Preparation method of gradient doped Nd: LuAG laser crystal

Abstract

The invention discloses a preparation method of a gradient doped Nd: LuAG laser crystal, in a single crystal growth process, a solute conservative system is obtained through a horizontal full melting mode, and compared with a traditional zone melting mode, the distribution of a solute in the crystal is more uneven, and a larger doping gradient can be obtained, so that a segregation coefficient k0lt is realized; the high-quality gradient-doped Nd: LuAG laser crystal is obtained by performing high-gradient doping on Nd < 3 + > (k0 = 0.18-0.25) with Nd < 3 + > (k0 = 0.18-0.25) of 1 in a LuAG matrix and reasonably controlling the seeding rate. In the single crystal growth process, melt rotation operation is avoided, the defects of a core, a side core and a cloud layer are avoided, high-quality crystals are easy to obtain, the machining allowance is small, and the production benefit is increased.

Description

Technical Field

[0001] The invention relates to the technical field of crystal growth, and in particular to a method for preparing a gradient-doped Nd:LuAG laser crystal. Background Art

[0002] Since the beginning of the 21st century, the repetition rate nanosecond high-energy diode-pumped solid-state laser (DPSSL) technology has rapidly emerged and has become a research hotspot in the laser field. This type of laser has shown great potential in many application fields, such as material processing, scientific research, and medical diagnosis. Among them, the gain medium is the core component of DPSSL technology, and its performance directly determines the output characteristics and stability of the laser.

[0003] Nd:LuAG crystals with medium saturation flux have become ideal gain media to meet the needs of high repetition rate nanosecond high energy DPSSL technology due to their unique physical and chemical properties. Specifically, Nd:LuAG has a moderate emission cross section and medium saturation flux at room temperature, which not only ensures that the laser can obtain higher gain, but also avoids the risk of film damage that may be caused by using high saturation flux gain media (such as Yb:YAG). Compared with low saturation flux media (such as Nd:YAG), Nd:LuAG can more effectively balance gain requirements and system efficiency, avoiding the problems of increased system complexity and reduced efficiency caused by low energy storage of a single chip. In addition, Nd:LuAG also has high thermal conductivity and good thermal shock resistance, which is crucial for effectively managing thermal effects in laser systems and ensuring stable operation of lasers.

[0004] However, despite the many advantages of Nd:LuAG crystals, they still face many challenges in practical applications. Traditional laser crystals use a uniformly doped design. Under high-energy repetitive pulse pumping, a population inversion will occur inside the gain medium, causing the energy of the pump light to be converted into laser energy storage. However, due to the quantum loss caused by the Stokes effect and the quantum efficiency of less than 1 in the fluorescence process, a large amount of pump light energy is eventually converted into heat and accumulated inside the medium. For thin-film lasers, this heat is mainly dissipated through the lower surface, which can easily form temperature gradients and thermal stresses in the thin film, which in turn seriously affects the optical quality of the laser. Therefore, the thermal management problem of the system under high repetition rate has become one of the key technical bottlenecks restricting the performance improvement of repetition rate high-energy pulse lasers.

[0005] In order to solve this problem, researchers began to explore the regulation of thermal energy deposition patterns in the medium through gradient doping from the perspective of laser crystals. Gradient doping technology is expected to significantly improve the heat dissipation effect, maintain the uniformity of energy storage, and thus improve the optical quality of the laser. However, in actual operation, the preparation of gradient-doped Nd:LuAG laser crystals faces many technical difficulties. As one of the most commonly used crystal growth methods, the pulling method is suitable for the preparation of Nd³⁺ gradient-doped crystals with a segregation coefficient less than 1. However, when growing LuAG crystals with a high melting point (2010°C), the crucible made of the precious metal iridium is prone to leaking, resulting in great technical difficulty and high production costs in the preparation of gradient-doped Nd:LuAG crystals.

[0006] In summary, the development of a new and efficient gradient-doped Nd:LuAG laser crystal growth method is of great significance for breaking through the current technical bottleneck of high-repetition-rate and high-energy pulse lasers and improving the overall performance of lasers. Summary of the invention

[0007] In view of the above-mentioned deficiencies in the prior art, the object of the present invention is to provide a method for preparing a gradient-doped Nd:LuAG laser crystal, so as to solve the problems that traditional laser crystals adopt a uniform doping design, the heat dissipation of the crystals is poor, which affects the performance of the laser, and the gradient-doped Nd:LuAG crystals are prone to leaking and are costly to prepare by the pulling method.

[0008] In order to solve the above technical problems, the technical solution adopted by the present invention is as follows: A method for preparing a gradient-doped Nd:LuAG laser crystal comprises the following steps: (1) Al2O3 polycrystalline material, Lu2O3 powder and Nd2O3 powder are uniformly mixed, melted and cooled to obtain a solid solution, and then the solid solution is broken into small pieces as pre-crystallization material; (2) Spread the pre-crystallization material flat in the boat-shaped crucible, then insert the seed crystal into the end of the boat-shaped crucible near the shoulder corner, put the boat-shaped crucible containing the pre-crystallization material and the seed crystal into the working furnace, and make the pre-crystallization material be located inside the heater and the seed crystal be located outside the heater, and then close the furnace; (3) Evacuate the working furnace. When the vacuum degree is less than 1×10 -3 Pa, turn on the heating power of the working furnace to heat the pre-crystallized material in the heater, adjust the heating power to make the pre-crystallized material completely melted, and keep warm for 10-20 minutes after the seed crystal is melted by 1 / 4~1 / 3; (4) Keeping the heating power constant, the crucible boat is slowly moved toward the end of the crucible boat containing the seed crystal at a seeding rate of 1-1.5 mm / h. After seeding and shoulder growth, the crucible boat is continuously moved toward the end of the crucible boat containing the seed crystal at a seeding rate of 1.5-2 mm / h for equal-width growth until all the melt is completely crystallized. After cooling and annealing, a gradient-doped Nd:LuAG crystal is obtained. During the movement, the melt in the crucible boat that is gradually away from the heater generates supercooling, which serves as a driving force for crystallization and promotes the production of single crystals.

[0009] Furthermore, the Al2O3 polycrystalline material, Lu2O3 powder and Nd2O3 powder in step (1) are prepared according to the chemical formula Lu 3(1-x) Nd 3x Al5O 12 The atomic ratio in is weighed, where x ranges from 0.1 to 2 at.%. x is the doping concentration. The doping concentration is controlled below 2 at.% because when growing a gradually doped crystal, if the doping concentration is high, the segregation effect of the solute is easily increased, which is not conducive to ensuring the integrity of the crystal.

[0010] Furthermore, the crystal orientation of the seed crystal in step (2) is

[111] . Generally, the crystal with LuAG as the matrix has the highest symmetry along the

[111] direction, and the defects generated during the growth process are relatively small. Therefore, when the seed crystal is grown, the crystal orientation of the seed crystal is

[111] , which makes it easier to obtain high-quality crystals.

[0011] Furthermore, the procedure of the cooling annealing in step (4) is as follows: first, the furnace temperature is lowered to 1700-1800°C at a cooling rate of 10-30°C / h, and kept at this temperature for 15-30h for in-situ annealing to reduce the internal thermal stress of the crystal, and then cooled to room temperature at a cooling rate of 40-60°C / h. When the power drops to 0 kW, the vacuum system is turned off after waiting for 15-30h. In the cooling annealing process, although the cooling rate is too fast or the holding time is too short, the crystal growth cycle can be reduced. However, the cooling rate is too fast and the holding time is too short, which will weaken the degree of stress elimination during the annealing process and increase the risk of cracking. Therefore, the present invention adopts a segmented cooling method for annealing, first cooling at a smaller rate and keeping warm for 15-30 hours, realizing in-situ annealing, reducing the internal thermal stress of the crystal, and then cooling to room temperature at a faster rate, improving the crystal preparation efficiency, and finally waiting for 15-30 hours at 0 kW. This is because when the power is reduced to 0 kW, the temperature inside the equipment is still high and cannot be immediately reduced to 0°C. If the equipment is opened to take out the crystal at this time, on the one hand, the temperature field parts inside the equipment are exposed to the air and are prone to oxidation, which reduces the service life of the parts; on the other hand, the crystal is taken out from the higher temperature furnace and placed in the lower temperature air. The large temperature difference easily causes the crystal to crack. The use of this in-situ annealing technology significantly improves the yield rate of the crystal and improves the stability and reliability of the overall production.

[0012] Furthermore, the shoulder release angle in the shoulder release stage is 80~120º. The shoulder release angle should not be too large or too small. When the shoulder release angle is too small, the part of the crystal before the shoulder release cannot be reasonably utilized, and the crystal utilization rate is low; if it is too large, it will cause difficulty in shoulder collection. When the shoulder release angle is 90º, the entire crystal ingot can be used most reasonably, which well solves the problem of crystal ingot utilization rate. At the same time, shoulder collection is also easier. Therefore, the preferred shoulder release angle is 90º.

[0013] Furthermore, the boat-shaped crucible is a boat-shaped molybdenum crucible. The molybdenum crucible used in the present invention has a melting point of 2630°C, is easy to grow high-melting-point crystals, and during the growth process, the crucible has high stability, is not easy to leak, and has low cost.

[0014] The preparation method of the gradient doped Nd:LuAG laser crystal of the present invention obtains a solute conservative system by horizontal full melting during the single crystal growth process. Compared with the traditional zone melting method, the distribution of the solute in the crystal is more uneven, and a larger doping gradient can be obtained, thereby achieving Nd with a segregation coefficient k0<1. 3+ (k0=0.18-0.25) high gradient doping in LuAG matrix and by reasonably controlling the seeding rate, high-quality gradient-doped Nd:LuAG laser crystals were obtained.

[0015] In the seeding process, the present invention first performs seeding and shoulder growth at a relatively small seeding rate of 1 to 1.5 mm / h, and then performs equal width growth at a relatively large seeding rate of 1.5 to 2 mm / h. This is because, compared with the horizontal zone melting crystallization method for preparing Nd:LuAG laser crystals uniformly doped with neodymium ions, the crystal is prepared by a horizontal full melting method. Under the condition of full melting, the temperature gradient in the melt is larger. According to the relationship between the crystal growth rate and the temperature gradient in the melt, when the temperature gradient in the crystal is constant, the larger the temperature gradient in the melt, the smaller the speed that can be adapted to crystal growth, and the quality of the seeding process will affect the subsequent crystal growth process. In order to ensure the stability of the solid-liquid interface as much as possible, thereby stabilizing the growth, the present invention initially performs seeding growth at a relatively low seeding rate, and subsequently, under the condition of stable crystal growth, appropriately increases the rate to increase the segregation effect of ions, forming a larger concentration gradient, thereby obtaining a high-quality gradient-doped Nd:LuAG laser crystal.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. The preparation method of the gradient-doped Nd:LuAG laser crystal of the present invention obtains a solute conservative system by horizontal full melting during the single crystal growth process. Compared with the traditional zone melting method, the distribution of the solute in the crystal is more uneven, and a larger doping gradient can be obtained, thereby achieving Nd with a segregation coefficient k0<1. 3+ (k0=0.18-0.25) high gradient doping in LuAG matrix, and by reasonably controlling the seeding rate, high-quality gradient doped Nd:LuAG laser crystals are obtained. The crystals prepared by the method of the present invention have a color change visible to the naked eye from the head to the tail, that is, from light purple to dark purple, and have no defects such as core and side core. The crystals have high transparency, good quality, and are easy to promote and apply.

[0017] 2. The molybdenum crucible used in the present invention has a melting point of 2630°C. During the crystal growth process, it is not easy to leak, which reduces the technical difficulty of growing high-temperature single crystals and makes it easy to grow Nd:LuAG high-temperature crystals. At the same time, it is low in price and easy to promote and apply. In the single crystal growth process, the present invention does not require melt rotation, which avoids the generation of core, side core and cloud defects, makes it easy to obtain high-quality crystals, and reduces processing allowances, thereby increasing production efficiency. In addition, during the single crystal cooling process, the present invention uses in-situ annealing to reduce the stress inside the crystal, reduce the possibility of crystal cracking, and easily improve the yield of the crystal. DETAILED DESCRIPTION

[0018] The specific implementation modes of the present invention are further described in detail below in conjunction with specific examples.

[0019] Example 1 This embodiment provides a method for preparing a gradient-doped Nd:LuAG laser crystal, comprising the following steps: (1) Preparation of pre-crystallization material: According to a certain stoichiometric ratio, weigh 599.90g of Al2O3 polycrystalline material, 1376.39g of Lu2O3 powder and 23.71g of Nd2O3 powder, each with a purity of 99.999%. Mix them evenly and put them into a rectangular molybdenum crucible. Then, place them in a single crystal furnace to melt and cool them into a solid solution. Then, break the solid solution into small pieces as pre-crystallization material for single crystal growth. (2) Furnace loading: Spread the pre-crystallized material flat in a molybdenum boat crucible, and insert a pure LuAG seed crystal into the seed crystal groove at one end of the molybdenum boat crucible near the shoulder corner. The seed crystal length is 36 mm and the crystal orientation is

[111] . Then, place the molybdenum boat crucible containing the pre-crystallized material and the seed crystal into the single crystal furnace, and place the pre-crystallized material inside the heater and the seed crystal outside the heater. Then, seal the single crystal furnace. (3) Melting crystals: After the furnace is loaded, turn on the high vacuum system. When the vacuum reaches 6×10 -4 Pa, turn on the heating system to heat the pre-crystallized material in the heater. When the power of the heating system reaches 28.5kW, the pre-crystallized material is completely melted, and the melt just contacts the seed crystal. After 20 minutes of heat preservation, there is no obvious melting phenomenon of the seed crystal. Increase the heating power by 0.2kW, and after 20 minutes, the seed crystal gradually melts. After another 20 minutes of heat preservation, the seed crystal melts 11mm in length, and the heat preservation is continued for 20 minutes. (4) Seeding, shouldering, and equal-width growth: Keep the heating power unchanged, turn on the automatic guide rail program, and move the boat-shaped molybdenum crucible toward the end of the boat-shaped molybdenum crucible with a seed crystal at a seeding speed of 1 mm / h, and perform seeding and shouldering operations in sequence, with a shouldering angle of 90°. After moving at a seeding speed of 1 mm / h for 50 hours, the crystal is about to enter the equal-width growth zone. Increase the seeding speed and continue to move the boat-shaped molybdenum crucible toward the end of the boat-shaped molybdenum crucible with a seed crystal at a seeding speed of 1.5 mm / h until all the melt is crystallized; during the movement, the melt in the boat-shaped molybdenum crucible that gradually moves away from the heater produces supercooling, which serves as a driving force for crystallization and promotes the production of single crystals.

[0020] (5) Cooling annealing: After the crystallization is completed, the cooling stage is entered. Three-stage cooling is used for in-situ annealing. First, the furnace temperature is lowered to 1700°C at a cooling rate of 10°C / h, kept at this temperature for 20 hours, and then cooled to room temperature at a cooling rate of 40°C / h. When the power drops to 0kW, the high vacuum is turned off after waiting for 20 hours, and the crystal is taken out to obtain a gradient-doped Nd:LuAG laser crystal.

[0021] The crystals grown in this embodiment show a color change visible to the naked eye, with the color changing from light purple to dark purple from the head to the tail, and have no defects such as core and side core. The crystals have high transparency and good quality, and the size is 200mm×100mm×30mm.

[0022] Example 2 This embodiment provides a method for preparing a gradient-doped Nd:LuAG laser crystal, comprising the following steps: (1) Preparation of pre-crystallization material: According to a certain stoichiometric ratio, weigh 659.53 g of Al2O3 polycrystalline material with a purity of 99.999%, 1520.92 g of Lu2O3 powder and 19.55 g of Nd2O3 powder, respectively, and then mix them evenly, put them in a rectangular molybdenum crucible, and then place them in a single crystal furnace together for high-temperature melting, rapid cooling, and solidification into a solid solution. The solid solution is then broken into small pieces as pre-crystallization material for single crystal growth; (2) Furnace loading: Spread the pre-crystallized material flat in a molybdenum boat crucible, and insert a piece of pure LuAG seed crystal into the seed crystal groove at one end of the molybdenum boat crucible near the shoulder corner, with

[111] as the seeding direction. Then, place the molybdenum boat crucible containing the pre-crystallized material and seed crystal in the single crystal furnace, with the pre-crystallized material located inside the heater and the seed crystal located outside the heater. Then, seal the single crystal furnace. (3) Melting crystal: After the furnace is loaded, high vacuum is drawn until the high vacuum reaches 1×10 -4 Pa, turn on the heating system to heat the pre-crystallized material in the heater until the set power of 28.5kW is reached. After keeping warm for 20 minutes, observe the shape of the solid-liquid interface and find that the shape of the solid-liquid interface is still moving forward. After keeping warm for another 20 minutes, the solid-liquid interface has not changed, and the solid-liquid interface is 10mm away from the seed crystal. Therefore, the heating power is increased by 0.4kW. After waiting for 20 minutes, the solid-liquid interface pushes toward the seed crystal, and the seed crystal gradually melts. After waiting for another 20 minutes, the melting length of the seed crystal is 1 / 3 of the original length. After waiting for another 20 minutes, there is basically no obvious fluctuation in the solid-liquid interface, which means that the crystal melting is completed. (4) Seeding, shoulder release, and equal-width growth: Keep the heating power unchanged, turn on the guide rail automatic program, and move the boat-shaped molybdenum crucible toward the end of the boat-shaped molybdenum crucible with the seed crystal at a seeding speed of 1 mm / h. Perform seeding and shoulder release operations in sequence, with a shoulder release angle of 90°. After 50 hours, the shoulder release stage is completed, and then continue to move toward the end of the boat-shaped molybdenum crucible with the seed crystal at a seeding speed of 1.5 mm / h to perform equal-width growth until all the melt is crystallized. During the movement, the melt in the boat-shaped molybdenum crucible that gradually moves away from the heater produces supercooling, which serves as a driving force for crystallization and promotes the production of single crystals.

[0023] (5) Cooling annealing: After the crystallization is completed, the cooling stage is entered. Three-stage cooling is used for in-situ annealing. First, the furnace temperature is lowered to 1700℃ at a cooling rate of 15℃ / h, kept at this temperature for 10h, and then cooled to room temperature at a cooling rate of 50℃ / h. When the power drops to 0kW, the high vacuum is turned off after waiting for 25h, and the crystal is taken out to obtain a gradient-doped Nd:LuAG laser crystal.

[0024] The crystals grown in this embodiment show a visible color change from head to tail, that is, from light purple to dark purple, and have no defects such as core and side core. The crystals have high transparency and good quality, and the size is 200mm×100mm×30mm.

[0025] The size of the crystal obtained by the present invention depends on the size of the crucible. The size of the crucible used in the experiment is 200mm×100mm×30mm, of which 200mm is the total length of the crucible (the total length of the crucible is the sum of the lengths of the shoulder area and the equal-width area). The size of the equal-width area can be calculated according to the shoulder angle, so a large-sized plate-shaped Nd:LuAG crystal of 200mm×100mm×30mm can be grown.

[0026] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit the technical solution. Those skilled in the art should understand that those modifications or equivalent substitutions of the technical solution of the present invention that do not depart from the purpose and scope of the technical solution should be included in the scope of the claims of the present invention.

Claims

1. A method for preparing a gradient-doped Nd:LuAG laser crystal, characterized in that: The following steps are involved: (1) Al2O3 polycrystalline material, Lu2O3 powder and Nd2O3 powder are uniformly mixed, melted and cooled to obtain a solid solution, and then the solid solution is broken into small pieces as pre-crystallization material; (2) Spread the pre-crystallization material flat in the boat-shaped crucible, then insert the seed crystal into the end of the boat-shaped crucible near the shoulder corner, put the boat-shaped crucible containing the pre-crystallization material and the seed crystal into the working furnace, and make the pre-crystallization material be located inside the heater and the seed crystal be located outside the heater, and then close the furnace; (3) Evacuate the working furnace. When the vacuum degree is less than 1×10 -3 Pa, turn on the heating power of the working furnace to heat the pre-crystallized material in the heater, adjust the heating power to make the pre-crystallized material completely melted, and keep warm for 10-20 minutes after the seed crystal is melted by 1 / 4~1 / 3; (4) Keeping the heating power constant, the crucible boat is slowly moved toward the end of the crucible boat containing the seed crystal at a seeding rate of 1-1.5 mm / h. After seeding and shoulder growth, the crucible boat is continuously moved toward the end of the crucible boat containing the seed crystal at a seeding rate of 1.5-2 mm / h for equal-width growth until all the melt is completely crystallized. After cooling and annealing, a gradient-doped Nd:LuAG crystal is obtained. During the movement, the melt in the crucible boat that is gradually away from the heater generates supercooling, which serves as a driving force for crystallization and promotes the production of single crystals.

2. The method for preparing the gradient-doped Nd:LuAG laser crystal according to claim 1, characterized in that: In step (1), the Al2O3 polycrystalline material, Lu2O3 powder and Nd2O3 powder are prepared according to the chemical formula Lu 3(1-x) Nd 3x Al5O 12 The atomic ratio in is weighed, where the value of x ranges from 0.1 to 2 at.%.

3. The method for preparing the gradient-doped Nd:LuAG laser crystal according to claim 1, characterized in that: The crystal orientation of the seed crystal in step (2) is [111].

4. The method for preparing the gradient-doped Nd:LuAG laser crystal according to claim 1, characterized in that: The procedure of the cooling annealing in step (4) is as follows: first, the furnace temperature is lowered to 1700-1800°C at a cooling rate of 10-30°C / h, and kept at this temperature for 15-30h for in-situ annealing to reduce the internal thermal stress of the crystal, and then cooled to room temperature at a cooling rate of 40-60°C / h. When the power drops to 0 kW, wait for 15-30h before turning off the vacuum system.

5. The method for preparing the gradient-doped Nd:LuAG laser crystal according to claim 1, characterized in that: The shoulder release angle during the shoulder release phase is 80~120º.

6. The method for preparing the gradient-doped Nd:LuAG laser crystal according to claim 1, characterized in that: The boat-shaped crucible is a boat-shaped molybdenum crucible.