JPH0575241B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention utilizes laser interference to perform high-precision, high-resolution length measurement using wavelength as a unit, as well as absolute length measurement. This relates to a length measuring device that can obtain output.

[Conventional technology]

Conventionally, high-precision length measuring instruments that utilize optical interference are
This type is called an incremental type, and the distance to the surface to be measured is determined by cumulatively counting pulse signals obtained according to the amount of displacement of the surface to be measured (displacement of interference fringes). For this reason, if the power is cut off during the length measurement operation, even if the power is turned on again, the amount measured up to that point will be reset, and the subsequent measured values will become completely meaningless.

In order to solve these problems, the applicant of the present application has already proposed a length measuring device that can obtain an absolute length measurement output. In a length measuring device that uses Michelson's interference optical system, it switches between at least two lights of 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. 2 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, HMR1
is a half mirror, AOM1 is an acousto-optic modulator (hereinafter abbreviated as AO modulator) that modulates the light on the reference side in order to heterodyne detect the phase delay amount of light;
2 is a modulation signal source that drives the AO modulator AOM1 at a constant frequency fb, PD1 is a photodetector, CC
1 and CC2 are cube corners, 3 is a phase detector that detects the amount of phase delay included in the output of photodetector PD1, and 4 is a phase detector that detects the amount of phase delay included in the output of photodetector PD1. This is an arithmetic circuit that calculates distance. The laser light source 1 is composed of, 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. In addition, the cube corner CC1 is a cube corner on the length measurement side that moves according to the length measurement operation.
CC2 is a cube corner on the reference side 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, its amplitude V ₀ is V ₀ = sinωt (1), and the modulation angular frequency in the AO modulator AOM1 is ωb (=2πfb), the amplitude V1 of the first-order diffracted light of the light modulated by the AO modulator AOM1 is V1=siw(ω+ωb)t (2). Further, the amplitude V2 of the light returning via the cube corner CC1 is V2=sin(ωt+φ) (3). Note that φ is the amount of phase delay that occurs in response to the difference in optical path length between the optical paths on the reference side and the length measurement side.

On the photodetector PD1, the above (2) and (3)
Since the two lights are superimposed as shown in the equation,
The amplitude of the incident light is the sum of V1 and V2, as V1 + V2 = sin (ω + ωb) t + sin (ωt + φ) = 2 sin (ωt + ωbt / 2 + φ) cos {(ωbt - φ) / 2} (4). Here, the output of photodetector PD1 is 2 times the amplitude of the incident light.
Theoretically, (V1+V2) ² =4sin ² {(ω+ωb/2)t+φ}・cos
² {(ωbt−φ)/2}(5) However, since the photodetector PD1 cannot respond to the frequency of light and will show the average value,
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 φ can be calculated from the value of the output Vp of the photodetector PD1.

Now, using the Michelson interference optical system, it is possible to measure the phase delay amount φ that changes depending on the distance as described above, but the value of this phase delay amount φ is (2πN + φ): Since N is equivalent to a natural number, the distance to the cube corner CC1 cannot be immediately determined from the magnitude of the phase delay amount φ.

Therefore, in the length measuring instrument shown in the figure, by changing the length measurement used for measurement, sequentially measuring the amount of phase delay φ corresponding to each wavelength, and solving these measurement results as simultaneous equations, the measurement target ( I am trying to find the distance to Cube Corner CC1).

Now, the distance from half mirror HMR1 to cube corner CC2 is d1, half mirror HMR1
Letting the distance from to cube corner CC1 be d2, the difference in each optical path length is 2d1-2d2=2d. Therefore, the wavelengths of the lights used for measurement are λ1, λ2, λ3, λ4 (λ1<λ2<λ3<λ4), and the phase delays obtained at this time are φ1, φ2, φ3, φ4 (φ1
~φ4 is 0 to 2π), the following equation is established from each measurement result.

2d=n1λ1+λ1φ1/2π (7) 2d=n2λ2+λ2φ2/2π (8) 2d=n3λ3+λ3φ3/2π (9) 2d=n4λ4+λ4φ4/2π (10) n1 to n4 are natural numbers. Also, from among these relational expressions, the above ( 7) and (8) to find d: d=A12(n1−n2) +A12(Φ1/λ1−Φ2/λ2) (11) In addition, A12=λ1/2・λ2/2/λ2 /2−λ1/
2 Φ1=λ1φ1/2π, Φ2=λ2φ2/2π. Here, A12 is the least common wavelength of the two wavelengths λ1 and λ2, and if the value of A12 is selected to be equal to or larger than the measurement range, A12(n1−n2) in the above equation (11) The value of the term can be specified, and the distance d can be uniquely calculated from the phase delay amounts φ1 and φ2 corresponding to these wavelengths λ1 and λ2. In other words, if the measurement range is narrower than the least common wavelength A12, the phase delay amount φ will always be between 0 and 2π, so there will be a one-to-one correspondence between the distance d and the phase delay amount φ. ,
The distance d can be immediately determined from the phase delay amount φ. For example, when the measurement range is 0 to 1000 mm, if the sizes of wavelengths λ1 and λ2 are selected so that the least common wavelength A12 is 1000 mm, the term A12 (n1 − n2) in equation (11) above becomes 0, and the distance d can be uniquely calculated from the phase delay amounts φ1 and φ2.

Next, the measurement location d ₁₂ is obtained by combining the wavelengths λ1 and λ2 as described above, and from the measurement accuracy (measurement error) at this time, the true value range of the measurement output d ₁₂ is d ₁₂ min < d ₁₂ Assuming that <d ₁₂ max is obtained, the next wavelength combination (λ1, λ3) is selected so that the true value range d ₁₂ min to _{d 12} max is the measurement range (least common wavelength). . Therefore, the measurement range is narrowed down, and higher resolution measurement becomes possible.

In this way, by using the above relationship to select a combination of wavelengths (least common wavelength) and sequentially narrowing down the measurement range, it is possible to obtain absolute measurement results over any measurement range. At the same time, the measurement resolution can be increased to the wavelength unit.

[Problem that the invention seeks to solve]

However, with the above-mentioned length measuring instruments, the measurement range (least common wavelength) is narrowed down from a wide state to the wavelength unit by sequentially changing the combination of wavelengths, so in order to cover a wide measurement range, it is necessary to , many wavelengths (laser light sources) must be used.

The present invention eliminates the drawbacks of the conventional device as described above, makes it possible to obtain absolute measurement output, and uses a smaller number of wavelengths (laser light source).
The aim is to realize a length measuring device with a simple configuration that can cover a wide measurement range.

[Means for solving problems]

The length measuring device of the present invention switches at least two or more lights of different wavelengths to sequentially measure the amount of phase delay of the light according to the distance to the measurement target, and
In a length measuring device that determines the distance to the measurement target from the relationship between these wavelengths and the amount of phase delay, a modulation means for FM modulating the light emitted from the laser light source is provided, and the phase delay is adjusted according to each wavelength. Before measuring the quantity, FM modulate the light of any wavelength,
The distance to the measurement target is measured from the amount of phase delay of this FM modulation signal, and the measurement range is roughly narrowed down.

[Effect]

In this way, if you measure the distance to the measurement target using the FM modulation signal before narrowing down the measurement range based on the relationship between wavelength and phase delay amount, you can roughly narrow down the measurement range from this measurement result. Since distance measurement using a combination of wavelengths can be started from a narrow measurement range, it is possible to obtain absolute measurement output over any measurement range, and it is also possible to obtain a wide measurement output with a smaller number of wavelengths (laser light source). The measurement range can be covered.

〔Example〕

FIG. 1 is a configuration diagram showing an embodiment of the length measuring device of the present invention. In the figure, the same parts as in FIG. 2 are designated by the same reference numerals. AOM2 is an AO modulator that performs FM modulation on the light emitted from the laser light source 1;
5 is an FM modulation signal source that supplies the FM modulation signal Fm to the AO modulator AOM2, 6 is a lens for aligning the emission direction of the light modulated (diffracted) by the AO modulator AOM2, and 7 is a 0 in the AO modulator AOM2. Stoppers SW1 and SW2 are switches that block the next diffracted light.

The operation of the length measuring device configured as described above is as follows.

First, it is performed before a length measurement operation using a combination of wavelengths. A length measurement operation using an FM modulated signal will be explained. In this case, switches SW1 and SW2
are both connected to the contacts on the a side, and the AO modulator AOM
2 modulates the light with a modulation signal of Fm=Δω ₁ , sinΩt (12). At this time, if the angular frequency (wavelength) of laser light source 1 is fixed at ω ₀ , then
Amplitude V ₀ of light modulated by AO modulator AOM2
is V ₀ = sin (ω ₀ t + Δω ₁ /Ω・sinΩt) (13). Here, since the drive signal fb is not applied to the AO modulator AOM1, the amplitude V1 of the light incident on the photodetector PD1 via the optical path on the reference side is equal to V ₀ in equation (13), and the measurement The amplitude V2 of the light incident on the photodetector PD1 through the long optical path is V2=sin {ω ₀ t + Δω ₁ /Ω・sin (Ωt +φ1) +φ2} (14) where, φ1=4πd/λf=2Ωd/ c φ2=4πd/λ0=2d( _ω0 + _Δω1 sinΩt)/ _C2ω0d /Cλf: Wavelength of FM modulation period λ0: Wavelength of light ( _ω0 ) C: Speed of light. Therefore, the amplitude of the light incident on photodetector PD1 is the sum of V1 and V2 above, and if this is expanded in the same way as equations (4) to (6) above, the output Vp obtained from photodetector PD1 is Vp = K [ cos{−2Δω ₁ d/C・cos(Ωt+
φ1/2)-2ω ₀ d/C}+1] K: Becomes a constant. Here, the term cos(Ωt+φ1/2) is −1
Since it changes in the range of +1 from
The number of zero crosses per period of FM modulation changes depending on the value of 2Δω ₁ d/C. Therefore, if the value of Δω ₁ is known, the distance d can be determined from the output Vp.

In this way, once the distance d is measured using the FM modulated signal, the measurement range can be roughly narrowed down depending on the measurement accuracy, so from now on, wavelength combinations based on this measurement range will be used. A length measurement operation is performed. In this case, switch SW
1 and SW2 are both switched to the b side contact,
A drive signal fb is supplied to the AO modulator AOM1, and a drive signal Fm of the AO modulator AOM2 is fixed at a constant frequency.

The following operation is the same as that of the apparatus shown in FIG. calculate. In this case, the wavelength used for measurement is not the wavelength itself generated by the laser light source 1, but a value modulated by a certain amount by the AO modulator AOM2.

In this way, if you measure the distance to the measurement target using the FM modulation signal before narrowing down the measurement range based on the relationship between wavelength and phase delay amount, you can roughly narrow down the measurement range from this measurement result. As a result, distance measurement based on wavelength combinations can be started from a narrow measurement range, and resolution down to wavelength units can be achieved with a small number of wavelengths (laser light sources). Here, in the length measuring device of the present invention, an FM modulation means must be provided in exchange for a reduction in the number of wavelengths (laser light sources) used, but compared to a laser light source with a large number of wavelengths, the FM Modulation means are cheaper and easier to implement.

In the above description, a light source composed of a single-wavelength light source and a wavelength shifter has been exemplified as the laser light source 1, but the means for generating a plurality of lights with different wavelengths is not limited to this, and for example, The same operation can be performed by using a plurality of laser light sources with different emission wavelengths and selectively emitting the output lights of these laser light sources. Further, the number of wavelengths used for measurement is determined depending on the desired resolution, and is not limited to four. Furthermore, the phase delay amount φ
The calculation procedure for determining the distance d from is not limited to the above method.

〔Effect of the invention〕

As explained above, in the length measuring device of the present invention, at least two or more lights with different wavelengths are switched, and the amount of phase delay of the light is sequentially measured according to the distance to the measurement target. In a length measuring device that determines the distance to the measurement target from the relationship with the amount of phase delay, a modulation means for FM modulating the light emitted from the laser light source is provided, and the amount of phase delay can be measured according to each wavelength. Before doing so, we FM-modulate light of an arbitrary wavelength and measure the distance to the measurement target from the amount of phase delay of this FM modulated signal to roughly narrow down the measurement range. Combination distance measurements can be started from a narrow measurement range, and absolute measurement output can be obtained.
A length measuring device that can cover a wide measurement range with a smaller number of wavelengths (laser light sources) can be realized with a simple configuration.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the length measuring device of the present invention, and FIG. 2 is a block diagram showing an example of the length measuring device already proposed by the applicant of the present invention. 1... Laser light source, 2... Modulation signal source, 3...
Phase detection circuit, 4... Arithmetic circuit, 5... FM modulation signal source, 6... Lens, 7... Stopper, HMR
1... Half mirror, PD1... Photo detector, AOM1, AOM2... AO modulator, CC1,
CC2...Cube corner, SW1, SW2...Switch.

Claims (1)

[Claims] 1. Use a Michelson interference optical system to switch at least two or more different wavelengths of light to sequentially measure the amount of phase delay of the light according to the distance to the measurement target, and to determine the relationship between these wavelengths and the amount of phase delay. In a length measuring device that calculates the distance from to the object to be measured, the light emitted from the laser light source is
It has a modulation means that performs FM modulation, and before measuring the amount of phase delay according to each wavelength, it modulates the light of any wavelength with FM, and calculates the distance to the measurement target from the amount of phase delay of this FM modulated signal. A length measuring instrument characterized in that it measures and roughly narrows down the measurement range.