HETERODYNE GRATING INTERFEROMETRIC METHOD AND SYSTEM FOR TWO-DEGREE-OF-FREEDOM WITH HIGH ALIGNMENT TOLERANCE

Abstract

Present disclosure relates to a heterodyne grating interferometric method and system for two-degree-of-freedom with high tolerance. The system comprises a separately modulated heterodyne laser (1), an optical prism (23) and a photoelectric detection and signal processing unit (4). The separately modulated heterodyne laser (1) simultaneously outputs two laser beams at different frequencies, which are incident in parallel to a first beamsplitting surface so as to be split, and then a part thereof is incident to a retro-reflector (233) to produce reference beams (53a, 53b), which are incident to a third beamsplitting surface, and the other part traverses a double-diffraction structure formed by a measured grating (3) and retro-reflectors (234a, 234b) to obtain two measured beams (59a, 59b), which are incident to a second beamsplitting surface and then are divided into two parts. Wherein one part is converged to form a first interference beam (61), and the other part is incident to the third beamsplitting surface and is converged with the corresponding reference beams (53a, 53b) to form second and third interference beams (62, 63). Photoelectric detection and signal processing is performed on the interference signals of the three interference beams (61, 62, 63), so as to calculate horizontal and vertical displacement of the grating (3). The present measurement method and system improve the angular tolerance of tip and tilt of the optical grating (3) while increasing the fold factors.

Claims

1. A heterodyne grating interferometric method for two-degree-of-freedom with high alignment tolerance, wherein a heterodyne laser outputs two laser beams at the same time, a first laser beam and a second laser beam, which are at a first frequency and a second frequency respectively, and are spatially separated and transmitted to a first beamsplitting surface to obtain the corresponding reference beams and measuring beams, wherein the two reference beams pass through the retro-reflector structure to form a first reference beam and a second reference beam, and is incident on a third beamsplitting surface; the two measuring beams are incident on the measured grating perpendicularly, pass through a double-diffraction structure formed by the grating and retro-reflector to obtain the corresponding first measuring beam and second measuring beam, and are divided into two parts by a second beamsplitting surface, one part thereof passes through a lateral displacement beamsplitter to form a first interference beam, and the other part thereof is incident on the third beamsplitting surface; on the third beamsplitting surface, the first reference beam and the second measuring beam, the second reference beam and the first measuring beam converge respectively to form a second interference beam and a third interference beam, optical beat frequency signals of the three interference beams each are detected and processed to obtain horizontal and vertical displacement information of the measured grating.

2. The heterodyne grating interferometric method for two-degree-of-freedom with high alignment tolerance and anti-aliasing according to claim 1 with high alignment tolerance and, the first, the second and the third beamsplitting surfaces can be realized by three different beamsplitting surfaces, or can be realized by two beamsplitting surfaces or one beamsplitting surface.

3. A heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance, comprising a separately modulated heterodyne laser source module (1), an optical prism (2), a measured grating (3), a photodetector and a signal processing module (4), characterized in that: a spatial beam is emitted by the single-frequency laser (11), coupled into the polarization-maintaining fiber through the coupler (12), and divided into a first beam and a second beam by a 1×2 fiber optic beamsplitter (13), which respectively enter into fiber acousto-optic modulators (15a) and (15b) with different modulation frequencies through fiber optic adapters (14a) and (14b), after modulation, the first beam and the second beam are respectively at the first frequency and the second frequency, enter into polarization-maintaining fibers (21a) and (21b) through fiber optic adapters (16a) and (16b), and are adjusted to ovally or circularly polarized collimated spatial beams by fiber collimators (22a) and (22b) with polarizers, which are named as the first incident beam (51a) and the second incident beam (51b) and enters into a prism (23); the first incident beam (51a) and the second incident beam (51b) are first converted by incident beam reflecting prisms (231a) and (231b) into a direction perpendicular to the grating, and pass through polarizing-beamsplitting surface of a central beamsplitting prism (232) to be divided into a first reflected beam (52a), a second reflected beam (52b), a first transmitted beam (54a) and a second transmitted beam (54b), and the two reflected beams (52a) and (52b) are cross-shifted after passing through the retro-reflector structure (233) to obtain a first reference beam (53a) and a second reference beam (53b), and on the other hand, the transmitted beams are incident on a grating (3) perpendicularly to undergo a first diffraction, produce the +1-order diffracted beam (55a) of the first transmitted beam and the −1-order diffracted beam (55b) of the second transmitted beam, are refracted and reflected by relevant surfaces of the retro-reflector structures (234a) and (234b) so as to obtain first and second retro-reflected beams (56a) and (56b) both with offsets, which are again obliquely incident on the grating (3) through the refraction surface to undergo a second diffraction, so as to produce a first (+1, +1)-order double-diffracted beam (57a) and a second (−1, −1)-order double-diffracted beam (57b) exited from the grating perpendicularly, then the double-diffracted beams (57a) and (57b) enter into the central beamsplitting prism (232) again and are split through non-polarizing-beamsplitting surfaces of the central beamsplitting prism, and reflected parts (58a) and (58b) thereof are incident on the lateral displacement beamsplitter (235) to produce a first interference beam (61); transmitted parts thereof are named as a first measuring beam (59a) and a second measuring beam (59b) and pass through the polarizing beamsplitting surface of the central beamsplitting prism again, here, the second measuring beam (59b) and the first reference beam (53a), the first measuring beam (59a) and the second reference beam (53b) are converged to produce a second interference beam (62) and a third interference beam (63), respectively, which pass through the interference beam reflecting prisms (236a) and (236b) to be adjusted to emit in a direction parallel to the grating, so far, the three interference beams (61), (62), and (63) respectively enter into fiber couplers (24a), (24b) and (24c) with analyzers to interfere and be coupled into multi-mode fibers (25a), (25b), (25c) correspondingly, which enter into cage structures (41a), (41b), and (41c) containing convergent fiber collimators after being transmitted through the multi-mode fibers, such that the beams are respectively focused on the surface of photodetectors (42a), (42b), (42c), collected and amplified, and then transmitted to a signal processing board (43) through wires for displacement calculation, to obtain horizontal and vertical displacement information of the measured grating.

4. The heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance and anti-aliasing according to claim 3, the beams (55a) and (55b) can respectively represent +1-order diffracted beam of the first transmitted beam and −1-order diffracted beam of the second transmitted beam, or can be configured to respectively represent −1-order diffracted beam of the first transmitted beam and +1-order diffracted beam of the second transmitted beam.

5. The heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance and anti-aliasing according to claim 3, a beamsplitting part of a single-frequency laser beam in the separately modulated heterodyne laser source module (1) can be realized by a fiber optic beamsplitter or a spatial beamsplitting prism; the frequency modulation part thereof can be realized by fiber acousto-optic modulator or acousto-optic modulator.

6. The heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance and anti-aliasing according to claim 3, the incident beam reflecting prisms (231a) and (231b) and the interference beam reflecting prisms (236a) and (236b) can be realized by reflecting prisms or mirrors.

7. The heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance and anti-aliasing according to claim 3, the central beamsplitting prism (232) can be realized by a special monolithic prism containing the polarizing-beamsplitting surfaces and non-polarizing-beamsplitting surfaces, or a combination of discrete beamsplitting prisms.

8. The heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance and anti-aliasing according to claim 3, the retro-reflector (233) of the reference beam can be realized by a retro-reflector structure such as a corner-cube prism or a combination of a convergent lens and a mirror.

9. The heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance and anti-aliasing according to claim 3, the measuring beam retro-reflectors (234a), (234b) can be realized by a retro-reflector structure such as any one of a standard corner-cube prism element, a prism equivalent to a corner-cube prism or a square mirror, or a combination of a convergent lens and a mirror.

10. The heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance and anti-aliasing according to claim 3, the incident beam reflecting prisms (231a), (231b), the reference beam retro-reflector (233), the measuring beam retro-reflectors (234a), (234b), the lateral displacement beamsplitter (235), the interference beam reflecting prisms (236a), (236b) can be directly bonded to the central beamsplitting prism (232) or bonded to it through an optical positioning part to form an integral optical prism and can be fixed through the central beamsplitting prism (232) bonded to a mechanical housing, or directly bonded to the mechanical housing in a form of multiple discrete prisms for fixing.

11. The heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance and anti-aliasing according to claim 3, the grating (3) is a reflective or transmissive grating, and/or is a linear grating or a planar grating.

12. The heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance and anti-aliasing according to claim 3, detection on the interference signal by the photodetectors (41a), (41b) and (41c) can be implemented by means of direct detection of the spatial beam by the photodetector, or can be implemented by means of remote reception by the fiber coupler and transmission to the photodetector.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1 is a schematic structural diagram of a heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance.

[0014] FIG. 2 is a schematic structural diagram of the prism of a grating reading head of the present invention.

[0015] FIG. 3 is a schematic diagram of a spatial beam path in the structure of the prism in FIG. 2. In the figure, an exploded view is appropriately processed for some optical components to clearly express each spatial beam path in the optical prism.

[0016] In the Figures: 1—separately modulated heterodyne laser, 11—single-frequency laser, 12—spatial beam and fiber coupler, 13—polarization maintaining fiber 1×2 beam splitter; 14a, 14b—fiber optic adapter; 15a, 15b—fiber optic acousto-optic modulator; 16a, 16b—fiber optic adapter, 2—grating reading head; 21a, 21b—polarization maintaining single-mode fiber; 22a, 22b—fiber collimator with a polarizer, 23—optical prism; 24a, 24b, 24c—fiber coupler with a analyzer; 25a, 25b, 25c—multimode fiber; 3—reflection grating; 41a, 41b, 41c—cage structure; 42a, 42b, 42c—photodetector; 43—signal processing board; 231a, 231b—incident beam reflecting prism; 232—central beamsplitting prism with both polarizing beamsplitting surface and non-polarizing-beamsplitting surface; 233—corner-cube prism; 234a, 234b—pentagonal corner-cube prism; 235—lateral displacement beamsplitter; 236a, 236b—interference beam reflecting prism; 51 to 59—each beam generated by a first incident beam; 51a to 59a—each beam generated by a second incident beam; 61, 62, 63—interference beam.

DETAILED DESCRIPTION OF EMBODIMENTS

[0017] The specific embodiments of the present invention will be further described in detail below in conjunction with the accompanying drawings.

[0018] A heterodyne grating interferometric method for two-degree-of-freedom with high alignment tolerance is disclosed. A heterodyne laser outputs two laser beams simultaneously, wherein a first laser beam and a second laser beam are at a first frequency and a second frequency respectively, and are spatially separated and transmitted to a first beamsplitting surface to obtain corresponding reference beams and measuring beams. The two reference beams pass through the retro-reflector structure to form a first reference beam and a second reference beam, and is incident on a third beamsplitting surface. The two measuring beams are incident on the measured grating perpendicularly, pass through a secondary diffraction structure formed by the grating and retro-reflectors to obtain corresponding first measuring beam and second measuring beam, and are divided into two parts by passing through a second beamsplitting surface, one part thereof passes through a lateral displacement beamsplitter to form a first interference beam, and the other part thereof is incident on a third beamsplitting surface. On the third beamsplitting surface, the first reference beam and the second measuring beam, the second reference beam and the first measuring beam respectively converge to form a second interference beam and a third interference beam. Optical beat frequency signals of the three interference beams each are detected and processed to obtain horizontal and vertical displacements of the measured grating.

[0019] In some embodiments, in the heterodyne grating interferometric method for two-degree-of-freedom with high alignment tolerance, the first, the second and the third beamsplitting surfaces can be realized by three different beamsplitting surfaces, or can be realized by two beamsplitting surfaces or one beamsplitting surface. The specific beamsplitting surface can be configured as a polarizing beamsplitting surface or a non-polarizing beamsplitting surface according to the actual optical structure.

[0020] A heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance is disclosed. Please refer to FIGS. 1-3, a spatial beam is emitted by the single-frequency laser 11, coupled into the polarization-maintaining fiber through the coupler 12, and then divided into a first beam and a second beam by a 1×2 fiber optic beamsplitter 13. The first beam and second beam respectively enter into fiber acousto-optic modulators 15a and 15b with different modulation frequencies through fiber optic adapters 14a and 14b. After modulation, the first beam and the second beam are respectively at the first frequency and the second frequency, enter into polarization-maintaining fibers 21a and 21b through fiber optic adapters 16a and 16b, and are adjusted to 45-degree circularly polarized collimated spatial beams by fiber collimators 22a and 22b with polarizers, which are named as the first incident beam 51a and the second incident beam 51b and enters into an optical prism 23. The first incident beam 51a and the second incident beam 51b are first converted by incident beam reflecting prisms 231a and 231b into a direction vertical to grating plane, and pass through polarizing-beamsplitting surface of a central beamsplitting prism 232 to be divided into a first reflected beam 52a, a second reflected beam 52b, a first transmitted beam 54a and a second transmitted beam 54b. The two reflected beams 52a and 52b are cross-shifted after passing through the retro-reflector structure to obtain a first reference beam 53a and a second reference beam 53b. And on the other hand, the transmitted beams are incident on a grating 3 perpendicularly to undergo a first diffraction, produce +1-order diffracted beam 55a of the first transmitted beam and −1-order diffracted beam 55b of the second transmitted beam, are refracted by a bottom surface of pentagonal prism retro-reflector structures 234a and 234b and are reflected three times by side surfaces and top surface of the pentagonal prism retro-reflector structures 234a and 234b, so as to obtain offset first and second retro-reflected beams 56a and 56b, which are obliquely incident on the grating 3 after through the refraction surface again to undergo a second diffraction, so as to produce a first (+1, +1)-order double-diffracted beam 57a and a second (−1, −1)-order double-diffracted beam 57b, which emit perpendicular to the grating, then the double-diffracted beams 57a and 57b enter into the central beamsplitting prism 232 again and are split through non-polarizing-beamsplitting surfaces of the central beamsplitting prism. The reflected parts 58a and 58b thereof are incident on the lateral displacement beamsplitter 235 to produce a first interference beam 61. The transmitted parts thereof are named as a first measuring beam 59a and a second measuring beam 59b and pass through the polarization beamsplitting surface of the central beamsplitting prism again, here, the second measuring beam 59b and the first reference beam 53a, the first measuring beam 59a and the second reference beam 53b are converged to produce a second interference beam 62 and a third interference beam 63, respectively, which pass through the interference beam reflecting prisms 236a and 236b to be adjusted to emit in a direction parallel to the grating; so far, the three interference beams 61, 62, and 63 respectively enter into fiber couplers 24a, 24b, and 24c with analyzers to interfere and be coupled into multi-mode fibers 25a, 25b, 25c respectively, and then respectively enter into cage structures 41a, 41b, and 41c containing convergent fiber collimators after being transmitted through the multi-mode fibers, such that the light beams are respectively focused on the surface of photodetectors 42a, 42b, 42c, collected and amplified, and then transmitted to a signal processing board 43 through wires for displacement calculation, to obtain horizontal and vertical displacement information.

[0021] In some embodiments, in the heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance, the beams 55a and 55b can respectively represent +1-order diffracted light of the first transmitted beam and −1-order diffracted beam of the second transmitted beam, or can be configured to respectively represent −1-order diffracted beam of the first transmitted beam and +1-order diffracted beam of the second transmitted beam. Correspondingly, the reference beam is reflected back to the measured grating via the retro-reflectors 234a and 234b placed on the optical path of the beams 55a and 55b to undergo a double-diffraction, and the vertically emitted double-diffracted beams 57a and 57b correspond to a first (+1, +1)-order double-diffracted beam and a second (−1, −1)-order double-diffracted beam, or a first (−1, −1)-order double-diffracted beam and a second (+1, +1)-order double-diffracted beam.

[0022] In some embodiments, in the heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance, the separately modulated heterodyne laser source module 1 can be a fiber transmission structure or a spatial beam structure. That is to say, the splitting part of a single-frequency laser beam can be realized by a fiber optic beamsplitter or a spatial beamsplitting prism; the frequency modulation part thereof can be realized by fiber acousto-optic modulator or spatial acousto-optic modulator.

[0023] In some embodiments, in the heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance, the incident beam reflecting prisms 231a and 231b and the interference beam reflecting prisms 236a and 236b can be realized by reflecting prisms or mirrors.

[0024] In some embodiments, in the heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance, the central beamsplitting prism 232 can be realized by a special monolithic prism containing the polarizing-beamsplitting surfaces and non-polarizing-beamsplitting surfaces, or a combination of discrete beamsplitting prisms.

[0025] In some embodiments, in the heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance, the retro-reflector 233 of the reference beam can be realized by a retro-reflector structure such as a corner-cube prism or a combination of a convergent lens and a mirror.

[0026] In some embodiments, in the heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance, the measuring beam retro-reflectors 234a, 234b can be realized by a retro-reflector structure such as any one of standard corner-cube prism element, a prism equivalent to a corner-cube prism or a square mirror, or a combination of convergent lens and a mirror.

[0027] In some embodiments, in the heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance, the grating 3 can be reflective or transmissive, and can be a linear grating or a planar grating, etc. When the grating is the reflective grating, the measuring beam retro-reflectors 234a, 234b are on the same side of a surface of the grating as the other elements of an optical prism. When the grating is the transmissive grating, the measuring beam retro-reflectors 234a, 234b and the other elements of the optical prism are on two sides of the surface of the grating respectively.

[0028] In some embodiments, in the heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance, the incident beam reflecting prisms 231a, 231b, the reference beam retro-reflector 233, the measuring beam retro-reflectors 234a, 234b, the lateral displacement beamsplitter 235, the interference beam reflecting prisms 236a, 236b can be directly bonded to the central beamsplitting prism 232 or bonded to it through an optical positioning part to form a monolithic prism (i.e., an integral optical prism) and can be fixed through the central beamsplitting prism 232 bonded to a mechanical housing, or directly bonded to the mechanical housing in a form of multiple discrete prisms for fixing.

[0029] In some embodiments, in the heterodyne grating interferometric system for two-degree-of-freedom with high alignment tolerance, the detection of the optical beat frequency signals by the photodetectors 41a, 41b and 41c can be implemented by means of direct detection of the spatial beam by the photodetector, or remote reception by the fiber coupler and transmission to the photodetector for detection.

[0030] It may be assumed that the frequency of the first beam output by the separately modulated heterodyne laser source module 1 is f.sub.1, the frequency of the second beam is f.sub.2, and f.sub.1>f.sub.2.

[0031] When the grating 3 is stationary, reflected beams 58a and 58b of the central beamsplitting prism, the first reference beam 53a and the second measuring beam 59b, the second reference beam 53b and the first measuring beam 59a produce interferometric beat-frequency signal, and the beat frequency thereof is f.sub.1−f.sub.2, that is, the difference between the modulation frequencies of the acousto-optic modulators 15a and 15b in the separately modulated heterodyne laser source module 1.

[0032] When the grating 3 produces horizontal and vertical motion, it may be assumed that the Doppler frequency shift caused by horizontal motion and vertical motion are f.sub.x and f.sub.z respectively, at this time, the frequency of the first reference beam 53a is still f.sub.1, the frequency of the second reference beam 53b is still f.sub.2, and the frequency of the first (+1, +1)-order double-diffracted beam 57a containing the Doppler frequency shift, the reflected beams 58a through the central beamsplitting prism, and the transmitted first measuring beam 59a is f.sub.1+2f.sub.x+kf.sub.z; the frequency of the second (−4, −1)-order double-diffracted beam 57b containing the Doppler frequency shift, the reflected beams 58b through the central beamsplitting prism, and the transmitted second measuring beam 59b is f.sub.2-2f.sub.x+kf.sub.z, k in the formula is the Doppler frequency shift coefficient in the vertical direction.

[0033] According to the geometrical relationship of the beam, there is k=1+1/cos θ, θ is a diffraction angle, which can be calculated based on the grating equation for normal incidence.

[0034] Therefore, the frequency of the first interference beam 61 is f.sub.1−f.sub.2+4f.sub.x; the frequency of the second interference beam 62 is f.sub.1−f.sub.2+2f.sub.x−kf.sub.z: the frequency of the third interference beam 63 is f.sub.1−f.sub.2+2f.sub.x+kf.sub.z.

[0035] In the signal processing board 43, the interference signals corresponding to the first interference beam 61 and the second interference beam 62 are demodulated to obtain a first demodulation signal with a frequency of 2f.sub.x+kf.sub.z; the interference signals corresponding to the first interference beam 61 and the third interference beam 63 are demodulated to obtain a second demodulation signal with a frequency of 2f.sub.x−kf.sub.z; the interference signals corresponding to the second interference beam 62 and the third interference beam 63 are demodulated to obtain a third demodulation signal with a frequency of 2kf.sub.z: the first demodulation signal and the second demodulation signal are demodulated again to obtain a secondary demodulation signal with a frequency of 4f.sub.x.

[0036] Phase calculation may be performed for the third demodulation signal and the secondary demodulation signal, so as to respectively obtain the horizontal displacement of the grating under optical configuration with fold factor of 4 and the vertical displacement of the grating under an optical subdivision with even higher fold factors.