JPH0781884B2 - Optical displacement measuring device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical displacement measuring device that detects movement of a scale by utilizing light interference, and sometimes uses a multimode semiconductor laser as a light source. The present invention relates to a measuring device.

[Conventional technology]

In an optical displacement measuring device in which a moving diffraction grating is used as a scale and diffracted light is interfered to detect a position change (moving distance) of the diffraction grating, it can be used by essentially allowing a wavelength change of a light source. 57 as a type
-81510 or Japanese Patent Laid-Open No. 58-191907, in which movement detection is performed by utilizing interference between plus and minus homo-diffracted lights, and Japanese Patent Laid-Open No. 60-98302 discloses. As described above, there is known one in which movement is detected by utilizing interference between first-order diffracted lights.

All of these conventional optical displacement measuring devices are designed so that the optical system is not affected by the change of the wavelength within the allowable range of the light source, such as the interferometer being excellent. As a result, there is an advantage that it is possible to use a compact and inexpensive one such as a semiconductor laser, which lacks wavelength stability.

However, on the other hand, in order to exhibit the desired characteristics in this optical system, the optical path lengths of the two beams until they are split by the beam splitter or diffraction grating of the interferometer and are diffracted again to interfere are It needs to be adjusted to ensure that they change equally. The reason for this is that if there is a difference in the optical path length, a phase change will occur in the interference signal as well as a phase change due to the movement of the diffraction grating to be measured due to the wavelength change, which will cause a measurement error.

The accuracy of adjusting the optical path lengths of the two beams in order to avoid such an adverse effect is determined by the allowable wavelength change, the required accuracy, and the movement amount of the diffraction grating corresponding to the period of one interference signal. However, although it cannot be generally stated, when the semiconductor laser is used without being subjected to temperature compensation, it must be within several tens of μm to several hundreds of μm, for example.

[Problems to be solved by the invention]

For that purpose, a very accurate optical system support or positioning jig was required, but the former becomes expensive and the latter requires precision in the adjustment work, and loosening of screws etc. hinders the optical system. Occurs, which causes a measurement error. Furthermore, when a light source with good coherence such as a single mode is used as in the conventional case, there is a possibility that a phase shift occurs in the interference signal due to unnecessary superposition of reflected light from the end face of the optical component. Therefore, it is necessary to apply an expensive antireflection coating to the optical components.

In addition, it is difficult to keep the adjustment accuracy within a certain range in spite of these measures.

[Means for solving problems]

The present invention has been made to address the above-mentioned problems, and the optical path length is adjusted by detecting the difference between the optical path lengths of the two beams so that the optical path length can be adjusted without requiring any special precision member or work. SUMMARY OF THE INVENTION It is an object of the present invention to provide an optical displacement measuring device which eliminates the above-mentioned problem and selectively causes interference between required light beams.

In order to achieve such an object, the present invention uses a multimode semiconductor laser having appropriate coherence as a light source.

〔Example〕

 Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing an optical displacement measuring apparatus according to an embodiment of the present invention, in which 1 is a light source composed of a multimode semiconductor laser, 2 is a beam splitter for dividing a light beam into two,
3 is a diffraction grating used as a scale, 4,7,13 are λ / 4
Plates, 5 and 6 are a pair of mirrors, 8 is a half mirror, 9 and 12 are deflecting plates, and 11 and 10 are photodetectors.

In the above configuration, the multimode semiconductor laser 1
The light emitted from the beam splitter 2 is incident on the beam splitter 2 and is split by A into light beams directed to B and C of the diffraction grating 3. Diffraction grating 3
Each of the light beams incident on is incident on the mirrors 5 and 6 via the λ / 4 plates 4 and 7, respectively, reflected on D and E, and incident on the diffraction grating 3 again. The beams diffracted by the diffraction grating 3 again return to the beam splitter 2, and after being interfered there, they are made incident on the photodetectors 10 and 11 via the half mirror 8 and the interference intensity thereof is detected and used as a scale. Displacement measurement of the diffraction grating 3 is performed.

Generally, the visibility of interference fringes in an interferometer is determined by the coherence of the light source and the difference in the optical path lengths of the two beams that interfere with each other, and in the case of a light source with good coherence, such as a laser performing single-mode oscillation, Even if the difference in optical path length is large, the visibility is not lost. On the other hand, it is known that in a light source with poor coherence, the visibility of interference fringes changes due to changes in the difference in optical path length.

In the present invention, this principle is applied to an optical displacement measuring device.

In FIG. 1, in order to prevent an error due to the wavelength variation of the multimode semiconductor laser 1, a path A → B → D and a path A of the light beam divided by the beam splitter 2 are used.
→ C → E must be adjusted equally. Here, the accuracy of the adjustment depends on the temperature conditions of the environment in which this optical system is used, but when taking a commercially available semiconductor laser as an example, it has a wavelength fluctuation characteristic of about 0.3 nm / ° C with respect to temperature changes, When the temperature width is ± 10 ° C. and the wavelength is 780 nm, the grating pitch of the diffraction grating is about 0.55 μm in this optical system.

If the required accuracy is 0.1 μm, the difference in optical path length Δl
Is Must be set to a value that satisfies Note that λ is the wavelength of the light source P is the grating pitch.

In this case, the allowable value of Δl is about 70 μm.

In order to monitor this, it is possible to detect this amount of deviation of Δl, and it is difficult to handle even if it is too sensitive to the value of Δl, so that for Δl below this value, it has an appropriate coherence that the visibility does not change much. You need to use a light source.

When an appropriate multimode semiconductor laser is used as in the present invention, this condition is satisfied, and a change in the modulation rate with respect to an appropriate change in Δl can be obtained. That is, a change in the difference in optical path length can be detected as a change in the modulation rate of the interference signal.

FIG. 2 shows the relationship between the experimentally determined difference Δl in optical path length and the change in the amplitude of the interference signal in the configuration of FIG.

As is apparent from FIG. 2, it is understood that the optical path length can be adjusted quite accurately.

In an optical system with low accuracy, it is sufficient to use a multimode having a smaller number of oscillation modes than the above-mentioned multimode semiconductor laser, and a gentler characteristic than that of the example of FIG. 2 can be obtained. Further, even when a single mode laser is used, the multimode laser may be attached only when the optical system is adjusted and the adjusted laser may be replaced.

However, when a multimode semiconductor laser is used, if a difference in optical path length occurs for some reason after adjustment, it can be detected and displayed as a drop in the output signal, so an error due to wavelength fluctuations will occur. You can avoid using it without worrying about it. Further, since only the interference between the beams having a small Δl is selectively detected, it is possible to avoid the fluctuation of the interference signal due to the interference of the unnecessary light reflected from the end surface of each optical component, so that the accuracy is improved. You can Further, since it is possible to omit the non-reflection coating on the optical component, it is possible to provide an inexpensive optical component. Furthermore, it is possible to improve the S / N ratio as compared with the case of using a single mode semiconductor laser.

〔The invention's effect〕

As is apparent from the above description, according to the present invention, in the optical displacement measuring device for obtaining the moving amount of the diffraction grating by the interference signal of the diffracted light using the moving diffraction grating as a scale,
Since the multi-mode semiconductor laser is required as the light source, the following effects can be obtained.

1. Since the difference in optical path length can be detected, the optical path length can be adjusted accurately and easily.

2. Since the adjustment status can be monitored, it is possible to avoid making measurements without knowing that errors due to wavelength fluctuations are occurring.

3. It is possible to prevent the interference signal from changing due to unnecessary light.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention, and FIG. 2 is a characteristic diagram for explaining the present invention. 1 ... Multimode semiconductor laser, 2 ... Beam splitter, 3 ... Diffraction grating, 10,11 ... Photodetector.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masaaki Toyama 3-9-17 Nishigotanda, Shinagawa-ku, Tokyo Toyo Building Sony Magnescale Co., Ltd. (56) Reference JP-A-60-98302 (JP, A) ) JP-A-58-191907 (JP, A) Actually developed Shou 57-81510 (JP, U)

Claims (3)

[Claims] 1. A movable diffraction grating used as a scale, a light source and a photodetector, and means for interfering two diffracted light beams emitted from a light source and diffracted by the diffraction grating. In an optical displacement measuring device that detects the amount of displacement of the diffraction grating by detecting the periodic change of the interference signal according to the intensity of the diffracted light due to the displacement of the diffraction grating, the interference of two light beams having the same optical path length A multi-mode semiconductor laser having appropriate coherence is used as the light source for selectively performing the light source, and a peak-to-peak amplitude difference of the interference signal based on a difference between optical path lengths of the two light beams to be interfered with each other. An optical displacement measuring device comprising recognition means for recognizing. 2. The optical displacement measuring device according to claim 1, wherein the recognizing means adjusts the optical path length so as to eliminate the difference in the amplitude. 3. The optical displacement measuring device according to claim 1, wherein the recognizing means monitors a change in the difference in the optical path length from the difference in the amplitude.