Beam transport model using thermal halo effect, detection device of thermal halo effect

Abstract

The application discloses a light beam transmission model using thermal blooming effect, a detection device of thermal blooming effect, and a transmission model comprising a plurality of groups of competition processes arranged in sequence on an optical path, wherein each group of the competition processes comprises an equivalent convex lens and an equivalent concave lens arranged in sequence on the optical path. The application has the advantages that the application can demonstrate that the thermal blooming effect has practical application potential of improving target irradiance and maintaining high-power transmission by establishing a light beam transmission model using thermal blooming effect, i.e., a "self-focusing-thermal blooming defocusing" competition model.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of thermo-optic and electro-optic, and particularly relates to a light beam transmission model using thermal blooming effect and a detection device of thermal blooming effect. BACKGROUND

[0002] Thermal blooming effect refers to the change of air refractive index on the cross section of emitted laser beam caused by the absorption of laser energy by atmospheric molecules and aerosols, so as to cause the bending and distortion of laser beam.

[0003] Since the 1970s, as an atmospheric effect, the interest in thermal blooming mostly comes from the desire to transmit laser radiation energy in the atmosphere. For a long time, thermal blooming has been considered as a harmful nonlinear effect in the fields of adaptive optics and high-energy laser. Even if the atmospheric window is selected, thermal distortion is still very serious for high enough laser power, which hinders the long-distance transmission of high-power and high-beam-quality laser in the atmosphere, thereby limiting the widespread use of remote diagnosis, laser communication and laser radar missions. At present, most of the work is focused on how to avoid or minimize the influence of thermal blooming on high-power laser transmission. SUMMARY

[0004] In order to solve the above technical problems, the present application provides a light beam transmission model using thermal blooming effect and a detection device of thermal blooming effect, and the specific technical solutions are as follows:

[0005] The light beam transmission model using thermal blooming effect comprises a plurality of groups of competition processes arranged in sequence on an optical path, and each group of competition processes comprises an equivalent convex lens and an equivalent concave lens arranged in sequence on the optical path.

[0006] Specifically, the transmission model satisfies the following formula:

[0007]

[0008] Wherein, β is the diffraction coefficient, β K is the nonlinear absorption coefficient of K photons, n=n0+n t , n t is the thermal induced refractive index, and the second to fifth terms represent diffraction, self-focusing, K-photon absorption and thermal blooming effect, respectively.

[0009] A detection device of thermal blooming effect comprises a first laser, a laser adjusting unit, a beam splitter, a first camera, a cuvette containing a sample liquid, and a second camera.

[0010] The laser is used to provide laser with set parameters; the laser adjustment unit includes a first attenuator OA, a polarizer P, a beam expander BE, a spatial light modulator SLM, a first aperture CA1, a lens L1, a ground glass RGGD, and an achromatic lens L2.

[0011] Specifically, the sample liquid is carbon disulfide solution.

[0012] Specifically, the laser adjustment unit includes a first attenuator OA, a polarizer P, a beam expander BE, a spatial light modulator SLM, a first aperture CA1, a lens L1, a ground glass RGGD, and an achromatic lens L2.

[0013] The first attenuator OA and the polarizer P are respectively used to adjust the light intensity and the polarization of the laser; the beam expander BE is used to expand the laser beam; the spatial light modulator SLM generates Gaussian beams with different ellipticity by loading different holograms; the first aperture CA1 is used to select the first-order diffracted light; the lens L1 is used to adjust the spot diameter generated by the spatial light modulator to be equal to the spot diameter incident on the surface of the ground glass RGGD; the ground glass RGGD is rotatable and is used to generate anisotropic Gaussian-Schell model AGSM beams from the input spot.

[0014] Specifically, the first laser is an ultrafast fiber laser, which is used to provide laser with set wavelength, set pulse width and set repetition frequency.

[0015] Specifically, the first camera is a digital camera, and the second camera is a CCD camera.

[0016] Specifically, the laser adjustment unit generates a plurality of different ellipticity beams with equal cross sections.

[0017] A Z-can open and closed hole experimental device includes a laser L2, an attenuator OA2, a beam splitter BS2, a plano-convex lens L3, a cuvette LC containing a sample liquid, an aperture CA4, a detector D1, and a detector D2. The laser L2 emits a light beam that passes through the attenuator OA2 and is split into two beams by the beam splitter BS2. One beam passes through the plano-convex lens L3, the cuvette LC containing the sample liquid, the aperture CA4, and the detector D1 in sequence, and the other beam is detected by the detector D2. The cuvette containing the sample liquid is used to simulate the above-mentioned light beam transmission model using the thermal blooming effect.

[0018] Specifically, the sample liquid is carbon disulfide, and the scanning range of the cuvette containing carbon disulfide is -25 to 25 mm, with the focal point of the plano-convex lens L3 as the 0 point position.

[0019] The advantage of the present application is that the present application can demonstrate the practical application potential of the thermal blooming effect in improving the target irradiance and maintaining high power transmission by establishing a light beam transmission model using the thermal blooming effect, i.e., a "self-focusing-thermal blooming defocusing" competition model. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 A model diagram using the thermal blooming effect in the present application.

[0021] Figure 2 A schematic diagram of a detection device for the thermal blooming effect in the present application.

[0022] Figure 3 The detection device for the thermal blooming effect in the present application loads 5 different computer holograms through a spatial light modulator.

[0023] Figure 4 The spot shape diagram taken by the first camera of the detection device for the thermal blooming effect in the present application.

[0024] Figure 5 The spot shape diagram taken by the second camera of the detection device for the thermal blooming effect in the present application.

[0025] Figure 6 A Z-can open and closed hole experimental device diagram for clarifying the defocusing mechanism in the present application.

[0026] Figure 7 A normalized transmittance measurement result diagram using the Z-can open and closed hole experimental device.

[0027] Figure 8 A comparison diagram of optical characterization of light intensity variation and transmission distance.

[0028] Figure 9 A simulation diagram of the focusing-defocusing dynamic balance cycle. DETAILED DESCRIPTION

[0029] With reference to the drawings of the embodiments of the present application, the technical solutions in the embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments of the present application, all other embodiments obtained by a person of ordinary skill in the art without creative work fall within the protection scope of the present application. In the description of the embodiments of the present application, it should be understood that the terms “first” and “second” are used only for the purpose of description, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Therefore, the features limited by “first” and “second” can explicitly or implicitly include one or more of the features. In the description of the embodiments of the present application, the meaning of “multiple” is two or more than two, unless otherwise explicitly and specifically limited.

[0030] In actual situations, the light beam will produce diffraction, self-focusing, multi-photon absorption and thermal blooming effect. When the self-focusing effect is greater than the diffraction effect, the light beam will continuously converge until the light beam radius is zero, and the light beam transmission terminates, and the transmission distance is the collapse length. And multi-photon absorption and thermal blooming effect will produce defocusing effect.

[0031] At present, most of the work is focused on how to avoid or minimize the impact of thermal blooming on high-power laser transmission. Researchers predict that femtosecond laser filamentation may be an effective method to break this “bottleneck”, because it can achieve high-power, long-distance transmission with almost constant spot size. It is worth mentioning that the relationship between the collapse length of the self-focusing light beam and the input power follows the semi-empirical Marburgur formula, and the collapse length decreases with the increase of the power, which means that increasing the input power makes the self-guiding process advance. On the one hand, the light beam collapses due to the termination of self-guiding transmission, and on the other hand, the target power is only close to the self-focusing power threshold due to the repetition of “focusing-defocusing” cycle. Therefore, whether it is thermal blooming effect or self-focusing effect, the problem of maximum irradiance of the target has not been fundamentally solved in the physical mechanism, and the limitation of power threshold has not been broken through.

[0032] For the inevitable thermal blooming problem, the traditional strategy is to minimize the impact of thermal blooming on laser beam transmission as much as possible. This not only needs to take advantage of the atmospheric window, but also needs to take advantage of the time difference of laser-medium interaction to reduce the thermal blooming effect, and the femtosecond laser filamentation follows this physical mechanism, that is, the laser pulse duration is less than the acoustic transit time. As mentioned earlier, filamentation itself also has certain limitations.

[0033] In order to take the thermal blooming effect as a beneficial effect of laser and medium, based on the physical mechanism that a part of laser energy absorbed by the transmission medium generates an equivalent thermal lens to resist the Kerr self-focusing, the feasibility of thermal blooming effect to improve the target light intensity irradiance and maintain high-power transmission is studied, such asFigure 1 As shown in the present application, a model using thermal blooming effect is disclosed, which comprises a plurality of groups of competition processes arranged in sequence on an optical path, each group of the competition processes comprising an equivalent convex lens and an equivalent concave lens arranged in sequence on the optical path.

[0034] The use of the above model can realize the confrontation between thermal blooming effect and self-focusing effect, so as to convert the adverse factors of thermal blooming effect into advantages. The thermal blooming replaces the defocusing effect in the filamentation process, forming a dynamic balance between self-focusing and thermal blooming defocusing, which means that the thermal blooming will no longer be a harmful factor for transmission, but has a significant advantage in increasing the maximum light intensity. However, this way not only returns to the time problem of laser and medium interaction, but also overcomes the limitation of self-focusing power threshold.

[0035] For the problem of self-focusing power threshold, the inventors recently use an alternative method to control the power threshold and collapse length by regulating the initial polarization state and initial spatial coherence length. For polarization state manipulation, the power threshold can be adjusted by changing the topological charge, but the disadvantage is that it cannot be continuously regulated; by changing the initial coherence length, not only the power threshold is continuously adjustable in theory, but it is also feasible in experiments. In principle, controlling the power threshold is an effective method to solve the problem of maximum irradiance. However, due to the invisibility of the initial spatial coherence length, it brings inconvenience to experimental observation, so an equivalent parameter with visualization and continuous controllability is crucial.

[0036] In order to cope with these challenges, therefore, based on the model of the competition between thermal blooming defocusing and self-focusing effect established in the present application, the power threshold of the laser beam input into the model is continuously manipulated by the observable ellipticity. Therefore, there are several issues that need to be explained: (i) for the establishment of the model, the laser satisfies two prerequisites: short pulse duration ~ fs and high repetition frequency ~ MHz, which ensures that the two effects are observed obviously, but avoids laser filamentation; (ii) for the operation of the power threshold, based on the van-Cittert-Zernike theorem, the anisotropic Gaussian-Schell model (AGSM) beam can be regarded as a function of beam ellipticity and spatial coherence length, so AGSM is conducive to verifying the ellipticity as a new physical quantity for continuous operation of the power threshold.

[0037] Since it is relatively complex to obtain an analytical solution from the nonlinear Schrödinger equation, only the diffraction and self-focusing effects are considered for simplicity, thereby obtaining the general expressions for the self-focusing power threshold and the beam collapse length;

[0038] The cross-spectral power density of AGSM is given by

[0039]

[0040] where (x1, y1) and (x2, y2) are any two points of the source field, w 0i is the beam width of AGSM along i direction, and δ 0i is the transverse coherence length of AGSM along i direction.

[0041] With the help of the literature

E. Wigner, On the quantum correction for thermodynamic equilibrium, Phys. Rev. 40 749-759 (1932)

[0042]

[0043] When γ 0i → ∞, w i = w j , the power threshold of AGSM beam can be simplified to that of Gaussian beam, i.e. n0and n2are the linear and nonlinear refractive indices, respectively.

[0044] According to the literature

W. Krolikowski, D. Edmundson, O. Bang, Unified Model for Partially Coherent Solitons in Logarithmically Nonlinear media, Phys. Rev. E 61, 3122-3126. (2000)

[0045]

[0046] where β is the diffraction coefficient.

[0047] According to

L. Lu, Z. Wang, Y. Cai, Self-focusing propagation characteristics of a radially-polarized beam in nonlinear media, Opt. Express 30(10), 15905-15912 (2022)

L. Lu, Z. Wang, J. Yu, C. Qiao, R. Lin and Y. Cai, Self-Focusing Property of Partially Coherent Beam with Non-Uniform Correlation Structure in Non-Linear Media, Front. Phys. 9, 807542 (2022)

[0048]

[0049] where the Rayleigh length along the i direction is z Ri = kw 0i δ 0i / 2, P in is the incident power.

[0050] Under the conditions of diffraction and self-focusing effect, the analytical expressions of the self-focusing power threshold and the collapse length are theoretically derived, and the relationship between the above analytical expressions and the beam ellipticity is determined.

[0051] In summary, in order to realize variable and observable ellipticity to test the model of competition between thermal blooming defocusing and self-focusing effect, as shown in Figure 2 The present application discloses a detection device for thermal blooming effect, which comprises a first laser, a laser adjusting unit, a beam splitter, a first camera, a cuvette containing sample liquid, and a second camera. The laser is used to provide laser with set parameters; the laser adjusting unit comprises a laser intensity adjusting unit and an ellipticity adjusting unit for adjusting the ellipticity of the laser beam; the beam splitter divides the light beam output by the laser adjusting unit into two beams, one of which passes through a corresponding diaphragm to be incident into the first camera, and the other of which passes through a corresponding diaphragm to irradiate the cuvette containing sample liquid, and the second camera is used to take pictures of the side wall of the cuvette. The cuvette containing sample liquid is used to generate thermal blooming effect, and in this embodiment, the sample liquid is carbon disulfide solution, which can also be sodium chloride solution.

[0052] In one of the embodiments, the first laser is a superfast fiber laser, the first laser is a superfast fiber laser for providing laser with a set wavelength, a set pulse width and a repetition frequency, in one of the embodiments, for providing laser with a center wavelength of 780 nm, a pulse width of 75 fs and a repetition frequency of 80 MHz. The model of the first laser is Rainbow 1550 Dichro.

[0053] In one of the embodiments, the laser adjusting unit comprises a first attenuator OA, a polarizer P, a beam expander BE, a spatial light modulator SLM, a first diaphragm CA1, a lens L1, a ground glass RGGD, an achromatic lens L2, the first attenuator OA and the polarizer P are respectively used for adjusting the light intensity and the polarization of the laser. The beam expander BE is used for expanding the laser beam. The spatial light modulator generates Gaussian beams with different ellipticities by loading different holograms. The first diaphragm CA1 is used to select the first-order diffracted light, the lens L1 adjusts the spot diameter generated by the spatial light modulator to be equal to the spot diameter incident on the surface of the ground glass RGGD, and the beam exits after rotating the ground glass RGGD, thereby generating an anisotropic Gaussian-Schell model AGSM beam. The AGSM beam passes through the achromatic lens L2 with a focal length of 60 mm. In one of the embodiments, in order to ensure consistent incident power, different calculated holograms are loaded through the spatial light modulator, as shown in Figure 3 Five special cases of A-E different ellipticity beams with equal cross sections generated by the laser adjusting unit are: A (0.4w0, w0), B (0.5w0, 0.8w0), C (0.4w0, 0.4w0), D (0.8w0, 0.5w0) and E (w0, 0.4w0).

[0054] Based on the five different ellipticity beams A-E, the beam output by the laser adjusting unit is divided into two beams by the beam splitter BS: one beam is imaged by the first camera after passing through the diaphragm CA2, as shown in Figure 4 The other beam is incident on the carbon disulfide cuvette through the diaphragm CA3 and is incident on the second camera perpendicular to the direction of the beam transmission, in one of the embodiments, the first camera is a digital camera with a model of Cam (Canon 850D), and the second camera is a CCD camera with a model of Sorebury BC106N-VIS / M), and other models of the first camera and the second camera can also be used, which all belong to the protection scope of the present application. The second camera captures a transmission dynamic image, as shown in Figure 5 .

[0055] Since the thermal blooming effect and the multi-photon absorption will both contribute to the defocusing, in order to verify the contribution ratio of the thermal blooming effect and the multi-photon absorption to the defocusing of the beam and to clarify the mechanism of the defocusing, the present application further discloses a Z-can open hole and closed hole experimental device, as shown inFigure 6 As shown, it comprises a laser L2, an attenuator OA2, a beam splitter BS2, a plano-convex lens L3, a cuvette LC containing sample liquid, an aperture CA4, a detector D1 and a detector D2, the light beam emitted by the laser L2 passes through the attenuator OA2 and is split into two beams by the beam splitter BS2, one of the beams passes through the plano-convex lens L3, the cuvette LC containing sample liquid, the aperture CA4 and the detector D1 in sequence, and the other beam passes through the detector D2 for detection.

[0056] Specifically, the second laser is a titanium sapphire laser (800 nm, 95±10 fs, 1 kHz), the plano-convex lens (f=150 mm) is focused to a 10 mm thick quartz cuvette containing carbon disulfide solution, the power value is obtained by the first detector, and the other laser beam passes through the second detector to obtain the power value. The scanning range of the cuvette containing carbon disulfide solution is -25 to 25 mm, and the focal point of the plano-convex lens L3 is taken as the 0 point position. The nonlinear refractive index is obtained by the closed aperture Z-scan, and the multiphoton absorption coefficient is calculated by the open aperture Z-scan, and the measurement result of the normalized transmittance is as shown in Figure 7 The results show that the nonlinear refractive index of carbon disulfide is n2=2.20×10 -15 cm 2 / W, the nonlinear absorption coefficient is β2=4.71×10 -13 cm / W, and it is proved that two-photon absorption occurs. Compared with the defocusing caused by thermal blooming effect, two-photon absorption is much smaller, and it can be known from the above that the defocusing effect is mainly caused by the thermal blooming effect.

[0057] The self-focusing-thermal blooming defocusing model established in the application satisfies the following formula:

[0058]

[0059] Wherein, β is the diffraction coefficient, β K is the K-photon nonlinear absorption coefficient, n=n0+n t , n t is the thermal induced refractive index, and the second to fifth terms represent diffraction, self-focusing, K-photon absorption (K=2, two-photon absorption is calculated by Z-scan experiment) and thermal blooming effect, respectively.

[0060] The theoretical, numerical and experimental results are compared (wherein the theoretical results are obtained by formula (4) and formula (5), the numerical results are calculated by Runge-Kutta distribution, and the experimental results are obtained by Figure 2Theoretical results and numerical values are obtained by Origin drawing software to draw the relationship between normalized light intensity and transmission distance. It is found that both numerical simulation and experimental measurement data show that thermal blooming effect increases the maximum light intensity and delays the appearance of maximum light intensity, and experimental measurement and numerical simulation are in good agreement, as shown in Figure 8 It can be seen that the "self-focusing-thermal blooming defocusing" model is reasonable. Comparing Figure 8 The data in Table 1 are obtained, as follows:

[0061]

[0062] Table 1

[0063] In addition, the present application verifies the feasibility of the "self-focusing-thermal blooming defocusing" multi-cycle; numerical calculation of the dynamic evolution of laser transmission in the medium again demonstrates the rationality of the "self-focusing-thermal blooming defocusing" model, and the thermal blooming effect has the practical application potential of improving the target irradiance and maintaining high-power transmission, as shown in Figure 9 .

[0064] The present application gives the transmission physical mechanism model constructed by "self-focusing-thermal blooming defocusing", and verifies the rationality and correctness of the model. Through the introduction of thermal blooming effect, it is further illustrated that thermal blooming effect has significant advantages in the transmission process.

[0065] In the description of the present application, the description of the terms "some embodiments" or "examples" means that the specific features, structures, materials or characteristics described in connection with the embodiments or examples are included in at least one embodiment or example of the present application. In the present application, the illustrative description of the above terms does not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described can be combined in any one or more embodiments in a suitable manner. In addition, those skilled in the art can combine and combine different embodiments or examples described in the present application.

[0066] The above is only the preferred embodiment of the present application, and does not limit the present application in any way. Any person skilled in the art can make any form of equivalent replacement or modification of the technical solutions and technical contents disclosed in the present application without departing from the scope of the technical solutions of the present application, which still belongs to the protection scope of the present application.

Claims

1. A model of beam transport using the thermal halo effect, characterized in that, The application comprises a plurality of groups of competition processes arranged in sequence on the light path, each group of the competition processes comprising an equivalent convex lens and an equivalent concave lens arranged in sequence on the light path; The transmission model satisfies the following formula: where β is the diffraction coefficient, β K is the K-photon nonlinear absorption coefficient, n = n0+ n t , n t is the thermal induced refractive index, and the second to fifth terms represent diffraction, self-focusing, K-photon absorption, and thermal blooming effects, respectively.

2. A device for detecting thermal blooming effects, characterized in that The application comprises a first laser, a laser adjusting unit, a beam splitter, a first camera, a cuvette containing a sample liquid, and a second camera. The laser is used to provide laser with set parameters; the laser adjusting unit comprises an attenuator OA and a polarizer P for adjusting the intensity and polarization of the laser; the beam splitter divides the light beam output by the laser adjusting unit into two beams, one of which is incident on the first camera through a corresponding diaphragm, and the other of which is incident on the cuvette containing the sample liquid through a corresponding diaphragm; the second camera is used to take a picture of the side wall of the cuvette; and the cuvette containing the sample liquid is used to realize the light beam transmission model using the thermal blooming effect according to the equivalent of claim 1.

3. A device for detecting thermal blooming effects according to claim 2, characterized in that The sample liquid is carbon disulfide solution.

4. The apparatus for detecting thermal blooming effects of claim 2, wherein, The laser adjusting unit comprises a first attenuator OA, a polarizer P, a beam expander BE, a spatial light modulator SLM, a first diaphragm CA1, a lens L1, a ground glass RGGD, and an achromatic lens L2. The first attenuator OA and the polarizer P are respectively used to adjust the intensity and polarization of the laser; the beam expander BE is used to expand the laser beam; the spatial light modulator SLM generates Gaussian beams with different ellipticities by loading different holograms; the first diaphragm CA1 is used to select the first-order diffracted light; the lens L1 is used to adjust the spot diameter generated by the spatial light modulator to be equal to the spot diameter incident on the surface of the ground glass RGGD; and the ground glass RGGD is rotatable and is used to generate an anisotropic Gaussian-Schell model AGSM light beam from the input spot.

5. The apparatus for detecting thermal blooming effects of claim 2, wherein, The first laser is an ultrafast fiber laser, which is used to provide laser with set wavelength, set pulse width, and set repetition frequency.

6. The apparatus for detecting thermal blooming effects of claim 2, wherein, The first camera is a digital camera, and the second camera is a CCD camera.

7. The apparatus for detecting thermal blooming effects of claim 2, wherein, The laser adjusting unit generates a plurality of different ellipticity beams with equal cross sections.

8. A Z-can open and closed cell experimental device characterized by, The application comprises a laser L2, an attenuator OA2, a beam splitter BS2, a plano-convex lens L3, a cuvette LC containing a sample liquid, a diaphragm CA4, a detector D1, and a detector D2; the light beam emitted by the laser L2 is divided into two beams by the attenuator OA2 and the beam splitter BS2; one of the two beams passes through the plano-convex lens L3, the cuvette LC containing the sample liquid, the diaphragm CA4, and the detector D1 in sequence, and the other beam is detected by the detector D2; and the cuvette LC containing the sample liquid is used to realize the light beam transmission model using the thermal blooming effect according to the equivalent of claim 1.

9. A Z-can open and closed pore test device according to claim 8, characterized in that The sample liquid is carbon disulfide, and the scanning range of the cuvette containing carbon disulfide is -25 to 25 mm, with the focal point of the plano-convex lens L3 as the 0-point position.

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