Polarization chromatic aberration compensation method for acousto-optic tuning spectral imaging

Abstract

The invention belongs to the technical field of spectral imaging and polarization optics, provides a polarization chromatic aberration compensation method for polarization multiplexing type acousto-optic tuned filtering (AOTF) spectral imaging, and designs a polarization chromatic aberration compensation module comprising a liquid crystal polarization grating according to the polarization dependence characteristic of an acousto-optic tuned filtering device. And chromatic aberration compensation of polarization multiplexing acousto-optic tuning is realized by matching the period of the liquid crystal polarization grating. Non-biased incident light enters an acousto-optic tuned filter through a front light path to generate two beams of polarization-state orthogonal narrow-band diffraction light, and the two beams of polarization-state orthogonal narrow-band diffraction light are collimated by a rear light path and screened by a shielding diaphragm, and then enter a liquid crystal polarization compensation module consisting of a liquid crystal wave plate and a liquid crystal polarization grating to be corrected; and finally, the image is focused to the detector through the imaging module to realize combined imaging. The method does not need mechanical movement or electric control adjustment, the structure is light and thin, and integration is easy.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of spectral imaging and polarization optics, and particularly relates to a polarization chromatic aberration compensation method for polarization multiplexing Acousto-Optic Tunable Filter (AOTF) spectral imaging. BACKGROUND

[0002] AOTF is a filter type spectral device based on the principle of acousto-optic diffraction, which has the advantages of high spectral resolution, fast wavelength switching speed, no mechanical moving parts, and can realize frame imaging, and is widely used in spectral detection, polarization imaging, beam deflection and optical filtering. The AOTF-based spectral imaging system can cover a wide spectral range from visible light to infrared, and can realize flexible spectral modulation and imaging detection in different application scenarios.

[0003] In the AOTF spectral imaging system, light flux is an important indicator affecting the detection distance, signal-to-noise ratio and spectral sensitivity. In the traditional AOTF system, due to the selective nature of the acousto-optic interaction process of the acousto-optic tunable filter to the incident light polarization state, only the light that meets the specific polarization state and Bragg matching condition is diffracted with high efficiency, and the diffracted light corresponding to the other polarization state often has different exit angles and imaging positions, which is difficult to co-image with the effective signal in the wide spectrum scanning process, and is easy to evolve into stray light or ghost and reduce the imaging contrast, so in engineering, only one diffracted light is usually selected as the effective imaging signal, and the polarization state of the incident light is limited or the other polarization component in the exit light is suppressed through a polarizer. But this kind of polarization selection structure will inevitably introduce significant light flux loss, thereby limiting the application performance of the system in long-distance detection and weak signal conditions. At present, the research on improving the light flux of AOTF system at home and abroad is mostly concentrated on the device level, such as optimizing the crystal cut parameters or acousto-optic driving structure to improve the diffraction efficiency, however, such methods have limited effect on the system level light flux improvement, and the device preparation and debugging are difficult.

[0004] To further improve the system light flux, a polarization multiplexing AOTF spectral imaging scheme is proposed. The scheme removes the polarizer and simultaneously uses the two orthogonal polarization diffracted lights: ordinary light (ordinary-o) and extraordinary light (extraordinary-e) generated by the AOTF to perform combined imaging, thereby significantly improving the energy utilization. However, in the polarization multiplexing mode, due to the anisotropic characteristics of the birefringent acousto-optic crystal used by the AOTF, the diffraction angles of the o light and the e light do not change consistently with the working wavelength, resulting in a shift in the spatial position of the two polarized lights on the detector with the wavelength, forming a so-called polarization chromatic aberration. The polarization chromatic aberration is defined as the image center shift of the two polarized diffracted lights of the AOTF on the imaging surface of the detector, wherein the image center refers to the intensity center position of the corresponding light spot on the imaging surface of the detector. This problem is particularly prominent in wide spectral range working conditions or multi-wavelength tuning applications, and will have a significant impact on the spatial imaging quality and spectral response consistency of the system.

[0005] To solve the above-mentioned polarization chromatic aberration problem, the existing compensation methods mainly include multi-mirror group optical chromatic aberration correction schemes, but these methods usually have complex structure, large volume and high optical insertion loss, which are difficult to meet the needs of high flux and compact system design. In addition, the system-level modeling of AOTF ray tracing under polarization multiplexing conditions is still not perfect, which also restricts the engineering application of related compensation schemes to some extent.

[0006] Therefore, in view of the polarization chromatic aberration problem of the polarization multiplexing AOTF spectral imaging system under wide spectral range working conditions, which is difficult to effectively correct, the light flux is limited, and the existing compensation structure is complex, it is urgent to propose a polarization chromatic aberration compensation method with thin structure, passive high transmittance, and effective compensation of the deflection difference between o light and e light, to improve the imaging performance and spectral detection capability of the polarization multiplexing AOTF spectral imaging system under different working wavebands, and expand its application range in multi-scene and high-sensitivity passive spectral detection. SUMMARY

[0007] The purpose of the present application is to provide a polarization chromatic aberration compensation method for polarization multiplexing acousto-optic tunable spectral imaging, which effectively corrects the polarization chromatic aberration generated by the acousto-optic tunable filter in the polarization multiplexing mode by introducing a liquid crystal polarization compensation module into the system optical path, thereby improving the spatial imaging quality and spectral detection performance of the system under visible light to medium long wave wide spectral range working conditions, while maintaining the high light flux advantage in the polarization multiplexing mode.

[0008] In a polarization multiplexed acousto-optic tunable filter spectral imaging system, due to the anisotropic characteristics of the birefringent acousto-optic crystal used by the acousto-optic tunable filter, the exit angle variation laws of the two orthogonal polarization diffraction lights (o light and e light) generated by the acousto-optic tunable filter are inconsistent under different working wavelengths, which causes the spatial positions of the two polarization lights on the detector to shift with the wavelength, forming polarization chromatic aberration. This problem is particularly prominent in wide spectral range or multi-wavelength tuning applications, and seriously affects the spatial resolution and spectral response consistency of the system. The existing chromatic aberration compensation methods mainly correct through a multi-mirror group structure, which not only has a complex structure and a large volume, but also introduces high optical insertion loss, and it is difficult to meet the requirements of high light flux, wide spectral range applicability and compact system design in the polarization multiplexing mode. Therefore, a polarization chromatic aberration correction method with simple structure, passive high transmittance and effective compensation of the deflection difference between o light and e light with the change of working wavelength is urgently needed.

[0009] To solve the above technical problems, the present application provides a polarization chromatic aberration compensation method for acousto-optic tunable spectral imaging, and the technical scheme is as follows:

[0010] A liquid crystal polarization compensation module is introduced in the post light path of the acousto-optic tunable filter before the imaging light path, and the liquid crystal polarization compensation module is composed of a liquid crystal wave plate and a liquid crystal polarization grating arranged in sequence; wherein the liquid crystal wave plate is used to adjust the polarization state of the diffraction light emitted by the acousto-optic tunable filter, so that it meets the diffraction polarization condition of the liquid crystal polarization grating; the liquid crystal polarization grating is a continuous orientation structure and works in the +1 order diffraction mode, and the liquid crystal polarization grating applies angle compensation in opposite directions to the diffraction o light and e light emitted by the acousto-optic tunable filter through the characteristic that the diffraction angle of the liquid crystal polarization grating changes with the wavelength. In summary, the polarization chromatic aberration compensation light path of the polarization multiplexed acousto-optic tunable filter spectral imaging system includes a front light path, an AOTF device, a post light path, a liquid crystal polarization compensation module and an imaging module.

[0011] The diffraction light exit angle of the acousto-optic tunable filter in the polarization multiplexing mode can be simulated and calculated by a three-dimensional light ray tracing model, and can also be inversely calculated by the measured pixel difference on the detector. Combined with the obtained acousto-optic tunable filter diffraction angle, the grating period of the liquid crystal polarization grating can be reasonably designed so that the rate of change of the diffraction angle with the wavelength matches the rate of change of the o light and e light exit angles of the acousto-optic tunable filter in the polarization multiplexing mode, so that the spatial position difference of the two polarization beams on the detector is significantly reduced or even coincides, achieving the purpose of compensating the polarization chromatic aberration under the condition of wide spectral range.

[0012] The liquid crystal polarization compensation module is a passive optical structure, which does not need external electrically controlled driving or mechanical adjusting devices, and can be directly integrated into the polarization multiplexed acousto-optic tunable filter spectral imaging system.

[0013] Compared with the prior art, the present application has the following beneficial effects:

[0014] (1) The present application proposes an effective correction method based on a liquid crystal polarization compensation module for the polarization chromatic aberration problem caused by the wavelength variation of o light and e light in a polarization multiplexed acoustic-optic tuning filter spectral imaging system, which can significantly improve the spatial imaging consistency of the system under wide spectral band working conditions.(2) The present application uses a passive liquid crystal polarization compensation module, which does not introduce mechanical movement or electrically controlled adjustment structure, has the advantages of thin structure, high reliability and easy system integration, and is suitable for engineering application.(3) The compensation method of the present application does not introduce additional polarization selection loss, which can correct the polarization chromatic aberration while maintaining the high light flux advantage in the polarization multiplexing mode, and is beneficial to improve the signal-to-noise ratio and detection sensitivity of the system.(4) The present application is suitable for acoustic-optic tuning filter spectral imaging systems under different working wavebands and different center wavelengths, and has good universality and expandability. BRIEF DESCRIPTION OF DRAWINGS

[0015] The application will be further described below in conjunction with the drawings:

[0016] Figure 1 is a flow chart of the polarization chromatic aberration compensation method;

[0017] Figure 2 is a schematic diagram of the characterization of polarization chromatic aberration in a polarization multiplexed AOTF spectral imaging system

[0018] Figure 3 is a schematic diagram of the vector layout of acoustic-optic interaction in an AOTF crystal;

[0019] Figure 4 is a schematic diagram of the structure of a polarization multiplexed AOTF spectral imaging system after adding a liquid crystal polarization compensation module;

[0020] Figure 5 is a schematic diagram of the polarization chromatic aberration of a polarization multiplexed AOTF spectral imaging principle prototype under different working wavelengths;

[0021] Figure 6 is a schematic diagram of the imaging of a polarization multiplexed AOTF spectral imaging principle prototype with a liquid crystal polarization compensation module under different working wavelengths. DETAILED DESCRIPTION

[0022] The polarization chromatic aberration compensation method of the acoustic-optic tuning spectral imaging system proposed by the present application will be further described in detail below in conjunction with the drawings.

[0023] As Figure 1As shown, the polarization chromatic aberration compensation method of the present application mainly includes theoretical calculation, experimental measurement, device preparation and chromatic aberration correction four parts, the purpose is: in the polarization multiplexing acousto-optic tuning spectral imaging system, set the liquid crystal polarization compensation module, compensate the combined image polarization chromatic aberration generated by the acousto-optic tunable filter (AOTF) in the polarization multiplexing mode, so as to improve the spatial imaging consistency and spectral imaging performance of the system under the condition of wide spectral range. Among them, the polarization chromatic aberration can be calculated by the number of pixels characterized, such as Figure 2 As shown.

[0024] For the structure of the polarization multiplexing acousto-optic tuning spectral imaging system. According to the function division, the system includes pre light path, AOTF device, post light path and imaging module. The pre light path is composed of entrance pupil and pre lens group, which is used to constrain the aperture angle of the incident light beam to meet the incident angle working condition of AOTF; the AOTF is used as the spectral element of the system, which selectively outputs the diffraction light corresponding to the center wavelength under the acoustic driving frequency tuning; the post light path includes optical components for collimating or shaping the AOTF exit light, and a light blocking diaphragm can be set to suppress the influence of transmission light or full color stray light on imaging; the imaging light path includes imaging lens group and detector, which is used to focus the monochromatic light shaped by the post light path on the detector target surface, so as to obtain spectral image data under different wavelengths.

[0025] Because the exit angle of the two orthogonal polarization diffraction lights (diffraction o light and diffraction e light) generated by AOTF is inconsistent under different working wavelength conditions, specifically, under the given acousto-optic interaction geometry and phase matching condition, the exit angle of the diffraction light generated by AOTF usually decreases with the increase of the working wavelength, that is, the diffraction angle is inversely related to the wavelength; because the effective refractive index dispersion characteristics of the diffraction o light and the diffraction e light are different, the slope of the exit angle change with wavelength is also inconsistent, thereby gradually introducing the relative angle deviation between the two polarization diffraction lights in the wide spectrum scanning process. The image center position of the two polarization lights on the detector is shifted with the wavelength, and then the polarization chromatic aberration is generated and affects the combined image imaging. The present application introduces a liquid crystal polarization compensation module between the post light path and the imaging light path of the system, which realizes the compensation of the polarization chromatic aberration. The specific steps of the method are described below.

[0026] Before implementing the present application, the structure parameters and working parameters of the polarization multiplexing acousto-optic tuning spectral imaging system need to be determined first. The structure parameters can include: the light aperture of the AOTF device , the cut type parameters of the crystal (the incident cut angle , the ultrasonic cut angle , the exit cut angle ), the optical structure of the pre and post light path of the system (the focal length of the pre lens and back lens focal length ) form and relative position (front lens distance from AOTF front surface and back lens distance from AOTF back surface ), imaging lens group focal length and detector pixel size , etc.

[0027] The working parameters can include: working wavelength , AOTF drive frequency , incident angle , detector pixel size . The above parameters are used to provide uniform boundary conditions for subsequent experimental measurements and theoretical calculations.

[0028] After the parameters are clear, the polarization multiplexing AOTF spectral imaging system optical path is built. Different from the traditional AOTF spectral imaging optical path, the optical path does not set an orthogonal polarizer for selecting a single polarization state in the front light path, so that the diffracted o light and the diffracted e light output by the AOTF can form a multiplexed image on the same detector. At the same time, a light-shielding diaphragm with appropriate size should be set in the rear light path to avoid the influence of transmitted light or full-color stray light on imaging.

[0029] When the optical path is built, the imaging data in the polarization multiplexing mode is collected under different working wavelengths (drive frequencies), and the image center positions of the diffracted o light and the diffracted e light on the detector under the same wavelength are extracted to obtain the corresponding image center offsets. In an embodiment, the polarization chromatic aberration is characterized by pixel difference , that is, the distance between the two polarization image centers in the detector coordinate system; in other embodiments, it can also be converted into actual displacement of the imaging plane or equivalent angle difference. Thus, the experimental data set of the polarization chromatic aberration with wavelength change is obtained.

[0030] While building the system optical path and collecting the measured polarization chromatic aberration, the modeling work of the three-dimensional ray tracing of the polarization multiplexing AOTF system is carried out. Based on the system structure and working parameters and the AOTF acousto-optic diffraction mechanism, a three-dimensional ray tracing model for the polarization multiplexing mode is established to describe the propagation direction and spatial position of the diffracted o light and the diffracted e light in the system. The model at least includes the following necessary links:

[0031] AOTF usually employs a birefringent crystal (e.g. TeO2) as the optical medium. The propagation of light inside the AOTF is affected by two main mechanisms: (1) the refraction at the entrance and exit faces of the crystal, which follows Snell's law; (2) the acousto-optic interaction inside the crystal, where the incident light couples with the ultrasonic wave and diffracts, thus changing its wave vector direction. In the polarization multiplexing mode, the unpolarized incident light first experiences birefringence and splits into two orthogonally polarized transmitted light components (o-light and e-light). The refraction of the transmitted light beams at the crystal surface follows Snell's law:

[0032] (1)

[0033] The refractive indices of the birefringent crystal medium are denoted as , which can be further separated into the ordinary refractive index and the extraordinary refractive index . Both of them are wavelength-dependent functions, representing the refractive response of the material perpendicular to the optical axis of the crystal. For TeO2 material, these refractive indices can be accurately calculated using the empirical Sellmeier dispersion equation.

[0034] These light beams interact with the ultrasonic wave in the acousto-optic interaction region, producing diffracted light beams through the acousto-optic interaction. Figure 3 The process of momentum matching for o-light incidence and e-light incidence is shown. Here, the incident shear angle is defined as the angle between the AOTF entrance face and the crystal

[110] axis, representing the inclination of the incident light beam in the crystal coordinate system. The ultrasonic shear angle is the angle between the surface of the piezoelectric transducer and the crystal optical axis

[001] , which determines the propagation direction of the acoustic wave and the direction of the acoustic energy vector inside the crystal. These two angles play a crucial role in determining the vector projection and geometric relationship of the acousto-optic interaction. In this interaction, the wave vectors , and represent the incident light, diffracted light and ultrasonic wave, respectively. When the following momentum matching conditions are met, the diffraction reaches the best state:

[0035] (2)

[0036] The incident light , diffracted light and ultrasonic wave can be calculated as:

[0037] (3)

[0038] The propagation directions of the transmitted and diffracted light in the AOTF crystal can be determined by the above-mentioned calculation tool combined with known system and working parameters. When the light beams reach the exit surface of the AOTF crystal, refraction will occur again according to Snell's law, and finally four light beams will be generated at the exit surface of the AOTF: two transmitted light beams (T-o-light and T-e-light) and two diffracted light beams (D-e-light and D-o-light). The four light beams exhibit spatial separation due to their different propagation angles. Using the calculation model proposed in the present patent, the exit angles (Θ 、 、 、 ) and spatial positions (Ψ 、 、 、 ) of the light beams can be accurately obtained. Thus, according to the calculated angle and position parameters, the necessary inputs can be provided for subsequent analysis of the separation conditions between the transmitted and diffracted light beams, the calculation of the optimal aperture of the obscuration stop, and the calculation of the matching period of the liquid crystal polarization grating. Meanwhile, based on the existing parameters, the image center positions of the diffracted o-light and e-light on the detector can also be calculated theoretically.

[0039] Next, for the calculation of the matching period of the liquid crystal polarization grating, two approaches can be used: forward derivation calculation based on ray tracing modeling and reverse calculation based on measured pixel difference. The specific algorithm is described as follows:

[0040] For the forward derivation calculation of the model, the known AOTF exit diffracted light 、 has different directions relative to the optical axis, so the directions are opposite. Taking the angle bisector direction of the two light beams as the new optical axis, the angles of the two light beams incident on the polarization grating should be:

[0041] (4)

[0042] According to the grating equation of the liquid crystal polarization grating, for the orthogonal polarized light with an incident angle of , if the ±1st order diffracted light ± of the two light beams is to be modulated to have the same propagation direction, the following condition should be satisfied:

[0043] (5)

[0044] After rearrangement, we get:

[0045] (6)

[0046] Accordingly, the matching period value of the liquid crystal polarization grating can be calculated based on the ray tracing model.​

[0047] And for the measured pixel difference back calculation, the idea is: first, the actual measurement of the detector on the diffraction o light and diffraction e light heart distance difference, and then back to the incident angle of different wavelengths of light incident camera lens, according to the grating period calculation formula, the calculation can be corrected to parallel light of the incident camera lens period value, different wavelengths of diffraction light can be parallel to each other incident camera, at the right focus, can realize the spectrum of the image, form a polarization multiplexing image, the specific derivation process is as follows:

[0048] In the wide spectrum application, under different wavelength conditions, the matched grating period when the polarization chromatic aberration is corrected is not completely consistent, so in the subsequent design process, it is also necessary to select a reasonable grating period value for the specific application spectrum, so as to obtain a grating period which makes the polarization chromatic aberration of the system after compensation minimum.

[0049] Now set the distance between the imaging lens and the image plane to be fixed and equal to the focal length of the lens According to the principle of geometric optics, we can calculate:

[0050] (7)

[0051] When using liquid crystal polarization compensation module to compensate for different wavelength deflection angles, the ideal situation is that the light beam deflection characteristics should satisfy the grating equation:

[0052] (8) Therefore, in order to compensate the light beam with arbitrary incident angle into parallel light beam incident into the imaging lens, the liquid crystal polarization grating period should satisfy:

[0053] (9)

[0054] The core idea of liquid crystal polarization compensation is to make the equivalent image plane chromatic aberration caused by the difference in diffraction angle of liquid crystal polarization grating with matching period at two characteristic wavelengths equal to the measured chromatic aberration difference of polarization multiplexing AOTF system. Now select two characteristic wavelengths to optimize the design of liquid crystal polarization grating period. The measured pixel difference of polarization multiplexing AOTF spectral imaging system ) is defined as:

[0055] (10)

[0056] In the formula and are the measured pixel differences of AOTF system at two characteristic wavelengths. At the same time, according to the grating equation, the diffraction angle difference of liquid crystal polarization grating at two characteristic wavelengths ) can be obtained as:

[0057] (11)

[0058] wherein and correspond to two characteristic wavelengths respectively. Further, according to the principle of geometrical optics, the equivalent to the multiplexing image aberration (MIA) is:

[0059] (12)

[0060] Combining the above formulas, and making the measured dispersion difference of the liquid crystal polarization grating and the polarization multiplexing AOTF system equal, i.e. , then the following can be obtained:

[0061] (13)

[0062] wherein, except that the grating period is a known value, the matching grating period value for simultaneously achieving polarization chromatic aberration correction of two characteristic wavelengths can be obtained by calculation. For the working condition of a wide spectral range, the matching grating period is not determined only according to the calculation of two characteristic wavelengths, but the center wavelength of the set spectral range is taken as the reference wavelength, the polarization chromatic aberration compensation requirements of each target working wavelength in the spectral range relative to the center wavelength are calculated respectively, and then the matching grating period values corresponding to different wavelengths are obtained. Through comprehensive evaluation of the above results, the grating period which can make the overall polarization chromatic aberration of each wavelength in the entire working spectral range minimum or remain within an acceptable range is selected as the final design parameter, so as to realize the overall optimization and compensation of the polarization chromatic aberration of the wide spectral range. The above is the algorithm for calculating the grating period by reverse calculation of the measured pixel difference.

[0063] According to the matching period parameters of the liquid crystal polarization grating determined by the above steps, the further work is to prepare the liquid crystal polarization grating and the liquid crystal 1 / 4 wave plate, and the two are combined to form a liquid crystal polarization compensation module. In one embodiment, the preparation of the liquid crystal polarization grating can adopt the orientation control method based on polarization modulation exposure. According to the grating period parameters calculated in the step, the related parameters in the polarization modulation exposure light path are set so that the spatial polarization distribution formed in the exposure process meets the grating period requirement, thereby guiding the liquid crystal molecules to form a periodic change in orientation structure on the substrate surface. In this way, the liquid crystal polarization grating can produce the expected diffraction angle characteristics in the designed working wavelength range, which is used for directional compensation of the diffracted light beam of the AOTF in the polarization multiplexing mode.

[0064] ​Meanwhile, to meet the compensation requirements of the system under specific central wavelength or wide spectrum conditions, the device thickness parameters of the liquid crystal polarization grating and the liquid crystal wave plate can be reasonably selected, so that the liquid crystal device meets the half-wave condition at the target working wavelength. By adjusting the thickness of the liquid crystal layer and the birefringence characteristics of the material, the polarization modulation requirements of the liquid crystal polarization compensation module under different central wavelengths can be realized. Meanwhile, the liquid crystal device can be designed to be wide-spectrum by means of chiral agent doping and multi-layer structure design, so as to improve the compensation stability and applicability of the liquid crystal device under wide spectrum conditions.

[0065] The liquid crystal polarization compensation module prepared in the above manner is preferably a passive optical structure, which does not require external electrically controlled driving or mechanical adjusting devices in the working process, has the advantages of thin structure, high stability and easy system integration, and is suitable for being integrated into a polarization multiplexing acoustic-optic tuning spectral imaging system to realize subsequent polarization chromatic aberration correction.

[0066] As shown in Figure 4 , the prepared liquid crystal polarization compensation module is introduced into the polarization multiplexing acoustic-optic tuning spectral imaging system and arranged between the rear light path and the imaging light path of the system. The specific position can be located after the rear collimating light path and before the imaging light path. Under this light path configuration, the diffracted o light and the diffracted e light output by the acoustic-optic tuning filter (AOTF) are collimated in the rear light path and then incident on the liquid crystal polarization compensation module with different propagation directions.

[0067] The liquid crystal wave plate in the liquid crystal polarization compensation module is used to modulate the polarization state of the incident light beam so as to meet the diffraction polarization condition of the liquid crystal polarization grating. The liquid crystal polarization grating exerts angle compensation with opposite directions and controllable sizes on the two orthogonal polarization diffracted lights according to the polarization selective diffraction characteristics. Through the compensation effect, the propagation directions of the diffracted o light and the diffracted e light, which originally have image center offsets on the detector imaging surface due to the birefringence characteristics of the AOTF, are corrected, so that the two polarization lights tend to propagate coaxially in the imaging light path. After being focused by the imaging light path, the image center offset of the two polarization lights on the detector target surface is significantly reduced, thereby realizing the correction of the combined image polarization chromatic aberration under the polarization multiplexing mode.

[0068] A prototype is built based on the technical scheme of the present application and experimental tests are carried out. As shown in Figure 5 Without introducing the liquid crystal polarization compensation module, the system produces obvious combined image polarization chromatic aberration on the detector imaging surface in the working wavelength range of 3.7-4.8 μm due to the difference in the exit angles of the two diffracted lights output by the AOTF under the polarization multiplexing mode. Taking a detector with an image element size of 30 μm as an example, the measured polarization chromatic aberration can reach the order of multiple pixels under some wavelength conditions, and the maximum chromatic aberration is greater than 6 pixels, which significantly affects the spatial imaging quality of the system.

[0069] Under the same experimental conditions, after introducing the liquid crystal polarization compensation module into the system, as shown in Figure 6 , the compensated polarization multiplexed image shows good spatial consistency in the whole 3.7-4.8 μm wavelength band. The experimental results show that after correction by the liquid crystal polarization compensation module, the image center shift of the two polarization lights on the imaging plane of the detector is significantly reduced, and the corresponding polarization chromatic aberration is less than 1 pixel in the whole working wavelength band, which is significantly better than that in the uncompensated state.

[0070] By comparing the imaging results of the system under different working wavelengths before and after introducing the liquid crystal polarization compensation module, it can be seen that the polarization chromatic aberration compensation method proposed in the present application can effectively suppress the spatial inconsistency caused by the change of polarization chromatic aberration with wavelength in the polarization multiplexing mode. At the same time, the compensation process does not need to introduce additional polarization selection elements, and will not destroy the high light flux advantage brought by simultaneously using two diffracted lights in the polarization multiplexing mode.

[0071] Through the above content, while maintaining the high flux characteristic of the polarization multiplexing acousto-optic tuning spectral imaging system, the present application realizes effective correction of the polarization chromatic aberration under wide spectral range conditions, thereby significantly improving the imaging quality and spectral imaging performance of the system.

[0072] The contents not described in detail in the specification of the present application belong to the prior art known to those skilled in the art.

Claims

1. A method for compensating polarization chromatic aberration in acousto-optic tuned spectral imaging, characterized in that, To address the polarization-dependent characteristics of the acousto-optic tuned filter (AOTF), a polarization chromatic aberration compensation module incorporating a liquid crystal polarization grating is designed. The chromatic aberration compensation for polarization multiplexing acousto-optic tuning is achieved by matching the period of the liquid crystal polarization grating. The polarization chromatic aberration compensation optical path includes a front optical path (1), an AOTF device (2), a rear optical path (3), a liquid crystal polarization compensation module (4), and an imaging module (5). The grating period of the liquid crystal polarization grating in the liquid crystal polarization compensation module (4) is designed to match the rate of change of its diffraction angle with wavelength with the rate of change of the angle of the two diffracted beams emitted from the AOTF. Utilizing the characteristic of the diffraction angle of the liquid crystal polarization grating changing with wavelength, directional compensation is performed on the two diffracted beams emitted from the AOTF, achieving spatial overlap of the two polarized beams on the detector plane. This enables polarization chromatic aberration correction under wide spectral conditions, improving the spatial consistency and spectral imaging performance of the system.

2. The polarization chromatic aberration compensation method for acousto-optic tuned spectral imaging as described in claim 1, characterized in that, The liquid crystal polarization compensation module is located between the AOTF rear optical path and the imaging optical path, and the liquid crystal waveplate and the liquid crystal polarization grating are sequentially arranged within the liquid crystal polarization compensation module.

3. The polarization chromatic aberration compensation method for acousto-optic tuned spectral imaging as described in claim 1, characterized in that, The liquid crystal polarization compensation module is a passive optical structure, requiring no external electronic drive or mechanical adjustment device.

4. The polarization chromatic aberration compensation method for acousto-optic tuned spectral imaging as described in claim 1, characterized in that, The liquid crystal waveplate is used to adjust the polarization state of the two diffracted beams emitted from the AOTF so that they meet the diffraction polarization conditions of the liquid crystal polarization grating.

5. The method for polarization chromatic aberration compensation in acousto-optic tuned spectral imaging as described in claim 1, characterized in that, The liquid crystal polarization compensation module provides angular compensation for the diffracted o-beam and diffracted e-beam of the AOTF in opposite directions. Through this module, the spatial position difference between the two polarized beams on the detector can be significantly reduced under broadband conditions.

6. The polarization chromatic aberration compensation method for acousto-optic tuned spectral imaging as described in claim 1, characterized in that, The liquid crystal polarization grating has a continuous orientation structure and operates in ±1 order diffraction modes to improve diffraction efficiency and compensation stability over a wide spectral range.

7. The method for polarization chromatic aberration compensation in acousto-optic tuned spectral imaging as described in claim 1, characterized in that, The polarization chromatic aberration compensation method is applicable to polarization multiplexing imaging modes operating under broadband conditions from visible light to medium and long wavelengths.

8. The polarization chromatic aberration compensation method for acousto-optic tuned spectral imaging as described in claim 1, characterized in that, The method can maintain the high luminous flux characteristics of the polarization multiplexing mode.

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