WO2020098227A1

WO2020098227A1 - Method and device for correcting non-linear errors of single-frequency laser interferometer

Info

Publication number: WO2020098227A1
Application number: PCT/CN2019/083984
Authority: WO
Inventors: 胡鹏程; 付海金; 王珂; 谭久彬
Original assignee: 哈尔滨工业大学
Priority date: 2018-11-14
Filing date: 2019-04-24
Publication date: 2020-05-22
Also published as: CN109539975A

Abstract

A method and device for correcting non-linear errors of a single-frequency laser interferometer, which relate to the technical field of laser measurement. The present invention uses a liquid crystal phase retarder to generate a continuous optical path difference variation between a measurement beam and a reference beam without needing to change positions of a first mirror and a second mirror, so that an interference signal generates a sufficient phase change. The pre-extraction of feature parameters of the interference signal is achieved, and thus non-linear errors in a displacement measurement process of the single-frequency laser interferometer are corrected by using the pre-extracted feature parameters so as to achieve high-precision displacement measurement. The present invention achieves the pre-extraction of the feature parameters of the interference signal of the single-frequency laser interferometer, may effectively solve the correction problem of non-linear errors in interference measurements, especially micro-displacement measurements, and has significant technical advantages in the field of precision measurement.

Description

Non-linear error correction method and device for single-frequency laser interferometer

Technical field

The invention belongs to the technical field of laser measurement, and mainly relates to a method and a device for correcting nonlinear errors of a single-frequency laser interferometer.

Background technique

With the rapid development of scientific research and the rapid improvement of industrial production levels, the scientific research and industrial fields have also put forward higher requirements for displacement measurement. The minimum change in displacement measurement is also developing in the direction of nanometers. Single-frequency laser interference is an instrument that uses the principle of laser interference for high-precision displacement measurement, and has the advantages of non-contact and high precision. A single-frequency laser interferometer contains at least one light source that can provide a single-frequency laser; a polarizing beam splitter that divides the single-frequency light source into a reference beam and a measurement beam; a first mirror that can reflect the reference beam; and one that can reflect the measurement beam The second mirror is usually fixed to the object to be measured and moves with the object to be measured; at least one photodetector capable of detecting interference signals, the interference signals are reflected by the first reflection The reference beam obtained by the mirror reflection interferes with the measurement beam reflected by the second mirror; and a signal processing unit, coupled to the photodetector, is adapted to collect the interference signal output by the photodetector; The reference beam and the measurement beam have the same frequency. Compared with the dual-frequency laser interferometer, it has many advantages such as simple structure, easy circuit processing, low environmental requirements, and unlimited measurement speed in principle. Therefore, it is more widely used in the field of displacement measurement. However, in practical applications, the existence of nonlinear errors has always been a key issue that limits the single-frequency laser interferometer to achieve high-precision measurements.

Figure 1 shows the structure of a typical single-frequency laser interferometer. The single-frequency laser emitted from laser 1 is split by a polarization beam splitter prism 2 into a reference beam and a measuring beam; the reflected beam is reflected by the plane mirror A 4 as a reference beam, and twice Through the 1/4 wave plate A 3, the transmitted beam is reflected by the plane mirror B 6 as the measuring beam, and after passing through the 1/4 wave plate B 5 twice, the reference beam and the measuring beam are transmitted and reflected respectively through the polarization beam splitter prism 2; The reference beam and the measurement beam pass through the 1/2 wave plate 7 and the polarization direction is rotated by 45 °. After being split by the non-polarization beam splitter prism 8, the transmitted light enters the photodetector through the 1/4 wave plate C 9 and the polarization beam splitter prism B 10 A 11 and photodetector B 12, the two signals are subtracted by the calculator A13 to obtain the interference signal _Ix ; the reflected light passing through the non-polarizing beam splitter prism 8 enters the photodetector C 15 through the polarizing beam splitter prism C 14 and The photodetector D 16, and the two signals pass the arithmetic unit B 17 to perform the subtraction operation to obtain the interference signal I _y . In an ideal state, I _x and I _y can be expressed as: (P. Hu, J. Zhu, X. Guo, and J. Tan, "Compensation for the Variable Cyclic Error in Homodyne Laser Interferometers," Sensors, 2015, 15 ( 2): 3090-3106.):

Among them, A is the AC amplitude of the interference signal, and φ is the phase difference between the reference optical path and the measurement optical path. It can be seen from this that I _x and I _y appear as a sine and cosine function about φ, and under ideal conditions, their amplitudes are equal, the DC offset is zero and they are orthogonal to each other. However, in the actual situation, due to the non-ideal optical devices, I _x and I _y can be expressed as:

Among them, A _x and A _y are DC offset errors, B _x and B _y are unequal amplitude errors, and δ is a non-orthogonal error. It can be seen from formula (2) that I _x and I _y actually appear as a sine and cosine function containing the above three differences. When the above two interfering signals with triple difference are directly used for displacement calculation, periodic nonlinear errors will occur, which will affect the measurement accuracy. Therefore, it is necessary to correct I _x and I _y by obtaining the characteristic parameters A _x , A _y , B _x , B _y and δ of the interference signal to obtain the ideal orthogonal interference signals cosφ and sinφ, thereby realizing the correction of the nonlinear error.

The method of correcting nonlinear errors was first proposed by Heydemann in 1981. He used least squares to ellipse fit interference signals with a period greater than one period to obtain the characteristic parameters of the interference signals, thereby realizing the correction of nonlinear errors (PLM Heydemann, Determination and correction of quadrature measurement errors in interferometers.Appl.Opt.1981, 20: 3382-3384), this method is a classic method of nonlinear error correction, and researchers have proposed a number of improved methods based on this method, all It can be called Heydemann correction method; Dai of the German Federal Institute of Physics extracts the nonlinear error parameters in real time by detecting the maximum and minimum values of each interference signal within a period, and realizes the real-time correction of nonlinear errors ( G.-L.Dai, F.Pohlenz, H.-U.Danzebrink, K.Hasche, G.Wilkening, Improving the performance of interferometers inmetrological scanning probe microscopes.Meas.Sci.Technol.2004,15: 444-450 ), Which is called the extreme value correction method. Although the above two methods achieve the correction of the nonlinear error, the prerequisite for its normal operation is that the phase change of the interference signal is not less than one cycle.

In order to achieve the above prerequisites, it is necessary to make the phase of the interference signal change by not less than one period (2π), that is, the change of the optical path difference between the reference optical path and the measurement optical path is not less than the laser wavelength. In practice, the method generally adopted is to move the first mirror or the second mirror, and to change the phase of the interference signal by changing the optical path of the measuring beam or the reference beam. But these two methods have certain defects in practice. The method of moving the second mirror is generally to control the movement of the object to be measured, so that the second mirror produces a displacement greater than the half wavelength of the laser, thereby acquiring an interference signal whose phase change is greater than one period. However, in actual situations, sometimes the displacement of the measured object that can move is smaller than the above-mentioned displacement size or even cannot be arbitrarily moved, so the above prerequisites cannot be met. In contrast, the method of moving the first mirror is generally to drive the first mirror by adding additional piezoelectric ceramics or other motion control elements, which also causes it to generate a displacement greater than the half-wavelength of the laser, because the magnitude of the displacement is relatively controllable Therefore, the above prerequisites can usually be met. However, this method also has certain problems: the additional motion control elements increase the complexity of the system and control, and inevitably affect the position stability of the first mirror, thereby introducing measurement errors.

In 2015, Zhu et al. Proposed a method for correcting nonlinear errors using optical switches. This method configures one optical switch for each of the reference and measurement optical paths. The combination of two optical switches "on" and "off" can be used in the test. Obtain some nonlinear error parameters in the interference signal when the object is at rest, (J.Zhu, P.Hu, J.Tan, Homodynelaservibrometercapableofdetectingnanometerdisplacementsaccuratelybyusingopticalshutters.Appl.Opt.2015,54 : 10196–10199). This method can achieve the correction of the nonlinear error in the measurement with displacement less than λ / 2 in conjunction with the specific optical path. However, this method also has certain shortcomings: First, this method can only obtain the DC offset error and unequal amplitude error parameters among the three differences of the characteristic parameters of the interference signal, but the non-orthogonal error parameters cannot be obtained, so it is necessary The measurement can only be carried out with a specific interference optical path structure, which is not universal; secondly, this method requires two optical switches, which leads to an increase in the volume of the device, and requires multiple operations on the two optical switches. The steps are complicated.

Summary of the invention

In view of the above-mentioned problems of the nonlinear correction method, the present invention proposes and develops a method and device for nonlinear error correction of a single-frequency laser interferometer based on a liquid crystal phase retarder. The present invention does not need to change the first mirror and the second Under the premise of the mirror position, by adding a liquid crystal phase retarder at the position of the common optical path of the reference beam and the measuring beam of the single-frequency laser interferometer, the polarization-related phase retardation characteristics of the liquid crystal phase retarder are used to make the The optical path difference between them produces a continuous change, so that the interference signal obtained by the detector produces sufficient phase change to realize the pre-extraction of the characteristic parameters of the interference signal, and the pre-extracted characteristic parameters are used to achieve the purpose of non-linear error correction during the measurement process. .

The object of the present invention is achieved by the following technical solutions:

A nonlinear error correction method for a single-frequency laser interferometer. The single-frequency laser interferometer includes: at least one light source capable of providing a single-frequency laser; an optical path including: a polarizing beam splitter, a first reflecting mirror, and a second A reflecting mirror, wherein the polarizing beam splitter is adapted to divide the single frequency light source into a reference beam and a measuring beam, the first reflecting mirror is adapted to reflect the reference beam, and the second reflecting mirror is adapted to reflect the The measuring beam; at least one photodetector capable of detecting an interference signal, the interference signal being formed by interference between the reference beam reflected by the first mirror and the measuring beam reflected by the second mirror.

The liquid crystal phase retarder is a polarization-dependent optical element. The phase retardation of the polarization component in the incident laser beam consistent with its slow axis direction is related to the operating voltage of the liquid crystal phase retarder, and the phase of the polarization component perpendicular to its slow axis direction The delay size has nothing to do with the working voltage of the liquid crystal phase retarder. Therefore, by adding at least one liquid crystal phase retarder in the optical path of the single-frequency laser interferometer, the liquid crystal phase retarder is adapted to change the phase difference between the reference beam and the measurement beam; An operating voltage of the liquid crystal phase retarder causes a continuous change in the phase difference between the reference beam and the measurement beam; the corresponding interference signal generates a corresponding phase change, thereby achieving the characteristic parameters of the interference signal Pre-extraction; in the displacement measurement process of single-frequency laser interferometer, the nonlinear error correction of the measured displacement can be achieved by using the pre-extracted nonlinear error parameters. In this process, the liquid crystal phase retarder should be made Operating voltage remains unchanged.

The position of the liquid crystal phase retarder is selected from between the light source and the polarization beam splitter and between the polarization beam splitter and the photodetector, at which position the reference beam and the measurement beam share a common optical path .

The slow axis direction of the liquid crystal phase retarder is the same as the polarization direction of the reference beam or measurement beam.

A nonlinear error correction device for a single-frequency laser interferometer, the device includes at least one light source capable of providing single-frequency laser; an optical path, the optical path includes: a polarizing beam splitter, a first reflecting mirror and a second reflecting mirror, wherein , The polarization beam splitter is adapted to divide the single frequency light source into a reference beam and a measurement beam, the first mirror is adapted to reflect the reference beam, and the second mirror is adapted to reflect the measurement beam; At least one photodetector capable of detecting interference signals formed by interference between the reference beam reflected by the first mirror and the measurement beam reflected by the second mirror; and at least one liquid crystal phase retardation Each of the liquid crystal phase retarders is placed in the optical path, and the liquid crystal phase retarder is adapted to change the phase difference between the reference beam and the measurement beam. The device further includes: a signal processing unit, coupled to the photodetector, adapted to collect the interference signal output by the photodetector, and the characteristic parameter of the interference signal indicates the displacement measurement process of the single-frequency laser interferometer Nonlinear error.

The invention has the following characteristics and good effects:

(1) Compared with the method of Heydemann or extremum correction, this method can produce a continuous between the measurement beam and the reference beam by using the liquid crystal phase retarder without changing the position of the first mirror and the second mirror The optical path difference changes to realize the pre-extraction of the characteristic parameters of the interference signal, so as to correct the non-linear errors in the displacement measurement process of the single-frequency laser interferometer to achieve high-precision displacement measurement. Compared with the above two methods, the technical solution of the present invention particularly solves the problem that the nonlinear error cannot be effectively compensated when the measured displacement is less than the half wavelength of the laser, and improves the accuracy of displacement measurement.

(2) Compared with the method of using optical switches for non-linear error correction, since only one optical element with a smaller volume is used, two optical switches with complicated mechanical structures are replaced, reducing the system volume and complexity, and The operation steps are simplified; because all the interference signal characteristic parameters can be extracted and do not depend on the specific interference optical path structure, the correction accuracy of the nonlinear error is improved, and the applicability of the correction method is improved.

(3) Place the liquid crystal phase retarder before the polarizing beam splitter or after the polarizing beam splitter. The position of the common beam path of the reference beam and the measuring beam can be effectively suppressed compared to placing the liquid crystal phase retarder in the reference beam or measuring beam. The phase delay drift introduced by the liquid crystal phase retarder due to temperature and other external environmental changes improves the accuracy of the phase measurement in the displacement measurement process.

BRIEF DESCRIPTION

1 is a schematic diagram of the configuration structure of a two-division optical path single-frequency interferometer composed of an existing polarization beam splitter prism and a plane mirror;

2 is a schematic diagram of the overall configuration of the present invention when it is applied to the single-frequency interferometer in FIG. 1 as an example;

Description of the part numbers in Figure 1: 1 single-frequency laser, 2 polarization beam splitter prism A, 3 1/4 wave plate A, 4 first mirror, 5 1/4 wave plate B, 6 second mirror, 7 1/2 Wave plate, 8 beam splitter prism, 9 quarter wave plate C, 10 polarization beam splitter prism B, 11 photodetector A, 12 photodetector B, 13 subtractor A, 14 polarizing beam splitter prism C, 15 photodetector C, 16 photodetector D, 17 subtractor B, 18 signal processing unit.

Part number description in Figure 2: 21 single frequency laser, 22 liquid crystal phase retarder, 23 polarization beam splitter prism A, 24 quarter wave plate A, 25 first mirror, 26 quarter wave plate B, 27 second reflection Mirror, 28 1/2 wave plate, 29 unpolarized beam splitter prism, 30 1/4 wave plate C, 31 polarized beam splitter prism B, 32 photodetector A, 33 photodetector B, 34 subtractor A, 35 polarized beam splitter prism C, 36 photodetector C, 37 photodetector D, 38 subtractor B, 39 signal processing unit, 40 position A.

detailed description

Since the single-frequency interferometer itself has different forms of optical path structures, the following uses a two-divided optical path single-frequency interferometer composed of a polarizing beam splitter prism and a plane mirror shown in FIG. 2 as an example to describe the embodiments of the present invention in detail.

A nonlinear error correction device for a single-frequency laser interferometer based on a liquid crystal phase retarder, the device includes a single-frequency laser 21, a liquid crystal phase retarder 22, a polarization beam splitter prism A 23, a quarter wave plate A 24, and a first reflection Mirror 25, 1/4 wave plate B 26, second mirror 27, 1/2 wave plate 28, non-polarization beam splitter prism 29, 1/4 wave plate C 30, polarization beam splitter prism B 31, photodetector A 32, Photodetector B 33, subtractor A 34, polarizing beam splitter prism C 35, photodetector C 36, photodetector D 37, subtracter B 38; on the exit optical path of the single-frequency laser 21, a polarizing beam splitter prism A 23 is arranged in this order , 1/4 wave plate B 26 and second mirror 27, the 1/4 wave plate B 26 is located in the x, y plane, and is coaxial with the polarization beam splitter prism A 23, 1/4 wave plate B 26 fast axis The direction is 45 ° counterclockwise with the y-axis, and the second mirror 27 is parallel to the 1/4 wave plate B 26; the 1/4 wave plate A 24 and the first reflection are sequentially arranged on the reflection optical path of the polarization beam splitter prism A 23 Mirror 25, the 1/4 wave plate A 24 is located in the y and z planes, and is coaxial with the polarization beam splitter prism A 23, and the fast axis direction of the 1/4 wave plate A 24 is 45 ° clockwise with the y axis. The first reflecting mirror 25 is parallel to the 1/4 wave plate A 24; on the opposite side of the polarizing beam splitting prism A 23 located on the opposite side of the first reflecting mirror 25, a 1/2 wave plate 28, a non-polarizing beam splitting prism 29, 1 are arranged in this order / 4 wave plate C 30, polarization beam splitter prism B 31, photodetector A 32, the 1/2 wave plate 28 is located in the y, z plane, and is coaxial with the polarization beam splitter prism A 23, 1/2 wave plate 28 The fast axis direction is 22.5 ° counterclockwise with the z axis. The 1/4 wave plate C 30 is located in the y and z planes and is coaxial with the polarization beam splitter prism A 23. The 1/4 wave plate C 30 fast axis direction is The axis is 45 ° counterclockwise; a photodetector B 33 is arranged on the reflected optical path of the polarizing beam splitter prism B 31; a polarizing beam splitter prism C 35 and a photodetector C are arranged in sequence on the reflected optical path of the non-polarizing beam splitter prism 29 36; a photodetector D 37 is arranged on the reflection optical path of the polarization beam splitter prism C 35; the interference signals detected by the photodetector A 32 and the photodetector B 33 are input to the subtractor A 34 to perform the subtraction operation to obtain interference Signal I _x ; the interference signals detected by the photodetector C 36 and the photodetector D 37 are input to the subtractor B 38 to perform the subtraction operation to obtain the interference signal I _y ; the liquid crystal phase retarder 22 and the quarter wave plate B 26 Parallel to each other and coaxially arranged between the single frequency laser 21 and the polarization beam splitter prism A 23, the slow axis direction of the liquid crystal phase retarder 22 is the same as the y axis direction, or the same as the x axis direction; the liquid crystal phase retarder 22 It can also be placed at position A 40, that is, parallel to 1/4 wave plate A 24 and coaxial Between the optical prism A 23 and the 1/2 wave plate 28, the slow axis direction may be either the y-axis direction or the x-axis direction.

The following also uses the two-division optical path single-frequency interferometer composed of the polarizing beam splitter prism and the plane mirror shown in FIG. 2 as an example to explain the steps of the method as follows:

(1) Turn on the single-frequency laser interference vibrometer, the single-frequency laser emits a single-frequency laser, and the laser is first incident vertically through the liquid crystal phase retarder, where the slow axis direction of the liquid crystal phase retarder is vertical, and the liquid crystal phase is delayed at this time The phase delay of the horizontal and vertical polarization components of the laser to the laser are respectively

with

After that, the polarization beam splitter prism separates the horizontal and vertical polarization components of the laser into a measurement beam and a reference beam; the reference beam passes through a 1/4 wave plate, and then is reflected by the reflector, then returns to the original path; meanwhile, the measurement beam passes through a 1/4 wave After the film, it illuminates the target to be measured (such as a plane reflector, a corner prism, and the surface of the object to be measured) and then reflects back along the original path; both the reference beam and the measurement beam pass through the 1/4 wave plate twice, and the polarization state is rotated After 90 °, it enters the polarization beam splitter prism again; the reference beams and measurement beams from the orthogonal horizontal and vertical polarization states exiting from the polarization beam splitter prism pass through the beam splitter prism and other devices, and finally pass through the detector and subtractor to obtain the formula (2) The two paths shown contain three-difference interference signals I _x and I _y ;

(2) Change the operating voltage of the liquid crystal phase retarder. According to the operating characteristics of the liquid crystal phase retarder, the phase retardation of the liquid crystal phase retarder to the horizontal polarization component of the laser does not change; Changed

Therefore, in this process, the amount of change in the optical path difference between the reference optical path and the measurement optical path is

The phase changes of the two corresponding interference signals I _x and I _y are also

Store the two interference signals I _x and I _y during the change; when

That is, when the variation of the optical path difference is greater than the laser wavelength λ, the phase changes of the two interference signals I _x and I _y exceed one period, and the Lissa plot is a complete ellipse pattern;

(3) According to the two interference signals I _x and I _y stored in step (2), and using the three-difference parameter extraction method, such as ellipse fitting method and extreme value detection method, the two interference signals I _x and I _y can be obtained The three-difference parameter of, that is, the characteristic parameters of the interference signal: A _x , B _x , A _y , B _y and δ;

(4) Keep the operating voltage of the liquid crystal phase shifter unchanged during the displacement measurement of the single-frequency laser interferometer, and use the characteristic parameters of the interference signal obtained in step (3) to perform the following operations:

The three differences in the interference signal can be eliminated, that is, the ideal orthogonal interference signals sin (φ) and cos (φ) can be obtained, thereby realizing the correction of the non-linear error during the interferometric measurement and improving the accuracy of the measurement.

Claims

A non-linear error correction method for a single-frequency laser interferometer. The single-frequency laser interferometer includes:

At least one light source capable of providing single frequency laser;

An optical path, the optical path includes: a polarizing beam splitter, a first reflecting mirror and a second reflecting mirror, wherein the polarizing beam splitter is adapted to divide the single frequency light source into a reference beam and a measuring beam, and the first reflection The mirror is adapted to reflect the reference beam, and the second mirror is adapted to reflect the measurement beam;

At least one photodetector capable of detecting an interference signal formed by interference of the reference beam reflected by the first mirror and the measurement beam reflected by the second mirror;

It is characterized in that the method includes:

Step 1: Place at least one liquid crystal phase retarder in the optical path of the single-frequency laser interferometer, the liquid crystal phase retarder is adapted to change the phase difference between the reference beam and the measurement beam;

Step 2: By changing the operating voltage of at least one of the liquid crystal phase retarders at least once, the phase difference between the reference beam and the measurement beam is continuously changed;

Step 3: Extract the characteristic parameters of the interference signal;

Step 4: Use the extracted feature parameters to correct the non-linear errors in the displacement measurement process of the single-frequency laser interferometer.
The non-linear error correction method for a single-frequency laser interferometer according to claim 1, wherein during the implementation of step 1, the position of the liquid crystal phase retarder is selected from the light source and the polarization beam splitter And between the polarization beam splitter and the photodetector.
The non-linear error correction method for a single-frequency laser interferometer according to claim 1, wherein during the implementation of step 1, the slow axis direction of the liquid crystal phase retarder is polarized with the reference beam or measurement beam The same direction.
The non-linear error correction method for a single-frequency laser interferometer according to claim 1, wherein during the implementation of step 4, the operating voltage of the liquid crystal phase retarder should be kept unchanged.
A nonlinear error correction device for a single-frequency laser interferometer, the device includes:

At least one light source capable of providing single frequency laser;

An optical path, the optical path includes: a polarizing beam splitter, a first reflecting mirror and a second reflecting mirror, wherein the polarizing beam splitter is adapted to divide the single frequency light source into a reference beam and a measuring beam, and the first reflection The mirror is adapted to reflect the reference beam, and the second mirror is adapted to reflect the measurement beam;

At least one photodetector capable of detecting an interference signal formed by interference of the reference beam reflected by the first mirror and the measurement beam reflected by the second mirror;

It is characterized in that the device further includes at least one liquid crystal phase retarder, each of the liquid crystal phase retarders is placed in the optical path, and the liquid crystal phase retarder is adapted to change between the reference beam and the measuring beam 'S phase difference.
The non-linear error correction device for a single-frequency laser interferometer according to claim 5, characterized in that the device further comprises: a signal processing unit, coupled to the photodetector, adapted to collect the output of the photodetector An interference signal, and the characteristic parameter of the interference signal indicates a nonlinear error in the displacement measurement process of the single-frequency laser interferometer.
The non-linear error correction device for a single-frequency laser interferometer according to claim 5, wherein the position of the liquid crystal phase retarder is selected between the light source and the polarization beam splitter and the polarization beam splitter and Between the photodetectors.
The non-linear error correction device for a single-frequency laser interferometer according to claim 5, wherein the slow axis direction of the liquid crystal phase retarder is the same as the polarization direction of the reference beam or measurement beam.