A system and method for measuring polarization aberrations of a high numerical aperture objective lens using photoelastic modulation

Abstract

The application belongs to the technical field of optical measurement, and particularly relates to a kind of elastic light modulation high numerical aperture objective lens polarization aberration measurement system and method, including detection light source, polarizer, half-wave plate, beam splitter, to-be-measured high numerical aperture objective lens, spherical mirror, elastic light modulator, analyzer, tube mirror, CMOS camera, computer and FPGA signal processing module;Spherical mirror center coincides with objective lens front focal point, and the polarization axis of polarizer and analyzer is-45° and 45° relative to the modulation fast axis of elastic light modulator.The Jones matrix of beam splitter is calibrated first when measuring, then linearly polarized light is incident at 0° and 90° in turn, the phase modulation amplitude of elastic light modulator is controlled, and the light intensity gray scale diagram when phase modulation value is 0, π / 2 and π is measured respectively, the amplitude ratio and retardation distribution of objective lens are obtained by using Jones matrix model and triangle identity equation to solve simultaneously.It can realize high sensitivity real-time measurement of small polarization aberration of high numerical aperture objective lens.

Description

Technical Field

[0001] This invention relates to the field of optical measurement technology, and in particular to a system and method for measuring polarization aberrations of a high numerical aperture objective lens with elastic-optical modulation. Background Technology

[0002] Polarization aberration is a key factor affecting imaging quality and measurement accuracy in high-precision optical systems, especially for those with stringent polarization requirements. Its root cause can be attributed to the asymmetry in amplitude and phase between s-polarized and p-polarized light when light is incident at a large angle to a medium interface. This is due to factors such as coatings and material birefringence, resulting in unequal transmission and reflection coefficients, leading to biaxial attenuation and phase delay. This effect is particularly pronounced in high numerical aperture (NA) optical systems because large incident angles amplify polarization-related optical path differences, thereby altering the system's effective focal length and imaging quality. In recent years, with the continuous increase in numerical aperture of high-end optical systems such as projection lithography and high-resolution microscopy, polarization aberration has become a critical factor restricting system imaging quality and manufacturing accuracy. Therefore, achieving highly sensitive and accurate measurement of minute polarization aberrations in high numerical aperture objectives is of great significance.

[0003] Currently, several methods exist for measuring polarization aberrations in high numerical aperture objectives. Nanoscale probe scanning achieves nanoscale spatial resolution polarization aberration detection by scanning the focused light field, but its accuracy is limited by the sub-nanometer positioning precision of the probe, and accurately recovering the wavefront from vector beam intensity measurements is challenging. Imaging methods measure structures obtained through photolithography and infer objective aberrations based on vector optical imaging theory, but require photoresist exposure and development, resulting in a long measurement cycle, and the photoresist processing introduces additional errors. Mueller matrix ellipsometers can directly measure the Mueller matrix at the system's back focal plane, providing complete polarization information including delay, dichroism, and depolarization effects, but it is usually difficult to directly resolve the wavefront phase, and it is susceptible to diffraction effects at the edge of the optical system's field of view. Interferometry is the primary method for phase measurement, but the system is complex and extremely sensitive to environmental interference. The Shaker-Hartmann wavefront sensor has a compact structure, but its spatial resolution is limited and it is susceptible to high-slope wavefront crosstalk. In addition, phase retrieval-based polarization aberration measurement methods have emerged in recent years, which transform the measurement into an optimization problem to invert the Jones matrix. However, these methods have high algorithmic complexity and rely on multi-plane intensity measurements. It is evident that existing polarization aberration measurement techniques generally involve complex equipment, are inconvenient to measure, or have low efficiency. Summary of the Invention

[0004] The purpose of this invention is to provide a system and method for measuring polarization aberrations of high numerical aperture objectives using elastic-optical modulation, aiming to solve the technical problems of existing high numerical aperture objectives polarization aberration measurement devices having complex structures, low measurement efficiency, poor anti-interference capabilities, and difficulty in simultaneously achieving high sensitivity and high precision in measuring minute polarization aberrations.

[0005] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a polarization aberration measurement system for an elastic-optical modulation high numerical aperture objective lens, comprising: A detection light source is used to emit a detection beam; a polarizer is placed in the output light path of the detection light source to convert the detection beam into linearly polarized light; a half-wave plate is placed in the output light path of the polarizer to adjust the polarization direction of the linearly polarized light; a beam splitter is used to reflect the linearly polarized light adjusted by the half-wave plate to the high numerical aperture objective lens under test; a spherical mirror, with its center coinciding with the front focal point of the high numerical aperture objective lens under test, is used to reflect the beam emitted from the high numerical aperture objective lens under test back to the high numerical aperture objective lens under test. The beam is incident on the objective lens and transmitted through it; a photoelectric modulator, positioned in the transmission and output path of the beam splitter, applies periodic phase modulation to the transmitted beam; the polarization axis of the polarizer is at -45° relative to the fast modulation axis of the photoelectric modulator; the analyzer, positioned in the output path of the photoelectric modulator, has its polarization axis at 45° relative to the fast modulation axis of the photoelectric modulator; a tube lens, positioned in the output path of the analyzer, has its front focal plane coincide with the rear focal plane of the high numerical aperture objective lens under test; CMOS A camera, positioned in the exit light path of the tube mirror, is used for image acquisition to obtain light intensity grayscale images. A computer, connected to both the FPGA signal processing module and the CMOS camera, receives the light intensity grayscale images uploaded by the CMOS camera, processes them using algorithms, and generates and displays the amplitude ratio and delay distribution maps of the high numerical aperture objective lens under test. The computer sends control signals to the FPGA signal processing module. The FPGA signal processing module includes an LC control submodule and a stroboscopic illumination submodule. The photoluminescent modulator includes an LC resonant high-voltage drive circuit. The LC control submodule is connected to the LC resonant high-voltage drive circuit of the photoluminescent modulator, providing a PWM drive signal to the LC resonant high-voltage drive circuit and simultaneously generating a fundamental frequency reference signal. The stroboscopic illumination submodule is connected to the detection light source. The control signal includes a stroboscopic phase control word. The computer digitally adjusts the stroboscopic phase by regulating the stroboscopic phase control word, thereby controlling the working state of the stroboscopic illumination submodule and controlling the stroboscopic illumination of the detection light source. This ensures that the detection light source illuminates only at a preset phase point, so that the illumination pulse is precisely synchronized with the fixed phase of the photoluminescent phase.

[0006] The detection light source is a light-emitting diode with a wavelength of 633nm.

[0007] Both the polarizer and the analyzer are calcite Glan Taylor polarizers with an extinction ratio better than 100,000:1.

[0008] The photoelastic modulator also includes a photoelastic crystal, the light-transmitting material of which is an isotropic fused silica single piezoelectric quartz driven long rod photoelastic modulator with a resonant frequency of 50kHz.

[0009] Secondly, the present invention also provides a method for measuring polarization aberration of a high numerical aperture objective lens with elastic-optical modulation, based on the aforementioned elastic-optical modulation high numerical aperture objective lens polarization aberration measurement system, comprising: S1, calibrating the Jones matrix of a beam splitter to obtain the Jones matrix parameters of the beam splitter in the reflection and transmission directions; S2, arranging the high numerical aperture objective lens to be measured and a spherical mirror in the measurement optical path, such that the center position of the spherical mirror coincides with the front focal point of the high numerical aperture objective lens to be measured, and that the front focal plane of the tube mirror coincides with the rear focal plane of the high numerical aperture objective lens to be measured. S3. Adjust the half-wave plate so that the linearly polarized light incident on the beam splitter is successively 0° linearly polarized light and 90° linearly polarized light. For each type of linearly polarized light, control the phase modulation amplitude of the light-elastic modulator to π, and collect the light intensity grayscale images of different phases obtained by the CMOS camera when the phase modulation values ​​of the light-elastic modulator are 0, π / 2 and π respectively; S4. Based on the light intensity grayscale images measured in step S3 and the Jones matrix parameters of the beam splitter obtained in step S1, calculate the amplitude ratio and retardation distribution of the high numerical aperture objective lens to be tested.

[0010] In step S1, calibrating the Jones matrix of the beam splitter includes: adjusting the half-wave plate so that the linearly polarized light incident on the beam splitter is 45° linearly polarized light; measuring three light intensity grayscale images when the phase modulation value of the light-elastic modulator is 0, π / 2, and π, respectively, from the reflection direction and the transmission direction of the beam splitter; and calculating the Jones matrix parameters of the beam splitter in the reflection direction and the Jones matrix parameters in the transmission direction based on the three light intensity grayscale images.

[0011] In step S4, the calculation of the amplitude ratio and retardation distribution map of the high numerical aperture objective lens to be tested includes: calculating the first ratio and the second ratio based on the grayscale images of each output light intensity measured in step S3; and solving the equations using trigonometric identities based on the first ratio and the second ratio to obtain the amplitude ratio and retardation distribution map.

[0012] In step S3, the phase modulation amplitude is controlled to be π by adjusting the driving voltage of the photoelectric modulator.

[0013] In step S3, the LC control submodule provides a PWM drive signal to the LC resonant high voltage drive circuit, while the strobe lighting submodule controls the detection light source to strobe under the control of the control signal sent by the computer, so that the detection light source is lit only at the preset phase point.

[0014] In step S1, when calibrating the beam splitter, the measurement optical path does not include the high numerical aperture objective lens and the spherical mirror to be measured.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This application employs an elastic-optical modulator as the core phase modulation element. By applying known high-frequency controllable phase modulation to the light wave, combined with a back-reflection optical path layout, the detection beam passes through the high numerical aperture objective lens under test twice, effectively multiplying the modulation effect of polarization aberrations on the beam, thereby significantly improving the system's detection sensitivity to minute polarization aberrations. Simultaneously, through the pre-calibration step of the beam splitter's Jones matrix, the system error introduced by the beam splitter's own polarization effect is subtracted from the final measurement result, further ensuring measurement accuracy. Furthermore, this invention utilizes the complete mathematical description of the Jones matrix, measuring multiple sets of light intensity grayscale images under different input polarization states and modulation phases, and combining this with a ratio calculation method to eliminate the influence of light source instability on the measurement results, ultimately obtaining the amplitude ratio and retardation distribution map of the high numerical aperture objective lens.

[0016] 2. This application employs an all-solid-state optical design with no moving mechanical components. The entire detection process requires no mechanical adjustment or scanning action, and all light intensity values ​​are acquired electronically in a very short time. Compared to traditional measurement schemes that rely on probe scanning or mechanical rotating components, this invention fundamentally eliminates the time overhead introduced by mechanical movement, enabling real-time and rapid measurement of polarization aberrations of the objective lens under test.

[0017] 3. This application employs a photoelastic modulator combined with an FPGA signal processing module for drive control. The photoelastic modulator operates at its mechanical resonant frequency (50kHz), and its modulation frequency is much higher than the frequency band of environmental vibrations and most noise sources. By extracting specific frequency components, various noise interferences that are not at the modulation frequency can be effectively suppressed. More importantly, the FPGA signal processing module of this invention is controlled by a field-programmable gate array, and the PWM drive signal of the LC resonant high-voltage drive circuit of the photoelastic modulator is also provided by the same field-programmable gate array. This ensures that the reference signal and the modulation base frequency signal are strictly in sync, fundamentally eliminating the demodulation error introduced by the asynchronous frequency drift between the reference signal and the modulation signal in traditional schemes, and significantly improving the measurement stability and reliability of the system in complex environments. Attached Figure Description

[0018] Figure 1 This is a schematic diagram of the structure of a photoelastic modulation high numerical aperture objective lens polarization aberration measurement system provided in an embodiment of this application; Figure 2 This is a schematic diagram of a device for calibrating a beam splitter provided in an embodiment of this application. Detailed Implementation

[0019] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0020] For example, refer to Figure 1 This application provides a schematic diagram of a polarization aberration measurement system for a high numerical aperture objective lens with elastic-optical modulation. The system includes a detection light source 1, a polarizer 2, a half-wave plate 3, a beam splitter 4, a high numerical aperture objective lens under test 5, a spherical mirror 6, an elastic-optical modulator 7, an analyzer 8, a tube lens 9, a CMOS camera 10, a computer 11, and an FPGA signal processing module 12. The FPGA signal processing module 12 includes an LC control submodule and a stroboscopic illumination submodule.

[0021] The detection light source 1 is used to emit a detection beam. In the embodiments provided in this application, the detection light source 1 is a light-emitting diode (LED) light source with good monochromaticity and stability, and its wavelength is 633nm. This wavelength is in the red light band where the human eye has low sensitivity, which facilitates the selection and matching of subsequent optical components. Compared with traditional gas lasers, LED light sources have advantages such as small size, low power consumption, and long lifespan, making them suitable for long-term stable operation in industrial environments. In some embodiments, the detection light source 1 can also be other wavelength-stable light sources, such as solid-state lasers or semiconductor lasers, to match different measurement requirements and the operating wavelength of optical components.

[0022] Polarizer 2 is positioned in the output optical path of the detection light source 1 to convert the detection beam into linearly polarized light. As one possible implementation, polarizer 2 is a calcite Glan-Taylor polarizer with an extinction ratio better than 100,000:1. The Glan-Taylor polarizer utilizes the birefringence of calcite crystals to convert incident unpolarized light into high-purity linearly polarized light output. Its excellent extinction ratio ensures extremely high polarization purity of the output beam, thus providing a reliable polarization reference for subsequent polarization measurements. In some embodiments, polarizer 2 can also be other types of polarization devices, such as a Glan-Thompson polarizer or a wire-grid polarizer, as long as a sufficiently high extinction ratio is provided.

[0023] A half-wave plate 3 is disposed in the output optical path of the polarizer 2 to adjust the polarization direction of linearly polarized light. The half-wave plate 3 is an optical element that generates a π phase delay between the two orthogonal polarization components of a light beam passing through it. When linearly polarized light passes through the half-wave plate at a certain angle, its polarization direction rotates, with the rotation angle being twice the angle between the fast axis of the half-wave plate and the incident polarization direction. In the embodiments provided in this application, by rotating the fast axis angle of the half-wave plate 3, the polarization direction of the linearly polarized light incident on the beam splitter 4 can be precisely adjusted, sequentially generating 0° linearly polarized light, 45° linearly polarized light, and 90° linearly polarized light to meet the different requirements for the input polarization state in different measurement stages (beam splitter calibration stage and objective lens measurement stage). The half-wave plate 3 has minimal attenuation of light intensity, enabling flexible control of the polarization direction without significantly affecting light energy utilization.

[0024] Beam splitter 4 is used to reflect the linearly polarized light adjusted by half-wave plate 3 to the high numerical aperture objective lens 5 under test, and simultaneously transmits the beam returning from the high numerical aperture objective lens 5 to the subsequent optical path. Specifically, the linearly polarized light from half-wave plate 3 first enters beam splitter 4, and after reflection by beam splitter 4, enters the high numerical aperture objective lens 5 under test; the beam returns along the original path after passing through the high numerical aperture objective lens 5 and spherical mirror 6, and then enters beam splitter 4 again after passing through the high numerical aperture objective lens 5 under test. At this time, the beam is transmitted through beam splitter 4 and enters the measurement arm optical path composed of elastic modulator 7 and analyzer 8. Those skilled in the art should understand that beam splitter 4 itself will generate polarization effect during reflection and transmission, that is, there are differences in reflectivity and transmittance for different polarization components, and it will introduce additional phase delay. In order to eliminate the influence of the polarization effect of beam splitter 4 itself on the measurement results, the Jones matrix of beam splitter 4 is pre-calibrated before formal measurement. The specific calibration method will be described in detail in the subsequent method embodiments.

[0025] The high numerical aperture objective lens 5 under test is the object of measurement in the system. Its numerical aperture (NA) is typically greater than 0.65, and its typical applications include high-resolution microscopic imaging systems and projection lithography objectives. The high numerical aperture objective lens 5 under test is fixed in the measurement optical path via a standard threaded interface or a special clamp, with its optical axis coinciding with the principal optical axis of the system optical path. Since the polarization aberration of high numerical aperture objectives is particularly sensitive to large-angle incident light, this application uses a back-reflection optical path to allow the detection beam to pass through the high numerical aperture objective lens 5 under test twice, effectively multiplying the modulation effect of polarization aberration on the beam, thereby improving the detection sensitivity of small polarization aberrations.

[0026] The spherical mirror 6 is used to reflect the focused beam emitted from the high numerical aperture objective lens 5 under test back to the high numerical aperture objective lens 5 under test, forming a back-reflected optical path. The center position (i.e., the center of the sphere) of the spherical mirror 6 is strictly coincident with the front focal point of the high numerical aperture objective lens 5 under test. In this configuration, the high numerical aperture objective lens 5 under test focuses the incident collimated beam at its front focal point, which falls exactly at the center of the sphere of the spherical mirror 6. After being reflected by the spherical mirror 6, the beam returns along the original path and re-enters the high numerical aperture objective lens 5 under test, forming a collimated beam after passing through the objective lens. Since the focal point of the light beam coincides with the center of the sphere of the spherical mirror, the Jones matrix of the spherical mirror 6 is proportional to the identity matrix, and its influence on the polarization state can be ignored in mathematical processing, which simplifies the subsequent Jones matrix calculation model. In some embodiments, the spherical mirror 6 is mounted on a three-dimensional precision adjustment frame, and the lens position is finely adjusted to ensure that the focal point of the objective lens is precisely coincident with the center of the concave sphere.

[0027] An optical-elastic modulator 7 is disposed in the transmission and output light path of the beam splitter 4 and is used to apply periodic phase modulation to the light beam transmitted through the beam splitter 4. The optical-elastic modulator 7 includes an optical-elastic crystal and an LC resonant high-voltage driving circuit. The light-transmitting material of the optical-elastic crystal is isotropic fused silica, which has good optical uniformity and high optical transmittance. In some embodiments, the optical-elastic modulator 7 is a single piezoelectric quartz-driven long rod-shaped optical-elastic modulator with a resonant frequency of 50 kHz.

[0028] The phase modulation principle of the photoelastic modulator 7 is as follows: When an alternating voltage matching the mechanical resonant frequency of the photoelastic modulator 7 is applied to the piezoelectric actuator inside the photoelastic modulator 7, the piezoelectric actuator undergoes periodic expansion and contraction based on the inverse piezoelectric effect. This movement is transmitted to the photoelastic crystal and precisely matches its inherent mechanical resonant frequency. Under resonant conditions, the driving force energy accumulates efficiently, thereby exciting a periodic stress with a significant amplitude inside the crystal with a small excitation. This periodic stress causes a periodic change in the refractive index of the photoelastic crystal through the photoelastic effect (i.e., the phenomenon that stress causes a change in the refractive index of an optical material), thus introducing a known, high-frequency, controllable phase modulation to the light wave passing through the crystal. The Jones matrix of the photoelastic modulator 7 can be expressed as:

[0029] in, , The phase modulation amplitude of the photoelectric modulator can be precisely controlled by adjusting the driving voltage. This is the resonant frequency of the photoelectric modulator (50kHz in this embodiment); It is a time variable; The unit is the imaginary unit. In the above expression, the component along the fast modulation axis of the optical modulator 7 is not phase modulated (phase delay is 0), while the component perpendicular to the fast modulation axis is periodically phase modulated (phase delay is 0). This allows for the periodic control of the phase difference between the two orthogonal polarization components of the passing beam.

[0030] In some embodiments, the FPGA signal processing module generates a frequency-adjustable square wave signal using Direct Digital Synthesis (DDS) technology and inputs it to the LC control submodule. The LC control submodule simultaneously outputs a PWM drive signal to the LC resonant high-voltage drive circuit of the photosensitive modulator 7 and generates a fundamental frequency reference signal. This ensures that the fundamental frequency reference signal and the modulated fundamental frequency signal of the photosensitive modulator are strictly in sync, effectively eliminating demodulation errors introduced by the asynchronous frequency drift between the reference signal and the modulated signal in traditional solutions.

[0031] As one possible implementation, the PWM drive signal sent by the FPGA signal processing module to the LC resonant high-voltage drive circuit is generated by the direct digital frequency synthesis (DDS) submodule within the FPGA signal processing module. Specifically, it is generated by the DDS submodule using DDS technology. The LC resonant high-voltage drive circuit then amplifies the PWM drive signal and performs an LC active inverter to ultimately generate high-voltage alternating current to drive the piezoelectric actuator.

[0032] The analyzer 8 is disposed in the output optical path of the photosensitive modulator 7. In some embodiments, the analyzer 8 is a calcite Glan Taylor polarizer of the same type as the polarizer 2, with an extinction ratio better than 100,000:1. The polarization axis of the analyzer 8 is at a 45° angle relative to the fast modulation axis of the photosensitive modulator 7. This 45° configuration is key to achieving efficient conversion of the photosensitive modulation signal into an intensity modulation signal: when the polarization axis of the analyzer is at a 45° angle to the fast modulation axis of the photosensitive modulator, the phase modulation introduced by the photosensitive modulator 7 can be converted into light intensity changes that can be detected by the CMOS camera 10 with maximum efficiency, thereby ensuring the system's sensitivity to small polarization aberrations.

[0033] The telescope 9 is positioned in the output optical path of the analyzer 8 to focus the light beam emitted by the analyzer 8 onto the photosensitive surface of the CMOS camera 10. The front focal plane of the telescope 9 is strictly coincident with the rear focal plane of the high numerical aperture objective lens 5 under test. With this configuration, the light field distribution at the rear focal plane of the high numerical aperture objective lens 5 under test can be accurately transmitted to the rear focal plane of the telescope 9 and received by the CMOS camera 10, ensuring the complete transmission of light field information.

[0034] A CMOS camera 10 is positioned on the outgoing light path of the tube lens 9. At specific phase points (e.g., 0°, 30°, 90°) of the PEM modulation cycle, it "freezes" the light intensity signal of the beam focused by the tube lens 9, converts it into a grayscale image of light intensity, and sends it to the computer 11 for further processing. The computer 11 is connected to both the CMOS camera 10 and the FPGA signal processing module 12. It receives the grayscale image of light intensity uploaded by the CMOS camera, runs the core processing algorithm, and ultimately calculates, generates, and displays the amplitude ratio and delay distribution map of the high numerical aperture objective lens 5 under test. Simultaneously, it transmits control signals to the stroboscopic illumination submodule in the FPGA signal processing module 12. For example, the control signals include a stroboscopic frequency control word, a stroboscopic phase control word, and a stroboscopic pulse width control word. The stroboscopic illumination submodule controls the stroboscopic movement of the detection light source 1 according to these words, determining when and for how long to strobe. For example, it can be set to perform strobe illumination and data acquisition by modulating the phase at 0°, 30°, and 90°.

[0035] The strobe illumination submodule in the FPGA signal processing module 12 is connected to the detection light source 1 and controls its strobe to make it light up only at a preset phase point; so that the illumination pulse is precisely synchronized with the fixed phase of the light; the LC control submodule in the FPGA signal processing module 12 is connected to the LC resonant high voltage drive circuit of the light modulator 7 and is used to provide PWM drive signals to the LC resonant high voltage drive circuit and generate a base frequency reference signal at the same time.

[0036] The workflow of the FPGA signal processing module 12 is as follows: The FPGA signal processing module 12 generates a fundamental frequency reference signal with a frequency equal to the resonant frequency of the photosensitive modulator (PPM), and simultaneously generates a PWM drive signal. This PWM drive signal is then output as a high-voltage sinusoidal voltage signal via the LC resonant drive circuit to drive the PEM. Since both the fundamental frequency reference signal and the PWM drive signal are generated by the LC control submodule, there are no frequency drift or phase asynchrony issues between them, ensuring the accuracy and stability of signal demodulation and output. Specifically, the LC control submodule generates a PWM drive signal with a frequency equal to the PEM resonant frequency (50kHz), and then sets the phase modulation amplitude of the PEM to a half-wave state (i.e., modulation amplitude) by changing the duty cycle of the PWM drive signal. =π); Finally, through resonant phase tracking stabilization control, the PEM is ensured to operate stably in this half-wave state. Thus, the instantaneous phase modulation values ​​generated by the PEM at the three drive phases of 0°, 30°, and 90° are 0, π / 2, and π, respectively, providing accurate time reference and phase conditions for subsequent stroboscopic illumination and image acquisition.

[0037] Along the optical path propagation direction, the spatial relationship of each optical element is as follows: detection light source 1 → polarizer 2 → half-wave plate 3 → beam splitter 4 → (after reflection by the beam splitter) high numerical aperture objective lens 5 under test → spherical mirror 6 → (after reflection by the spherical mirror and returning along the original path) high numerical aperture objective lens 5 under test → beam splitter 4 → (after transmission by the beam splitter) light-sensitive modulator 7 → analyzer 8 → tube mirror 9 → CMOS camera 10. Specifically, the polarization axis of polarizer 2 is at -45° relative to the fast modulation axis of light-sensitive modulator 7; the center of spherical mirror 6 coincides with the front focal point of high numerical aperture objective lens 5 under test; the front focal plane of tube mirror 9 coincides with the rear focal plane of high numerical aperture objective lens 5 under test; the polarization axis of analyzer 8 is at 45° relative to the fast modulation axis of light-sensitive modulator 7; and the fast modulation axis of light-sensitive modulator 7 coincides with the horizontal direction.

[0038] Before actually measuring the high numerical aperture objective lens to be measured, the Jones matrix of beam splitter 4 needs to be calibrated to eliminate the interference of the beam splitter's own polarization effect on the measurement results. Figure 2 This is a schematic diagram of the device used for calibrating the beam splitter.

[0039] like Figure 2 As shown, the beam splitter calibration device does not include the high numerical aperture objective lens 5 and the spherical mirror 6. The test beam passes through the polarizer 2 and the half-wave plate 3 and is directly incident on the beam splitter 4. Then, it enters the detection arm, which consists of an elastic modulator 7, an analyzer 8, a tube mirror 9, and a CMOS camera 10, from the reflection direction and the transmission direction of the beam splitter, respectively. The specific calibration steps are as follows: First, configure the incident polarization state: Adjust half-wave plate 3 so that the linearly polarized light incident on beam splitter 4 is 45° linearly polarized light. Using the Jones vector notation, the Jones vector of the 45° linearly polarized light is:

[0040] in, To detect the total light intensity of the beam after it passes through the half-wave plate.

[0041] Next, measurements are taken from both the reflection and transmission directions: first, the reflection direction of beam splitter 4 is measured. The 45° linearly polarized light, after reflection by beam splitter 4, enters the optical modulator 7, then passes through analyzer 8, and is finally received by CMOS camera 10. The Jones vector of the outgoing light at this point can be expressed as:

[0042] in, The Jones matrix of the analyzer; The Jones matrix of the elastic modulator; The Jones matrix of the beam splitter, with subscripts... or These represent the directions of reflection ( ) and transmission direction ( The Jones matrix of the beam splitter can be expressed as:

[0043] in, and Here are the Jones matrix parameters for the beam splitter. It reflects the ratio (i.e., amplitude ratio) of the amplitude transmission (or reflection) coefficients of the two orthogonal polarization components of the beam splitter. It reflects the difference in phase delay between two orthogonal polarization components by the beam splitter.

[0044] The Jones matrix of analyzer 8 is:

[0045] Substituting the Jones matrix above, the expression for the light intensity received by the CMOS camera 10 can be derived as follows:

[0046] in, This represents the instantaneous phase modulation amount of the photoelastic modulator.

[0047] Then, the light intensity is measured at a specific modulation phase point: the driving voltage of the light-elastic modulator 7 is controlled to adjust its phase modulation amplitude. It equals π (i.e., 180°). Under this condition, when the instantaneous phase modulation value of the photoelastic modulator... When the angles are 0, π / 2 (i.e., 90°), and π (i.e., 180°), respectively, the corresponding light intensity grayscale images are obtained from the CMOS camera 10, and are denoted as follows: , and (The angle values ​​in the subscripts here represent the phase angle of the light-sensitive modulator drive signal corresponding to the moment of strobe illumination; under this drive phase, the instantaneous phase modulation values ​​generated by the light-sensitive modulator are respectively the corresponding...) ).

[0048] Substituting the three phase modulation values ​​into the light intensity expression, we get:

[0049]

[0050]

[0051] Finally, the Jones matrix parameters of the beam splitter are calculated: to eliminate the incident light intensity The impact of instability on the solution results is addressed by calculating the ratio in the above formula. First, we have:

[0052]

[0053] Therefore, the Jones matrix parameters of the beam splitter in the reflection or transmission directions can be obtained respectively. and :

[0054]

[0055] In the above calculation process, the instability of the detection light source is effectively eliminated through ratio calculation, thus ensuring the accuracy of the calibration results. By repeating the above steps from both the reflection and transmission directions of the beam splitter, the Jones matrix parameters of the beam splitter in the reflection direction can be obtained. (corresponding parameters A and B) and the Jones matrix parameters in the transmission direction ( (corresponding to parameters C and D), used for error correction in subsequent objective lens measurements.

[0056] After calibrating the beam splitter 4, the high numerical aperture objective lens 5 and the spherical mirror 6 are installed in the measurement optical path to form a complete measurement system (e.g., Figure 1 As shown in the figure, the formal measurement of the polarization aberration of the objective lens under test begins. The specific steps are as follows: First, configure the measurement optical path: Place the high numerical aperture objective lens 5 and the spherical mirror 6 in the measurement optical path, ensuring that the center of the spherical mirror 6 precisely coincides with the front focal point of the high numerical aperture objective lens 5, and that the front focal plane of the tube mirror 9 precisely coincides with the rear focal plane of the high numerical aperture objective lens 5. Fine-tune the position of the spherical mirror using a high-precision three-dimensional adjustment frame until the light spot received on the detector reaches its brightest and most symmetrical state. This indicates that the center of the spherical mirror and the focal point of the objective lens are precisely aligned.

[0057] Next, measurements were performed using linearly polarized light incident at 0° and 90° respectively. The half-wave plate 3 was adjusted so that the linearly polarized light incident on the beam splitter 4 was initially 0° linearly polarized. The Jones vector of the 0° linearly polarized light is:

[0058] Under 0° linearly polarized light incidence, the emitted light, after being reflected by the beam splitter → objective lens under test → spherical mirror → objective lens under test → beam splitter transmission → optical modulator → analyzer, reaches the CMOS camera with the following Jones vector:

[0059] in, and These are the Jones matrices of the beam splitter in the reflection and transmission directions, respectively (obtained during the calibration process). The Jones matrix of the high numerical aperture objective lens to be tested; For the Jones matrix of a spherical mirror, since the focal point of the light rays coincides with the center of the concave mirror sphere, It is proportional to the identity matrix and can be ignored in calculations; The rotation angle is The Jones rotation transformation matrix is ​​used to transform the Cartesian coordinate system containing linearly polarized light into the coordinate system containing p / s polarized light of the microscope objective. This is the corresponding inverse rotation transformation.

[0060] The Jones matrix of the high numerical aperture objective lens under test is:

[0061] in, and These are the Jones matrix parameters of the objective lens under test, which are the amplitude ratio and retardation that this invention ultimately needs to measure.

[0062] The Jones matrices of the beam splitter in the reflection and transmission directions are as follows:

[0063]

[0064] Where A, B, C, and D are the elements of the Jones matrix of the beam splitter obtained in the calibration step.

[0065] The rotation transformation matrix is:

[0066] Substituting the Jones matrix above, the expression for the light intensity received by the CMOS camera can be derived. The phase modulation amplitude is adjusted by controlling the driving voltage of the photoelectric modulator 7. Equal to π, the light intensity grayscale images obtained by the CMOS camera are recorded when the instantaneous phase modulation value of the photoelectric modulator is 0, π / 2, and π, respectively, and denoted as π / 2, ... , and .

[0067] Then, adjust the half-wave plate 3 so that the linearly polarized light incident on the beam splitter 4 is 90° linearly polarized light. The Jones vector of the 90° linearly polarized light is:

[0068] Under the condition of 90° linearly polarized light incidence, the above measurement process is repeated to obtain the Jones vector of the outgoing light. And the light intensity grayscale images at the three phase modulation points of the elastic modulator, respectively denoted as , and .

[0069] Finally, the amplitude ratio and retardation of the objective lens under test are calculated. The first ratio is calculated based on the measured light intensity grayscale images. Second ratio :

[0070]

[0071] Based on the two ratio expressions above, combined with the trigonometric identities... Solving the system of equations simultaneously, we get:

[0072]

[0073] make Then the amplitude ratio and delay They can be solved using the following formulas:

[0074]

[0075] In the above calculation process, the incident light intensity The parameters of the Jones matrix of the beam splitter cancel each other out in the ratio calculation, thus effectively eliminating the influence of the instability of the detection light source and the polarization effect of the beam splitter itself, and finally obtaining the amplitude ratio. The retardation distribution map accurately reflects the polarization aberration characteristics of the high numerical aperture objective lens under test.

[0076] It should be understood that in one embodiment provided in this application, an LED light source with a wavelength of 633nm is used for detection. In other embodiments, other stable light sources with different wavelengths can be selected according to the operating wavelength and measurement accuracy requirements of the objective lens under test. For example, for projection lithography objectives operating in the ultraviolet band, an ultraviolet light source (such as a solid-state laser with a wavelength of 355nm or 266nm) can be selected; for microscope objectives operating in the near-infrared band, a near-infrared light source (such as a laser with a wavelength of 780nm or 1064nm) can be selected. The selection of different wavelengths does not affect the basic measurement principle and device structure of this invention; only the coating and operating parameters of the optical elements need to be adjusted accordingly.

[0077] In this application example, the polarization axes of polarizer 2 and analyzer 8 are set at -45° and 45° relative to the fast modulation axis of optical modulator 7, respectively. This configuration, verified by experiments, achieves the maximum modulation efficiency. In some examples, the polarization axes of polarizer and analyzer are set at 45° and -45° relative to the fast modulation axis of optical modulator 7, respectively. In other embodiments, other angle configurations can also be used, such as 30° or 60° for polarization and analysis. In this case, the efficiency of converting the optical modulated signal into an intensity modulated signal will be reduced. However, by increasing the laser emission power, increasing the camera gain, or extending the effective exposure time, the signal intensity loss can still be compensated for, and effective sample polarization imaging detection can still be completed. The corresponding Jones matrix expression and calculation need to be adaptively adjusted according to the actual angle.

[0078] In the description of this specification, specific features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

[0079] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A device for measuring polarization aberration of a high numerical aperture objective lens with elastic-optical modulation, characterized in that, include: A detection light source (1) is used to emit a detection beam; a polarizer (2) is disposed on the output light path of the detection light source (1) to convert the detection beam into linearly polarized light; a half-wave plate (3) is disposed on the output light path of the polarizer (2) to adjust the polarization direction of the linearly polarized light; a beam splitter (4) is used to reflect the linearly polarized light adjusted by the half-wave plate (3) to the high numerical aperture objective lens (5) to be tested; a spherical mirror (6) is disposed with its center position coinciding with the front focal point of the high numerical aperture objective lens (5) to reflect the beam emitted from the high numerical aperture objective lens (5) back to the high numerical aperture objective lens (5) to be tested. After passing through the high numerical aperture objective lens (5), the light beam is incident on the beam splitter (4) and transmitted through the beam splitter (4); a light-sensitive modulator (7) is disposed on the transmission output light path of the beam splitter (4) and is used to apply periodic phase modulation to the light beam transmitted through the beam splitter (4); the polarization axis direction of the polarizer (2) is at -45° relative to the fast modulation axis direction of the light-sensitive modulator (7); an analyzer (8) is disposed on the output light path of the light-sensitive modulator (7), and the polarization axis direction of the analyzer (8) is at 45° relative to the fast modulation axis direction of the light-sensitive modulator (7); a tube lens (9) is disposed on the output light path of the analyzer (8), and its front focal plane is parallel to the beam beam. The back focal plane of the high numerical aperture objective (5) under test coincides; a CMOS camera (10) is set on the output light path of the tube lens (9) for image acquisition to obtain a light intensity grayscale image; a computer (11) is connected to the FPGA signal processing module (12) and the CMOS camera (10) respectively, for receiving the light intensity grayscale image uploaded by the CMOS camera (10), and after algorithm calculation, generating and displaying the amplitude ratio and delay distribution map of the high numerical aperture objective (5) under test, and the computer (11) sends control signals to the FPGA signal processing module (12); the FPGA signal processing module (12) includes an LC control submodule and a stroboscopic illumination submodule; the The light-sensitive modulator (7) includes an LC resonant high-voltage drive circuit. The LC control submodule is connected to the LC resonant high-voltage drive circuit of the light-sensitive modulator (7) and is used to provide a PWM drive signal to the LC resonant high-voltage drive circuit and generate a base frequency reference signal at the same time. The strobe lighting submodule is connected to the detection light source (1). The control signal includes a strobe phase control word. The computer (11) digitally adjusts the strobe phase by regulating the strobe phase control word, thereby controlling the working state of the strobe lighting submodule and controlling the detection light source (1) to strobe, so that the detection light source (1) is lit only at the preset phase point, so that the lighting pulse is accurately synchronized with the fixed phase of the light-sensitive modulator.

2. The device for measuring polarization aberration of a high numerical aperture objective lens with elastic-optical modulation according to claim 1, characterized in that, The detection light source (1) is a light-emitting diode light source with a wavelength of 633nm.

3. The device for measuring polarization aberration of a high numerical aperture objective lens with elastic-optical modulation according to claim 1, characterized in that, Both the polarizer (2) and the analyzer (8) are calcite Glan Taylor polarizers with an extinction ratio better than 100,000:

1.

4. The device for measuring polarization aberration of a high numerical aperture objective lens with elastic-optical modulation according to claim 1, characterized in that, The photoelastic modulator (7) also includes a photoelastic crystal, the light-transmitting material of which is an isotropic fused silica single piezoelectric quartz driven long rod photoelastic modulator with a resonant frequency of 50kHz.

5. A method for measuring polarization aberration of a high numerical aperture objective lens with elastic-optical modulation, based on a polarization aberration measuring device for a high numerical aperture objective lens with elastic-optical modulation as described in any one of claims 1-4, characterized in that, include: S1. The Jones matrix of the beam splitter (4) is calibrated to obtain the Jones matrix parameters of the beam splitter (4) in the reflection direction and the transmission direction. S2. Arrange the high numerical aperture objective (5) to be measured and the spherical mirror (6) in the measurement optical path, so that the center position of the spherical mirror (6) coincides with the front focal point of the high numerical aperture objective (5) to be measured, and the front focal plane of the tube mirror (9) coincides with the back focal plane of the high numerical aperture objective (5) to be measured; S3. Adjust the half-wave plate (3) so that the linearly polarized light incident on the beam splitter (4) is successively 0° linearly polarized light and 90° linearly polarized light. For each type of linearly polarized light, control the phase modulation amplitude of the elastic modulator (7) to π, and collect the light intensity grayscale images obtained by the CMOS camera (10) at different phases when the phase modulation values ​​of the elastic modulator (7) are 0, π / 2 and π respectively; S4. Based on the light intensity grayscale images measured in step S3 and the Jones matrix parameters of the beam splitter (4) obtained by calibration in step S1, calculate the amplitude ratio and retardation distribution of the high numerical aperture objective lens (5) to be tested.

6. The method for measuring polarization aberration of a high numerical aperture objective lens with elastic-optical modulation according to claim 5, characterized in that, In step S1, calibrating the Jones matrix of the beam splitter (4) includes: adjusting the half-wave plate (3) so that the linearly polarized light incident on the beam splitter (4) is 45° linearly polarized light; measuring three light intensity grayscale images of the phase modulation value of the light-elastic modulator (7) at 0, π / 2 and π respectively from the reflection direction and the transmission direction of the beam splitter (4); and calculating the Jones matrix parameters of the beam splitter (4) in the reflection direction and the Jones matrix parameters in the transmission direction based on the three light intensity grayscale images.

7. The method for measuring polarization aberration of a high numerical aperture objective lens with elastic-optical modulation according to claim 5, characterized in that, In step S4, calculating the amplitude ratio and retardation distribution map of the high numerical aperture objective lens (5) to be tested includes: calculating the first ratio and the second ratio based on the grayscale images of the output light intensity measured in step S3; and solving the amplitude ratio and the retardation distribution map by combining the first ratio and the second ratio using trigonometric identities.

8. The method for measuring polarization aberration of a high numerical aperture objective lens with elastic-optical modulation according to claim 5, characterized in that, In step S3, the phase modulation amplitude is controlled to be π by adjusting the driving voltage of the elastic modulator (7).

9. The method for measuring polarization aberration of a high numerical aperture objective lens with elastic-optical modulation according to claim 5, characterized in that, In step S3, the LC control submodule provides a PWM drive signal to the LC resonant high voltage drive circuit, while the strobe lighting submodule controls the detection light source (1) to strobe under the control of the control signal sent by the computer, so that the detection light source (1) is lit only at the preset phase point.

10. The method for measuring polarization aberration of a high numerical aperture objective lens with elastic-optical modulation according to claim 5, characterized in that, In step S1, when calibrating the beam splitter (4), the measurement optical path does not include the high numerical aperture objective lens (5) to be measured and the spherical mirror (6).

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