Method and apparatus for measuring roll angle of diffraction grating heterodyne interferometry based on half-wave plate

Abstract

This invention discloses a method and apparatus for measuring the roll angle of a diffraction grating heterodyne interference based on a half-wave plate. The apparatus includes a non-polarizing beam-splitting prism disposed in the output optical path of a laser source; two parallel output optical paths each equipped with an acousto-optic modulator; a first non-polarizing beam-splitting prism disposed in the output optical path of the acousto-optic modulator; and a first quarter-wave plate, a first half-wave plate, a first transmission grating, a first combined reflector, a second non-polarizing beam-splitting prism, and a third non-polarizing beam-splitting prism disposed in the transmission optical path. In the reflected optical path, a first right-angle prism, a second quarter-wave plate, a second half-wave plate, a second transmission grating, a second combined reflector, a second right-angle prism, a third right-angle prism, and an optical space exchange are disposed. The output optical path of the first non-polarizing beam-splitting prism is equipped with a first polarizer, a first photodetector, a second polarizer, and a second photodetector. The detection signals of the two photodetectors are compared and calculated by a phase meter.

Description

Technical Field

[0001] This invention relates to an optical measurement technique, and more particularly to a method and apparatus for measuring the roll angle of a diffraction grating heterodyne interference based on a half-wave plate. Background Technology

[0002] Because the plane of motion of the roll angle is orthogonal to the measurement plane, it is difficult to induce changes in optical path. Therefore, in precision machinery and engineering applications, such as CNC machine tools, coordinate measuring machines (CMMs), long-range surface mapping instruments (LTPs), or nano-optical-mechanical systems (NOMS), the measurement of roll angle during linear motion remains a challenging problem. In many practical applications, roll angle measurement is either ignored or performed using simple instruments such as gauges and levels. Therefore, there is an urgent need for an advanced and reliable roll measurement method to meet the significant demands of engineering applications.

[0003] Research indicates that there are currently three main types of methods for measuring roll angle:

[0004] The first category is non-optical methods. The most common non-optical method uses an electronic level to detect roll by referencing the direction of gravity. Due to the limitations of gravity, these methods cannot achieve real-time roll angle measurement and high sampling rate compensation, and are incompatible with linear motions outside the horizontal plane, such as vertical linear displacement. Another non-optical method is a composite measurement based on the position of a standard unit and a capacitive sensor. The disadvantages of non-optical methods are the existence of physical connections and the influence of the flatness of the measured surface. Although cables can now be replaced by wireless communication, the additional power supply required in wireless communication mode is an extra burden attached to the measured object.

[0005] The second type, the self-collimation method, is based on geometric principles and utilizes the collimation properties of lasers. A specially designed angle sensor reflects a pair of parallel beams, and a position-sensitive sensor (PSD) detects changes in the spatial position of the reflected beams to measure the roll angle. This method is more sensitive to air disturbances due to the extended optical path, and the measured displacement is not only caused by roll but is also typically affected by pitch and yaw. Therefore, it is necessary to find ways to eliminate the influence of pitch and yaw angles.

[0006] The third category, optical interferometry, including heterodyne interferometry, is a precise roll measurement method. In this method, a specific optical element is attached to the object being measured, and the optical path distance is modulated by rotating this attached optical element. The measurement principle limits the measurement range of optical interferometry. Roll angle measurement methods based on the interference principle generally convert the angle to be measured into parameters involved in the interference phenomenon for measurement. These can be broadly divided into interferometry based on optical path difference and interferometry based on beat frequency signal phase detection. Interferometry based on optical path difference typically uses a wedge prism to convert the roll angle into optical path difference, offering relatively high measurement accuracy and wide application, but it is more susceptible to air disturbances and temperature changes.

[0007] Overall, optical interferometry has broad application prospects in the real-time measurement of the roll angle of objects moving in a straight line. Existing interferometry methods offer high accuracy and good real-time performance in measuring the roll angle.

[0008] For phase detection methods based on polarization direction, patent publication CN102654392A discloses that, based on the aforementioned patent, multiple reflections are incorporated into the measurement of the roll angle of polarized light, significantly improving its measurement resolution. Building upon this, patent publication CN104180776A discloses that the measurement resolution is improved by using the reflection effect of two sets of planar mirror arrays to make the measurement light pass back and forth multiple times through a half-wave plate. However, this method requires high precision in the debugging and installation of the measurement system. Patent publication CN10544698A discloses that the polarization modulation effect of the polarized beam reflection by two mutually perpendicular but tilted high-reflectivity mirrors is used to further increase the magnification of the measurement system. However, this system has high requirements for the installation of the high-reflectivity mirrors, and the approximate linear region during measurement is very narrow. These methods not only require numerous optical components, resulting in complex structures and difficult adjustments, but also make it difficult to guarantee measurement accuracy in practical applications.

[0009] Regarding the diffraction grating heterodyne interferometry, patent publication CN109443249A discloses a method using dual diffraction beams for measurement. This method features a compact differential structure, high resolution, and effectively avoids the influence of other degrees of freedom. Patent publication CN 108775878A discloses a method using dual diffraction gratings based on an existing commercial dual-frequency laser interferometer, further improving resolution through higher optical subdivision. The above-mentioned diffraction grating differential arrangements show broad application prospects, but further improvements in system resolution are still needed. Summary of the Invention

[0010] To address the aforementioned issues, this invention provides a method for measuring roll angle using a diffraction grating heterodyne interference based on a half-wave plate. The aim is to further improve the sensor multiplication factor, thereby enhancing the sensitivity of real-time measurement of minute roll angles during linear motion of an object.

[0011] To achieve the above objective, the method for measuring the roll angle of a diffraction grating heterodyne interferometry based on a half-wave plate is implemented as follows:

[0012] Step 1: The laser source is split into two parallel beams by a combination beam splitter.

[0013] Step 2: The two outgoing beams pass through an acousto-optic modulator and are modulated into a first polarized beam and a second polarized beam with different frequencies and parallel linear polarization directions. The frequency of the first polarized modulated beam is f1, and the frequency of the second polarized modulated beam is f2.

[0014] Step 3: The first polarized light and the second polarized light are divided into transmitted light and reflected light equally by the first unpolarized beam splitter;

[0015] Step 4: The transmitted light of the first polarized beam passes through the first quarter-wave plate, the first half-wave plate, the first transmission grating, and the first combined reflector, and then passes through the first transmission grating and the first half-wave plate again. This optical path is called the first measuring arm.

[0016] Step 5: The reflected light of the first polarized beam passes through a right-angle prism, a second quarter-wave plate, a second half-wave plate, a second transmission grating, and a second combined reflector, and then passes through the second transmission grating and the second half-wave plate again. This optical path is called the second measuring arm.

[0017] Step 6: The transmitted light of the second polarized beam passes through the first quarter-wave plate, the first half-wave plate, the first transmission grating, and the first combined reflector, and then passes through the first transmission grating and the first half-wave plate again. This optical path is called the third measuring arm.

[0018] Step 7: The reflected light of the second polarized beam passes through the right-angle prism, the second quarter-wave plate, the second half-wave plate, the second transmission grating, and the second combined mirror, and then passes through the second transmission grating and the second half-wave plate again. This optical path is called the fourth measuring arm.

[0019] Step 8: The second and fourth measuring arms exchange their spatial positions with the optical space exchanger without changing the polarization state of the light, so that the beams output by the first and fourth measuring arms enter the first photodetector, and the beams output by the second and third measuring arms enter the second photodetector.

[0020] Step 9: The beam output from the fourth measuring arm is reflected by the second right-angle prism and combined with the beam output from the first measuring arm at the second unpolarized beam splitter to form the first measuring beam, which is polarized by the first analyzer and received by the first photodetector to generate the first beat frequency signal.

[0021] Step 10: The beam output from the second measuring arm is reflected by the third right-angle prism and combined with the beam output from the third measuring arm at the third unpolarized beam splitter to form the second measuring beam, which is polarized by the second analyzer and received by the second photodetector to generate the second beat frequency signal.

[0022] Step 11: The first beat frequency signal and the second beat frequency signal are compared by a phase meter, and the data is transmitted to the computer. The computer obtains the roll angle based on the relationship between the change in the phase difference between the first beat frequency signal and the second beat frequency signal, the roll angle of the half-wave plate, and the grating displacement.

[0023] The method for measuring the roll angle of a diffraction grating heterodyne interferometry based on a half-wave plate involved in this invention also includes the following features:

[0024] In step four, the fast axis of the first quarter-wave plate makes an angle θ with the polarization direction of the polarized light, and in step five, the fast axis of the second quarter-wave plate makes an angle -θ with the polarization direction of the polarized light.

[0025] The first half-wave plate, the second half-wave plate, the first transmission grating, and the second transmission grating are disposed on the object under test and move together with the object. The grating constants of the first transmission grating and the second transmission grating are the same.

[0026] The relationship between the phase difference and the roll angle in step eleven is as follows:

[0027]

[0028] in Let α be the phase difference between the first beat frequency signal and the second beat frequency signal, α be the roll angle of the object under test, m be the diffraction order of the grating, d be the grating constant of the selected grating, and L be the distance between the first and fourth measuring arms.

[0029] The present invention also relates to an apparatus for the above-described measurement method, characterized in that it comprises:

[0030] A laser source and a non-polarized beam splitter prism disposed in the output light path of the laser source are provided. A first acousto-optic modulator and a second acousto-optic modulator are respectively disposed in the two parallel output light paths of the non-polarized beam splitter prism. The output light path of the acousto-optic modulator is provided with the first non-polarized beam splitter prism. The transmission light path of the first non-polarized beam splitter prism is provided with a first quarter-wave plate, a first half-wave plate, a first transmission grating, a first combined reflector, a second non-polarized beam splitter prism, and a third non-polarized beam splitter prism. The reflection light path of the non-polarized beam splitter prism is provided with a first right-angle prism, a second quarter-wave plate, a second half-wave plate, a second transmission grating, a second combined reflector, a second right-angle prism, a third right-angle prism, and an optical space exchanger. The output light path of the first non-polarized beam splitter prism is provided with a first polarizer, a first photodetector, a second polarizer, and a second photodetector. The detection signals of the first photodetector and the second photodetector are compared by a phase meter, and the data is transmitted to a data processing unit (computer) for calculation.

[0031] Because this invention employs a structure that combines a half-wave plate and a grating to detect the roll angle, the following beneficial effects can be achieved:

[0032] The combination of a half-wave plate and a grating for detecting roll angle can further increase the multiplication factor of the roll angle based on the original half-wave plate as a sensor, thereby improving the measurement resolution.

[0033] The gratings are arranged differentially, which can effectively avoid the influence of optical path caused by other degrees of freedom. By increasing the grating spacing or selecting a larger grating constant, the measurement sensitivity can be effectively improved, thereby improving the measurement resolution. Attached Figure Description

[0034] Figure 1 This is a schematic diagram of the structure of the diffraction grating heterodyne interference roll angle measurement device based on a half-wave plate in the embodiment.

[0035] Figure 2 This is a schematic diagram of the optical path of the diffraction grating heterodyne interference roll angle measurement device based on a half-wave plate in the embodiment.

[0036] Figure 3 This is a top view of a diffraction grating heterodyne interference roll angle measurement device based on a half-wave plate.

[0037] Figure 4 This is a schematic diagram of the grating and half-wave plate rolling with the object under test in the embodiment.

[0038] Figure reference numerals: 101-Laser source, 102-Unpolarized beam splitter prism, 103a-First acousto-optic modulator, 103b-Second acousto-optic modulator, 104-First unpolarized beam splitter prism, 106a-First quarter-wave plate, 107a-First half-wave plate, 108a-First transmission grating, 110a-First combined mirror, 113a-Second unpolarized beam splitter prism, 113b-Third unpolarized beam splitter prism, 104a-First... A non-polarizing beam splitter, 105-first right-angle prism, 106b-second quarter-wave plate, 107b-second half-wave plate, 108b-second transmission grating, 110b-second combined mirror, 111-optical space exchanger, 112a-second right-angle prism, 112b-third right-angle prism, 114a-first polarizer, 114b-second polarizer, 115a-first photodetector, 115b-second photodetector. Detailed Implementation

[0039] The working principle of the present invention will be further described in detail with reference to the accompanying drawings. The description is intended to explain the present invention and is not intended to limit it.

[0040] A device for measuring the roll angle of a diffraction grating heterodyne interferometry based on a half-wave plate, such as... Figure 1 As shown, the feature is that it includes a laser source 101, a non-polarizing beam splitter prism 102 disposed in the output optical path of the laser source 101, a first acousto-optic modulator 103a and a second acousto-optic modulator 103b respectively disposed in the two parallel output optical paths of the non-polarizing beam splitter prism 102, a first non-polarizing beam splitter prism 104 disposed in the output optical path of the acousto-optic modulator, and a first quarter-wave plate 106a, a first half-wave plate 107a, a first transmission grating 108a, a first combined reflector 110a, a second non-polarizing beam splitter prism 113a, and a third non-polarizing beam splitter prism 104 respectively disposed in the transmission optical path of the first non-polarizing beam splitter prism 104. A light prism 113b is provided. Along the reflected light path of the first unpolarized beam splitter 104, a first right-angle prism 105, a second quarter-wave plate 106b, a second half-wave plate 107b, a second transmission grating 108b, a second combined reflector 110b, an optical space exchanger 111, a second right-angle prism 112a, and a third right-angle prism 112b are respectively provided. The output light path of the second unpolarized beam splitter 113a is provided with a first polarizer 114a and a first photodetector 115a. The output light path of the third unpolarized beam splitter 113b is provided with a second polarizer 114b and a second photodetector 115b. The first half-wave plate 107a, the second half-wave plate 107b, the first transmission grating 108a, and the second transmission grating 108b are disposed on the object under test 109 and move together with the object under test 109.

[0041] Laser source 101 is used to provide a frequency-stable single-frequency linearly polarized laser source.

[0042] The non-polarized beam splitter prism 102 is used to divide the emitted beam of the laser source 101 into transmitted light and reflected light equally through the non-polarized beam splitter prism. The reflected light is then emitted parallel to the transmitted light of the non-polarized beam splitter prism through a right-angle prism, thereby achieving the function of parallel beam splitting of the laser source.

[0043] The acousto-optic modulator 103 is used to modulate the frequency of the incident beam. The outgoing beam is only a first-order diffracted beam after passing through the aperture. The frequency of the first diffracted beam generated by the acousto-optic modulator 103a is f1, and the frequency of the second diffracted beam generated by the acousto-optic modulator 103b is f2.

[0044] The laser beam output from the laser source 101 is parallelly split by the non-polarized beam splitting combination prism 102 and then passes through the acousto-optic modulator. The laser frequency is modulated into two lasers with different frequencies and parallel linear polarization directions. The frequency of the first polarized light is f1, and the frequency of the second polarized light is f2. The frequency of the beat frequency signal corresponds to the frequency difference (f1-f2) between the first polarized light and the second polarized light.

[0045] The first unpolarized beam splitter 104 is used to divide linearly polarized light with frequencies f1 and f2 into transmitted light and reflected light equally.

[0046] The f1 transmitted light from the first non-polarizing beam splitter 104 passes sequentially through the first quarter-wave plate 106a, the first half-wave plate 107a, and the first transmission grating 108a, and is reflected twice by the combined reflector 110a to make the incident light return in parallel and then pass sequentially through the first transmission grating 108a and the first half-wave plate 107a to generate the first measuring arm.

[0047] The f1 reflected light from the first non-polarizing beam splitter 104 is reflected by the right-angle prism 105 and then passes sequentially through the second quarter-wave plate 106b, the second half-wave plate 107b, and the second transmission grating 108b. After two reflections by the combined reflector 110b, the incident light is paralleled back and then passes sequentially through the second transmission grating 108b and the second half-wave plate 107b to generate the second measuring arm.

[0048] The f2 transmitted light from the first non-polarizing beam splitter 104 passes sequentially through the first quarter-wave plate 106a, the first half-wave plate 107a, and the first transmission grating 108a, and is reflected twice by the combined reflector 110a to make the incident light return in parallel and then pass sequentially through the first transmission grating 108a and the first half-wave plate 107a to generate the third measuring arm.

[0049] The f2 reflected light from the first non-polarizing beam splitter 104 is reflected by the right-angle prism 105 and then passes sequentially through the second quarter-wave plate 106b, the second half-wave plate 107b, and the second transmission grating 108b. After two reflections by the combined reflector 110b, the incident light is paralleled back and then passes sequentially through the second transmission grating 108b and the second half-wave plate 107b to generate the fourth measuring arm.

[0050] The quarter-wave plate is used to change the polarization state of the beam. For example, linearly polarized light is incident perpendicularly to the wave plate and the polarization direction is at a small angle θ with the optical axis, and the outgoing light is microelliptically polarized. The angle between the fast axis of the first quarter-wave plate and the polarization direction of the incident light is θ1, and the angle between the fast axis of the second quarter-wave plate and the polarization direction of the incident light is θ2, and θ1 = -θ2.

[0051] A half-wave plate is used as the first sensor of the roll angle; the micro-ellipsoidally polarized light emitted from the quarter-wave plate can be converted into a phase change by the half-wave plate. A transmission grating is used as the second sensor of the roll angle; the phase change of the transmitted light after +1 order diffraction is... Where m is the diffraction order, S is the grating displacement, and d is the grating constant.

[0052] After passing through the space exchanger 111, the fourth measuring arm is reflected by the second right-angle prism 112a and then combined with the first measuring arm at the second unpolarized beam splitter 113a to generate the first measuring light.

[0053] After passing through the space exchanger 111, the second measuring arm is reflected by the third right-angle prism 112b and then combined with the third measuring arm at the third unpolarized beam splitter 113b to generate the second measuring light.

[0054] The first measuring light is polarized by the polarizer 114a and then received by the first photodetector 115a to generate the first beat frequency signal.

[0055] The second measuring light is polarized by the polarizer 114b and then received by the second photodetector 115b to generate the second beat frequency signal.

[0056] The first beat frequency signal and the second beat frequency signal are phase-calculated by phase meter 115.

[0057] Phase difference between the first beat frequency signal and the second beat frequency signal for:

[0058]

[0059] Where α is the roll angle of the object, the derivation process of equation (1) is as follows:

[0060] Using the Jones matrix to analyze this method, the light vector of the output light source of laser source 101 is expressed as:

[0061]

[0062] After modulation by an acousto-optic modulator:

[0063]

[0064] The Jones matrix derivation for the first beat frequency signal is as follows:

[0065] For the first measuring arm with frequency f1:

[0066]

[0067] in, First rotation matrix Jones matrix of the first quarter-wave plate Jones matrix of the first half-wave plate Phase change introduced by the first transmission grating L is the spatial distance between the first measuring arm and the fourth measuring arm, such as Figure 4 As shown; polarizer Substituting the above into equation (3), we can obtain:

[0068]

[0069] In the formula

[0070] For the fourth measuring arm with frequency f2:

[0071]

[0072] in, Second rotation matrix Jones matrix of the second quarter-wave plate Jones matrix of the second half-wave plate Phase change introduced by the second transmission grating L is the distance between the first measuring arm and the fourth measuring arm, such as Figure 3 As shown; polarizer Substituting the above into equation (6), we can obtain:

[0073]

[0074] In the formula

[0075] The first beat frequency signal measured by the photodetector is:

[0076]

[0077] in

[0078]

[0079] The Jones matrix derivation for the second beat frequency signal is as follows:

[0080] For the second measuring arm with frequency f1:

[0081]

[0082] in, Second rotation matrix Jones matrix of the first quarter-wave plate Jones matrix of the first half-wave plate Phase change introduced by the first transmission grating L is the spatial distance between the second and third measuring arms, and its magnitude is the same as the distance between the first and fourth measuring arms, such as... Figure 4 As shown; polarizer Substituting the above into equation (11), we can obtain:

[0083]

[0084] In the formula

[0085] For the third measuring arm with frequency f2:

[0086]

[0087] in, First rotation matrix Jones matrix of the second quarter-wave plate Jones matrix of the second half-wave plate Phase change introduced by the second transmission grating L is the distance between the second and third measuring arms, and its magnitude is the same as the distance between the first and fourth measuring arms, such as... Figure 4 As shown; polarizer Substituting the above into equation (14), we can obtain:

[0088]

[0089] In the formula

[0090] The second beat frequency signal measured by the photodetector is:

[0091]

[0092] in

[0093]

[0094] The second beat frequency signal and the first beat frequency signal can be compared using a phase meter to obtain:

[0095]

[0096] Angular magnification K is:

[0097]

[0098] From equation (20), it can be seen that when α is in a small range near 0, equation (20) can be linearly approximated as:

[0099]

[0100] Where 16cotθ represents the gain effect brought by the half-wave plate. The gain effect brought about by grating displacement is considered if a quarter-wave plate with a fast axis angle θ = 2°, a measuring arm spacing of 10 mm, and a grating constant are selected. For a light source with λ = 633 nm, the ±1st order diffraction angle is 3.63°. Theoretically, the angular magnification of the composite sensor can reach more than 26,000 times. Using a phase meter with a resolution of 0.01°, the theoretical resolution of the roll angle α of the object being measured by the diffraction grating heterodyne interference roll angle measurement device based on a half-wave plate is 0.001″.

[0101] Although specific embodiments of the invention have been disclosed for illustrative purposes to aid in understanding and implementing the invention, those skilled in the art will understand that various substitutions, variations, and modifications are possible without departing from the spirit and scope of the invention and the appended claims. Therefore, the invention should not be limited to the content disclosed in the preferred embodiments, and the scope of protection claimed by the invention is defined by the claims.

Claims

1. A method for measuring the roll angle of a diffraction grating heterodyne interferometry based on a half-wave plate, comprising the following steps: 1) The laser source is split into two parallel beams by a beam splitter; 2) Modulate the two outgoing beams to obtain two beams with different frequencies and parallel linear polarization directions, namely a first polarized beam and a second polarized beam; the frequency of the first polarized modulated beam is f1, and the frequency of the second polarized modulated beam is f2. 3) The first polarized light and the second polarized light are split into transmitted light and reflected light by the first unpolarized beam splitter prism; 4) The optical path of the transmitted light of the first polarized beam, passing through the first quarter-wave plate, the first half-wave plate, the first transmission grating, the first combined reflector, and then passing through the first transmission grating and the first half-wave plate again, is denoted as the first measuring arm; the optical path of the reflected light of the first polarized beam, passing through the right-angle prism, the second quarter-wave plate, the second half-wave plate, the second transmission grating, the second combined reflector, and then passing through the second transmission grating and the second half-wave plate again, is denoted as the second measuring arm; the optical path of the transmitted light of the second polarized beam, passing through the first quarter-wave plate, the first half-wave plate, the first transmission grating, the first combined reflector, and then passing through the first transmission grating and the first half-wave plate again, is denoted as the third measuring arm; the optical path of the reflected light of the second polarized beam, passing through the right-angle prism, the second quarter-wave plate, the second half-wave plate, the second transmission grating, the second combined reflector, and then passing through the second transmission grating and the second half-wave plate again, is denoted as the fourth measuring arm; 5) The beam output from the second measuring arm and the beam output from the fourth measuring arm exchange their spatial positions via an optical space exchanger, so that the beam output from the fourth measuring arm is reflected by the second right-angle prism and combined with the beam output from the first measuring arm at the second unpolarized beam splitter to form the first measuring beam; the beam output from the second measuring arm is reflected by the third right-angle prism and combined with the beam output from the third measuring arm at the third unpolarized beam splitter to form the second measuring beam. 6) The first measurement light is polarized by the first analyzer and then input into the first photodetector to generate a first beat frequency signal; the second measurement light is polarized by the second analyzer and then input into the second photodetector to generate a second beat frequency signal. 7) Input the first beat frequency signal and the second beat frequency signal into a phase meter for phase comparison to obtain the phase difference. Phase difference The data is transmitted to the calculation unit to calculate the roll angle α of the object under test; wherein, the first half-wave plate, the second half-wave plate, the first transmission grating, and the second transmission grating are disposed on the object under test and move together with the object under test.

2. The method according to claim 1, characterized in that, The fast axis of the first quarter-wave plate forms an angle θ with the polarization direction of the transmitted light of the first polarized beam, and the fast axis of the second quarter-wave plate forms an angle -θ with the polarization direction of the reflected light of the first polarized beam; the first transmission grating and the second transmission grating have the same structure and a phase difference. Where m is the diffraction order of the first transmission grating, d is the grating constant of the selected grating, and L is the distance between the first and fourth measuring arms.

3. The method according to claim 2, characterized in that, The beat frequencies of the first beat frequency signal and the second beat frequency signal correspond to the frequency difference (f1-f2).

4. The method according to claim 1, 2, or 3, characterized in that, One of the two emitted beams is modulated using a first acousto-optic modulator, and the other of the two emitted beams is modulated using a second acousto-optic modulator. The first and second acousto-optic modulators emit only first-order diffraction light, and the radio frequency quantities of the first and second acousto-optic modulators are different. The difference between the two radio frequency quantities is within the detection limit of the photodetector, resulting in two beams of first polarization and second polarization with different frequencies and parallel linear polarization directions.

5. The method according to claim 1, 2, or 3, characterized in that, The beam splitting unit is a non-polarized beam splitting combination prism, which is composed of a non-polarized beam splitting prism and a right-angle prism.

6. A device for measuring the roll angle of a diffraction grating heterodyne interferometry based on a half-wave plate, characterized in that, The system includes a laser source, a beam splitting unit, a modulation unit, a first unpolarized beam splitting prism; a first quarter-wave plate, a first half-wave plate, a first transmission grating, a first combined mirror, a second unpolarized beam splitting prism, and a third unpolarized beam splitting prism; a first right-angle prism, a second quarter-wave plate, a second half-wave plate, a second transmission grating, a second combined mirror, a second right-angle prism, and a third right-angle prism; an optical space exchange, a first polarizer, a first photodetector, a second polarizer, a second photodetector, a phase meter, and a data processing unit; wherein the first half-wave plate, the second half-wave plate, the first transmission grating, and the second transmission grating are disposed on the object under test and move together with the object under test; The laser source is used to output stable linearly polarized light of a single frequency and incident it onto the beam splitting unit. The beam splitting unit is used to split the incident laser into two parallel outgoing beams and input them into the modulation unit; The modulation unit is used to modulate the two outgoing beams to obtain two beams of first polarization and second polarization with different frequencies and parallel linear polarization directions; the frequency of the first polarization modulated light is f1, and the frequency of the second polarization modulated light is f2; the first polarization light and the second polarization light are respectively split into corresponding transmitted light and reflected light by the first non-polarization beam splitter prism. The optical path of the transmitted light of the first polarized beam, passing through the first quarter-wave plate, the first half-wave plate, the first transmission grating, and the first combined reflector, and then passing through the first transmission grating and the first half-wave plate again, is denoted as the first measurement arm; the optical path of the reflected light of the first polarized beam, passing through the right-angle prism, the second quarter-wave plate, the second half-wave plate, the second transmission grating, and the second combined reflector, and then passing through the second transmission grating and the second half-wave plate again, is denoted as the second measurement arm; the optical path of the transmitted light of the second polarized beam, passing through the first quarter-wave plate, the first half-wave plate, the first transmission grating, and the first combined reflector, and then passing through the first transmission grating and the first half-wave plate again, is denoted as the third measurement arm; the optical path of the reflected light of the second polarized beam, passing through the right-angle prism, the second quarter-wave plate, the second half-wave plate, the second transmission grating, and the second combined reflector, and then passing through the second transmission grating and the second half-wave plate again, is denoted as the fourth measurement arm; The optical spatial exchanger is used to exchange the spatial positions of the light beams output from the second measuring arm and the fourth measuring arm, so that the light beam output from the fourth measuring arm is reflected by the second right-angle prism and combined with the light beam output from the first measuring arm at the second unpolarized beam splitter to form the first measuring light; the light beam output from the second measuring arm is reflected by the third right-angle prism and combined with the light beam output from the third measuring arm at the third unpolarized beam splitter to form the second measuring light. The optical spatial exchanger only exchanges the spatial positions of the two parallel incident lights and does not change the polarization state of the two parallel incident lights. The first measurement light is polarized by the first analyzer and then input into the first photodetector to generate a first beat frequency signal; the second measurement light is polarized by the second analyzer and then input into the second photodetector to generate a second beat frequency signal. The phase meter is used to compare the input first beat frequency signal and the second beat frequency signal to obtain the phase difference. Calculation unit, used to calculate based on phase difference The roll angle α of the object under test is calculated.

7. The apparatus according to claim 6, characterized in that, The fast axis of the first quarter-wave plate forms an angle θ with the polarization direction of the transmitted light of the first polarized beam, and the fast axis of the second quarter-wave plate forms an angle -θ with the polarization direction of the reflected light of the first polarized beam; the first transmission grating and the second transmission grating have the same structure and a phase difference. Where m is the diffraction order of the first transmission grating, d is the grating constant of the selected grating, and L is the distance between the first and fourth measuring arms.

8. The apparatus according to claim 7, characterized in that, The frequencies of the first beat frequency signal and the second beat frequency signal correspond to the frequency difference (f1-f2).

9. The apparatus according to claim 6, characterized in that, The modulation unit includes two acousto-optic modulators, wherein the first acousto-optic modulator modulates one of the two emitted beams, and the second acousto-optic modulator modulates the other of the two emitted beams. The first and second acousto-optic modulators emit only first-order diffraction light, and the radio frequency quantities of the first and second acousto-optic modulators are different. The difference between the two radio frequency quantities is within the detection limit of the photodetector, resulting in two beams of first polarization and second polarization with different frequencies and parallel linear polarization directions.

10. The apparatus according to claim 6, characterized in that, The beam splitting unit is a non-polarized beam splitting combination prism, which is composed of a non-polarized beam splitting prism and a right-angle prism.

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