JPH02236102A - Laser interferometer - Google Patents

Abstract

PURPOSE:To prevent deterioration of the S/N ratio thereby to reduce factors of errors even when an incident beam is diffused at large angles by rotating an angle of a plane of polarization of said light incident upon an interferometer. CONSTITUTION:An interferometer 6 consists of a polarizing beam splitter (PBS) 6' and a quarter-wave plate 6''. A laser light incident upon the interferometer 6 is divided into two, measuring and reference lights having approximately the same strength of beams by the PBS6'. The components of the reference light and measuring light are further separated by a beam splitter 8 in front of a polarizing plate 17 and detected respectively by photodetectors 10, 11. A differential amplifier 12 detects the difference of the strength of the components and consequently a current controlling circuit 13 is controlled to that the difference becomes zero. An angle of the plane of polarization is thus rotated. Accordingly, the strength of the reference light and that of the measuring light at a photodetector 14 can be made equal to each other, whereby the interference efficiency and visibility are improved, and measuring errors are reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a laser length measuring device that utilizes interference of orthogonal linearly polarized light.

[Prior Art] Conventionally, in this type of length measuring instrument, linearly polarized light from a laser is incident on a polarizing beam splitter that constitutes an interferometer, and is divided into a reference light and a sub-constant light that is directed toward the measurement target. There are heterodyne types (for example, see Nikkei Microdevices, October 1985 issue, pages 67-77) and homodyne types.

In recent years, it has become common practice to guide light from a laser oscillator to an interferometer using an optical fiber.

[Problem to be Solved by the Invention] In the prior art, the angle of the plane of polarization of the IiA polarized light directly incident on the interferometer is fixed, and the intensity ratio of the measurement light and reference light changes as the measurement target moves. Measurement errors caused by fluctuations were not taken into account.

For example, if the beam divergence angle of the laser oscillator that is the light source is small, even if the target object is displaced and the optical path length of the measurement light becomes longer, the beam diameter of the measurement light will not expand much at the receiver position. However, in the case of a laser oscillator with a large beam divergence angle, or as has become popular recently, interference fringes from the laser oscillator via an optical fiber can be obtained. If the beam spread is large due to the performance of the collimation lens at the exit of the fiber in a system that guides the laser beam to the interferometer, there is no particular problem when the object to be measured is close to the interferometer, but if the object is When the distance from the interferometer increases due to displacement, and the measurement optical path length increases, the diameter of the measurement light beam at the position of the receiver that detects the interference light increases, causing a decrease in the power density of the beam and a decrease in interference efficiency. This results in deterioration of the visibility of the interference light. In this case, the S/N of the interference signal after photoelectric conversion by the photoreceiver becomes poor, and 81! This may lead to a fixed error.

An object of the present invention is to maintain good visibility of interference light and prevent deterioration of the S/N ratio of interference signals even when the incident beam has a spread and the measurement light beam diameter at the receiver changes. The goal is to provide a laser interferometric length measurement device that can be used to obtain laser interferometers.

[Means for Solving the Problems] The laser interferometric length measuring device of the present invention has the configuration described in each claim of the claims.

[Function] In the laser interferometric length measuring device according to claim 1, 2, 3, or 4, by rotating the angle of the polarization plane of the light incident on the interferometer, the reference light and measurement light are separated by the interferometer. In the laser interferometric length measuring device according to claim 5, the intensity ratio of
By rotating the angle of the interference means, the intensity ratio of the reference light and measurement light when interference occurs in the interference means is controlled. This control is performed based on the detection result of the monitor light as described in each claim, and thereby the intensity ratio at both ends is maintained so as to improve interference.

[Example] FIG. 1 shows an example of the present invention. 1 is a wavelength-stabilized laser oscillator that oscillates linearly polarized light, 3 is a polarization-maintaining fiber, and 5 is a Faraday element that controls the angle of the polarization plane.
A coil l8 for magnetic field control is wound around it. 6 is an interferometer that measures the displacement of the measurement object 7, and 8 is a beam splitter (
9 is a polarizing beam splitter (abbreviated as PBS) that separates the monitoring beam taken out from BS8 into a reference light component and an all constant light component; 10.11
12 is a differential amplifier that outputs the intensity difference between the two optical components, and 13 is a current control circuit that controls the current to the coil 18 wound around the Faraday element 5. . 17 is a polarizing plate, 14 is a photodetector, and 15 is a phase difference measuring device.

The operation will be explained below. The laser beam from the laser oscillator 1 is focused by the collimator 2 onto the input side end face of the polarization maintaining fiber 3, enters the same fiber, and is collimated by the output side collimator 4 to maintain its polarization state (linear polarization). The light is emitted as almost parallel light. However, due to the characteristics of the collimating lens and its imperfect alignment with the fiber, this emitted beam generally has a somewhat larger divergence angle than that of the laser oscillator itself, and the longer the optical path length, the larger the beam diameter. This laser light enters an interferometer 6 via a Faraday element 5. Interferometer 6 is a polarizing beam splitter (13BS) 6'
, 1/4 wave plate 6#, 6'' is attached, and the plane of polarization of the laser beam incident on this 'f interferometer 6 is set at approximately 45 degrees with respect to the plane of the paper. The interferometer 6 is divided into two beams of approximately equal intensity by the Pn S 6', which constitutes the 41ff beam, and becomes a measurement beam and a reference beam, respectively.The measurement beam is reflected by the measurement object 7 and passes through the interferometer 6 again. The light then returns to the polarizing plate 17 via the beam splitter (BS) 8.

On the other hand, the reference light is reflected by the reference surface 16 provided on one end face of the 174-wave plate 6 of the interferometer 6 and reaches the polarizing plate 17 via the same <Bs8. Since the planes of polarization of the lights are orthogonal to each other, they do not interfere, but their common components are extracted and interfere with each other in the polarizing plate 17, and this interference light is received by the photodetector 14 and photoelectrically converted, resulting in a change in the signal. is taken into the phase difference measuring device 15 and processed to obtain the measured object displacement measurement output X.

Here, the optical path lengths of the reference light and measurement light differ by twice the distance dead path between the interferometer 6 and the measurement object 7. When the measurement object 7 is displaced and the dead path becomes longer, only the beam diameter of the measurement light at the position of the photodetector 14 increases due to the beam spreading as described above, and therefore, it is natural that the measurement at this position will not be possible. The power density within the beam of light also decreases,
As a result, the interference efficiency of the interference light deteriorates, and visibility decreases. In this state, the S/N ratio of the interference signal that is input to the phase difference measuring device 15 deteriorates, which becomes a cause of error in the displacement measurement output X.

Therefore, in the present invention, by controlling the angle of the polarization plane of the light incident on the interferometer 6, the split intensity ratio of the reference light and measurement light is adjusted appropriately, so that highly visible interference fringes are always obtained. .

This will be explained next. In the case of Fig. 1, if the angle between the plane of polarization of the light incident on the interferometer 6 and the plane of the paper is θ, then the P of the interferometer 6 is
The intensity ratio of the reference light and measurement light divided by B96' is sin
θ: cos O. When O=45'', the two are equal, but the beam for intensity ratio monitoring is taken out by BS8 placed in front of the polarizing plate 17, and the reference light component of this monitoring beam is converted by r'ns9 to the constant light component. When reseparated and detected by photodetectors 10 and 11,
There, the intensity of the measurement light component is lower than the intensity of the reference light component due to the beam expansion due to the dead path.

Therefore, this difference in intensity is detected by the differential amplifier 12 and set to 0 so that this difference becomes zero, that is, the intensity of the 21+'l constant light component detected by the photodetector 10 and I1 and the reference light component are equal. The intensity ratio of the reference light and measurement light separated by the interferometer 6 is adjusted by changing the intensity ratio. By doing this,
The intensity of the reference light and the measurement light at the photodetector 14 can be made equal, and as a result, the interference efficiency of the resulting interference light improves, and the visibility improves. In this embodiment, for example, a Faraday cable 5 is used as a means for changing θ. This is an element that can change the angle of the plane of polarization of linearly polarized light traveling inside it depending on the strength of the applied magnetic field.In the case of this embodiment, the current flowing through the coil 18 is controlled by the current control circuit 13. By constructing a closed-loop system that controls the magnetic field inside the Faraday element, the above-mentioned intensity ratio adjustment is achieved.

FIG. 2 shows another embodiment of the invention. In this figure, the same parts as in FIG. 1 are designated by the same reference numerals. The configuration of this embodiment is almost the same as the previous embodiment, and the only difference is the means for changing the polarization plane angle 0 of the linearly polarized light incident on the f-intermeter 6. In this embodiment, the means for changing θ is 2/
A one-wavelength plate 20 is used, and its installation angle is adjusted by a motor 21 such as a stamping motor. The control is carried out in almost the same way as in the case of Fig. 1, by detecting the difference in intensity between the measurement light component and the reference light component received by the photodetection signal 10 and 11 using the differential amplifier 12, and controlling the motor control circuit so that this difference signal becomes zero. 19 constitutes a closed loop control system.

Next, another embodiment of the present invention will be described with reference to FIG. In this figure as well, the same reference numerals are used for the same parts as in the previous figure. This embodiment differs from the previous embodiments in that the interferometer 6
No means for changing the angle of polarization plane of the incident light is provided on the incident side of the polarizing plate 17, and a part of the interference light after interference with the polarizing plate 17 is divided for monitoring purposes, so that the amplitude of this interference signal is maximized. The point is to rotate the polarizing plate 17 to control the angle of its transmission axis. That is, the interference light that has interfered at the polarizing plate 17 is split into two by the beam splitter 8. One of these beams is used as a monitoring beam, which is photoelectrically converted by the photodetector 10, the point where the amplitude becomes maximum is detected by the interference signal maximum amplitude detection circuit 24, and the motor control circuit is configured so that the amplitude is always the maximum. 19 and the motor 21 to control the angle of the transmission axis of the polarizing plate 17, the same effects as in the previous embodiment can be obtained.

In addition, as shown in FIG. 4 as a modified example of FIG. 1,
The interference signal is taken into the interference signal maximum amplitude detection circuit 24 in the same manner as shown in FIG. 3, and the current control circuit 13 controls the current applied to the coil 18 wound around the Faraday element 5 so that the amplitude is always the maximum The rotation angle of the linearly polarized light incident on the interferometer 6 may be adjusted.

In this case, the polarization plane rotating means may use a l/2 wavelength plate as shown in FIG. 2 instead of the Faraday element.

As mentioned above, the method according to any of the embodiments is applicable to the patent application No. 63-027025 and No.
The present invention can also be applied to a laser interferometric length measuring device such as that shown in No. 44183, in which the measurement light and the reference light each consist of two beams, and the same effects as those described above can be expected in this case as well.

Furthermore, each of the above embodiments uses homodyne interference di! I
Although a long instrument has been disclosed, it can also be applied to a heterodyne type interferometric length measuring instrument.

[Effects of the Invention] According to the present invention, even when the divergence angle of the incident beam to the T-intermeter is large, the intensity of the measurement light and the reference light at the detection position of the interference light are always equal, and maximum visibility can be obtained. Therefore, in a device that measures phase difference, that is, displacement from interference light intensity, the S/N ratio is improved and error factors can be reduced.

[Brief explanation of drawings]

FIG. 1, FIG. 2, FIG. 3, and FIG. 4 are block diagrams of different embodiments of the present invention, respectively. 1... Laser oscillator 3... Optical fiber 5...
Faraday element 6...Interferometer 7...Measurement target
8...BS9...PBS 10,1
1... Photodetector 12... Differential amplifier 13...
Current control circuit 14...Photodetector 15...Phase difference fll'l constant device 17...Polarizing plate 19...
・Motor control circuit 20...l/2 wavelength plate 21・
...Motor 24...Interference signal maximum amplitude detection circuit Fig. Fig. Displacement measurement output

Claims (1)

[Claims] 1. Linearly polarized light from a laser oscillator is made incident on an interferometer having a polarization beam splitter function, where it is split into a measurement beam and a reference beam, and the two beams pass through an optical path for the measurement beam and an optical path for the reference beam, causing interference between the two beams. In a laser interferometric length measuring instrument configured to introduce the output light from the interferometer into the interference means and obtain the length measurement output from the light receiving measuring means that receives the interference light after passing through the interference means, a part of the output light from the interferometer is used. means for extracting the extracted monitor light as a monitor light at a stage before the interference means, a monitor means for separating the extracted monitor light into a measurement light component and a reference light component and detecting the intensity relationship between them; and control means for rotating the angle of the plane of polarization of the linearly polarized light incident on the interferometer so as to maintain the intensity relationship at a predetermined value. 2. The linearly polarized light from the laser oscillator is input into an interferometer with a polarization beam splitter function to split it into a measurement beam and a reference beam, and the output light from the interferometer for both beams passes through the measurement beam optical path and the reference beam optical path. In a laser interferometric length measuring device configured to obtain a length measurement output from a light receiving measuring means that is introduced into an interference means and receives the interference light that has passed through the interference means, a part of the interference light that has passed through the interference means is taken out as monitor light. means and
a monitor means for receiving the extracted interference light as the monitor light and detecting the amplitude of the interference signal; and a linearly polarized light input to the interferometer so as to maximize the amplitude of the interference signal detected by the monitor means. A laser interferometric length measuring device comprising: control means for rotating the angle of the plane of polarization. 3. The laser interferometric length measuring device according to claim 1 or 2, wherein a Faraday element is used as the optical means for rotating the angle of the polarization plane of the linearly polarized light incident on the interferometer. 4. A rotatably supported 1/
3. The laser interferometric length measuring device according to claim 1, which uses a two-wavelength plate. 5 Linearly polarized light from a laser oscillator is input into an interferometer with a polarization beam splitter function to split it into a measurement beam and a reference beam, and the output light from the interferometer for both beams passes through the measurement beam optical path and the reference beam optical path. In a laser interferometric length measuring device configured to obtain a measurement output from a light reception measuring means that is introduced into an interference means and receives the interference light that has passed through the interference means, means for extracting a part of the interference light that has passed through the interference means as monitor light. and,
A monitor means for receiving the interference light as the extracted monitor light and detecting the amplitude of the interference signal, and rotating the angle of the interference means so that the amplitude of the interference signal detected by the monitor means is maximized. A laser interferometric length measuring device characterized by comprising a control means.