A method and system for measuring the central phase of a thermal lens

Abstract

The present invention discloses a method and system for measuring the center phase of a thermal lens, and relates to the field of optical measurement. The method for measuring the center phase of a thermal lens includes obtaining an image halo and its actual geometric size after being modulated by a thermal lens; establishing a verification relationship between the center phase of the thermal lens and the geometric size of the image halo; substituting different center phase values ​​of the thermal lens into the verification relationship to obtain different theoretical calculation values ​​of the geometric size of the image halo; when the theoretical calculation value of the geometric size of the image halo is consistent with the actual geometric size, determining the actual center phase of the thermal lens. The present invention establishes a verification relationship between the geometric size of the image halo formed by the thermal lens and the center phase of the thermal lens, and tries to adjust the center phase of the thermal lens until it is the same as the actual geometric size of the image halo, thereby obtaining the actual center phase of the thermal lens. The method has a simple optical path, low implementation cost, and is not affected by vibration.

Description

Technical Field

[0001] The invention relates to the technical field of optical measurement, and in particular to a method and system for measuring the central phase of a thermal lens. Background Art

[0002] The thermal lens effect is when the laser irradiates the surface of a material or transmits through a sample, the material's thermal absorption and thermal expansion cause the surface of the material to produce a lens-like response, which in turn changes the phase distribution of the light wave. Phase measurement can directly reflect this phase change and further understand the corresponding physical mechanism of thermal lensing.

[0003] Thermal lens center phase measurements are usually performed using the following method:

[0004] 1. Interference method: The phase change of the light beam at the center of the lens can be measured by an interferometer. By placing the lens in one arm of the interferometer, the phase of the center of the lens can be measured by observing the movement of the interference fringes. This method requires a sophisticated interferometer, and the equipment is complex and expensive. For large-aperture lenses or high-frequency wavefronts, the interference fringes may be limited by spatial coherence, resulting in reduced sensitivity of the measurement.

[0005] 2. Fresnel lens method: Use a series of radial Fresnel lenses to generate an approximate plane wave front, and then use interference technology to measure the phase change of the tested lens. This method requires high-precision processing technology and is costly. The manufacturing error of the lens and the imaging accuracy of the Fresnel lens will affect the measurement accuracy.

[0006] 3. Aberration expansion method: By imaging a point light source onto the image plane of the lens under test, the phase of the lens center is inferred by observing the change in aberration. This method is usually less sensitive to phase measurement and is suitable for some larger wavefront distortions.

[0007] 4. Dual-focus method: Utilize the dual-focus property of the lens, adjust the position of the imaging plane, and observe the movement of the image point to determine the phase of the lens. This method requires an accurate imaging system, has high requirements on the position of the light source and the imaging system, and the measurement accuracy is limited by the resolution and sensitivity of the imaging system.

[0008] 5. Point spread interferometry: Use a point light source to illuminate the lens under test, and observe the formation of interference fringes to measure the phase of the lens. This method may cause phase discontinuity, especially for complex wavefront structures. At the same time, it is susceptible to environmental interference and system errors, such as vibration, thermal drift, etc., which may affect the measurement results.

[0009] The purpose of the present invention is to provide a method for measuring the central phase of a thermal lens by imaging the Bessel annular ring of an axicon diffraction. Compared with the existing method, this method only needs beam expansion, collimation and imaging optical paths, and the thermal lens phase information can be obtained through image processing technology. The optical path is simple, the implementation cost is low, and it is not affected by vibration. Summary of the invention

[0010] In view of the problems existing in the above-mentioned prior art, the present invention is proposed. The present invention provides a method and system for measuring the central phase of a thermal lens, which can measure the phase information of the thermal lens without being affected by vibration and has low implementation cost.

[0011] To solve the above technical problems, in a first aspect, the present invention provides a method for measuring the central phase of a thermal lens, which comprises obtaining an image halo and its actual geometric dimensions after being modulated by a thermal lens; establishing a verification relationship between the central phase of the thermal lens and the geometric dimensions of the image halo; substituting different central phase values ​​of the thermal lens into the verification relationship to obtain different theoretical calculated values ​​of the geometric dimensions of the image halo; and determining the actual central phase of the thermal lens when the theoretical calculated value of the geometric dimensions of the image halo is consistent with the actual geometric dimensions.

[0012] As a preferred solution of the thermal lens central phase measurement method described in the present invention, the method for obtaining the image halo after being modulated by the thermal lens includes obtaining a ring-shaped light spot; passing the ring-shaped light spot through a 4f optical system to obtain the image halo; and placing the thermal lens at the confocal point of two convex lenses in the 4f optical system.

[0013] As a preferred solution of the method for measuring the central phase of a thermal lens described in the present invention, the method for obtaining the annular light spot includes allowing the expanded collimated light beam to pass through an axicon and a convex lens in sequence.

[0014] As a preferred solution of the thermal lens center phase measurement method of the present invention, the image light ring is imaged on a charge coupled device.

[0015] As a preferred solution of the thermal lens center phase measurement method of the present invention, the actual geometric size of the image light ring is obtained by Hough transformation.

[0016] As a preferred solution of the thermal lens central phase measurement method of the present invention, the actual geometric size of the image light ring is the radius or diameter of the image light ring.

[0017] As a preferred solution of the thermal lens center phase measurement method of the present invention, the verification relationship is a function of the relationship between the thermal lens center phase and the geometric size of the image halo.

[0018] In a second aspect, the present invention provides a thermal lens center phase measurement system, which is suitable for the above-mentioned measurement method. The system includes an annular spot generating unit for generating an annular spot, which includes a laser, an axicon and a first convex lens arranged in sequence along the optical path; an imaging unit, including a second convex lens, a thermal lens and a third convex lens arranged in sequence along the optical path; and an image acquisition unit, including a neutral attenuation plate and a charge coupler arranged in sequence along the optical path.

[0019] As a preferred solution of the thermal lens center phase measurement system described in the present invention, the second convex lens and the third convex lens form an f optical system, and the thermal lens is placed at the common focus of the second convex lens and the third convex lens.

[0020] As a preferred solution of the thermal lens center phase measurement system of the present invention, the annular light spot is formed at the front focal plane of the second convex lens, and the charge coupler is located at the back focal plane of the third convex lens.

[0021] The beneficial effects of the present invention are as follows: the present invention establishes a corroborative relationship between the geometric size of the image light ring formed by the thermal lens and the central phase of the thermal lens, and tries to adjust the central phase of the thermal lens until it is the same as the actual geometric size of the image light ring, thereby obtaining the actual central phase of the thermal lens. Compared with the existing method, this method only needs beam expansion, collimation and imaging light path, and the thermal lens phase information can be obtained through image processing technology. The optical path is simple, the implementation cost is low, and it is not affected by vibration. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0023] Figure 1 This is the optical path structure diagram of the thermal lens center phase measurement system.

[0024] Figure 2 Schematic diagram of the image halo of the receiving surface.

[0025] Figure 3 Schematic diagram of the structural parameters of the axicon mirror.

[0026] In the figure: 100, annular spot generating unit; 101, laser; 102, axicon; 103, first convex lens; 200, imaging unit; 201, second convex lens; 202, thermal lens; 203, third convex lens; 300, image acquisition unit; 301, neutral attenuation plate; 302, charge coupler; 400, Bessel ring. DETAILED DESCRIPTION

[0027] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the specific embodiments of the present invention are described in detail below in conjunction with the accompanying drawings.

[0028] In the following description, many specific details are set forth to facilitate a full understanding of the present invention, but the present invention may also be implemented in other ways different from those described herein, and those skilled in the art may make similar generalizations without violating the connotation of the present invention. Therefore, the present invention is not limited to the specific embodiments disclosed below.

[0029] Secondly, the term "one embodiment" or "embodiment" as used herein refers to a specific feature, structure or characteristic that may be included in at least one implementation of the present invention. The term "in one embodiment" that appears in different places in this specification does not necessarily refer to the same embodiment, nor is it an embodiment that is mutually exclusive with other embodiments, either individually or selectively.

[0030] Example 1, reference Figure 1~Figure 3 , is the first embodiment of the present invention, which provides a method for measuring the central phase of a thermal lens, and the method for measuring the central phase of a thermal lens includes:

[0031] S1: Obtain the image halo after thermal lens modulation and its actual geometric size.

[0032] Specifically, in order to better describe the measurement method of this embodiment, it is now described based on a set of thermal lens center phase measurement system, such as Figure 1 As shown, the system includes a laser 101, an axicon 102, a first convex lens 103, a second convex lens 201, a thermal lens 202, a third convex lens 203, a neutral attenuation plate 301 and a charge coupler 302 which are sequentially arranged along the optical path.

[0033] Among them, the second convex lens 201 and the third convex lens 203 form a 4f imaging system. The laser 101 expands the collimated light beam through the axicon 102 and the first convex lens 103 to form an annular Bessel light ring 400, and the Bessel light ring 400 is used as the object to form an image through the 4f system. The Bessel light ring 400 completes a Fourier transform through the second convex lens 201, and the focal plane light intensity is Gaussian distributed. The focal light intensity acts on a liquid sample that has a certain absorption of the incident laser, that is, the thermal lens 202. Due to the non-uniform distribution of the incident sample light intensity, a temperature gradient is formed in the thermal lens 202, thereby forming a density gradient. According to the positive and negative photothermal coefficient of the thermal lens 202, a divergent lens or a condenser lens is formed in the illumination area, and the light beam at the focus is divergent or converged by the thermal lens 202. The modulated light beam completes the inverse Fourier transform through the third convex lens 203, and is imaged on the target surface of the charge coupler 302, thereby obtaining an image light ring. The charge coupled device 302 is connected to a computer and calculates the actual geometric size of the image halo through Hough transformation.

[0034] S2: Establishing a corroborative relationship between the central phase of the thermal lens and the geometric size of the image halo.

[0035] Specifically, the incident light after collimation and expansion is assumed to be parallel light, and its amplitude can be calculated by measuring the incident energy and the spot size. First, assume that the incident parallel light has unit amplitude, the optical axis direction is the z-axis, and establish a coordinate system. The polar coordinate plane of the apex of the through-axis conic mirror 102 is (r, θ), and the polar coordinate plane at the z position behind the apex is The optical path function of the axicon 102 is:

[0036] ,

[0037] in, is the change in the optical path length of the light beam at the radial position r after passing through the axicon; n is the refractive index of the axicon material; h is the height of the axicon; α is the opening angle of the axicon; r is the radial distance from the center of the axicon to the position of the light beam on the lens.

[0038] The corresponding complex amplitude transmittance function can be expressed as:

[0039] ,

[0040] in, is the complex amplitude transmittance function of the light beam at the radial position r, which describes the phase change of the light beam after passing through the axicon; j is an imaginary unit used to represent the phase change; k is the wave number; n, h, α, and r are the same as above, representing the material and geometric parameters of the axicon.

[0041] Ignore the constant phase factor and de-paraxialize:

[0042] ,

[0043] The diffraction field distribution of the light beam at z after passing through the axicon can be obtained by the Fresnel diffraction formula:

[0044] ,

[0045] in, is the light field in coordinates Field distribution at is the radial coordinate, indicating the radial position of the light beam on the imaging plane; is the angular coordinate; z is the distance the light beam propagates, the optical path distance from the lens to the imaging plane; is the wavelength of the light wave; k is the wave number , is the polar coordinate angle. The light intensity distribution of the axicon at the vertex perpendicular to the optical axis is obtained by multiplying the uniform incident light by the axicon transmittance function: ,in is the incident parallel light electric field intensity, which can be calculated by measuring the incident light energy and the incident light spot size.

[0046] Here the light intensity does not affect the subsequent measurement results. ; Substitute the axon transmittance function, the incident axis axon focal length The light wave field distribution at is:

[0047] ,

[0048] R is the maximum radius of the halo, the upper limit of the integral; is the zero-order Bessel function of the first kind with scaling in polar coordinates.

[0049] The cross-sectional light intensity distribution at the focus of the corresponding axicon is:

[0050] ,

[0051] in, To receive the polar coordinates of the focal plane of the axicon, , z, k are the same as above, which are the wavelength, optical path distance and wave number of light wave respectively. r is the radial coordinate integral variable; is the Bessel function, which describes the diffraction characteristics of the light beam on the lens; R is the maximum radius of the light ring, the upper limit of the integral.

[0052] After passing through the first convex lens 103, the light ring 400 at the rear focal plane can be regarded as After a Fourier transform, we get:

[0053] ,

[0054] in is the electric field intensity distribution of the light beam at position 400 of the halo, 400 halo plane polar coordinates , are the radial and angular coordinates of the focal plane of the axon. The light intensity distribution at the position of the ring 400 is:

[0055] ,

[0056] pass The diameter information of the 400 halo can be obtained.

[0057] The position of the thermal lens 202 is the Nessel ring 400, which is Fourier transformed again. Therefore, the light intensity distribution on the front surface of the thermal lens 202 is There is a difference of one scaling factor. If the convex lens has the same focal length, the electric field intensity on the front surface of the thermal lens 202 is the same as the light intensity expression. The electric field intensity of the light beam on the rear surface of the thermal lens 202 can be expressed as:

[0058] ,

[0059] in is the light field distribution after being modulated by the thermal lens effect; is the field distribution of the beam before modulation by the thermal lens effect. Under the conditions of same; is the light absorption coefficient, which describes the absorption loss when the light beam passes through the thermal lens; L is the thickness of the thermal lens; is the phase distribution of the thermal lens, and its numerical distribution is proportional to the light intensity distribution on the front surface of the thermal lens.

[0060] .

[0061] in It is the phase of the center point of the sample. It can give an initial value for numerical fitting. is the maximum light intensity at the center of the front surface of the sample, which can be obtained through numerical simulation. is the light intensity distribution on the front surface of the sample.

[0062] The light intensity distribution on the CCD receiving surface is: After a Fourier transform:

[0063] ,

[0064] in, stands for Inverse Fourier Transform; is the coherent optical transfer function of an ideal lens, ; , Receiving plane rectangular coordinate value. N A is the numerical aperture of the lens; G is the magnification of the optical system, and when the focal length of the lens is the same, G=1. and The light ring on the receiving plane can be simulated and calculated.

[0065] S3: Substituting different central phase values ​​of the thermal lens into the verification relationship, and calculating and obtaining different theoretical values ​​of the geometric dimensions of the image halo; when the theoretical values ​​of the geometric dimensions of the image halo are consistent with the actual geometric dimensions, determining the actual central phase of the thermal lens.

[0066] The specific steps can be divided into the following steps:

[0067] Step 1: After adjusting the optical path, take a photo of the light ring without a thermal lens and calculate the diameter of the light ring .

[0068] Step 2: Place the thermal lens, and after stabilization, take a photo of the light ring under the thermal lens modulation condition and calculate the diameter of the light ring . Calculate the ratio of the two diameters .

[0069] Step 3: Calculate the light intensity at the 400 light ring position based on the light path parameters . Extract the diameter of the halo .

[0070] Step 4: Give a trial phase , calculate the CCD image plane light intensity distribution , extract the diameter of the halo .

[0071] Step 5: Calculate the ratio of the diameters of the two plane numerical rings: .

[0072] Step 6: Compare k with ,if , reduce the trial phase Value, recalculate .if , increase the trial phase Value until and The only difference is that the error requirement is met. This is the measured value.

[0073] Example 2, reference Figure 1 , which is the second embodiment of the present invention, and is based on the previous embodiment. This embodiment provides a thermal lens center phase measurement system.

[0074] Specifically, Figure 1 As shown, the system includes a laser 101, an axicon 102, a first convex lens 103, a second convex lens 201, a thermal lens 202, a third convex lens 203, a neutral attenuation plate 301 and a charge coupler 302 which are sequentially arranged along the optical path.

[0075] The working principle of this system is explained by taking the determination of the central phase of the thermal lens formed by irradiating a copper sulfate solution with a thickness of 1mm with a 632.8nm, 3mW laser as an example:

[0076] First, the laser 101 is expanded to obtain a collimated parallel light beam. To ensure the focusing effect, the diameter of the emitted light spot is greater than 3 mm. The axicon 102 is adjusted, and a symmetrical annular light spot appears in the far field.

[0077] Next, the first convex lens 103 is placed and adjusted so that the Bessel light ring 400 is received at the rear focal plane of the first convex lens 103 .

[0078] The second convex lens 201 and the third convex lens 203 are placed, and the distance between the second convex lens 201 and the third convex lens 203 is adjusted to be the sum of the focal lengths of the two convex lenses.

[0079] The intensity of the outgoing light is adjusted by the neutral attenuation plate 301, and the rear focal plane of the third convex lens 203 is placed by the charge coupler 302 to receive the outgoing image light ring.

[0080] The current image aura is collected, and the parameters are numerically calculated and optimized based on the incident light spot size and the optical device parameters, so that the calculated aura size is the same as the photographed aura size.

[0081] A copper sulfate sample, i.e., a thermal lens 202, is placed on the confocal plane of the second convex lens 201 and the third convex lens 203, and the absorption coefficient of the liquid to be measured is calculated by measuring the light intensity on the front and rear surfaces of the sample. The size of the annular spot after passing through the sample to be measured is recorded, and the central phase of the thermal lens is tried by numerical fitting so that the calculated value is close to the photographed value. At this time, the trial value is the central phase of the thermal lens. The specific method is as described in Example 1.

[0082] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present invention, which should all be included in the scope of the claims of the present invention.

Claims

1. A method for measuring the central phase of a thermal lens, characterized in that: include, Obtain the image halo after thermal lens modulation and its actual geometric size; Establishing a corroborative relationship between the central phase of the thermal lens and the geometric size of the image halo; Substituting different central phase values ​​of the thermal lens into the verification relationship, the theoretical calculated values ​​of different geometric dimensions of the image halo are calculated; When the theoretical calculated value of the geometric size of the image light ring is consistent with the actual geometric size, determining the actual central phase of the thermal lens; The method for obtaining the image halo after being modulated by the thermal lens comprises: Obtain an annular light spot; Passing the annular light spot through a 4f optical system to obtain the image light ring; The thermal lens is placed at the confocal position of two convex lenses in the 4f optical system; The verification relationship is a function of the relationship between the central phase of the thermal lens and the geometric size of the image halo.

2. The method for measuring the central phase of a thermal lens according to claim 1, wherein: The method for obtaining the annular light spot comprises allowing the expanded collimated light beam to pass through an axicon and a convex lens in sequence.

3. The method for measuring the central phase of a thermal lens according to claim 2, characterized in that: The image light ring is imaged on a charge coupled device.

4. The method for measuring the central phase of a thermal lens according to claim 3, characterized in that: The actual geometric size of the image halo is obtained through Hough transformation.

5. The method for measuring the central phase of a thermal lens according to claim 4, characterized in that: The actual geometric size of the image halo is the radius or diameter of the image halo.

6. A thermal lens center phase measurement system, characterized in that: Applicable to the thermal lens center phase measurement method as described in any one of claims 1 to 5, the system comprises: An annular light spot generating unit (100), used for generating an annular light spot, comprising a laser (101), an axicon (102), and a first convex lens (103) arranged in sequence along an optical path; An imaging unit (200) comprising a second convex lens (201), a thermal lens (202), and a third convex lens (203) which are sequentially arranged along an optical path; The image acquisition unit (300) comprises a neutral attenuation plate (301) and a charge coupler (302) which are sequentially arranged along an optical path.

7. The thermal lens center phase measurement system according to claim 6, characterized in that: The second convex lens (201) and the third convex lens (203) form a 4f optical system, and the thermal lens (202) is placed at the common focus of the second convex lens (201) and the third convex lens (203).

8. The thermal lens center phase measurement system according to claim 7, characterized in that: The annular light spot is formed at the front focal plane of the second convex lens (201), and the charge coupler (302) is located at the back focal plane of the third convex lens (203).

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