Method for estimating focal length of a crystal thermal lens

Abstract

The application discloses a method for deducing the focal length of a crystal thermal lens. Firstly, the spot data of a measuring end are measured and corrected, so that the distance from the light outlet of a detector to a focusing lens is consistent with the actual measuring distance, and the beam waist radius of the light outlet of a detection laser is deduced. Then, in the case that a crystal is put in but not pumped, the inherent lens-like focal length of the crystal caused by clamping stress or residual stress during growth is deduced. Finally, in the case that pumping light irradiates the crystal, the spot sizes of the detection light passing through both sides of the crystal are made consistent, the total lens-like focal length value of the crystal is deduced by a curvature radius formula deduction method, the influence of the inherent lens-like focal length of the crystal caused by clamping stress or residual stress during growth is excluded by a combined lens focal length formula, and finally the thermal lens focal length value of the crystal is obtained. The application has a smaller measurement error and is suitable for the measurement of the thermal lens focal length of any strength crystal.

Description

Technical Field

[0001] This invention belongs to the field of solid-state laser technology and relates to a method for estimating the focal length of a crystal thermal lens. Background Technology

[0002] In recent years, semiconductor laser-pumped solid-state lasers have developed rapidly and have been widely used in industrial processing, long-distance communication, precision detection, clinical medicine, microelectronics manufacturing, and defense industries. However, solid-state lasers generate certain thermal effects along with laser light, including thermal lensing, thermal damage, and thermally induced birefringence, and the higher the laser output power, the more severe the thermal effects. Among these, the thermal lensing effect seriously affects many performance characteristics of semiconductor laser-pumped all-solid-state lasers. To date, many methods have been developed to measure the thermal lens focal length of all-solid-state lasers. However, in some special cases, the thermal lens focal length of the crystal in a solid-state laser is relatively large, especially for crystals exhibiting negative thermal lensing effects. Traditional helium-neon laser methods and parallel plane mirror methods are insufficient to detect the thermal lens focal length in these cases, hindering the further development of weakly thermally lensed lasers and negatively thermally lensed lasers. Therefore, accurately and effectively measuring the thermal lens focal length of the crystal in these situations has become a crucial problem that urgently needs to be solved in the field of solid-state lasers.

[0003] In the actual measurement of the focal length of weak and negative thermal lenses, the inherent lensing effect caused by factors such as growth stress or clamping stress of the crystal itself must be considered. This is because, without pump light illumination, the crystal itself is not an ideal transparent medium, but rather a lens-like object with a certain focal length. Furthermore, the crystal itself can degrade the beam quality of the probe light passing through it. Therefore, using traditional or simple direct methods to measure the focal length of thermal lenses will result in significant measurement errors. Summary of the Invention

[0004] (I) Purpose of the Invention

[0005] The purpose of this invention is to provide a method for estimating the focal length of a crystal thermal lens, which solves the problems of difficulty in measuring the focal length of weak thermal lenses and negative thermal lenses, as well as the inherent lens-like properties and beam degradation caused by the crystal itself. Within the allowable range of measurement error, the method enables the measurement of the focal length of thermal lenses of laser crystals with weak thermal lens effects and negative thermal lens effects.

[0006] (II) Technical Solution

[0007] To address the aforementioned technical problems, this invention provides a system for estimating the focal length of a crystal thermal lens, comprising: a semiconductor laser pump source 1, a collimating focusing lens group 2, a 45° dichroic mirror 3, a crystal with thermal lensing effect 4, a first silver mirror 5, a second silver mirror 6, a probe laser 7, a focusing lens 8 with a known focal length, a wedge mirror 9, and a CMOS spot detector 10; the 45° dichroic mirror 3 and the first silver mirror 5 are disposed on both sides of the crystal 4 with thermal lensing effect; the focusing lens group 2 is disposed to the left of the 45° dichroic mirror 3; the pump source 1 is disposed to the left of the focusing lens group 2; the focusing lens 8 with a known focal length is disposed above the 45° dichroic mirror 3; the wedge mirror 9 is disposed above the focusing lens 8 with a known focal length, and the CMOS spot detector 10 is placed below the wedge mirror 9; the second silver mirror 6 is disposed below the first silver mirror 5; and the probe laser 7 is disposed to the left of the second silver mirror.

[0008] like Figure 2 As shown, when the crystal 4 is removed from the estimation system and the pump light intensity of the semiconductor laser pump source 1 is zero, the probe light emitted by the probe laser 7 is reflected sequentially by the second silver mirror 6 and the first silver mirror 5, then incident on the focusing lens 8 through the 45° dichroic mirror 3, and then reflected by the wedge mirror 9 into the CMOS spot detector 10.

[0009] like Figure 3 , 5 As shown, in the estimation system, when the crystal 4 is arranged and the pump light intensity of the semiconductor laser pump source 1 is zero, the probe light emitted by the probe laser 7 is reflected sequentially by the second silver mirror 6 and the first silver mirror 5, passes through the crystal 4, is incident on the focusing lens 8 through the 45° dichroic mirror 3, and is then reflected by the wedge mirror 9 into the CMOS spot detector 10.

[0010] like Figure 4 , 6 As shown, the pump light emitted by the pump laser 1 is focused onto the center of the crystal 4 by the collimating and focusing lens group 2 and the 45° dichroic mirror 3 in sequence, causing the crystal to produce a thermal lensing effect; the probe light emitted by the probe laser 7 is reflected by the second silver mirror 6 and the first silver mirror 5 in sequence, and then enters the crystal 4 with the thermal lensing effect. After being reflected by the 45° dichroic mirror 3, it passes through the focusing lens 8 and is then reflected by the wedge mirror 9 into the CMOS spot detector 10.

[0011] The crystal 4 has a thermal lensing effect.

[0012] The detection laser 7 emits visible light, which is beneficial for researchers to observe and adjust the experimental optical path.

[0013] This invention also provides a method for estimating the focal length of a crystal thermal lens, comprising three steps:

[0014] First, the magnification was obtained by correcting the measurement error of the beam size. Then, the corrected beam waist radius was fine-tuned so that the distance from the exit mirror of the probe laser where the beam waist is located to the focusing lens is consistent with the measured distance, and thus the beam waist radius of the probe laser was derived.

[0015] Second, a crystal is added to the optical path. After measuring and correcting the beam size, the corrected beam waist radius is finely adjusted to ensure that the spot size before and after the crystal is consistent. Then, the beam waist transformation formula and the radius of curvature formula are used to derive the radius of curvature of the wavefront on the incident and exit sides of the crystal. Based on this, the inherent lens-like focal length of the crystal due to clamping stress or residual stress during growth is derived.

[0016] Third, after the pump light irradiates the crystal, the beam size is measured and corrected, and the corrected beam waist radius is finely adjusted to ensure that the spot size before and after the crystal is consistent. Then, the curvature radii of the wavefront on the incident and exit sides of the crystal are derived from the beam waist transformation formula and the radius of curvature formula. Based on this, the total focal length of the lens-like effect generated by the thermal lensing effect after the crystal is pumped is derived. Then, the influence of the inherent lens-like effect caused by clamping stress or residual stress during growth is eliminated, and the actual thermal lens focal length value of the crystal is derived using the combined lens formula.

[0017] (III) Beneficial Effects

[0018] The method for estimating the focal length of a crystal thermal lens provided by the above technical solution has the following beneficial effects:

[0019] (1) In this invention, the absorption peak of the laser crystal is near the center wavelength of the semiconductor laser, and the crystal will generate a thermal lensing effect after being pumped by the semiconductor laser.

[0020] (2) In this invention, a laser with a wavelength in the visible light range is used as the probe light, which enables the CMOS spot detector to directly measure the beam waist position and beam waist radius of the probe light, and facilitates the experimenter to observe and adjust the optical path.

[0021] (3) In this invention, a CMOS detector is used to measure the size of the light spot beam, and the beam waist radius, beam waist position and beam transmission factor are fitted by the least squares method, which can effectively reduce the error in the light spot measurement and improve the accuracy.

[0022] (4) In this invention, all measured beam sizes are corrected using the assumed correction magnification K in the x and y directions, and the beam transmission factor M2 is corrected to the factory value of the probe laser. Based on this correction magnification K, the beam sizes of all subsequent measurements are corrected, which can reduce the measurement error when the spot detector reads data.

[0023] (5) In this invention, considering that the M2 of the probe light will deteriorate after passing through the crystal, the curvature radius deduction method is used to derive the focal length of the crystal's thermal lens, which can eliminate the influence of different M2 before and after the crystal, thereby obtaining a more accurate focal length of the crystal's thermal lens.

[0024] (6) In this invention, under the premise of no pump light irradiation, by measuring and analyzing the beam waist data when there is no crystal in the optical path, the influence of the crystal-like lens effect caused by growth defects or clamping can be evaluated, thereby reducing the error of crystal thermal lens focal length measurement.

[0025] (7) In this invention, the focal length of the crystal thermal lens is derived by using the combined lens focal length formula by obtaining the inherent lens focal length value of the crystal and the overall lens focal length value of the crystal. This eliminates the influence of the crystal lens effect caused by growth defects or clamping, thereby realizing the precise measurement of the focal length of weak thermal lenses and negative thermal lenses.

[0026] (8) In this invention, when evaluating the effect of crystal-like lens effect caused by growth defects and clamping, and the overall lens focal length generated by the crystal after pumping, it is necessary to finely adjust the beam waist radius obtained at the measuring end to ensure that the spot size of the probe light incident on and out of the crystal is consistent, which is beneficial to reduce experimental error. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the process for measuring the focal length of a crystal thermal lens;

[0028] Figure 2 It measures M1 of the probe light in the absence of a crystal and pump in the optical path. 2 Experimental setup diagram for Z1 and ω1;

[0029] Figure 3 It measures the M2 of the probe light when a crystal is placed in the optical path but there is no pump. 2 Experimental setup diagrams for Z2 and ω2;

[0030] Figure 4 It measures the probe light under conditions where the crystal is inserted and pumped. 2 Experimental setup diagrams for Z3 and ω3;

[0031] Figure 5 This is a schematic diagram of the inherent lens-like structure of a crystal and beam transmission in the absence of a pump.

[0032] Figure 6 This is a schematic diagram of the overall lens-like structure of a crystal under pumped conditions and the beam transmission. Detailed Implementation

[0033] To make the objectives, contents, and advantages of the present invention clearer, the specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples.

[0034] In the description of this invention, it should be noted that the terms "left side", "right side", "both sides", and "lower side" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, and are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation.

[0035] like Figure 1 The diagram shows a flowchart of a technique for estimating the focal length of a thermal lens in a crystal exhibiting a thermal lensing effect. The technique consists of three parts. The experimental setup diagrams for each part are shown below. Figure 2 , 3 As shown in Figure 4, the device includes: a semiconductor laser pump source 1, a collimating and focusing lens group 2, a 45° dichroic mirror 3, a crystal with thermal lensing effect 4, a first silver mirror 5, a second silver mirror 6, a probe laser 7, a focusing lens 8 with a known focal length, a wedge mirror 9, and a CMOS spot detector 10; the 45° dichroic mirror 3 and the first silver mirror 5 are distributed on both sides of the crystal 4 with thermal lensing effect; the focusing lens group 2 is arranged on the left side of the 45° dichroic mirror 3; the pump source 1 is arranged on the left side of the focusing lens group 2; the focusing lens 8 with a known focal length is arranged on the upper side of the 45° dichroic mirror 3; the wedge mirror 9 is arranged on the upper side of the focusing lens 8 with a known focal length, and the CMOS spot detector 10 is placed on the lower side of the wedge mirror 9; the second silver mirror 6 is arranged on the lower side of the first silver mirror 5; and the probe laser 7 is arranged on the left side of the second silver mirror.

[0036] The crystal 4 has a thermal lensing effect.

[0037] The detection laser 7 emits visible light, which is beneficial for researchers to observe and adjust the experimental optical path.

[0038] Based on the above estimation system, the present invention also provides a method for estimating the focal length of a crystal thermal lens, comprising the following steps:

[0039] First, measure the beam waist radius at the laser emission point.

[0040] like Figure 2 As shown, in the case of no crystal in the optical path and zero pump light intensity, the probe light emitted by the probe laser 7 is reflected by silver mirrors 6 and 5, then incident on the focusing lens 8 through the 45° dichroic mirror 3, and reflected by the wedge mirror 9 into the CMOS spot detector 10; the beam size of the probe light after the focusing lens 8 is measured at the detector end, and the beam quality M1 is fitted. 2Beam waist radius ω1 and beam waist position Z1. When processing the data, all measured beam dimensions are corrected using assumed correction magnification K in the x and y directions, and the beam transmission factor M is adjusted. 2 Corrected to factory settings, based on the correction magnification K in the x and y directions, the corrected beam waist radius ω1 is obtained using the least squares method. Then, the beam waist radius ω1 is finely adjusted to ensure that the distance from the probe laser beam waist to the incident end of the focusing lens, obtained based on theoretical calculations, is equal to the actual measured distance. Finally, the beam waist transformation formula is used... The beam waist radius ω at the laser emission point can be obtained from the corrected beam waist data. x0 and ω y0 .

[0041] Second, assess the influence of the inherent lens-like effect of the crystal caused by clamping stress or residual stress during growth.

[0042] like Figure 3 , 5 As shown, with a crystal placed in the optical path and the pump light intensity zero, the probe light emitted by the probe laser 7 is reflected by silver mirrors 6 and 5, incident on crystal 4, and then incident on focusing lens 8 via 45° dichroic mirror 3. It is then reflected by wedge mirror 9 into CMOS spot detector 10. The beam size of the probe light after focusing lens 8 is measured at the detector end, and the beam quality M2 is fitted. 2 The beam waist radius ω2 and beam waist position Z2 are used to correct the beam size at each point using a correction magnification K to obtain the new M2. 2 After adjusting the obtained beam waist radius ω2, the spot size of the probe light on both sides of the crystal is made consistent. Then, the beam waist transformation formula and the crystal curvature radius formula are used. The radius of curvature R2 of the wavefront at the crystal's exit side is obtained. Additionally, the beam waist radius ω at the laser's exit point, obtained in the first part, is used. x0 and ω y0 By combining this with the formula for the radius of curvature of a lens, the radius of curvature R1 of the wavefront on the incident side of the crystal can be obtained. Finally, from formula 1 / f c = 1 / R1 + 1 / R2, and by working backward, we can obtain the inherent lens-like focal length f in the x and y directions of the crystal due to clamping stress or residual stress during growth. c .

[0043] Third, measure the focal length of the thermal lens generated by the laser crystal after being pumped by the pump light.

[0044] like Figure 4 , 6As shown, the pump light emitted by pump laser 1 is focused at the center of crystal 4 by collimating and focusing lens group 2 and 45° dichroic mirror 3, causing the crystal to produce a thermal lensing effect; the probe light emitted by probe laser 7 is reflected by silver mirror 6 and silver mirror 5, incident on crystal 4 with thermal lensing effect, reflected by 45° dichroic mirror 3, passed through focusing lens 8, and reflected by wedge mirror 9 into CMOS spot detector 10; the beam size of the probe light after passing through the crystal when the pump is not zero is measured at the detector end, and the beam quality M3 is fitted. 2 The beam waist radius ω3 and beam waist position Z3 are then used to correct the beam size at each point using a correction magnification K to obtain the new M3. 2 After adjusting the obtained beam waist radius ω3, the spot size of the probe light passing through both sides of the crystal is made consistent. Then, the curvature radius R3 of the wavefront on the exit side of the crystal is obtained by using the beam waist transformation formula and the crystal curvature radius formula; from formula 1 / f all = 1 / R1 + 1 / R3, and working backwards, we can obtain the total focal length f of the crystal lens. all Finally, the focal length formula for a combined lens is used, i.e., 1 / f. all =1 / f T +1 / f c After removing the inherent focal length f of the lens c The focal length f of the crystal thermo-induced lens can be calculated after considering the influence of the lens. T This enables the measurement of the focal length of crystals exhibiting thermal lensing effects.

[0045] As can be seen from the above technical solutions, the present invention has the following significant features:

[0046] 1. In this invention, the intrinsic absorption peak of the crystal is near the output center wavelength of the semiconductor laser, which enables the crystal to produce a thermally induced lensing effect after being pumped.

[0047] 2. In this invention, a CMOS detector is used to measure the size of the light spot beam, and a statistical method is used to fit the beam waist radius, beam waist position and beam transmission factor, which helps to reduce the error during light spot measurement.

[0048] 3. In this invention, by modifying the spot size in the x and y directions, M is made... 2 When corrected to the factory value, the corresponding correction factor K is obtained. Using this correction factor K, the beam size of all measured detection spots is corrected, which helps to effectively reduce the measurement error when the spot detector measures the beam size.

[0049] 4. In this invention, due to the inherent characteristics of the crystal, the M wavefront on the incident and exit sides of the crystal will be affected. 2 To mitigate the degradation of the beam transmission factor, the radius of curvature of the wavefront before and after passing through the crystal is calculated using the radius of curvature derivation method. This method is then used to calculate the focal length of the crystal's thermal lens, which helps to reduce the impact of beam transmission factor degradation.

[0050] 5. In this invention, by measuring and analyzing the spot data in the light path without a pump and whether a crystal is placed, it is beneficial to evaluate the influence of the crystal's inherent lens-like properties caused by factors such as the clamping stress of the crystal or the residual stress during the crystal's growth, and to improve the accuracy of the thermal lens focal length measurement.

[0051] 6. In this invention, by measuring and deriving the inherent lens-like focal length value and the overall lens focal length value of the crystal, and by using the combined lens focal length formula to deduce the thermally induced lens focal length value of the crystal, it is beneficial to eliminate the influence of the crystal-like lens effect caused by growth defects or clamping, and to realize the measurement of the focal length value of crystals with weak thermal lenses or negative thermal lenses.

[0052] 7. In this invention, when measuring and deriving the inherent lens focal length value and the overall lens focal length value of the crystal, it is necessary to finely adjust the waist radius of the measuring end so that the spot size of the probe light passing through both sides of the crystal is consistent. This helps to reduce experimental errors and thus ensures the accuracy of the crystal thermal focal length measurement.

[0053] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the technical principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for estimating the focal length of a crystal thermal lens, characterized in that, The estimation is achieved based on a crystal thermal lens focal length estimation system, which includes: a semiconductor laser pump source (1), a collimating focusing lens group (2), a 45° dichroic mirror (3), a crystal with thermal lensing effect (4), a first silver mirror (5), a second silver mirror (6), a probe laser (7), a focusing lens with a known focal length (8), a wedge mirror (9), and a CMOS spot detector (10); the 45° dichroic mirror (3) and the first silver mirror (5) are distributed on both sides of the crystal with thermal lensing effect (4); the 45° A focusing lens group (2) is set on the left side of the dichroic mirror (3); a semiconductor laser pump source (1) is set on the left side of the focusing lens group (2); a focusing lens (8) with a known focal length is set on the upper side of the 45° dichroic mirror (3); a wedge mirror (9) is set on the upper side of the focusing lens (8) with a known focal length, and a CMOS spot detector (10) is placed on the lower side of the wedge mirror (9); a second silver mirror (6) is set on the lower side of the first silver mirror (5); a probe laser (7) is set on the left side of the second silver mirror to estimate the focal length of the crystal thermal lens; The estimation method includes three steps: First, measure the beam waist radius at the laser emission point. The correction magnification is obtained by correcting the measurement error of the beam size. Then, the corrected beam waist radius is finely adjusted so that the distance from the exit mirror of the probe laser where the beam waist is located to the focusing lens is consistent with the measured distance. Thus, the beam waist radius at the beam exit of the probe laser is derived. Second, assess the influence of the inherent lens-like effect of the crystal caused by clamping stress or residual stress during growth. After adding a crystal into the optical path, measuring and correcting the beam size, fine-tuning the corrected beam waist radius to ensure that the spot size before and after the crystal is consistent, and then deriving the curvature radius of the wavefront on the incident and exit sides of the crystal from the beam waist transformation formula and the curvature radius formula, and on this basis, deriving the inherent lens-like focal length of the crystal due to clamping stress or residual stress during growth. Third, measure the focal length of the thermal lens generated by the laser crystal after being pumped by the pump light. After the pump light irradiates the crystal, the beam size is measured and corrected, and the corrected beam waist radius is finely adjusted to ensure that the spot size before and after the crystal is consistent. Then, the curvature radii of the wavefront on the incident and exit sides of the crystal are derived from the beam waist transformation formula and the radius of curvature formula. Based on this, the total focal length of the lens-like effect generated by the thermal lensing effect after the crystal is pumped is derived. Then, the influence of the inherent lens-like effect caused by clamping stress or residual stress during growth is eliminated, and the actual thermal lens focal length value of the crystal is derived using the combined lens formula.

2. The method for estimating the focal length of a crystal thermal lens as described in claim 1, characterized in that, In the first step, with no crystal in the optical path and the pump light intensity zero, the probe light emitted by the probe laser (7) is reflected by the second silver mirror (6) and the first silver mirror (5), then incident on the focusing lens (8) through the 45° dichroic mirror (3), and reflected by the wedge mirror (9) into the CMOS spot detector (10); the beam size of the probe light after the focusing lens (8) is measured at the detector end, and the beam quality is fitted. M 1 2 Waist radius w 1 and waist position Z 1 When processing data, use assumed correction factors in the x and y directions. K Correct all measured beam dimensions and adjust beam transmission factor. M 2 Corrected to factory settings, based on this x and y Directional correction factor K The corrected waist radius was obtained using the least squares method. w 1 Then fine-tune the waist radius. w 1 This ensures that the distance from the probe laser beam waist to the incident end of the focusing lens, obtained based on theoretical calculations, is equal to the actual measured distance. Finally, the beam waist radius at the light output point of the probe laser is obtained using the beam waist transformation formula and the corrected beam waist data. w x0 and w y0 .

3. The method for estimating the focal length of a crystal thermal lens as described in claim 2, characterized in that, In the second step, with a crystal placed in the optical path and the pump light intensity zero, the probe light emitted by the probe laser (7) is reflected by the second silver mirror (6) and the first silver mirror (5), passes through the crystal (4), enters the focusing lens (8) through the 45° dichroic mirror (3), and is reflected by the wedge mirror (9) into the CMOS spot detector (10). The beam size of the probe light after the focusing lens (8) is measured at the detector end, and the beam quality is fitted. M 2 2 Waist radius w 2 and waist position Z 2 Corrected magnification K Correcting the beam size at each point yields a new M 2 2 , w 2 Then, fine-tune the obtained waist radius. w 2 To ensure that the spot size of the probe light is consistent on both sides of the crystal, the radius of curvature of the wavefront on the exit side of the crystal is obtained using the beam waist transformation formula and the crystal radius of curvature formula. R 2 The beam waist radius at the laser emission point obtained in Part 1 was used. w x0 and w y0 The radius of curvature of the crystal's incident wavefront is obtained by combining the lens's radius of curvature formula. R 1 Finally, by formula 1 / f c = 1 / R 1 +1 / R 2 By working backwards, the inherent lens-like focal lengths in the x and y directions of the crystal, caused by clamping stress or residual stress during growth, can be obtained. f c .

4. The method for estimating the focal length of a crystal thermal lens as described in claim 3, characterized in that, In the third step, the pump light emitted from the semiconductor laser pump source (1) is focused onto the center of the crystal (4) by the collimating focusing lens group (2) and the 45° dichroic mirror (3), causing the crystal to produce a thermal lensing effect; the probe light emitted from the probe laser (7) is reflected by the second silver mirror (6) and the first silver mirror (5), and then incident on the crystal (4) with the thermal lensing effect. After being reflected by the 45° dichroic mirror (3), it passes through the focusing lens (8) and is reflected by the wedge mirror (9) into the CMOS spot detector (10); the beam size of the probe light after passing through the crystal when the pump is not zero is measured at the detector end, and the beam quality is fitted. M 3 2 Waist radius w 3 and waist position Z 3 After adjusting the magnification K Correcting the beam size at each point yields a new M 3 2 , w 3 Then, fine-tune the obtained waist radius. w 3 To ensure that the spot size of the probe light passing through both sides of the crystal is consistent, the radius of curvature of the wavefront on the exit side of the crystal is obtained using the beam waist transformation formula and the crystal radius of curvature formula. R 3 From the formula 1 / f all = 1 / R 1 +1 / R 3 The total focal length of the crystal lens can be obtained by working backwards. f all Finally, the focal length formula for combined lenses is used, i.e. 1 / f all =1 / f T +1 / f c In removing the inherent focal length of the lens f c The focal length of the crystal thermo-induced lens was calculated after considering the influence of the thermo-induced effect. f T This enables the measurement of the focal length of crystals exhibiting thermal lensing effects.

5. The method for estimating the focal length of a crystal thermal lens as described in claim 4, characterized in that, When the crystal (4) is removed from the optical path of the system and the pump light intensity of the semiconductor laser pump source (1) is zero, the probe light emitted by the probe laser (7) is reflected sequentially by the second silver mirror (6) and the first silver mirror (5), then incident on the focusing lens (8) through the 45° dichroic mirror (3), and then reflected by the wedge mirror (9) into the CMOS spot detector (10).

6. The method for estimating the focal length of a crystal thermal lens as described in claim 5, characterized in that, Assuming that the system is equipped with a crystal (4) and the pump light intensity of the semiconductor laser pump source (1) is zero, the probe light emitted by the probe laser (7) is reflected sequentially by the second silver mirror (6) and the first silver mirror (5), passes through the crystal (4), enters the focusing lens (8) through the 45° dichroic mirror (3), and is reflected by the wedge mirror (9) into the CMOS spot detector (10).

7. The method for estimating the focal length of a crystal thermal lens as described in claim 6, characterized in that, The pump light emitted from the semiconductor laser pump source (1) is focused onto the center of the crystal (4) by the collimating focusing lens group (2) and the 45° dichroic mirror (3) in sequence, causing the crystal to produce a thermal lensing effect; the probe light emitted from the probe laser (7) is reflected by the second silver mirror (6) and the first silver mirror (5) in sequence, and is incident on the crystal (4) with the thermal lensing effect. It is reflected by the 45° dichroic mirror (3) and passed through the focusing lens (8), and then reflected by the wedge mirror (9) into the CMOS spot detector (10).

8. The method for estimating the focal length of a crystal thermal lens as described in claim 7, characterized in that, The detection laser (7) emits visible light.

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