# US 11,307,018 B2 · Two-degree-of-freedom heterodyne grating interferometry measurement system > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 11,307,018 B2 - Document ID: US11307018B2 - Status: Granted - Kind: B2 - Application number: 17/257,219 - Filed: 2019-06-26 - Published: 2021-06-03 - Granted: 2022-04-19 - Earliest claimed: 2018-07-02 - Assignees: Tsinghua University; Beijing U-Precision Tech Co., Ltd. - Applicants: Tsinghua University; Beijing U-Precision Tech Co., Ltd. - Inventors: Yu Zhu; Ming Zhang; Leijie Wang; Fuzhong Yang; Rong Cheng; Xin Li; Weinan Ye; Jinchun Hu - Web page: https://patentgrep.com/patent/US11307018B2 ## CPC - G01B 9/02003: using beat frequencies - G01B 9/02027: {Two or more interferometric channels or interferometers} - G01B 9/02051: {Integrated design, e.g. on-chip or monolithic} - G01B 11/0641: {with measurement of polarization} - G01B 11/0675: {using interferometry} - G01B 2290/70: Using polarization in the interferometer - G01B: MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS ## Abstract A two-degree-of-freedom heterodyne grating interferometry measurement system, comprising: a single-frequency laser device for emitting a single-frequency laser, and the single-frequency laser can be split into a beam of reference light and a beam of measurement light; an interferometer mirror group and a measurement grating for forming a reference interference signal and a measurement interference signal from the reference light and the measurement light; and a receiving optical fiber for receiving the reference interference signal and the measurement interference signal, wherein a core diameter of the receiving optical fiber is smaller than a width of an interference fringe of the reference interference signal and the measurement interference signal, so that the receiving optical fiber receives a part of the reference interference signal and the measurement interference signal. The measurement system has advantages of insensitivity to grating rotation angle error, small volume, light weight, and a facilitating arrangement. ## Claims ### Claim 1 A two-degree-of-freedom heterodyne grating interferometry measurement system, comprising: a single-frequency laser device for emitting a single-frequency laser, the single-frequency laser split into a beam of reference light and a beam of measurement light; an interferometer mirror group and a measurement grating for forming a reference interference signal and a measurement interference signal from the beam of reference light and the beam of measurement light, a plurality of receiving optical fibers for receiving the reference interference signal and the measurement interference signal, wherein a core diameter of each of the receiving optical fibers is smaller than widths of interference fringes of the reference interference signal and the measurement interference signal, so that the plurality of receiving optical fibers receive parts of the reference interference signal and the measurement interference signal, and wherein the interferometer mirror group, in sequence from one side to an opposing side, comprises: a reflecting mirror, a refractive element, a quarter wave plate, a beam splitting prism, and a polarization splitting prism, wherein the beam splitting prism is positioned on the polarization splitting prism. ### Claim 2 (depends on 1) The grating interferometry measurement system according to claim 1, wherein the plurality of receiving optical fibers are multimode optical fibers, wherein the reference interference signal comprises one path of reference interference signal, the measurement interference signal comprises two paths of measurement interference signals, and wherein the two paths of measurement interference signals and the one path of reference interference signal are coupled to and received by the multimode optical fibers, respectively, to output three optical signals. ### Claim 3 (depends on 2) The grating interferometry measurement system according to claim 2, further comprising: a photoelectric conversion unit and an electronic signal processing component, wherein the photoelectric conversion unit receives the optical signals output by the multimode optical fibers, converts the optical signals into electrical signals, and inputs the electrical signals to the electronic signal processing component, and wherein the electronic signal processing component receives the electrical signals to calculate a linear displacement of the measurement grating. ### Claim 4 (depends on 1) The grating interferometry measurement system according to claim 1, wherein the measurement grating performs linear motions with a degree of freedom in a horizontal direction and a degree of freedom in a vertical direction with respect to the interferometer mirror group. ### Claim 5 (depends on 2) The grating interferometry measurement system according to claim 2, wherein the measurement grating performs linear motions with a degree of freedom in a horizontal direction and a degree of freedom in a vertical direction with respect to the interferometer mirror group. ### Claim 6 (depends on 1) The grating interferometry measurement system according to claim 1, wherein the beam of reference light is split into three beams of light after passing through the beam splitting prism, and the three beams of light are used as the reference light of three interference signals after being reflected by the polarization splitting prism, respectively, wherein: the beam of measurement light is split into three beams of light after passing through the beam splitting prism, and two beams of the split measurement light are reflected by the polarization splitting prism, then pass through the quarter wave plate and the refractive element in sequence, and then are incident on the measurement grating to be diffracted, and then return back along an original optical path, and then are transmitted by the polarization splitting prism, and then interfere with two beams of the reference light used as the reference light of the three interference signals, to form two measurement interference signals, and a third beam of the split measurement light is reflected by the polarization splitting prism, then passes through the quarter wave plate, and then is reflected by the reflecting mirror to return back along an original optical path, and then passes through the quarter wave plate and is transmitted by the polarization splitting prism again, and then interferes with another a third beam of the reference light used as the reference light of the three interference signals, to form one reference interference signal. ### Claim 7 (depends on 6) The grating interferometry measurement system according to claim 6, wherein the two beams of the split measurement light pass through the refractive element, then are incident on the measurement grating in an incident light path of a specific angle, wherein the specific angle causes a diffracted light path to overlap with the incident light path, and wherein the diffracted light path passes through the refractive element to interfere with and in parallel with two beams of the reference light used as the reference light of the three interference signals, to form the two measurement interference signals. ### Claim 8 (depends on 1) The grating interferometry measurement system according to claim 1, wherein components in the interferometer mirror group are adjacent and fixed to each other and are integrated into an integrated structure. ### Claim 9 (depends on 1) The grating interferometry measurement system of claim 1, wherein a cross section of the refractive element is an isosceles trapezoid, and the measurement light is refracted when transmitted through both sides of the trapezoid and is reflected when transmitted through a top of the trapezoid. ### Claim 10 (depends on 1) The grating interferometry measurement system of claim 1, wherein the interferometry measurement system further comprises: an acousto-optic modulator for performing frequency shifting of the single-frequency laser.