High-precision wave plate detection method and system

Abstract

The application discloses a high-precision wave plate detection method and system, and the high-precision wave plate detection method is realized based on a polarized light measurement system and comprises the following steps: a rough measurement step, a wave plate to be detected is placed on a sample table at an arbitrary azimuth angle, a Mueller matrix is acquired through the polarized light measurement system, and an initial azimuth angle value and an initial phase retardation difference value are calculated; a polarized device rotation step, the polarizer and the analyzer are synchronously rotated to the initial azimuth angle according to the initial azimuth angle value; and a fine measurement step, the Mueller matrix is re-acquired at the rotated azimuth angle, a measured azimuth angle value and a measured phase retardation difference value are calculated, and the measured azimuth angle value and the measured phase retardation difference value are output. The two-step measurement strategy of rough measurement-rotation-fine measurement is adopted, the polarizer and the analyzer are aligned with the optical axis of the wave plate, the influence of the wave plate placement azimuth angle on the measurement precision is reduced, and high-precision measurement of the wave plate placed at an arbitrary azimuth angle is realized.

Description

Technical Field

[0001] This application relates to the fields of waveplate detection technology and elliptic measurement technology, specifically to a high-precision waveplate detection method and system. Background Technology

[0002] A waveplate is an important optical element made of birefringent crystal material, which can introduce a specific phase delay between two orthogonal polarization directions. Key parameters of a waveplate include azimuth angle, phase delay difference, and transmittance. High-precision measurement of waveplate parameters is of great significance in fields such as optical precision measurement, laser technology, and optical communication.

[0003] Currently, waveplate measurement systems based on the Mueller matrix principle are one of the mainstream waveplate detection methods. These systems retrieve various parameters of the waveplate by measuring its Mueller matrix. However, since waveplates are anisotropic materials, their optical properties depend on the relative angles between the light propagation direction and polarization direction and the crystal's optical axis. When there is an angle between the waveplate's optical axis and the reference direction of the detection system, the measurement results will deviate with changes in angle.

[0004] In existing waveplate testing instruments, the measurement accuracy of the waveplate is affected by the azimuth angle of the waveplate being tested. Current calibration methods mainly consist of two parts: one is calibrating the installation angle deviations of optical components such as polarizers, analyzers, and compensators; the other is calibrating the overall system error using standard materials. However, calibration results using standard materials cannot fully calibrate all types of waveplates and all measurement states of the waveplates. Standard material calibration is only more accurate at a specific angle; at other angles, it introduces significant errors.

[0005] For example, for a zero-order waveplate, when the waveplate (41) under test is freely placed in the range of 0° to 360°, the measured value of its phase delay difference can vary by ±1°, which seriously limits the accuracy and consistency of the measurement results. Summary of the Invention

[0006] The purpose of this invention is to provide a high-precision waveplate detection method and system to solve the problem that the measurement accuracy of waveplates is affected by the placement azimuth angle in the prior art, and to achieve high-precision measurement of waveplates under any azimuth angle.

[0007] The high-precision waveplate detection method and system provided in this application adopts the following technical solution:

[0008] A high-precision waveplate detection method is provided, the method being implemented based on a polarization measurement system. The polarization measurement system comprises, sequentially along the optical path, a light source, a polarizer, a sample stage for placing the waveplate under test, an analyzer, and a detection device. The polarizer and the analyzer are respectively mounted on an adjustable azimuth rotation drive mechanism. The method includes the following steps:

[0009] Coarse measurement steps: Place the waveplate to be tested on the sample stage at an arbitrary azimuth angle, obtain the Mueller matrix of the waveplate to be tested through the polarization measurement system, and calculate the initial value of the azimuth angle of the waveplate to be tested based on the Mueller matrix. Initial value of phase delay difference ;

[0010] Polarizing device rotation steps: Based on the initial azimuth angle value The rotary drive mechanism synchronously rotates the azimuth angles of the polarizer and the analyzer to their initial values. ;

[0011] Precision measurement steps: Under the azimuth angles of the rotated polarizer and analyzer, the Mueller matrix of the waveplate under test is re-obtained through the polarization measurement system, and the azimuth angle measurement value of the waveplate under test is calculated based on the Mueller matrix. and phase delay difference measurement This is output as the final measurement result.

[0012] In some embodiments, the polarization measurement system is a dual-rotation compensator ellipsometric measurement system, which sequentially includes a light source, a polarizer, a first compensator, a sample stage, a second compensator, an analyzer, and a spectrometer as the detection device along the optical path; wherein the first compensator and the second compensator operate at different fixed rotational speeds. , Continuous rotation, and and The ratio is a ratio of coprime integers.

[0013] In some embodiments, a method for obtaining the Mueller matrix of the waveplate under test includes:

[0014] During the continuous rotation of the first compensator and the second compensator, the spectrometer collects multiple sets of emitted light intensity data at equal time intervals within a complete rotation cycle, and the number of data points collected is no less than the number of unknowns to be determined in the Mueller matrix.

[0015] Fourier analysis was performed on the emitted light intensity data to extract the DC component and the cosine and sine coefficients of each harmonic frequency component determined by the rotational speed ratio. The harmonic frequencies are... , Integer multiples of and their sum frequency and difference frequency;

[0016] A system of linear equations is established between the coefficients of each frequency component and the 16 elements of the Mueller matrix of the waveplate under test. The system of linear equations is solved by the least squares method, and the Mueller matrix of the waveplate under test is obtained by inversion calculation.

[0017] In some embodiments, an instrument polarization device azimuth angle calibration step is included before the coarse measurement step, the instrument polarization device azimuth angle calibration step including:

[0018] Using a standard polarizer fixture with a known polarization direction, the installation angle deviation of the polarizer, the analyzer, the first compensator and the second compensator are calibrated respectively.

[0019] The installation angle deviation of each polarization device is determined by the extinction method: rotate the polarization device to be calibrated, record the relationship curve between the rotation angle and the light intensity of the detection device, fit the curve according to Malus's law to obtain the rotation angle corresponding to the lowest light intensity, and calculate the installation angle deviation based on the rotation angle corresponding to the lowest light intensity and the known azimuth angle of the standard polarizer fixture.

[0020] In some embodiments, the installation angle deviation of the polarizer The calibration method is as follows: Construct a transmission optical path consisting of a light source, polarizer, standard polarizer fixture, and spectrometer. Let the azimuth angle of the standard polarizer fixture be... Rotate the polarizer to obtain the rotation angle corresponding to the lowest light intensity. ; Calculate the installation angle deviation of the polarizer: =b-(a+90°);

[0021] Installation angle deviation of the first compensator The calibration method is as follows: Construct the transmission optical path of the light source—polarizer—first compensator—standard polarizer fixture—spectrometer; rotate the polarizer to the calibrated angle. Rotate the first compensator and fit the rotation angle corresponding to the lowest light intensity. ; Calculate the installation angle deviation of the first compensator: =d-(a+90°).

[0022] In some embodiments, an instrument system error calibration step is included before the coarse measurement step:

[0023] To obtain the measured Mueller matrix, a standard transmission sample with a known Mueller matrix is ​​used. ;

[0024] According to the standard Mueller matrix of the standard transmission sample and the measured Mueller matrix Calculate the system error calibration matrix ;

[0025] In the subsequent coarse and fine measurement steps, the measured Mueller matrix is ​​corrected using the systematic error calibration matrix dM. The corrected Mueller matrix... .

[0026] In some embodiments, the method for calculating the azimuth angle and phase delay difference of the waveplate from the Mueller matrix in the coarse measurement step and the fine measurement step includes:

[0027] Based on the elements of the Mueller matrix Calculate the phase delay difference : Its value ranges from 0° to 180°;

[0028] Based on the elements of the Mueller matrix and Calculate the azimuth angle using the ratio : and according to the elements , , , Determining the sign of the sign The quadrant in which it is located makes The value can be unique;

[0029] In the precision measurement step, the polarizer and analyzer have been rotated to an orientation aligned with the optical axis of the waveplate under test. The waveplate under test is equivalent to being placed at zero azimuth in the system coordinate system. At this time, the Mueller matrix of the pure phase retarder satisfies , , Based on this, respectively , Calculated , , and according to The combined estimated value was calculated. Then, weight the values ​​according to the reciprocal of the variance of each calculated value. , , Take the weighted average value as the phase delay difference measurement. Cross-validation is performed; the error variance of each calculated value is obtained by taking the sample variance of each calculated value after repeating the measurement of the same waveplate a preset number of times, or by calculating the measurement noise variance of each element of the Mueller matrix using the error propagation formula of the corresponding arcsine function.

[0030] In some embodiments, the final measurement result corresponds to a specified wavelength within the spectral range of the light source, and the phase delay difference measurement value The phase delay difference at the specified wavelength; the spectrometer has a spectral range of 380nm~100nm, a spectral resolution better than 1nm, and can simultaneously measure light intensity signals at multiple wavelengths to achieve broadband ellipsometric measurement.

[0031] In some embodiments, during the polarization device rotation step, the initial azimuth angle value obtained in the coarse measurement step is used. Compared with the azimuth measurement value obtained from the precision measurement step The absolute value of the difference represents the initial value of the azimuth angle. The error is such that when the error is no greater than ±5°, the precision measurement step is in the state of making the Mueller matrix elements... , The optimal measurement state with the highest measurement accuracy; the coarse measurement step, the polarization device rotation step, and the fine measurement step are automatically controlled and executed by the control software in the control and processing module.

[0032] A high-precision waveplate inspection system, comprising:

[0033] The polarization measurement module, along the optical path, sequentially includes a light source, a polarizer, a first compensator, a sample stage, a second compensator, an analyzer, and a spectrometer. The polarizer and analyzer are each driven by their respective rotary drive mechanisms to achieve precise azimuth angle adjustment. The first and second compensators are each driven by their respective rotary drive mechanisms at different fixed rotation speeds. , Continuous rotation, and and The ratio is a ratio of coprime integers;

[0034] A control and processing module is connected to the polarizer rotation drive mechanism, the first compensator rotation drive mechanism, the second compensator rotation drive mechanism, the analyzer rotation drive mechanism, and the spectrometer, respectively, and is used to control the movement of each of the rotation drive mechanisms and collect data from the spectrometer, thereby executing the high-precision waveplate detection method according to any one of claims 1 to 9;

[0035] The angular resolution of each rotary drive mechanism is better than 0.01°; the system also includes a standard polarizer fixture with a known polarization direction, which is used as an angular reference in the azimuth angle calibration step of the instrument polarization device.

[0036] In summary, this application includes at least one of the following beneficial technical effects:

[0037] 1. A two-step measurement strategy of "coarse measurement - rotation - fine measurement" is adopted. First, the initial value of the azimuth angle of the wave plate under test is obtained through coarse measurement. Then, the polarizer and analyzer are rotated synchronously to the initial value of the azimuth angle, so that the optical axis of the wave plate under test is aligned with the polarization reference direction of the system. This reduces the influence of the azimuth angle of the wave plate placement on the phase delay difference measurement results, and allows the wave plate to be measured under any azimuth angle.

[0038] 2. In the precision measurement step, select the Mueller matrix from the following: , Elements and Combinations Calculate the phase delay difference and take a weighted average based on the reciprocal of the error variance of each calculated value. Multi-element cross-validation is used to reduce the measurement error introduced by single-element calculation.

[0039] 3. The coarse measurement step, the polarization device rotation step, and the fine measurement step are automatically controlled and executed by the control software in the control and processing module. There is no need for manual rotation of the waveplate or sample stage, which reduces the manual intervention and facilitates batch testing. Attached Figure Description

[0040] Figure 1 This is a schematic diagram of the structure of a high-precision waveplate detection system provided in one embodiment of this application;

[0041] Figure 2 This is a schematic flowchart of a high-precision waveplate detection method provided in one embodiment of this application;

[0042] Figure 3 This is a schematic diagram of the optical path for polarizer mounting angle deviation calibration provided in one embodiment of this application;

[0043] Figure 4 This is a schematic diagram of the optical path for calibrating the installation angle deviation of the first compensator according to an embodiment of this application;

[0044] Explanation of reference numerals in the attached figures: 1. Light source; 2. Polarizer; 21. Polarizer rotation drive mechanism; 3. First compensator; 31. First compensator rotation drive mechanism; 4. Sample stage; 41. Waveplate under test; 5. Second compensator; 51. Second compensator rotation drive mechanism; 6. Analyzer; 61. Analyzer rotation drive mechanism; 7. Spectrometer; 8. Control and processing module; 9. Standard polarizer fixture. Detailed Implementation

[0045] The technical solutions in the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. The described embodiments are only possible technical implementations of the present invention, but are not limited thereto. Other embodiments obtained by those skilled in the art in conjunction with the embodiments of the present invention without creative effort are also within the protection scope of the present invention.

[0046] This application mainly adopts a two-step measurement strategy of "coarse measurement-rotation-fine measurement", which achieves the effect of high-precision measurement of waveplates at any azimuth angle. The following is a further detailed description of this application.

[0047] Example 1

[0048] Please refer to Figure 1 The high-precision waveplate detection method provided in this application embodiment is based on a polarization light measurement system. The polarization light measurement system is sequentially arranged along the optical path direction, including a light source 1, a polarizer 2, a sample stage 4 for placing the waveplate 41 under test, an analyzer 6, and a detection device. The polarizer 2 and the analyzer 6 are respectively mounted on adjustable azimuth angle rotation drive mechanisms 21 and 61. The method includes a coarse measurement step, a polarization device rotation step, and a fine measurement step. First, the initial azimuth angle and initial phase delay difference of the waveplate 41 under test are obtained through the coarse measurement step. Then, the azimuth angles of the polarizer 2 and the analyzer 6 are rotated synchronously according to the initial azimuth angle values. Finally, a fine measurement is performed at the rotated azimuth angle to obtain the final measurement result. This achieves the effect of eliminating the influence of the waveplate placement azimuth angle on the measurement accuracy and realizing high-precision measurement. This is because rotating the polarizer 2 and the analyzer 6 to align with the optical axis of the waveplate puts the system in the optimal measurement state, reducing measurement errors.

[0049] In a preferred embodiment, the polarization measurement system is a dual-rotation compensator ellipsometric measurement system, which sequentially includes a light source 1, a polarizer 2, a first compensator 3, a sample stage 4, a second compensator 5, an analyzer 6, and a spectrometer 7 serving as the detection device along the optical path; wherein the first compensator 3 and the second compensator 5 operate at different fixed rotational speeds. , Continuous rotation, and and The ratio is a coprime integer ratio. This setting generates a sufficient number of independent harmonic frequency components during the continuous rotation of the two compensators, so that the detected light intensity signal carries complete Mueller matrix information, thereby ensuring that all 16 Mueller matrix elements of the waveplate 41 under test can be solved independently and completely, improving the accuracy of Mueller matrix inversion.

[0050] Specifically, the polarization measurement system includes, in sequence along the optical path, a light source 1, a polarizer 2, a sample stage 4 for placing the waveplate 41 under test, an analyzer 6, and a detection device. The light source 1 can be a broadband halogen lamp, whose emitted light, after passing through a collimating lens group, forms a parallel beam that propagates vertically downwards. The collimating lens group is installed between the light source 1 and the polarizer 2 to ensure that the incident light is a collimated parallel beam, improving measurement accuracy. Alternatively, other types of light sources, such as laser light sources, can be used. The polarizer 2 converts the natural light emitted by the light source 1 into linearly polarized light and can employ a Glan-Thomson prism with an extinction ratio superior to [missing information]. The polarizer achieves precise azimuth adjustment through a polarizer rotation drive mechanism 21. This mechanism can employ a high-precision stepper motor with an angular resolution better than 0.01°; other types of polarizers, such as polarizers, can also be used. The first compensator 3 modulates the polarization state of the linearly polarized light, employing a quartz crystal phase delay plate. It operates at a fixed rotation speed via the first compensator rotation drive mechanism 31. The first compensator rotation drive mechanism 31, which operates continuously, employs a high-precision stepper motor with an angular resolution better than 0.01° and ensures speed stability through closed-loop control. Other types of compensators can also be used. The sample stage 4 has a platform structure with positioning pin holes on its surface for fixing the waveplate 41 under test. The waveplate 41 can be placed on the sample stage 4 at any azimuth angle without angular alignment. The effective aperture of the sample stage 4 is not less than 50mm. The second compensator 5 is used to perform a second polarization modulation on the light after passing through the waveplate 41 under test. It also employs a quartz crystal phase delay waveplate, and the rotation drive mechanism 51 of the second compensator operates at a fixed speed. Continuous rotation, where The second compensator rotation drive mechanism 51 uses a high-precision stepper motor with an angular resolution better than 0.01°; other types of compensators can also be used. The analyzer 6 is used to analyze the polarization state of the emitted light. It can also use a Glan-Thomson prism, and the azimuth angle is precisely adjusted through the analyzer rotation drive mechanism 61. The analyzer rotation drive mechanism 61 uses a high-precision stepper motor with an angular resolution better than 0.01°; other types of analyzers can also be used. The detection device can be a spectrometer 7, using a grating spectrometer. The light emitted from the analyzer 6 is guided into the entrance slit of the spectrometer 7 through an optical fiber, allowing simultaneous measurement of light intensity signals at multiple wavelengths to achieve spectral ellipsometric measurement. The spectrometer 7 has a spectral range of 380nm~1100nm and a spectral resolution better than 1nm; other types of light intensity detection devices can also be used.

[0051] The working process of the above-mentioned polarization measurement system is as follows: Natural light emitted from light source 1 is collimated and then converted into linearly polarized light by polarizer 2. The linearly polarized light sequentially passes through a continuously rotating first compensator 3, the waveplate under test 41, and a continuously rotating second compensator 5. After being continuously modulated, it is emitted through analyzer 6 and finally received by spectrometer 7 and converted into an outgoing light intensity signal that varies with time. Since the first compensator 3 and the second compensator 5 rotate continuously at a rotation speed of coprime integer ratio, the outgoing light intensity signal contains multiple independent harmonic components, carrying the complete polarization information of the waveplate under test 41, thus providing a data basis for the subsequent inversion of the Mueller matrix.

[0052] For further details, please refer to Figure 2 , Figure 3 and Figure 4Before the coarse measurement step, there are also instrument polarization device azimuth angle calibration steps and instrument system error calibration steps to eliminate the influence of installation angle deviation of each polarization device and system comprehensive error on the measurement results.

[0053] The azimuth angle calibration steps for the instrument's polarization devices are as follows: using a standard polarizer fixture 9 with a known polarization direction, the installation angle deviations of the polarizer 2, analyzer 6, first compensator 3, and second compensator 5 are calibrated respectively. The installation angle deviation of each polarization device is determined using the extinction method: rotating the polarization device to be calibrated, recording the relationship curve between the rotation angle and the light intensity of the detection device, fitting the curve according to Malus's law to obtain the rotation angle corresponding to the lowest light intensity, and calculating the installation angle deviation based on the rotation angle corresponding to the lowest light intensity and the known azimuth angle of the standard polarizer fixture 9.

[0054] For details, please refer to Figure 3 The installation angle deviation of the polarizer 2 The calibration method is as follows: Construct a transmission optical path from light source 1—polarizer 2—standard polarizer fixture 9—spectrometer 7, and set the azimuth angle of the standard polarizer fixture 9 as... Rotate polarizer 2. When the polarization direction of polarizer 2 is orthogonal to the standard polarizer fixture 9, the light intensity received by spectrometer 7 is at its lowest. The rotation angle corresponding to this lowest light intensity is obtained by fitting the data. Based on this, the installation angle deviation of the polarizer 2 is calculated: =b-(a+90°). Please refer to [the original text]. Figure 4 The installation angle deviation of the first compensator 3 The calibration method is as follows: Assemble the transmission optical path from light source 1—polarizer 2—first compensator 3—standard polarizer fixture 9—spectrometer 7; rotate polarizer 2 to the calibrated angle. Rotate the first compensator 3 until its fast axis is parallel or perpendicular to the polarization direction, at which point the light intensity is minimum. Fit the rotation angle corresponding to this minimum light intensity. Based on this, the installation angle deviation of the first compensator 3 is calculated: =d-(a+90°). The calibration method for the installation angle deviation of the analyzer 6 and the second compensator 5 is the same as the method described above. This calibration step uses a standard polarizer fixture 9 with a precisely known polarization direction as an angle reference to calibrate the actual installation angle of each polarization device one by one, eliminating the angle system error caused by mechanical assembly and improving the reference accuracy of azimuth angle measurement.

[0055] The instrument system error calibration steps are as follows: Measure a standard transmission sample with a known Mueller matrix to obtain the measured Mueller matrix. According to the standard Mueller matrix of the standard transmission sample. and the measured Mueller matrix Calculate the system error calibration matrix In the subsequent coarse and fine measurement steps, the measured Mueller matrix is ​​corrected using the systematic error calibration matrix dM. The corrected Mueller matrix... By correcting each measurement result using this calibration matrix, the overall system error, such as residual deviations of optical components and uneven detection response, can be compensated, further improving the accuracy of the measurement.

[0056] Specifically, the coarse measurement steps include placing the waveplate 41 to be tested 4 at an arbitrary azimuth angle on the sample stage 4, obtaining the Mueller matrix of the waveplate 41 to be tested 41 through a polarization light measurement system, and calculating the initial value of the azimuth angle of the waveplate 41 to be tested based on the Mueller matrix. Initial value of phase delay difference .

[0057] When obtaining the Mueller matrix of the waveplate 41 under test, during the continuous rotation of the first compensator 3 and the second compensator 5, multiple sets of emitted light intensity data at equal time intervals within a complete rotation cycle are collected by the spectrometer 7. The number of data points collected is no less than the number of unknowns in the Mueller matrix. Fourier analysis is performed on the emitted light intensity data to extract the DC component and the cosine and sine coefficients of each harmonic frequency component determined by the rotation speed ratio. The harmonic frequencies are... , The integer multiples of the frequency components and their sum and difference frequencies are used. A system of linear equations is established between the coefficients of each frequency component and the 16 elements of the Mueller matrix of the waveplate 41 under test. The system of linear equations is solved by the least squares method, and the Mueller matrix of the waveplate 41 under test is obtained by inversion calculation. This method expands the light intensity signal modulated by the rotating compensator in the frequency domain, so that each element of the Mueller matrix corresponds one-to-one with different frequency components. Then, the least squares method is used to suppress random noise, thereby stably and accurately inverting the complete Mueller matrix.

[0058] As a supplementary explanation, the polarization measurement system is not limited to the dual rotating compensator structure. Any polarization measurement system capable of measuring the complete Mueller matrix of the waveplate 41 under test (such as a rotating analyzer combined with a fixed compensator, or a rotating polarizer combined with a rotating analyzer, etc.) can be used to implement the coarse measurement step and the fine measurement step. This application does not impose any restrictions on this.

[0059] When calculating the azimuth and phase delay difference of a waveplate from the Mueller matrix, the elements in the Mueller matrix are used... Calculate the phase delay difference : Its value ranges from 0° to 180°; based on the elements of the Mueller matrix and Calculate the azimuth angle using the ratio : and according to the elements , , , Determining the sign of the sign The quadrant in which it is located makes The value is unique. The initial value of the calculated phase delay difference is... With azimuth initial value It is also used to determine whether the preliminary measurement results are abnormal. If the value exceeds the reasonable range of the nominal value of the waveplate under test, it should be reset and the coarse measurement repeated.

[0060] Specifically, the polarization device rotation step includes adjusting the initial azimuth angle value. The azimuth angles of the polarizer 2 and the analyzer 6 are synchronously rotated to their initial values ​​via the rotary drive mechanisms 21 and 61. .

[0061] The rotary drive mechanisms 21 and 61 can be high-precision stepper motors with an angular resolution better than 0.01°, capable of accurately rotating the polarizer 2 and analyzer 6 to a specified azimuth angle; other types of rotary drive devices, such as servo motors, can also be used. When the initial azimuth angle value... Compared with the azimuth measurement value obtained by precision measurement When the absolute value of the difference is no greater than ±5°, the fine measurement step is in the state of making the Mueller matrix elements , The optimal measurement state yields the highest measurement accuracy. The control software uses this angle difference as the convergence criterion: if it is not satisfied, then... After updating the alignment, the rotation and fine measurement are repeated until the criterion is met. By synchronously rotating the polarizer 2 and analyzer 6 to align with the optical axis of the waveplate 41 under test, the polarization reference direction of the system adaptively follows the optical axis of the waveplate, thereby placing the waveplate in the optimal measurement state with the most sensitive phase delay difference and the smallest error, providing the most accurate measurement conditions for the fine measurement step.

[0062] Specifically, the precision measurement steps include re-obtaining the Mueller matrix of the waveplate 41 under test using the polarization measurement system under the azimuth angle of the rotated polarizer 2 and analyzer 6, and calculating the azimuth angle measurement value of the waveplate 41 under test based on the Mueller matrix. and phase delay difference measurement This is output as the final measurement result.

[0063] In the precision measurement step, polarizer 2 and analyzer 6 have been rotated to an orientation aligned with the optical axis of the waveplate 41 under test. The waveplate 41 under test is equivalent to being placed at zero azimuth angle in the system coordinate system, and its Mueller matrix satisfies , , Based on this, the phase delay difference measurement value The calculations were performed using the following methods respectively. , Elements and Combinations Calculations were performed to obtain , , And weighted according to the reciprocal of the error variance of each calculated value , , Take the weighted average value as the phase delay difference measurement. Cross-validation is performed. The error variance is obtained by statistically analyzing the same waveplate 41 after a preset number of repeated measurements, or by calculating the noise variance of each Mueller matrix element using the error propagation formula. The final measurement result corresponds to a specified wavelength within the spectral range of light source 1, and the measured value of the phase delay difference is... This represents the phase delay difference at the specified wavelength. A multi-element weighted average is used, utilizing... , The mutual verification between combined elements reduces random errors caused by noise from individual elements, and by using the inverse of the error variance as a weight, the more accurate calculated values ​​are given greater weight, thereby improving the phase delay difference measurement. Reliability and repeatability.

[0064] For further details, please refer to Figure 2 The coarse measurement step, the polarization device rotation step, and the fine measurement step are all automatically controlled and executed by the control software in the control and processing module 8, without manual intervention. Combining the above steps, the overall working process of the high-precision waveplate detection method in this embodiment is as follows: First, the instrument polarization device azimuth angle calibration step (S1) is performed, calibrating the installation angle deviation of the polarizer 2, analyzer 6, first compensator 3, and second compensator 5 using the extinction method; then, the instrument system error calibration step (S2) is performed, measuring the standard transmission sample and calculating the system error calibration matrix dM; then, the waveplate 41 to be tested is placed on the sample stage 4 at an arbitrary azimuth angle; subsequently, the coarse measurement step (S3) is performed, measuring and... After correcting the Mueller matrix, the initial azimuth angle was calculated. Initial value of phase delay difference Determine the initial value of the azimuth angle. If the error is not greater than ±5°, repeat the coarse measurement. If it is satisfied, proceed to the polarization device rotation step (S4), and rotate the azimuth angles of polarizer 2 and analyzer 6 synchronously to... Finally, proceed to the fine measurement step (S5), where the Mueller matrix is ​​remeasured under optimal measurement conditions, and the azimuth measurement value is calculated. and with , , The phase delay difference measurement is obtained by weighted averaging. The final measurement result is output. The entire process is automatically executed sequentially by the control and processing module 8, achieving automated measurement without human intervention.

[0065] The implementation principle of this embodiment is as follows: through a two-step measurement strategy of "coarse measurement-rotation-precise measurement", the approximate value of the waveplate azimuth angle is first quickly obtained, and then the polarizer 2 and analyzer 6 are rotated to align with the optical axis of the waveplate, so that the system is in the optimal measurement state. Finally, the precise measurement is performed in the optimal state, which effectively eliminates the influence of the waveplate placement azimuth angle on the measurement accuracy and realizes high-precision measurement of the waveplate under any azimuth angle. Compared with the prior art, the fluctuation of the phase delay difference measurement value under any azimuth angle is reduced from ±1° of the existing zero-order waveplate. Moreover, the entire measurement process can be automatically controlled by software, which is simple to operate and suitable for industrial batch testing applications.

[0066] Example 2

[0067] The difference between this embodiment and the above embodiments is that this embodiment adopts a method of specifying the measurement direction. That is, after the coarse measurement is completed, the control software prompts the operator to rotate the waveplate 41 to be measured to a specific angle (for example, to align the optical axis of the waveplate with the initial reference direction of the system), and then perform the measurement again.

[0068] In this method, the control software runs on the industrial control computer in the control and processing module 8 and is operated by the operator through a graphical interface. However, this method requires manual intervention, is relatively cumbersome, and has limited accuracy in manually rotating the angle.

[0069] The implementation principle of this embodiment is as follows: by specifying the measurement direction, the optical axis of the waveplate is aligned with the initial reference direction of the system, which can reduce the influence of the azimuth angle of the waveplate placement on the measurement accuracy to a certain extent. However, since manual intervention is required, the operation is not simple enough, and the accuracy of manual rotation is difficult to guarantee. Therefore, the accuracy and consistency of the measurement are relatively low.

[0070] Example 3

[0071] The difference between this embodiment and the above embodiment is that this embodiment does not rotate the polarizer 2 and the analyzer 6, but achieves a similar effect by automatically rotating the sample stage 4.

[0072] The sample stage 4 needs to be equipped with a high-precision rotating mechanism to ensure that the waveplate 41 under test can be rotated accurately. However, this method requires a high degree of coaxiality of the rotation center of the sample stage 4; otherwise, the beam will deviate from the central region of the waveplate 41 under test after rotation, introducing new measurement errors.

[0073] The implementation principle of this embodiment is as follows: by rotating the sample stage 4, the optical axis of the waveplate is aligned with the system reference direction, achieving an effect similar to that of the rotating polarizer 2 and the analyzer 6. However, due to the high requirements for the rotation mechanism and coaxiality of the sample stage 4, the complexity and cost of the system are increased, and new errors are easily introduced. Therefore, in practical applications, it is not as good as the solution of rotating polarizer 2 and analyzer 6.

[0074] Example 4

[0075] Please refer to Figure 1 The high-precision waveplate detection system provided in this application includes a polarization light measurement module and a control and processing module 8. The polarization light measurement module is arranged with optical elements sequentially along the optical path. The control and processing module 8 is connected to each of the rotating drive mechanisms 21, 31, 51, 61 and the spectrometer 7, respectively. It controls the movement of each rotating drive mechanism and collects data from the spectrometer 7, and executes the high-precision waveplate detection method. This achieves the effect of high-precision measurement of the waveplate. This is because the system can accurately measure the Mueller matrix of the waveplate through reasonable optical element layout and precise control, and thus obtain various parameters of the waveplate.

[0076] Specifically, the polarization measurement module, along the optical path, includes, in sequence, a light source 1, a polarizer 2, a first compensator 3, a sample stage 4, a second compensator 5, an analyzer 6, and a spectrometer 7. The light source 1 can be a broadband halogen lamp, whose emitted light, after passing through a collimating lens group, forms a parallel beam that propagates vertically downwards. The collimating lens group is installed between the light source 1 and the polarizer 2 to ensure that the incident light is a collimated parallel beam, improving measurement accuracy. Other types of light sources, such as laser sources, can also be used. The polarizer 2 converts the natural light emitted by the light source 1 into linearly polarized light and can use a Glan-Thomson prism with an extinction ratio better than... The polarizer achieves precise azimuth adjustment through a polarizer rotation drive mechanism 21. This mechanism can employ a high-precision stepper motor with an angular resolution better than 0.01°; other types of polarizers, such as polarizers, can also be used. The first compensator 3 modulates the polarization state of the polarized light, employing a quartz crystal phase delay plate, and operates at a fixed speed via the first compensator rotation drive mechanism 31. The first compensator rotation drive mechanism 31, which operates continuously, employs a high-precision stepper motor with an angular resolution better than 0.01°. Closed-loop control ensures rotational speed stability. Other types of compensators can also be used. The sample stage 4 has a platform structure with positioning pin holes on its surface for fixing the waveplate 41 under test. The waveplate 41 can be placed on the sample stage 4 at any azimuth angle without angular alignment. The effective aperture of the sample stage 4 is not less than 50mm. The second compensator 5 is used for a second polarization modulation of the light after passing through the waveplate 41. It also employs a quartz crystal phase delay waveplate, and the rotation drive mechanism 51 maintains a fixed rotational speed. Continuous rotation, where The speed ratio of the first compensator 3 and the second compensator 5 The ratio of coprime integers is used to generate a sufficient number of independent frequency components for the complete solution of the Mueller matrix; other types of compensators can also be used. The analyzer 6 is used to analyze the polarization state of the emitted light. It can also use a Glan-Thomson prism, with precise azimuth adjustment achieved through the analyzer rotation drive mechanism 61. The analyzer rotation drive mechanism 61 uses a high-precision stepper motor with an angular resolution better than 0.01°; other types of analyzers can also be used. The spectrometer 7 is used to detect the intensity information of the emitted light. It uses a grating spectrometer, with the light emitted from the analyzer 6 guided through an optical fiber into the entrance slit of the spectrometer 7. It can simultaneously measure the intensity signals at multiple wavelengths, achieving spectral ellipsometric measurement. The spectral range of the spectrometer 7 is 380nm~1100nm, with a spectral resolution better than 1nm; other types of light intensity detection devices can also be used.

[0077] The control and processing module 8 is connected to the polarizer rotary drive mechanism 21, the first compensator rotary drive mechanism 31, the second compensator rotary drive mechanism 51, the analyzer rotary drive mechanism 61, and the spectrometer 7, respectively. It controls the movement of each rotary drive mechanism and collects data from the spectrometer 7 to execute a high-precision waveplate detection method. The control and processing module 8 includes an industrial control computer and corresponding control software. It connects to the spectrometer 7 via a USB or Ethernet interface and to each rotary drive mechanism 21, 31, 51, and 61 via a motion control card to achieve synchronous control and data acquisition. The angular resolution of each rotary drive mechanism 21, 31, 51, and 61 is better than 0.01°, ensuring sufficiently high angular accuracy for the azimuth adjustment of the polarizer 2 and analyzer 6, as well as the continuous rotation of the first compensator 3 and the second compensator 5.

[0078] The system also includes a standard polarizer fixture 9, which has a known polarization direction and is used as an angular reference in the azimuth calibration step of the instrument's polarization device. The standard polarizer fixture 9 uses a high extinction ratio polarizer, whose polarization direction is precisely calibrated to an accuracy better than 0.05°, and is equipped with a dedicated fixing clamp for accurately placing the standard polarizer fixture 9 in the optical path of the sample stage 4.

[0079] The overall working process of this system is as follows: Before measurement, the control and processing module 8 controls each rotation drive mechanism 21, 31, 51, and 61 to work with the standard polarizer fixture 9 to complete the azimuth angle calibration of the instrument's polarization device and the system error calibration; during measurement, the waveplate 41 to be measured is placed on the sample stage 4 at an arbitrary azimuth angle, and the control and processing module 8 drives the first compensator 3 and the second compensator 5 to rotate at different speeds. , The light emitted from light source 1 rotates continuously. After passing through polarizer 2, first compensator 3, waveplate 41, second compensator 5, and analyzer 6, the emitted light intensity data is collected by spectrometer 7. The control and processing module 8 performs Fourier analysis and least squares inversion on the data to obtain the Mueller matrix and completes the coarse measurement. Then, it drives polarizer rotation drive mechanism 21 and analyzer rotation drive mechanism 61 to synchronously rotate polarizer 2 and analyzer 6 to the initial azimuth angle value. Then, under optimal measurement conditions, a fine measurement is performed, and finally, the azimuth angle measurement value of the waveplate 41 under test is output. and phase delay difference measurement .

[0080] The implementation principle of this embodiment is as follows: the system uses a reasonable optical element layout and precise control to generate a sufficient number of independent frequency components using a dual rotating compensator to ensure the complete solution of the Mueller matrix. At the same time, the rotatable design of the polarizer 2 and the analyzer 6 enables the system to adaptively adjust the polarization reference direction according to the coarse measurement results, thereby realizing high-precision measurement of the waveplate at any azimuth angle. Furthermore, the accuracy and reliability of the measurement are improved by calibrating with a standard polarizer fixture 9.

[0081] The specific embodiments described above do not constitute a limitation on the scope of protection of this application. Any other corresponding changes and modifications made based on the technical concept of this application should be included within the scope of protection of this application.

Claims

1. A high-precision waveplate detection method, characterized in that, The method is based on a polarization measurement system, which is provided with a light source (1), a polarizer (2), a sample stage (4) for placing the waveplate (41) to be measured, an analyzer (6), and a detection device in sequence along the optical path. The polarizer (2) and the analyzer (6) are respectively mounted on a rotary drive mechanism (21, 61) with adjustable azimuth angle. The method includes the following steps: Coarse measurement steps: Place the waveplate (41) to be tested on the sample stage (4) at an arbitrary azimuth angle, obtain the Mueller matrix of the waveplate (41) to be tested through the polarization measurement system, and calculate the initial value of the azimuth angle of the waveplate (41) to be tested based on the Mueller matrix. Initial value of phase delay difference ; Polarizing device rotation steps: Based on the initial azimuth angle value The azimuth angles of the polarizer (2) and the analyzer (6) are synchronously rotated to their initial values ​​via the rotary drive mechanisms (21, 61). ; Precision measurement steps: Under the azimuth angle of the rotated polarizer (2) and analyzer (6), the Mueller matrix of the waveplate (41) under test is obtained again through the polarization measurement system, and the azimuth angle measurement value of the waveplate (41) under test is calculated based on the Mueller matrix. and phase delay difference measurement This is output as the final measurement result.

2. The high-precision waveplate detection method according to claim 1, characterized in that, The polarization measurement system is a dual-rotation compensator ellipsometric measurement system, which includes, in sequence along the optical path, a light source (1), a polarizer (2), a first compensator (3), a sample stage (4), a second compensator (5), an analyzer (6), and a spectrometer (7) serving as the detection device; wherein the first compensator (3) and the second compensator (5) operate at different fixed rotation speeds. , Continuous rotation, and and The ratio is a ratio of coprime integers.

3. The high-precision waveplate detection method according to claim 2, characterized in that, The method for obtaining the Mueller matrix of the waveplate (41) to be tested includes: During the continuous rotation of the first compensator (3) and the second compensator (5), the spectrometer (7) collects multiple sets of emitted light intensity data at equal time intervals within a complete rotation cycle, and the number of data points collected is not less than the number of unknowns to be determined in the Mueller matrix; Fourier analysis was performed on the emitted light intensity data to extract the DC component and the cosine and sine coefficients of each harmonic frequency component determined by the rotational speed ratio. The harmonic frequencies are... , Integer multiples of and their sum frequency and difference frequency; Establish a system of linear equations between the coefficients of each frequency component and the 16 elements of the Mueller matrix of the wave plate (41) under test. Solve the system of linear equations by the least squares method and inversely calculate the Mueller matrix of the wave plate (41) under test.

4. The high-precision waveplate detection method according to claim 2, characterized in that, Prior to the coarse measurement step, an instrument polarization device azimuth angle calibration step is included, which includes: Using a standard polarizer fixture (9) with a known polarization direction, the installation angle deviation of the polarizer (2), the analyzer (6), the first compensator (3), and the second compensator (5) is calibrated respectively. The installation angle deviation of each polarization device is determined by the extinction method: rotate the polarization device to be calibrated, record the relationship curve between the rotation angle and the light intensity of the detection device, fit the rotation angle corresponding to the lowest light intensity according to Malus's law, and calculate the installation angle deviation based on the rotation angle corresponding to the lowest light intensity and the known azimuth angle of the standard polarizer fixture (9).

5. The high-precision waveplate detection method according to claim 4, characterized in that, The installation angle deviation of the polarizer (2) The calibration method is as follows: Construct a transmission optical path from light source (1) to polarizer (2), standard polarizer fixture (9), and spectrometer (7), assuming the azimuth angle of the standard polarizer fixture (9) is... ; By rotating the polarizer (2), the rotation angle corresponding to the lowest light intensity is obtained by fitting the data. ; Calculate the installation angle deviation of the deflector (2): =b-(a+90°); Installation angle deviation of the first compensator (3) The calibration method is as follows: construct the transmission optical path of light source (1) — polarizer (2) — first compensator (3) — standard polarizer fixture (9) — spectrometer (7); rotate the polarizer (2) to the calibrated angle. Rotate the first compensator (3) and fit the rotation angle corresponding to the lowest light intensity. ; Calculate the installation angle deviation of the first compensator (3): =d-(a+90°).

6. The high-precision waveplate detection method according to claim 1, characterized in that, The instrument system error calibration step is included before the coarse measurement step: To obtain the measured Mueller matrix, a standard transmission sample with a known Mueller matrix is ​​used. ; According to the standard Mueller matrix of the standard transmission sample and the measured Mueller matrix Calculate the system error calibration matrix ; In the subsequent coarse and fine measurement steps, the measured Mueller matrix is ​​corrected using the systematic error calibration matrix dM. The corrected Mueller matrix... .

7. The high-precision waveplate detection method according to claim 1, characterized in that, The methods for calculating the azimuth angle and phase delay difference of the waveplate from the Mueller matrix in the coarse measurement step and the fine measurement step include: Based on the elements of the Mueller matrix Calculate the phase delay difference : Its value ranges from 0° to 180°; Based on the elements of the Mueller matrix and Calculate the azimuth angle using the ratio : and according to the elements , , , Determining the sign of the sign The quadrant in which it is located makes The value can be unique; In the precision measurement step, the polarizer (2) and analyzer (6) have been rotated to an orientation aligned with the optical axis of the waveplate (41) under test. The waveplate (41) under test is equivalent to being placed at zero azimuth in the system coordinate system, and its Mueller matrix satisfies , , ; Measurement of phase delay difference Selecting Muller matrix respectively , Two elements , Perform calculations and select element combinations. according to Calculations were performed to obtain , , The weights are calculated using the reciprocal of the variance of each calculated value. , , Take the weighted average value as the phase delay difference measurement. Cross-validation is performed; the error variance of each calculated value is obtained by statistical analysis after repeated measurement of the same waveplate (41) a preset number of times, or by calculation of the noise of the Mueller matrix elements using the error propagation formula.

8. The high-precision waveplate detection method according to claim 2, characterized in that, The final measurement result corresponds to a specified wavelength within the spectral range of the light source (1), and the phase delay difference measurement value The phase delay difference at the specified wavelength; the spectrometer (7) has a spectral range of 380nm~1100nm and a spectral resolution better than 1nm. It can simultaneously measure the light intensity signal at multiple wavelengths and realize broadband ellipsometric measurement.

9. The high-precision waveplate detection method according to claim 1, characterized in that, In the polarization device rotation step, the initial azimuth angle value obtained in the coarse measurement step is used. Compared with the azimuth measurement value obtained from the precision measurement step The absolute value of the difference represents the initial value of the azimuth angle. The error is such that when the error is no greater than ±5°, the precision measurement step is in the state of making the Mueller matrix elements... , The optimal measurement state with the highest measurement accuracy; the coarse measurement step, the polarization device rotation step and the fine measurement step are automatically controlled and executed by the control software in the control and processing module (8).

10. A high-precision waveplate detection system, characterized in that, include: The polarization measurement module, along the optical path, includes, in sequence, a light source (1), a polarizer (2), a first compensator (3), a sample stage (4), a second compensator (5), an analyzer (6), and a spectrometer (7); wherein the polarizer (2) and the analyzer (6) are driven by their respective rotary drive mechanisms (21, 61) to achieve precise adjustment of the azimuth angle, and the first compensator (3) and the second compensator (5) are driven by their respective rotary drive mechanisms (31, 51) at different fixed rotation speeds. , Continuous rotation, and and The ratio is a ratio of coprime integers; The control and processing module (8) is connected to the polarizer rotation drive mechanism (21), the first compensator rotation drive mechanism (31), the second compensator rotation drive mechanism (51), the analyzer rotation drive mechanism (61), and the spectrometer (7), respectively, and is used to control the movement of each of the rotation drive mechanisms and collect data from the spectrometer (7), thereby executing the high-precision waveplate detection method according to any one of claims 1 to 9; The angular resolution of each of the rotary drive mechanisms (21, 31, 51, 61) is better than 0.01°; the system also includes a standard polarizer fixture (9) having a known polarization direction, which is used as an angular reference in the azimuth angle calibration step of the instrument polarization device.

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