Homodyne three-axis grating ruler

Abstract

A homodyne three-axis grating ruler comprises a laser, a reflector, five polaroids, a partial reflector, three quarter-wave plates, a composite prism, a two-dimensional diffraction grating for measurement, a diffraction grating for beam combination, three phase shift gratings, nine detectors and a signal acquisition and processor, and can realize X / Y / Z three-axis displacement real-time measurement of relative movement of the two-dimensional grating. The grating ruler system is compact and simple in overall structure, insensitive to environmental influence, few in used optical devices, easy to manufacture and low in cost.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of precision optical measuring instruments, and particularly relates to a homodyne three-axis grating ruler for three-degree-of-freedom displacement measurement. TECHNICAL BACKGROUND The grating ruler, also known as an optical encoder, is an important displacement sensor in the field of precision measurement. With its high precision, high resolution, large range, and strong anti-interference ability, it plays a vital role in many application scenarios.

[0002] With the rapid development of science and technology, the demand for multi-axial and high-precision measurement systems in various industries is increasingly urgent. Traditional single-axis grating rulers have been difficult to meet the requirements of multi-dimensional displacement measurement under complex working conditions. For example, in semiconductor manufacturing, the photolithography machine requires six-degree-of-freedom ultra-precision positioning technology with sub-nanometer displacement measurement precision for the wafer table to ensure high integration and high performance of chip manufacturing. In this context, multi-axis grating rulers have emerged. Multi-axis grating rulers can accurately measure displacement in multiple directions, providing more comprehensive and accurate position feedback to devices, effectively improving the overall performance and automation level of the devices.

[0003] However, existing multi-axis grating rulers still have certain limitations in terms of anti-interference ability. In complex industrial environments, fluctuations in environmental factors such as temperature, humidity, and air pressure, as well as the vibration of the equipment itself, can all affect the measurement results, leading to increased measurement errors.

[0004] Heidenhain, Germany, was the first company to introduce a grating ruler based on diffraction gratings. Its patent US4776701 proposed a method of using a beam to pass through a refractive grating and a reflective grating to achieve coherent superposition and optical phase shift for X-axis displacement measurement. This method does not require additional phase shift elements, so the system is relatively small in size. However, the installation distance of the reading head is strictly required, and the system can only be used for X-direction displacement measurement.

[0005] The patent US7362446B2 of ASML, Netherlands, proposes a method of measuring the displacement of a grating ruler in the X and Y directions using a grating diffraction encoder and an interferometer principle. This position measurement system achieves the purpose of reducing the size and mass of the unit and compact structure through special prism structure design. However, the prism structure is complex, and the system resolution is not known.

[0006] In 2014, the US patent (US8885172B2) of Zygo, USA, disclosed a grating interferometer that eliminates periodic nonlinear errors. This structure uses a prism pair as an output component, and through angle separation and polarization control, it suppresses stray light and polarization mixing errors. However, this scheme cannot simultaneously realize the measurement of multiple degrees of freedom.

[0007] Patent CN102865817A of Mitutoyo Corporation proposes a two-dimensional sensor, but the structure depends on high-precision lenses, prisms and other elements, and needs to suppress the crosstalk between diffraction orders, the hardware cost is high, and the system volume is large.

[0008] Chinese patent CN106017308B gives a six-axis grating ruler for six-degree-of-freedom measurement, the reading head is composed of three grating reading heads and three heterodyne laser reading heads, and the measurement grating array is composed of three measurement grating assemblies and three heterodyne laser mirrors, the structure is complex, the volume is large, and it is not easy to manufacture.

[0009] In Chinese patent CN104596425A, a three-dimensional displacement measurement system based on Michelson interferometer and multi-diffraction grating interference principle is proposed to measure the displacement of the scale grating in X / Y / Z direction. The position measurement system uses the light splitting characteristics of the grating itself to realize optical phase shift, and the measurement signals in X, Y and Z directions are independent. However, the structure has high installation precision requirement, the separation and detection of nine measurement beams depend on accurate grating parameter design, which increases the manufacturing difficulty, and the measurement of Z axis is easily disturbed by the environment.

[0010] Therefore, in the implementation of multi-axis grating ruler in the prior art, it is difficult to balance the system simplicity, environmental robustness, manufacturing cost and measurement stability. Therefore, there is an urgent need for a grating ruler system with simple structure, strong anti-interference ability, easy manufacturing and high-precision three-axis synchronous measurement. SUMMARY

[0011] To solve the problems of complex system structure, sensitivity to environmental disturbance and high manufacturing cost in the multi-axis grating ruler in the prior art, the present application proposes a zero-difference three-axis grating ruler based on diffraction grating with simple and compact structure, small volume and not sensitive to environmental disturbance, which can realize high-precision real-time three-dimensional displacement measurement of X / Y / Z axis of two-dimensional grating.

[0012] The technical solution of the present application is as follows: A zero-difference three-axis grating ruler, comprising: a single-frequency laser light source, a mirror, a composite prism, a two-dimensional measurement grating, a beam combining grating, a partial mirror, a first quarter-wave plate, a second quarter-wave plate, a third quarter-wave plate, a first polarizer, a second polarizer, a third polarizer, a fourth polarizer, a fifth polarizer, a first phase shift grating, a first detector, a second detector, a third detector, a second phase shift grating, a fourth detector, a fifth detector, a sixth detector, a third phase shift grating, a seventh detector, an eighth detector, a ninth detector, a signal acquisition and processor. The single-frequency laser light source emits a laser beam containing P light and S light polarization components, which is reflected by a mirror and vertically incident on the composite prism, and after passing through the one-dimensional grating along the y-axis direction in the middle grating line of the composite prism, it diffracts in the X / Z plane to generate three beams of 0th, ±1st order diffracted light; Wherein the 0th order diffracted light in the X / Z plane is partially reflected and partially transmitted after passing through the partial mirror. The transmitted light only leaves P light after passing through the first polarizer, and then is incident on the two-dimensional measurement grating to diffract after passing through the first quarter-wave plate, generating three beams of 0th, ±1st order diffracted light in the Y / Z plane, denoted as (0, 0) T , (0, +1) T , (0, -1) T , T represents transmitted light. Among them, the diffracted light (0, 0) T returns along the original path and passes through the first quarter-wave plate again, and the polarization direction is rotated by 90° after passing through the first quarter-wave plate twice, so it cannot pass through the first polarizer and is isolated. The diffracted light (0, +1) T and (0, -1) T are respectively changed from circularly polarized light to S light after passing through the second and third quarter-wave plates, and are transmitted through the bottom surface of the composite prism, and are incident on the reflecting surfaces on the left and right sides of the composite prism through the lower surface of the composite prism. After reflection, it is incident on the beam combining grating through the upper surface of the composite prism to diffract and combine, and the combined light is incident on the first phase shift grating to form three beams of interference light, which are detected by the first detector, the second detector and the third detector respectively, and after photoelectric conversion, signal acquisition and processor processing and calculation, the Y-axis displacement of the two-dimensional measurement grating can be obtained. The reflected light passing through the partial mirror returns to the one-dimensional grating in the middle grating line of the composite prism again, generating ±1st order diffracted light, denoted as (0, +1) R , (0, -1) R , R represents reflected light. The diffracted light (0, +1) R passes through the fourth polarizer to only retain S light, and then is incident on the second phase shift grating. The diffracted light (0, -1) R passes through the fifth polarizer to only retain P light, and then is incident on the third phase shift grating.

[0013] Wherein the ±1st order diffracted light generated by the one-dimensional grating of the composite prism in the X / Z plane is incident on the two-dimensional measurement grating at the Littrow angle after passing through the second polarizer and the third polarizer, the +1st order diffracted light becomes P light after passing through the second polarizer, and the -1st order diffracted light becomes S light after passing through the third polarizer, and after diffraction by the two-dimensional measurement grating, it returns to the one-dimensional grating in the middle of the composite prism along the original path to generate diffracted light (+1, 0, 0), (-1, 0, 0); the diffracted light (-1, 0, 0) and (0, +1) RThe combined beam is incident on the second phase-shifting grating, and diffracts to form three interference beams, which are detected by the fourth, fifth, and sixth detectors respectively; the diffracted beams are (+1, 0, 0) and (0, -1). R The combined beam is incident on the third phase-shifting grating to form three interference beams, which are detected by the seventh, eighth, and ninth detectors, respectively. The interference signals received by the fourth, fifth, and sixth detectors, along with those received by the seventh, eighth, and ninth detectors, are converted into photoelectric signals. After signal acquisition and processing, the X-axis and Z-axis displacements of the two-dimensional diffraction grating used for measurement can be obtained. The formulas for calculating the X-axis and Z-axis displacements are as follows: X-axis displacement: ; Z-axis displacement: ; In the formula S 1 represents the displacement calculation of the interference signals detected by the fourth, fifth, and sixth detectors. S 2 represents the displacement calculation of the interference signals received by the seventh, eighth, and ninth detectors.

[0014] The upper surface of the composite prism has a one-dimensional grating with grating lines along the Y-axis in the middle, and the one-dimensional grating is transmissive. The left and right side surfaces of the prism are reflective surfaces, and the upper and lower surfaces of the prism are transmissive surfaces.

[0015] The two dimensions of the two-dimensional measurement grating have orthogonal grating line directions, which are respectively set as the X-axis and Y-axis in the three axes. The plane formed by the two dimensions of the two-dimensional grating grating is perpendicular to the Z-axis of the corresponding rectangular coordinate system. The two-dimensional measurement grating is reflective.

[0016] The first phase-shifting grating, the second phase-shifting grating, and the third phase-shifting grating are one-dimensional diffraction gratings.

[0017] Each phase grating diffracts the incident light to form three interference beams, and the optimal phase difference angle of the three interference beams is 120°.

[0018] The beam combining grating is a one-dimensional diffraction grating, and the beam combining grating is a transmission type.

[0019] Compared with the prior art, the technical effects of the present invention are as follows: The zero-difference three-axis grating ruler optical system adopts a simple composite prism, a two-dimensional measurement grating and a light path combination of phase shift gratings to realize three-axis displacement measurement. The zero-difference interference technology is adopted to avoid a complex and high-cost external difference frequency modulation device. In combination with a polarizer and a quarter-wave plate, effective separation of light paths of different measurement axes, stray light isolation and polarization pairing of reference light and measurement light are realized, so that the independence and low crosstalk between signals of different axes are ensured. The structure is simple and compact, easy to manufacture, low in cost and not sensitive to the environment. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 It is a schematic diagram of the zero-difference three-axis grating ruler optical system. DETAILED DESCRIPTION

[0021] The specific embodiments of the present application will be further described in detail below in combination with the drawings, but the protection scope of the present application should not be limited thereby.

[0022] Figure 1 It is a structural schematic diagram of the zero-difference three-axis grating ruler, comprising a single-frequency laser light source 1, a mirror 2, a composite prism 3, a two-dimensional measurement grating 4, a partial mirror 5, a first polarizer 6, a first quarter-wave plate 7, a second quarter-wave plate 8, a third quarter-wave plate 9, a second polarizer 10, a third polarizer 11, a fourth polarizer 12, a fifth polarizer 13, a beam combining grating 14, a first phase shift grating 15, a second phase shift grating 16, a third phase shift grating 17, a first detector 18, a second detector 19, a third detector 20, a fourth detector 21, a fifth detector 22, a sixth detector 23, a seventh detector 24, an eighth detector 25, a ninth detector 26 and a signal acquisition and processing device 27.

[0023] The middle region of the upper surface of the composite prism 3 is a one-dimensional grating 3-1, and the grating line direction is strictly parallel to the Y-axis of the measurement coordinate system. The one-dimensional grating is a one-dimensional transmission type phase grating or amplitude grating. The lower surface of the composite prism 3 is a transmission surface allowing the light beam to freely pass through. The left surface of the composite prism 3 is a left reflection surface 3-2, and the right surface is a right reflection surface 3-3, which are two mutually parallel optical planes, and the surfaces are coated with a high-reflection film for the working wavelength or satisfy the optical total reflection condition, and form a specific angle with the upper and lower surfaces, for guiding the Y-axis measurement light beam.

[0024] The period of the one-dimensional grating 3-1 on the composite prism, the X / Y period of the two-dimensional measurement grating 4, the period of the beam combining grating 14 and the periods of the first phase shift grating 15, the second phase shift grating 16 and the third phase shift grating 17 need to be designed to meet the specific diffraction angle (such as the Littrow angle), the light beam coincidence and the interference condition.

[0025] The single frequency laser source 1 emits a laser beam containing P and S polarized components, which is reflected by the mirror 2 and then vertically incident on the compound prism 3. After passing through the one-dimensional grating 3-1 along the y-axis direction in the middle grating line of the compound prism 3, the laser beam diffracts in the X / Z plane to generate three beams of 0th, ±1st order diffracted light.

[0026] The 0th order diffracted light in the X / Z plane is partially transmitted and partially reflected by the partial mirror 5. The transmitted light passes through the first polarizer 6, leaving only P light, and then passes through the first quarter-wave plate 7 to be incident on the two-dimensional measurement grating 4 to diffract and generate three beams of 0th, ±1st order diffracted light in the Y / Z plane, denoted as (0, 0) T , (0, +1) T , (0, -1) T , T represents the transmitted light through the partial mirror 5. The diffracted light (0, 0) T returns along the original path and passes through the first quarter-wave plate 7 again, with the polarization direction rotated by 90°, so it cannot pass through the first polarizer 6 and is isolated. The diffracted light (0, +1) T and (0, -1) T passes through the second quarter-wave plate 8 and the third quarter-wave plate 9, respectively, to change from circularly polarized light to S light. After being incident on the left reflecting surface 3-2 and the right reflecting surface 3-3 on the left and right sides of the compound prism 3 through the lower surface of the compound prism 3, the light is reflected and then incident on the beam combining grating 14 through the upper surface of the compound prism 3 to diffract and combine. After being diffracted by the first phase grating 15, three beams of interference light are formed, which are detected by the first detector 18, the second detector 19, and the third detector 20, respectively. After photoelectric conversion, signal acquisition, and processing by the signal processor 27, the Y-axis displacement of the two-dimensional measurement grating 4 can be obtained. The reflected light through the partial mirror 5 returns to the one-dimensional grating 3-1 along the y-axis direction in the middle grating line of the compound prism 3, generating ±1st order diffracted light, denoted as (0, +1) R , (0, -1) R , R represents the reflected light through the partial mirror. The diffracted light (0, +1) R passes through the fourth polarizer 12, leaving only S light, and then is incident on the second phase grating 16. The diffracted light (0, -1) R passes through the fifth polarizer 13, leaving only P light, and then is incident on the third phase grating 17.

[0027] Wherein the incident light generated by the grating 3-1 of the compound prism in the X / Z plane is incident on the two-dimensional measurement grating 4 at the Littrow angle after passing through the second polarizer 10 and the third polarizer 11, the +1 order diffracted light becomes P light after passing through the second polarizer 10, and the -1 order diffracted light becomes S light after passing through the third polarizer 11, and after being diffracted by the two-dimensional measurement grating 4, the diffracted light returns to the one-dimensional grating 3-1 of the compound prism in the X / Z plane, and the diffracted light (+1, 0, 0), (-1, 0, 0) is generated, and the diffracted light (-1, 0, 0) and (0, +1) R The combined beams are incident on the second phase shift grating 16 to form three interference beams, which are detected by the fourth detector 21, the fifth detector 22, and the sixth detector 23 respectively; the diffracted light (+1, 0, 0) and (0, -1) R The combined beams are incident on the third phase shift grating 17 to form three interference beams, which are detected by the seventh detector 24, the eighth detector 25, and the ninth detector 26 respectively; the interference signals received by the fourth detector 21, the fifth detector 22, and the sixth detector 23 and the interference signals received by the seventh detector 24, the eighth detector 25, and the ninth detector 26 are processed and calculated by the signal acquisition and processor 27 after photoelectric conversion, and the X-axis displacement and the Z-axis displacement of the two-dimensional measurement grating 4 used for measurement can be obtained. The X-axis displacement and the Z-axis displacement calculation formula is as follows: X-axis displacement: ; Z-axis displacement: ; In the formula S 1 is the displacement calculation amount of the interference signals detected by the fourth, fifth, and sixth detectors, S 2 is the displacement calculation amount of the interference signals received by the seventh, eighth, and ninth detectors.

[0028] The grating line directions of the two dimensions of the two-dimensional measurement grating 4 are orthogonal and are respectively set as the X-axis and the Y-axis in the three-axis, and the plane composed of the grating line directions of the two dimensions of the two-dimensional measurement grating 4 is perpendicular to the Z-axis of the corresponding rectangular coordinate system, and the two-dimensional measurement grating 4 is a reflective type.

[0029] The first phase shift grating 15, the second phase shift grating 16, and the third phase shift grating 17 are one-dimensional diffraction gratings.

[0030] Each phase shift grating diffracts the incident light to form three interference beams, and the optimal phase difference angle of the three interference beams is 120°.

[0031] The combined grating is a one-dimensional diffraction grating or a two-dimensional diffraction grating, and the combined grating is a transmissive type.

[0032] The specific embodiments of the present application are described above. It needs to be understood that the present application is not limited to the specific embodiments described above, and various modifications or changes can be made by those skilled in the art within the scope of the claims, which do not affect the essence of the present application. The above embodiments and features in the embodiments can be combined with each other without conflict, if necessary.

Claims

1. A zero-difference triaxial grating ruler, characterized in that, include: A single-frequency laser source, a reflector, a compound prism, a two-dimensional measurement grating, a beam combiner grating, a partial reflector, a first quarter-wave plate, a second quarter-wave plate, a third quarter-wave plate, a first polarizer, a second polarizer, a third polarizer, a fourth polarizer, a fifth polarizer, a first phase-shifting grating, a first detector, a second detector, a third detector, a second phase-shifting grating, a fourth detector, a fifth detector, a sixth detector, a third phase-shifting grating, a seventh detector, an eighth detector, and a ninth detector, as well as a signal acquisition and processor respectively connected to the first to the ninth detectors; The composite prism is a one-piece transparent prism. Its upper surface has a central region with a one-dimensional transmission diffraction grating whose grating line direction is parallel to the Y-axis of the measurement coordinate system. Its lower surface is a transmission surface, and its left and right sides are total reflection surfaces coated with high reflectivity films or optical surfaces that meet the conditions for total optical reflection. The working surface of the two-dimensional measurement grating is parallel to and opposite to the lower surface of the composite prism, and the surface is engraved with diffraction grating lines that are orthogonal to each other and parallel to the X-axis and Y-axis of the measurement coordinate system, respectively. The single-frequency laser source emits a laser beam containing P- and S-polarization components. After being reflected by a mirror, it is incident perpendicularly onto a composite prism. After passing through a one-dimensional grating along the y-axis in the middle of the upper surface of the composite prism, it diffracts in the X / Z plane, producing three diffracted beams of the 0th order and ±1st order. The 0th order diffracted beam is partially transmitted and partially reflected after passing through some mirrors. The reflected beam is diffracted again by the one-dimensional grating in the middle of the composite prism, producing ±1st order diffracted beams. These beams are then incident on the second and third phase-shifting gratings, respectively, after passing through the fourth and fifth polarizers. The transmitted beam passes through the first polarizer and then through the first quarter-wave plate, and is incident on a two-dimensional measurement grating, where it diffracts again, producing three diffracted beams of the 0th order and ±1st order in the Y / Z plane. The 0th order diffracted beam returns along its original path and passes through the first quarter-wave plate again, but its polarization direction is rotated by 90° and it cannot pass through the first polarizer again. The ±1st order diffracted light in the Y / Z plane is converted from circularly polarized light to S-light after passing through the second and third quarter-wave plates, respectively. It passes through the bottom surface of the composite prism, is reflected by the reflective surfaces on the left and right sides of the prism, and then passes through the upper surface of the prism and is incident on the beam combining grating to form an interference signal. The combined light is incident on the first phase-shifting grating and diffracts to form three interference beams, which are detected by the first detector, the second detector, and the third detector respectively. After photoelectric conversion, the signal is acquired and processed by the processor and calculated to obtain the Y-axis displacement of the two-dimensional diffraction grating for measurement. The ±1st order diffracted light in the X / Z plane, diffracted by the one-dimensional grating in the middle of the compound prism, is incident on the two-dimensional measurement grating at a Littrow angle. The +1st order diffracted light becomes P light after passing through the second polarizer, and the -1st order diffracted light becomes S light after passing through the third polarizer. After diffracting by the two-dimensional measurement grating, the light returns along the original path in the X / Z plane to the one-dimensional grating in the middle of the compound prism, where it is diffracted again. The two 0th order diffracted lights are combined with the ±1st order diffracted light reflected by the partial reflector and respectively, and then incident on the first phase-shifting grating and the second phase-shifting grating. The interference signals received by the fourth, fifth, sixth, seventh, eighth, and ninth detectors are converted by photoelectric conversion and then processed and calculated by the signal acquisition and processor to obtain the X-axis displacement and Z-axis displacement of the two-dimensional diffraction grating used for measurement.

2. The zero-difference triaxial grating ruler according to claim 1, characterized in that, The signal acquisition and processor is configured to: calculate the displacement of the two-dimensional measurement grating in the Y-axis direction based on the signals received by the first detector, the second detector and the third detector; calculate the first displacement calculation amount S1 based on the signals received by the fourth detector, the fifth detector and the sixth detector; and calculate the second displacement calculation amount S2 based on the signals received by the seventh detector, the eighth detector and the ninth detector. The displacement of the two-dimensional measurement grating in the X and Z axis directions: X-axis displacement: ; Z-axis displacement: .

3. The zero-difference triaxial grating ruler according to claim 1, characterized in that, The first phase-shifting grating, the second phase-shifting grating, and the third phase-shifting grating are all one-dimensional transmission diffraction gratings and are configured to diffract an incident light beam to generate three interference beams, with an optimal phase difference of 120° between the three interference beams.

4. The zero-difference triaxial grating ruler according to claim 1, characterized in that, The beam combining grating is a one-dimensional transmission diffraction grating or a two-dimensional transmission diffraction grating.

5. The zero-difference triaxial grating ruler according to claim 1, characterized in that, The fast axis direction of the first quarter-wave plate forms a 45° angle with the polarization direction of the P-light transmitted by the first polarizer; the fast axis directions of the second and third quarter-wave plates are orthogonal to the fast axis direction of the first quarter-wave plate.

6. The zero-difference triaxial grating ruler according to claim 1, characterized in that, An appropriate gap is provided between the working surface of the two-dimensional measurement grating and the lower surface of the composite prism, and the two remain parallel.

7. The zero-difference triaxial grating ruler according to claim 1, characterized in that, The single-frequency laser source, reflector, partial reflector, compound prism, first to fifth polarizers, first to third quarter-wave plates, beam combiner grating, and first to third phase-shifting grating are integrated into a single reading head module; the two-dimensional measurement grating is an independent moving component that undergoes relative displacement with the reading head module.

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