A Dynamic Thermal Lens Compensation Method Based on an Unstable Resonant Cavity

Abstract

This invention discloses a dynamic thermal lens compensation method based on an unstable resonant cavity, comprising: (1) deriving the abcd matrix of light traveling one round trip in the cold cavity according to a repetition frequency of 1 Hz, and calculating its geometric magnification; (2) measuring the focal length of the thermal lens at different repetition frequencies; (3) fixing the distances from the output mirror and the original total reflection mirror in the resonant cavity to the end face of the crystal, and replacing the original total reflection mirror with a planar total reflection mirror; (4) deriving the ABCD matrix of light traveling one round trip in the compensation cavity according to the focal length of the thermal lens corresponding to different repetition frequencies, and calculating its geometric magnification; (5) setting the distance between the planar total reflection mirror and the left end face of the crystal as a variable, making the geometric magnification of the compensation cavity equal to that of the cold cavity, and calculating the corresponding distance between the planar total reflection mirror and the left end face of the crystal; (6) constructing the final compensation unstable cavity according to the distance between the planar total reflection mirror and the left end face of the crystal, thus completing the thermal lens compensation. This invention achieves dynamic compensation of the focal length of the thermal lens corresponding to different repetition frequencies using simple operations.

Description

Technical Field

[0001] This invention relates to the field of solid-state laser technology, and in particular to a dynamic thermal lens compensation method based on an unstable resonant cavity. Background Technology

[0002] High-energy, high-beam-quality lasers have wide applications in industry, military, and biomedicine, such as industrial welding, military weaponry, and photoacoustic imaging detection. These beams are typically generated by unstable resonators. Generating high-beam-quality lasers using stable cavities usually requires sacrificing laser energy, while unstable cavities can generate large fundamental mode volumes and possess strong mode discrimination capabilities, making it easier to obtain high-energy, high-beam-quality lasers from them.

[0003] Common cavity types suitable for high-energy unstable cavities include rod imaging cavities, near-concentric cavities, and positive branch confocal cavities. The first two types have the advantage of being less affected by thermal lensing effects; however, both have a focal point within the cavity, which can easily damage internal components if handled improperly or during Q-switching. Positive branch confocal cavities do not have a focal point and can produce an automatically collimated output beam with an ideally very small divergence angle. However, this type of cavity is very sensitive to changes in thermal lensing, causing the resonant cavity's amplification to easily decrease as the thermal effect intensifies, ultimately leading to a deterioration in the output beam quality.

[0004] Traditional thermal compensation methods typically combine an unstable cavity total reflection mirror with a thermal lens, replacing the original total reflection mirror with a new total reflection mirror with equivalent curvature to achieve thermal lens compensation. This method usually uses concave or convex mirrors with large curvatures, which are difficult to manufacture, have long customization cycles, are costly, and can only provide static compensation for specific thermal lens focal lengths.

[0005] A paper published in the *Acta Physica Sinica*, 2000, 49(8): 1495-1498, entitled "A Solid-State Laser Resonator with Adaptive Compensation for Thermal Lensing Effect," describes the addition of a short-focal-length lens inside the laser resonator, fixed to one end of the laser rod. The thermal lens of the laser rod is located at the focal point of this short-focal-length lens, forming a lens group. The short-focal-length lens is designed to compensate for the thermal lensing effect generated by the laser at its normal operating temperature.

[0006] However, in practical applications, resonators typically need to operate at different repetition frequencies, and the focal length of the thermal lens will also change dynamically accordingly. How to achieve dynamic and precise compensation of the thermal lens using more readily available components and simpler devices is one of the important problems in the field of high-energy, high-beam-quality laser applications. Summary of the Invention

[0007] This invention provides a dynamic thermal lens compensation method based on an unstable resonant cavity. It can use readily available components and devices to accurately compensate the corresponding thermal lens focal length at different repetition frequencies, thereby ensuring high-energy, high-beam-quality laser output.

[0008] A dynamic thermal lens compensation method based on an unstable resonant cavity includes the following steps:

[0009] (1) Define the resonant cavity without the thermal lens as the cold cavity. Based on the repetition frequency of 1Hz, derive the abcd matrix of the light propagating from the original total reflection mirror to the right and back to the original total reflection mirror in one round trip, and calculate the geometric magnification of the cold cavity.

[0010] (2) Measure the focal length of the thermal lens at different repetition frequencies;

[0011] (3) Fix the distance from the output mirror and the original total reflection mirror in the resonant cavity to the crystal end face, replace the original total reflection mirror with a plane total reflection mirror, and set a thermal lens in the resonant cavity and define it as a compensation cavity;

[0012] (4) Based on the focal length of the thermal lens corresponding to different repetition frequencies, derive the ABCD matrix of the light propagating from the plane total reflection mirror to the right and back to the plane total reflection mirror in one round trip, and calculate the geometric magnification of the compensation cavity.

[0013] (5) Set the distance between the plane total reflection mirror and the left end face of the crystal as a variable, and make the geometric magnification of the compensation cavity equal to that of the cold cavity, and further calculate the corresponding distance between the plane total reflection mirror and the left end face of the crystal.

[0014] (6) Construct the final compensation unstable cavity according to the distance between the plane total reflection mirror and the left end face of the crystal to complete the thermal lens compensation.

[0015] In step (1), when the resonant cavity contains only a crystal, the abcd matrix is ​​represented as:

[0016]

[0017] In the formula, R1 and R2 are the radii of curvature of the output mirror and the original total reflection mirror, respectively; L1 and L2 are the distances from the output mirror to the right end face of the crystal and from the original total reflection mirror to the left end face of the crystal, respectively; l is the crystal length; and n is the crystal refractive index.

[0018] The formula for calculating the geometric magnification of the cold cavity is:

[0019]

[0020] In the formula, G1 is obtained from the abcd matrix of the cold cavity: G1=(a+d) / 2.

[0021] In step (2), the focal length of the thermal lens at different repetition frequencies is measured using a side-pumping method, specifically as follows:

[0022] A collimated and expanded He-Ne laser beam is injected into a pumped crystal rod at maximum pulse energy. The focal length of the thermal lens at different repetition frequencies is obtained by measuring the focal position of the emitted He-Ne laser.

[0023] In step (4), when the thermal lens is a thin lens and located at the center of the crystal, the ABCD matrix is ​​represented as:

[0024]

[0025] In the formula, R1 is the radius of curvature of the output mirror, L1 and L3 are the distances from the output mirror to the right end face of the crystal and from the planar total reflection mirror to the left end face of the crystal, respectively, l is the crystal length, n is the crystal refractive index, and f is the refractive index. T This is the focal length of the crystal thermal lens.

[0026] The formula for calculating the geometric magnification of the compensation cavity is:

[0027]

[0028] G2 is obtained from the ABCD matrix of the compensation cavity: G2 = (A + D) / 2.

[0029] In step (6), dynamic compensation for the focal length of the thermal lens corresponding to different repetition frequencies is achieved by fixing the output mirror and adjusting the distance between the planar total reflection mirror and the left end face of the crystal.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] 1. This invention utilizes the ABCD matrix of the resonant cavity to restore the geometric magnification of the unstable cavity containing a hot lens to be consistent with that of the cold cavity, effectively solving the problem of decreased output beam quality of the unstable cavity due to decreased magnification.

[0032] 2. Compared with traditional thermal lens compensation methods, the planar total reflection mirror used in this invention is easier and cheaper to obtain than large curvature concave or convex mirrors.

[0033] 3. In practical applications, light sources typically need to operate at different repetition frequencies, and the corresponding focal length of the thermal lens varies continuously in different applications. The method proposed in this invention, which involves shifting the plane total reflection mirror to change the distance between it and the left end face of the crystal, allows for a wide range of changes in the magnification of the unstable cavity, and provides corresponding compensation positions for different thermal lens focal lengths.

[0034] 4. The dynamic compensation method proposed in this invention is simple and reliable to operate. It introduces minimal and common components, and compensation for different thermal lens focal lengths can be achieved through simple operation, making it more suitable for applications at different operating repetition frequencies of the same light source. Attached Figure Description

[0035] Figure 1 This is a flowchart of a dynamic thermal lens compensation method based on an unstable resonant cavity according to the present invention.

[0036] Figure 2 This is a schematic diagram of an unstable cavity device constructed without a thermal lens in an embodiment of the present invention.

[0037] Figure 3 This is a schematic diagram of a compensated resonant cavity containing a thermal lens and a beam quality measurement device in an embodiment of the present invention.

[0038] Figure 4 This is a graph showing the beam quality measurement results under the optimal compensation condition with a repetition frequency of 30Hz in an embodiment of the present invention.

[0039] Figure 5 This is a graph showing the beam quality measurement results under the optimal compensation condition with a repetition frequency of 20Hz in an embodiment of the present invention.

[0040] In the diagram: 1-Original total reflection mirror; 2-Crystal; 3-Xenon flash lamp; 4-Output mirror; 5-Planar total reflection mirror; 6-Thermal lens; 7-Dynamic thermal lens compensation unstable cavity; 8-Wedge mirror; 9-Reflecting mirror; 10-Convex lens; 11-Attenuator assembly; 12-CCD camera. Detailed Implementation

[0041] The present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be noted that the embodiments described below are intended to facilitate the understanding of the present invention and do not constitute any limitation thereof.

[0042] like Figure 1 As shown, a dynamic thermal lens compensation method based on an unstable resonant cavity includes the following steps:

[0043] First, derive the abcd matrix of light propagating from the original total reflection mirror 1 to the right and returning to the original total reflection mirror 1 in the case of a repetition frequency of 1 Hz and no thermal lens. Taking the case where the resonant cavity contains only a crystal as an example, the matrix can be expressed as:

[0044]

[0045] In the formula, R1 and R2 are the radii of curvature of the output mirror 4 and the original total reflection mirror 1, respectively; L1 and L2 are the distances from the output mirror 4 to the right end face of crystal 2 and from the original total reflection mirror 1 to the left end face of crystal 2, respectively; l is the length of crystal 2; and n is the refractive index of crystal 2. The geometric magnification of the unstable cavity can be calculated by formula (2):

[0046]

[0047] In the formula, G1 can be obtained from the abcd matrix of the cold cavity: G1=(a+d) / 2.

[0048] Then, taking the side-pumping method as an example, a collimated and expanded He-Ne laser beam is injected into the pumped crystal rod at the maximum pulse energy. The focal length of the thermal lens at different repetition frequencies can be obtained by measuring the focal position of the emitted He-Ne laser.

[0049] With the distance between the two arms and the output mirror unchanged, the original total reflection mirror 1 is replaced with a planar total reflection mirror 5. Then, the ABCD matrix of one round trip of the unstable cavity containing the thermal lens 6 is derived. Assuming the thermal lens 6 is a thin lens located at the center of crystal 2, the matrix of the compensation cavity after replacing the original total reflection mirror 1 with the planar total reflection mirror 5 can be expressed as:

[0050]

[0051] In the formula, R1 is the radius of curvature of the output mirror 4, L1 and L3 are the distances from the output mirror 4 to the right end face of crystal 2 and from the planar total reflection mirror 5 to the left end face of crystal 2, respectively, l is the length of crystal 2, n is the refractive index of crystal 2, and f is the refractive index of crystal 2. T Let L3 be the focal length of thermal lens 6. Setting L3 as a variable, the expression for the magnification of the unstable cavity containing thermal lens 6 is:

[0052]

[0053] G2 is obtained from the ABCD matrix of the compensation cavity: G2 = (A + D) / 2

[0054] Then, setting G2 = G1, the magnification of the compensation cavity containing the hot lens is restored to be consistent with that of the cold cavity, and the left arm distance L3 corresponding to the focal length of the hot lens is solved. Finally, the corresponding compensation cavity is constructed using the solved L3 to complete the compensation of the hot lens. By fixing the output mirror 4 and adjusting the distance between the planar total reflection mirror 5 and the left end face of the crystal 2, dynamic compensation for the focal length of the hot lens corresponding to different repetition frequencies can be achieved.

[0055] To verify the effectiveness of this invention, a Gaussian mirror positive branch confocal unstable cavity is constructed for testing. A dynamic thermal lens compensation method based on an unstable resonant cavity includes:

[0056] Step 1: Assemble the following setup at 1Hz without a thermal lens. Figure 2The Gaussian mirror positive branch confocal unstable cavity shown has the following characteristics: the original total reflection mirror 1 is a 1064nm concave total reflection mirror with a radius of curvature of 3m; the output mirror 4 is a 1064nm convex Gaussian mirror with a radius of curvature of -1.5m and a film spot radius of 4.25mm, and the confocal cavity length is 815mm; the distance from the output mirror 4 to the end face of crystal 2 is 127.5mm, and the distance from the original total reflection mirror 1 to the end face of crystal 2 is 542.5mm; crystal 2 is an Nd:YAG crystal with a size of φ7mm×145mm and a refractive index of 1.82. The abcd matrix of its round trip is derived using equation (1), and G1=(a+d) / 2 is substituted into equation (2) to calculate the magnification M1=2 of the cold cavity.

[0057] Step 2: Adjust the pump single pulse energy to the maximum, and inject a collimated and expanded He-Ne laser beam into the crystal 2 pumped by the xenon flash lamp 3. By observing the focal point position of the outgoing laser beam, the focal length of the thermal lens is measured to be 3.1m at a repetition frequency of 30Hz and 4.5m at a repetition frequency of 20Hz.

[0058] Step 3: Keeping the distance between the two arms and the output mirror 4 unchanged, replace the original total reflection mirror 1 with a 1064nm planar total reflection mirror 5. The distance between the two arms refers to the distance from the output mirror 4 and the original total reflection mirror 1 to the end face of the crystal 2, respectively.

[0059] Step 4: Set the distance L3 between the planar total reflection mirror 5 and the left end face of the crystal 2 as a variable. Substitute the focal length of the thermal lens and the resonant cavity parameters at 30Hz and 20Hz into equation (3) to derive the ABCD matrix of the compensation cavity for one round trip. Substitute G2=(A+D) / 2 into equation (2) to derive the expression for the magnification M2 of the compensation cavity.

[0060] Step 5: Make M2 of the compensation cavity equal to M1 of the cold cavity, thus completing the amplification recovery of the hot cavity. Solve for L3 = 213.5mm and 96mm at 30Hz and 20Hz respectively.

[0061] Step 6: Construct compensation devices for the two repetition frequencies according to the above parameters, as shown in the attached diagram. Figure 3 As shown. The 1064nm laser emitted from the unstable cavity 7, compensated by a dynamic thermal lens, is reflected and attenuated by a wedge mirror 8. After being reflected by a 1064nm, 45° reflecting mirror 9, it is focused by a convex lens 10 with a focal length of 500mm. After being attenuated by an attenuator assembly 11, it enters the CCD camera 12. The CCD camera 12 moves back and forth around the laser beam waist to collect the spot size at different positions, and finally fits and calculates the beam quality.

[0062] The compensation results are attached. Figure 4 and 5As shown, the output energy is 170 mJ and the beam quality is 1.97 at 30 Hz; the output energy is 200 mJ and the beam quality is 1.7 at 20 Hz. This represents a significant improvement compared to the original uncompensated cavity's energy of 145 mJ and beam quality of 4.35 at 30 Hz and 200 mJ and beam quality of 2.39 at 20 Hz.

[0063] The embodiments described above provide a detailed explanation of the technical solutions and beneficial effects of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, and equivalent substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A dynamic thermal lens compensation method based on an unstable resonant cavity, characterized in that, Includes the following steps: (1) Define the resonant cavity without the thermal lens as the cold cavity. Based on the repetition frequency of 1Hz, derive the abcd matrix of the light propagating from the original total reflection mirror to the right and back to the original total reflection mirror in one round trip, and calculate the geometric magnification of the cold cavity. (2) Measure the focal length of the thermal lens at different repetition frequencies; (3) Fix the distance from the output mirror and the original total reflection mirror in the resonant cavity to the crystal end face, replace the original total reflection mirror with a plane total reflection mirror, and set a thermal lens in the resonant cavity and define it as a compensation cavity; (4) Based on the focal length of the thermal lens corresponding to different repetition frequencies, derive the ABCD matrix of the light propagating from the plane total reflection mirror to the right and back to the plane total reflection mirror in one round trip, and calculate the geometric magnification of the compensation cavity. (5) Set the distance between the plane total reflection mirror and the left end face of the crystal as a variable, and make the geometric magnification of the compensation cavity equal to that of the cold cavity, and further calculate the corresponding distance between the plane total reflection mirror and the left end face of the crystal. (6) Construct the final compensation unstable cavity according to the distance between the plane total reflection mirror and the left end face of the crystal to complete the thermal lens compensation.

2. The dynamic thermal lens compensation method based on an unstable resonant cavity according to claim 1, characterized in that, In step (1), when the resonant cavity contains only a crystal, the abcd matrix is ​​represented as: In the formula, R1 and R2 are the radii of curvature of the output mirror and the original total reflection mirror, respectively; L1 and L2 are the distances from the output mirror to the right end face of the crystal and from the original total reflection mirror to the left end face of the crystal, respectively; l is the crystal length; and n is the crystal refractive index.

3. The dynamic thermal lens compensation method based on an unstable resonant cavity according to claim 2, characterized in that, In step (1), the geometric magnification of the cold cavity is calculated using the following formula: In the formula, G1 is obtained from the abcd matrix of the cold cavity: G1=(a+d) / 2.

4. The dynamic thermal lens compensation method based on an unstable resonant cavity according to claim 1, characterized in that, In step (2), the focal length of the thermal lens at different repetition frequencies is measured using a side-pumping method, specifically as follows: A collimated and expanded He-Ne laser beam is injected into a pumped crystal rod at maximum pulse energy. The focal length of the thermal lens at different repetition frequencies is obtained by measuring the focal position of the emitted He-Ne laser.

5. The dynamic thermal lens compensation method based on an unstable resonant cavity according to claim 1, characterized in that, In step (4), when the thermal lens is a thin lens and located at the center of the crystal, the ABCD matrix is ​​represented as: where R1 is the radius of curvature of the output mirror, L1 and L3 are the distances from the output mirror to the right end face of the crystal and from the planar total reflection mirror to the left end face of the crystal, respectively, / is the length of the crystal, n is the refractive index of the crystal, and f T is the focal length of the thermal lens of the crystal.

6. The dynamic thermal lens compensation method based on an unstable resonant cavity according to claim 5, characterized in that, In step (4), the geometric magnification of the compensation cavity is calculated using the following formula: G2 is obtained from the ABCD matrix of the compensation cavity: G2 = (A + D) / 2.

7. The dynamic thermal lens compensation method based on an unstable resonant cavity according to claim 1, characterized in that, In step (6), dynamic compensation for the focal length of the thermal lens corresponding to different repetition frequencies is achieved by fixing the output mirror and adjusting the distance between the planar total reflection mirror and the left end face of the crystal.

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