JPH0511881B2 - - Google Patents

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention is a laser interference device used to measure, for example, the moving direction and moving distance of a moving object with high precision. Concerning things that use light.

[Conventional technology]

An example of a conventional laser interference device of this type is shown in FIG. This device is also called a single beam interferometer and is constructed as follows.

That is, from the laser light source 2, first and second linearly polarized lights that are orthogonal to each other and have different frequencies, for example, as shown by the black circles in the figure, the first linearly polarized light 41v whose polarization direction is perpendicular to the plane of the paper and whose frequency is ₁ are emitted. Then, as shown by the bar line in the figure, the second linearly polarized light 42h whose polarization direction is horizontal to the plane of the paper and whose frequency is ₂ (≠ ₁ ), both of which have stabilized frequencies, are output. The first linearly polarized light 41v is transmitted through a polarizing beam splitter 6 that transmits the vertically polarized light and reflects the horizontally polarized light.
and the second linearly polarized light 42h reflected there.

Then, the divided first linearly polarized light 41v is divided into 1/
The light is transmitted through a phase shifter 8 such as a four-wavelength plate and converted into circularly polarized light 41c as shown by a circle in the figure, which is then incident on a reflector 10 such as a plane mirror attached to a moving object (not shown). Therefore, it is reflected onto the same optical path as when it was incident, and transmitted through the phase shifter 8 again to return the linearly polarized light 41h, which has a polarization direction 90 degrees different from the first linearly polarized light 41v (that is, the polarization direction is horizontal). , this linearly polarized light 41h is reflected by the polarization beam splitter 6, transmitted through the polarizer 12, and made incident on the detector 14.

On the other hand, the second beam split by the polarizing beam splitter 6
The linearly polarized light 42h is transmitted through a phase shifter 16 such as a 1/4 wavelength plate and converted into circularly polarized light 42c, which is then incident on a reflector 18 such as a plane mirror where it is reflected on the same optical path as when it was incident. and pass through the phase shifter 16 again to make the polarization direction different from the second linearly polarized light 42h by 90 degrees (that is, the polarization direction is perpendicular).
The linearly polarized light 42v is returned to the linearly polarized light 42v, and the linearly polarized light 42v is transmitted through the polarization beam splitter 6 and polarized by the polarizer 12.
is transmitted and incident on the detector 14.

Since the polarizer 12 transmits light in an oblique direction of 45 degrees, both linearly polarized lights 41h and 42v
The same components are superimposed on each other and cause interference. The detector 14 detects this superimposed light 43.

In that case, when the reflector 10 moves in the optical axis direction (X direction in the figure) of the circularly polarized light 41c incident thereon,
Due to the Doppler effect of light, the frequency of the returning circularly polarized light 41c and linearly polarized light 41h is ₁ ±Δ ₁ , so the frequency of the superimposed light 43 is = ₂
−( ₁ ±Δ ₁ ). Since these ₁ and ₂ are known, the moving direction of the reflector ₁₀ , that is, the moving object to which it is attached, can be determined by ±Δ ₁ , that is, the polarity (±) of Δ 1, and the moving direction of the moving object can be determined by the absolute value of Δ ₁ . The moving speed and the moving distance of the moving object can be measured by the integral value of _Δ1 .

[Problem that the invention seeks to solve]

As can be seen from the above, in order to perform measurements accurately, two linearly polarized lights 4 output from the laser light source 2 are required.
It is important that the stability of both frequencies ₁ and ₂ of 1v and 41h is high, and therefore the laser light source 2 has traditionally been stabilized to ensure frequency stability of, for example, 1×10 ^-8 . .

However, the polarizing beam splitter 6 in the laser interference device does not accurately reflect 100% of the linearly polarized light 41h returning from the reflector 10 side, and when its extinction ratio is, for example, 400:1, 1 /Four
00 is transmitted, and this transmitted linearly polarized light 41h' returns to the laser light source 2, which causes the operation of the laser light source 2 to become unstable and the measurement accuracy to deteriorate.

SUMMARY OF THE INVENTION An object of the present invention is to provide a laser interference device that stabilizes the operation of a laser light source and improves measurement accuracy by minimizing the amount of light returned to the laser light source.

[Means for solving problems]

The laser interference device of the present invention has first and second beams that are orthogonal to each other and have different frequencies from a laser light source.
outputs linearly polarized light, and splits these into first linearly polarized light that is transmitted through it and second linearly polarized light that is reflected by a first polarization beam splitter, and the split first linearly polarized light is transmitting the second polarizing beam splitter and transmitting the retarder to convert the circularly polarized light into circularly polarized light; the circularly polarized light is reflected by the first movable reflector onto the same optical path as the incident optical path; and The linearly polarized light is transmitted through the phase shifter again and returned to a linearly polarized light having a polarization direction different by 90 degrees from the first linearly polarized light, and this linearly polarized light is reflected by the second polarized beam splitter and enters a beam combiner. and the second linearly polarized light split by the first polarizing beam splitter is reflected by a second reflector and incident on the beam superimposer where it is superimposed with the linearly polarized light, and this superposition is performed. The present invention is characterized in that it is configured such that the light emitted by the light is detected by the detector.

[Effect]

According to the above configuration, the light returning to the laser light source passes through both the second polarizing beam splitter and the first polarizing beam splitter among the light returning from the first movable reflector side. Since it is attenuated by the product of the extinction ratios of both polarizing beam splitters, it becomes almost negligible.

〔Example〕

FIG. 1 is a schematic configuration diagram showing a laser interference device according to an embodiment of the present invention. The same reference numerals are given to the same or corresponding parts as those in the apparatus shown in FIG. 2, and the differences from the conventional example will be mainly explained below.

The apparatus of this embodiment is constructed as follows. That is, the polarization direction as described above from the laser light source 2 is, for example, perpendicular to the plane of the paper, and the frequency is, for example, _1.
The first linearly polarized light 41v and the second linearly polarized light 42h whose polarization direction is, for example, horizontal to the plane of the paper and whose frequency is, for example, ₂ , both of which have stabilized frequencies, are output, and these are outputted, for example, in the vertical direction. A first polarizing beam splitter 61 that transmits linearly polarized light and reflects horizontally polarized light splits the beam into first linearly polarized light 41v that is transmitted therethrough and second linearly polarized light 42h that is reflected there.

This laser light source 2 may be a laser that can output two-frequency laser light by itself, or a laser that outputs one-frequency laser light and a converter that separates it into two-frequency laser light. A combination or the like may also be used. Further, as the polarizing beam splitter 61, a cube-shaped or planar polarizing beam splitter can be used. The second
The same is true for the polarization beam splitter 62 of.

Then, the first linearly polarized light 41v divided as described above is transmitted through a second polarizing beam splitter 62 having the same function as the first polarizing beam splitter 61, and for example, a 1/4 wavelength plate. It is transmitted through a phase shifter 8 such as 8, converted into circularly polarized light 41c, and is made incident on a first reflector 10, such as a plane mirror, attached to a moving object (not shown), where it is reflected on the same optical path as the time of incidence. The first linearly polarized light 41v and the polarization direction are reflected and transmitted through the phase shifter 8 again.
The linearly polarized light 41h is returned to the linearly polarized light 41h that is different by 90 degrees (that is, the polarization direction is horizontal in this example), and this linearly polarized light 41h is
h is reflected by the second polarization beam splitter 62 and incident on the beam combiner 20.

In addition to the above-mentioned plane mirror, the first reflector 10 may be a retroreflector such as a corner cube prism, a lens surface, or the like. Further, as the beam superimposer 20, a non-polarizing beam splitter (also called a semi-transparent mirror) or the like can be used.

On the other hand, the second linearly polarized light 42h split by the first beam splitter 61 is reflected by a second reflector 22, such as a plane mirror, and is incident on the beam superimposer 20, where the linearly polarized light is 41h
Overlap with. In this case, both linearly polarized light 41
Since the polarization directions of light beams h and 42h are the same (in this case, horizontal to the plane of the paper), interference occurs even without using a polarizer 12 as in the conventional case. This superimposed light 44 is then detected by a detector 14 such as a photoelectric converter.

In the case of this embodiment, as in the case of the conventional example,
When the reflector 10 moves in the optical axis direction (X direction in the figure) of the circularly polarized light 41c incident thereon, the frequency of the superimposed light 44 incident on the detector 14 is = ₂
−( ₁ ±Δ ₁ ), and due to this ±Δ ₁ , reflector 1
0, that is, the moving direction, moving speed, and moving distance of the moving object to which it is attached can be measured.

On the other hand, regarding the return light to the laser light source 2, if the extinction ratios of the polarizing beam splitters 61 and 62 are both 400:1, for example, as in the conventional case, the polarizing beam splitter 62 will pass through the straight line returning from the reflector 10 side. Only 1/400 of the polarized light 41h is transmitted, and the polarization beam splitter 61 transmits an additional 1/400 of the linearly polarized light 41h', which returns to the laser light source 2. 10
It is attenuated to (1/400) ² of the linearly polarized light 41h returning from the side, and becomes almost negligible. As a result, the operation of the laser light source 2 is stabilized compared to the conventional example, and high frequency stability can be ensured, resulting in improved measurement accuracy.

In the conventional example, the amount of linearly polarized light 41h or 42v incident on the detector 14 is halved by the polarizer 12 in the 45-degree direction.
It becomes about 1/2 of each. On the other hand, in this embodiment as well, there are two polarization beam splitters (i.e., 6
1 and 62), but the amount of linearly polarized light 41h or 42h incident on the detector 14 is
The beam is only halved by the beam combiner 20, such as a non-polarized beam splitter, and the laser light source 2
Approximately 1/2 of each of the linearly polarized light 41v or 42h outputted from the optical system can be secured, which is no inferior to the conventional example.

〔Effect of the invention〕

As described above, according to the present invention, since the return light to the laser light source is reduced to a negligible level, the operation of the laser light source is stabilized and measurement accuracy is improved.

[Brief explanation of the drawing]

FIG. 1 is a schematic configuration diagram showing a laser interference device according to an embodiment of the present invention. FIG. 2 is a schematic configuration diagram showing an example of a conventional laser interference device. 2... Laser light source, 41v... First linearly polarized light, 42h... Second linearly polarized light, 61... First beam splitter, 62... Second beam splitter, 8
... Phaser, 10 ... First reflector, 14 ... Detector, 20 ... Beam superimposer, 22 ... Second
reflector.

Claims (1)

[Claims] 1 A laser light source outputs first and second linearly polarized lights that are orthogonal to each other and have different frequencies, and these are separated by a first polarization beam splitter into which the first linearly polarized light is transmitted and the second linearly polarized light is reflected therefrom. The first linearly polarized light is divided into the second linearly polarized light.
the polarizing beam splitter and the retarder to convert the circularly polarized light into circularly polarized light, and the circularly polarized light is reflected by a first movable reflector onto the same optical path as the incident light, and transmitting the polarized light again to return linearly polarized light having a polarization direction different from the first linearly polarized light by 90 degrees, and reflecting this linearly polarized light by the second polarization beam splitter and making it incident on a beam combiner; and the second linearly polarized light split by the first polarizing beam splitter is reflected by a second reflector and incident on the beam superimposer where it is superimposed with the linearly polarized light, and the superimposed light is A laser interference device characterized in that it is configured to be detected by a detector.