JPH0695003B2 - Length measuring instrument - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention is capable of performing highly accurate and high-resolution length measurement in units of wavelength by utilizing the interference of laser light, and is also an absolute measurement. The present invention relates to a length measuring instrument that can obtain a long output.

[Conventional technology]

Conventionally, a high-precision length-measuring device that uses the interference of light is called an incremental type, and the pulse signal obtained according to the movement amount (displacement of interference fringes) of the surface to be measured is integrated and counted. The displacement of the target surface is obtained. Therefore, if the power is cut off during the length measurement operation,
Even if the power is turned on again, the measured amount up to that point is reset, and the measured value after that becomes completely meaningless.

In order to solve such a problem, the applicant of the present application has already proposed, as Japanese Patent Application No. 60-277380, a length measuring instrument capable of obtaining an absolute length measuring output. This is a length measuring instrument using Michelson's interference optical system, in which at least two or more different wavelengths of light are switched to sequentially measure the phase delay amount of each light according to the distance to the measurement target, The distance to the measurement target is obtained from the relationship between the wavelength and the phase delay amount.

[Problems to be solved by the invention]

However, in such a length measuring device, a simultaneous equation is set up from the relationship between a plurality of wavelengths and the amount of phase delay, and the distance to the measurement target is calculated. Must measure the phase delay amount with respect to the light of each wavelength with high accuracy, and the phase measuring means becomes complicated.

INDUSTRIAL APPLICABILITY The present invention eliminates the above-mentioned drawbacks of the conventional device, can obtain an absolute length measurement output, and realizes a length measuring device with a simple configuration that can perform length measurement with high accuracy and high resolution. This is the purpose.

[Means for solving problems]

The length measuring device of the present invention utilizes Michelson's interference optical system to measure the amount of phase delay of each light according to the distance to the measurement target by using two lights of different wavelengths and And the amount of phase delay, the length measuring device determines the distance to the object to be measured, and the difference between the respective amounts of phase delay of the two lights on the interference surface in the interference optical system is always a constant value. The phase delay amount of one or both of the two lights is changed so that the distance to the measurement target is determined by the frequency difference between the two wavelength lights and the set phase difference. It is calculated.

[Action]

In this way, when the wavelength difference (frequency difference) of the two lights is changed so that the difference between the phase delay amounts on the interference surface becomes a constant value, the two light beams are changed in accordance with the distance to the measurement target. Since the length of the synthetic wavelength by light is changed in a fixed relationship, the subsequent arithmetic processing becomes easy. Further, since it is only necessary to measure the difference in the amount of phase delay between the lights of two wavelengths, the configuration of the phase detecting means can be simplified. In particular, when the difference in the amount of phase delay is set to 0 or 2π, the absolute value of the amount of phase delay with respect to the light of each wavelength does not matter, so the phase detection means can be configured by a simple circuit such as a mixer. it can.

〔Example〕

FIG. 1 is a block diagram showing an embodiment of the length measuring device of the present invention. In the figure, 1 is a laser light source that emits coherent light, and 2 is a frequency shift to the light emitted from the laser light source 1, which is divided into two lights having different wavelengths (frequency) and the wavelength difference An acousto-optic modulator (hereinafter abbreviated as AO modulator) that is controlled arbitrarily, 3 is a voltage controlled oscillator (hereinafter abbreviated as VCO) that drives this AO modulator 2, 4
Is a λ / 2 plate that rotates the polarization plane of one light by 90 °, 5 is a mirror, 6 and 7 are half mirrors, 8 is a cube corner on the length measuring side that moves according to the length measuring operation, and 9 is a fixed distance Cube corner on the reference side fixed to, 10 is an AO modulator that modulates the light on the reference side for heterodyne detection of the phase delay amount of the light, 11 is a modulation signal that drives the AO modulator 10 at a constant angular frequency Δω A source, 12 is a polarization beam splitter that transmits or reflects light depending on the direction of the plane of polarization, 13 and 14 are photodetectors, and 15 is a photodetector 13 or 14, which detects the amount of phase delay in light of two wavelengths. It is a phase detection circuit that generates a signal Vθ according to the difference in the amount of phase delay.
Here, the AO modulator 2 and the VCO 3 constitute a phase varying means for two lights, and by changing the drive frequency of the AO modulator 2, the wavelength difference (frequency difference) between the two lights is changed, The amount of phase delay in each light is changed. The output signal Vθ of the phase detection circuit 15 is fed back to VCO3, and the oscillation frequency of VCO3 is controlled so that the difference in the amount of phase delay between the two lights becomes an arbitrary constant value Δθ. Reference numeral 16 is an arithmetic circuit for obtaining the distance to the cube corner 8 from the relationship between the wavelength difference (frequency difference Δf) of light according to the drive frequency of the AO modulator 2 and the set phase difference Δθ at that time, 17
Is a display for displaying the calculation result, and 18 is the set phase difference Δθ.
Is a digital-to-analog converter that generates an offset signal VΔθ in order to change

The lights divided into two wavelengths λ1 and λ2 by the AO modulator 2 are superimposed by the λ / 2 plate 4 and the half mirror 6 with their polarization planes orthogonal to each other, and are composed of the half mirror 7 and the cube corners 8 and 9, etc. It is incident on the interference optical system. In addition, the light that has passed through this interference optical system is polarized beam splitter.
The light is separated by 12 according to the direction of the polarization plane, and enters the photodetectors 13 and 14, respectively. That is, the wavelength λ
The light of No. 1 enters the photodetector 13 after passing through the interference optical system, and the light of the wavelength λ2 enters the photodetector 14 after passing through the interference optical system. Therefore, in the length measuring device configured in this way, two lights having different wavelengths enter the photodetectors 13 and 14 without interfering with each other, so that light having different wavelengths (λ1, λ2) Therefore, it can be considered that independent measurement systems are respectively configured, and the measurement of the phase delay amount with respect to the light of two wavelengths (λ1, λ2) can be simultaneously performed under the same condition. Therefore, it is possible to realize a length measuring instrument that is hardly affected by fluctuations of air in the measurement optical path.

Now, with respect to the two lights of wavelengths λ1 and λ2 that are incident on the interference optical system, their amplitudes are respectively V ₁ = A ₁ sin (ω ₁ t + φ ₁ ) V ₂ = A ₂ sin (ω ₂ t + φ ₂ ) φ ₁ , φ When ₂ is set to the initial phase, in the light of V _1, the amplitude V ₁₁ on the interference plane passing through the interference optical system d ₁ : Reference side optical path length d ₂ : Measurement side optical path length. If this is received by the photo detector 13 and only the AC component is taken, Becomes

Similarly, for the light of V _{2, when} the output obtained from the photodetector 14 is obtained, Becomes

Therefore, V ₁₂ , V ₂₂ detected by the phase detection circuit 15
The phase difference θ of And from this, C: speed of light Δf: frequency difference between two lights Λ: synthetic wavelength Is required.

Here, the output Vθ of the phase detection circuit 15 is fed back to VCO3 together with the output VΔθ of the digital-analog converter 18, so that the phase difference θ becomes equal to the set phase Δθ.
Since the drive frequency of is changed, the distance from the oscillation frequency of VCO3 to the measurement target can be obtained. That is, when the oscillation frequency of the VCO 3 is determined, the difference (frequency difference Δf) between the two wavelengths λ1 and λ2 generated by the AO modulator 2 is determined. Therefore, the arithmetic circuit 16 determines the frequency difference Δf and the set phase difference Δθ. By substituting into the equation (4), the distance to the measurement target can be obtained.

When Δθ = 2π, (d ₁ −d ₂ ) = C / 2Δf, and if Δf is calculated from the driving frequency of the AO modulator 2,
The distance to the measurement target can be calculated immediately. At this time, the phase detection circuit 15 only needs to detect the difference in the amount of phase delay between the light of two wavelengths, and the absolute value of each amount of phase delay does not matter. Therefore, the phase detection circuit 15 can be replaced by a simple circuit such as a mixer. Can be configured. In such a case, since it is not necessary to superimpose the offset signal VΔθ on the output Vθ of the phase detection circuit 15, the digital / analog converter 18 can be omitted.

Thus, the difference in the amount of phase delay between the two wavelengths is
By controlling the difference of two wavelengths (frequency difference) so that the phase difference becomes a constant value, the length of the combined wavelength of the two lights can be changed according to the distance to the measurement target. The distance to the measurement target can be easily known from the frequency difference at this time.

2 and 3 are configuration diagrams showing another embodiment of the length measuring device of the present invention. In the figure, the same parts as those in FIG. 1 are designated by the same reference numerals.

The length measuring device shown in FIG. 2 uses an optical crystal 19 such as a quartz crystal formed in a tapered shape as a means for varying the phase delay amount (phase of the combined wavelength) between two lights. By moving this, the phase delay amount of each light is changed. That is,
By moving the optical crystal 19 and changing the incident position of light,
Since the length of the optical crystal 19 inserted in the optical path changes,
The optical path length d ₁ on the reference side can be changed equivalently, and the phase delay amount of light can be changed. Further, in the length measuring device in the figure, the interference fringes are formed by the light passing through the interference optical system, and the positions of the interference fringes that change according to the phase delay amount of this light are detected by using the photodiode arrays 21 and 22. I try to detect it. That is, by slightly tilting the polarization plane of the light returning from the cube corner 8 and the polarization plane of the light returning from the cube corner 9, interference fringes can be formed on the photodiode arrays 21 and 22. Therefore, by detecting the position (movement) of the interference fringes, the amount of phase delay in light of each wavelength can be measured.

Generally, in the photodiode arrays 21 and 22, by scanning the photodiode elements forming the photodiode arrays 21 and 22 at a constant speed, the photodiode arrays 21 and 22 themselves can be provided with a spatial filter characteristic, and the AO modulator or the like is provided. The position (movement) of the interference fringes according to the phase delay amount can be easily detected without using. Reference numeral 23 is a drive signal source for scanning the photodiode arrays 21 and 22 at a constant speed. Reference numeral 24 is an oscillator that drives the AO modulator 2 at a constant frequency.

In addition, the length measuring device shown in FIG.
Phase shift circuit for changing phase of drive signal in modulator 2
25 is used. That is, in the AO modulator 2, when the phase of the drive signal is changed, the diffracted light other than the 0th order undergoes a phase shift according to the phase of the drive signal, so that the phase of the combined wavelength can be changed.

Further, in the example of FIG. 3, as a means for detecting the amount of phase delay of light, the mirror 26 arranged on the reference side is vibrated (displaced) at a constant frequency by the piezoelectric element 27, and this vibration frequency is used. Heterodyne detection of the phase delay amount of light.

In the above description, the laser light source 1 for one wavelength is
Although the case where two lights having different wavelengths (λ1, λ2) are generated by the AO modulator 2 is illustrated, the means for generating two lights having different wavelengths is not limited to this. Also, λ
/ 2 plate 4 and polarization beam splitter 12 are used to measure the amount of phase delay for two wavelengths of light at the same time, but the measurement of the amount of phase delay for each light is not superposed. The same operation can be performed by alternately performing. Further, the combination of the phase varying means and the phase delay amount measuring means is not limited to the illustrated contents.

〔The invention's effect〕

As described above, the length measuring device of the present invention uses Michelson's interference optical system and uses two lights having different wavelengths to determine the phase delay amount of each light according to the distance to the measurement target. In a length measuring device for measuring and determining a distance to the measurement target from the relationship between the wavelength and the amount of phase delay, in the interferometric surface of the interference optical system,
The phase delay amount of one or both of the two lights is changed so that the difference between the phase delay amounts of the two lights is always a constant value. Since the distance to the measurement target is calculated from the difference and the set phase difference, the length of the combined wavelength of the two lights can be changed to a fixed relationship according to the distance to the measurement target. By simply measuring the difference in the amount of phase delay between two wavelengths of light, an absolute length measurement output can be obtained, and a length measuring device that can perform high precision and high resolution length measurement can be configured with a simple configuration. Can be realized.

[Brief description of drawings]

1 to 3 are block diagrams showing an embodiment of the length measuring device of the present invention. 1 ... Laser light source, 2, 10 ... AO modulator, 3 ... Voltage controlled oscillator, 4 ... λ / 4 plate, 5, 26 ... Mirror, 6, 7 ... Half mirror,
8, 9 ... Cube corner, 11 ... Modulation signal source, 12 ... Polarization beam splitter, 13, 14 ... Photo detector, 15 ... Phase detection circuit, 16 ... Arithmetic circuit, 17 ... Display, 18 ... Digital / analog converter, 19 ... optical crystal, 20 ... driving unit, 21,22 ... photodiode array, 23 ... driving signal source, 24 ... oscillator,
25 ... Phase shift circuit, 27 ... Piezoelectric element.

Claims (1)

[Claims] 1. A Michelson interference optical system is used to measure the amount of phase delay of each light according to the distance to the object to be measured using two lights of different wavelengths, and the wavelength and the amount of phase delay are also measured. In the length measuring device for obtaining the distance to the measurement target from the relationship with, the difference between the phase delay amounts of the two lights on the interference surface in the interference optical system is always a constant value. A length measuring instrument, wherein a phase delay amount of one or both of the two lights is changed.