lights with different wavelengths and sequentially measures the amount of phase delay of the lights depending on the distance to the measurement target. The distance to the measurement target is determined from the relationship between the wavelength and the amount of phase delay.

FIG. 4 shows the configuration of this length measuring device.

In the figure, 1 is a laser light source that selectively generates multiple coherent lights with different wavelengths, HMRI is a half mirror, and AOMl is an acousto-optic modulator that modulates the light on the reference side to heterodyne detect the amount of phase delay of the light. Vessel [hereinafter referred to as A
2 is AO variable 1Jll device AQII
11 is a modulation signal source driven at a constant frequency fb, PCI is a photodetector, CC1 and CC2 are cube corners,
3 is a phase detection circuit that detects the amount of phase delay included in the output of the photodetector PCI, and 4 is an arithmetic circuit that calculates the distance to the cube corner CC1 from the relationship between the wavelength of the light used for measurement and the amount of phase delay at that time. It is. The laser light source 1 is constituted by, for example, a light source of a constant wavelength and a wavelength shifter that shifts the wavelength by an arbitrary amount, and sequentially generates light of an arbitrary wavelength. Further, the cube corner CCI is a cube corner on the length measurement side that moves according to the length measurement operation, and the cube corner CC2 is a cube corner on the reference side that is fixed at a constant distance.

In the length measuring device configured in this way, the angular frequency of the light emitted from the laser light source 1 is ω, and the amplitude Vo is Vo=sinωt
(1) and the modulation angular frequency in the AO modulation AOMI is ωb (=2wfb), then the AO modulator AO
Amplitude of first-order diffracted light of light changed by MI
1 is Vl=5in(ω+ωb)t
(2) becomes. Also, the amplitude v2 of the light returning via the cube corner CC1 is 'V2! 5jn(ωt+g) (
a). Note that φ is the amount of phase delay that occurs in response to the difference in optical path length between the reference side and sub-length fil+ optical paths.

On the photodetector PDI, the above (2), (
3) Since the two lights shown in the formula are multiplied by 1, the amplitude of the incident light is V1+V2-sin(ω+ωb)t+5in(ωt
+c6)-2sin(ωt + (ωbt+cb)/2
)・cost(ωbt-tb)/2) α
), it is the sum of vl and v2. Here, since the output of photodetector PD1 is proportional to the square of the amplitude of the incident light, theoretically (V1+V2)"=4 sin" (ra+t+
(ωbt + d))/2)・cos”((ωbt −
ds )/2) (5) However, the photodetector PDI cannot respond to the frequency of light and shows an average value, so its output Vp is VP! l + cos(ωbt −h)
(6) becomes.

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

Now, using the Michelson interference optical system, it is possible to measure the minimum phase lag that changes depending on the distance as described above, but the value of this minimum phase lag is (2frN+#): N is equivalent to a natural number, so the distance to the cube corner CCI cannot be immediately determined from the magnitude of the phase delay amount 6.

So, what about the figure? In the ItIl long instrument, the measurement target (cube corner ccl
) to find the distance.

Now, cube corner cc2 from half mirror HMRI
Assuming that the distance P1 to the corner CC1 is dl, and the distance from the half mirror HMRI to the cube corner CC1 is d2, the difference between these optical path lengths is 2dl-2d2-2d. Therefore, the wavelength of the light used for measurement is λ1.
λ2. λ3. Let λ4 (λ1 × λ2 × λ3 × λ4), and the amount of phase delay obtained at this time is i 1. Lb2. d)
Assuming 3°d+4' (0 to 2w for 61 to 4th grade), the following equation is established from each measurement result.

2d=nlλ1+λ1 d)1/2w (7
)2d=n2λ2+λ2small 2/2tr (8)
2d=n3 A3 +λ3 Small 3 /2w
(9) 2d=n4 A4 +λ4c64/2w
QΦn1 to TI4 are natural numbers, and from among these relational expressions, the above (7) and (8
) formula to find d, d = A 12 (nl - n2) + A12 (Φ1/λ1 - Φ2/λ2) (11) A
1 A2 2 In addition, A12- - A2 A1 Φl#λ1c61/2vr, Φ2-λ2m2/2τ. Here, A12 has two wavelengths λ1. It is the shell minor common wavelength at λ2, and if the value of A12 is selected to be equal to or larger than the measurement range, the above (11
) in the term A 12 (nl - n2) can be specified, and these wavelengths λ1. Phase delay amount corresponding to λ2 d)1. The distance d can be uniquely calculated from the second grade. In other words, the measurement range is the minimum public wavelength A12
If it is narrower, the minimum phase delay at this time is always between 0 and 2f, so there is a one-to-one correspondence between the distance fed and the phase delay amount 6, and the minimum phase ′an amount immediately changes to the distance i.
d can be specified. For example, set the measurement range to 0 to 1.
000mm, the minimum public wavelength A12 is 1000
The wavelength λ1. If the size of λ2 is selected, A 12(nl −n2) in the above equation (11)
The term becomes 0, and the distance ad can be uniquely calculated from the phase delay ff1tf, 1.62.

Next, the wavelength λ1 as described above. By adjusting λ2, the measured value d II is obtained, and the measurement accuracy (measurement error) at this time is
If the true value range for the measurement output d 1 m is found as d+o1IIIn<d+*<d+, max, then the next wavelength combination (^1.λ3) is within the true value range d r * m 1 n-d + t m
It is chosen so that aX is the measurement range [minimum public wavelength]. Therefore, the measurement range is narrowed down, and higher resolution measurement becomes possible.

In this way, using the above relationship, the wavelength combination (a
By selecting one wavelength (lower common wavelength) and sequentially narrowing down the measurement range, it is possible to obtain absolute measurement results over any measurement range, and it is also possible to increase the measurement resolution to the wavelength unit.

[Problem that the invention seeks to solve]

However, with the above-mentioned length measuring device, the wavelength of light is sequentially switched and the amount of phase delay at this time is measured.
This measurement data is used to calculate the distance to the measurement target, so if there are fluctuations in the air in the interference optical system or changes in the refractive index, the measurement conditions for each wavelength of light will be changed. This changes each time a measurement is made, making distance calculation extremely difficult.

The purpose of the present invention is to eliminate the drawbacks of the conventional design as described above, and to realize a sub-length instrument with a simple structure that can obtain absolute measurement output and is not easily affected by air fluctuations. This is what I did.

[Means for solving problems]

The length measuring device of the present invention utilizes a Michelson interference optical system to measure the amount of phase delay of the light depending on the distance to the measurement target using at least two or more lights of different wavelengths, and In a length measuring device that determines the distance to the measurement target from the relationship between the wavelength and the amount of phase delay, two lights with different wavelengths are superimposed with their polarization planes orthogonal, and the superposition is made to enter the interference optical system. means, a spectroscopic means for separating the light obtained through the interference optical system according to the direction of the polarization plane, and a spectroscopic means for receiving the separated light by the spectroscopic means and measuring the amount of phase delay in each light. The apparatus further includes a phase measuring means.

[For production]

In this way, if two lights with different wavelengths are superimposed using the difference in polarization plane and are made to enter the interference optical system,
Since the light returned via the interference optical system can be divided according to the direction of its polarization plane and received by separate measurement means, measurements for two wavelengths can be performed simultaneously. Therefore, if the distance is calculated using measurement data obtained at the same time, fluctuations and other fluctuations can be avoided.
To realize a length measuring device with a simple configuration that can obtain values close to the true value and is not easily affected by air fluctuations because calculations can be performed between measurement data that are equally affected by 'zllll. I can do it.

〔Example〕

FIG. 1 is a block diagram showing an embodiment of the sub-length device of the present invention. In the figure, the same parts as in FIG. 4 are designated by the same reference numerals. For example, 11.12 has a wavelength of λ1. λ2 becomes 2
A laser light source that emits two lights, 5 has a polarization plane of light of 90°.
λ/2 plate to be rotated, MRl is a mirror, PBSI,
PH10 is a polarizing beam splitter that transmits or reflects light depending on the direction of the polarization plane, and PO2 is a photodetector. As shown in the figure, polarizing beam splitter PB
The SI, together with the λ/2 plate 5, constitutes a superimposing means (14) that multiplies two lights with different wavelengths with their polarization planes perpendicular to m, and the superimposed light undergoes interference formed by cube corners CC1, CC2, etc. It is incident on the optical system. Further, the polarizing beam splitter PBS2 is a spectroscopic means that separates the interference light obtained through the interference optical system according to the direction of the polarization plane, and the separated light is incident on the photodetectors PDI and PO2. Photodetector PD1. The outputs of the PDs 2 are respectively given to phase measuring means consisting of a phase detection circuit 3. That is, the light of wavelength λ1 emitted from the laser light source 11 passes through the interference optical system and then enters the photodetector PDl, and the light of wavelength λ2 emitted from the laser light source 12 passes through the interference optical system and then enters the photodetector PDl. The light will be incident on the photodetector PD2.

Therefore, in the length measuring device configured as described above, two lights of different wavelengths enter the respective photodetectors PCI and PO2 without interfering with each other, so that the lights of different wavelengths (λ1, λ2) On the other hand, it can be considered that each independent measurement system is configured. In addition, the operation in each measurement system is described in the fourth section above.
The device is similar to the one shown in the figure, and the phase detection circuit 3 outputs a signal corresponding to the difference in phase delay amount of light of each wavelength.

In this way, the length measuring device of the present invention has two wavelengths (λ1...
It is possible to simultaneously measure the amount of phase delay for the light of λ2) under the same conditions. Here, the amount of phase delay measured at the same time is equally affected by air fluctuations, etc., so if you use this measurement data to calculate the distance, even though it is affected by fluctuations, etc. Measurement output close to the true value can be obtained. Therefore, by repeating measurements using the same combination of wavelengths many times and averaging the obtained measurement outputs, it is possible to remove the effects of fluctuations and perform highly accurate measurements.

Furthermore, like the apparatus shown in FIG. 4 described above, by changing the combination of wavelengths, it is possible to narrow down the measurement range and obtain an absolute and high-resolution measurement output.

FIGS. 2 and 3 are configuration diagrams showing an embodiment of the length measuring device pond of the present invention.

The length measuring device shown in FIG. 2 uses an AO modulator AOM2 to convert light emitted from a laser light source 11 into two lights of different wavelengths (λ1, ^3), and a light shielding plate 7 to convert the light into two lights of different wavelengths. The combination of these is changed. That is, by using the zero-order and first-order diffracted lights of the AO modulator AOM2, lights with different wavelengths can be easily obtained. Moreover, by switching the position of the light shielding plate 7, the light with the wavelength λ2 emitted from the laser light source 12 can be made to enter the polarizing beam splitter PBSl instead of the light with the wavelength λ3, and the combination of wavelengths to be superimposed can be easily changed. can do.

6 is a lens for returning the diffracted light of the AO modulation modulation AOM2 to a parallel beam, and HMR2 is a half mirror.

Furthermore, in the example shown in FIG. 2, as a means for detecting the amount of phase delay of light, the cube corner CC2 on the reference side is connected to the piezoelectric element 8.
This vibration frequency is used to heterodyne detect the amount of phase delay of light.

The principle of distance measurement is the same as that of the apparatus shown in FIG.

In addition, in the example shown in FIG. 3, interference fringes are formed by light passing through an interference optical system, and the position of the interference fringes, which changes depending on the amount of phase delay of this light, is controlled by a photodiode array PDAI.
, which is detected using PDA2. That is, by slightly tilting the wavefront of the light returning from the cube corner CC1 and the wavefront of the light returning from the cube corner CC2, interference fringes can be formed on the photodiode arrays PDAI and PDA2. By detecting the position (movement) of this interference fringe, it is possible to measure the amount of phase delay depending on the distance.

Generally, photodiode array PDAI, PDA
In 2, by scanning the photodiode elements that make up the photodiode elements at a constant speed, the photodiode arrays PDAI and PDA2 themselves can have spatial filter characteristics, and the phase delay can be achieved without using an AO modulator. The position (movement) of interference fringes depending on the amount can be easily detected.

In addition, in the above description, the laser light source 11 of − wavelength
and AO modulation modulation AOM2 produce two different wavelengths (λ1
．． Although the case where light of wavelength λ3) is generated has been exemplified, the means for generating a plurality of lights having different wavelengths is not limited to this. Furthermore, the number of wavelengths used for measurement is determined depending on the desired resolution, and is not limited to three or four.

〔Effect of the invention〕

As explained above, the length measuring device of the present invention utilizes the Michelson interference optical system to use at least two or more lights of different wavelengths, and the phase delay of the light is determined according to the distance to the measurement target. In a length measuring device, the distance to the object to be measured is determined from the relationship between these wavelengths and the amount of phase delay. ff15E means for inputting the light into the interference optical system; a spectroscopic means for separating the light returned via the interference optical system according to the direction of the polarization plane; Since it is equipped with a phase measuring means for measuring the amount of phase delay in light, it is possible to measure two wavelengths at the same time, and distances can always be calculated using measurement data obtained at the same time. By doing so, it is possible to obtain an absolute measurement output and to realize a length measuring device with a simple configuration that is not easily affected by air fluctuations.

[Brief explanation of drawings]

1 to 3 are block diagrams showing one embodiment of the length measuring device of the present invention, and FIG. 4 is a block diagram showing an example of the length measuring device already proposed by the applicant of the present invention. 1.11.12... Laser light source, 2... Modulation signal source, 3... Phase detection circuit, 4... Arithmetic circuit, 5...
λ/2 plate, 6...lens, 7...light shielding plate, 8...
Piezoelectric element, )IMRI, )1MR2...Half mirror-1PD1. PO2...Photodetector, AOM
I, AOM2...AO modulator, CCI, CC
2...Cube Corner, PBSI, PBS2...
・Polarizing beam splitter, MHI...mirror. Figure 1

Claims (1)

[Claims] Using a Michelson interference optical system, the amount of phase delay of each light is measured according to the distance to the measurement target using at least two or more lights with different wavelengths, and the relationship between these wavelengths and the amount of phase delay. The length measuring device is configured to determine the distance from the object to the measurement target, comprising: a superimposing means for superimposing two lights of different wavelengths with their polarization planes perpendicular to each other and inputting the superimposed light into the interference optical system; and a phase measuring means that receives the light separated by the spectroscopic means and measures the amount of phase delay in each beam. Length measuring device.