An experimental device and method for realizing and measuring thermal lens effect of vector light beam

Abstract

The application discloses an experimental device and method for realizing and measuring thermal lens effect of a vector light beam. The device comprises a first laser for radiating a detection laser to a third direction, and a second laser for radiating a heating laser to the third direction, wherein the heating laser is a vector laser; the detection laser and the heating laser satisfy that the product of the detection laser power and the absorption rate of the detection laser in an anisotropic sample material is less than the product of the heating laser power and the absorption rate of the heating laser in the material by 5 orders of magnitude; and the anisotropic sample material, a filter and a detector are sequentially arranged along the third direction; the filter is used for filtering the heating laser emitted from the sample to be measured; and the detector is used for receiving the light signal of the detection laser. The application can realize the generation, regulation and measurement of the thermal lens effect of the vector light beam, and provides a convenient experimental device for the application research of the vector light beam in the thermal lens effect.

Description

Technical Field

[0001] The present invention relates to the technical field of thermal lens effect, and in particular to an experimental device and method for realizing and measuring the thermal lens effect of a vector beam. Background Art

[0002] When a laser beam is irradiated onto a sample and absorbed, due to the specific distribution of light intensity, the light energy absorbed at different locations of the sample medium is also different, which is accompanied by an increase in the sample temperature and the existence of a temperature gradient. Due to the existence of the thermo-optic coefficient, the uneven distribution of temperature will cause an uneven distribution of the refractive index of the transparent medium, thereby modulating the intensity distribution of the transmitted light. As early as 1965, Gordon et al. first experimentally discovered the thermal lens effect (TLE) of a Gaussian beam, and confirmed through theoretical calculations that it can be equivalent to a lens under certain conditions [JP Gordon, RCC Leite, RS Moore, et al, Journal of Applied Physics, 36(1), 3-8(1965).]. This marked the beginning of the research and application of the thermal lens effect in various fields of optics.

[0003] Thermal lens spectroscopy [R.C.C. Leite, R.S. Moore, J.R. Whinnery, Applied Physics Letters, 5(7), 141-143 (1965).] is one of the important methods for precise measurement of medium absorption spectrum due to its high sensitivity [M.E. Long, R.L. Swofford, A.C. Albrecht, 191(4223), 183-185 (1976).] In addition, thermal lens technology has important applications in chemical analysis, environmental monitoring, biomedicine and other fields. However, with the advent of high-power lasers, laser thermal lens effect may damage optical elements, reduce beam quality and stability [J. Foster, L.J. J. o. A. P. Osterink, 41(9), 3656-3663 (1970); D. Metcalf, P. de Giovanni, J. Zachorowski, et al, 26(21), 4508-4517 (1987); M. Innocenzi, H. Yura, C. Fincher, et al, 56(19), 1831-1833 (1990); Y.-F. Chen, T. Huang, C. Kao, et al, 33(8), 1424-1429 (1997); F. Hoos, S. Li, T. Meyrath, et al, 16(9), 6041-6049 (2008).] In order to solve this problem, in recent years, our country has published a number of patents focusing on reducing the laser thermal lens effect: a method for reducing the thermal lens effect of a laser crystal [invention patent CN111884022A], a compensation method for high-power laser thermal lens effect [invention patent CN108988114A], a compensation method for thermal lens effect of a laser rod [invention patent CN212626497U].

[0004] In recent years, structured light field has attracted widespread attention. LG beams with orbital angular momentum (OAM) will produce unique thermal lens effect different from Gaussian beams [Y. Li, W. Zhang, L.J. O. E. Chen, 30(17), 30210-30218 (2022).]. However, the thermal lens effect based on vector beams has not been studied. SUMMARY

[0005] Therefore, the purpose of the present application is to provide an experimental device and method for realizing the thermal lens effect and measurement of vector beams, which can provide a convenient experimental device for the generation, control and measurement of vector beam thermal lens effect, and facilitate the application research of vector beams in the aspect of thermal lens effect.

[0006] According to one aspect of the present application, an experimental device for realizing vector beam thermal lens effect and measurement is provided, comprising:

[0007] a first laser for radiating a probe laser to a third direction,

[0008] a second laser for radiating a heating laser to the third direction, the heating laser being a vector laser;

[0009] and the probe laser and the heating laser satisfy that the product of the probe laser power and its absorption rate in the material is less than the product of the heating laser power and its absorption rate in the material by 5 orders of magnitude in the same anisotropic sample material;

[0010] and,

[0011] an anisotropic material sample, a filter, and a detector arranged in sequence along the third direction

[0012] the filter is used for filtering the heating laser emitted from the sample to be measured;

[0013] the detector is used for receiving the optical signal of the probe laser.

[0014] In the above technical solution, the anisotropic material is heated by the vector beam to realize the thermal lens effect. The thermal lens effect of the anisotropic material is used to realize the shaping of the probe beam. Further, the device can realize the generation, control and measurement of the vector beam thermal lens effect, and provides a train of thought for the miniaturization and integration of the optical field unidirectional control device. Further, the device also has potential applications in the measurement of the physical properties of anisotropic substances. Further, the probe laser and the heating laser satisfy that the product of the probe laser power and its absorption rate in the material is less than the product of the heating laser power and its absorption rate in the material by 5 orders of magnitude in the same anisotropic sample material; the purpose is to observe the novel thermal lens effect similar to the column lens effect based on the characteristics of the anisotropic material sample, and considering the absorption rate and thermal absorption rate of the anisotropic material sample to different wavelengths, the above limiting relationship can be set to observe the novel thermal lens effect similar to the column lens effect.

[0015] In some embodiments, the first laser radiates the probe laser along a first direction, and a first filtering system and an attenuating sheet are arranged in sequence along the first direction, the probe laser passes through the first filtering system and the attenuating sheet, and is reflected to a third direction;

[0016] the second laser radiates the heating laser along a second direction, and a second filtering system, a second polarizing sheet, a vector beam adjusting device, and a converging system are arranged in sequence along the second direction, the heating laser passes through the second filtering system, the second polarizing sheet, the vector beam adjusting device, and the converging system, and is reflected to the third direction.

[0017] In the technical solution, the probe laser and the irradiation laser pass through a filtering system respectively for shaping, so that the laser is changed into a standard Gaussian beam, which is conducive to realizing the effect of a cylindrical lens. Further, the probe laser passes through a first polarizer for polarization, so that the probe light maintains a uniform polarization state, thereby reducing the influence of unnecessary light beams and facilitating accurate calculation of the focal length of the thermal lens. The purpose of such arrangement is that the light emitted by the laser is linearly polarized, and therefore, rotating the polarizer can conveniently adjust the energy of the incident light, thereby complementing the attenuation of the attenuator for precise and continuous adjustment of the light intensity. Further, the attenuator is used to attenuate the energy of the probe laser to avoid overexposure of the detector. Further, the heating laser first passes through a second polarizer and then passes through a vector beam adjustment device, so that the heating laser can be adjusted into different types of vector beams. The vector beam adjustment device can be a vortex wave plate, or can be generated by using a spatial light modulator, a metasurface structure, a diffractive optical element, and other phase modulation methods, which are not limited here.

[0018] In some embodiments, the device further comprises a beam splitter; the probe laser and the heating laser are combined via the beam splitter, and then the combined laser is irradiated along a third direction.

[0019] In the technical solution, the beam splitter serves as a combiner, and by simply adjusting the reflection mode, the optical axes of the detection laser and the heating laser beam can be made to coincide after passing through the beam splitter.

[0020] In some embodiments, the first filtering system comprises a first convex lens and a second convex lens arranged in sequence along a first direction.

[0021] The positions of the first convex lens and the second convex lens relative to the anisotropic material sample satisfy the following relationship condition:

[0022] The radius of the probe laser spot on the anisotropic material sample is smaller than the radius of the heating laser spot, and the sample is located outside the Rayleigh distance of the probe laser beam waist.

[0023] In the technical solution, considering that the temperature field distribution of the anisotropic material after thermal absorption is not uniform from the inside to the outside, and a good cylindrical lens effect is generated in the central region, the size of the probe beam is smaller than that of the heating beam, which is conducive to improving the detection accuracy. Further, the sample is located outside the Rayleigh distance of the probe laser beam waist, so as to facilitate the calculation of the equivalent focal length of the thermal lens using geometric methods.

[0024] In some embodiments, the vector beam is a radial polarization vector beam or an angular polarization vector beam.

[0025] In the above technical solution, the device can achieve a cylindrical lens-like effect with continuously adjustable equivalent focal length by varying the heating light power. Based on a second polarizer and a vector beam adjustment device, the device can adjust a radially polarized vector beam or an angularly polarized vector beam. The thermal lens induced by the radially polarized vector beam can achieve unidirectional divergence or convergence of the scalar light field, while the angularly polarized vector beam achieves unidirectional control of the scalar light field in a direction perpendicular to the radially polarized vector beam.

[0026] In some embodiments, the vector beam adjustment device uses a vortex wave plate.

[0027] In the above technical solution, the use of vortex wave plates is less invasive to the overall optical system and the method is simple.

[0028] In some embodiments, the distance between the sample and the detector is about 50-60 cm.

[0029] In the above technical solution, considering that the distribution of the temperature field of the anisotropic material after heat absorption is uneven from the inside to the outside, setting the distance between the sample and the detector helps to improve the detection accuracy.

[0030] In some embodiments, the ratio of the heating laser spot diameter to the detection laser spot diameter is 2:1 to 4:1.

[0031] In the above technical solution, in order to achieve a better thermal lens effect, if the radius of the heating laser and the detection laser are consistent, the observation effect will be poor. As a preferred value, it is set to 2:1.

[0032] According to another aspect of the present invention, an experimental method for realizing and measuring the thermal lens effect of a vector beam is provided, based on the above-mentioned experimental device for realizing and measuring the thermal lens effect of a vector beam; the method comprises the following steps:

[0033] Based on the experimental device, coaxial detection laser and heating laser are radiated to the sample, and the detection laser spot after emission is recorded by a detector;

[0034] Based on the detector recording results, the thermal lens equivalent focal length F is calculated by the following formula:

[0035]

[0036] Wherein, w is the spot radius of the detection laser at the sample, w1 is the spot radius of the detection laser when no sample is placed in the device, w2 is the spot radius of the detection laser when the sample is placed in the device, and z is the distance between the sample and the detector.

[0037] In the technical solution, the anisotropic material is heated by changing the light field distribution and the polarization state and using the vector light beam to realize the thermal lens effect different from the Gaussian light beam. Further, the device can realize the generation, regulation and measurement of the vector light beam thermal lens effect. Further, the device provides a train of thought for realizing the miniaturization and integration of the light field one-way regulation device, and has potential application in the measurement of the physical properties of the anisotropic material. The specific principle of calculating the equivalent focal length F of the thermal lens will be described below, and will not be repeated here.

[0038] In some embodiments, the experimental device radiates coaxial probe laser and heating laser to the sample, and records the probe laser spot after emission by using a detector, specifically:

[0039] Adjust the power parameter of the second laser, and modulate the heating laser into a radial polarization vector light beam or an angular polarization vector light beam.

[0040] Turn on the first laser, adjust the sample position to be outside the Rayleigh distance after the probe laser beam focal point, and set the probe laser spot radius on the anisotropic material sample to be smaller than the heating laser spot radius.

[0041] Turn on the second laser, and record the probe laser spot by using a detector.

[0042] In the technical solution, the device realizes the one-way divergence or convergence of the scalar light field based on the polarization phase modulation and the thermal lens induced by the radial polarization vector light beam. The angular polarization vector light beam realizes the one-way regulation of the scalar light field in the direction perpendicular to the former. BRIEF DESCRIPTION OF DRAWINGS

[0043] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiment or prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0044] Figure 1 is a light path schematic diagram of an embodiment of the experimental device for realizing the vector light beam thermal lens effect and measurement of the present application;

[0045] Figure 2 is a flowchart of an embodiment of the experimental method for realizing the vector light beam thermal lens effect and measurement of the present application;

[0046] Figure 3is an embodiment of the experimental method for realizing the thermal lens effect and measurement of a vector light beam, after different polarization states of the vector light beam heat an anisotropic medium, the temperature and refractive index distribution of the sample, the light intensity distribution of the detection light beam modulated by the thermal lens, and the schematic diagram of the equivalent cylindrical lens;

[0047] Figure 4 is the theoretical simulation (left) and experimental results (right) of the light intensity distribution of the detection light beam modulated by the thermal lens in an embodiment of the experimental method for realizing the thermal lens effect and measurement of a vector light beam;

[0048] Figure 5 is a schematic diagram of the relationship curve between the equivalent focal length of the thermal lens induced by the vector light beam of different polarization states and the heating light power in an embodiment of the experimental method for realizing the thermal lens effect and measurement of a vector light beam;

[0049] Figure 6 is a physical model diagram for calculating the equivalent focal length of the thermal lens in an embodiment of the experimental method for realizing the thermal lens effect and measurement of a vector light beam. DETAILED DESCRIPTION

[0050] The application will be further described below in conjunction with the drawings and embodiments. It is particularly pointed out that the following embodiments are only used to illustrate the application, but do not limit the scope of the application. Similarly, the following embodiments are only part of the embodiments of the application, not all embodiments, and all other embodiments obtained by those skilled in the art without creative labor are within the scope of the application.

[0051] The application provides an experimental device for realizing the thermal lens effect and measurement of a vector light beam, which can provide a convenient experimental device for the generation, control and measurement of the thermal lens effect of a vector light beam, and facilitate the application research of the vector light beam in the thermal lens effect.

[0052] One of the embodiments

[0053] Please refer to Figure 1 An experimental device for realizing the thermal lens effect and measurement of a vector light beam.

[0054] The first laser 1 is used to radiate a detection laser to a third direction C. In this embodiment, the first laser 1 is a small-power helium-neon laser with a wavelength of 632.8 nm.

[0055] The second laser 2 is used to radiate a heating laser to a third direction C, and the heating laser is a vector laser. In this embodiment, the second laser 2 is a large-power laser 2 with a wavelength of 405 nm.

[0056] and the probe laser and the heating laser satisfy: the product of the probe laser power and its absorption rate in the sample material is less than the product of the heating laser power and its absorption rate in the sample material by 5 orders of magnitude; in the embodiment, based on the anisotropic characteristics of the liquid crystal molecules, and considering that the liquid crystal molecules have good absorption rate at 405 nm wavelength and poor thermal absorption rate at 633 nm, the above wavelengths are selected as the heating and probe beams, respectively, so that the novel thermal lens effect similar to the cylindrical lens effect can be observed. It should be noted that the probe laser and the heating laser satisfy the above relationship to achieve the corresponding effect, and the liquid crystal molecules are used as the sample in the embodiment, and therefore the above selection is made.

[0057] and,

[0058] The anisotropic material sample 19, the filter 20, and the detector 21 are sequentially arranged along the third direction C.

[0059] The filter 20 is configured to filter the heating laser emitted from the sample 19.

[0060] The detector 21 is configured to receive the optical signal of the probe laser. In the embodiment, the detector 21 is a CMOS camera.

[0061] In the embodiment, the first laser 1 radiates the probe laser along a first direction A, and a first filtering system (the first convex lens 3, the pinhole 4, and the second convex lens 5), the first polarizer 6, and the attenuator 7 are sequentially arranged along the first direction A. The probe laser passes through the first filtering system, the first polarizer 6, and the attenuator 7, and then is radiated to a third direction C through the first mirror 8 and the 50:50 non-polarized beam splitter 18. The detection laser passing through the filter is irradiated on the detector 21, and then is stored into a computer.

[0062] The second laser 2 radiates heating laser along a second direction B, and a second filtering system (third convex lens 9, pinhole 10, fourth convex lens 11), a second polarizer, a vector beam adjusting device are sequentially arranged along the second direction B. The heating laser passes through the second filtering system, the second polarizer 12, the vector beam adjusting device, and then is radiated to a third direction through the second mirror 16, the third mirror 17, and the 50:50 non-polarized beam splitter 18. The heating laser is filtered out by the filter 20. In the embodiment, the vector beam adjusting device uses a vortex wave plate 13. The vortex wave plate has less invasiveness to the overall optical system and a simple method. In the embodiment, a converging lens group (fifth convex lens 14, sixth convex lens 15) is further arranged after the vector beam adjusting device along the second direction. It should be noted that the converging lens group can also be arranged on the probe laser path, as long as the ratio of the heating laser spot size to the probe laser spot size is within the range of 2:1 to 4:1, and the lens effect is best within the range. This is because in order to achieve better thermal lens effect, if the radius of the heating laser and the probe laser is consistent, the observation effect is not good. As a preferred value, it is set to 2:1.

[0063] In the embodiment, the probe laser and the radiated laser respectively pass through a filtering system for shaping, so that the laser is changed into a standard Gaussian beam. Such an arrangement is conducive to achieving a cylindrical lens effect. Further, the probe laser passes through a first polarizer for polarization, so that the probe light maintains a uniform polarization state, thereby reducing the influence of unnecessary beams and facilitating accurate calculation of the thermal lens focal length. The purpose of such an arrangement is that the laser exit light is linearly polarized. Therefore, rotating the polarizer can also conveniently adjust the energy of the incident light, thereby achieving precise and continuous adjustment of the light intensity complementary to the attenuator. Further, the attenuator is used to attenuate the probe laser energy to avoid overexposure of the detector. Further, the heating laser passes through the second polarizer and then passes through the vector beam adjusting device, so that the heating laser can be adjusted into different types of vector beams. The vector beam adjusting device can be a vortex wave plate, and can also use a spatial light modulator, a metasurface structure, a diffractive optical element, and other phase modulation methods. Here, no limitation is made.

[0064] In the embodiment, the beam combination uses a 50:50 non-polarization beam splitter 18. After the probe laser and the heating laser pass through the beam splitter, the combined laser is radiated along a third direction C. It should be noted that there are many ways of beam combination. In the embodiment, the beam splitter is used for the sake of simple experimental arrangement. The beam splitter is used as a beam combiner. The detection laser and the heating laser beam can be made to have the same optical axis by simply adjusting the reflection mode. Those skilled in the art can select a suitable beam combination optical path scheme according to actual needs. At the same time, the distance between the sample and the detector is about 50-60 cm. Considering that the temperature field distribution of the anisotropic material after heat absorption is not uniform from the inside to the outside, setting the distance between the sample and the detector helps to improve the detection accuracy.

[0065] In the embodiment, the light emitted by the second laser 2 is incident on a spatial filtering system comprising a third convex lens 9, a pinhole 10, and a fourth convex lens 11, and is shaped into a standard Gaussian beam. A polarizer 12 controls the linear polarization of the light. The light is then normally incident on the center of a vortex wave plate 13 with l=1. Changing the 0° fast axis direction of the vortex wave plate to be parallel or perpendicular to the linear polarization direction can respectively obtain a radial polarization beam or an angular polarization beam. A fifth convex lens 14 and a sixth convex lens 15 reduce the radius of the heating vector beam to a suitable size. Adjusting the second mirror 16 and the third mirror 17 makes the heating laser normally incident on the beam splitter 18.

[0066] In the embodiment, the laser beam of the first laser 1 and the laser beam of the second laser 2 have the same optical axis after passing through the beam splitter 18. Adjusting the first polarizer 6 can change the polarization direction of the detection laser. An attenuator 7 is used to reduce the detection laser to a suitable intensity. Adjusting the positions of the first convex lens 3, the second convex lens 5 in the spatial filtering system, and the sample makes the spot radius projected onto the sample about 1 / 2 of the vector beam, and ensures that the sample is located outside the Rayleigh distance of the detection beam waist. At this time, the center of the 405 nm vector beam is a Gaussian beam with a smaller radius of 632.8 nm. Considering that the temperature field distribution of the anisotropic material after heat absorption is not uniform from the inside to the outside, and a good cylindrical lens effect is generated in the central region, the detection beam size is smaller than the heating beam, which is conducive to improving the detection accuracy. Further, the sample is located outside the Rayleigh distance of the detection laser beam waist, which is to facilitate the calculation of the equivalent focal length of the thermal lens using the geometric method.

[0067] In the embodiment, the first laser 1 and the second laser 2 are turned off, a laser power meter is placed between the beam splitter 18 and the sample 19, the second laser 2 is turned on, and the output power is adjusted so that the output power P is the value to be studied. The second laser 2 is turned off, and the output power adjustment knob position is kept unchanged.

[0068] Open the first laser 1, after it is stable, open the CMOS camera 21 and start recording, open the light of the second laser 2. When the CMOS camera 21 monitors the window to show that the light spot is stable, stop the recording of the CMOS camera 21 and save. The recorded data is processed by software to obtain the focal length of the thermal lens.

[0069] Example two

[0070] See Figure 2 An experimental method for realizing the thermal lens effect of a vector light beam and measurement, based on the device described in example one.

[0071] S1, radiate coaxial probe laser and heating laser to the sample based on the experimental device, and record the probe laser spot after emission with a detector;

[0072] S2, calculate the equivalent focal length F of the thermal lens by the following formula based on the detector recording result:

[0073]

[0074] In the formula, w is the spot radius of the probe laser at the sample, w1 is the spot radius of the probe laser without placing the sample in the device, w2 is the spot radius of the probe laser when the sample is placed in the device, and z is the distance between the sample and the detector.

[0075] In this embodiment, the specific steps of S1 are as follows:

[0076] (1) Use an experimental device for realizing the thermal lens effect of a vector light beam and measurement as described in example one, align the optical path so that the optical axes of the two lasers coincide after passing through the beam splitter 18; the specific method for aligning the optical path so that the optical axes of the two lasers coincide after passing through the beam splitter 18 is as follows: the heating laser emits light after passing through a spatial filter, which is modulated into a standard Gaussian light beam, then passes through a polarizer to obtain linearly polarized light, and then is vertically incident to the center of the l=1 vortex wave plate 13, and then passes through a lens group to shrink the heating beam, and is reflected by a mirror into the beam splitter 18. The detection beam passes through a spatial filter, a polarizer, and an attenuator in turn, and is also reflected by a mirror into the beam splitter 18. Adjust the mirror and the beam splitter 18 so that the detection laser and the heating laser beam coincide in optical axis after passing through the beam splitter. At this time, the center of the 405nm vector light beam is a smaller radius 632.8nm detection light beam.

[0077] (2) Turn on the heating laser, make the linearly polarized light pass through the vortex wave plate 13 to generate a radial polarization vector beam or an angular polarization vector beam, place a laser power meter in front of the sample to measure, and adjust the laser power to the value to be measured, then turn off the heating laser; in this embodiment, when the polarization direction of the linearly polarized light is parallel to the 0° fast axis of the vortex wave plate 13, a radial polarization vector beam is generated; when the polarization direction of the linearly polarized light is perpendicular to the 0° fast axis of the vortex wave plate 13, an angular polarization vector beam is generated. When the second laser is turned off, the position of the light output power adjusting knob is kept unchanged, and at this time, the light output power P is the value of the independent variable to be studied.

[0078] (3) Turn on the first laser to generate a detection laser, and adjust the radius to 1 / 2 of the second laser; adjust the attenuation sheet and the CMOS exposure parameters so that the spot exposure is appropriate, and the highest brightness is about 2 / 3 of the dynamic range.

[0079] (4) Adjust the sample position to be outside the Rayleigh distance behind the focus point of the detection beam, while ensuring that the radius of the detection beam incident on the medium surface of the sample is about 1 / 2 of the vector beam; in this embodiment, the sample is placed outside the Rayleigh distance behind the waist spot of the first laser, so as to facilitate the calculation of the equivalent focal length of the thermal lens using the geometric method.

[0080] (5) Turn on the CMOS recording, turn on the heating laser, and stop recording and turn off the heating laser after the spot in the CMOS monitoring window is stable.

[0081] (6) Use software to process to obtain the equivalent focal length of the thermal lens. In this embodiment, the equivalent focal length of the thermal lens is calculated by changing the beam waist of the detection beam, i.e. the spot of the Gaussian beam of 632.8 nm, irradiated on the thermal lens. Changing the heating laser power can also study the relationship between the equivalent focal length of the thermal lens and the heating power.

[0082] Compared with the prior art, the advantages of the present application are:

[0083] The present application changes the light field distribution and the polarization state, and uses a vector beam to heat an anisotropic material, so as to realize different thermal lens effects from a Gaussian beam. The present application changes the heating light power to realize the continuous adjustable cylindrical lens effect of the equivalent focal length. The thermal lens induced by the radial polarization vector beam can realize the one-way divergence or convergence of the scalar light field, and the angular polarization vector beam realizes the one-way regulation of the scalar light field in the direction perpendicular to the former. The present application can realize the generation, regulation and measurement of the vector beam thermal lens effect, and the experimental device design. The present application provides a train of thought for realizing the miniaturization and integration of the light field one-way regulation device. It also has potential applications in the measurement of the physical properties of anisotropic substances.

[0084] The working principle of this embodiment is as follows:

[0085] The thermal lensing effect is influenced by three factors: the physical properties of the sample, the parameters of the light beam that produces the thermal lens, and the environment the sample is in. This experiment uses a vector beam to heat an anisotropic medium and measures the thermal lensing effect of the sample at different heating light powers.

[0086] The change in refractive index of a material exhibiting thermo-optical effect with temperature has the following form:

[0087]

[0088] Where n0 is the refractive index at the initial temperature, is the linear variation coefficient of the material's refractive index with temperature.

[0089] It can be seen that the refractive index distribution of the material is determined by its initial refractive index, temperature distribution and thermo-optic coefficient. For the anisotropic material nematic liquid crystal MBBA, it has different initial refractive indices and thermo-optic coefficients in the directions parallel or perpendicular to the average orientation of the liquid crystal molecules, and both satisfy the above refractive index change formula.

[0090] The temperature distribution inside the material is determined by the heat source distribution, material properties and boundary conditions, and is solved by the heat conduction equation. For anisotropic materials, the heat conduction equation is:

[0091]

[0092] If a very thin sample is selected, the temperature of the sample along the propagation direction of the beam can be approximated to be constant. Therefore, in the specific calculation, the three-dimensional space can be simplified to two dimensions, and the temperature distribution of the incident two-dimensional plane can be calculated. The electric vector of the incident vector beam can be expressed as:

[0093]

[0094] represents the CV beam and the LG with p=0 l0 The beams have the same amplitude distribution, and is the Jones vector of the CV beam. The linear polarization direction and angular coordinate of the CV beam are γ represents the angle between the linear polarization direction of the CV beam and the horizontal direction when the angular coordinate is 0.

[0095] The intensity of light incident on the sample surface can be expressed as:

[0096]

[0097] If the absorbance of the material is not affected by temperature and light intensity, according to the Beer-Lambert law, the density of the body heat source generated in the sample is:

[0098] I=I x0exp(-α x z)+I y0 exp(-α y z)

[0099] In the low absorption approximation, the heat generated by the laser in a small volume element dV of the anisotropic medium per unit time can be written as:

[0100]

[0101] The incident vector field is decomposed into a superposition of different directionally polarized light at each point, and the heat source contributions are added, so that the heat source distribution caused by the incident light can be obtained

[0102] In the laboratory environment, the boundary conditions can be expressed as:

[0103] (1) The temperature change far from the center of the incident light source is 0, that is, T=T0(r→∞);

[0104] (2) At the initial moment, the temperature of the whole sample is the same, that is, T=T0(t=0);

[0105] (3) Since the sample is heated to a temperature higher than the surrounding environment, the sample will heat the environment on both sides of the end face, and the heat dissipation rate of the surface element ds is proportional to the temperature difference between the sample and the environment, which can be expressed as: dΦ=η(T-T0)ds, η is the heat exchange coefficient Φ is the heat flow (W).

[0106] So far, the conditions required to solve the equation have been met, and the finite element difference method can be used to solve the equation. For example Figure 3 After different polarization state vector beams heat the anisotropic medium, the sample temperature distribution can be obtained by solving the heat conduction equation, and then the refractive index distribution can be obtained. According to the refractive index distribution, the light intensity distribution of the detection beam modulated by the thermal lens can be calculated.

[0107] Figure 4 The left side shows the theoretical simulation results of the light intensity distribution of the detection beam modulated by the thermal lens under the conditions of different heating light powers and different polarization states.

[0108] In this experimental device, the focal length of the thermal lens can be calculated by changing the beam waist of the 632.8nm Gaussian beam spot irradiated on the thermal lens. For example Figure 5 The relationship curve between the equivalent focal length of the thermal lens induced by the vector beam of different polarization states and the heating light power.

[0109] In this embodiment, the specific steps of S2 are as follows:

[0110] Method for calculating the equivalent focal length:

[0111] The thermal lens effect of the vector light beam modulates the detection light beam as Figure 6 .

[0112] If the sample is placed at a distance larger than the Rayleigh distance behind the waist of the detection light beam, the imaging rule of the Gaussian light beam after the thermal lens can be approximated by the geometric optical imaging formula:

[0113]

[0114] First, remove the sample, and use the CMOS to collect the spot size w of the detection light beam expected to be incident on the sample surface at the selected sample position, and then use the CMOS to collect the spot size w1 of the detection light beam after propagating a distance z. According to this, the divergence half-angle θ of the detection light beam not modulated by the thermal lens effect satisfies:

[0115]

[0116] Similarly, after placing the sample at the selected position, use the CMOS to collect the spot size w2 of the detection light beam after propagating a distance z.

[0117] According to this, the divergence half-angle θ of the detection light beam modulated by the thermal lens effect satisfies: ′

[0118]

[0119] Further, we can calculate the distance of the waist of the detection light beam from the sample, and the waist distance without thermal lens modulation is:

[0120]

[0121] And the waist distance modulated by the thermal lens is:

[0122]

[0123] According to the geometric imaging formula, the equivalent focal length of the thermal lens can be obtained as:

[0124]

[0125] The application discloses an experimental device capable of realizing thermal lens effect of a vector light beam and measurement. A heating laser and a detection laser are different in wavelength, a spatial filter is used to form a standard Gaussian mode of the light beams, a vortex wave plate is used to generate a radial or angular polarization vector light beam, a polarizer is used to control the linear polarization direction of the detection light beam, an attenuator is used to reduce the light intensity of the detection light beam, a plurality of mirrors and a beam splitter are used to combine the two light beams and irradiate the anisotropic material, a filter is used to filter out the laser participating in heating in the transmitted light, a CMOS is used to collect the spot of the detection laser in the transmitted light, and a computer is used for processing and calculation.​

[0126] The present application studies the thermal lens effect of the vector light beam, and finds the light field regulation method similar to the cylindrical lens effect by heating the anisotropic medium with the vector light beam. The thermal lens induced by the radial polarization vector light beam can realize the one-way divergence or convergence of the scalar light field, while the angular polarization vector light beam realizes the one-way regulation of the scalar light field in the direction perpendicular to the former. Moreover, by changing the heating light power, the cylindrical lens effect with continuously adjustable equivalent focal length can be realized. This provides a train of thought for the miniaturization and integration of the one-way regulation device of the light field. We also believe that it has potential applications in the measurement of the physical properties of anisotropic substances. In addition, the generalized vector light beam containing radial polarization and angular polarization components is expected to realize the one-way regulation of the scalar light field in any direction, which will greatly improve the use range of the thermal lens effect induced by the vector light beam.

[0127] The above only describes some embodiments of the present application, and does not limit the protection scope of the present application, and any equivalent device or equivalent flow transformation made by using the content of the specification and drawings, or directly or indirectly applied in other related technical fields, are also included in the patent protection scope of the present application.

Claims

1. An experimental device for realizing and measuring the thermal lens effect of a vector beam, characterized in that: include: a first laser, configured to radiate detection laser light in a third direction; A second laser is used to radiate heating laser light in a third direction, where the heating laser light is a vector laser light; The detection laser and the heating laser satisfy the following conditions: in the same anisotropic sample material, the product of the detection laser power and its absorptivity in the material is less than 5 orders of magnitude of the product of the heating laser power and its absorptivity in the material; as well as, Arranged in sequence along the third direction: an anisotropic material sample, a filter, and a detector; A filter, used for filtering the heating laser emitted from the sample to be tested; a detector, for receiving an optical signal of a detection laser; The first laser radiates detection laser light along a first direction. A first filter system and an attenuation plate are sequentially arranged along the first direction. After the detection laser light passes through the first filter system and the attenuation plate, it is reflected and radiated to a third direction. The second laser radiates heating laser light in a second direction. A second filter system, a second polarizer, and a vector beam adjustment device are sequentially arranged along the second direction. After the heating laser light passes through the second filter system, the second polarizer, and the vector beam adjustment device, it is reflected and radiated in a third direction. The device further includes a beam splitter; after the detection laser and the heating laser are combined through the beam splitter, the laser is radiated along a third direction; The first filtering system includes a first convex lens and a second convex lens arranged in sequence along a first direction; The positions of the first convex lens, the second convex lens and the anisotropic material sample satisfy the following relationship conditions: The detection laser spot radius on the anisotropic material sample is smaller than the heating laser spot radius, and the sample is located outside the Rayleigh distance behind the detection laser beam waist spot; The vector light beam is a radially polarized vector light beam or an angularly polarized vector light beam.

2. The experimental device for realizing and measuring the thermal lens effect of a vector beam as claimed in claim 1, characterized in that: The vector beam adjustment device adopts a vortex wave plate.

3. The experimental device for realizing and measuring the thermal lens effect of a vector beam as claimed in claim 1, characterized in that: The distance between the sample and the detector is about 50 to 60 cm.

4. The experimental device for realizing and measuring the thermal lens effect of a vector beam as claimed in claim 1, characterized in that: The ratio of the heating laser spot diameter to the detection laser spot diameter is 2:1 to 4:

1.

5. An experimental method for realizing and measuring the thermal lens effect of a vector beam, characterized in that: An experimental device for realizing and measuring the thermal lens effect of a vector beam according to any one of claims 1 to 4; the method comprising the following steps: Based on the experimental device, coaxial detection laser and heating laser are radiated to the sample, and the detection laser spot after emission is recorded by a detector; Based on the detector recording results, the thermal lens equivalent focal length F is calculated by the following formula: Wherein, w is the spot radius of the detection laser at the sample, w1 is the spot radius of the detection laser when no sample is placed in the device, w2 is the spot radius of the detection laser when the sample is placed in the device, and z is the distance between the sample and the detector.

6. The experimental method for realizing and measuring the thermal lens effect of a vector beam as claimed in claim 5, characterized in that: Based on the experimental device, coaxial detection laser and heating laser are radiated to the sample, and the detector is used to record the detection laser spot after emission. Specifically: adjusting the power parameters of the second laser and modulating the heating laser into a radially polarized vector beam or an angularly polarized vector beam; Turning on the first laser, adjusting the sample position to a Rayleigh distance behind the focus of the detection laser beam, and setting the probe laser spot radius on the anisotropic material sample to be smaller than the heating laser spot radius; The second laser is turned on, and the laser spot is recorded by a detector.

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