JPS63266302A - Length measuring instrument by laser - Google Patents

Abstract

PURPOSE:To reduce a quantization error to raise the measurement accuracy by using a semiconductor laser light which is applied with frequency modulation continuously within a prescribed range in which a mode hop does not occur, and calculating and deriving an absolute distance from a period of a beat signal being proportional to a distance. CONSTITUTION:A laser light which is applied with frequency modulation continuously within a prescribed range in which a mode hop does not occur is generated from a semiconductor laser oscillator 2. This laser light passes through a collimator lens 3, allowed to branch into two by a beam splitter 4, and one light beam irradiates a reference mirror 5 and reflected, passes through the beam splitter 4, and made incident on a photodetector 7. The other light beam irradiates a moving mirror 6 and reflected, curved by the beam splitter 4 and made incident on the photodetector 7. Subsequently, an output of the photodetector 7 is supplied to a period counting circuit 8 and a period of a beat signal is measured, this period data is supplied to a distance calculating circuit 9, and a signal corresponding to a distance is outputted an output circuit 10.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a laser length measuring device used, for example, when accurately positioning a moving body of an X-Y stage.

[Conventional technology]

The third example shows the configuration of a conventional laser length measuring device that uses optical interference.
As shown in the figure. In this method, a single wavelength laser beam is emitted from a He-Ne laser oscillator 20, and a beam splitter 4
One of the light beams passes through the reference optical path and reaches the reference mirror 5, and the other light beam passes through the movable mirror 6 fixed on the movable stage 11, and the reflected light from each mirror is converted into a beam. The interference light generated by this is mixed through the splitter 4 and is incident on the light receivers 22 and 23 through the splitting mirror 21, resulting in two waves E whose phases are shifted by 90 degrees.
, F is generated. This output is amplified by the preamplifier 24.

The acceleration counter 25 measures the pulse P1 generated at the zero cross point of the wave E by a logical operation with 1 or 0 of the pulse P2 generated on the positive side of the wave F depending on the direction of movement, and the distance calculation circuit 26 measures the pulse P1 generated at the zero cross point of the wave E. It calculates the amount and direction of movement. 12 is a motor for moving the stage.

[Problem that the invention seeks to solve]

However, since this method is an incremental measurement in which pulse signals obtained by interference waves from the optical receiver are counted in each direction of stage movement, the accumulated data will be updated if any obstacle passes through the optical path even momentarily. This has the disadvantage that the position information of the stage is lost, making measurement impossible.

Furthermore, since the laser used is a gas laser, the shape is large and the optical system is complicated, making the device large and expensive.

[Object of the present invention]

An object of the present invention is to provide an inexpensive and compact laser length measuring device that is resistant to disturbance noise.

[Means for solving problems]

To achieve the above objective, a semiconductor laser beam modulated so that one frequency is continuously increased and decreased at a constant rate within a certain frequency range is used to irradiate the reference mirror and the moving mirror of the object to be measured, respectively. The light reflected from the mirror is mixed, thereby generating a beat wave proportional to the difference between the reference optical path length of the reference mirror and the measurement optical path length of the moving mirror, and by measuring the period of the beat wave, the absolute distance of the measurement optical path can be determined. I am trying to detect it.

[Effect]

Therefore, even if the laser beam is blocked by an obstacle temporarily passing through the optical path, when the laser beam is irradiated again, the absolute distance is detected using the beat wave generated by the modulated laser beam, so the position information can be obtained. Precise measurements can be made continuously without losing any data.

〔Example〕

The present invention will be explained in detail below using examples.

FIG. 1 is a diagram showing the configuration of a laser length measuring device according to the present invention.

Reference numeral 1 denotes a frequency modulation circuit that continuously modulates the wavelength of the laser light emitted from the semiconductor laser oscillator 2 at a constant rate. For this reason, the current injected into the semiconductor laser oscillator 2 is continuously changed at a constant rate. 3 is a collimator lens that makes the emitted light from the semiconductor laser oscillator parallel; 4 is a beam that distributes the laser light to both the reference mirror 5 and the movable mirror 6, and also mixes the light reflected from each mirror. Splitter, 7 is a light receiver that detects the beat signal generated by mixing, 8 is a period coefficient circuit that counts the period of the beat signal using a sufficiently high frequency clock, and 9 is used to calculate the position of the movable mirror from the period data. A distance calculation circuit, 10 is an output circuit.

11 is a moving stage, and 12 is a motor for moving the stage.

The operation will be explained below.

Laser light is emitted from the semiconductor laser oscillator 2.

When the injection current from the frequency modulation circuit 1 is varied linearly, for example in a triangular waveform, within a frequency range (0.1% or less) that does not cause mode hops in which the frequency of the semiconductor laser light changes rapidly, the injection current changes. Proportionally, the frequency of the semiconductor laser light changes and is frequency modulated at a frequency f expressed by linear equation (1).

r=fo+tt...
......(1) (where f. is the frequency of the modulation start point, i is the frequency change rate, and t is time) This semiconductor laser light passes through the collimator lens 3 and is split into two by the beam splitter 4. One of the light beams whose direction is bent by 90 degrees is reflected by the reference mirror 5 in the standard optical path, passes through the beam splitter 4, and enters the light receiver 7. This reference light A is expressed by the following equation (2).

Δ=A1exp (-2πj (fo+4t) t)
・+−+・−・(2) (However, AI is the intensity of the initial reference light) The other light beam that passed through the beam splitter 4 hits the movable mirror 6 and is reflected, and the direction is changed by the beam splitter 4. The light is bent by 90 degrees and enters the light receiver 7. This measurement light B is expressed by the following equation (3), where τ is the time delay with respect to the reference light A, and R is the optical path difference.

B=Biexp [2πj (f+f (t r)
)×(t-τ)] ・・・・・・・・・
・(3) (where B is the initial measurement light intensity, τ=2R/
C and C are the speeds of light) In the light receiver 7, the reference light A and the measurement light B are mixed.

V1= l A+B l =AI +B1 +
2 AIBtCos (2tt (fr +lr t,
rτ2) ) ・===...I4) However,
? Since τ2<<fr, it can be ignored.

If we take out only the alternating current component of equation (4), the output of the photodetector v2
becomes the signal of the following equation (5).

V2= 2AIBICos [2x (fr +fr
(5) The second item in equation (5) is the frequency fb of the beat signal, which is expressed by the following equation (6).

= i−τ=i・2 R/C・・・・・・・・・(6)
As can be seen from equation (6), the beat frequency is proportional to the optical path difference R, and is expressed as R=c-rb/2i.

However, when counting the frequency fb of the beat signal.

If the amount of frequency deviation is ΔF, a quantization error of C/(4·ΔF) will occur, and if ΔF is 50 GHz, this quantization error will be 1.5 mm, which is a large error value.

Therefore, instead of counting one frequency fb, the period Tb (= 1 /fb) of the beat signal is measured, and this period data is given to the distance calculation circuit 9, and R=C/2? The optical path difference R is calculated by the path Tb, and a signal corresponding to the distance is outputted via the output circuit 10.

Therefore, when counting is performed using a clock having a beat period Tc, the quantization error ΔR is expressed by the following equation.

△R/R=Tc/Tb= 1/N Now, if R=10mm and N=105, the quantization error is △
R becomes 0.1 μm.

Therefore, the clock period Tc is set to 1/N of the beat period Tb when the optical path difference R is maximum (for example, N=10' to 1
If the value is set to correspond to 05, the quantization error ΔR can be kept at a small value and accurate measurement can be performed.

Another example of application of the present invention is shown in FIG.

In this application example, a polarization-constant optical fiber 13 is provided in the optical path between the beam splitter 4 and the movable mirror 6, and its end is fixed opposite to the movable mirror.

The constant polarization optical fiber can transmit optical phase information, and there is no need to position the movable mirror to face the beam splitter, allowing the movable stage and optical system to be freely arranged.

Note that if the measurement optical path is lengthened by separating the movable stage, the beat period will shorten and there is a risk that the quantization error will increase due to the ratio with the one-period counting clock, but a constant polarization optical fiber is also used for the reference optical path.

A coating 14 is applied to the tip of the optical fiber for total reflection, and the beat frequency is lowered by making the optical fiber as long as the polarization constant optical fiber of the measurement optical path.

It is possible to prevent quantization errors from increasing.

〔Effect of the invention〕

In this way, according to the present invention, the absolute distance is calculated and derived from the period of the beat signal proportional to the distance using a semiconductor laser light that is continuously frequency-modulated within a certain range that does not cause mode hopping. Even if the sensor temporarily passes through the optical path, the position information is not lost and nine cycles are measured. Therefore, the measurement clock cycle can be shortened to reduce quantization errors, making it possible to perform precise measurements. Yes, even more.

Since the optical system is simple, it is possible to provide a compact and inexpensive length measuring device.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the configuration of a laser length measuring device of the present invention, Fig. 2 is a block diagram showing main parts of another embodiment of the present invention, and Fig. 3 is a block diagram showing the configuration of a conventional laser length measuring device. FIG. 4 is a block diagram showing a waveform of interference light and a time chart of pulses in a conventional device. 1 is a frequency modulation circuit, and 2 is a semiconductor laser oscillator. 4 is a beam splitter, 5 is a reference mirror, and 6 is a moving mirror. 7 is a light receiver, 8 is a period counting circuit, 9 is a distance calculation circuit, 1
3 indicates a constant polarization optical fiber. Figure 1 Figure 2 Circle Figure 4 Circle ゛

Claims (1)

[Claims] In a length measuring device that uses optical interference of laser light, a semiconductor laser light whose frequency is modulated so that the frequency is continuously increased or decreased by a certain percentage within a range that does not cause mode pop is split into two directions, and one light is The beam is irradiated onto a reference mirror installed at the end of the reference optical path, and the other light beam is irradiated onto a reflecting mirror fixed to the object to be measured, and the respective reflected lights are mixed. A laser length measuring device characterized by calculating the distance between mirrors.