JPH02157601A - Reference optical path length extending method for laser measuring instrument - Google Patents

Abstract

PURPOSE:To suppress the occurrence of a measurement error low and to reduce the size of the measuring instrument by providing an extending means which extends the optical path length of a laser beam between a beam splitter and a reference mirror and nearly equalizing the length of a reference optical path to that of a measurement optical path. CONSTITUTION:The laser beam of an LD 1 is split by a beam splitter 2 and one beam is projected in the direction of the reference mirror 3. Extension mirrors 6a and 6b are provided at proper positions of the beam splitter 2 and reference mirror 3 and set at proper angles and then the projected beam travels forward and backward between them to reach the reference mirror 3. The optical path length is extended corresponding to the reciprocation and the extended reference optical path length Ly' is about three times as large as the original optical path length Ly. The Ly' is nearly equalized to the measurement optical path length Lx by setting the Ly properly. Both beams which are reflected by the reference mirror 3 and measurement mirror 4 return to the beam splitter 2 and are put together and photodetected by a photodetector 5, which outputs a signal corresponding to the position shift of interference fringes formed as the measurement mirror 4 moves.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a method for extending the optical path length in a laser length measuring device, and more specifically, a method for effectively increasing the optical path length between a beam splitter that splits a laser beam and a reference mirror. It is about how to extend it.

[Prior Art] Fig. 4 shows a schematic configuration to explain the principle of a laser length measuring device, in which the laser beam from L D 1 is split into two by a beam splitter 2, and one beam is split into two in the perpendicular direction. The light is projected onto a reference mirror 3 provided at an appropriate position. The other beam is fixed to the moving object to be measured in front.
Beams A and B, which are projected onto the constant mirror 4 and reflected by the measurement mirror and the reference mirror, are combined at the beam splitter 2. Since both beams A and B are from the same light source, they interfere with each other and produce interference fringes. Here, when the measurement mirror moves due to the movement of the object to be measured, the position of the interference fringes moves, and when this is detected by the light receiver 5,
A signal S corresponding to the intensity change of the interference fringes is obtained. Signal S
The distance traveled is measured by counting the number of waves.

Now, the oscillation wavelength of LDI changes depending on temperature and injection current. When the wavelength changes, the position of the interference fringes changes as described above even when the measurement mirror does not move. This will be explained below using mathematical formulas.

Now, the laser beam oscillated by the LD is sinωt...(1)(
ω=2πC/λ0. λ0: wavelength, C: speed of light).

As shown in FIG. 4, when the optical path lengths for both beams A and H are Lx and Ly, respectively, the intensity of the combined beam C of both beams is expressed by the following equation, omitting the time variation.

C= (A2 +B2)/2+ABcos Φo・
...(2) ΦO=2πΔL/λ0 ・
(3) Here, ΔL is the optical path difference between both beams.

ΔL=Lx −Ly ・・・・・・
(4) When the LD's oscillation wavelength λ0 changes by δλ for some reason, the second term of equation (2) and Φ0 of equation (3) become Φ0′ of the following equation: Φ0′→Φo (1−δλ /λ0) = ΦO−2πΔLδλ/λo2 (5) In other words, when the wavelength changes by δλ, 2πΔLδλ/λ
o2...The phase change (6) occurs. Now, when δλ changes over time, the phase changes according to equation (6), so the position of the interference fringes in FIG. be done.

This clearly results in a measurement error.

As shown in equation (6), the phase change that causes the 1τ1 difference is directly proportional to the optical path difference ΔL, so if ΔL is set to 0, the error also becomes O. However, ΔL is the difference between LX and Ly, and since Lx is a variable, it cannot always be made equal, but it can be made as small as possible to reduce the error.

Now, in FIG. 4, it is possible to make the reference optical path length Ly of the reference mirror 3 almost equal to the measurement optical path length Lx of the measurement mirror 4, but the position of the object to be measured with respect to the laser length measuring device is restricted to some extent. Since Lx is
It cannot be shortened to C.

Therefore, when Ly is made equal to this value, it becomes too large for the length measuring device, which prevents downsizing of the length measuring device.

On the other hand, if the optical path length Ly can be effectively extended by some means, the same effect as described above should be obtained.

The present invention has been made in view of the above, and an object of the present invention is to provide an extension method for extending the reference optical path length so as to make it approximately equal to the measurement optical path length.

[Means for Solving the Problems] This invention splits the laser beam of an LD using a beam splitter, one of which is attached to a reference mirror fixed at an appropriate position, and the other is attached to a measuring device attached to a fixed moving body. A method for extending the reference optical path length in a laser beam length device, in which the laser beams are projected onto each mirror, the two laser beams reflected by each mirror are combined using a beam splinter, and the interference fringes of both beams are counted to measure the displacement of a moving body. An extension means for extending the optical path length of the laser beam is provided between the beam splitter and the reference mirror, so that the extended reference optical path length and the measurement optical path length between the beam splitter and the measurement mirror are approximately equal to each other. It is to make them equal.

A preferred embodiment of the above-mentioned optical path length extension means is to provide extension mirrors for reciprocating the laser beam at respective positions of the beam splitter and the reference mirror. In another embodiment, an optical medium having a suitable refractive index is inserted between the beam splitter and the reference mirror to equimetrically multiply the optical path length by the refractive index.

[Function] According to the method for extending the standard optical path length according to the present invention, the reference optical path length is made almost equal to the measurement optical path length by the extension means, and the fluctuation of the LD's oscillation wavelength due to temperature, injection current, etc. The phase fluctuation of the interference fringes is eliminated or reduced, thereby eliminating the problem of measurement errors.

Note that there is no difference in the lengthening effect whether the optical path length is extended using an extension mirror or an optical medium, so whatever is actually suitable may be used, and in some cases, a combination of both may be used. IJJ ability, in which case there is a synergistic effect between the two.

[Example] FIG. 1 shows the first method of extending the reference optical path length according to the present invention.
In the configuration diagram of the embodiment, the laser beam of LD1 is split by a beam splitter 2, and one is projected toward a reference mirror 3. Extension mirrors 8a and 6b are attached to appropriate locations on the beam splitter 2 and the reference mirror 3, respectively. When the angles of the mirrors are set appropriately, the projected beam travels back and forth between them and reaches the reference mirror. Therefore, the optical path length is extended by the round trip, and the extended reference optical path length Ly' becomes about three times the original optical path length Ly.

Ly is set appropriately so that Ly' is approximately equal to the measurement optical path length Lx. Both beams reflected by the reference mirror and the measurement mirror are directed to the beam splitter 2, where they are combined and received by the light receiver 5. A signal corresponding to a change in the position of the interference fringes that occurs as the measurement mirror 4 moves is outputted from the light receiver. In this case, since both optical path lengths are approximately equal, the error due to wavelength change is small or reduced to 0.

FIG. 2 shows a second embodiment, in which an optical medium 7 with a refractive index na is inserted between the beam splitter 2 and the reference mirror 3, and the optical path length is isometrically multiplied by n6.

For example, if you use a refractive index of about 1.5,
Ly ”F l , 5Ly, which is set approximately equal to L× in the same way as above.

FIG. 3 shows a third embodiment, which is a combination of the first and second embodiments described above. In this case, the optical path length will be approximately 3 times the round trip, the refractive index will be approximately 1.5 times, and the synergistic effect will be approximately 4.5 times.
Since the length of the optical medium 7 is doubled, the length of the optical medium 7 can be significantly shortened. Note that both sides of the optical medium 7 are finished at appropriate angles, and aluminum is deposited thereon to form the mirror 6.

[Effects of the Invention] As is clear from the following explanation, according to the method for extending the reference optical path length in a laser length measuring device of the present invention, the spatially short reference optical path length is extended by the extension means, and the iil' l Constant optical path length By increasing the color optical path length, the occurrence of the measurement IL-L difference due to fluctuations in the LD's oscillation wavelength is reduced (reduced, reliability is improved, and the size of the laser length measuring device is reduced. The contributing effects are significant.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a first embodiment of the method for extending the reference optical path length of a laser length measuring device according to the present invention, and FIG. FIG. 3 is a block diagram of the third embodiment of the method for extending the reference optical path length of a laser length measuring device according to the present invention, and FIG.
The figure is an explanatory diagram of the length measurement principle of a conventional laser length measuring device and measurement errors caused by wavelength changes. 1... Semiconductor laser element (LD), 2... Beam splitter, 3... Reference mirror 4...
・Measurement mirror 5...Receiver, 8a, 6b...
- Extension mirror, 7... optical medium.

Claims (3)

[Claims] (1) A laser beam output from a semiconductor laser device (hereinafter abbreviated as LD) is split by a beam splitter, one of the splits is attached to a reference mirror fixed at an appropriate position, and the other is attached to a moving object to be measured. The two laser beams reflected by the reference mirror and the measurement mirror are combined in the beam splitter, and the interference fringes of the two laser beams are counted to measure the displacement of the moving body. In the laser length measuring device, an extension means for extending the optical path length of the laser beam is provided between the beam splitter and the reference mirror, and a distance between the extended reference optical path length and the beam splitter and the measurement mirror is provided. A method for extending a reference optical path length in a laser length measuring device, characterized by making the optical path length substantially equal to the measurement optical path length. (2) Extension of the reference optical path length in the laser length measuring instrument according to claim 1, wherein the optical path length extension means is provided with an extension mirror for reciprocating the laser beam at each position of the beam splitter and the reference mirror. Method. (3) The optical path length extending means inserts an optical medium having an appropriate refractive index between the beam splitter and the reference mirror to equivalently multiply the optical path length by the refractive index. How to extend the reference optical path length in a laser length measuring device.