JPH0641844B2 - 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 a phase delay amount, the distance measuring device is configured to obtain the distance to the measurement target,
The wavelengths of the two lights are changed so that the respective phase delay amounts of the two lights are always constant, and the wavelength difference between the two lights at this time and the set phase are set. The distance to the measurement target is calculated.

[Action]

In this way, when the wavelengths of the two lights are changed so that the respective phase delay amounts of the lights of the two wavelengths become constant values, the relationship between these wavelengths and the distance to the measurement target is always constant. Can be maintained and the subsequent arithmetic processing becomes easy. Further, since it is only necessary to measure a certain amount of phase delay with respect to the light of two wavelengths, it is possible to simplify the configuration of the phase detecting means. In particular, when the phase delay amount is set to 0 or 2π, the absolute value of the phase delay amount 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.

〔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 and 3 are acousto-optic modulators that give a frequency shift to the light emitted from the laser light source 1 to generate two lights with different wavelengths (frequency). (Hereinafter abbreviated as AO modulator), 4 and 5 are voltage controlled oscillators (hereinafter abbreviated as VCO) that drive the AO modulators 2 and 3, 6 is a mirror,
Reference numerals 7 and 8 are half mirrors, 9 is a shutter for selectively allowing two lights to enter the interference optical system, 10 is a cube corner on the length measuring side that moves according to the length measuring operation, and 11 is a fixed distance. Cube corner on the reference side, 12 is an AO modulator that modulates the light on the reference side for heterodyne detection of the phase delay amount of light, 13 is a modulation signal source that drives the AO modulator 12 at a constant angular frequency ω, and 14 is A photodetector, 15 detects a phase delay amount in the light of two wavelengths via the photodetector 14, and generates a signal Vθ corresponding to the phase delay amount, and 16 is an output Vθ of the phase detection circuit 15.
Is a switch that selectively feeds back to VCOs 4 and 5. Here, the AO modulators 2 and 3 and the VCOs 4 and 5 constitute wavelength tunable means for two lights.
By changing the driving frequency of No. 3, the wavelength (frequency) of the two lights is changed, and the phase delay amount of each light is changed. Further, the output signal Vθ of the phase detection circuit 15 is fed back to the VCOs 4 and 5 via the switch 16,
V is set so that the phase delay amount of the two lights becomes an arbitrary constant value θ.
The oscillation frequencies of CO4 and CO5 are controlled. Reference numeral 17 is a counter for measuring the difference between the driving frequencies of the AO modulators 2 and 3 according to the wavelength difference of light, and 18 is the cube corner 10 from the relationship between the wavelength difference of light (frequency difference Δ) and the set phase θ at that time. An arithmetic circuit for obtaining the distance to, 19 is a display for displaying the arithmetic result.

Now, the light whose wavelength is changed by the AO modulators 2 and 3 and which is divided into two by the half mirror 7 (λ
1, λ2) are selectively made incident on the interference optical system including the half mirror 8 and the cube corners 10 and 11 according to the state of the shutter 9. The light that has passed through this interference optical system is incident on the photodetector 14.

Now, assuming that the side a of the shutter 9 is open, the wavelength λ1 frequency-shifted by the AO modulator 2 is assumed.
Light will enter the interference optical system. The switch 16 is connected to the VCO 4 side. Here, assuming that the angular frequency of light is ω0, the amplitude V _{1 of} the light returning through the reference side optical path is V ₁ = sin {(ω0 + ω) t + φ ₁ } (1) φ ₁ = (ω0 + ω) d ₁ / C φ ₁ : phase delay amount C: speed of light d ₁ : the optical path length on the reference side. On the other hand, the amplitude V _{2 of} the light returning through the measurement-side optical path is V ₂ = sin (ω0t + φ ₂ ) (2) φ ₂ = ω 0 · d ₂ / C φ ₂ : Phase delay amount d ₂ : Optical path on the measurement side Be long. Therefore, the output P of the photodetector 14 on which these lights are superposed and incident is P = A [1 + cos (ωt + φ ₁ −φ ₂ )] = A [1 + cos {ωt + ω 0 (d ₁ −d ₂ ) / C + ω · d ₁ / C}] (3)

At this time, the phase detection circuit 15 compares the phase of the drive signal of the modulation signal source 13 with the phase of the output P. For example, the phase delay amount (φ ₁ −φ ₂ ) becomes 0, and ω0 (d ₁ −d ₂ ) / C = 2π (d ₁ −d ₂ ) / λ 1 = 2n ₁ π (4) n ₁ : When the oscillation frequency of the VCO 4 is changed so that it becomes an integer, the relationship between the wavelength λ1 of light and the distance to the measurement target. Becomes (d ₁ −d ₂ ) = n ₁ λ1 (5).

Similarly, switch the shutter 9 to the b side and switch 16 to V
If connected to the CO5 side, the oscillation frequency of the VCO 5 starts from direct current in accordance with the output Vθ of the phase detection circuit 15 and increases the wavelength λ2 from a value equal to λ1, (d ₁ −d ₂ ) = n ₂ λ2 (6) n ₂ : Locked to an integer value.

Therefore, from the above equations (5) and (6), (d ₁ −d ₂ ) = n ₁ λ1 = n ₂ λ ₂ (7) | n ₁ −n ₂ | = 1 is obtained. Therefore, the oscillation frequency of VCO4, 5
_{If 1} and ₂ are measured by the counter 17, the wavelength λ at this time is
1, it is possible to know exactly the _{λ2, (d 1 -d 2)} = C / - since (1 2) _{(8) 1,} the _second value the value of _(d 1 -d ₂₎ easily Can be calculated. At this time, the phase detection circuit 15
It suffices to detect whether or not the phase delay amounts of light of two wavelengths each have a constant value. Since the absolute value of each phase delay amount does not matter, the phase detection circuit 15 using a simple circuit such as a mixer. Can be configured.

Furthermore, when high-precision measurement output is required, λ1, λ
Since 2 is known and n ₁ and n ₂ can be determined from this, n ₁ , λ1 or n ₂ λ2 may be calculated by the arithmetic circuit 18.

In this way, the phase delay amounts at the two wavelengths λ1 and λ2 are fed back to the VCOs 4 and 5, respectively, and the oscillation frequencies of the VCOs 4 and 5 are adjusted so that the phase delay amounts are always constant.
_{When 1} and ₂ (wavelengths λ1 and λ2) are controlled, the wavelength λ
The relationship between 1 and λ2 and the distance to the measurement target can always be kept constant, and the distance to the measurement target can be easily calculated from the frequency difference at this time. Also, λ2 is λ
Since the value of 1 is continuously changed, | n ₁ −n ₂ |
Can always be 1, and an absolute length measurement output can be obtained.

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 is a device in which two light beams are made to have their polarization planes orthogonal to each other, superposed on each other, and simultaneously made to enter the interference optical system. In the figure, 20 is a λ / 2 plate that rotates the polarization plane of one light by 90 °, 21 is a polarization beam splitter that transmits or reflects light depending on the direction of the polarization plane, and 22 is a photodetector. The light split into two by the half mirror 7 is superimposed by the λ / 2 plate 20 and the half mirror 7 with their polarization planes orthogonal to each other, and is incident on the interference optical system. The light that has passed through the interference optical system is separated by the polarization beam splitter 21 according to the direction of the plane of polarization, and enters the photodetectors 14 and 22, respectively. That is, the light of wavelength λ1 enters the photodetector 14 after passing through the interference optical system, and the light of wavelength λ2 enters the photodetector 22 after passing through the interference optical system. Therefore, in the length measuring device configured in this way, two lights having different wavelengths enter the respective photodetectors 14 and 22 without interfering with each other, so that light having different wavelengths (λ1, λ2) Therefore, it can be considered that independent measurement systems are configured, and two wavelengths (λ
It is possible to simultaneously measure the amount of phase delay with respect to the light of 1, λ2) under the same conditions. Therefore, it is possible to realize a length measuring instrument that is hardly affected by fluctuations of air in the measurement optical path.

Further, in the example of FIG. 2, the AO modulators 2 and 3 are both inserted after the half mirror 7, and wavelengths λ1 and λ are independently provided.
The value of 2 is controlled. For example, in such cases, VC
If the OCOs 4 and 5 are started from the same frequency and the VCO 4 is changed in the direction of increasing the frequency and the VCO 5 is changed in the direction of decreasing the frequency, the oscillation frequency is locked at the first | n ₁ -n ₂ | Since the relation of = 1 is established, an absolute measurement output can be obtained.

Further, the length measuring device shown in FIG. 3 is the same as the length measuring device shown in FIG. 2 except that an interference fringe is formed by the light passing through the interference optical system, and the interference fringe changing depending on the phase delay amount of this light. The position is detected by using the photodiode arrays 23 and 24. That is, by slightly tilting the polarization plane of the light returning from the cube corner 10 and the polarization plane of the light returning from the cube corner 11, it is possible to form interference fringes on the photodiode arrays 23, 24. Therefore, by detecting the position (movement) of the interference fringes, the amount of phase delay in light of each wavelength can be measured.

In general, in the photodiode arrays 23 and 24, by scanning the photodiode elements constituting the photodiode arrays 23 and 24 at a constant speed, the photodiode arrays 23 and 24 themselves can be provided with a spatial filter characteristic, and an 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 25 is a drive signal source for scanning the photodiode arrays 23 and 24 at a constant speed.

In the above description, the laser light source 1 having one wavelength and the AO modulators 2 and 3 have two different wavelengths (λ1, λ).
Although the case where the light of 2) is generated is shown as an example, 2
The means for generating the two lights is not limited to this.
Further, the combination of the wavelength varying means and the phase delay amount measuring means is not limited to the illustrated contents.

〔The invention's effect〕

As described above, in the length measuring device of the present invention, the Michelson interference optical system is used, and two light beams having different wavelengths are used to measure the phase delay amount of each light beam according to the distance to the measurement target. In a length measuring device that measures the distance to the measurement target from the relationship between these wavelengths and the amount of phase delay, so that the amount of phase delay of each of the two lights is always a constant value, The wavelengths of the two lights are changed, and the distance to the measurement target is calculated from the wavelength difference between the two wavelengths of light at this time and the set phase. The wavelengths of the two lights can be changed in a fixed relationship according to the distance of, and an absolute measurement output can be obtained by simply measuring the wavelength difference between the lights of the two wavelengths. It can be realized by a simple structure length measuring device capable of performing the resolution measurement.

[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, 3, 12 ... AO modulator, 4, 5
...... Voltage controlled oscillator, 6 …… Mirror, 7,8 …… Half mirror, 9 …… Shutter, 10,11 …… Cube corner, 1
3 …… Modulation signal source, 14,22 …… Photo detector, 15 ……
Phase detection circuit, 16 ... Switch, 17 ... Counter, 18 ...
Computational circuit, 19 Display, 20 λ / 2 plate, 21 Polarizing beam splitter, 23, 24 Photodiode array, 25 Drive signal source.

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 that obtains the distance to the measurement target from the relationship with, the wavelengths of the two lights are changed so that the phase delay amounts of the two lights are always constant values. This is a length measuring instrument.