Method for measuring thermal conversion coefficient of end-pumped rod-shaped MOPA laser amplifier

Abstract

The invention discloses a method for measuring the thermal conversion coefficient of an end-pumped rod-shaped MOPA laser amplifier, and the method comprises the steps: placing a ZnSe reflector on the front end surface of a crystal at an angle of 45 degrees, and reflecting a pumping light beam and a signal light beam to be amplified into the crystal, so as to simulate the actual working state of the laser amplifier; a thermal infrared imager is placed behind the ZnSe reflector and faces the front end face of the crystal, and the temperature of the end face of the crystal is measured and collected; constructing a crystal steady-state temperature field equation containing a thermal conversion coefficient, wherein the equation is used for calculating crystal temperature distribution; and performing similarity calculation on the obtained crystal temperature distribution and the measurement result of the thermal infrared imager by continuously adjusting the value of the thermal conversion coefficient until the similarity reaches a set threshold value, thereby obtaining the final thermal conversion coefficient. The method can be embedded into a designed light path of the laser amplifier, and it is ensured that the measured value conforms to the actual working condition of the laser amplifier.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of solid-state laser, in particular to a method for measuring thermal conversion coefficient of end-pumped rod MOPA laser amplifier. BACKGROUND

[0002] The master oscillator power amplifier (MOPA) with rod crystal as gain medium has a high peak power and high beam quality, and is widely used in industrial, medical and military fields, such as industrial material cutting, military weapon confrontation and optical biological detection. For example, a power amplifier of a master oscillator and a master control oscillator is disclosed in Chinese patent document CN117559208A.

[0003] However, part of the pump energy absorbed by the gain medium is converted into heat and deposited inside the crystal. This part of heat is usually represented by the thermal conversion coefficient, which is defined as the proportion of the absorbed pump energy converted into heat. These heat will cause the temperature of the crystal to rise, resulting in many adverse thermal effects such as deterioration of beam quality, reduction of extraction efficiency and crystal fracture. The definition of the value of the thermal conversion coefficient is very important for the design of solid-state lasers.

[0004] The traditional method for measuring the thermal conversion coefficient is to measure the thermal lens focal length of the crystal to deduce the thermal conversion coefficient. This method depends on many crystal parameters, including but not limited to the thermal conductivity of the crystal, the thermo-optic coefficient and the thermal expansion coefficient, which limits the accuracy of the deduced value to some extent. For example, a measuring device and method for the thermal conversion coefficient of a grazing incidence slab structure laser amplifier are disclosed in Chinese patent document CN107560825A.

[0005] In addition, there are many factors affecting the thermal conversion coefficient, including the doping concentration of the crystal, the pump wavelength, the pump power and the signal light power. For laser amplifiers under different working conditions, the above parameters usually have certain differences, resulting in changes in the thermal conversion coefficient.

[0006] Therefore, it is of great practical significance to find a method for accurately measuring the thermal conversion coefficient of a laser under different working conditions for the development of the field of solid-state laser technology. SUMMARY

[0007] The present application provides a method for measuring the thermal conversion coefficient of an end-pumped rod MOPA laser amplifier, which can be embedded in the design optical path of the laser amplifier to ensure that the measured value fits the actual working conditions of the laser amplifier.

[0008] A method for measuring the thermal conversion coefficient of an end-pumped rod MOPA laser amplifier, comprising: (1) A ZnSe mirror is placed at 45° in front of the crystal end face to reflect the pump beam and the signal beam to be amplified into the crystal to simulate the actual working state of the laser amplifier; (2) An infrared thermal imager is placed behind the ZnSe mirror, facing the front end face of the crystal, to measure and collect the crystal end face temperature; (3) A crystal steady-state temperature field equation containing a heat conversion coefficient is constructed to calculate the crystal temperature distribution; (4) The similarity of the obtained crystal temperature distribution and the infrared thermal imager measurement result is calculated by continuously adjusting the value of the heat conversion coefficient until the similarity reaches the set threshold, and the final heat conversion coefficient is obtained.

[0009] In step (1), the specific process of reflecting the pump beam and the signal beam to be amplified into the crystal is as follows: The pump beam first passes through the transformation of the pump coupling mirror, and then transmits through the dichroic mirror; the signal beam is reflected by the dichroic mirror and coupled with the pump beam, and propagates coaxially; and is reflected into the crystal by the ZnSe mirror.

[0010] In step (1), the film parameters of the face of the ZnSe mirror facing the crystal are as follows: high reflection film for the pump beam and the signal beam, and high transmission film for the 8-14 μm wave band; The film parameters of the face of the ZnSe mirror facing away from the crystal are as follows: 8-14 μm wave band anti-reflection film.

[0011] In step (2), the infrared thermal imager is used to collect the center highest temperature and the surrounding boundary temperature of the crystal.

[0012] In step (3), a crystal steady-state temperature field equation containing a heat conversion coefficient is constructed, which is as follows: ; The boundary conditions are as follows: ; Wherein, is the crystal temperature distribution, is the thermal conductivity of the crystal in the direction of the x-axis, is the thermal conductivity of the crystal in the direction of the y-axis, is the thermal conductivity of the crystal in the direction of the z-axis, is the thermal conductivity of the crystal in the direction of the x-axis, is the thermal conductivity of the crystal in the direction of the y-axis; is the thermal conductivity of the crystal in the direction of the z-axis; is the three-dimensional space coordinate; is the absorption coefficient of the crystal to the pump light, is the heat conversion coefficient to be fitted, is the distribution of the pump light intensity in the crystal; is the heat exchange coefficient between the crystal and the heat dissipation fixture, is the heat exchange coefficient of the crystal end face and air, is the temperature of the heat sink clamp, is the temperature of the air, is the length of the crystal. is the length of the crystal.

[0013] Distribution of pump light intensity in the crystal The expression is: ; ; wherein, is the pump beam radius in the crystal, is the pump beam waist diameter, is the super-Gaussian coefficient of the pump intensity distribution; is the pump wavelength; is the pump light beam quality factor; is the refractive index of the crystal; is the normalization coefficient.

[0014] The expression of the normalization coefficient is: ; wherein, is the incident pump light power.

[0015] In step (4), the obtained crystal temperature distribution and the infrared thermal imager measurement result are calculated for similarity, specifically including: calculating the similarity of the highest central temperature, the similarity of the four surrounding boundary temperatures and the overall similarity.

[0016] Compared with the prior art, the present application has the following beneficial effects: 1. The present application utilizes the characteristic that the ZnSe reflector can transmit infrared waves, embeds the measuring device into the amplifier optical path, realizes the measurement of the thermal conversion coefficient of the amplifier under any working condition, has a wide practical range and high measurement authenticity.

[0017] 2. Compared with the traditional measurement method, the present application only needs to consult the thermal conductivity of the crystal, greatly reduces the error sources.

[0018] 3. The infrared thermal imager temperature measurement method adopted by the present application has high resolution, higher precision and sensitivity in measuring the thermal conversion coefficient, and can distinguish the slight change of the thermal conversion coefficient. BRIEF DESCRIPTION OF DRAWINGS

[0019] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of the device for measuring the temperature distribution on the end face of a crystal in an embodiment of the present invention.

[0021] Figure 2 This is the camera temperature distribution captured by the infrared thermal imager in an embodiment of the present invention.

[0022] Figure 3 This represents the crystal temperature distribution calculated after fitting in this embodiment of the invention.

[0023] Figure 4 This embodiment of the invention measures the relationship between the thermal conversion coefficient and the signal light. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] It should be noted that, unless otherwise specified, the features in the following embodiments and implementation methods can be combined with each other.

[0026] like Figure 1 As shown, in this embodiment, the pump source 1 is an optical fiber-coupled laser diode source with a pump wavelength of 888nm, a pump power of 230W, an optical fiber core diameter of 400μm, and a numerical aperture of 0.22. The focal length of the first pump coupling mirror 3 is f1=12mm, and the focal length of the second pump coupling mirror 4 is f1=75mm. After being transformed by the first pump coupling mirror 3 and the second pump coupling mirror 4, the pump beam 2 is focused onto the front end face of the crystal at the beam waist position. The measured pump beam waist diameter is 2.2mm.

[0027] Signal beam 9 is a 1064nm continuous laser with an output power of 150W and a beam quality factor M. 2 <1.3, collimated with a diameter of 2.2mm. Pump beam 2 and signal beam 9 are coupled through dichroic mirror 5 and propagate coaxially.

[0028] The Nd:YVO4 crystal was cut along the a-axis. 3+The doping concentration of ions is 0.5 at.%. The size of the Nd:YVO4 crystal 8 is 4 mm x 4 mm x 5 mm, and the two ends are coated with 888 nm and 1064 nm anti-reflective films. The Nd:YVO4 crystal 8 is installed in a copper clamp after being coated with a liquid metal alloy. The copper clamp is cooled by water, and the temperature of the cooling water is set to = 20℃, the ambient temperature = 20℃. The heat exchange coefficient of the crystal around the copper clamp = 0.02 W / (mm 2 x K), and the heat exchange coefficient of the crystal end face with air is = 0.00001 W / (mm 2 x K).

[0029] The size of the ZnSe mirror 6 is Ф25.4 mm x 2 mm. The film parameters of the face S1 of the ZnSe mirror 6 facing the crystal are as follows: high reflection film of 888 nm-1064 nm, high transmission film of 8-14 μm wave band; the film parameters of the face S2 of the ZnSe mirror 6 away from the crystal are as follows: 8-14 um wave band anti-reflective film. The ZnSe mirror 6 is placed close to the crystal, and the coupled pump beam 2 and signal beam 9 are reflected into the Nd:YVO4 crystal 8.

[0030] The infrared thermal imager 7 is placed behind the ZnSe mirror 6 facing the front end face of the crystal, with a distance of 32 mm from the front end face of the crystal, for measuring the temperature distribution of the crystal end face. The measurement field of view of the infrared thermal imager 7 is 5.2 mm x 4.1 mm, the single pixel resolution is 8 μm x 8 μm, the temperature measurement range is -20℃-150℃, and the working distance is 32 mm.

[0031] The temperature distribution of the crystal is collected under the conditions of pump beam power of 230 W and signal beam power of 150 W, as shown in Figure 2 . The center temperature of the Nd:YVO4 crystal 8 is 90.6℃, and the four boundary temperatures are 36.4℃ (left), 38.2℃ (up), 39.6℃ (right), and 38.6℃ (down), respectively.

[0032] The thermal conductivity coefficient of the crystal is obtained by consulting literature, and the crystal steady-state temperature field equation is constructed by using simulation software according to the crystal thermal effect theory to calculate the temperature distribution of the crystal for fitting the thermal conversion coefficient.

[0033] The constructed crystal steady-state temperature field equation is as follows: ; The boundary conditions are as follows: ; wherein, is the temperature distribution of the crystal, crystal axial direction, crystal axial direction, crystal axial direction; three-dimensional spatial coordinates; absorption coefficient of the crystal to the pump light; thermal conversion coefficient to be fitted, distribution of the pump light intensity inside the crystal; heat exchange coefficient of the crystal periphery and the heat dissipation fixture, heat exchange coefficient of the crystal end face and air, temperature of the heat dissipation fixture, temperature of the air, side length of the crystal cross section, length of the crystal.

[0034] distribution of the pump light intensity inside the crystal is expressed as: ; ; wherein, pump light beam radius inside the crystal, pump beam waist diameter, in the embodiment =2.2mm; super-Gaussian coefficient of the pump intensity distribution, in the embodiment =6; absorption coefficient of the crystal to the pump light, in the embodiment =0.066mm -1 ; pump wavelength, =888nm; pump light beam quality factor, in the embodiment =140; refractive index of the crystal, in the embodiment =2.17; normalization coefficient.

[0035] normalization coefficient is expressed as: ; wherein, incident pump light power, in the embodiment =230W.

[0036] The thermal conversion coefficient is continuously corrected until the maximum goodness of fit is reached. In this embodiment, the goodness of fit is evaluated using the similarity, which takes into account the highest temperature in the center of the crystal, the four boundary temperatures and the overall similarity. For the case of the embodiment, the thermal conversion coefficient value obtained by fitting is 0.1630, and the temperature distribution calculated at this value is shown in Figure 3

[0037] Figure 4 The trend of the measured thermal conversion coefficient with the signal beam is shown, all measured at a pump beam of 230 W. In this embodiment, the thermal conversion coefficient is also measured for each case with variations in the pump power, pump diameter, signal light diameter and other common parameters of a MOPA amplifier.

[0038] The above-described embodiments have described the technical solutions and beneficial effects of the present application in detail. It should be understood that the above-described is only a specific embodiment of the present application and is not intended to limit the present application. Any modification, supplement and equivalent replacement made within the principle range of the present application should be included in the protection scope of the present application.​

Claims

1. A method for measuring the thermal conversion coefficient of an end-pumped rod-shaped MOPA laser amplifier, characterized in that, include: (1) Place a ZnSe mirror at 45° on the front face of the crystal to reflect the pump beam and the signal beam to be amplified into the crystal to simulate the actual working state of the laser amplifier. (2) Place the infrared thermal imager behind the ZnSe reflector, facing the front end of the crystal, and measure and collect the temperature of the crystal end face; (3) Construct a steady-state temperature field equation for the crystal that includes the thermal conversion coefficient, and use it to calculate the temperature distribution of the crystal; (4) By continuously adjusting the value of the thermal conversion coefficient, the similarity between the obtained crystal temperature distribution and the infrared thermal imager measurement results is calculated until the similarity reaches the set threshold, and the final thermal conversion coefficient is obtained.

2. The method for measuring the thermal conversion coefficient of an end-pumped rod-shaped MOPA laser amplifier according to claim 1, characterized in that, In step (1), the specific process of reflecting the pump beam and the signal beam to be amplified into the crystal is as follows: The pump beam is first transformed by the pump coupling mirror and then transmitted through the dichroic mirror; the signal beam is reflected by the dichroic mirror and coupled with the pump beam, and propagates coaxially; it is then reflected into the crystal by the ZnSe mirror.

3. The method for measuring the thermal conversion coefficient of an end-pumped rod-shaped MOPA laser amplifier according to claim 1, characterized in that, In step (1), the coating parameters of the ZnSe mirror facing the crystal are: a high-reflectivity film for the pump beam and signal beam wavelengths, and a high-transmission film for the 8-14μm band. The coating parameters for the ZnSe mirror back-to-crystal surface are: 8-14μm band antireflective coating.

4. The method for measuring the thermal conversion coefficient of an end-pumped rod-shaped MOPA laser amplifier according to claim 1, characterized in that, In step (2), the infrared thermal imager is used to collect the highest temperature at the center of the crystal and the temperature at the surrounding boundaries.

5. The method for measuring the thermal conversion coefficient of an end-pumped rod-shaped MOPA laser amplifier according to claim 1, characterized in that, In step (3), the steady-state temperature field equation of the crystal, which includes the thermal conversion coefficient, is constructed as follows: ； The boundary conditions are: ； in, For the temperature distribution of the crystal, Crystal Thermal conductivity in the axial direction, Within the crystal Thermal conductivity in the axial direction, Within the crystal Thermal conductivity in the axial direction; Three-dimensional spatial coordinates; The absorption coefficient of the crystal for pump light is denoted as . The heat conversion coefficient to be fitted is . This represents the distribution of pump light intensity within the crystal. The heat transfer coefficient between the crystal's perimeter and the heat dissipation fixture is given. The heat transfer coefficient between the crystal end face and the air is denoted as . For the temperature of the heat dissipation fixture, For air temperature, The side length of the crystal cross section The length is the crystal length.

6. The method for measuring the thermal conversion coefficient of an end-pumped rod-shaped MOPA laser amplifier according to claim 5, characterized in that, Distribution of pump light intensity within the crystal The expression is: ； ； in, The radius of the pump beam inside the crystal. The pump waist diameter, The super-Gaussian coefficients of the pump intensity distribution; The pump wavelength; The pump beam quality factor; The refractive index of the crystal; This is the normalization coefficient.

7. The method for measuring the thermal conversion coefficient of an end-pumped rod-shaped MOPA laser amplifier according to claim 6, characterized in that, Normalization coefficient The expression is: ； in, The incident pump light power.

8. The method for measuring the thermal conversion coefficient of an end-pumped rod-shaped MOPA laser amplifier according to claim 1, characterized in that, In step (4), the similarity between the obtained crystal temperature distribution and the infrared thermal imager measurement results is calculated, specifically including: calculating the similarity of the highest temperature at the center, the similarity of the temperature at the surrounding boundaries, and the overall similarity.

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