An optical limiting method based on reversible saturated high nonlinear fluorescence process

Abstract

This invention relates to the interdisciplinary field of nanophotonics, nonlinear optics, and optical limiting technology, specifically to an optical limiting method based on a reversible saturated highly nonlinear fluorescence process. The invention constructs reversible saturated fluorescent nanoparticles with multiple photonic avalanche engines of different excitation thresholds, and disperses them in an optically transparent solid matrix to create an optical limiting element. Utilizing a multi-level intelligent optical limiting system comprising a beam splitter, photodetector, and central controller, the incident laser power is progressively increased while simultaneously acquiring incident light intensity, transmitted light intensity, and fluorescence spectral characteristics. This clamps the output light intensity to multiple preset safety levels throughout the high-power range, obtaining a multi-level stepped limiting curve. This invention achieves multi-level adaptive limiting of incident light intensity, possessing advantages such as multi-level limiting, intelligent adaptation, reversible saturation, and high cyclic stability, and can be widely applied in fields such as high-power laser protection and intelligent optical windows.

Description

Technical Field

[0001] This invention relates to the interdisciplinary field of nanophotonics, nonlinear optics, and optical limiting technology, specifically to an optical limiting method based on a reversible saturated highly nonlinear fluorescence process. Background Technology

[0002] An optical limiter is a nonlinear optical device that requires maintaining high transmittance under low incident light intensity to avoid affecting normal optical observation or signal transmission, while automatically reducing transmittance under high incident light intensity to clamp the output light intensity below a safe threshold, thereby protecting downstream components such as the human eye, photoelectric sensors, and precision optical elements from damage caused by strong lasers. As an important laser protection method, optical limiter technology has wide applications in military laser countermeasures, industrial laser processing, medical laser surgery, and scientific research laser systems. Since the concept of two-photon absorption optical limiter was proposed, researchers have developed optical limiter technologies based on various physical mechanisms such as reversible saturable absorption, nonlinear scattering, stimulated Brillouin scattering, and two-photon or multi-photon absorption. For example, the interaction between the focusing field of a radially polarized beam and a two-photon absorbing material can significantly reduce the limiting threshold; suspensions based on transition metal sulfide nanosheets (such as TiS2 and TaS2) exhibit excellent optical limiting performance under nanosecond laser irradiation; the concentration of graphene oxide solution can be precisely controlled using microfluidic technology to achieve continuous adjustment of the limiting threshold; and cascaded optical limiters based on stimulated Brillouin scattering (SBS) flatten the limiting curve by connecting two SBS cells with different thresholds in series. These technological advancements provide diverse solutions for laser protection.

[0003] However, the aforementioned optical limiting technologies all suffer from several common limitations. Traditional optical limiters typically only activate limiting at a fixed power density. Once the incident light intensity exceeds this threshold, although the output light intensity is clamped, the limiting depth is often insufficient when the light intensity increases further, or even fails due to material damage. Furthermore, the effective operating range of optical limiters usually only covers a narrow range of incident power, making it difficult to adapt to the wide dynamic range protection requirements from continuous weak light to strong pulsed lasers. Current limiters cannot provide stepped attenuation according to different levels of incident light intensity, often resulting in over-protection (sacrificing normal light transmission) at medium light intensities or insufficient protection at high light intensities. In addition, while organic materials have fast responses, they are prone to photobleaching, while inorganic materials, although stable, have slow responses, and most materials experience irreversible damage accumulation at high power. These limitations severely restrict the performance and reliability of optical limiters in practical applications. Therefore, there is an urgent need for an optical limiting method that can achieve multi-level, reversible, highly stable, and adaptive capabilities. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the present invention aims to provide an optical limiting method based on a reversible saturated high nonlinear fluorescence process. It utilizes the nonlinear absorption characteristics of reversible saturated fluorescent nanoparticles that are sequentially and explosively enhanced in multiple independent power ranges to achieve multi-level adaptive limiting of incident light intensity, thus overcoming the bottlenecks of traditional optical limiters, such as single threshold, narrow dynamic range, and lack of hierarchical capability.

[0005] To address the aforementioned problems, the present invention proposes the following technical solution: a light limiting method based on a reversible saturated highly nonlinear fluorescence process, comprising the following steps: S1. Construct and prepare reversible saturated fluorescent nanoparticles, wherein multiple photonic avalanche engines with different excitation thresholds are integrated inside the nanoparticles, and each engine is individually activated at its respective threshold power to produce an explosive excited-state absorption enhancement. S2. Disperse the nanoparticles in an optically transparent solid matrix to fabricate an optical limiting element; S3. Construct a multi-level intelligent optical limiting optical path system, place the optical limiting element in the incident optical path, and set a beam splitter and a photodetector before and after it respectively to form a closed-loop feedback control system. S4. The limiting characteristic curve of the optical limiting element is tested using the multi-level intelligent optical limiting optical path system. The incident laser power is gradually increased while the incident light intensity, transmitted light intensity and fluorescence spectral characteristics are collected simultaneously. The intensity ratio of at least two characteristic wavelengths in the fluorescence spectrum is used to help confirm the currently activated engine level. As the absorption intensity of the optical limiting element increases sharply at each threshold, as the incident light intensity crosses each excitation threshold, the output light intensity is clamped at multiple preset safety levels throughout the high power range. This makes the transmittance change curve with incident light intensity present a multi-level stepped shape composed of multiple clamping platforms that appear in sequence, thereby completing the quantitative characterization of the multi-level limiting performance of the optical limiting element.

[0006] Furthermore, the excitation threshold of each photon avalanche engine in the reversible saturated fluorescent nanoparticle can be independently controlled by selecting different lanthanide ion doping systems, different host lattices, and adjusting the doping concentration and spatial distribution.

[0007] Furthermore, in the optical limiting device based on the reversible saturated high nonlinear fluorescence process, all nanophotonic avalanche engines are not activated under low incident light intensity, the nanoparticles absorb the incident light weakly, and the device maintains a high transmission state. When the incident light intensity increases to the first threshold, the lowest threshold engine is activated, the excited state absorption is greatly enhanced, and the device transmittance drops sharply to the first-level low value. As the incident light intensity continues to increase, the first engine gradually saturates and the absorption weakens, but at this time the second engine is triggered, which again reduces the transmittance to the second-level low value. And so on, so that the output light intensity is effectively clamped at multiple preset safety levels throughout the high power range, realizing a multi-level, stepped limiting curve.

[0008] Furthermore, the limiting curve of the optical limiting device exhibits a unique multi-level stepped shape, and the output light intensity level corresponding to each clamping platform is determined by the excited state absorption intensity of each engine. By analyzing this curve, the key performance parameters of the optical limiting element can be obtained, including the limiting threshold at each level, the clamping light intensity at each level, the dynamic range, and the limiting depth.

[0009] Furthermore, the optical limiting optical path system includes an incident light source, an optical limiting element, a first beam splitter, a second beam splitter, a main energy detector, a reference energy detector, a spectrometer, a central controller, and an aperture. The incident laser is split into two paths by the first beam splitter. The main optical path passes through the optical limiting element and is then split again by the second beam splitter. One path enters the main energy detector to record the output energy after limiting, while the other path enters the spectrometer to monitor the anti-saturation fluorescence characteristic peak to determine the currently activated engine level. The reference light split from the first beam splitter enters the reference energy detector to normalize the incident light intensity fluctuation. All detector signals are connected to the central controller.

[0010] Furthermore, the intelligent limiting control algorithm executed by the central controller includes: real-time acquisition of the intensity ratio of at least two characteristic wavelengths in the fluorescence spectrum, comparing it with a pre-established engine level-ratio mapping relationship to determine which engine level's operating range is currently in; comparing the incident light intensity with pre-calibrated threshold values ​​for each engine, and sending a corresponding preset attenuation value to the back-end variable attenuator according to the identified engine level, so that the output light intensity is clamped within the safe range corresponding to that level; when the incident light intensity exceeds the saturation threshold of the highest-level engine and continues to rise, issuing a high-risk alarm signal and triggering the aperture to completely cut off the optical path.

[0011] Furthermore, the optical limiting element is integrated into the fiber end face, microcavity resonator, or waveguide structure, or the polymer film doped with the reversible saturated fluorescent nanoparticles is directly coated onto the fiber end face or fused into the fiber link to form an all-fiber optical limiter.

[0012] Due to the adoption of the above technical solution, the beneficial effects of the optical limiting method based on a reversible saturated highly nonlinear fluorescence process of the present invention are as follows: 1. By utilizing the multi-threshold characteristics of reversible saturated fluorescent nanoparticles, nonlinear absorption is achieved by increasing the incident light intensity in segments. The output light intensity is repeatedly clamped in multiple power ranges, effectively expanding the dynamic range of the limiting. Moreover, the limiting curve exhibits a unique "step-like" characteristic, which is different from the single clamping platform of traditional optical limiters.

[0013] 2. The threshold values ​​of each engine in reversible saturated fluorescent nanoparticles can be pre-programmed through material and structural design, enabling the limiter to automatically switch the protection depth according to different levels of incident light intensity. It can achieve graded adaptive response of "high transmittance in weak light, medium resistance in medium light, and deep resistance in strong light" without the need for external control circuits. This is an intelligent feature that traditional optical limiters do not have.

[0014] 3. The reversible saturated fluorescence process is essentially reversible: after the excitation light is removed, the excited-state particles return to the ground state through radiative and non-radiative relaxation, and the nanoparticles recover to their initial low absorption state. Therefore, it has excellent cycle stability, which is far superior to the fatigue resistance of organic light limiting materials.

[0015] 4. Employing an all-inorganic fluoride matrix and lanthanide ions, the material itself possesses high thermal and chemical stability, with a damage threshold far exceeding that of organic optical limiting materials, making it suitable for high-power continuous-wave lasers and repetitive-frequency pulsed laser systems. Furthermore, by selecting different lanthanide ion doping systems, it can cover multiple important laser bands in the near-infrared range (such as 852nm, 965nm, 1064nm, 1450nm, etc.).

[0016] 5. Threshold Programmability and Design Flexibility: By changing the type and concentration of dopant ions, the type of host lattice, and the spatial arrangement of each engine, the start-up power and limiting depth of each limiting stage can be precisely set, enabling customized designs for different application scenarios. This programmability provides a completely new approach to the development of intelligent optical limiters. Attached Figure Description

[0017] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings: Figure 1 This is a basic schematic diagram of the optical limiting method based on a reversible saturated highly nonlinear fluorescence process according to the present invention.

[0018] Figure 2 The theoretical fluorescence intensity of the reversible saturated fluorescence optical limiting device based on high-order nonlinearity in the optical limiting method based on reversible saturated high nonlinearity of the present invention is shown in the figure.

[0019] Figure 3 This is a nanostructure diagram of nanoparticles based on a reversible saturated high nonlinear fluorescence process, provided in Example 1 of the optical limiting method based on a reversible saturated high nonlinear fluorescence process of the present invention.

[0020] Figure 4 This is a transmission electron microscope (TEM) image of nanoparticles based on a reversible saturated high nonlinear fluorescence process, provided in Example 1 of the optical limiting method based on a reversible saturated high nonlinear fluorescence process of the present invention.

[0021] Figure 5 This is a spectrum of nanoparticles based on a reversible saturated high nonlinear fluorescence process under 852 nm continuous laser excitation, provided in Example 1 of the optical limiting method based on a reversible saturated high nonlinear fluorescence process of the present invention.

[0022] Figure 6 This is a schematic diagram of the optical path system of the optical limiting method based on the reversible saturated highly nonlinear fluorescence process in the present invention.

[0023] Figure 7 This is a graph showing the absorption intensity of the optical limiting element versus the excitation intensity in Example 4 of the optical limiting method based on a reversible saturated highly nonlinear fluorescence process of the present invention.

[0024] Figure 8 This is a simulated normalized transmittance curve of the optical limiting element versus incident intensity in Example 4 of the optical limiting method based on a reversible saturated high nonlinear fluorescence process of the present invention. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0026] This invention provides a light limiting method based on a reversible saturated highly nonlinear fluorescence process, comprising the following steps: S1. Construction and preparation of reversible saturated high nonlinear fluorescent nanoparticles: The reversible saturated high nonlinear fluorescent nanoparticles integrate three or more photonic avalanche engines with different excitation thresholds. Each engine is individually activated at its respective threshold power, generating explosive excited-state absorption enhancement, causing the overall absorption coefficient of the nanoparticles to increase stepwise, thus producing reversible saturated fluorescence with high-order nonlinearity. The nanoparticles maintain a low absorption and high transmission state under low incident light intensity; as the incident light intensity successively reaches the excitation threshold of each engine, the absorption coefficient of the nanoparticles increases sharply at each level, while the transmittance decreases sharply at each level, thereby achieving multi-level confinement.

[0027] S2. Reversible saturated highly nonlinear fluorescent nanoparticles were diluted and dispersed in an optically transparent solid matrix to fabricate thin films or bulk optical limiting elements. The theoretical fluorescence intensity versus excitation light intensity curve is shown in the figure below. Figure 2 As shown. The doping concentration of the nanoparticles is controlled between 1% and 10%. The thickness of the optical limiting element is adjusted according to the target limiting power range and response speed requirements.

[0028] S3. A multi-level intelligent optical limiting optical path system is constructed using the multi-threshold characteristics of reversible saturated fluorescent nanoparticles. The optical limiting element is placed in the incident optical path, and a beam splitter and a photodetector are respectively set before and after it to form a closed-loop feedback control system. The specific optical path is as follows: Figure 3 The structure is as follows: The incident laser is split into two paths by the first beam splitter. The main beam passes through the optical limiting element and is split again by the second beam splitter. One path enters the main energy detector to record the output energy after limiting, and the other path enters the spectrometer to monitor the anti-saturation fluorescence characteristic peak to determine the currently activated engine level. The reference light split from the first beam splitter enters the reference energy detector to normalize the incident light intensity fluctuation. All detector signals are connected to the central controller, which analyzes the incident light intensity, transmitted light intensity and fluorescence spectral characteristics in real time.

[0029] S4. The limiting characteristic curve of the optical limiting element is tested using the multi-level intelligent optical limiting optical path system. The excitation light source is turned on and the incident laser power is gradually increased. At the same time, the central controller synchronously collects real-time data from the reference energy detector, the main energy detector, and the spectrum analyzer. Under low incident light intensity, all photon avalanche engines in the nanoparticles are not activated, the nanoparticles absorb the incident light weakly, and the element maintains a high transmission state. When the incident light intensity rises to the first threshold, the lowest threshold engine is activated, the excited state absorption is greatly enhanced, and the element transmittance drops sharply to the first-level low value. As the incident light intensity continues to rise, the first engine gradually saturates and the absorption weakens, but at this time the second engine is triggered, which again reduces the transmittance to the second-level low value. This process continues, so that the output light intensity is effectively clamped to multiple preset safety levels throughout the high-power range, realizing a multi-level, stepped limiting curve. The central controller plots the incident and output light intensity data recorded throughout the process into a limiting characteristic curve. This curve exhibits a unique "multi-stage stepped" shape, with the output light intensity level corresponding to each clamping platform determined by the excited-state absorption intensity of each engine. By analyzing this curve, key performance parameters of the optical limiting element can be obtained, including the limiting threshold at each stage, the clamping light intensity at each stage, the dynamic range, and the limiting depth, thereby completing a quantitative characterization of the multi-stage limiting performance of the optical limiting element.

[0030] The excitation threshold of each photon avalanche engine in the reversible saturated fluorescent nanoparticles is independently controlled by selecting different lanthanide ion doping systems (such as Pr). 3+ -Yb 3+ Co-doped system, high concentration of Tm 3+ Doped systems, Ho 3+ Doping systems, etc.); Select different host lattices (such as low phonon energy fluorides such as NaYF4, NaLuF4, NaGdF4, etc.) to achieve threshold differentiation by utilizing the significant influence of different host lattices on the photon avalanche threshold; (finely control the activation power of each engine by adjusting the doping concentration and spatial distribution).

[0031] The incident laser can be selected from multiple important near-infrared laser bands such as 852nm, 965nm, 1064nm, and 1450nm, depending on the different lanthanide ion doping systems.

[0032] The optically transparent solid matrix is ​​selected from one or more of polymethyl methacrylate, polyvinyl alcohol, or optical epoxy resin.

[0033] The optical limiting element can be integrated into the fiber end face, microcavity resonator, or waveguide structure to achieve a miniaturized, on-chip integrated optical limiter.

[0034] Polymer films doped with reversible saturated fluorescent nanoparticles can be directly coated onto the end face of an optical fiber or fused into the optical fiber link to form an all-fiber optical limiter.

[0035] The incident light intensity can be compared with the pre-determined threshold values ​​of each engine in the reversible saturated fluorescent nanoparticles to determine which engine level is currently in operation. Simultaneously, the intensity ratio of specific wavelengths in the fluorescence spectrum helps confirm the engine's activation status, avoiding misjudgments caused by power fluctuations.

[0036] Based on the identified engine level, the controller sends the corresponding preset attenuation value to the back-end variable attenuator (such as an electro-optic modulator or tunable filter) to clamp the output light intensity to the safety level corresponding to that level.

[0037] When the incident light intensity exceeds the saturation threshold of the highest-level engine and continues to rise, the controller issues a high-risk alarm signal and can trigger the aperture to completely cut off the light path.

[0038] Example 1 This embodiment provides a method for preparing nanoparticles based on a reversible saturated highly nonlinear fluorescence process. This embodiment also provides a method using Pr... 3+ -Yb 3+ As engine ions for multi-threshold photonic avalanche engines, nanoparticles prepared based on reversible saturated highly nonlinear fluorescence processes, such as nanostructures... Figure 4 As shown. The specific steps are as follows: The first step was to synthesize the core structure of the nanoparticles: At room temperature (2325℃), 5 mL of 0.2 mL of 0.2 Mn(CH3COO)3 solution (Ln = Y / Yb / Pr) was added to a 100 mL round-bottom flask, followed by the sequential addition of 7.5 mL of oleic acid and 17.5 mL of 1-octadecene. The reaction was carried out at 150℃ for 45 minutes to form the precursor. The heating mantle was removed, and the reaction mixture was allowed to cool to 40℃ while stirring. A mixture of 10 mL of 0.4 M NH4F methanol solution and 2.5 mL of 1 M NaOH methanol solution was rapidly added, and the reaction was carried out at 40℃ for at least 2 hours. Subsequently, the methanol was removed by reacting under vacuum at 100℃ for 30 minutes. After the methanol evaporated, the temperature was raised to 300℃ under an argon atmosphere and the reaction was carried out at this temperature for 1.5 hours. Remove the heating mantle and allow the reactants to cool to room temperature while stirring. Then add 15 mL of anhydrous ethanol and centrifuge at 7500 rpm for 5 minutes. Remove the supernatant and collect the product. Wash with a mixture of ethanol and cyclohexane to obtain the core NaYF4:Yb / Pr of the upconversion nanoparticles, which are dispersed in 8 mL of cyclohexane. Adjust the Y... 3 + Pr 3+ and Yb 3+ The concentration ratio is such that the doping component is NaYF4:Yb / Pr (25 / 0.5%) nanoparticles.

[0039] The synthesis method for each shell structure thereafter is largely the same as the core layer synthesis method described above. The difference lies in the addition of different volumes of lanthanide ion acetate solution according to the composition of each layer, and the addition of the previously synthesized nanoparticles after reacting at 150°C for 45 minutes and cooling to 90°C. The final six-layer core-shell nanoparticles are as follows: NaYF4:Yb,Pr(25%,0.5%)@NaYF4:Yb,Tm(10%,4%)@NaLuF4:Yb,Pr(15%,0.5%)@NaLuF4:Yb,Tm(10%,4%)@NaLuF4:Yb,Pr(30%,0.5%)@NaLuF4:Yb,Tm(10%,4%). During the shell synthesis process, the volume ratio of each layer was changed by varying the amount of core nanoparticles added. In this example, the feed ratio used was 1:0.5:1.5:1:2:2. Transmission electron microscopy (TEM) images of the successfully synthesized multilayer core-shell nanoparticles are shown below. Figure 5 As shown, the spectrum is as follows Figure 6 As shown.

[0040] Example 2 This embodiment provides a method for fabricating an optical limiting device based on a reversible saturated highly nonlinear fluorescence process. The reversible saturated fluorescent nanoparticles prepared in Example 1 are composited with a polymethyl methacrylate (PMMA) matrix to fabricate a thin-film optical limiting element. The specific steps are as follows: 1. Take 5 mL of the cyclohexane dispersion of nanoparticles prepared in Example 1 (concentration approximately 50 mg / mL), add 10 mL of anhydrous ethanol, sonicate for 5 minutes, centrifuge at 8000 rpm for 10 minutes, discard the supernatant, and collect the precipitate. Repeat the above washing process twice to remove residual reaction byproducts and excess oleic acid. Redisperse the washed nanoparticles in 10 mL of chloroform and sonicate for 30 minutes to obtain a clear and transparent chloroform dispersion of nanoparticles.

[0041] 2. Weigh 1g of polymethyl methacrylate (PMMA, Mw≈120,000, optical grade) granules, add them to 10mL of chloroform, and stir magnetically for 2 hours at room temperature until the PMMA is completely dissolved, forming a colorless and transparent viscous solution.

[0042] 3. Slowly add the nanoparticle chloroform dispersion prepared in step one to the PMMA chloroform solution prepared in step two, while stirring vigorously. The doping concentration of the nanoparticles is controlled by adjusting the amount added. In this embodiment, 2 mL of the nanoparticle chloroform dispersion is added to the PMMA solution, so that the mass fraction of the nanoparticles in the final solid film is 10%. Continue stirring for 1 hour to ensure that the nanoparticles are fully dispersed in the PMMA solution and form a uniform composite solution. To ensure the uniformity of the film, the composite solution is placed in an ultrasonic cleaner for ultrasonic degassing for 15 minutes to remove air bubbles introduced during stirring and further promote the dispersion of nanoparticles.

[0043] 4. Preparation of Nanoparticle-PMMA Composite Film. Place a clean glass petri dish on a level platform and calibrate it with a level to ensure the bottom of the dish is absolutely horizontal. Slowly pour the composite solution prepared in step 3 into the petri dish and gently shake it to spread the solution evenly. Place the petri dish in a fume hood, cover it with the lid (leaving a small gap to allow the solvent to evaporate slowly), and let it stand at room temperature for 48 hours to allow the chloroform solvent to evaporate slowly. A slow solvent evaporation rate is beneficial for forming a film with uniform thickness and a smooth surface, avoiding film shrinkage and cracking or surface roughness caused by rapid solvent evaporation. After the solvent has completely evaporated, place the petri dish in a vacuum drying oven at 60℃ for 2 hours to completely remove residual solvent. Then peel the film off from the bottom of the petri dish to obtain a circular composite film with a diameter of approximately 50 mm and a thickness of approximately 100 μm.

[0044] Example 3 This embodiment provides a specific method for constructing a multi-stage intelligent optical limiting optical path system based on a reversible saturated highly nonlinear fluorescence process optical limiting method. This system can be used to test the limiting characteristic curve of the optical limiting element prepared in Example 2 and to realize closed-loop feedback control. The basic principle is as follows: Figure 3 As shown, The system includes a laser generation module, an optical path control module, an optical limiting device, a photoelectric detection module, and a central processing unit.

[0045] The excitation optical path, used to generate single-wavelength excitation light, includes a near-infrared continuous laser 1, a half-glass slide 2, a polarizing beam splitter 3, a 25mm focusing lens 4, a pinhole filter 5, a 75mm focusing lens 6, and a quarter-glass slide 7. The near-infrared laser produces a continuous Gaussian laser output with a wavelength of 852nm to match the excited-state absorption of Pr ions. The half-glass slide 4, used in conjunction with the linear polarizer 5, can be used to adjust the power of the laser beam. The focusing lens group 4 and 6, along with the pinhole filter 5, are used to remove high-frequency components of the laser and expand the excitation beam. The quarter-glass slide 7 is used to adjust the phase of the excitation light.

[0046] The optical path control module is used to control the laser irradiation of the sample and transmit the laser to the photodetector module. It includes a first beam splitter 8, a motorized control aperture 9, a motorized control attenuator 10, a second beam splitter 12, and an 820nm long-pass filter 16. The electrically controlled aperture is used to control the laser beam. The laser beam, after power adjustment, is incident on the first beam splitter 8. The first beam splitter 8 splits the incident laser into two paths: the main path (accounting for 90% of the energy) is transmitted and then perpendicularly incident on the optical limiting element 11. The optical limiting element is fixed on a three-dimensional precision displacement stage, and its position can be adjusted to align the laser spot with the center region of the thin film; the reference path (accounting for 10% of the energy) is transmitted and then enters the reference energy detector. After passing through the optical limiting element, the main path laser is split again by the second beam splitter 12 (each beam splitting by 50%): one path is filtered by the 820nm long-pass filter 14 to emit fluorescence, and the remaining signal is incident on the main energy detector; the other path is received by the spectrometer. The electrically controlled aperture 9 is used to control the main optical path switch. When the incident light intensity exceeds the saturation threshold of the highest-level engine and continues to rise, the central controller issues a high-risk alarm signal and triggers the aperture to completely cut off the optical path. The electrically controlled attenuator 10 is used to control the excitation light power in real time.

[0047] The photoelectric detection module is used to collect the reference laser signal and the signal after the excitation light passes through the optical limiting device 11, as well as the fluorescence signal of the optical limiting device. It includes a reference energy detector 15, a focusing lens 14, a spectrometer 13, and a main energy detector 17. The main energy detector 17 is used to record the output energy after limiting, the spectrometer 13 is used to monitor the anti-saturated fluorescence characteristic peak to determine the currently activated engine level, and the reference energy detector 15 is used to normalize the incident light intensity of the reference optical path.

[0048] All detector signals are connected to the central controller 18, which calculates the incident light intensity, transmitted light intensity, and fluorescence spectral characteristics in real time.

[0049] Example 4 This embodiment utilizes the multi-level intelligent optical limiting optical path system built in Embodiment 3 to test the limiting characteristics of the optical limiting element. The specific steps are as follows: Before testing, complete system warm-up, zero-point calibration, and power density calibration, and set the incident light intensity from 0.01 kW / cm². 2 Up to 3000kW / cm 2 A logarithmic scan is performed, with a 2-second dwell time at each point to ensure a steady-state response. The main energy detector detects Pr in real time. 3+The fluorescence intensity of the 609 nm emission peak was measured and recorded by a spectrometer. The central controller synchronously acquired data from the reference energy detector, main energy detector, and spectrometer. Six stages of phenomena were observed during the test: In the low-intensity incident region, all photon avalanche engines in the nanoparticles based on the reversible saturated highly nonlinear fluorescence process were inactive, resulting in low absorption intensity of excitation light and extremely low emission fluorescence intensity; when the incident light intensity reached the first threshold, the first engine located in the core layer was activated, the transmittance dropped sharply, and the absorption intensity showed an S-shaped jump; subsequently, the first engine gradually saturated; when the incident light intensity reached the second threshold, the second engine located in the shell layer was activated, the transmittance dropped sharply a second time, and the fluorescence intensity jumped again; when the incident light intensity reached the third threshold, the third engine was activated, and the transmittance dropped sharply a third time; when the incident light intensity exceeded 2500 kW / cm², the fluorescence intensity increased; when the incident light intensity exceeded 2500 kW / cm², the fluorescence intensity decreased; when the incident light intensity reached the third threshold, the third engine was activated, and the transmittance dropped sharply a third time; when the incident light intensity exceeded 2500 kW / cm², the fluorescence intensity decreased; when the incident light intensity reached the third threshold ... 2 Afterwards, all three engines were saturated, and the transmittance stabilized at a low level. The absorption intensity curve of the optical limiting element as a function of excitation intensity is shown below. Figure 7 As shown; the simulated normalized transmittance versus incident intensity curve is as follows. Figure 8 As shown.

[0050] The central controller uploads all data for post-processing, obtaining threshold values ​​of 500 kW / cm for each level. 2 1000kW / cm 2 and 2000kW / cm 2 The absorption nonlinearity orders calculated near each threshold point were 21, 23, and 20, respectively, which are much higher than the linear response S=1, confirming the high nonlinearity characteristics of each engine during activation; the dynamic range of the incident light intensity response was 0.1 kW / cm². 2 Up to 3000kW / cm 2 (Spanning more than two orders of magnitude). Through the above tests and quantitative characterization, the effectiveness of the optical limiting method based on reversible saturated highly nonlinear fluorescence processes proposed in this invention in achieving multi-level adaptive limiting has been fully verified.

[0051] The present invention and its embodiments have been described above. This description is not restrictive. In short, if a person skilled in the art is inspired by this description and designs a similar structure and embodiment without departing from the spirit of the present invention, such design should fall within the protection scope of the present invention.

Claims

1. A light limiting method based on a reversible saturated highly nonlinear fluorescence process, comprising the following steps: S1. Construct and prepare reversible saturated fluorescent nanoparticles, wherein multiple photonic avalanche engines with different excitation thresholds are integrated inside the nanoparticles; each engine is individually activated at its respective threshold power to produce an explosive excited-state absorption enhancement. S2. Disperse the nanoparticles in an optically transparent solid matrix to fabricate an optical limiting element; S3. Construct a multi-level intelligent optical limiting optical path system, place the optical limiting element in the incident optical path, and set a beam splitter and a photodetector before and after it respectively to form a closed-loop feedback control system. S4. The limiting characteristic curve of the optical limiting element is tested using the multi-level intelligent optical limiting optical path system. The incident laser power is gradually increased while the incident light intensity, transmitted light intensity and fluorescence spectral characteristics are collected simultaneously. The intensity ratio of at least two characteristic wavelengths in the fluorescence spectrum is used to help confirm the currently activated engine level. As the absorption intensity of the optical limiting element increases sharply at each threshold, as the incident light intensity crosses each excitation threshold, the output light intensity is clamped at multiple preset safety levels throughout the high power range. This makes the transmittance change curve with incident light intensity present a multi-level stepped shape composed of multiple clamping platforms that appear in sequence, thereby realizing the quantitative characterization of the multi-level limiting performance of the optical limiting element.

2. The optical limiting method based on a reversible saturated highly nonlinear fluorescence process according to claim 1, characterized in that: In the optical limiting device based on the reversible saturated high nonlinear fluorescence process, all nanophotonic avalanche engines were not activated under low incident light intensity, the nanoparticles had weak absorption of incident light, and the device maintained a high transmission state. When the incident light intensity rises to the first threshold, the lowest threshold engine is activated, the excited state absorption is greatly enhanced, and the element transmittance drops sharply to the first-level low value. As the incident light intensity continues to rise, the first engine gradually saturates and the absorption weakens, but at this time the second engine is triggered, which again reduces the transmittance to the second-level low value. And so on, so that the output light intensity is effectively clamped at multiple preset safety levels throughout the high power range, realizing a multi-level, stepped limiting curve.

3. The optical limiting method based on a reversible saturated highly nonlinear fluorescence process according to claim 1, characterized in that: The limiting curve of the optical limiting device exhibits a unique multi-step shape, and the output light intensity level corresponding to each clamping platform is determined by the excited state absorption intensity of each engine. By analyzing this curve, the key performance parameters of the optical limiting element can be obtained, including the limiting threshold at each level, the clamping light intensity at each level, the dynamic range, and the limiting depth.

4. The optical limiting method based on a reversible saturated highly nonlinear fluorescence process according to claim 1, characterized in that: The optical limiting optical path system includes an incident light source, an optical limiting element, a first beam splitter, a second beam splitter, a main energy detector, a reference energy detector, a spectrometer, a central controller, and an aperture. The incident laser is split into two paths by the first beam splitter. The main optical path passes through the optical limiting element and is then split again by the second beam splitter. One path enters the main energy detector to record the output energy after limiting, while the other path enters the spectrometer to monitor the anti-saturation fluorescence characteristic peak to determine the currently activated engine level. The reference light split from the first beam splitter enters the reference energy detector to normalize the incident light intensity fluctuation. All detector signals are connected to the central controller.

5. The optical limiting method based on a reversible saturated highly nonlinear fluorescence process according to claim 4, characterized in that: The intelligent limiting control algorithm executed by the central controller includes: real-time acquisition of the intensity ratio of at least two characteristic wavelengths in the fluorescence spectrum, comparing it with a pre-established engine level-ratio mapping relationship to determine which engine level is currently in the working range; comparing the incident light intensity with the pre-calibrated threshold values ​​of each engine, and sending the corresponding attenuation preset value to the back-end variable attenuator according to the identified engine level, so that the output light intensity is clamped within the safe range corresponding to that level; when the incident light intensity exceeds the saturation threshold of the highest level engine and continues to rise, a high-risk alarm signal is issued, and the aperture is triggered to completely cut off the optical path.

6. The optical limiting method based on a reversible saturated highly nonlinear fluorescence process according to claim 1, characterized in that: The optical limiting element is integrated into the fiber end face, microcavity resonator, or waveguide structure, or the polymer film doped with the reversible saturated fluorescent nanoparticles is directly coated onto the fiber end face or fused into the fiber link to form an all-fiber optical limiter.

Images