A method for growing rare-earth gradient-doped fluoride laser crystals

Abstract

This invention relates to a method for growing rare-earth gradient-doped fluoride laser crystals, belonging to the field of crystal growth technology. Addressing the technical problem of the difficulty in artificially designing or controlling the rare-earth ion concentration distribution in rare-earth doped fluoride crystals, this invention employs a microporous crucible comprising an upper loading chamber, a lower loading chamber, and a single-crystal growth through-hole connecting the two chambers and extending vertically. First, a high-doped rare-earth doped fluoride laser crystal raw material block is placed in the lower loading chamber, and a low-doped rare-earth doped fluoride laser crystal raw material powder is placed in the upper loading chamber, filling the single-crystal growth through-hole. Then, using a temperature gradient method or a crucible lowering method, the small-diameter through-hole in the crucible inhibits melt convection, thereby growing a rare-earth gradient-doped fluoride laser crystal. Furthermore, the concentration gradient of the grown rare-earth doped crystal can be artificially controlled by adjusting parameters such as the holding time.

Description

Technical Field

[0001] This invention belongs to the field of crystal growth technology, specifically relating to a method for growing rare-earth gradient-doped fluoride laser crystals. Background Technology

[0002] Fluoride crystals, as matrix materials for laser crystals, have advantages such as low effective phonon energy, high transmittance, and wide transmission band. Rare earth doped fluoride crystals such as Yb:CaF2, Nd:LiYF4, and Tm:LiYF4 are important laser gain materials.

[0003] In the design and application of solid-state lasers using rare-earth-doped crystals as the gain medium, it is generally desirable for the rare-earth-doped crystal to have a suitable doping concentration gradient along the pump light propagation direction. That is, the crystal has a lower doping concentration at the pump light incident end, and the doping concentration gradually increases along the pump light propagation direction. The benefits are that the pump light is uniformly absorbed in the gain crystal along the propagation direction, and the heat and stress distribution in the gain crystal is more uniform during laser operation. This can effectively improve the crystal thermal effect and increase the laser's output power, optical conversion efficiency, and beam quality.

[0004] Currently, the main methods for preparing rare-earth-doped fluoride crystals include the crucible lowering method, the temperature gradient method, and the Czochralski method and micro-lowering method. All of these crystal preparation methods are based on the principle of raw material melting and recrystallization. The distribution of the incorporated rare-earth ions in the crystal mainly depends on the segregation characteristics of the rare-earth ions in the matrix crystal. Since the segregation coefficient of rare-earth ions in the crystal is fixed, the concentration distribution of rare-earth ions in rare-earth-doped fluoride crystals is difficult to design or control artificially. Summary of the Invention

[0005] To address the aforementioned problems, this invention proposes a method for growing rare-earth gradient-doped fluoride laser crystals. This method can prepare rare-earth-doped fluoride laser crystals with gradient doping characteristics along the crystal growth direction. The doping concentration distribution is mainly determined by the diffusion process of rare-earth ions in the melt. By controlling the crystal growth process, the concentration distribution of rare-earth ions in the crystal can be regulated to a certain extent. By designing a suitable crucible and temperature field, the growth of multiple rare-earth gradient-doped fluoride crystals with different doping compositions can be achieved in a single operation.

[0006] In a first aspect, the present invention provides a method for growing rare-earth gradient-doped fluoride laser crystals, wherein the rare-earth gradient-doped fluoride laser crystals are grown using a microporous crucible; the structure of the microporous crucible includes a crucible body; the crucible body includes at least one independent crystal growth space; each independent crystal growth space includes an upper loading chamber, a lower loading chamber, and a single crystal growth through-hole connecting the upper loading chamber and the lower loading chamber and extending in the vertical direction; The method includes: (1) The raw material block formed by pressing the raw material powder of rare earth doped fluoride laser crystal with doping content c1 and the raw material powder of rare earth doped fluoride laser crystal with doping content c2 are placed in the lower loading bin and the upper loading bin of the microporous crucible, respectively, and the raw material powder of rare earth doped fluoride laser crystal with doping content c2 is kept to fill the single crystal growth through hole, where c1>c2. (2) The rare earth gradient doped fluoride laser crystal is grown by using the temperature gradient method or the crucible descent method.

[0007] Preferably, in step (1), the raw material block formed by pressing the raw material powder of the rare earth doped fluoride laser crystal with a doping content of c1 occupies at least 95% of the volume of the lower loading hopper; the pressing pressure is 100-150 MPa.

[0008] Preferably, in step (1), the rare earth-doped fluoride laser crystal comprises a fluoride matrix and rare earth doping elements; the fluoride matrix comprises at least one of CaF2, SrF2, BaF2, MgF2, CeF3, and LaF3; and the rare earth doping elements comprise at least one of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y.

[0009] Preferably, c1 is 0.1–20 at.%; c2 is 0–10 at.%.

[0010] Preferably, PbF2 powder is added as an oxygen scavenger to the raw material powder of the rare earth doped fluoride laser crystal with a doping content of c1, and the amount of oxygen scavenger added is 0.5 wt.% to 1.2 wt.% of the mass of the raw material powder of the rare earth doped fluoride laser crystal with a doping content of c1. PbF2 powder is added as an oxygen scavenger to the raw material powder of rare earth doped fluoride laser crystal with a doping content of c2. The amount of oxygen scavenger added is 0.5 wt.% to 1.2 wt.% of the mass of the raw material powder of rare earth doped fluoride laser crystal with a doping content of c2.

[0011] Preferably, the structure of the microporous crucible includes: a crucible body, an upper crucible cover, a lower crucible cover, and a crucible support.

[0012] Preferably, the diameter of the single crystal growth via is ≤6mm and the length is ≤100mm.

[0013] Furthermore, preferably, when the diameter of the single crystal growth through-hole is <2mm, an exhaust hole is provided between the upper and lower loading bins; the diameter of the exhaust hole is ≥2mm.

[0014] Preferably, the parameters of the temperature gradient method include: first, heating to a temperature T1 10–30°C higher than the melting point of the raw material powder to completely melt the raw material and form a doping concentration gradient in the single crystal growth vias; then cooling at a rate V1 of 0.2–5°C / hour for 100–200°C to complete the melt crystallization process; finally cooling to room temperature; the atmosphere of the temperature gradient method is a vacuum atmosphere with a vacuum degree of 10... -3 Below Pa.

[0015] Furthermore, preferably, the heating rate is 20–60 °C / h; The raw material is kept at a temperature of T1 for 5–50 h to allow it to melt completely and form a doping concentration gradient in the through-hole of the single crystal growth. The cooling rate to room temperature is 5–30 °C / hour.

[0016] Preferably, the parameters of the crucible lowering method include: first, heating to a temperature T1 10–30°C higher than the melting point of the raw material powder to completely melt the raw material and form a doping concentration gradient in the single crystal growth via; then, starting the crucible lowering process and controlling the lowering rate at 0.2–5 mm / h to begin crystal growth; finally, after completing the lowering process, cooling to room temperature; the atmosphere of the crucible lowering method is a vacuum atmosphere with a vacuum degree of 10... -3 Below Pa.

[0017] Furthermore, preferably, the heating rate is 20–60 °C / h; The raw material is kept at a temperature of T1 for 5–50 h to allow it to melt completely and form a doping concentration gradient in the through-hole of the single crystal growth. The cooling rate to room temperature is 5–30 °C / hour.

[0018] Preferably, before heating to temperature T1, the temperature is first kept at 150℃~250℃ for 5~15h, and then the temperature is further increased to 800~900℃ and kept constant for 5~15h.

[0019] Secondly, the present invention provides a rare-earth gradient-doped fluoride laser crystal prepared according to the above method.

[0020] Beneficial effects 1. This invention utilizes the effect of small-diameter through-holes in the crucible to suppress melt convection, thus achieving the simple and efficient preparation of rare-earth gradient-doped fluoride crystals. 2. This invention adjusts the concentration gradient of the grown rare earth-doped crystals by adjusting the heat preservation time; 3. By designing the temperature field of the crucible and crystal growth furnace, this invention can complete the growth of multiple rare earth gradient doped crystals with different doping compositions in a single operation. Attached Figure Description

[0021] Figure 1 This is a front cross-sectional view of the microporous crucible with only one independent crystal growth space used in this invention; where ① to ④ are, in order, the crucible body, the lower crucible cover, the upper crucible cover, and the crucible support, and ⑤ to ⑦ are, respectively, the lower loading chamber, the upper loading chamber, and the single crystal growth through hole of the crucible body; Figure 2 This is a front cross-section and a top view of the microporous crucible with four independent crystal growth spaces used in this invention. Figure 3 A photograph of the gradient-doped Er:CaF2 crystal prepared in Example 1; Figure 4 This is a schematic diagram of the axial distribution of Er ion concentration in the gradient-doped Er:CaF2 crystal prepared in Example 1. Figure 5 A photograph of the gradient-doped Tm:SrF2 crystal prepared in Example 2; Figure 6 This is a schematic diagram of the axial distribution of Tm ion concentration in the gradient-doped Tm:SrF2 crystal prepared in Example 2. Figure 7 A photograph of the gradient-doped Tm:SrF2 crystal prepared for Comparative Example 1; Figure 8 A schematic diagram of the axial distribution of Tm ion concentration in the gradient-doped Tm:SrF2 crystal prepared for Comparative Example 1. Detailed Implementation

[0022] The present invention will be further illustrated by the following embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the present invention.

[0023] This invention employs a microporous crucible for growing rare-earth gradient-doped fluoride crystals. The crucible design is based on the patent "Apparatus and Method for High-Throughput Fabrication of Single-Crystal Fibers" (Patent No. ZL 202011482671.8). Its structure includes a crucible body, an upper crucible cover, a lower crucible cover, and a crucible support. The crucible body may contain one or more independent crystal growth spaces. Each independent growth space within the crucible body consists of an upper loading chamber, a lower loading chamber, and a single-crystal growth through-hole connecting the upper and lower loading chambers. This invention can be used to grow rare-earth gradient-doped fluoride crystals, wherein the fluoride matrix crystal includes CaF2, SrF2, BaF2, MgF2, CeF3, and LaF3, and the doped rare-earth elements include at least one selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y.

[0024] The diameter of the single-crystal growth through-hole connecting the upper and lower loading hoppers should not exceed 6 mm, and the length should not exceed 100 mm. Preferably, when the diameter of the through-hole connecting the upper and lower loading hoppers is less than 2 mm, an additional vent hole with a diameter of not less than 2 mm should be designed between the upper and lower loading hoppers.

[0025] In this invention, the method for growing gradient-doped rare-earth-doped fluoride laser crystals mainly includes: preparing raw materials; loading the material; loading the furnace; evacuating the furnace; heating to completely melt the raw materials in the crucible and form a doping concentration gradient in the melt through the crucible's through-holes; and slowly cooling or lowering the crucible until crystal growth is complete. The following is an exemplary description of the method for growing rare-earth-doped fluoride laser crystals.

[0026] Raw material preparation. Prepare crystal growth raw materials with higher doping concentrations (c1) and lower doping concentrations (c2). Based on the stoichiometry of rare earth-doped fluoride crystals and the volumes of the upper and lower hoppers, weigh and prepare powder raw materials of rare earth-doped fluoride crystals with doping concentrations of c1 and c2, respectively, and add 0.5 wt.% to 1.2 wt.% PbF2 powder as an oxygen scavenger. Grind and mix the raw materials evenly. Preferably, the powder raw material with doping concentration c1 is pressed into a raw material block using a hydraulic press, which has the same shape as the lower hopper of the crucible but is slightly smaller in size.

[0027] Loading. Load the pressed raw material block with a doping concentration of c1 into the lower loading bin from the bottom of the crucible, and then cover the upper and lower crucibles. Load the powder raw material with a doping concentration of c2 into the upper loading bin from the top of the crucible. Gently tap or shake the crucible to allow the powder raw material in the upper loading bin to enter and fill the through-hole connecting the upper and lower loading bins. Then cover the upper crucible with the upper lid and install the crucible body on the crucible support.

[0028] It should be noted that if the raw material powder of concentration C1 is directly loaded into the lower charging hopper, the volume of the powder after melting will be smaller than the volume of the lower charging hopper because the powder is relatively loose. The extra volume will be filled by powder / melt of concentration C2. As the raw material is diluted, the doping concentration of the raw material in the lower charging hopper will be reduced during the actual crystal growth process (the actual doping concentration is between C1 and C2), and the concentration gradient of the crystal in the through hole will be smaller.

[0029] Crystal growth. The crucible is installed in the crystal growth furnace. The position of the crucible in the temperature field is adjusted by adjusting the height of the seed crystal rod. The insulation layer is installed, the furnace lid is closed, and the vacuum inside the furnace chamber is evacuated to 10°C. -3 After the Pa is below the target value, an automatic temperature control program is set according to the crystal melting point, and heating is started. Rare earth gradient doped fluoride laser crystals are prepared using the temperature gradient method or the crucible descent method.

[0030] In an optional embodiment, the temperature gradient method includes: first, heating to a temperature T1 10-30°C above the melting point of the raw material at a rate of 20-60°C / h, and holding at this temperature for 5-50h to allow the raw material in the crucible to completely melt and form a doping concentration gradient in the through-hole. Then, cooling to 100-200°C at a cooling rate V1 of 0.2-5°C / h. Finally, cooling to room temperature at a cooling rate of 5-30°C / h. In a more preferred embodiment, when growing crystals using the temperature gradient method, before raising the temperature to T1, the temperature should be held at 150-250°C for 5-15h to remove residual adsorbed water in the crucible. The temperature is then raised to 800-900°C and held for 5-15h to allow the PbF2 in the raw material to preferentially react with oxygen-containing substances such as water and oxygen, preventing the fluoride raw material from being oxidized.

[0031] In an optional embodiment, the crucible lowering method includes: first, heating the crucible at a rate of 20–60°C / h to a temperature T1 10–30°C higher than the melting point of the raw material, and holding the temperature for 5–50 hours to allow the raw material in the crucible to completely melt and form a doping concentration gradient in the through-holes. Then, the crucible is lowered, and the lowering rate is controlled at 0.2–5 mm / h. After the lowering process is completed, the temperature is reduced to room temperature at a rate of 5–30°C / h. In a more preferred embodiment, when growing crystals using the crucible lowering method, before the temperature reaches T1, it should first be held at 150°C–250°C for 5–15 hours to remove residual adsorbed water in the crucible. The temperature is then further increased to 800–900°C and held for 5–15 hours to allow the PbF2 in the raw material to preferentially react with oxygen-containing substances such as water and oxygen, preventing the fluoride raw material from being oxidized.

[0032] During the isothermal stage, the principle of the formation of a doping concentration gradient in the melt within the through-hole of single crystal growth is as follows: the small diameter of the through-hole inside the crucible inhibits the convection of the melt between the upper / lower feed chamber and the through-hole, as well as within the through-hole. Due to the difference in doping concentration between the melt in the feed chamber and the melt within the through-hole, doped ions diffuse vertically from the feed chamber toward the through-hole, thereby forming a concentration gradient in the melt within the through-hole where the doping concentration gradually decreases along the crucible axis. The doping concentration gradient of the grown crystal can be adjusted by adjusting the holding time, the cooling rate V1 of the temperature gradient method, or the crucible descent rate.

[0033] In this invention, the crucible used can be designed with an independent number of crystal growth spaces and a number of through holes between the upper and lower material bins according to actual needs, so that multiple rare earth gradient doped crystals with different doping compositions can be grown in a single process.

[0034] The following examples further illustrate the present invention in detail. It should also be understood that the following examples are only for further explanation of the present invention and should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made by those skilled in the art based on the above description of the present invention are within the scope of protection of the present invention. The specific process parameters, etc., in the following examples are merely examples within a suitable range; that is, those skilled in the art can make appropriate selections within the appropriate range based on the description herein, and are not intended to be limited to the specific values ​​in the examples below.

[0035] Example 1 Er:CaF2 crystals were grown using a temperature gradient method, and the graphite crucible used was as follows: Figure 1 As shown, it includes the following steps.

[0036] (1) Raw material preparation Based on the stoichiometric ratio of 5 at.% Er:CaF2, calculate the required mass of CaF2 and ErF3 raw materials according to a total mass of 10g. Weigh the corresponding mass of CaF2 and ErF3 raw materials and 0.1g of PbF2. Grind the above three raw materials and mix them evenly, and press them into cylindrical raw material blocks of Φ20mm×10mm. Weigh 40g of CaF2 raw material and 0.4g of PbF2, grind the above two raw materials and mix them evenly.

[0037] (2) Loading Pack the pressed 5 at.% Er:CaF2 raw material block into the lower loading bin of the crucible, cover the upper and lower crucible lids, and the filling ratio is close to 100%; pack the CaF2 powder raw material into the upper loading bin, gently tap or shake the crucible to allow the powder raw material to enter and fill the through hole, and cover the upper crucible lid; install the crucible body on the crucible support.

[0038] (3) Crystal growth by temperature gradient method Place the crucible into the self-made temperature gradient furnace, adjust the height of the seed crystal rod to position the crucible appropriately, install the insulation layer, close the furnace sealing chamber, and turn on the vacuum pump to evacuate the furnace cavity to a vacuum level of 10. -3 Below Pa. Heating was initiated, and the crucible was heated at a rate of 50°C / h to 180°C, then held at that temperature for 5 hours. The temperature was then increased to 800°C at a rate of 50°C / h and held for 15 hours. Subsequently, the crucible was heated to 1420°C at a rate of 30°C / h and held for 12 hours. Immediately afterwards, the temperature was decreased to 1270°C at a rate of 1°C / h to complete the melt crystallization process. Finally, the temperature was decreased to 1100°C at a rate of 5°C / h, and then to room temperature at a rate of 20°C / h. The gradient-doped Er:CaF2 crystal growth process was completed. The gradient-doped Er:CaF2 crystal and the axial gradient distribution of rare earth elements obtained in Example 1 are shown in [Figure 1]. Figure 3 and Figure 4 .

[0039] Example 2 Gradient-doped Tm:SrF2 crystals were grown using the crucible descent method. The graphite crucible used was, for example... Figure 2 As shown, it includes the following steps.

[0040] (1) Raw material preparation Based on the stoichiometric ratio of 5 at.% Tm:SrF2, calculate the required mass of SrF2 and TmF3 raw materials according to a total mass of 15g. Weigh the corresponding mass of SrF2 and TmF3 raw materials and 0.1g of PbF2. Grind and mix the above three raw materials evenly, and press them into cylindrical raw material blocks of Φ20mm×10mm. Based on the stoichiometric ratio of 0.5 at.% Tm:SrF2, calculate the required mass of SrF2 and TmF3 raw materials according to a total mass of 50g. Weigh the corresponding mass of SrF2 and TmF3 raw materials and 0.4g of PbF2. Grind and mix the above three raw materials evenly.

[0041] (2) Loading The pressed (c1) 5at.%Tm:SrF2 raw material block is loaded into the lower loading bin of the crucible, and all lower crucible lids are closed, with a filling ratio close to 100%; the (c2) 0.5at.%Tm:SrF2 powder raw material is loaded into the upper loading bin, and the crucible is gently tapped or shaken to allow the powder raw material to enter and fill the through hole, and the upper crucible lid is closed; the crucible body is installed on the crucible support.

[0042] (3) Crystal growth by crucible lowering method Place the crucible into the self-made crucible lowering furnace, adjust the height of the seed crystal rod to position the crucible at a suitable height, install the insulation layer, close the furnace sealing chamber, and turn on the vacuum pump to evacuate the vacuum level in the furnace cavity to 10. -3 Below Pa. Heating was initiated, and the crucible was heated at a rate of 50°C / h to 180°C, then held at that temperature for 5 hours. The temperature was then increased to 800°C at a rate of 50°C / h and held for 10 hours. Subsequently, the crucible was heated to 1490°C at a rate of 30°C / h and held for 15 hours. The crucible was then lowered at a rate of 1 mm / h, maintaining a constant temperature of 1490°C during the descent. After the crucible had descended 100 mm, the melt crystallization process was complete. The temperature was then lowered to 1000°C at a rate of 5°C / h, and then to room temperature at a rate of 20°C / h. The gradient-doped Tm:SrF2 crystal growth process was then complete. The gradient-doped Tm:SrF2 crystal and rare earth element gradient distribution obtained in Example 2 are shown in [reference needed]. Figure 5 and Figure 6 .

[0043] Comparative Example 1 The preparation process of gradient-doped Tm:SrF2 crystal (c1 = 5 at.%, c2 = 1 at.%) in Comparative Example 1 is the same as in Example 2, except that: 5 at.% Tm:SrF2 raw material powder is directly loaded into the lower loading bin of the crucible, completely filling it but not compacting it.

[0044] The gradient-doped Tm:SrF2 crystal and the axial gradient distribution of rare earth elements obtained in Comparative Example 1 are shown in the figure. Figure 7 and Figure 8 .

[0045] The above specific embodiments only illustrate the principle of rare-earth gradient-doped fluoride crystal growth using the temperature gradient method and the crucible descent method. In practical applications, the furnace temperature field, crucible material, crucible structure and size, crystal growth process parameters, etc., can be adjusted according to specific needs. Improvements based on the same principle are still within the scope of protection of this patent.

Claims

1. A method for growing rare-earth gradient-doped fluoride laser crystals, characterized in that, The rare-earth gradient-doped fluoride laser crystal is grown using a microporous crucible. The structure of the microporous crucible includes a crucible body. The crucible body contains at least one independent crystal growth space. Each independent crystal growth space includes an upper loading chamber, a lower loading chamber, and a single crystal growth through-hole that connects the upper loading chamber and the lower loading chamber and extends in the vertical direction. The method includes: (1) The raw material block formed by pressing the raw material powder of rare earth doped fluoride laser crystal with doping content c1 and the raw material powder of rare earth doped fluoride laser crystal with doping content c2 are placed in the lower loading bin and the upper loading bin of the microporous crucible, respectively, and the raw material powder of rare earth doped fluoride laser crystal with doping content c2 is kept to fill the single crystal growth through hole, where c1>c2. (2) The rare earth gradient doped fluoride laser crystal is grown by using the temperature gradient method or the crucible descent method.

2. The method for growing rare-earth gradient-doped fluoride laser crystals according to claim 1, characterized in that, In step (1), the raw material block formed by pressing the raw material powder of rare earth doped fluoride laser crystal with a doping content of c1 occupies at least 95% of the volume of the lower loading hopper; the pressing pressure is 100-150 MPa.

3. The method for growing rare-earth gradient-doped fluoride laser crystals according to claim 1, characterized in that, In step (1), the rare earth-doped fluoride laser crystal includes a fluoride matrix and rare earth doping elements; the fluoride matrix includes at least one of CaF2, SrF2, BaF2, MgF2, CeF3, and LaF3; the rare earth doping elements include at least one of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y.

4. The method for growing rare-earth gradient-doped fluoride laser crystals according to claim 1, characterized in that, c1 is 0.1–20 at.%; c2 is 0–10 at.%.

5. The method for growing rare-earth gradient-doped fluoride laser crystals according to claim 1, characterized in that, PbF2 powder is added as an oxygen scavenger to the raw material powder of rare earth doped fluoride laser crystal with a doping content of c1. The amount of oxygen scavenger added is 0.5 wt.% to 1.2 wt.% of the mass of the raw material powder of rare earth doped fluoride laser crystal with a doping content of c1. PbF2 powder is added as an oxygen scavenger to the raw material powder of rare earth doped fluoride laser crystal with a doping content of c2. The amount of oxygen scavenger added is 0.5 wt.% to 1.2 wt.% of the mass of the raw material powder of rare earth doped fluoride laser crystal with a doping content of c2.

6. The method for growing rare-earth gradient-doped fluoride laser crystals according to claim 1, characterized in that, The diameter of the single crystal growth via is ≤6mm and the length is ≤100mm.

7. The method for growing rare-earth gradient-doped fluoride laser crystals according to claim 1, characterized in that, The parameters of the temperature gradient method include: first, heating to a temperature T1 10–30°C higher than the melting point of the raw material powder to completely melt the raw material and form a doping concentration gradient in the single crystal growth vias; then cooling at a rate V1 of 0.2–5°C / hour for 100–200°C to complete the melt crystallization process; finally, cooling to room temperature; the atmosphere of the temperature gradient method is a vacuum atmosphere with a vacuum degree of 10. -3 Below Pa.

8. The method for growing rare-earth gradient-doped fluoride laser crystals according to claim 1, characterized in that, The parameters of the crucible lowering method include: first, heating to a temperature T1 10–30°C higher than the melting point of the raw material powder to completely melt the raw material and form a doping concentration gradient in the single crystal growth via; then, starting the crucible lowering process and controlling the lowering rate at 0.2–5 mm / h to begin crystal growth; finally, after completing the lowering process, cooling to room temperature; the atmosphere of the crucible lowering method is a vacuum atmosphere with a vacuum degree of 10. -3 Below Pa.

9. The method for growing rare-earth gradient-doped fluoride laser crystals according to claim 7 or 8, characterized in that, Before heating to temperature T1, keep the temperature at 150℃~250℃ for 5~15h, then continue heating to 800~900℃ and keep the temperature constant for 5~15h.

10. A rare-earth gradient-doped fluoride laser crystal prepared by the method for growing rare-earth gradient-doped fluoride laser crystal according to claim 1.

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