Optical measurement method and system of liquid thermal diffusivity

Abstract

The application discloses a kind of optical measurement method and system of liquid thermal diffusion coefficient, method includes: using focused laser beam to irradiate liquid to be measured, produce temperature rise amount that changes with time and radial distribution;Make probe light beam pass through temperature rise area, based on thermo-optic effect to make it carry phase change information, obtain first light beam;First light beam is spatially filtered, and phase change information is converted into light intensity distribution change, obtain second light beam;Second light beam is converted into electrical signal, and light intensity distribution change is acquired;According to the linear correspondence of light intensity distribution change and temperature rise amount and the linear variation law of temperature rise amount with time logarithm, calculate thermal diffusion coefficient.The application adopts non-contact optical measurement, avoids the disturbance of sensor to liquid temperature field, measurement precision is high, response is fast, optical path structure is simple, data processing is convenient, suitable for transparent and translucent liquid, has scientific research and teaching application value.

Description

Technical Field

[0001] This invention relates to the field of liquid thermophysical parameter measurement technology, and in particular to an optical measurement method and system for the thermal diffusivity of a liquid. Background Technology

[0002] The thermal diffusivity of a liquid is an important thermophysical parameter characterizing the rate of heat transfer in a liquid, and it has wide applications in many fields. For example, in the heat treatment of materials, liquid thermal diffusivity is used to achieve quenching, tempering, and annealing to improve material properties; in drug development, the principle of liquid thermal diffusivity is used to control the diffusion rate of drugs in solution to achieve sustained drug release; and in food sterilization, liquid thermal diffusivity is used to transfer heat to the interior of food to kill bacteria and microorganisms. Therefore, accurate measurement of the liquid thermal diffusivity has important application reference value in basic research, chemical process design and optimization, and new material development.

[0003] Current common methods for measuring the thermal diffusivity of liquids have the following main shortcomings: The laser-induced thermal grating method uses a laser beam to form a pulse, which is split into two coherent beams by a beam splitter. These beams intersect in a film-like sample, causing interference and forming a thermal grating in the liquid film. The thermal diffusivity is then calculated based on the decay law of the thermal grating. This method involves complex experimental setups, requires stringent environmental conditions, and presents significant challenges in data analysis.

[0004] The transient hot-wire method, based on the theory of unsteady-state heat conduction, uses a short-time pulse to heat a hot wire to measure the thermal diffusivity of a liquid. To avoid natural convection caused by excessive temperature rise, the initial stage of measurement must be avoided, and a sufficient time must be allowed to satisfy the approximate solution conditions. This method has high requirements for the sample; the sample must have homogeneous properties, and there are certain limitations on its size and shape, increasing the difficulty of sample preparation. Furthermore, good thermal contact between the hot wire and the sample must be ensured during the measurement process; poor contact will introduce significant errors.

[0005] The transient planar heat source method uses a planar probe with both heat source and temperature sensor functions to calculate the thermal diffusivity by recording temperature and response time. This method also places extremely high demands on the materials used and the accuracy and stability of the probe.

[0006] Most of the existing methods mentioned above require immersing contact sensors such as thermocouples or hot wires in the liquid for measurement. The heat capacity and thermal conduction of the sensor itself can disturb the fluid temperature field. Especially for low-viscosity liquids, natural or forced convection around the sensor can lead to measurement signal distortion, failing to accurately reflect the pure thermal diffusion process. If the liquid is corrosive or prone to crystallization, the sensor is more susceptible to contamination or damage, affecting long-term measurement stability. Furthermore, existing devices are complex in structure and expensive, making it difficult to meet the needs of easy-to-operate, high-precision measurement devices in everyday teaching scenarios. Summary of the Invention

[0007] Therefore, the technical problem to be solved by the present invention is to overcome the problems of existing technologies, such as the disturbance of the fluid temperature field and the distortion of the measurement signal caused by the insertion of contact sensors into the liquid, as well as the complexity of the experimental device.

[0008] To address the aforementioned technical problems, this invention provides an optical measurement method for the thermal diffusivity of a liquid, comprising the following steps: S1: The liquid to be tested is irradiated with a focused laser beam, causing the liquid to absorb laser energy and generate a temperature increase that varies with time and is radially distributed in space. S2: Pass the probe beam through the region in the liquid to be tested that generates the temperature increase, so that the probe beam carries phase change information and obtains a first beam; S3: Spatial filtering is performed on the first beam to convert the phase change information carried by the first beam into a change in light intensity distribution, thus obtaining the second beam; S4: Receive the second light beam and convert it into an electrical signal, and obtain the change in light intensity distribution based on the electrical signal; S5: Based on the correspondence between the change in light intensity distribution and the increase in temperature, as well as the logarithmic change of the increase in temperature over time, the thermal diffusivity of the liquid to be tested is calculated.

[0009] In one embodiment of the present invention, in step S2, the method for passing the probe beam through the region in the liquid to be tested that generates the temperature increase, so that the probe beam carries phase change information, to obtain the first beam is as follows: The probe beam passes through the heating region, and the temperature rises by a certain amount. The change in refractive index of the liquid under test modulates the phase of the probe beam, which carries the phase change information as it exits, thus obtaining the first beam; the phase change of the exiting light With temperature increase The relationship between them is represented as follows: , in, The optical path length of the probe beam within the temperature rise region. The wavelength of the probe beam. The thermo-optic coefficient of the liquid to be tested is denoted as .

[0010] In one embodiment of the present invention, in step S3, the method for spatially filtering the first beam and converting the phase change information carried by the first beam into a change in light intensity distribution to obtain the second beam is as follows: The first beam is introduced into a 4f phase coherent imaging system. A phase stop is placed on the incident surface of the 4f phase coherent imaging system. The phase stop consists of a circular stop and a circular phase object located at the center of the circular stop. The circular phase object applies a phase shift to a portion of the beam incident on it. Without changing the amplitude, the first beam passing through the phase aperture is subjected to Fourier transform by a lens and then incident on the liquid to be tested located on the spectral plane. The spectrum is filtered by the thermally induced nonlinear phase shift generated by the liquid to be tested. On the image plane, the phase delay of the circular phase object is converted into a change in light intensity distribution to obtain the second beam.

[0011] In one embodiment of the present invention, the change in light intensity distribution ∆I satisfies the following relationship with the phase change ∆φ of the first light beam: , in, For geometric proportions, , Let be the radius of the circular phase object. Let be the radius of the circular aperture. The difference in Taylor expansion coefficients.

[0012] In one embodiment of the present invention, in step S4, the second light beam is received and converted into an electrical signal, and the method for obtaining the change in light intensity distribution based on the electrical signal is as follows: Receive the second beam and convert it into a voltage signal. The voltage signal Changes in light intensity distribution with the second beam They satisfy a linear relationship ,in is the conversion factor.

[0013] In one embodiment of the present invention, in step S5, the method for calculating the thermal diffusivity of the liquid to be tested based on the correspondence between the change in light intensity distribution and the increase in temperature, and the logarithmic change of the increase in temperature over time, is as follows: The change in light intensity distribution and the temperature increase satisfy a linear correspondence, and the temperature increase and the logarithm of time satisfy a linear relationship; the change in light intensity distribution is converted into the temperature increase corresponding to different times. In terms of temperature increase The vertical axis represents the logarithm of time. Plot a curve on the x-axis, determine the linear interval of the curve, and fit the curve to obtain the slope. The thermal diffusivity is calculated by substituting the slope into the following formula. : , in, To focus the heating power of the laser beam, The density of the liquid to be tested, is the specific heat capacity of the liquid to be tested.

[0014] In one embodiment of the present invention, the liquid to be tested is a transparent or semi-transparent liquid, including water and organic solvents.

[0015] The present invention also provides an optical measurement system for the thermal diffusivity of a liquid, comprising: The heating module is used to irradiate the liquid to be tested with a focused laser beam, so that the liquid absorbs the laser energy and generates a temperature increase that varies with time and is radially distributed in space. A detection optical module is used to pass the detection beam through the region in the liquid to be tested that generates the temperature rise, so that the detection beam carries phase change information to obtain a first beam. A spatial filtering module is used to perform spatial filtering on the first beam, converting the phase change information carried by the first beam into changes in light intensity distribution to obtain a second beam. The signal receiving module is used to receive the second light beam and convert it into an electrical signal, and to obtain the change in light intensity distribution based on the electrical signal; The data processing module is used to calculate the thermal diffusivity of the liquid under test based on the correspondence between the change in light intensity distribution and the increase in temperature, as well as the logarithmic change of the increase in temperature over time.

[0016] In one embodiment of the present invention, the spatial filtering module includes a 4f phase coherent imaging system, the 4f phase coherent imaging system including a first lens, a liquid placement surface to be tested, a second lens arranged sequentially along the optical path, and a phase stop disposed on the incident surface of the 4f phase coherent imaging system; the phase stop is composed of a circular stop and a circular phase object located at the center of the circular stop.

[0017] In one embodiment of the present invention, the signal receiving module is disposed on the image plane of the 4f phase coherence imaging system.

[0018] The technical solution of the present invention has the following advantages compared with the prior art: The optical measurement method and system for the thermal diffusivity of liquids described in this invention employs non-contact optical measurement, avoiding the disturbance to the liquid temperature field and the resulting measurement errors caused by traditional contact sensors. This method is particularly suitable for low-viscosity, corrosive, or easily crystallizing liquids. Based on the thermo-optic effect, this invention linearly converts the temperature rise into a phase change, and then linearly converts it into a change in light intensity distribution through a 4f phase coherent imaging system. The entire signal conversion chain is linear, resulting in high measurement accuracy, fast response speed, and real-time dynamic monitoring. Compared with existing technologies, this invention eliminates the need for complex interference optical paths and reference beams, resulting in a simple optical path structure, convenient operation, and data processing requiring only linear fitting. This significantly reduces experimental difficulty and offers dual application value for both high-precision scientific research measurement and everyday teaching demonstrations. Attached Figure Description

[0019] To make the content of this invention easier to understand, the invention will be further described in detail below with reference to specific embodiments and accompanying drawings.

[0020] Figure 1 This is a schematic flowchart of the optical measurement method for the thermal diffusivity of a liquid in an embodiment of the present invention. Figure 2 This is a schematic diagram of a focused laser beam passing through the central region of a sample dish containing the liquid to be tested, as described in an embodiment of the present invention. Figure 3 This is a schematic diagram of the 4f phase coherence imaging system in an embodiment of the present invention; Figure 4 This is a schematic diagram of the phase stop in an embodiment of the present invention; Figure 5 This is a typical 4f phase coherent imaging intensity distribution diagram in an embodiment of the present invention; Figure 6 yes Figure 5 Cross-sectional view.

[0021] Explanation of reference numerals in the accompanying drawings: 1. First lens; 2. Second lens; 3. Third lens; 4. Laser system; 5. First beam splitter; 6. Second beam splitter; 7. Phase stop; 8. Liquid to be tested; 9. Filter; 10. First mirror; 11. Second mirror; 12. CCD. Detailed Implementation

[0022] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0023] Example 1: like Figure 1 As shown, the present invention provides an optical measurement method for the thermal diffusivity of a liquid, comprising the following steps: S1: The liquid to be tested is irradiated with a focused laser beam, causing the liquid to absorb laser energy and generate a temperature increase that varies with time and is radially distributed in space. S2: Pass the probe beam through the region in the liquid to be tested that generates the temperature increase, so that the probe beam carries phase change information and obtains a first beam; S3: Spatial filtering is performed on the first beam to convert the phase change information carried by the first beam into a change in light intensity distribution, thus obtaining the second beam; S4: Receive the second light beam and convert it into an electrical signal, and obtain the change in light intensity distribution based on the electrical signal; S5: Based on the correspondence between the change in light intensity distribution and the increase in temperature, as well as the logarithmic change of the increase in temperature over time, the thermal diffusivity of the liquid to be tested is calculated.

[0024] This invention utilizes a focused laser beam to irradiate the liquid under test, causing it to absorb laser energy and generate a temperature rise that varies over time and is spatially radially distributed. A probe beam is then passed through this temperature rise region, and the thermo-optical effect allows the probe beam to carry phase change information related to the temperature rise, thus obtaining a first beam. The first beam is then spatially filtered to linearly convert the phase change information into a change in light intensity distribution, resulting in a second beam. This second beam is then converted into an electrical signal, from which the change in light intensity distribution is obtained. Based on the linear correspondence between the change in light intensity distribution and the temperature rise, as well as the linear variation of the temperature rise with the logarithm of time, the thermal diffusivity coefficient is calculated. This method is entirely based on non-contact optical measurement, avoiding the disturbance to the liquid temperature field caused by traditional contact sensors. The entire signal conversion chain is linear, resulting in high measurement accuracy and fast response speed. Furthermore, it eliminates the need for complex interference optical paths, simplifying the optical path structure and making operation convenient. Data processing only requires linear fitting, significantly reducing operational difficulty. It possesses dual application value for high-precision scientific research and everyday teaching demonstrations.

[0025] In this embodiment of the invention, in step S1, a focused laser beam is used to irradiate the liquid to be tested, and a controllable thermal disturbance is applied to the liquid to be tested, so that the liquid to be tested absorbs laser energy and generates a temperature increase that varies with time and is radially distributed in space.

[0026] In practice, a focused laser beam (i.e., the heating source, also known as the pump beam) is converged through a convex lens, with its beam waist positioned inside the liquid sample to be tested. The liquid sample exhibits intrinsic absorption of the laser energy or achieves characteristic absorption through the addition of specific dyes. The absorbed light energy is converted into heat through a non-radiative relaxation process, thus forming a localized heat source in the liquid sample whose temperature varies with time. Since the focused laser beam is typically a Gaussian beam, its intensity distribution near the focal point is approximately Gaussian, resulting in a significant temperature increase. The temperature is radially distributed in space, meaning it is highest at the center and decreases radially outwards, with the temperature increase varying over time. It is constantly evolving.

[0027] The thermal diffusion process in the heated liquid follows an unsteady-state heat conduction equation, which can be expressed as: (1), in, For temperature, The thermal diffusivity of the liquid being tested is . For the Laplace operator.

[0028] The definition of thermal diffusivity α is: (2), Where λ is the thermal conductivity of the liquid being tested (unit: 1000 m³ / s). ), The density of the liquid to be tested (in units of 1000 kJ / m³) ), The specific heat capacity of the liquid being tested (unit: ) Thermal diffusivity The unit is .

[0029] The initial temperature of the liquid to be tested is denoted as . The temperature rise caused by heating from a focused laser beam at any time and any position is defined as: (3), The heat diffusion equation can then be rewritten as relating the amount of temperature increase. Format: (4); The corresponding initial and boundary conditions are set as follows: the temperature rise is zero at the initial moment and before, the temperature rise at infinity is zero, and the heat flux boundary condition is satisfied at the center of the heat source. Specifically, this is expressed as: (5), (6), (7), in, The radial distance from any location to the center of the heat source (in units of 1000 ppm). ); The laser power absorbed per unit length along the focused laser beam direction by the liquid under constant laser power (unit: ). ).

[0030] Assuming all physical parameters are constant, the above system of equations is solved using the Laplace transform to obtain the temperature rise. The solution is: (8), in, For an exponential integral function, its series expansion is: (9), In the formula, , where is Euler's constant.

[0031] The actual focused laser beam is equivalent to a radius of... cylindrical heat source ( That is, the beam waist radius of the focused laser beam, when its surface temperature is... When the fluid temperature is the same at a given location, the fluid in The temperature rise at that location is: (10).

[0032] When the waist radius of the focused laser beam Small enough, heating time Given a sufficiently long time, the second-order expansion term in the above equation... This is negligible. Furthermore, the radius of the Gaussian beam changes very little near the focal point, and the beam cross-section remains almost constant. Therefore, the temperature rise can be simplified to: (11), in, , where is a constant. When the above approximation conditions are met, the temperature rise ΔT of the liquid under test exhibits a linear relationship with the logarithm of time, ln t, providing a theoretical basis for subsequently calculating the thermal diffusivity by measuring the temperature rise. The liquid under test can be a transparent or translucent liquid, including water and organic solvents.

[0033] Furthermore, in step S2, the probe beam is passed through the region in the liquid to be tested where the temperature rise occurs, so that the probe beam carries phase change information to obtain a first beam. The optical properties of the liquid to be tested may be changed by the temperature rise through the probe beam.

[0034] In actual optical paths, the probe beam is usually also a laser beam, and its propagation characteristics follow the laws of Gaussian beams.

[0035] For a bundle along A Gaussian beam propagating along its axis has a beam radius With propagation distance The pattern of change is described by the following formula: (12) in, The propagation distance along the beam propagation direction is given by the beam waist position as the origin. ), For a Gaussian beam at a propagation distance of The beam radius at that location Let be the waist radius of the Gaussian beam. The Rayleigh length of a Gaussian beam is defined as: (13) To detect the wavelength of the light beam. Rayleigh length. This characterizes the property of maintaining beam collimation: in Within the range, beam radius The beam spread is not significant and can be approximated as a parallel beam.

[0036] Specifically, a probe beam is collimated and precisely passes through the temperature rise region in the liquid to be tested, which is heated by the focused laser beam and generates the temperature increase. The liquid sample is placed near the waist of the probe beam, and the inner diameter of the sample dish... (That is, the optical path length of the probe beam in the heated region) is much shorter than the Rayleigh length. At that time, the approximate conditions for thin samples are met.

[0037] like Figure 2 As shown, in order to intuitively understand the geometric relationship of a focused laser beam as an equivalent heat source, Figure 2 This demonstrates the effect of a focused laser beam passing through the central region of a sample dish containing the liquid to be tested. At this point, the beam radius... With propagation distance The variation pattern of the inner diameter of the sample dish Within the range, the beam radius of the detection beam The change is extremely small, can be considered a constant, and is approximately equal to the waist radius. Therefore, the optical path length of the probe beam in the heating region... The inner diameter of the sample dish can be easily taken as the inner diameter.

[0038] The amount of temperature increase in the tested liquid within the heating zone The effect of its refractive index Changes will occur; this phenomenon is known as the thermo-optic effect. The mathematical expression for the thermo-optic effect is: (14) in, For the liquid to be tested at the initial temperature The refractive index below, The thermo-optic coefficient of the liquid to be tested (unit: ). ), is a physical property parameter that characterizes the rate of change of refractive index with temperature.

[0039] When the probe beam passes through this heated region where the refractive index changes, its optical path length changes, causing a phase delay in its outgoing wavefront, i.e., phase modulation. The increase in temperature causes a change in the refractive index of the liquid being tested, modulating the phase of the probe beam and causing it to carry phase change information upon exiting, thus obtaining the first beam.

[0040] Phase change carried by the first beam of the outgoing light With temperature increase The relationship between them is determined by the optical path difference and can be expressed as: (15) Or equivalent to: (16).

[0041] Further, in step S3, the first beam is spatially filtered to convert the phase change information carried by the first beam into a change in light intensity distribution, thus obtaining the second beam.

[0042] In this embodiment of the invention, a 4f phase coherence imaging system is used to achieve this conversion. Specifically: the first beam is introduced into the 4f phase coherence imaging system, and a phase stop is placed on the incident surface of the 4f phase coherence imaging system. The phase stop consists of a circular stop and a circular phase object located at the center of the circular stop. The circular phase object applies a phase shift to the portion of the beam incident on it without changing the amplitude. The first beam passing through the phase stop is subjected to Fourier transform by a lens and then incident on the liquid to be measured located on the spectral plane. The spectrum is filtered by the thermally induced nonlinear phase shift generated by the liquid to be measured. On the image plane, the phase delay of the circular phase object is converted into a change in light intensity distribution to obtain the second beam.

[0043] like Figure 3 As shown, the 4f phase coherent imaging system uses laser system 4 as its light source. The output laser beam is first split into two paths by the first beam splitter 5. One path serves as the probe beam entering the main imaging optical path, while the other path serves as the reference beam entering the reference optical path composed of the first reflecting mirror 10, the second reflecting mirror 11, and the third lens 3. The probe beam is incident on the phase stop 7 placed on the incident surface of the 4f system. This phase stop consists of a circular stop and a circular phase object located at its center. The circular phase object only applies pressure to the portion of the beam incident on it. The phase shift does not change the amplitude, allowing the beam passing through the phase stop 7 to carry the initial phase modulation information. Subsequently, the beam undergoes a Fourier transform via the first lens 1, forming a spatial spectrum on the back focal plane (spectral plane) of the first lens 1. The liquid to be measured 8 is precisely placed on this spectral plane; the thermally induced nonlinear phase shift generated after absorbing the focused laser energy filters the incident spatial spectrum, further converting the refractive index change caused by temperature variation into phase modulation information. The filtered spectrum then undergoes an inverse Fourier transform via the second lens 2, forming an image conjugate to the incident plane on the back focal plane (image plane) of the second lens 2, thus converting the phase delay of the phase stop 7 into a change in light intensity distribution. A filter 9 is installed on the output light path of the second lens 2 to adjust the light intensity and filter out stray light to improve the imaging signal-to-noise ratio. The beam is then split by the second beam splitter 6 into one path for real-time observation, and the other path serves as a second beam carrying the light intensity distribution change information, which is then incident on the signal receiving device. The light beam in the reference optical path changes its propagation direction after passing through the first reflecting mirror 10 and the second reflecting mirror 11, and is collimated and expanded by the third lens 3. It then coherently superimposes with the probe beam on the image plane, further enhancing the contrast and observability of the image. Finally, the CCD 12 set on the image plane completes the acquisition and conversion of the optical signal.

[0044] like Figure 4 As shown, the phase stop 7 consists of a radius of A circular aperture and at its center, with a radius of It consists of a circular phase object (PO). A circular aperture is used to limit the aperture of the incident beam, while a circular phase object is typically made by depositing a dielectric film of a specific thickness and refractive index at the center of a transparent glass substrate. The circular phase object applies a preset phase delay to the portion of the beam incident on it, but does not change its amplitude.

[0045] The transmittance of the entire phase stop 7 can be expressed as: (17) in, The phase delay of the phase object.

[0046] Let the monochromatic linearly polarized plane light field incident on the incident plane of the 4f system be: (18) in, It is angular frequency. It's a wave arrow. It is the electric field amplitude that includes time.

[0047] The transmission function of the circular aperture at the incident surface of the 4f system is: (19) Among them, circular functions Defined as when The value is 1 when the value is less than 1, and 0 elsewhere. Therefore, the transmission function of a phase-controlled object aperture can be written as: (20).

[0048] The light field distribution on the incident surface of the system then becomes After passing through the first lens 11, the light field distribution on the system's spectral plane is as follows: (twenty one), in Represents Fourier transform, and f1 is the spatial frequency on the focal plane, and f1 is the focal length of the first lens 11. λ is the wavelength of the incident laser.

[0049] At this point, the liquid under test 8, located on the spectral plane, has undergone a thermally induced nonlinear phase shift (i.e., a change in refractive index due to the increase in temperature) caused by the heating of the focused laser beam. This thermally induced nonlinear phase shift acts as a phase filter, filtering the spatial spectrum. It is assumed that the nonlinear sample (i.e., the liquid under test 8) has linear absorption. Nonlinear absorption and nonlinear refraction When the nonlinear sample can be considered as a thin sample, the light field distribution at the sample's exit surface is as follows: (twenty two), in, , , It is the thickness of the sample.

[0050] The filtered spectrum is then subjected to an inverse Fourier transform by the second lens 2, forming the final image on the image plane. The light intensity distribution detected by CCD 12 on the exit plane of the 4f system can be expressed as: (twenty three), in, Represents the inverse Fourier transform. It is the coherent optical transfer function of the optical lens system. It is the numerical aperture of the first lens 1. It refers to the magnification of the optical system.

[0051] Using the above information, the image on the exit surface of the 4f system can be calculated. For example... Figure 5 The image shows the light intensity distribution of a typical 4f phase-coherent imaging system. The three concentric circles (white, gray, and black, respectively) represent three different levels of light intensity distribution. Transmittance variation... Defined as the difference between the average light intensity inside and outside the phase object region. Because Nonlinear phase shift Related, when When positive, the image intensity in the PO region will be enhanced. On the contrary, when When the value is negative, the image intensity in the PO region will decrease. Physically speaking, this behavior can be understood as relying on the pattern formed by constructive or destructive interference between spatial frequencies of nonlinear phase-shifted symbols being filtered. According to... Nonlinear phase shift The relationship can be obtained The value of . Figure 5 This is a numerical simulation image. In the simulation process, the γ sign is positive, and you can see that the light intensity at the location of the phase object in the image is stronger than the surrounding area.

[0052] Three concentric circular regions with different brightness levels are represented, with the innermost layer corresponding to the PO region and the middle layer representing the area from the circular phase object to the edge of the circular aperture. The average light intensity difference between the inner and outer regions within this area represents the change in light intensity distribution. .

[0053] like Figure 6 What is shown is Figure 5 A radial intensity profile, with the horizontal axis representing the distance from the center and the vertical axis representing the normalized light intensity. The figures are labeled... This refers to the change in light intensity distribution, which more clearly quantifies this key parameter.

[0054] The 4f phase coherence imaging system essentially applies the inverse of the phase contrast principle: the photoelectric point (PO) is placed on the incident plane of the 4f system, while the nonlinear sample is placed on the Fourier plane. Under high-intensity illumination, the nonlinear sample will generate a nonlinear phase shift, filtering the incident light. The result is that the phase delay of the PO is converted into amplitude changes in the nonlinear image. The amplitude change in the PO region on the nonlinear image is related to the sign of the sample's nonlinear refractive index. When the nonlinear refractive index is positive, the intensity of the PO region increases, forming a convex image with a higher center than the surrounding area; conversely, when the nonlinear refractive index is negative, a concave image is formed. Thus, the sign of the nonlinear refraction can be determined by whether the center of the nonlinear image is convex or concave. By comparing the numerical simulation image with the experimental nonlinear image, the magnitude of the material's nonlinear refractive index can be obtained. Measuring nonlinear refraction using 4f phase coherence imaging technology has advantages such as simple optical path, high sensitivity, simultaneous measurement of the magnitude and sign of nonlinear refraction, single-pulse measurement, no sample movement required, and low requirements for light source energy stability.

[0055] A light wave that can be generated at the center bright spot is placed on the front focal plane of the 4F system. A phase-delayed phase filter can alter the light intensity distribution on the image plane. This type of phase filter is made by depositing a film of appropriate thickness at the center of a transparent substrate. When the phase filter produces a bright spot at the center... When the phase delay is , the light intensity distribution on the image plane is as follows: (twenty four), It can be seen that when When the light intensity is small, the change in the light intensity of the image is linearly related to the phase change of the object. Therefore, the phase shift of a transparent phase object can be detected by the receiver through changes in light intensity.

[0056] Incident electric field It can be decomposed into two top-hat beams, TH1 and TH2. Taking the Taylor expansion of the amplitude and phase of TH1 and TH2 and retrieving the first two terms, we get: (25), (26).

[0057] The 4f back focal plane contrast signal is defined as the difference in average intensity between the inside and outside of the PO, i.e., the grayscale ratio between the center and the outer ring: (27); A further approximation yields the relationship between the grayscale signal and the phase change: (28); in, For changes in light intensity distribution, The ratio of PO to the aperture radius. The phase shift of the phase object in phase stop 7; The difference in coefficients in the Taylor expansions of TH1 and TH2. This represents the phase change carried by the first beam.

[0058] Equation (28) clearly demonstrates that, through this 4f phase coherent imaging system, the phase change information carried by the first beam is linearly converted into a directly measurable change in light intensity distribution. Thus, a second beam is obtained.

[0059] In practical implementation, a Michelson interferometer is preferred to realize the function of converting phase changes into light intensity distribution changes in the aforementioned 4f phase coherent imaging system. The Michelson interferometer splits the first beam carrying phase change information into two paths: one as a probe beam and the other as a reference beam. By adjusting the optical path difference in the reference path, interference occurs when the two beams are combined, thus converting the minute phase change in the first beam caused by the thermo-optical effect of the liquid under test into a detectable change in light intensity distribution in the interference fringes, forming the second beam. This scheme utilizes the high sensitivity and ease of adjustment of the Michelson interferometer, simplifies the optical path structure, and facilitates flexible adjustment and acquisition of coherent light signals under different phase differences in teaching and research scenarios, effectively avoiding the influence of environmental and human factors.

[0060] Further, in step S4, the second beam is received and converted into an electrical signal, and the change in light intensity distribution is obtained based on the electrical signal.

[0061] Specifically, the second beam is received and converted into a voltage signal. The voltage signal and the change in light intensity distribution of the second beam They satisfy a linear relationship: (29) in, This is the voltage signal output by the photodetector. The conversion factor is determined by the responsivity of the photodetector and the gain of the subsequent amplification circuit, and can be determined through system calibration.

[0062] Further, in step S5, the thermal diffusivity of the liquid 8 to be tested is calculated based on the correspondence between the change in light intensity distribution and the increase in temperature, as well as the logarithmic change of the increase in temperature over time.

[0063] Based on the previous derivation, the change in light intensity distribution... With the temperature increase There is a linear correspondence between them, and the temperature increase is... logarithm with time They follow a linear variation law.

[0064] By combining the linear relationships in steps S2, S3, and S4, the voltage signal can be established. With the temperature increase A direct linear correspondence.

[0065] The phase change carried by the first beam of emitted light in step S2 With temperature increase The relationship is based on the thermo-optic effect. When a small amount of the test liquid is heated by a laser, its refractive index changes, modulating the phase of the emitted light. The relationship is as follows: (30) in, To detect the optical path length of the beam in the region of temperature increase, To detect the wavelength of the light beam, The thermo-optic coefficient of the liquid to be tested, 8.

[0066] Substituting equation (30) into the light intensity distribution change in step S3 Phase change with the first beam According to the relationship (28), based on the phase contrast principle, the 4f optical path with a phase object can convert the small phase change of the outgoing light into a grayscale change. The relationship is as follows: (31), The relationship between the change in light intensity distribution ΔI and the increase in temperature ΔT can be obtained as follows: (32), Combining this with step S4 (29), the electrical signal is obtained. With the temperature increase Relationship: (33), Therefore, electrical signals With temperature increase They satisfy a linear correspondence.

[0067] On the other hand, the temperature rise has been obtained from the heat conduction model derivation in step S1. logarithm with time It exhibits a linear variation law. As shown in equation (11), when an electrical signal is used... Characterizing the amount of temperature rise At that time, the linear relationship still holds, that is and They also exhibit a linear relationship within the same approximate interval.

[0068] Specifically, the signal receiving module acquires The results were obtained through calculation and simulation using equations (30) and (31). The curve is then substituted into the transient hot-wire method model to calculate the thermal diffusivity: (34), in, Let be the thermal diffusivity of the liquid 8 to be tested. The heating power of the focused laser beam is the heating power per unit length of the focused laser beam in the liquid under test (8). The slope is obtained by fitting the curve to a linear interval. The density of the liquid to be tested, 8 is the specific heat capacity of the liquid 8 to be tested.

[0069] Combining equations (11), (30), and (31), we can obtain the final formula for calculating the thermal diffusivity: (35); in, It is a constant. Thermo-optic coefficient, This refers to the heat absorption power of the liquid. The wavelength of the laser. For conversion factors, For voltage, For time.

[0070] In practical applications, the instrument constant can be calibrated by measuring a standard liquid (such as pure water) with a known thermal diffusivity. Conversion coefficient The combination of factors eliminates the need to measure each parameter individually, simplifying the measurement process and ensuring measurement accuracy.

[0071] The optical measurement method for the thermal diffusivity of liquids described in this invention uses a transparent or semi-transparent liquid 8 as the test liquid, including water and organic solvents such as ethanol, acetone, ethylene glycol, etc., as well as mixtures or solutions thereof, and has a wide range of applications.

[0072] Example 2: The present invention also provides an optical measurement system for the thermal diffusivity of a liquid, comprising: The heating module is used to irradiate the liquid to be tested with a focused laser beam, so that the liquid absorbs the laser energy and generates a temperature increase that varies with time and is radially distributed in space. A detection optical module is used to pass the detection beam through the region in the liquid to be tested that generates the temperature rise, so that the detection beam carries phase change information to obtain a first beam. A spatial filtering module is used to perform spatial filtering on the first beam, converting the phase change information carried by the first beam into changes in light intensity distribution to obtain a second beam. The signal receiving module is used to receive the second light beam and convert it into an electrical signal, and to obtain the change in light intensity distribution based on the electrical signal; The data processing module is used to calculate the thermal diffusivity of the liquid under test based on the correspondence between the change in light intensity distribution and the increase in temperature, as well as the logarithmic change of the increase in temperature over time.

[0073] The heating module is used to irradiate the liquid under test with a focused laser beam, causing the liquid to absorb laser energy and generate a temperature increase that varies over time and is radially distributed in space. The heating module consists of a focused laser beam (i.e., the heating source, also known as the pump beam) and a focusing lens. The focused laser beam is converged through a convex lens, with its beam waist located inside the liquid sample under test, applying a controllable thermal disturbance to the liquid and generating a localized heat source with a temperature that varies over time and is radially distributed in space. The working principle of this module and the solution process of the heat diffusion equation are exactly the same as step S1 in Example 1. The heat diffusion equation is shown in Equation (1), and the final simplified expression for the temperature increase is shown in Equation (11), which will not be repeated here.

[0074] The probe light module is used to pass the probe beam through the region in the liquid under test that generates the temperature rise, so that the probe beam carries phase change information to obtain the first beam. The light source beam expanding and collimating module consists of a laser generator, a convex lens group and a beam expander, and is responsible for generating and providing high-quality parallel probe light. This module collimates a probe beam and makes it pass precisely through the heated region in the liquid under test. When the probe beam passes through the heated region, based on the thermo-optic effect, the change in refractive index of the liquid under test caused by the temperature rise will modulate the phase of the probe beam, so that the outgoing light carries phase change information related to the temperature rise, and obtains the first beam. The working principle of this module and the relationship between phase change and temperature rise are exactly the same as step S2 in Example 1. The thermo-optic effect relationship is shown in Equation (14), and the relationship between phase change and temperature rise is shown in Equation (15), which will not be repeated here.

[0075] The spatial filtering module is used to spatially filter the first beam, converting the phase change information carried by the first beam into changes in light intensity distribution to obtain the second beam. The 4f phase-contrast imaging module is the core module of the system, consisting of a pinhole aperture, a convex lens group, and a reflecting mirror.

[0076] Specifically, the spatial filtering module includes a 4f phase coherent imaging system, such as... Figure 3 As shown, the 4f phase coherent imaging system includes a first lens 1, a surface on which the liquid to be measured is placed (i.e., the spectral surface), a second lens 2, and a phase stop 7 disposed on the incident surface of the 4f phase coherent imaging system, arranged sequentially along the optical path. Figure 4 The phase stop consists of a circular stop and a circular phase object (PO) located at the center of the circular stop. The circular stop is used to limit the aperture of the incident beam, and its radius is [missing information]. Circular phase objects are typically made by depositing a thin dielectric film of specific thickness and refractive index at the center of a transparent glass substrate, with a radius of [missing information]. It applies a preset phase delay to the portion of the beam incident on it without changing its amplitude.

[0077] The 4f phase-coherent imaging system uses laser system 4 as its light source. The output laser beam is first split into two paths by the first beam splitter 5. One path serves as the probe beam and enters the main imaging optical path, while the other path serves as the reference beam and enters the reference optical path composed of the first reflector 10, the second reflector 11, and the third lens 3. The probe beam is incident on the phase stop 7 placed on the incident surface of the 4f system, and the beam passing through the phase stop 7 carries the initial phase modulation information.

[0078] The light beam undergoes a Fourier transform via the first lens 1, forming a spatial spectrum on the back focal plane (spectral plane) of the first lens 1. The liquid to be measured 8 is precisely placed on this spectral plane, and the thermally induced nonlinear phase shift generated after absorbing the focused laser energy filters the incident spatial spectrum. The filtered spectrum then undergoes an inverse Fourier transform via the second lens 2, forming an image conjugate to the incident plane on the back focal plane (image plane) of the second lens 2, thus converting the phase delay of the phase stop 7 into a change in light intensity distribution. A filter 9 is installed on the output light path of the second lens 2 to adjust the light intensity and filter out stray light to improve the imaging signal-to-noise ratio. The beam is then split into two paths by the second beam splitter 6: one for real-time observation and the other as a second beam carrying information about the change in light intensity distribution, which is then incident on the signal receiving device. The beam in the reference path has its propagation direction changed by the first reflector 10 and the second reflector 11, and after being collimated and expanded by the third lens 3, it coherently superimposes with the probe beam on the image plane, further enhancing the contrast and observability of the image.

[0079] The working principle of this module and the relationship between the change in light intensity distribution and the phase change are exactly the same as step S3 in Example 1. The transmittance function of the phase stop 7 is shown in Equation (17), the incident light field is shown in Equation (18), the light field distribution of the spectral surface is shown in Equation (21), the light field distribution of the liquid under test 8 after filtering is shown in Equation (22), the light intensity distribution of the image surface is shown in Equation (23), and the linear relationship between the final change in light intensity distribution and the phase change of the first beam is shown in Equation (28), which will not be repeated here.

[0080] In practical implementation, the spatial filtering module can employ a Michelson interferometer to achieve the functions of the aforementioned 4f phase coherent imaging system. The Michelson interferometer includes a beam splitter, a stationary mirror, and a moving mirror. It splits the first beam carrying phase change information into a probe beam and a reference beam. A black dot with a diameter less than 1 mm is installed at the center of the stationary mirror to block the central bright fringe. A small aperture is placed in front of the moving mirror to make the transmitted light the same size as the central bright fringe. By adjusting the position of the moving mirror, the optical path difference between the two beams is changed, converting the minute phase change caused by the thermo-optical effect of the liquid under test into a change in the brightness of the interference light, outputting a second beam carrying information on the change in light intensity distribution. The scheme using a Michelson interferometer has the advantages of a simpler optical path structure, lower device cost, and convenient operation and adjustment, making it particularly suitable for teaching demonstration scenarios.

[0081] The signal receiving module receives the second light beam and converts it into an electrical signal, obtaining the change in light intensity distribution based on the electrical signal. The signal receiving module is positioned on the image plane of the 4f phase-coherent imaging system. The signal receiving module consists of an aperture stop and a photosensor; the signal receiver can be a photodiode, photomultiplier tube, or photodetector from a CCD / CMOS camera. This module receives the second light beam and converts it into a voltage signal. The voltage signal and the change in the light intensity distribution of the second beam They satisfy a linear relationship As shown in equation (29), where The conversion factor is determined by the responsivity of the photodetector and the gain of the subsequent amplification circuit. The CCD12, as the core component of the signal receiving module, completes the acquisition and conversion of optical signals, transforming changes in light intensity distribution. It is converted into an electrical signal that can be processed later.

[0082] The data processing module is used to calculate the thermal diffusivity of the liquid under test based on the correspondence between the change in light intensity distribution and the increase in temperature, as well as the logarithmic change of the increase in temperature over time. This data processing module is typically implemented by a computer with dedicated data processing software installed. The data processing module first calculates the thermal diffusivity of the liquid under test using equations (30) and (31) from the waveform obtained by the signal receiving module. The curve is then substituted into the transient hot-wire method model. The specific data processing flow and calculation formula are exactly the same as step S5 in Example 1, and finally the direct calculation formula (35) for the thermal diffusivity is obtained.

[0083] Example 3: This embodiment uses pure water as the liquid to be tested, and specifically describes the complete experimental procedure, data recording, and result analysis for actual measurement using the optical measurement system for the thermal diffusivity of liquids described in this invention.

[0084] First, prepare for the experiment and debug the system. Clean the surface of the optical platform with alcohol and a lint-free cloth, and check the tightness of all adjustment bracket fixing screws to ensure the stability of the optical platform. Pour the pure water sample into a clean quartz cuvette. The inner diameter of the sample cuvette (i.e., the optical path length of the probe beam in the heating region) is... Precise measurements are required beforehand. Place the cuvette containing pure water on the sample holder.

[0085] Turn on the laser generator and preheat for 5 minutes until the output stabilizes, ensuring that the stability of the laser output power meets the measurement requirements. In this embodiment, the laser generator simultaneously provides a heating beam (pump beam) and a probe beam, or these can be provided by two independent lasers respectively.

[0086] Use a piece of white paper to check if the laser beam path matches the calibration optical path. Confirm that the laser beam passes completely through the center of all optical components, through the center of the liquid sample, and into the photodetector. Adjust the sample position so that the beam waist of the focused laser beam (pump beam) is accurately located inside the liquid sample to be tested, while simultaneously ensuring that the probe beam passes through the heated region.

[0087] A phase stop is mounted on the incident plane of a 4f phase-coherent imaging system using precision instruments. The phase stop consists of a circular aperture and a circular phase object formed by depositing a dielectric thin film at its center. The radius of the circular phase object... and the radius of the circular aperture Given the design parameters, adjust the position so that the circular phase object is at the center of the optical path. Observe the image that appears on a white screen to confirm that the zero-frequency component has been accurately phase-shifted.

[0088] Adjust the optical system and observe the light emitted from the liquid on the screen. Check if the brightness of the central bright fringe changes. If it does, the phenomenon is correct and the system is working properly.

[0089] Preferably, when using a Michelson interferometer, a black dot with a diameter of less than 1 mm is installed at the center of the stationary mirror of the Michelson interferometer using a precision instrument. The small aperture in front of the moving mirror is adjusted so that the transmitted light is the same size as the central bright fringe. The interference fringes are observed on a white screen. If the central bright fringe is just blocked, it indicates that the position of the black dot is correct. The interferometer knob is adjusted to change the optical path difference between the two beams, and the interference fringes are observed on the screen to see if the brightness of the central bright fringe changes. If a change in brightness occurs, it indicates that the phenomenon is correct and the system is working properly. After finding a suitable phase for observation and fixing the position of the interferometer knob, the measurement begins.

[0090] Cover the detector with a piece of white paper and observe the changes on the oscilloscope. This helps eliminate experimental errors caused by the oscilloscope and issues with its own sensitivity, and allows you to check if the oscilloscope is functioning correctly. If the oscilloscope image changes from a curve to a horizontal straight line when the paper is covered, it indicates that the oscilloscope is functioning correctly and has a response time of 1.2ms. The signal should return to zero the instant the paper is covered and recover the instant it is removed. This verifies the response speed and integrity of the signal receiver and oscilloscope system, indicating that the signal receiving module is sensitive and has no significant lag, ensuring that its response time meets the millisecond-level measurement requirements.

[0091] Next, proceed to the formal measurement. Fine-tune the position of the liquid to be measured, moving it forward, backward, left, and right, to find the focal point of the light formed by the sample liquid, and precisely place the sample at the focal point. Adjust the system to find a suitable phase for observation (generally the phase at its brightest / darkest; this example demonstrates the phenomenon at the brightest phase).

[0092] Change the adjustment knob and observe the amplitude changes of the oscilloscope at different phases. Record and compare the changes at the highest and lowest amplitudes. Adjust the system to the highest position to obtain the best signal contrast.

[0093] At a certain moment, a baffle is used to momentarily block the focused laser beam (pump optical path) directed at the liquid under test. After the pump optical path is blocked, the forced heating of the liquid stops, and thermal diffusion begins to dominate the temperature change. Observe the changes in the oscilloscope image; if the signal shows a certain amplitude decrease, it indicates that thermal diffusion is occurring in the liquid. Record the changes in the oscilloscope at this time, including the maximum value, minimum value, and change time. During the process of blocking the pump optical path by manual or automatic shutter, the intensity of the central bright spot in the CCD-acquired image begins to decrease.

[0094] From the moment the pump optical path is blocked, the oscilloscope will record the voltage signal. Over time A continuous decay curve. This decay curve corresponds to the temperature rise during the cooling process. logarithm over time The relationship between the changes. Continuously record data until the signal tends to stabilize.

[0095] Change the adjustment knob and observe the amplitude changes of the oscilloscope at different phases. Record and compare the changes at the highest and lowest amplitudes. Repeat the above experimental steps multiple times, observe the changes in the oscilloscope graph, and integrate the experimental data to obtain multiple sets of measurement data for subsequent statistical analysis and uncertainty assessment.

[0096] Further, data processing and analysis are performed, importing the electrical signal-time data collected by the signal receiving module into the computer. Based on the linear relationship chain established in Example 1, the voltage signal... Linear conversion to temperature rise In terms of temperature increase The vertical axis represents the logarithm of time. Plotting the x-axis curve.

[0097] Based on the conditions for the approximate solution of the heat diffusion equation, the interval of the curve that approximates a straight line is selected as the linear interval. After determining the linear interval of the curve, the slope is obtained by linear fitting of the linear interval using the least squares method. .

[0098] Furthermore, the slope obtained from the fitting... Heating power of focused laser beam and the known density of pure water and specific heat capacity Substituting into the thermal diffusivity calculation formula (34), the thermal diffusivity of the liquid to be tested (pure water) can be calculated. Table 1 shows the analysis results of 16 sets of repeated measures data.

[0099] Table 1:

[0100] Based on the data in Table 1, the arithmetic mean of the thermal diffusivity obtained from 16 measurements is: The result is in good agreement with the standard value of the thermal diffusivity of pure water at room temperature, verifying the accuracy and reliability of the method of the present invention.

[0101] Further uncertainty analysis was conducted, including: Type A uncertainty (caused by the repeatability of measurement data): The calculation demonstration will be conducted using the first set of data, i.e., the first eight data points. Multiple repeated measurements (n=8) are performed on the same liquid to be tested, resulting in a set of thermal diffusivity measurements. Calculate their arithmetic mean: .

[0102] The experimental standard deviation is: ; The Type A standard uncertainty is: ; .

[0103] Type B uncertainty (mainly caused by instrument error): Assume the time resolution of the oscilloscope is The time resolution of the detector is If treated as a uniform distribution, then: The Type B uncertainty component introduced by the oscilloscope is: ; The Type B uncertainty component introduced by the detector is: ; The combined standard uncertainty of type B is: .

[0104] Combined standard uncertainty: The combined standard uncertainty is composed of Type A and Type B uncertainties combined by the root sum of squares: .

[0105] Expanded uncertainty: Taking a confidence factor k = 2 (corresponding to a confidence probability of approximately 95%), the expanded uncertainty is: , Therefore, the final measurement result of the thermal diffusivity of pure water in this embodiment can be expressed as: .

[0106] The measurement results are consistent with the accepted standard value of the thermal diffusivity of pure water within the error range, fully verifying the accuracy of the method of the present invention. Experimental results show that the non-contact optical measurement system described in this invention can effectively avoid the disturbance of the liquid temperature field by traditional contact sensors. The linear relationship of the entire signal conversion link ensures high-precision measurement. Moreover, the experimental device has a simple structure and is easy to operate, making it fully suitable for daily teaching demonstrations and scientific research measurement scenarios.

[0107] In summary, through actual measurements using pure water as the test liquid, the operation process, data processing method, and measurement accuracy of the optical measurement method and system for the thermal diffusivity of liquids described in this invention have been fully demonstrated, proving the feasibility and superiority of this invention.

[0108] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. An optical measurement method for the thermal diffusivity of a liquid, characterized in that, Includes the following steps: S1: The liquid to be tested is irradiated with a focused laser beam, causing the liquid to absorb laser energy and generate a temperature increase that varies with time and is radially distributed in space. S2: Pass the probe beam through the region in the liquid to be tested that generates the temperature increase, so that the probe beam carries phase change information and obtains a first beam; S3: Spatial filtering is performed on the first beam to convert the phase change information carried by the first beam into a change in light intensity distribution, thus obtaining the second beam; S4: Receive the second light beam and convert it into an electrical signal, and obtain the change in light intensity distribution based on the electrical signal; S5: Based on the correspondence between the change in light intensity distribution and the increase in temperature, as well as the logarithmic change of the increase in temperature over time, the thermal diffusivity of the liquid to be tested is calculated.

2. The optical measurement method for the thermal diffusivity of a liquid according to claim 1, characterized in that: In step S2, the method for passing the probe beam through the region in the liquid to be tested that generates the temperature increase, so that the probe beam carries phase change information, to obtain the first beam is as follows: The probe beam passes through the heating region, and the temperature rises by a certain amount. The change in refractive index of the liquid under test modulates the phase of the probe beam, which carries the phase change information as it exits, thus obtaining the first beam; the phase change of the exiting light With temperature increase The relationship between them is represented as follows: ， in, The optical path length of the probe beam within the temperature rise region. The wavelength of the probe beam. The thermo-optic coefficient of the liquid to be tested is denoted as .

3. The optical measurement method for the thermal diffusivity of a liquid according to claim 1, characterized in that: In step S3, the first beam is spatially filtered to convert the phase change information carried by the first beam into a change in light intensity distribution, thereby obtaining the second beam. The first beam is introduced into a 4f phase coherent imaging system. A phase stop is placed on the incident surface of the 4f phase coherent imaging system. The phase stop consists of a circular stop and a circular phase object located at the center of the circular stop. The circular phase object applies a phase shift to a portion of the beam incident on it. Without changing the amplitude; The first beam, passing through the phase aperture, is subjected to Fourier transform by a lens and then incident on the liquid to be tested located on the spectral plane. The spectrum is filtered by the thermally induced nonlinear phase shift generated by the liquid to be tested. On the image plane, the phase delay of the circular phase object is converted into a change in light intensity distribution to obtain the second beam.

4. The optical measurement method for the thermal diffusivity of a liquid according to claim 3, characterized in that: The change in light intensity distribution ∆I and the phase change ∆φ of the first beam satisfy: ， in, For geometric proportions, , Let be the radius of the circular phase object. Let be the radius of the circular aperture. The difference in Taylor expansion coefficients.

5. The optical measurement method for the thermal diffusivity of a liquid according to claim 1, characterized in that: In step S4, the second light beam is received and converted into an electrical signal. The method for obtaining the change in light intensity distribution based on the electrical signal is as follows: Receive the second beam and convert it into a voltage signal. The voltage signal Changes in light intensity distribution with the second beam They satisfy a linear relationship ,in is the conversion factor.

6. The optical measurement method for the thermal diffusivity of a liquid according to claim 1, characterized in that: In step S5, the method for calculating the thermal diffusivity of the liquid under test based on the correspondence between the change in light intensity distribution and the increase in temperature, and the logarithmic change of the increase in temperature over time, is as follows: The change in light intensity distribution and the temperature increase satisfy a linear correspondence, and the temperature increase and the logarithm of time satisfy a linear relationship; the change in light intensity distribution is converted into the temperature increase corresponding to different times. In terms of temperature increase The vertical axis represents the logarithm of time. Plot a curve on the x-axis, determine the linear interval of the curve, and fit the curve to obtain the slope. The thermal diffusivity is calculated by substituting the slope into the following formula. : ， in, To focus the heating power of the laser beam, The density of the liquid to be tested, is the specific heat capacity of the liquid to be tested.

7. The optical measurement method for the thermal diffusivity of a liquid according to claim 1, characterized in that: The liquid to be tested is a transparent or semi-transparent liquid, including water and organic solvents.

8. An optical measurement system for the thermal diffusivity of a liquid, characterized in that, include: The heating module is used to irradiate the liquid to be tested with a focused laser beam, so that the liquid absorbs the laser energy and generates a temperature increase that varies with time and is radially distributed in space. A detection optical module is used to pass the detection beam through the region in the liquid to be tested that generates the temperature rise, so that the detection beam carries phase change information to obtain a first beam. A spatial filtering module is used to perform spatial filtering on the first beam, converting the phase change information carried by the first beam into changes in light intensity distribution to obtain a second beam. The signal receiving module is used to receive the second light beam and convert it into an electrical signal, and to obtain the change in light intensity distribution based on the electrical signal; The data processing module is used to calculate the thermal diffusivity of the liquid under test based on the correspondence between the change in light intensity distribution and the increase in temperature, as well as the logarithmic change of the increase in temperature over time.

9. The optical measurement system for the thermal diffusivity of a liquid according to claim 8, characterized in that: The spatial filtering module includes a 4f phase coherent imaging system, which includes a first lens, a liquid placement surface, a second lens arranged sequentially along the optical path, and a phase stop disposed on the incident surface of the 4f phase coherent imaging system; the phase stop consists of a circular stop and a circular phase object located at the center of the circular stop.

10. The optical measurement system for the thermal diffusivity of a liquid according to claim 9, characterized in that: The signal receiving module is disposed on the image plane of the 4f phase coherent imaging system.

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