Apparatus and method for suppressing spurious tilt errors in differential phase
By designing a dual-frequency laser source and an optical path interference module, and combining it with post-processing subtraction technology, beam tilt and lateral offset errors in DWS technology are suppressed, improving angle detection accuracy and anti-interference capability, and expanding the application range.
Patent Information
- Application Number
- CN202411091031.2
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2024-08-09
- Publication Date
- 2025-12-12
- Estimated Expiration
- 2044-08-09
AI Technical Summary
In existing DWS technology, parasitic tilt errors caused by beam tilt and lateral offset affect the accuracy and stability of angle measurement, and there is currently a lack of effective suppression methods.
By employing a dual-frequency laser source module, an optical path interference module, and a precision measurement and control module, and through frequency modulation, beam correction, and interference signal processing, combined with post-processing subtraction techniques, the coupling effects of beam tilt and lateral offset on the differential phase are suppressed.
It improves the sensitivity and anti-interference capability of DWS angle detection, and expands the application of DWS technology in angle measurement when the beam is misaligned and lateral offset measurement when the beam is tilted.
Smart Images
Figure CN118857162B_ABST
Abstract
Description
TECHNICAL FIELD
[0001] The application belongs to the field of spatial laser interference technology and optical precision measurement technology, and particularly relates to a device and method for suppressing parasitic tilt error in differential phase. BACKGROUND
[0002] Differential wavefront sensing (DWS) technology is an angle-sensitive technology based on heterodyne interference. When there is a relative angle between the reference beam and the measurement beam, the wavefront component of the laser beam will tilt accordingly following the misalignment angle, thereby causing the phase of the beat frequency detected by each quadrant of the detector to change accordingly. By detecting the differential phase of the beat frequency signal and combining the angle measurement algorithm of the DWS technology, the measurement of the beam angle can be realized.
[0003] As a phase-sensitive angle measurement technology, DWS has a phase angle amplification factor of three to four orders of magnitude, which can achieve a measurement precision of urad level to detect a pointing angle of nrad level. In addition, while realizing angle measurement, DWS technology can realize ranging through average phase, which can effectively avoid the increase of light intensity dispersion and shot noise caused by independent measurement of distance and angle, and realize the design of a system integrating angle measurement and ranging. Moreover, DWS technology has a simple interference optical path and optical system, so the background noise is low.
[0004] Based on the above three advantages, DWS technology is widely used in earth gravity field test such as: Earth Gravity Field Recovery and Climate Experiment Follow-on Mission (GRACE-FO) and Next Generation Gravity Mission (NGGM), almost all the gravitational wave interference plans such as: Laser Interferometer Gravitational-Wave Observatory (LIGO), European Virgo, 600-meter arm length laser interferometer gravitational wave detector (GEO600), Kamioka Gravitational Wave Detector (KAGRA), DECI-hertz interferometer Gravitational wave Observatory (DECIGO), The Laser Interferometer Space Antenna (LISA), Taiji Program in Space (Taiji), TianQin Program in Space (TianQin). In summary, due to high precision, low noise, multi-freedom and adaptability of long distance, DWS technology has gradually become the core technology in the field of space laser interference technology and optical precision measurement technology.
[0005] Ideally, the differential phase information of a four-quadrant photodetector (QPD) has a linear relationship with the beam angle, however, any factor that causes the laser intensity distribution to be uneven and the degree of spot overlap to be reduced inevitably affects the linearity of the differential phase detection of the DWS, and further affects the DWS angle measurement range and stability. The angle measurement value in the DWS technology is obtained by decoupling the differential phase. According to the Gaussian beam electric field distribution and the coordinate transformation, the beam tilt angle can be obtained, and the lateral offset of the beam, the longitudinal offset of the beam, the waist radius of the Gaussian beam and the photosensitive area of the detector are all coupled with the differential phase, and it is difficult to directly derive an analytical expression. Therefore, in addition to the beam tilt angle, other influencing factors become noise sources of the parasitic tilt error and the non-linear error of the DWS. Among them, the lateral offset of the beam has the largest contribution to the error of the DWS, and it is manifested as a parasitic tilt error, and there is currently no related technology that can well solve the parasitic tilt error in the DWS technology. SUMMARY
[0006] In view of the above technical problems, the present application provides a device and method for suppressing parasitic tilt error in differential phase, which can improve the sensitivity of DWS angle detection.
[0007] The technical solutions provided by the present application are as follows:
[0008] According to the first aspect of the present application, a device for suppressing parasitic tilt error in differential phase is provided, which comprises a dual-frequency laser light source module, an optical path interference module, and a precision measurement and control module, wherein the dual-frequency laser light source module comprises a 1064nm laser, an isolator, a first light splitting prism, a first acousto-optic modulator, a second acousto-optic modulator, a first wedge-shaped piece, and a second wedge-shaped piece.
[0009] The optical path interference module comprises a first diaphragm, a second diaphragm, a first linear polarizer, a second linear polarizer, a piezoelectric fast mirror, a reflecting prism, and a second light splitting prism.
[0010] The precision measurement and control module comprises a four-quadrant photodetector, a lock-in meter, data processing, and a controller.
[0011] The method comprises: a 1064nm laser generates a stable light signal, which is divided into two branches through a first light splitting prism after passing through an isolator; the light signal of the first branch is first passed through a first acousto-optic modulator to generate Bragg diffraction effect to frequency modulate the laser; then, the light signal is passed through a first wedge to correct the laser tilt deviation caused by the frequency shift of the first acousto-optic modulator; the light signal is continuously passed through a first diaphragm, and the first-order Bragg diffraction light modulated by the acousto-optic modulator is selected at the first diaphragm, and other-order Bragg diffraction light and stray light are isolated; then, the light signal is passed through a first linear polarizer to be transmitted to a reflecting prism as one input of the interference light signal in a specific polarization direction; the light signal of the second branch is sequentially passed through a second acousto-optic modulator, a second wedge, a second diaphragm, a second linear polarizer and a piezoelectric fast mirror as the other input of the interference light signal in the same way; then, the output light signals of the two branches are first interfered by a second light splitting prism and then transmitted to a four-quadrant detector; the four-quadrant detector outputs four beat frequency electric signals; the four-quadrant detector is connected to a phase-locked counter through a radio frequency connection line; the phase-locked counter outputs four phase signals; a data processing module is used for calculating a differential phase value; and a controller is used for controlling the deflection or translation of the piezoelectric fast mirror according to the differential phase output by the data processing module.
[0012] In addition, the data processing module further comprises: through post-processing subtraction, laser beam angle measurement when two light beams are misaligned based on the DWS technology and spot lateral shift measurement in a tilted propagation mode are realized.
[0013] According to the second aspect of the present application, a method for suppressing parasitic tilt error in differential phase is provided, which is executed by the above-mentioned apparatus for suppressing parasitic tilt error in differential phase, and the method comprises:
[0014] A 1064nm laser generates a stable light signal, which is divided into two branches through a first light splitting prism after passing through an isolator; the light signal of the first branch is first passed through a first acousto-optic modulator to generate Bragg diffraction effect to frequency modulate the laser; then, the light signal is passed through a first wedge to correct the laser tilt deviation caused by the frequency shift of the first acousto-optic modulator; the light signal is continuously passed through a first diaphragm, and the first-order Bragg diffraction light modulated by the acousto-optic modulator is selected at the first diaphragm, and other-order Bragg diffraction light and stray light are isolated; then, the light signal is passed through a first linear polarizer to be transmitted to a reflecting prism as one input of the interference light signal in a specific polarization direction; the light signal of the second branch is sequentially passed through a second acousto-optic modulator, a second wedge, a second diaphragm, a second linear polarizer and a piezoelectric fast mirror as the other input of the interference light signal in the same way; then, the output light signals of the two branches are first interfered by a second light splitting prism and then transmitted to a four-quadrant detector; the four-quadrant detector outputs four beat frequency electric signals; the phase-locked counter outputs four phase signals; a data processing module is used for calculating a differential phase value; and a controller is used for controlling the deflection or translation of the piezoelectric fast mirror according to the differential phase output by the data processing module.
[0015] The data processing is performed by the data processing module: the laser beam angle measurement based on the DWS technology when two light beams are misaligned is realized by post-processing subtraction, and the spot lateral shift measurement in the tilted propagation mode is realized.
[0016] Compared with the prior art, the present application has the beneficial effects that:
[0017] The present application separately studies the coupling relationship between the beam tilt and the lateral shift and the DWS differential phase, and on this basis, simultaneously studies the output influence of the beam tilt and the lateral shift on the DWS differential phase. It is obtained that under the conditions of the beam tilt in the mrad level and the lateral shift in the mm level, the output influence of the beam tilt and the lateral shift on the DWS differential phase shows a superposition relationship, and therefore the post-processing subtraction parasitic tilt error suppression method is proposed. The method not only can improve the sensitivity of the DWS angle detection, but also can improve the anti-interference ability of the DWS technology, and is helpful to popularize and apply the DWS technology to the angle measurement when the light beams are misaligned and the lateral shift measurement when the light beams are tilted. BRIEF DESCRIPTION OF DRAWINGS
[0018] Figure 1 Fig. 1 is a structural diagram of a parasitic tilt error suppression device according to an embodiment of the present application;
[0019] Figure 2 Fig. 2 is a structural schematic diagram of a four-quadrant photodetector according to an embodiment of the present application;
[0020] Figure 3 Fig. 3 is an interference beam wavefront schematic diagram under different Gaussian beam propagation models;
[0021] Fig. 4 (a) shows the response curve of the DWS differential phase and the laser beam angle;
[0022] Fig. 4 (b) shows the relationship between the DWS phase error and the angle before and after the lateral shift compensation;
[0023] Fig. 5 (a) shows the response curve of the DWS differential phase and the beam lateral shift;
[0024] Fig. 5 (b) shows the relationship between the DWS phase error and the lateral shift before and after the angle compensation.
[0025] Wherein: 1064nm laser 101, isolator 102, first light splitting prism 103, reflecting prism 104, second light splitting prism 105, four-quadrant detector 106, phase-locked meter 107, data processing module 108, controller 109, piezoelectric fast mirror 110.
[0026] The first acousto-optic modulator 1001, the first wedge 1002, the first aperture stop 1003, and the first linear polarizer 1004.
[0027] The second acousto-optic modulator 2001, the second wedge 2002, the second aperture stop 2003, and the second linear polarizer 2004. DETAILED DESCRIPTION
[0028] Exemplary embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. While exemplary embodiments of the present disclosure are shown in the drawings, it is understood that the present disclosure can be embodied in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art.
[0029] Figure 1 A structural diagram of a device for suppressing parasitic tilt error in differential phase according to an embodiment of the present disclosure. As shown in the figure, the device includes a dual-frequency laser light source module, an optical path interference module, and a precision measurement and control module. The dual-frequency laser light source module, the optical path interference module, and the precision measurement and control module are sequentially connected. Figure 1
[0030] The dual-frequency laser light source module includes a 1064nm laser 101, an isolator 102, a first beam splitter 103, a first acousto-optic modulator (AOM) 1001, a second acousto-optic modulator 2001, a first wedge 1002, and a second wedge 2002.
[0031] The optical path interference module includes a first aperture stop (AS) 1003, a second aperture stop 2003, a first linear polarizer 1004, a second linear polarizer 2004, a piezoelectric fast mirror 110, a reflecting prism 104, and a second beam splitter (BS) 105.
[0032] The precision measurement and control module includes a four-quadrant detector 106, a lock-in amplifier 107, a data processing module 108, and a controller 109.
[0033] A 1064nm laser 101 generates a stable optical signal, which is then split into two branches by a first beam splitter 103 after passing through an isolator 102. The optical signal of the first branch first passes through a first acousto-optic modulator 1001, where Bragg diffraction modulates the laser frequency. Next, it passes through a first wedge 1002 to correct the laser tilt caused by the frequency shift of the first acousto-optic modulator 1001. Continuing through a first aperture 1003, the first-order Bragg diffraction light modulated by the acousto-optic modulator is selected, while other orders of Bragg diffraction light and stray light are isolated. Then, it passes through a first linear polarizer 1004 and is transmitted in a specific polarization direction to a reflecting prism 104, becoming one input of the interference optical signal. Similarly, the optical signal of the second branch passes sequentially through a second acousto-optic modulator 2001, a second wedge 2002, a second aperture 2003, a second linear polarizer 2004, and a piezoelectric fast reflector 110, becoming the other input of the interference optical signal. The optical signals from the two output branches first interfere through the second beam splitter 105, and then are transmitted to the four-quadrant detector 106. The four-quadrant detector 106 outputs four beat frequency electrical signals. The four-quadrant detector 106 is connected to the phase-locked loop (PLL) 107 through an RF connection line. The PLL 107 demodulates the four beat frequency electrical signals, obtains and outputs four phase signals. The data processing module 108 calculates the measured value of the laser beam angle. The controller 109 is used to control the deflection of the piezoelectric fast reflector 110 according to the angle output by the data processing module 108. Specifically, the angle output by the data processing module 108 is decoupled by differential phase. The differential phase is a linear combination of the four phase signals output by the PLL, including differential phases in the left and right quadrants and differential phases in the upper and lower quadrants.
[0034] The following description Figure 1 The design methods of each module are shown.
[0035] The design of a dual-frequency laser source module includes generating two beams with a stable frequency difference. The laser source is designed. The optical path interferometry module is designed to generate the heterodyne interference optical signal to be detected. The precision measurement and control module is designed to detect the interference optical signal and provide feedback control for the system.
[0036] The following describes a specific implementation of the dual-frequency laser source module design. First, according to Maxwell's equations, the electric field of a Gaussian beam propagating in free space... The description is as follows:
[0037] (1)
[0038] and, The phase of a Gaussian beam is described as follows:
[0039] (2)
[0040] in, is an arbitrary point described in the fixed coordinate system of the light beam, and t represents time, represents the laser frequency, and i represents the imaginary unit, represents the laser frequency, represents the laser power, represents the wave number, represents the wavefront curvature, represents the Gouy phase.
[0041] The first acousto-optic modulator 1001 is a tunable frequency acousto-optic modulator, and the modulation frequency is set to be the frequency hopping , wherein is the modulation frequency of the first acousto-optic modulator, denoted as the frequency hopping, is the fixed frequency difference between the first acousto-optic modulator and the second acousto-optic modulator, is the modulation frequency of the second acousto-optic modulator, denoted as the point frequency. The first branch in which the first acousto-optic modulator 1001 is located is the branch through which the measurement light beam passes. After the first acousto-optic modulator 1001, the measurement light beam then passes through the first diaphragm 1003. represents the electric field of the first-branch Gaussian light beam. The second acousto-optic modulator 2001 is a fixed frequency acousto-optic modulator, and the modulation frequency is set to be . The second branch in which the second acousto-optic modulator 2001 is located is the branch through which the reference light beam passes. The reference light beam refers to the second-branch light beam. After the reference light beam passes through the second acousto-optic modulator 2001, it then passes through the second diaphragm 2003. represents the electric field of the second-branch Gaussian light beam. At this time, the two light beams obtain a fixed frequency difference and continue to transmit to the optical path interference module;
[0042] A specific implementation of the design of the optical path interference module is described below.
[0043] The first output signal (the electric field is ) passing through the first diaphragm 1003 passes through the first linear polarizer 1004 in the branch, is incident from the inclined surface of the reflection prism 104 at an incident angle of 45°, the second output signal (the electric field is ) passing through the second diaphragm 2003 passes through the second linear polarizer 2004 in the branch, is incident from the inclined surface of the piezoelectric fast mirror 110 at an incident angle of 45°, and then the first output signal and the second output signal are transmitted to the second beam splitter 105 and are converted into the first light beam and the second light beam by the second beam splitter 105, respectively, and the first light beam and the second light beam are parallelly emitted.
[0044] The electric field of the heterodyne interference signal of the first light beam and the second light beam generated by the second beam splitter 105 is represented as , wherein the superscript The local intensity of the heterodyne interference signal, representing a vector, is expressed as: The heterodyne interference signal, also known as the interference optical signal, is then transmitted to the precision measurement and control module.
[0045] The following describes a specific implementation method for the design of the precision measurement and control module.
[0046] The interference light signal emitted after passing through the second beam splitter 105 is transmitted to the four-quadrant detector 106 in the precision measurement and control module. The signal output by the four-quadrant detector 106 is called the heterodyne interference beat frequency signal. In one embodiment, the structural schematic diagram of the four-quadrant detector 106 is shown in Figure 2. Its four output ports are connected to the four input ports of the phase-locked loop (PLL) meter via connecting lines. The output port of the PLL meter 107 is connected to the controller 109 via the data processing module 108. Any quadrant of the four-quadrant detector 106... Heterodyne interference beat frequency signal optical power Described as:
[0047] (3)
[0048] in, and These represent the optical power of the measurement beam and the reference beam, respectively. and These represent the beam waist radii of the measurement beam and the reference beam, respectively. and These represent the frequency difference and phase difference between the measurement beam and the reference beam, respectively. This indicates the distance from any point on the measurement beam to the center of the beam spot. This represents the distance from any point on the reference beam to the center of the beam spot. It is the x-coordinate of the measured light spot. It is the ordinate of the measured light spot. It is the x-coordinate of the reference light spot. y is the ordinate of the reference light spot, and t represents time. This indicates any quadrant in the four-quadrant detector. The photosensitive surface area, I, II, III, IV represent the four quadrants of the four-quadrant detector.
[0049] The photodiodes of the four-quadrant detector convert the optical signals from the four quadrants into four current signals under reverse bias conditions, and these signals are then passed through the corresponding load resistors. Any quadrant Voltage of heterodyne interference beat frequency signal Described as:
[0050] (4)
[0051] wherein, represents photoelectric conversion efficiency, represents quantum efficiency of the photodiode, is the electronic charge amount, is the Planck constant, is the frequency of the heterodyne interference optical signal, i.e. the fixed frequency difference of the two light signals before interference;
[0052] According to the addition theorem of trigonometric functions , the heterodyne interference beat signal voltage is expressed as:
[0053] (5)
[0054] wherein, represents a proportional coefficient, described as:
[0055] (6)
[0056] The signal output by the four-quadrant detector is transmitted to a phase-locked calculator. The phase-locked calculator comprises a frequency meter, a voltage-controlled oscillator (VCO) and a digital phase-locked loop. The phase-locked calculator obtains the initial frequency of the to-be-detected signal (the to-be-detected signal refers to the heterodyne interference beat signal output by the four-quadrant detector 106) through the frequency meter , generates the in-phase signal and the anti-phase signal of the to-be-detected signal by the voltage-controlled oscillator (VCO), and further transmits the to-be-detected signal to the digital phase-locked loop. The outputs of the voltage-controlled oscillator are respectively multiplied with the output signals in the digital phase-locked loop, filtered after multiplication inside the phase-locked calculator, and the in-phase output end voltage is , wherein represents the four-way output of the four-quadrant detector, represents the amplitude value of the interference signal in a certain quadrant, represents the phase value of the interference signal in a certain quadrant. The anti-phase output end voltage is . According to the in-phase and anti-phase output end voltage values inside the phase-locked loop 107, the arctangent is performed to obtain the four-quadrant phase , and the linear combination is performed on the four-quadrant phase signals inside the data processing module 108 to obtain the differential phase signal. According to the DWS technology, the laser beam angle measurement value is obtained as , i.e. the laser beam angle measurement value and the differential phase are in a certain proportional coefficient, wherein is the photosensitive surface radius of the four-quadrant detector, represents the wavelength of the laser beam.
[0057] Before operating the above device, the optical path is calibrated using a precision measurement and control module. By setting the differential power sensor (DPS) readout to zero, the incident points of both the incident and reflected beams are located at the origin of the four-quadrant detector, achieving an aligned propagation mode. By setting the differential phase readout to zero, a tilt-free propagation mode is achieved. Furthermore, by suppressing parasitic tilt errors, differential wavefront sensing technology can be extended to measure the laser beam angle when two beams are misaligned, as well as to measure the lateral offset of the beam spot in tilt mode. The following description, with reference to Figures 4(a), 4(b), and 5(a), 5(b), illustrates how the relative angle of the laser beams and parasitic tilt errors are simulated before operating the above device.
[0058] Figure 4(a) shows the response curve of the DWS differential phase and the angle between the laser beams; Figure 4(b) shows the relationship between the DWS phase error and the angle before and after lateral offset compensation. As shown in Figure 4(a), the dashed line represents the relationship between the differential phase and the angle between the laser beams when the center point of the measured beam is offset laterally by xim = 0.1 mm from the center point of the reference beam; the star-shaped line represents the relationship between the differential phase and the angle between the laser beams after post-processing subtraction to compensate for parasitic tilt error. Specifically, by making the differential phase readout zero, a tilt-free propagation model is constructed as follows: Figure 3 The first figure shows this. Based on this, the position of the fast-reflecting mirror in the branch containing the measurement beam is adjusted, changing the relative distance between the fast-reflecting mirror and the first linear polarizer, thereby modulating the lateral offset of the measurement beam on the four-quadrant detector. Since the lateral offset is coupled with the radius of curvature of the Gaussian beam as... Figure 3 The second figure shows a parasitic tilt error, which, according to differential wavefront sensing (DWS) technology, is represented by the differential phase of the yaw angle in the left quadrant of the four-quadrant detector (…). Figure 2 The total voltage in quadrants II and III and the right quadrant (shown) Figure 2 The phase difference modulation of the total voltage in quadrants I and IV (shown) is represented by the modulation coefficient, which is the coupling coefficient between the lateral shift of the light spot and the differential phase under the no-tilt propagation model. Therefore, in the no-tilt propagation mode, the influence of the lateral shift of the light spot on the differential phase can be studied separately as follows: Figure 3 The second figure shows the calibration of parasitic tilt error.
[0059] Figure 5(a) shows the response curve of DWS differential phase to beam lateral shift; Figure 5(b) shows the relationship between DWS phase error and lateral shift before and after angle compensation. As shown in Figure 5(a), the dashed line represents the relationship between differential phase and spot lateral shift when the measurement beam is tilted by a = 0.1 mrad in the horizontal direction compared with the reference beam; the star line represents the relationship between differential phase and spot lateral shift after compensating for the phase error caused by tilting using post-processing subtraction. Specifically, for the four voltage signals output by the four-quadrant detector 106, the differential power sensing (DPS) technology is used to make the differential power reading zero, and the alignment propagation model is constructed as Figure 3 As shown in the first graph, the measurement spot and the reference spot of the four-quadrant detector 106 are both located at the origin of the four-quadrant detector. On this basis, according to the differential wavefront sensing (DWS) technology, the coupling relationship between the relative angle of the laser beams in the horizontal direction and the differential phase of the four-quadrant detector is ; therefore, in the alignment propagation mode, the effect of spot lateral shift on the differential phase is studied separately as Figure 3 As shown in the third graph, the calibration of displacement error is realized.
[0060] Finally, as shown in Figure 3 the fourth and fifth graphs, the effects of beam tilting and lateral shift on the differential phase are studied simultaneously, and as shown in Figures 4 and 5, the response curves with xim= 0.1 mm and a = 0.1 mrad show that in the output effect on the differential phase, the beam tilting of mrad level and the lateral shift of mm level show a superposition relationship, and in the data processing module 108, the laser beam angle measurement based on the DWS technology when the two beams are misaligned, and the spot lateral shift measurement in the tilting propagation mode are realized through post-processing subtraction.
[0061] When the two beams are misaligned, the yaw angle and the pitch angle of the measurement beam compared with the reference beam are described as:
[0062] (7)
[0063] (8)
[0064] wherein, represents the laser beam angle caused by the horizontal swing of the piezoelectric fast mirror, i.e. the real yaw angle, is the yaw angle information calculated from the differential phase, i.e. the measured yaw angle; represents the false tilt caused by the horizontal lateral shift, which is the disturbance value of the yaw angle, and is called the parasitic tilt error of the yaw angle; is the differential phase corresponding to the measured yaw angle; represents the coupling coefficient of the horizontal transverse offset and the differential phase, whose value is calibrated in the alignment mode in advance; represents the distance of the horizontal transverse offset; similarly, represents the laser beam included angle caused by the vertical oscillation of the piezoelectric fast mirror, i.e. the real pitch angle, is the pitch angle information calculated from the differential phase, i.e. the measured pitch angle; represents the false tilt caused by the vertical transverse offset, which is the disturbance value of the pitch angle, called the parasitic tilt error of the pitch angle; is the differential phase corresponding to the measured pitch angle; represents the coupling coefficient of the vertical transverse offset and the differential phase, whose value is calibrated in the alignment mode in advance; represents the distance of the vertical transverse offset; is the photosensitive surface radius of the four-quadrant detector, represents the wavelength of the laser beam; as shown in FIG. 4(a) and FIG. 4(b), taking the horizontal misalignment propagation mode as an example, i.e. the distance of the horizontal transverse offset , the measured value of the yaw angle , the parasitic tilt error is subtracted from the posteriori , the parasitic tilt error in the differential phase is suppressed from 14.6 mrad to 0.3 mrad, and the tilt measurement of the DWS technology in the beam misalignment is realized.
[0065] Similarly, when the beam is tilted, the transverse offset of the measured beam center point on the detection surface compared with the reference beam center point in the horizontal direction and the vertical direction is described as:
[0066] (9)
[0067] (10)
[0068] wherein, and respectively represent the inverse of the coupling coefficient of the transverse offset and the differential phase in the horizontal direction and the vertical direction, and respectively represent the differential phase change caused by the oscillation of the piezoelectric fast mirror in the horizontal direction and the vertical direction. As shown in FIG. 5(a) and FIG. 5(b), taking the tilt propagation mode as an example, i.e. the real yaw angle , the differential phase change caused by the oscillation of the piezoelectric fast mirror in the horizontal direction is subtracted from the posteriori , the displacement error in the differential phase is suppressed from 0.27 rad to 3.4 mrad, and therefore, the DWS technology can also be extended to the measurement of the transverse offset in the beam tilt propagation mode.
[0069] While the foregoing detailed description of the application has been described with particularity, it should be readily apparent to those having ordinary skill in this art that various changes can be made without deviating from the spirit and scope of the application, which is defined by the appended claims.
Claims
1. An apparatus for suppressing spurious tilt errors in differential phase, characterized by, The device comprises a dual-frequency laser light source module, an optical path interference module, and a precision measurement and control module, wherein, The dual-frequency laser light source module comprises a 1064nm laser (101), an isolator (102), a first light splitting prism (103), a first acousto-optic modulator (1001), a second acousto-optic modulator (2001), a first wedge-shaped piece (1002), and a second wedge-shaped piece (2002); The optical path interference module comprises a first light barrier (1003), a second light barrier (2003), a first linear polarizer (1004), a second linear polarizer (2004), a piezoelectric fast mirror (110), a reflecting prism (104), and a second light splitting prism (105); The precision measurement and control module comprises a four-quadrant detector (106), a lock-in amplifier (107), a data processing module (108), and a controller (109); The 1064nm laser (101) generates a stable light signal, which is split into two branches by the first light splitting prism (103) after passing through the isolator (102). The light signal of the first branch is first passed through the first acousto-optic modulator (1001) to generate Bragg diffraction effect for frequency modulation. Then, it passes through the first wedge-shaped piece (1002) to correct the laser tilt caused by the frequency shift of the first acousto-optic modulator (1001). It continues to pass through the first light barrier (1003), where the first-order Bragg diffraction light is selected, and other orders of Bragg diffraction light and stray light are isolated. Then, it passes through the first linear polarizer (1004) with a specific polarization direction to the reflecting prism (104) as one input of the interference light signal. The light signal of the second branch is sequentially passed through the second acousto-optic modulator (2002), the second wedge-shaped piece (2002), the second light barrier (2003), the second linear polarizer (2004), and the piezoelectric fast mirror (110) as the other input of the interference light signal. The output light signals of the two branches are first interfered by the second light splitting prism (105) and then transmitted to the four-quadrant detector (106). The four-quadrant detector (106) outputs four beat frequency electrical signals. The four-quadrant detector (106) is connected to the lock-in amplifier (107) through a radio frequency connection line. The lock-in amplifier (107) outputs four phase signals. The data processing module (108) calculates the differential phase value. The controller (109) controls the deflection or translation of the piezoelectric fast mirror according to the differential phase output by the data processing module (108). The data processing module (108) is used for post-processing subtraction to realize laser beam angle measurement when two light beams are misaligned based on DWS technology and spot lateral shift measurement in tilted propagation mode.
2. The device for suppressing parasitic tilt error in differential phase according to claim 1, wherein In the dual-frequency laser source module, the first acousto-optic modulator (1001) is a frequency-adjustable acousto-optic modulator, and the modulation frequency is set as frequency hopping , wherein is the modulation frequency of the first acousto-optic modulator, is a fixed frequency difference of the first acousto-optic modulator and the second acousto-optic modulator, is the modulation frequency of the second acousto-optic modulator, recorded as the dot frequency, the first branch in which the first acousto-optic modulator (1001) is located is a branch through which the measurement light beam passes; the second acousto-optic modulator (2001) is a fixed-frequency acousto-optic modulator, and the modulation frequency is set as , the second branch in which the second acousto-optic modulator (2001) is located is a branch through which the reference light beam passes.
3. The device for suppressing parasitic tilt error in differential phase according to claim 2, wherein The setting in the optical path interference module includes: the first output signal passing through the first light barrier (1003) passes through the first linear polarizer (1004) in the branch, is incident from the inclined plane of the reflecting prism (104) at an incident angle of 45°, the second output signal passing through the second light barrier (2003) passes through the second linear polarizer (2004) in the branch respectively, is incident from the inclined plane of the piezoelectric fast mirror (110) at an incident angle of 45°, then the first output signal and the second output signal are transmitted to the second light splitting prism (105), and the first light beam and the second light beam are converted by the second light splitting prism (105) and are parallelly emitted.
4. The apparatus for suppressing parasitic tilt error in differential phase according to claim 2, wherein, Four output ports of the four-quadrant detector (106) are connected to four input ports of the phase detector (107) through connecting lines, an output port of the phase detector (107) is connected to the controller (109) through the data processing module (108), the phase detector (107) comprises a frequency meter, a voltage controlled oscillator and a digital phase-locked loop, the phase detector (107) obtains the initial frequency of the to-be-measured signal through the frequency meter, generates the in-phase signal and the anti-phase signal of the to-be-measured signal through the voltage controlled oscillator, and further transmits the to-be-measured signal to the digital phase-locked loop, the output of the voltage controlled oscillator is multiplied with the output signal in the digital phase-locked loop, filtering is performed inside the phase detector (107), the in-phase output port voltage and the anti-phase output port voltage are obtained, and finally the arctangent is performed inside the phase detector (107) to obtain the phase error signal, the differential phase signal is obtained through the data processing module (108) and is transmitted to the controller (109).
5. The apparatus for suppressing parasitic tilt error in differential phase according to claim 1, wherein, The process of performing data processing through the data processing module includes: When the two light beams are misaligned, the yaw angle and the pitch angle of the measurement light beam relative to the reference light beam are described as: （7） （8） wherein, represents the laser beam included angle caused by the horizontal direction swing of the piezoelectric fast mirror, i.e. the real yaw angle, is the yaw angle information solved by the differential phase, i.e. the measured yaw angle; represents the false tilt caused by the horizontal direction lateral displacement, which is the disturbance value of the yaw angle, and is called the parasitic tilt error of the yaw angle; is the differential phase corresponding to the measured yaw angle; represents the coupling coefficient of the horizontal direction lateral displacement and the differential phase, the value of which is calibrated in advance in the alignment mode; represents the distance of the horizontal direction lateral displacement; similarly, represents the laser beam included angle caused by the vertical direction swing of the piezoelectric fast mirror, i.e. the real pitch angle, is the pitch angle information solved by the differential phase, i.e. the measured pitch angle; represents the false tilt caused by the vertical direction lateral displacement, which is the disturbance value of the pitch angle, and is called the parasitic tilt error of the pitch angle; is the differential phase corresponding to the measured pitch angle; represents the coupling coefficient of the vertical direction lateral displacement and the differential phase, the value of which is calibrated in advance in the alignment mode; represents the distance of the vertical direction lateral displacement; is the photosensitive surface radius of the four-quadrant detector, represents the wavelength of the laser beam; According to the formulas (7) and (8), the differential phase change caused by the transverse offset is subtracted from the differential phase corresponding to the yaw angle measurement value through post-processing subtraction, so that the laser beam included angle measurement when the two light beams are misaligned based on the DWS technology is realized; When the light beams are tilted, the transverse offset of the center point of the measurement light beam on the detection surface relative to the center point of the reference light beam in the horizontal direction and the vertical direction is described as: （9） （10） wherein and denote the inverse of the coupling coefficients of the lateral offset and the differential phase in the horizontal and vertical direction, respectively; According to the formulas (9) and (10), the differential phase change caused by the tilt of the piezoelectric fast mirror, that is, the disturbance value of the displacement, is subtracted from the differential phase corresponding to the yaw angle measurement value through post-processing subtraction, so that the transverse offset measurement of the light spot in the tilt propagation mode is realized.
6. A method of suppressing spurious tilt errors in differential phase, characterized by, The method is performed by the apparatus for suppressing parasitic tilt error in differential phase according to any one of claims 1-5, and the method comprises: 1064nm laser (101) generates a stable optical signal through the isolator (102), after the first light splitting prism (103) is divided into two branches, the first branch of optical signal first through the first acousto-optic modulator (1001), the Bragg diffraction effect occurs on the laser frequency modulation; Then, through the first wedge (1002) to correct the laser tilt caused by the first acousto-optic modulator (1001) frequency shift; Continue through the first light barrier (1003), the acousto-optic modulation of the first Bragg diffraction light is selected at the first light barrier (1003), and other levels of Bragg diffraction light and stray light are isolated; Then through the first linear polarizer (1004) to transmit to the reflecting prism (104) with a specific polarization direction to become one of the input of the interference optical signal, in the same way, the second branch of the optical signal passes through the second acousto-optic modulator (2002), the second wedge (2002), the second light barrier (2003), the second linear polarizer (2004) and the piezoelectric fast mirror (110) in turn, and becomes another input of the interference optical signal. After that, the output of the two branches of the optical signal first passes through the second light splitting prism (105) to interfere, and then transmits to the four-quadrant detector (106). The four-quadrant detector (106) outputs four frequency mixing electrical signals, the phase-locked meter (107) outputs four phase signals, the data processing module (108) calculates the differential phase value, and the controller (109) is used to control the deflection or translation of the piezoelectric fast mirror according to the differential phase output by the data processing module (108). The data processing module performs data processing: through post-processing subtraction, the laser beam angle measurement when the two beams are misaligned based on the DWS technology and the spot lateral shift measurement in the tilted propagation mode are realized.
7. The method of claim 6, wherein, The process of performing data processing by the data processing module includes: When the two beams are misaligned, the yaw angle and the pitch angle of the measurement beam compared with the reference beam are described as: （7） （8） wherein, represents the laser beam angle caused by the horizontal direction swing of the piezoelectric fast mirror, i.e. the real yaw angle, is the yaw angle information calculated from the differential phase, i.e. the measured yaw angle; represents the false tilt caused by the horizontal direction lateral displacement, which is the disturbance value of the yaw angle, and is called the parasitic tilt error of the yaw angle; is the differential phase corresponding to the measured yaw angle; represents the coupling coefficient of the horizontal direction lateral displacement and the differential phase, the value of which is calibrated in advance in the alignment mode; represents the distance of the horizontal direction lateral displacement; similarly, represents the laser beam angle caused by the vertical direction swing of the piezoelectric fast mirror, i.e. the real pitch angle, is the pitch angle information calculated from the differential phase, i.e. the measured pitch angle; represents the false tilt caused by the vertical direction lateral displacement, which is the disturbance value of the pitch angle, and is called the parasitic tilt error of the pitch angle; is the differential phase corresponding to the measured pitch angle; represents the coupling coefficient of the vertical direction lateral displacement and the differential phase, the value of which is calibrated in advance in the alignment mode; represents the distance of the vertical direction lateral displacement; is the photosensitive surface radius of the four-quadrant detector, represents the wavelength of the laser beam; According to formulas (7) and (8), through post-processing subtraction, the differential phase change caused by the lateral shift is subtracted from the differential phase corresponding to the yaw angle measurement value, and the laser beam angle measurement when the two beams are misaligned based on the DWS technology is realized; When the beam is tilted, the lateral shift of the center point of the measurement beam on the detection surface compared with the center point of the reference beam in the horizontal direction and the vertical direction is described as: （9） （10） wherein and denote the inverse of the coupling coefficients of the lateral offset and the differential phase in the horizontal and vertical direction, respectively; According to formulas (9) and (10), through post-processing subtraction, the differential phase change caused by the tilt of the piezoelectric fast mirror, i.e. the disturbance value of the displacement, is subtracted from the differential phase corresponding to the yaw angle measurement value, and the spot lateral shift measurement in the tilted propagation mode is realized.