Method and device for recovering polarization of deep medium based on feedback of polarized photoacoustic signal

Abstract

The present application relates to the technical field of optical regulation, in particular to a deep medium polarization recovery method and device based on polarization photoacoustic signal feedback, comprising: pulsed laser is incident into a first SLM, and the absolute phase of light is updated by photoacoustic signal to realize the regulation of absolute phase; a half-wave plate, a second SLM and a quarter-wave plate constitute a polarization regulation channel, and the polarization state of light is updated by linear polarization degree to realize the polarization regulation of relative phase; and the additional absolute phase caused in polarization regulation is calculated and reversely applied on the first SLM for compensation, and finally the polarization regulation is realized. The method proposed in the present application overcomes the limitation that the traditional optical imaging is difficult to realize the simultaneous regulation of phase-polarization, and through accurate phase-polarization regulation, different feedback signals are continuously iterated to realize the light energy enhancement and polarization state recovery of deep scattering medium, which can be applied to the polarization detection field of biomedicine.

Description

Technical Field

[0001] This invention relates to the field of optical control technology, and in particular to a method and apparatus for deep medium polarization recovery based on polarized optical acoustic signal feedback. Background Technology

[0002] The extreme sensitivity of light signals to changes or inhomogeneities in the medium makes light a promising tool for detecting or processing target media. Many exciting optical technologies and instruments have been invented that utilize this sensitivity, making significant contributions to the advancement of modern science and technology. More importantly, the vector properties of light play an indispensable role in light-matter interactions, light transmission, and imaging. However, when light propagates through strongly deep scattering media such as biological tissues, it undergoes multiple scattering, leading to wavefront distortion and energy defocusing. Simultaneously, the incident polarization state is randomly disturbed. Especially when propagation perturbations occur in deep regions (typically greater than one scattering mean free path), the absolute phase of the light is disrupted, and polarization information is almost completely lost, severely limiting the penetration depth and imaging quality of polarization-dependent optical imaging and detection technologies.

[0003] In existing technologies, wavefront shaping or polarization manipulation techniques are commonly used to address the multiple scattering problem of light in deep scattering media. However, wavefront shaping techniques often use light intensity, fluorescence, or speckle as feedback signals to compensate for scattering phase distortion. This approach only achieves phase focusing and lacks polarization recovery capabilities. Polarization manipulation techniques often rely on fixed waveplates and polarizers or vector transfer matrices to control the polarization of scattered light. This approach lacks phase compensation, suffers from high energy loss, and requires precise matrix measurement, making it difficult to achieve dynamic polarization optimization under conditions of strong deep scattering. For example, patent document (CN114894799A) discloses a focusing device and method for penetrating deep scattering media based on a polarization transfer matrix. This method uses coaxial interferometry to measure the transfer matrix to study the polarization characteristics of deep scattering media, effectively restoring the disturbance of polarization information by the deep scattering media and achieving focusing through the deep scattering media. However, the aforementioned existing technical solutions can only achieve phase focusing or polarization focusing, failing to meet the dual requirements of polarization detection and high-energy focusing in deep scattering media. Summary of the Invention

[0004] Based on this, and addressing the shortcomings of existing technologies such as polarization disorder, energy defocusing, and the inability to coordinate phase-polarization optimization in deep scattering media, this invention provides a method and apparatus for deep media polarization recovery based on polarized photoacoustic signal feedback. Using pulsed laser as the excitation source, and leveraging the phase delay principle of anisotropic molecules on light, combined with the Jones matrix method, simultaneous control of phase and polarization is achieved. Through feedback of different signals, the phase or polarization of light is iteratively updated, maximizing the photoacoustic signal and linear polarization degree, thus realizing synchronous closed-loop optimization of light energy enhancement and polarization state recovery within the deep scattering medium.

[0005] In one embodiment, the present invention provides a deep medium polarization restoration method based on polarized photoacoustic signal feedback, comprising: S10, control the pulsed laser to emit pulsed laser and obtain small spot linearly polarized light after beam expansion and collimation, and perform absolute phase modulation on the small spot linearly polarized light through the first spatial light modulator, so that the long axis of the liquid crystal molecules of the first spatial light modulator is consistent with the electric vector direction of the small spot linearly polarized light; S20, a polarization control channel is formed by the first polarization controller, the second spatial light modulator and the second polarization controller, and the polarization control channel is used to control the small spot linearly polarized light emitted from the first spatial light modulator by the Jones vector matrix. S30, calculate the geometric absolute phase introduced during polarization modulation, and invert the calculated geometric absolute phase to the first spatial light modulator, and end when it is determined that the enhancement factor and linear polarization degree of the acquired photoacoustic signal have reached their respective convergence thresholds.

[0006] Furthermore, S10 includes: S101, The energy of the pulsed laser emitted by the pulsed laser is controlled to obtain horizontally polarized light; S102, the horizontally polarized light obtained after energy control is expanded and collimated to obtain small-spot linearly polarized light, and the small-spot linearly polarized light obtained after expansion and collimation is incident on the first spatial light modulator for absolute phase modulation. S103, under the modulation of the third polarization controller, the photoacoustic signals generated by the anisotropic absorber at four different rotation angles are acquired, and these signals are used as feedback signals to iteratively update the absolute phase distribution of the first spatial light modulator to achieve absolute phase modulation.

[0007] Furthermore, in S103, a continuous sequence iterative algorithm is used to iteratively update the absolute phase distribution of the first spatial light modulator; wherein, the optimal phase calculation formula is as follows:

[0008] in, This represents the optimal phase of the i-th group in the first spatial light modulator. , , , Let i represent the photoacoustic signals corresponding to different rotation angles, and let i be the number of macro-pixel groups divided by the first spatial light modulator.

[0009] Furthermore, S20 includes: S201, establish the Jones vector matrices of the small-spot linearly polarized light incident on the polarization control channel, the first polarization controller, the second spatial light modulator, and the second polarization controller, and obtain the Jones vector matrix of the outgoing small-spot linearly polarized light based on the Jones vector matrices of the incident small-spot linearly polarized light, the first polarization controller, the second spatial light modulator, and the second polarization controller. S202, Based on the Jones vector matrix corresponding to the emitted small-spot linearly polarized light, establish the mapping relationship between the phase of the second spatial light modulator and the target polarization angle of the emitted small-spot linearly polarized light; S203. Based on the mapping relationship between the phase of the second spatial light modulator and the target polarization angle of the emitted small spot linearly polarized light, under the modulation of the third polarization controller, the photoacoustic signals generated by the anisotropic absorber at four different rotation angles are obtained. The photoacoustic Stokes vector is constructed according to the photoacoustic signals at different rotation angles and the degree of linear polarization is calculated. The calculated degree of linear polarization is used as a feedback signal to iteratively update the phase of the second spatial light modulator until the degree of linear polarization converges to the maximum value.

[0010] Furthermore, in S201, the Jones vector matrix of the small-spot linearly polarized light incident on the polarization control channel is represented as:

[0011] The major axis is at an angle to the horizontal direction. The Jones vector matrix of the first polarization controller is represented as:

[0012] The Jones vector matrix of the second spatial light modulator is represented as follows:

[0013] The major axis is at an angle to the horizontal direction. The Jones vector matrix of the second polarization controller is represented as:

[0014] The Jones vector matrix of the emitted small-spot linearly polarized light is then expressed as:

[0015] In S202, the mapping relationship between the phase of the second spatial light modulator and the target polarization angle of the emitted small-spot linearly polarized light is expressed as follows:

[0016] in, This refers to the phase of the second spatial light modulator. The target polarization angle is the linearly polarized light emitted as a small spot.

[0017] Furthermore, in S203, photoacoustic signals under different polarization states are modulated by the third polarization controller to construct Stokes vectors; the constructed Stokes vectors are represented as follows:

[0018]

[0019]

[0020] in, , , , These are the photoacoustic signals generated by the anisotropic absorber at rotation angles of 0°, 45°, 90°, and 135°, respectively. linear polarization degree The calculation formula is as follows: .

[0021] Furthermore, in S30, the formula for calculating the geometric absolute phase is as follows:

[0022] in, The Jones vector matrix represents the small-spot linearly polarized light incident on the polarization control channel. The Jones vector matrix representing the polarization state emitted from the target. This represents the conjugate transpose, used to calculate the inner product of two polarization states. This indicates that the phase value is taken.

[0023] In one embodiment, the present invention also provides a deep medium polarization restoration device based on polarized photoacoustic signal feedback, used to implement the deep medium polarization restoration method described in the above embodiments, the device comprising: Pulsed laser excitation module, used to emit pulsed laser light; The control module, connected to the pulsed light excitation module, is used to control the phase and polarization of the pulsed laser. A photoacoustic detection module, connected to the control module, is used to generate a photoacoustic signal under the action of the pulsed laser and to collect and process the generated photoacoustic signal; The signal processing module is connected to the pulsed light excitation module, the modulation module, and the photoacoustic detection module, respectively, and is used to acquire photoacoustic signals at different rotation angles, and use the acquired photoacoustic signals at different rotation angles as feedback signals to control and update the phase and polarization of the pulsed laser.

[0024] Furthermore, the pulsed light excitation module includes a pulsed laser, an aperture, a polarizer, and a beam expander collimator connected in sequence; the pulsed laser output from the pulsed laser passes through the aperture, the polarizer, and the beam expander collimator in sequence to obtain parallel small-spot linearly polarized light; The control module includes a first reflecting mirror, a first spatial light modulator, a first polarization controller, a second reflecting mirror, a second spatial light modulator, a second polarization controller, and a third polarization controller connected in sequence; wherein, the first reflecting mirror is connected to the beam expander collimator, and the incident small-spot linearly polarized light passes through the first reflecting mirror, the first spatial light modulator, the first polarization controller, the second reflecting mirror, the second spatial light modulator, the second polarization controller, and the third polarization controller in sequence to obtain the outgoing small-spot linearly polarized light; The photoacoustic detection module includes a deep scattering medium, an anisotropic absorber, a focused ultrasound probe, a filter, and an amplifier. The third polarization controller is connected to the deep scattering medium, and the deep scattering medium and the anisotropic absorber are arranged adjacent to each other along the optical axis. The small-spot linearly polarized light emitted from the third polarization controller passes through the deep scattering medium and the anisotropic absorber, causing the anisotropic absorber to generate a photoacoustic signal. The focused ultrasound probe collects the generated photoacoustic signal, which is then amplified by the filter and the amplifier to obtain a processed photoacoustic signal. The signal processing module includes a computer and a rotating platform. The computer is connected to the pulsed laser, the first spatial light modulator, and the second spatial light modulator, and is also connected to the rotating platform via a motion control card. The anisotropic absorber is mounted on the rotating platform. The computer drives the anisotropic absorber to rotate around the optical axis to different rotation angles by controlling the rotation of the rotating platform. The computer uses the photoacoustic signals of the anisotropic absorber at different rotation angles as feedback signals to control the first spatial light modulator to iteratively optimize the corresponding absolute phase distribution, thereby achieving energy focusing. The computer constructs a photoacoustic Stokes vector based on the photoacoustic signals at different rotation angles and calculates the degree of linear polarization. Using the calculated degree of linear polarization as feedback signals, the computer controls the second spatial light modulator to iteratively optimize the corresponding polarization modulation phase, thereby achieving polarization state recovery.

[0025] In one embodiment, the present invention also provides a computer device, including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps of the deep dielectric polarization recovery method described in the above embodiments.

[0026] Compared with the prior art, the present invention has the following beneficial effects: 1. Strong deep polarization recovery capability: It adopts polarized photoacoustic signal (Stokes vector / degree of linear polarization) as wavefront shaping feedback, which breaks through the limitation of traditional optical feedback that cannot penetrate deep scattering medium, and realizes active recovery of deep polarization information of biological tissue.

[0027] 2. Phase-polarization synergistic optimization: Dual SLMs construct phase control channels and polarization control channels respectively. With geometric phase compensation, precise phase-polarization control is achieved through continuous iteration of different feedback signals, eliminating the interference of polarization modulation on phase focusing and simultaneously realizing deep light energy focusing and maximum recovery of polarization state.

[0028] 3. High iteration efficiency and good stability: The two-stage serial iteration mechanism first focuses the phase and then optimizes the polarization. After compensation, the amplitude fluctuation is controllable, the anti-noise ability is strong, and the convergence speed is fast.

[0029] 4. Simple optical path and wide applicability: Based on the combination of standard spatial light modulator and waveplate, it has a compact structure and low energy loss. It can be adapted to femtosecond / nanosecond pulse light sources and is suitable for fields such as biomedical polarization detection and deep tissue imaging. Attached Figure Description

[0030] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Furthermore, the accompanying drawings are not drawn to a 1:1 scale, and the relative dimensions of the various components are shown in the drawings only as examples and not necessarily to actual scale.

[0033] Figure 1 A schematic diagram of a deep medium polarization restoration device based on polarized photoacoustic signal feedback provided in an embodiment of the present invention; Figure 2A schematic flowchart of a deep medium polarization restoration device based on polarized photoacoustic signal feedback provided in an embodiment of the present invention; Figure 3 This is a polarization detection diagram of the emitted small-spot linearly polarized light according to an embodiment of the present invention; Figure 4 This is a structural block diagram of a computer device provided in an embodiment of the present invention.

[0034] Explanation of reference numerals in the attached figures: 1-1 pulsed laser, 1-2 aperture, 1-3 polarizer, 1-4 beam expander and collimator, 2-1 first reflecting mirror, 2-2 first spatial light modulator (SLM1), 2-3 first polarization controller, 2-4 second reflecting mirror, 2-5 second spatial light modulator (SLM2), 2-6 second polarization controller, 2-7 third polarization controller, 3-1 deep scattering medium, 3-2 anisotropic absorber, 3-3 focusing ultrasound probe, 3-4 filter, 3-5 amplifier, 4-1 computer, 4-2 rotating platform. Detailed Implementation

[0035] To make the above-mentioned objects, features, and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. Many specific details are set forth in the following description to provide a thorough understanding of the present invention. However, the present invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0036] The deep medium polarization restoration method and device based on polarization photoacoustic signal feedback proposed in this invention overcomes the limitation of traditional optical imaging in achieving simultaneous phase-polarization control. Through precise phase-polarization control and continuous iteration with different feedback signals, the light energy enhancement and polarization state restoration of the deep scattering medium can be achieved, which can be applied to polarization detection fields such as biomedicine.

[0037] Specifically, please refer to Figure 1 , Figure 1A schematic diagram of the architecture of a deep medium polarization restoration device based on polarization photoacoustic signal feedback according to an embodiment of the present invention is shown, including a pulsed light excitation module, a modulation module, a photoacoustic detection module, and a signal processing module; the pulsed light excitation module, the modulation module, and the photoacoustic detection module are connected sequentially, and the signal processing module is connected to the pulsed light excitation module, the photoacoustic detection module, and the modulation module respectively; wherein, the pulsed light excitation module is used to emit pulsed laser, the modulation module is used to modulate the phase and polarization of the pulsed laser; the photoacoustic detection module is used to generate photoacoustic signals under the action of pulsed laser and to collect and process the generated photoacoustic signals; the signal processing module is used to acquire photoacoustic signals at different polarization angles and use the acquired photoacoustic signals at different polarization angles as feedback signals to control and update the modulation of the phase and polarization of the pulsed laser.

[0038] Furthermore, the pulsed light excitation module includes a pulsed laser 1-1, an aperture 1-2, a polarizer 1-3, and a beam expander / collimator 1-4 connected in sequence. In this embodiment, a suitable incident light polarization state can be obtained through the pulsed light excitation module. The pulsed laser output from the pulsed laser 1-1 passes sequentially through the aperture 1-2 and the polarizer 1-3 for energy control to filter out stray light and generate fixed horizontally linearly polarized light. The horizontally linearly polarized light obtained after energy control is shaped into parallel small-spot horizontally linearly polarized light by the beam expander / collimator 1-4, providing a stable incident light source for subsequent adjustment. In this embodiment, the beam expander / collimator 1-4 is composed of a first lens, a pinhole, and a second lens, and its beam expansion ratio is determined by the focal lengths of the first and second lenses.

[0039] Optionally, the pulsed laser is a fiber pulsed laser with a wavelength of 1035 nm and a pulse width of 300 fs; the polarizer 1-3 is a linear polarizer with its major axis in the horizontal direction, which can output pure linearly polarized small spot pulsed light; the first lens and the second lens of the beam expander collimator 1-4 are both convex lenses, with the first lens having a focal length of 40 mm and the second lens having a focal length of 60 mm, and the beam expansion ratio is 1.5 times.

[0040] Furthermore, the control module includes a first reflector 2-1, a first spatial light modulator (SLM1) 2-2, a first polarization controller 2-3, a second reflector 2-4, a second spatial light modulator (SLM2) 2-5, a second polarization controller 2-6, and a third polarization controller 2-7 connected in sequence; wherein, SLM1 is used for absolute phase control and geometric phase compensation; and SLM2 is used for polarization state modulation.

[0041] Specifically, in this embodiment, to achieve coordinated control of phase and polarization, the device employs a dual-pure-phase spatial light modulator (SLM) architecture. The horizontally linearly polarized light of the small spot obtained after beam expansion and collimation is incident on SLM1 via the first reflecting mirror 2-1. The long axis of the SLM1 liquid crystal is parallel to the polarization direction of the incident horizontally linearly polarized light, which can cause changes in the absolute phase at different positions on the entrance pupil plane. By applying absolute phase modulation to the light field using SLM1, focusing of light energy within the deep scattering medium can be achieved. Subsequently, the beam of the horizontally linearly polarized light of the small spot sequentially passes through the first polarization controller 2-3, the second reflecting mirror 2-4, SLM2, the second polarization controller 2-6, and the third polarization controller 2-7 for polarization control. The first polarization controller 2-3, SLM2, and the second polarization controller 2-6 are combined to form a polarization control channel. The incident small-spot horizontal linearly polarized light passes through the first polarization controller 2-3, SLM2, and the second polarization controller 2-6 in sequence. The outgoing small-spot horizontal linearly polarized light changes with the phase of the light. As SLM2 changes from 0 to 2π, the polarization state also changes from 0 to π. Thus, the polarization control channel formed above achieves active control of the linear polarization state. The third polarization controller 2-7 is used to correct the direction of the linear polarization state of the outgoing small-spot linearly polarized light, eliminate the polarization reference offset introduced by SLM2 modulation, and ensure that the polarization direction of the incident light into the deep scattering medium is aligned with the system coordinate system, thereby improving the polarization recovery accuracy and optical path stability.

[0042] Optionally, SLM1 and SLM2 are liquid crystal spatial light modulators with wavelengths from 1000 nm to 1100 nm and frequencies of 60 Hz, capable of external triggering synchronization control; the first polarization controller 2-3 is a half-wave plate with a major axis at an angle of 22.5° to the horizontal direction, the second polarization controller 2-6 is a quarter-wave plate with a major axis at an angle of 45° to the horizontal direction, and the third polarization controller 2-7 is a half-wave plate with a major axis at an angle of 22.5° to the horizontal direction.

[0043] Furthermore, the photoacoustic detection module includes a deep scattering medium 3-1, an anisotropic absorber 3-2, a focusing ultrasound probe 3-3, a filter 3-4, and an amplifier 3-5; the deep scattering medium 3-1 and the anisotropic absorber 3-2 are arranged adjacent to each other along the optical axis. The small-spot linearly polarized light emitted from the third polarization controller 2-7 is incident on the deep scattering medium 3-1, undergoing multiple scattering and wavefront distortion, and then incident on the anisotropic absorber 3-2; the anisotropic absorber 3-2 is excited by the scattered light field to generate a photoacoustic signal, which is collected by the focusing ultrasound probe 3-3, filtered and denoised by the filter 3-4, and amplified by the amplifier 3-5 to obtain a processed photoacoustic signal for subsequent detection and feedback control.

[0044] Optionally, the deep scattering medium 3-1 is a 5 mm thick deep scattering medium, the anisotropic absorber 3-2 is a directional carbon nanotube absorber, the filter is a 1-10 MHz filter, and the amplifier has a gain of 325.

[0045] Furthermore, the signal processing module includes a computer 4-1 and a rotating platform 4-2, with the computer 4-1 connected to the rotating platform 4-2 via a motion control card. The deep scattering medium 3-1 is used to simulate a strong deep scattering medium environment; the anisotropic absorber 3-2 is mounted on the rotating platform 4-2. The computer 4-1 controls the rotating platform 4-2 to rotate to 0°, 45°, 90°, and 135°, thereby driving the anisotropic absorber 3-2 to rotate around the optical axis. This allows the focused ultrasound probe 3-3 to acquire, in real time, the photoacoustic signals generated by the anisotropic absorber 3-2 at different rotation angles and the polarization angle of the emitted light. The computer 4-1 receives photoacoustic signals acquired by the focused ultrasound probe 3-3 at different rotation angles, filters them for noise reduction using the filter 3-4, and amplifies them using the amplifier 3-5. On one hand, it uses the photoacoustic signals at different rotation angles as feedback signals to control the SLM1 to iteratively optimize the corresponding absolute phase distribution, achieving energy focusing. On the other hand, it constructs photoacoustic Stokes vectors based on the photoacoustic signals at different rotation angles and calculates the degree of linear polarization (DOP). Using the calculated DOP as feedback signals, it controls the SLM2 to iteratively optimize the corresponding polarization modulation phase, achieving polarization state recovery. In this embodiment, the rotating platform is a rotary motor with a rotation angle accuracy of 0.003 rad and a maximum pulse frequency of 1000 Hz, triggered externally by the motion control card of the computer 4-1.

[0046] In this embodiment, the pulsed light excitation module outputs linearly polarized pulsed light, which is then phase-polarization coordinated and incident on the deep scattering medium and anisotropic absorber of the photoacoustic detection module after being controlled by the modulation module. The photoacoustic signal generated by the anisotropic absorber is acquired, amplified, and processed by the focusing ultrasonic probe, filter, and amplifier of the photoacoustic detection module and then sent to the signal processing module. The computer of the signal processing module provides feedback based on the photoacoustic amplitude and linear polarization degree of the photoacoustic signal, and iteratively controls SLM1 and SLM2 to achieve energy focusing and polarization recovery of the deep scattering medium.

[0047] Furthermore, based on the aforementioned deep medium polarization restoration device based on polarization-optical signal feedback, an embodiment of the present invention provides a deep medium polarization restoration method based on polarization-optical signal feedback. Figure 2 A flowchart illustrating a deep medium polarization restoration method based on polarized photoacoustic signal feedback according to an embodiment of the present invention is shown, including: S10, control the pulsed laser to emit pulsed laser and obtain small spot linearly polarized light after beam expansion and collimation. The small spot linearly polarized light is absolutely phase-modulated by the first spatial light modulator so that the long axis of the liquid crystal molecules of the first spatial light modulator is consistent with the electric vector direction of the small spot linearly polarized light. Understandably, the phase change of the long axis of liquid crystal molecules causes a change in the wavefront shape of light. When a collimated beam with a flat wavefront passes through a medium (first spatial light modulator) with a spatially varying refractive index (liquid crystal molecules), the liquid crystal molecules undergo orientation changes under voltage control, resulting in differences in the equivalent refractive index at different positions. Since the optical path length (OPL) is proportional to the refractive index, the optical path length (OPL) experienced by the beam at different pixel positions is also different. The phase of the beam passing through the first spatial light modulator is no longer flat, thus introducing a spatially distributed phase difference. The mathematical relationship between the phase and the optical path length is:

[0048] in, For the phase of the beam, Where is the wavelength of the incident light. This is the optical path length of the light beam as it propagates through the medium.

[0049] Understandably, by applying different voltages to different pixels of the SLM1, independent phase control of each position of the beam can be achieved, thereby shaping the flat wavefront into the target wavefront shape. In this invention, the first spatial light modulator (SLM1) compensates for the wavefront distortion introduced by the deep scattering medium by iteratively optimizing the phase modulation amount of each pixel, thereby achieving focusing of light energy within the deep scattering medium.

[0050] Specifically, step S1 includes: S101, The energy of the pulsed laser emitted by the pulsed laser is controlled to obtain horizontally polarized light; Specifically, in this embodiment, the pulsed laser 1-1 emits a pulsed laser with a wavelength of 1035 nm and a pulse width of 300 fs. After controlling the pulsed laser 1-1 to emit the pulsed laser, the energy of the emitted pulsed laser is controlled to generate energy suitable for the peak power of the first spatial light modulator 2-2. The energy of the pulsed laser can be controlled by a combination of the aperture 1-2 and the polarizer 1-3. The aperture 1-2 is used to change the polarization state of the incident pulsed laser. After the pulsed laser with its polarization state changed passes through the polarizer 1-3, only the linearly polarized light component can be determined. Therefore, by changing the polarization state of the pulsed laser, energy control can be achieved to obtain horizontally polarized light. It is understood that the fast axis direction of the polarizer 1-3 is consistent with the long axis direction of the SLM1.

[0051] S102, the horizontally polarized light obtained after energy control is expanded and collimated to obtain small-spot linearly polarized light, and the small-spot linearly polarized light obtained after expansion and collimation is incident on the first spatial light modulator for absolute phase modulation. Specifically, the linearly polarized light after energy control is expanded and collimated by the beam expander and collimator 1-4 to obtain a light spot size (small spot linearly polarized light) that can be controlled by the SLM1. After being reflected by the first mirror 2-1, it is then incident on the first spatial light modulator SLM1. Subsequently, the small spot linearly polarized light is absolutely phase-controlled by the first spatial light modulator 2-2, so that the long axis of the liquid crystal molecules of the first spatial light modulator 2-2 is consistent with the electric vector direction of the small spot linearly polarized light.

[0052] S103, under the modulation of the third polarization controller, acquires the photoacoustic signals generated by the anisotropic absorber at four different rotation angles, uses them as feedback signals, and iteratively updates the absolute phase distribution of SLM1 to achieve absolute phase control of the pulsed laser. Specifically, in this embodiment, when the small-spot linearly polarized light obtained after beam expansion and collimation is incident on SLM1, the small-spot linearly polarized beam is incident on the deep scattering medium 3-1 (simulating biological tissue, 5 mm thick) and the anisotropic absorber 3-2 (directionally arranged carbon nanotube film) after passing through the control module. The anisotropic absorber 3-2 generates photoacoustic signals. The computer 4-1 drives the anisotropic absorber 3-2 to rotate around the optical axis by different rotation angles through the control of the rotating platform 4-2. The focusing ultrasound probe 3-3 collects the photoacoustic signals of the anisotropic absorber at different rotation angles and filters and amplifies them in sequence through the filter 3-4 and the amplifier 3-5. The computer 4-1 acquires the processed photoacoustic signals in real time and uses them as feedback signals. The absolute phase distribution of SLM1 is continuously optimized through the feedback signals, thereby maximizing the generated photoacoustic signals and ultimately maximizing the intensity of the pulsed laser, thus achieving deep light energy focusing. In this embodiment, when using SLM1 for absolute phase modulation, the phase of SLM2 is frozen to zero. At this time, only the phase of SLM1 is optimized so that the photoacoustic signal converges to the maximum value, thus completing phase focusing.

[0053] In this embodiment, preferably, the Continuous Sequence Iteration (SPGD) algorithm is used to iteratively update the absolute phase distribution of SLM1. For example, the initial phase map of SLM1 is set to a random distribution (0-2π). The N control units of SLM1 are divided into M groups of macropixels (each group contains N / M adjacent units). In each iteration, a random small phase perturbation is applied to all macropixels of SLM1. For example, four test phase values ​​of 0, π / 2, π, and 3π / 2 are applied sequentially to the i-th group of macropixels. At the same time, the corresponding photoacoustic signals A0, A1, A2, and A3 are acquired synchronously. The optimal phase of the group is calculated by the four-step phase shift method. The formula for calculating the optimal phase is as follows:

[0054] Specifically, The signal is applied to the i-th group, and all M groups of macropixels are traversed sequentially to complete one iteration; this iteration is repeated until the stopping condition is met. Understandably, when the enhancement factor of the photoacoustic signal is less than the first convergence threshold, it indicates that the focusing performance is stabilizing and the photoacoustic signal has essentially converged to its maximum value. Therefore, the stopping condition for iteration can be: the enhancement factor of the photoacoustic signal is less than the first convergence threshold or the preset maximum number of iterations K_max is reached. In this embodiment, the first convergence threshold is preferably 5%, and K_max is preferably 20 iterations.

[0055] S20, a polarization control channel is formed by the first polarization controller, the second spatial light modulator and the second polarization controller, and the polarization control channel is used to control the small spot linearly polarized light emitted from the first spatial light modulator by the Jones vector matrix. In this embodiment, the first polarization controller uses a half-wave plate, and the second polarization controller uses a quarter-wave plate. The half-wave plate, SLM2, and quarter-wave plate form a polarization control channel to achieve polarization control of the relative phase. The specific implementation of S20 includes: S201, establish the Jones vector matrices of the small-spot linearly polarized light incident on the polarization control channel, the first polarization controller, the second spatial light modulator, and the second polarization controller, and obtain the Jones vector matrix of the outgoing small-spot linearly polarized light based on the Jones vector matrices of the small-spot linearly polarized light, the first polarization controller, the second spatial light modulator, and the second polarization controller. Specifically, in order to establish the mapping relationship between the target linear polarization direction and the phase loaded on SLM2 to achieve vector polarization control, this invention uses a Jones matrix to model the entire polarization control optical path; the Jones vector matrix of the incident small-spot linearly polarized light is expressed as:

[0056] First, the incident small-spot linearly polarized light is irradiated onto the half-wave plate, with its major axis at an angle to the horizontal direction. The Jones vector matrix of the half-wave plate is represented as:

[0057] Phase modulation was then performed using SLM2, with the direction of the liquid crystal molecules being the x-direction, and the modulation was introduced only in the x-direction. For the commonly used reflective SLM2, phase modulation is equivalent to a pure phase modulation device plus a reflector. Therefore, the Jones vector matrix of the reflective SLM2 is expressed as:

[0058] The light after passing through SLM2 is polarized light with various ellipticities. Therefore, it must pass through a quarter-wave plate to convert the light with different ellipticities into linearly polarized light, thereby achieving precise control over the target polarization direction. The major axis is at an angle to the horizontal direction. The Jones vector matrix of the quarter-wave plate is:

[0059] The Jones vector matrix of the emitted small-spot linearly polarized light is represented as follows:

[0060] S202, Based on the Jones vector matrix corresponding to the emitted small-spot linearly polarized light, establish a mapping relationship between the phase of the second spatial light modulator and the target polarization angle of the emitted small-spot linearly polarized light, and perform polarization control on the emitted small-spot linearly polarized light according to the established mapping relationship; Target linear polarization angle The ratio of the y-direction component to the x-direction component in the Jones vector matrix of the emitted small-spot linearly polarized light is calculated by taking the arctangent of the ratio. The formula is as follows:

[0061] in, , These are the x and y components of the Jones vector representing the polarization state of the emitted linearly polarized light spot. To take the real part of a complex number, This indicates taking the imaginary part of a complex number.

[0062] Based on the above model, the phase loaded by SLM2 can be derived. (0-2π) and the target polarization angle of the emitted linearly polarized light The mapping relationship (0°~180°) is specifically represented as follows:

[0063] Therefore, when the polarization state of the emitted light When the temperature is between 135 degrees and 180 degrees, the phase modulation amount applied to the SLM2 can be calculated based on the above mapping relationship. Corresponding to 0 to 90 degrees (radian value), when the polarization state of the emitted light... When the temperature is between 0 and 135 degrees, the phase modulation amount applied to the SLM2 can be calculated based on the above mapping relationship. This corresponds to 90 to 360 degrees (radian value).

[0064] Understandably, SLMs can only be modulated along a single axis. When light is incident on an SLM2 with a 45° polarization, it can be decomposed into two components: x (parallel to the major axis of the SLM2) and y (perpendicular to the major axis of the SLM2). The SLM2 can only modulate the phase of the x component, while the phase of the y component remains unchanged. The phase difference between the two components determines the final polarization direction. The phase difference between the orthogonal components is twice the phase applied by the SLM. Therefore, the SLM2 can achieve uninterrupted control of the linear polarization direction within the range of 0~180° through phase modulation of 0-2π. The specific polarization control results are as follows: Figure 3 As shown.

[0065] S203. Based on the established mapping relationship between the phase of the second spatial light modulator and the target polarization angle of the emitted small spot linearly polarized light, the photoacoustic signals generated by the anisotropic absorber at four different rotation angles are acquired. The photoacoustic Stokes vector is constructed according to the photoacoustic signals at different rotation angles and the degree of linear polarization is calculated. The calculated degree of linear polarization is used as a feedback signal to iteratively update the polarization control phase of SLM2 until the degree of linear polarization converges to the maximum value.

[0066] Specifically, a polarizer is mounted on the rotating platform 4-2. During signal acquisition, the polarizer rotates at four angles: 0°, 45°, 90°, and 135°, driven by the rotation of the platform. At each rotation angle, the photoacoustic signal generated by the anisotropic absorber 3-2 under pulsed laser light is acquired. Understandably, different target polarization angles of the emitted small-spot linearly polarized light result in different photoacoustic signals generated by the anisotropic absorber 3-2. Different emitted polarization angles incident on the anisotropic absorber sample will produce different photoacoustic signals. Therefore, based on the established mapping relationship between the phase of the second spatial light modulator and the target polarization angle of the emitted small-spot linearly polarized light, different target polarization angles can be obtained by feedback fine-tuning the phase loaded onto the second spatial light modulator, thereby generating different photoacoustic signals. Furthermore, a photoacoustic Stokes vector is constructed based on the photoacoustic signals generated by the anisotropic absorber 3-2, and the degree of linear polarization is calculated. The calculated degree of linear polarization is used as a feedback signal for polarization control. The absorption coefficient of the anisotropic absorber satisfies the following polarization dependence:

[0067] in, The average absorption coefficient, This is the anisotropic difference quantity. The target polarization angle of the emitted small-spot linearly polarized light. The optical axis direction of the anisotropic absorber.

[0068] The constructed Stokes vector is represented as follows:

[0069]

[0070]

[0071] in, , , , These are the photoacoustic signals generated by the anisotropic absorber at polarization angles of 0°, 45°, 90°, and 135°, respectively. linear polarization degree The calculation formula is as follows:

[0072] in, The value ranges from 0 to 1, with values ​​closer to 1 indicating purer linear polarization of the light. The degree of linear polarization... The feedback signal for polarization modulation iteratively updates the SLM2 phase until... Converging to the maximum value; understandably, the degree of linear polarization. When the difference between adjacent iterations is less than the second convergence threshold, it indicates that the polarization state is approaching the optimal value, at which point the linear polarization degree... It also converges to the maximum value. In this embodiment, the second convergence threshold is preferably 0.001.

[0073] In this embodiment, the phase loaded by the second spatial light modulator has a fixed mapping relationship with the target polarization angle of the emitted small-spot linearly polarized light. The target polarization angle directly determines the subsequently calculated linear polarization degree value. This method aims to maximize the linear polarization degree. It calculates the linear polarization degree by real-time acquisition of photoacoustic signals, iteratively corrects the phase value of SLM2 based on the numerical change trend, and continuously optimizes the emitted polarization state until the linear polarization degree meets the convergence threshold, thus completing the optimal phase update.

[0074] S30, calculate the geometric absolute phase introduced during polarization modulation, and invert the calculated geometric absolute phase to compensate the first spatial light modulator. The process ends when the enhancement factor and linear polarization degree of the acquired photoacoustic signal both reach their respective convergence thresholds.

[0075] Understandably, when the polarization modulation module modulates the polarization state of the incident light, it introduces an additional geometric phase shift. This phase shift is superimposed on the focusing phase modulated by the first spatial light modulator (SLM1), disrupting the optimized light energy focusing state. To eliminate this effect, this invention calculates the geometric absolute phase introduced by polarization modulation using the Jones vector inner product, and compensates for and corrects the loading phase of SLM1. The formula for calculating the geometric absolute phase is as follows:

[0076] in, The Jones vector matrix represents the small spot linearly polarized light incident on the polarization control channel. The Jones vector matrix representing the polarization state emitted from the target. This represents the conjugate transpose, used to calculate the inner product of two polarization states. This indicates that the phase value is taken.

[0077] Understandably, when When , it indicates that the two polarization states are not perpendicular. The geometric phase introduced during the polarization state change can be calculated using the argument of their inner product. When the incident and emitted polarization states are orthogonal, the inner product is 0, the geometric phase is 0, and no compensation is needed. Therefore, by calculating the geometric phase... The phase is inverted and compensated into the phase distribution of SLM1, meaning the actual loaded phase of SLM1 is... The compensation phase This eliminates the interference of polarization control on the focusing phase of the optical field, and achieves decoupled synergistic optimization of phase focusing and polarization control.

[0078] In this embodiment, the coordination mechanism for phase optimization and polarization optimization adopts a two-stage serial optimization plus compensation iteration: In the first stage, when using SLM1 for absolute phase control, the phase map of SLM2 is frozen (set to all zero phase, fixing the SLM phase and polarization angle), and only the absolute phase of SLM1 is optimized to maximize the acquired photoacoustic signal and complete phase focusing; In the second stage, when using SLM2 for polarization control, the current phase of SLM1 is frozen, and the polarization control phase of SLM2 is adjusted and optimized based on the piecewise function to maximize the calculated linear polarization degree; At the same time, after each polarization update, the additional geometric absolute phase caused by polarization control is calculated by the Jones vector matrix, and the additional geometric absolute phase is superimposed on the current phase of SLM1 for compensation, and the compensated photoacoustic signal is verified; If the acquired photoacoustic signal drops more than a preset threshold after compensation, the first stage is returned to readjust the absolute phase of SLM1 before continuing polarization optimization; When both the photoacoustic signal enhancement factor and the linear polarization degree reach their respective convergence thresholds, the overall optimization is considered to be complete.

[0079] The compensation mechanism of this invention achieves complete decoupling of the functions of SLM1 and SLM2. SLM1 is only responsible for focusing light energy, and SLM2 is only responsible for polarization state control. The optimization processes of the two do not interfere with each other, ensuring the synchronous improvement of focusing efficiency and polarization recovery accuracy.

[0080] It should be understood that, although Figure 2 The steps in the flowchart are shown sequentially as indicated by the arrows, but these steps are not necessarily executed in the order indicated by the arrows. Unless otherwise specified herein, there is no strict order in which these steps are executed, and they can be performed in other orders. Figure 2 At least some of the steps in the process may include multiple steps or multiple stages. These steps or stages are not necessarily completed at the same time, but may be executed at different times. The execution order of these steps or stages is not necessarily sequential, but may be executed in turn or alternately with other steps or at least some of the steps or stages in other steps.

[0081] In one embodiment, a computer device is also provided, including a memory and a processor, wherein the memory stores a computer program, and the processor executes the computer program to implement the steps in the above method embodiments.

[0082] In one embodiment, a computer device is provided, which may be a terminal, and its internal structure diagram may be as follows: Figure 4As shown, the computer device includes a processor, memory, communication interface, display screen, and input devices connected via a system bus. The processor provides computing and control capabilities. The memory includes non-volatile storage media and internal memory. The non-volatile storage media stores the operating system and computer programs. The internal memory provides an environment for the operation of the operating system and computer programs stored in the non-volatile storage media. The communication interface is used for wired or wireless communication with external terminals; wireless communication can be achieved through Wi-Fi, carrier networks, NFC (Near Field Communication), or other technologies. When executed by the processor, the computer program implements a deep scattering medium polarization recovery method based on polarized optical acoustic signal feedback. The display screen can be an LCD screen or an e-ink screen. The input devices can be a touch layer covering the display screen, buttons, a trackball, or a touchpad on the computer device casing, or an external keyboard, touchpad, or mouse.

[0083] Those skilled in the art will understand that Figure 4 The structure shown is merely a block diagram of a portion of the structure related to the present invention and does not constitute a limitation on the computer device to which the present invention is applied. A specific computer device may include more or fewer components than those shown in the figure, or combine certain components, or have different component arrangements.

[0084] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0085] The embodiments described above are merely illustrative of several implementations of the present invention, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this patent should be determined by the appended claims.

Claims

1. A deep medium polarization recovery method based on polarization photoacoustic signal feedback, characterized in that, include: S10, control the pulsed laser to emit pulsed laser and obtain small spot linearly polarized light after beam expansion and collimation, and perform absolute phase modulation on the small spot linearly polarized light through the first spatial light modulator, so that the long axis of the liquid crystal molecules of the first spatial light modulator is consistent with the electric vector direction of the small spot linearly polarized light; S20, a polarization control channel is formed by the first polarization controller, the second spatial light modulator and the second polarization controller, and the polarization control channel is used to control the small spot linearly polarized light emitted from the first spatial light modulator by the Jones vector matrix. S30, calculate the geometric absolute phase introduced during polarization modulation, and invert the calculated geometric absolute phase to the first spatial light modulator, and end when it is determined that the enhancement factor and linear polarization degree of the acquired photoacoustic signal have reached their respective convergence thresholds. S20 includes: S201, establish the Jones vector matrices of the small-spot linearly polarized light incident on the polarization control channel, the first polarization controller, the second spatial light modulator, and the second polarization controller, and obtain the Jones vector matrix of the outgoing small-spot linearly polarized light based on the Jones vector matrices of the incident small-spot linearly polarized light, the first polarization controller, the second spatial light modulator, and the second polarization controller; wherein, the Jones vector matrix of the small-spot linearly polarized light incident on the polarization control channel is expressed as: The major axis is at an angle to the horizontal direction. The Jones vector matrix of the first polarization controller is represented as: The Jones vector matrix of the second spatial light modulator is represented as follows: The major axis is at an angle to the horizontal direction. The Jones vector matrix of the second polarization controller is represented as: The Jones vector matrix of the emitted small-spot linearly polarized light is then expressed as: in, The imaginary unit satisfies , used to characterize the complex amplitude phase of the optical field; S202, based on the Jones vector matrix corresponding to the emitted small-spot linearly polarized light, establish the mapping relationship between the phase of the second spatial light modulator and the target polarization angle of the emitted small-spot linearly polarized light; the mapping relationship is expressed as: in, This refers to the phase of the second spatial light modulator. The target polarization angle of the emitted small-spot linearly polarized light; S203. Based on the mapping relationship between the phase of the second spatial light modulator and the target polarization angle of the emitted small-spot linearly polarized light, under the modulation of the third polarization controller, photoacoustic signals generated by the anisotropic absorber at four different rotation angles are acquired. Photoacoustic Stokes vectors are constructed based on the photoacoustic signals at different rotation angles, and the degree of linear polarization is calculated. The calculated degree of linear polarization is used as a feedback signal to iteratively update the phase of the second spatial light modulator until the degree of linear polarization converges to its maximum value. The photoacoustic signals at different polarization states obtained by the third polarization controller are used to construct the photoacoustic Stokes vectors. The constructed photoacoustic Stokes vectors are represented as follows: in, , , , These are the photoacoustic signals generated by the anisotropic absorber at rotation angles of 0°, 45°, 90°, and 135°, respectively. linear polarization degree The calculation formula is as follows: 。 2. The deep dielectric polarization restoration method according to claim 1, characterized in that, S10 includes: S101, The energy of the pulsed laser emitted by the pulsed laser is controlled to obtain horizontally polarized light; S102, the horizontally polarized light obtained after energy control is expanded and collimated to obtain small-spot linearly polarized light, and the small-spot linearly polarized light obtained after expansion and collimation is incident on the first spatial light modulator for absolute phase modulation. S103, under the modulation of the third polarization controller, the photoacoustic signals generated by the anisotropic absorber at four different rotation angles are acquired, and these signals are used as feedback signals to iteratively update the absolute phase distribution of the first spatial light modulator to achieve absolute phase modulation.

3. The deep dielectric polarization recovery method according to claim 2, characterized in that, In step S103, a continuous sequence iterative algorithm is used to iteratively update the absolute phase distribution of the first spatial light modulator; the optimal phase calculation formula is as follows: in, This represents the optimal phase of the i-th group in the first spatial light modulator. , , , Let i represent the photoacoustic signals corresponding to different rotation angles, and let i be the number of macro-pixel groups divided by the first spatial light modulator.

4. The deep dielectric polarization restoration method according to claim 1, characterized in that, In S30, the formula for calculating the geometric absolute phase is as follows: in, The Jones vector matrix represents the small-spot linearly polarized light incident on the polarization control channel. The Jones vector matrix representing the polarization state emitted from the target. This represents the conjugate transpose, used to calculate the inner product of two polarization states. This indicates that the phase value is taken.

5. A deep medium polarization restoration device based on polarized photoacoustic signal feedback, used to implement the deep medium polarization restoration method according to any one of claims 1-4, characterized in that, The device includes: Pulsed laser excitation module, used to emit pulsed laser light; The control module, connected to the pulsed light excitation module, is used to control the phase and polarization of the pulsed laser. A photoacoustic detection module, connected to the control module, is used to generate a photoacoustic signal under the action of the pulsed laser and to collect and process the generated photoacoustic signal; The signal processing module is connected to the pulsed light excitation module, the modulation module, and the photoacoustic detection module, respectively, and is used to acquire photoacoustic signals at different rotation angles, and use the acquired photoacoustic signals at different rotation angles as feedback signals to control and update the phase and polarization of the pulsed laser.

6. The deep dielectric polarization restoration device according to claim 5, characterized in that, The pulsed light excitation module includes a pulsed laser, an aperture, a polarizer, and a beam expander / collimator connected in sequence; the pulsed laser output from the pulsed laser passes through the aperture, the polarizer, and the beam expander / collimator in sequence to obtain parallel small-spot linearly polarized light. The control module includes a first reflecting mirror, a first spatial light modulator, a first polarization controller, a second reflecting mirror, a second spatial light modulator, a second polarization controller, and a third polarization controller connected in sequence; wherein, the first reflecting mirror is connected to the beam expander collimator, and the incident small-spot linearly polarized light passes through the first reflecting mirror, the first spatial light modulator, the first polarization controller, the second reflecting mirror, the second spatial light modulator, the second polarization controller, and the third polarization controller in sequence to obtain the outgoing small-spot linearly polarized light; The photoacoustic detection module includes a deep scattering medium, an anisotropic absorber, a focused ultrasound probe, a filter, and an amplifier. The third polarization controller is connected to the deep scattering medium, and the deep scattering medium and the anisotropic absorber are arranged adjacent to each other along the optical axis. The small-spot linearly polarized light emitted from the third polarization controller passes through the deep scattering medium and the anisotropic absorber, causing the anisotropic absorber to generate a photoacoustic signal. The focused ultrasound probe collects the generated photoacoustic signal, which is then amplified by the filter and the amplifier to obtain a processed photoacoustic signal. The signal processing module includes a computer and a rotating platform. The computer is connected to the pulsed laser, the first spatial light modulator, and the second spatial light modulator, and is also connected to the rotating platform via a motion control card. The anisotropic absorber is mounted on the rotating platform. The computer drives the anisotropic absorber to rotate around the optical axis to different rotation angles by controlling the rotation of the rotating platform. The computer uses the photoacoustic signals of the anisotropic absorber at different rotation angles as feedback signals to control the first spatial light modulator to iteratively optimize the corresponding absolute phase distribution, thereby achieving energy focusing. The computer constructs a photoacoustic Stokes vector based on the photoacoustic signals at different rotation angles and calculates the degree of linear polarization. Using the calculated degree of linear polarization as feedback signals, the computer controls the second spatial light modulator to iteratively optimize the corresponding polarization modulation phase, thereby achieving polarization state recovery.

7. A computer device comprising a memory and a processor, wherein the memory stores a computer program, characterized in that, When the processor executes the computer program, it implements the steps of the deep medium polarization recovery method according to any one of claims 1 to 4.

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