JPH05302809A - Two wave length interference length measuring device - Google Patents

Abstract

PURPOSE:To enable accurate displacement measurement even in the case with a difference in functions for two wave lengths in an optical element by providing a polarization means which is penetrative for only polarization component crossing perpendicularly to each other in the light paths of two wave lengths measuring beam and two wave lengths reference beam. CONSTITUTION:Two wave lengths incidence beam 14 is separated into two wave lengths measuring beam 17 and two wave lengths reference beam 18 by a polarization beam splitter 15. The beams 17 and 18 after going into corner cube prisms 19 and 23 respectively are returned again to the beam splitter 15 and become two wave lengths measuring output beam 24 and two wave lengths reference output beam 25. As the light elimination ratio in the beam splitter 15 is poor, the beams 24 and 25 become elliptic polarization light. When both beams 24, 25 are let go into a polarization means 26 which is a combination of a pair of polarization element having different penetration polarization azimuth for 90 degree, the penetrated two beams 27, 28 become linear polarization lights crossing perpendicularly to each other, combined in one by beam combination element 29 and become two wave length single output beam 32 so that accurate measurement becomes possible.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-wavelength interferometric length measuring apparatus used for equipment which requires highly accurate length measuring measurement.

[0002]

2. Description of the Related Art With the progress of ultra-precision processing technology, high-precision displacement measurement using light wave interferometry is widely used. A problem in high-precision displacement measurement using light wave interferometry is a measurement error due to air fluctuations. As a method for solving the problem caused by the air fluctuation, a method has been studied in which light having two wavelengths is used to obtain a displacement for each light and a displacement for which the air fluctuation is corrected is obtained from the displacement.

[0003]

In the measuring method using such a two-wavelength interferometer, since two wavelengths of light are passed through the interferometer coaxially, each optical element has the same function for two wavelengths. Need to be configured. However, in practice, it is difficult to cause each optical element to function ideally for two wavelengths, so there is a problem that a subtle difference in function for two wavelengths affects measurement errors.

An object of the present invention is to provide an interferometer which enables highly accurate displacement measurement even when there is a difference in the function for two wavelengths in an optical element.

[0005]

The two-wavelength interferometer according to the present invention comprises (a) light source means for generating a coaxial two-wavelength light beam, and (b) most preferably a polarizing beam splitter. Polarizing beam splitting means for splitting a two-wavelength input beam obtained from the light source means into a two-wavelength measuring beam having polarization states substantially orthogonal to each other and a two-wavelength reference beam, and (c) most preferably a right-angle prism or a corner cube prism Or a quarter-wave plate and a retro-reflecting device, but a length-measuring beam reflecting means for returning the two-wavelength measuring beam so as not to overlap the two-wavelength input beam so as to be emitted from the polarized beam splitting means. (D) Most preferably a right-angle prism or a corner cube prism, but the two-wavelength reference beam is bent by the two-wavelength measuring beam reflecting means. A fixed reference beam reflecting means that is returned so as to be separated from the two-wavelength measuring beam that is output from the polarized beam splitting means and that does not overlap with the two-wavelength input beam; Preferably, the polarizers are such that the transmission polarization directions are perpendicular to each other and are integrated, but two wavelength measurement beams separated from each other by the measurement beam reflection means and the reference beam reflection means and emitted from the polarization beam splitting means. A pair of polarizing means that passes only polarization components orthogonal to each other in the optical paths of the long beam and the two-wavelength reference beam; and (f) a two-wavelength measuring beam and a two-wavelength reference beam that pass through the pair of polarizing means. Beam combining means for converting to a dual wavelength single output beam, and (g) most preferably a dichroic mirror, said dual wavelength single output beam A means for separating the respective wavelength ranges, and detection means for detecting a displacement amount of the measurement beam reflector means causes interference respective beams separated by (h) the wavelength region separating means.

As the detecting means for detecting the displacement amount of the length measuring beam reflecting means, the orthogonal polarization components of the separated output beams of the respective wavelengths are interfered with each other and the interference having the phases of 0 °, 90 ° and 180 ° is performed. Interference means for producing light, light detection means for receiving the three interference lights and outputting signals of three phases, 0 °, 1 from a signal of 90 ° phase from the output signal of the three phases
Subtracting means for subtracting the 80 ° phase signal to obtain two output signals having a DC level of zero and a 90 ° phase difference, and computing means for calculating the displacement of the object to be measured from the two output signals obtained by the subtracting means. , And correction means for obtaining a displacement in which air fluctuations are corrected from the two displacement amounts obtained by the displacement measuring means for two wavelengths.

[0007]

According to the above structure, the two-wavelength measuring beam and the two-wavelength reference beam reflected by the length measuring beam reflecting means and the fixed reference beam reflecting means and emitted from the polarization beam splitting means are polarized with respect to each other. Since the light beams pass through the pair of polarization means whose axes are orthogonal to each other, the polarization states of the respective beams are aligned to be linearly polarized lights which are completely orthogonal to each other, whereby the polarization separation action of the polarization beam splitting means is not perfect for each of the two wavelengths. In addition, accurate measurement is possible.

[0008]

FIG. 1 shows a schematic structure of a first embodiment in which the two-wavelength measuring beam reflecting means of the present invention is composed of a corner cube prism. The coaxial two-wavelength input beam 14 is obtained from the light source 13, and it is desirable that the light source 13 is a two-wavelength light source utilizing the second harmonic generation and having a rotatable polarization plane. The two-wavelength input beam 14 is split by the polarization beam splitter 15 into a two-wavelength measuring beam 17 and a two-wavelength reference beam 18. Since the polarization beam splitting surface 16 of the polarization beam splitter 15 used here is generally formed of a multilayer film, it is difficult to have a perfect polarization splitting function for both wavelengths, and the extinction ratio is not ideal. Absent. Therefore, the split dual-wavelength measuring beams 17 and 2
The wavelength reference beam 18 is slightly elliptically polarized instead of being perfectly linearly polarized orthogonal to each other.

Then, the two-wavelength measuring beam 17 is incident on the movable corner cube prism 19, is returned to the polarization beam splitter 15 again by the corner cube prism 19, passes through the polarization splitting surface 16, and is transmitted to the two-wavelength measuring output beam 2.
It becomes 4. On the other hand, the two-wavelength reference beam 18 enters the corner cube prism 23, is returned again by the corner cube prism, is reflected by the polarization splitting surface 16, and becomes the two-wavelength reference output beam 25.

Since the extinction ratio of the polarization beam splitter 15 is poor in the two-wavelength measurement output beam 24, the two-wavelength measurement output beam 24 is elliptically polarized light having a major axis parallel to the paper surface, and the two-wavelength reference output beam 25 is also for the same reason. It becomes elliptically polarized light with its long axis perpendicular to the plane of the paper. The two-wavelength measurement output beam 24 and the two-wavelength reference output beam 25 are a polarization unit 2 in which a pair of polarizers whose transmission polarization directions differ by 90 ° in the upper and lower parts in the drawing are combined.
6, the two-wavelength measurement output beam 24 is incident on the lower part of the transmission polarization direction parallel to the paper surface, and the two-wavelength reference output beam 25 is incident on the upper part of the transmission polarization direction perpendicular to the paper surface. Therefore, the two beams 27 and 28 that have passed through the polarization means 26 become linearly polarized lights that are orthogonal to each other, and are combined by the beam combining element 29 to form a two-wavelength single output beam 32.
Becomes The two-wavelength measurement output beam 27 is transmitted through the polarization splitting surface 30 of the beam combining means for converting into a two-wavelength single output beam and becomes a linear polarization component (P-polarized light) parallel to the paper surface, and the two-wavelength reference output beam 28 is reflected. The light is reflected by the surface 31 and further reflected by the polarization splitting surface 30 to become a linearly polarized light component (S polarized light) perpendicular to the paper surface.

The phase difference between the polarization components orthogonal to each other of the dual wavelength single output beam 32 changes in proportion to the optical path difference between the dual wavelength measurement beam and the dual wavelength reference beam. The dual wavelength single output beam 32 is incident on the dichroic mirror 33 and is split into beams 34A and 34B of respective wavelengths. The separated beams 34A and 34B generate polarization components that are orthogonal to each other by the half-wave plates 35A and 35B.
Rotate 5 °, non-polarizing beam splitter 36A, 36
B is branched in two directions. Beams 37A, 3 going straight
7B are polarization beam splitters 39A and 39B, respectively.
Incident on and interferes with polarized light. Assuming that the phase of the interference light 40A, 40B incident on the photodetectors 45A, 45B is 0 °, the interference light 41A, 41B incident on the photodetector 46A, 46B.
Has a phase of 180 ° with respect to the interference lights 40A and 40B.

On the other hand, the non-polarizing beam splitter 36A,
Beams 38A and 3 separated in the 90 ° direction by 36B
8B is incident on the quarter-wave plates 42A and 42B, and the phases thereof are further shifted by 90 ° between polarization components orthogonal to each other.
The beam whose phase is further shifted by 90 ° between the orthogonal polarization components is a polarizer 43A, 4 having a transmission polarization direction parallel to the paper surface.
It is incident on 3B and causes polarization interference. The interference lights 44A and 44B have a phase of 90 ° with respect to the interference lights 40A and 40B. Photodetectors 45A, 45B, 46A, 46B, 47A, 47B
Outputs a three-phase signal having a variable gain and the same DC level and AC level. The output signals of the three phases are subtracted by the subtracters 48A and 48B from the signals of the 90 ° phase to the signals of the 0 ° and 180 ° phases, and two signals having a DC level of zero and a 90 ° phase difference are output. The output signal enters the calculators 49A and 49B and the displacement of the corner cube prism 19 is calculated and output. Then, the displacement amount calculated for each wavelength enters a calculator 50 as a correction means, and is calculated and output as a displacement amount in which the air fluctuation is corrected.

FIG. 2 is a schematic diagram showing an embodiment in which the two-wavelength measuring beam reflecting means of the present invention comprises a quarter-wave plate and a retro-reflecting device. The difference from the first embodiment is that the corner cube prism 19 of FIG. 1 is replaced with a quarter wave plate 20, a plane mirror 21 and a corner cube prism 22. Polarization splitting surface 16
Of elliptically polarized light having a major axis parallel to the plane of the paper divided by
The wavelength measuring beam 17 passes through the quarter-wave plate 20 and is reflected by the plane mirror 21 and again passes through the quarter-wave plate 20 to become elliptically polarized light having a long axis perpendicular to the paper surface. Further, the beam returned to the polarization beam splitter 15 is reflected by the polarization splitting surface 16, enters the corner cube prism 22, and is returned to the polarization beam splitter 15 again. After that, the light is reflected by the polarization splitting surface 16, and is reflected by the quarter-wave plate 20 and the plane mirror 21 as elliptically polarized light whose major axis is parallel to the paper surface in the same manner as before. The light further passes through the polarization splitting surface 16 and exits from the polarization beam splitter 15. The subsequent process is the same as in the first embodiment.

Also in the configuration of the embodiment shown in FIG. 2, similarly, the displacement amount of the plane mirror 21 calculated for each wavelength output from the calculators 49A and 49B is calculated as the correction means. It enters the device 50 and is calculated and output as a displacement amount in which air fluctuations are corrected. In the configuration of each of the above-mentioned embodiments, the two-wavelength measuring beam and the two-wavelength measuring beam are used.
The configuration is such that the wavelength reference beam and the orthogonal polarization component having a phase difference proportional to the optical path difference are converted into a dual wavelength single output beam, but the optical path difference between the dual wavelength measurement beam and the dual wavelength reference beam is detected. For this purpose, it is possible to use a so-called heterodyne system in which beat signals of two wavelengths which differ from each other by a minute wavelength are used for detection. When this method is adopted, for example, in order to compensate for the incompleteness of the extinction ratio of the polarization beam splitter 15 in the configuration shown in FIG.
It is effective to insert polarizing plates whose polarization axes are orthogonal to each other in the optical paths of the two-wavelength measuring beam 17 and the two-wavelength reference beam 18 immediately after being emitted from the polarization beam splitter 15. That is, a polarizing plate that passes polarized light (P-polarized light) whose polarization axis is parallel to the paper surface is arranged at the position of the two-wavelength measuring beam 17, and polarized light perpendicular to the paper surface (S-polarized light) at the position of the two-wavelength reference beam 18. By arranging a polarizing plate that allows
In the heterodyne method, it is possible to prevent the intrusion of noise light into each other while aligning the polarizations of lights having slightly different wavelengths, and improve the measurement accuracy.

[0015]

As described above, even when there is a slight difference in the performance of the polarized beam splitting means for two wavelengths,
Since the polarization separation between the two-wavelength measuring beam and the two-wavelength reference beam can be performed completely, a two-wavelength interferometric measuring device capable of highly accurate displacement measurement can be achieved.

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram of a two-wavelength interferometer length measuring device according to a first embodiment of the present invention.

FIG. 2 is a schematic configuration diagram of a two-wavelength interferometer length measuring device according to a second embodiment of the present invention.

[Explanation of symbols]

13 ... Two-wavelength light source, 15 ... Polarizing beam splitter 19 ... Movable corner cube prism 20 ... Quarter wave plate 21 ... Movable plane mirror 22, 23 ... Fixed corner cube prism 26 ... Polarizing means 29 ... Beam combining element 34 ... Dichroic mirror 35A, 35B ... Half wave plate 36A, 36B ... Non-polarizing beam splitter 39A, 39B ... Polarizing beam splitter 42A, 42B ... Quarter wave plate 43A , 43B ... Polarizers 45A, 45B, 46A, 46B, 47A, 47B ...
Light detecting means 48A, 48B, 49A, 49B, 50 ... Computing means

Claims (4)

[Claims] 1. A light source means for generating a coaxial two-wavelength light beam, and a two-wavelength measuring beam having a polarization state in which a two-wavelength input beam obtained from the light source means is substantially orthogonal to each other.
A polarized beam splitting means for splitting the two-wavelength measuring beam into a wavelength reference beam; a measuring beam reflecting means for returning the two-wavelength measuring beam so as to be emitted from the polarized beam splitting means without overlapping with the two-wavelength input beam; The reference beam is turned back by the two-wavelength measuring beam reflecting means and turned back so as to be emitted in a state separated from the two-wavelength measuring beam emitted from the polarization beam splitting means and without overlapping with the two-wavelength input beam. In the optical paths of the fixed reference beam reflecting means, the two-wavelength measuring beam and the two-wavelength reference beam, which are folded back by the length measuring beam reflecting means and the reference beam reflecting means and emitted from the polarized beam splitting means, respectively. A pair of polarizing means that pass only the polarization components orthogonal to each other, and a two-wavelength measuring device that passes the pair of polarizing means. A beam and two-wavelength reference beam 2
Beam combining means for converting into a single wavelength output beam, wavelength range separating means for separating the two wavelength single output beam into respective wavelength ranges, and respective beams separated by the wavelength range separating means A two-wavelength interferometric length measuring apparatus, comprising: a detecting unit that detects a displacement amount of the length measuring beam reflecting unit. 2. The detecting means interferes the orthogonal polarization components of the respective beams separated by the wavelength band separating means to make 0 °, 90 ° with respect to the displacement of the length measuring beam reflecting means.
Interfering means for producing interference light having a phase of °, 180 ° and 0 °, 90 °, 180 for receiving the interference light, respectively.
A light detecting means for outputting three signals having a phase of 0 °, and a signal having a phase of 90 ° from the three outputs, 0 °,
Subtracting means for subtracting a 180 ° phase signal and outputting two signals having a DC level of zero and quadrature with each other, and an operation for calculating the displacement of the length measuring beam reflecting means from the two output signals obtained by the subtracting means. 2. The two-wavelength interferometer length measuring apparatus according to claim 1, further comprising: a means and a correction means for calculating a displacement amount obtained by correcting the air fluctuation from the two displacement amounts obtained by the arithmetic means for two wavelengths. apparatus. 3. The two-wavelength interferometer according to claim 1, wherein the two-wavelength measuring beam reflecting means comprises a right-angle prism or a corner cube prism which can be displaced in the optical axis direction. 4. The two-wavelength measuring beam reflecting means comprises one plane mirror which is displaceable in the optical axis direction, and functions for two wavelengths between the polarized beam splitting means and the plane mirror. The two-wavelength interferometer according to claim 1 or 2, wherein a quarter-wave plate is arranged.