JPH0377921B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is an interferometric length measuring device that easily generates a dual-frequency laser beam using a single-frequency laser and uses the dual-frequency laser beam to measure minute displacements of an object to be measured. This is related to.

Conventional interferometric length measuring devices that use dual-frequency laser light to measure minute displacements of objects to be measured use a laser device that generates light of two different frequencies, such as the one shown in Utility Model Publication No. 53-9343. Those that use the emitted beam, or those that use the special public
Some use a single frequency laser device to generate a dual frequency laser, as shown in the 45198 gas laser. However, the former method requires a laser device that generates light of two different frequencies, which makes the device large and expensive, and the latter method also requires a complicated mechanism, so the device is large and expensive. The drawback was that it had to be expensive. This invention solves the above-mentioned drawbacks of the conventional device, and uses a simple mechanism to separate the light emitted from a single frequency laser into two different frequency lights, and uses this laser light to perform the measurement under measurement. The present invention is designed to measure minute displacements of objects, and provides an interferometric length measuring device that is compact and lightweight as a whole and is inexpensive.

An embodiment of the present invention will be described below with reference to the drawings. First, a device that separates light emitted from a single frequency laser into light of two frequencies will be described. In the area surrounded by the dashed line in Fig. 1, the light beam of frequency f ₀ emitted from the linearly polarized laser 1 enters the polarized beam splitter 2, and the P wave/f ₀ (P) is horizontal to the plane of the paper and the light beam is perpendicular to the plane of the paper. The P wave is separated into an S wave and f ₀ (S), and the P wave travels straight, and the S wave is turned 90° to the right in the figure and enters the acousto-optic modulators 2 and 4, respectively. The acousto-optic modulators 3 and 4 are shown in FIG. 2, and when a high frequency voltage f is applied, the diffracted light changes its direction with respect to the direction of the incident light. ₀ , the diffracted light can be changed into a frequency (f ₀ +f).

Therefore, a high frequency voltage (frequency f ₁ ) is applied to the acousto-optic modulator 3 into which the P liquid is incident, and a high frequency voltage (frequency f ₂ ) is applied to the acousto-optic modulator 4 into which the S wave is incident.
When a voltage is applied, the frequency is modulated, and from the respective acousto-optic modulators 3 and 4, light (f ₀ +f ₁ )(P) and (f ₀ +f ₂ )(S) is obtained. Here, the high frequency voltages applied to the two acousto-optic modulators 3 and 4 are appropriately selected so that the frequency difference between the two frequencies after frequency modulation becomes a countable frequency. For example, an acousto-optic modulator 3
If 81MHz is set for 4, and 80MHz is set for 4, the frequency difference between the two frequencies is 1MHz, which is readable.

Compared to the conventional devices, the linearly polarized laser light source and the acousto-optic modulator are much cheaper and can be made smaller overall.

Next, a length measuring device using light sources of two different frequencies generated by the above device will be explained. In this case, there are two cases: a case in which the displacement of the object to be measured is carried out in the direction of the optical axis by a reflector provided perpendicularly to the optical axis, and a case in which the displacement is carried out in the direction of the optical axis by a cube corner. There are two possibilities.

What is done using the reflector will now be explained with reference to FIG. The light output from the acousto-optic modulators 3 and 4 described above is divided into a reference signal system and a length measurement system. Straight light that exits the acousto-optic modulator 3 (f ₀ +
f ₁ )(P) passes through the non-polarizing beam splitter 7,
Half of them turn left and half of them go straight. Also,
The light (f ₀ +f ₂ ) (S) that has exited the acousto-optic modulator 4 enters the non-polarized beam splitter 7 from the right through the two reflectors 5 and 6, and half of it travels straight, and the light (f ₀
+f ₁ ) overlaps with (P). Then, half of the light changes its direction downward in the figure and overlaps with the light (f ₀ +f ₁ )(P).
Now move to the left (f ₀ + f ₁ ) (P) (f ₀ + f ₂ ) (S)
interferes by passing through the polarizing plate 13.
Therefore, this is detected by the photodetector 14 as a sine wave of frequency (f ₁ -f ₂ ). This is the reference signal.

Next, the length measurement system will be explained. The light outputted from the non-polarizing beam splitter 7 and traveling downward in the figure is separated into (f ₀ + f ₁ ) (P) and (f ₀ + f ₂ ) (S) by the polarizing beam splitter 8, and ( f ₀ + f ₂ ) (S)
Proceeds to the right, passes through the λ/4 plate 9, becomes circularly polarized light, is reflected by the reflector 10, and passes through the λ/4 plate 9 again, resulting in a P wave or beam whose direction is changed by 90° (f ₀ + f ₂ ) (P) and passes straight to the left through the polarizing beam splitter 8.

It also enters the polarizing beam splitter 8 from above (f ₀
+f ₁ ) (P) travels straight and becomes circularly polarized light when it passes through the λ/4 plate 11, is reflected by the movable reflector 12, and when it passes through the λ/4 plate 11 again, it becomes an S wave whose direction has been changed by 90°, i.e. The beam becomes (f ₀ +f ₁ )(S).
Here, the movable reflector 12 is displaced in the optical axis direction, so if the frequency corresponding to the amount of displacement is ±△f, the light entering the polarizing beam splitter 8 from below is (f ₀ + f ₁ ) (S) ± Δf. This light is reflected by the polarizing beam splitter 8 and becomes the aforementioned light (f ₀ +
The beam that overlaps with f ₂ ) (P) and travels leftward through the polarizing beam splitter 8 is (f ₀ + f ₂ ) (P), (f ₀ + f ₁ )
(S)±△f. When this beam passes through the polarizing plate 15, it interferes, and is detected by the detector 16 as a sine wave of frequency (f ₁ −f ₂ ±Δf).
Therefore, this signal is compared with the reference signal (f ₁ −f ₂ ) detected by the detector 14 by the counter 17,
By determining the phase difference ±Δf, the amount of displacement of the movable reflection plate 12 can be determined with high precision and a high division value. Since this first embodiment uses a movable reflector that is flat with respect to the optical axis, it is suitable for measuring the amount of displacement at a small point by attaching a condenser lens or the like to this, for example.

Next, as another embodiment of the present invention, an interferometric length measuring device in which a cube corner is provided in the displacement section will be described with reference to FIG. Here, the process is the same as that in FIG. 2 until it passes through the non-polarizing beam splitter 7. (The same part numbers as in Fig. 2 are indicated with the same numbers. In this case, the cube corner 23 is displaced, and it is displaced by ±△f approximately in the optical axis direction. The non-polarizing beam splitter 7 is The beams (f ₀ + f ₁ ) (P) and (f ₀ + f ₂ ) (S) that advance to enter the polarizing beam splitter 21, and (f ₀ + f ₂ ) (S) turns right and enters the cube corner 22 and is reflected. The light then enters the polarizing beam splitter 21 again, is turned upward, and enters the polarizing plate 15. It also passes straight through the polarizing beam splitter 21 (f ₀ + f ₁ ) (P)
is determined by the movable cube corner 23 (f ₀ + f ₁ )
(P) ±Δf and is reflected and incident on the polarizing plate 15. Both signals incident on the polarizing plate 15 interfere,
It is detected by the photodetector 16 as a sine wave of frequency (f ₁ −f ₂ ±△f). This signal is combined with the reference signal (f ₁ −f ₂ ) detected by the detector 14 and the counter 17
The first step is to compare and find the phase difference △±f.
This is the same as the first embodiment of the present invention shown in the figures.

In this second embodiment, since a cube corner is used, even if the cube corner is mounted with a slight inclination with respect to the optical axis, there is an effect that the amount of displacement is not affected at all.

As described in detail above, according to the present invention, a linearly polarized laser generated from a single frequency laser device is divided into two frequencies of light using a simple mechanism, and an interference device using this light is configured to By making it possible to measure minute displacements, it has become possible to obtain an interferometric length measuring device whose entire device is compact, lightweight, and inexpensive.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the configuration of the present invention, FIG. 2 is an explanatory diagram of an acousto-optic modulator, and FIG. 3 is an explanatory diagram showing another embodiment of the length measuring device. 1: linearly polarized laser generator, 2: polarized beam splitter, 3, 4: acousto-optic modulator, 7: non-polarized beam splitter, 8: polarized beam splitter,
9, 11: λ/4 plate, 11: Movable reflector, 13,
15: polarizing plate, 14, 15: photodetector, 17: counter, 21: polarizing beam splitter, 22: cube corner, 23: movable cube corner.

Claims (1)

[Claims] 1. A polarizing beam splitter that separates a linearly polarized light beam of frequency f ₀ emitted from a light source into frequencies f ₀ (P) and f ₀ (S), and a reference light detection optical system consisting of an acousto-optic modulator that modulates at frequencies f ₁ and f ₂ ; a reflector, a non-polarizing beam splitter, a polarizing plate, and a detector for interference light (f ₁ − _{f 2} ); , a λ/4 plate that passes the optical beam of frequency (f ₀ + f ₂ ) (S) wave separated and reflected by the polarizing beam splitter and returns it as (f ₀ + f ₂ ) (P) wave, and a fixed reflector,
The frequency (f ₀
+f ₁ ) (P) wave light beam passes through, (f ₀ +f ₁ )
(S) A λ/4 plate that returns the light beam as a wave, a movable reflector for the light beam, and the transmitted light (f ₀ + f ₂ ).
(P) Polarizing plate and _interference light (f ₁ - f _{2 ±} △
A light wave interferometric length measuring device comprising a length measuring light detection optical system consisting of the detector f) and a circuit for calculating ±Δf corresponding to the amount of displacement of the movable reflecting plate from the phase difference between detection frequencies of both systems. 2. A polarizing beam splitter that separates the linearly polarized light beam with frequency f ₀ emitted from the light source into frequencies f ₀ (P) and f 0 (S), and splits the separated light beam into frequencies f ₁ and f ₂ . an acousto-optic modulator that modulates the light beam, a reference light detection optical system consisting of a reflector, a non-polarizing beam splitter, a polarizing plate and a detector for interference light (f ₁ - f ₂ ), and a polarizing beam splitter. _The frequency ( _{f 0}
+f ₂ ) A movable cubic corner that reflects and returns the light beam of the (P) wave, a polarizing plate that causes the transmitted light and the reflected light to interfere, and interference light f ₁ −f ₂ ±
Equipped with a length measurement light detection optical system consisting of a △f detector, ±△ corresponds to the amount of displacement of the movable cube corner.
A light wave interferometric length measurement device having a circuit that calculates f from the phase difference between detection frequencies of both systems.