Rare earth doped alkaline earth fluoride / alkaline earth fluoride bonded crystal and preparation method thereof

Abstract

The invention relates to a rare earth doped alkaline earth fluoride / alkaline earth fluoride bonded crystal and a preparation method thereof. The preparation method of the rare earth doped alkaline earth fluoride / alkaline earth fluoride bonding crystal comprises the following steps: (1) polishing and cleaning a bonding surface to prepare a pre-bonding piece of a rare earth doped alkaline earth fluoride RE: MF2 crystal and an alkaline earth fluoride MF2 crystal; (2) placing the pre-bonding piece of the RE: MF2 crystal and the pre-bonding piece of the MF2 crystal on an optical cement leaning body in the direction that the bonding surfaces are oppositely attached to each other, heating, and applying pressure to enable the bonding surfaces of the two crystals to tightly abut against each other to obtain a crystal optical cement pre-bonding piece; and (3) fixing the crystal optical cement pre-bonding piece, applying pressure of 0.2-5 MPa perpendicular to the bonding surface again through a stainless steel spring, embedding the crystal optical cement pre-bonding piece by adopting MF2 crystal powder, and carrying out heat treatment in a muffle furnace at the temperature of 400-650 DEG C to obtain the rare earth doped alkaline earth fluoride / alkaline earth fluoride bonding crystal.

Description

Technical Field

[0001] The invention belongs to the technical field of laser crystal processing, and in particular relates to a rare earth doped alkaline earth fluoride / alkaline earth fluoride bonded crystal and a preparation method thereof. Background Art

[0002] Alkaline earth fluoride (MF2) crystals are face-centered cubic fluorite structures with optical isotropy, high thermal conductivity, wide transmission spectrum range, low effective phonon energy, and the ability to be doped with high concentrations of rare earth ions. They are a type of matrix crystal with excellent comprehensive performance. At present, rare earth-doped alkaline earth fluoride (RE:MF2) crystals have achieved excellent laser performance reports in the visible light band, 1 micron near-infrared band, and 2-3 micron mid-infrared band. Especially in the mid-infrared band, Tm:SrF2 and Er:SrF2 crystals are the laser gain materials with the highest laser efficiency among similar crystals reported so far.

[0003] However, the doping of rare earth ions will significantly reduce the thermal conductivity of MF2 crystals, and the thermal expansion coefficient of this type of crystal is large. Under high-power optical pumping conditions, the thermal effects of the crystal (including thermal lensing and thermally induced stress birefringence) will seriously deteriorate the quality of the generated laser beam, which is an important factor currently affecting its application in high-power lasers.

[0004] Since the undoped crystal has high thermal conductivity and extremely weak absorption of pump light, by bonding the doped crystal to the undoped crystal of the same matrix, the undoped crystal can be used as a heat sink for the doped crystal, thereby improving the heat dissipation capacity of the pump end face of the laser gain crystal element. This solution has been proven to significantly improve the thermal effect of the laser gain crystal. At present, there are literature and patents reporting the process of bonding rare earth doped crystals to pure crystals, including Nd:YAG / YAG, Nd:YVO4 / YVO4, Tm:LiYF4 / LiYF4, etc., but there is still a lack of effective process methods for RE:MF2 / MF2 crystal bonding. Summary of the invention

[0005] In view of the above technical problems, the object of the present invention is to provide a rare earth doped alkaline earth fluoride / alkaline earth fluoride bonded crystal and a preparation method thereof, so as to further prepare RE:MF2 / MF2 crystal bonding elements with high bonding strength and good optical uniformity.

[0006] In a first aspect, the present invention provides a method for preparing a rare earth-doped alkaline earth fluoride / alkaline earth fluoride bonded crystal, the preparation method comprising the following steps: (1) polishing and cleaning the bonding surface to prepare a pre-bonded piece of rare earth-doped alkaline earth fluoride RE:MF2 crystal and alkaline earth fluoride MF2 crystal; (2) placing the pre-bonded parts of RE:MF2 crystal and MF2 crystal on the optical adhesive body with the bonding surfaces facing each other, heating and applying pressure to make the bonding surfaces of the two crystals closely contact each other, thereby obtaining the crystal optical adhesive pre-bonded parts; (3) Fix the crystal photoresist pre-bonded part and apply a pressure of 0.2-5 MPa again perpendicular to the bonding surface through a stainless steel spring, embed the crystal photoresist pre-bonded part with MF2 crystal powder, and heat treat it at 400-650° C. in a muffle furnace to obtain the rare earth-doped alkaline earth fluoride / alkaline earth fluoride bonded crystal.

[0007] Preferably, in step (1), the alkaline earth fluoride MF2 includes CaF2, SrF2, BaF2, PbF2, and the doped rare earth element RE includes at least one of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y.

[0008] Preferably, in step (1), the pre-bonding surface is a square cross-section with a side length ≤ 20 mm or a circular cross-section with a diameter ≤ Φ20 mm; preferably, the roughness of the polished bonding surface is less than 1 nm and the flatness is less than λ / 10.

[0009] Preferably, in step (1), the cleaning process comprises: first ultrasonic cleaning in deionized water for 1-2 hours, then soaking in a 3-7wt.% NaOH ethanol solution for 15-60 minutes, then washing with deionized water and then soaking in a 10-15wt.% HCl aqueous solution for 10-60 minutes, and finally washing with ethanol and drying; Preferably, the concentration of the NaOH ethanol solution is 4-5wt.%, and the soaking time is 30 minutes; the concentration of the HCl aqueous solution is 10-13wt.%, and the soaking time is 30 minutes.

[0010] Preferably, in step (2), the heating temperature is 100-110° C., and the heating time is 10-30 minutes, preferably 10-15 minutes.

[0011] Preferably, in step (2), the direction of applying pressure is perpendicular to the bonding surface, and the applied pressure is 0.2-10 MPa, preferably 0.5-1 MPa.

[0012] Preferably, in step (3), the particle size of the MF2 crystal powder used for pre-bonding of the embedded crystal optical glue is less than 1 mm.

[0013] Preferably, in step (3), the heat treatment process is: first heat to 400-650°C at a heating rate of 2-10°C / hour, keep warm for 10-30 hours, and then cool to room temperature at 1-5°C / hour to complete the heat treatment and annealing of the crystal optical resin pre-bonded parts.

[0014] In a second aspect, the present invention provides a rare earth-doped alkaline earth fluoride / alkaline earth fluoride bonded crystal obtained according to the above preparation method.

[0015] Beneficial Effects The RE:MF2 / MF2 crystal bonding element prepared by the bonding process method of rare earth-doped alkaline earth fluoride crystals and pure alkaline earth fluoride crystals provided by the present invention has high bonding strength and good optical uniformity; wherein, heat treatment does not need to be carried out under high vacuum and fluorination conditions, and oxidation of the bonded crystal can be effectively avoided, and heat treatment and annealing of the bonded crystal are completed in one step, which greatly improves the bonding efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 This is a photo of the crystal optical adhesive pre-bonded part after the heat treatment in Example 1; Figure 2 This is a photo of the Er:SrF2 / SrF2 bonded crystal element prepared after shaping in Example 1; Figure 3 This is a photo of the Er:SrF2 / SrF2 bonded crystal element obtained in Comparative Example 1. DETAILED DESCRIPTION

[0017] The present invention is further described below by the following embodiments. It should be understood that the following embodiments are only used to illustrate the present invention, but not to limit the present invention.

[0018] The following is an exemplary description of the preparation method of the rare earth doped alkaline earth fluoride / alkaline earth fluoride bonded crystal provided by the present invention. The preparation method may include the following steps: (1) processing the blanks of rare earth-doped alkaline earth fluoride RE:MF2 crystal and alkaline earth fluoride MF2 crystal to the required size and polishing the bonding surfaces of the two crystals, and then cleaning them to obtain a pre-bonded part; (2) placing the pre-bonded pieces of RE:MF2 crystal and MF2 crystal on an optical adhesive body with the bonding surfaces facing each other and heating them on a temperature-controlled heating plate, while applying pressure perpendicular to the bonding surfaces so that the bonding surfaces of the two crystals are closely abutted and connected together through intermolecular forces, thereby obtaining a crystal optical adhesive pre-bonded piece; (3) Fix the crystal photoresist pre-bonded part and apply a pressure of 0.2-5 MPa again perpendicular to the bonding surface through a stainless steel spring, embed the crystal photoresist pre-bonded part with MF2 crystal powder, and perform heat treatment at 400-650° C. in a muffle furnace to obtain the rare earth-doped alkaline earth fluoride / alkaline earth fluoride bonded crystal.

[0019] In some embodiments, in step (1), the alkaline earth fluoride MF2 may include CaF2, SrF2, BaF2, PbF2, and the doped rare earth element RE may include at least one of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y; preferably, the doping molar percentage of the rare earth element in the rare earth-doped alkaline earth fluoride RE:MF2 crystal may be 0.1-20%.

[0020] In some embodiments, in step (1), the pre-bonding surface may be a square cross section with a side length of ≤20 mm or a circular cross section with a diameter of ≤Φ20 mm; preferably, the roughness of the bonding surface after polishing is less than 1 nm and the flatness is less than λ / 10. Improper bonding surface shape, roughness, and flatness may lead to unsatisfactory optical bonding effect, easy formation of pores between optical bonding surfaces, or low optical bonding strength and debonding during heat treatment.

[0021] In some embodiments, in step (1), the cleaning process may include: first ultrasonic cleaning in deionized water for 1-2 hours, then soaking in a 3-7wt.% NaOH ethanol solution for 15-60 minutes, then washing with deionized water and then soaking in a 10-15wt.% HCl aqueous solution for 10-60 minutes, and finally washing with ethanol and drying; preferably, the concentration of the NaOH ethanol solution is 4-5wt.%, and the soaking time is 30 minutes; the concentration of the HCl aqueous solution is 10-13wt.%, and the soaking time is 30 minutes.

[0022] The cleaning process can help remove particles and contamination on the pre-bonding surface, and obtain a relatively ideal clean pre-bonding surface. Improper cleaning will lead to unsatisfactory optical bonding effect, easy appearance of pores between optical bonding surfaces, or low optical bonding strength and debonding during heat treatment.

[0023] In addition, through the process of crystal photoresist, the pre-bonding surfaces can be bonded together through intermolecular forces, so that a strong bond can be formed through further heat treatment. Directly contacting the pre-bonding surfaces and then annealing cannot achieve a good bonding effect.

[0024] In some embodiments, in step (2), the temperature of the temperature-controlled heating plate may be 100-110° C., and the heating time on the temperature-controlled heating plate may be 10-30 minutes, preferably 10-15 minutes. By adjusting the temperature of the heating plate, water and organic solvents on the pre-bonded surface can be effectively removed. Too short a temperature / time may result in incomplete removal, while too long a temperature / time may affect efficiency.

[0025] In some embodiments, in step (2), the pressure applied perpendicular to the bonding surface may be 0.2-10 MPa, preferably 0.5-1 MPa.

[0026] In some embodiments, in step (3), the crystal photoresist pre-bonded component may be embedded with MF2 crystal powder with a particle size of less than 1 mm before heat treatment.

[0027] It should be noted that, in general, fluoride crystals heated under air conditions will react with oxygen in the air, causing the optical quality of the crystals to decrease. Therefore, heat treatment of fluoride crystals is usually carried out under high vacuum or special atmosphere. However, the technical solution disclosed in the present invention performs heat treatment on bonded crystals under air conditions. On the one hand, this is because the heat treatment temperature adopted by the present invention is relatively low and the MF2 crystal reacts very slowly with oxygen at this temperature. On the other hand, by using MF2 crystal powder to embed the bonded crystals (the powder is easier to react with oxygen), the oxidation of the bonded crystals by oxygen can be further reduced. Moreover, by limiting the particle size of the MF2 crystal powder to less than 1 mm, the powder can embed the bonded crystals more densely, and the anti-oxidation effect can be better.

[0028] In some embodiments, in step (3), the heat treatment process can be: first heat to 400-650°C at a heating rate of 2-10°C / hour, keep warm for 10-30 hours, and then cool to room temperature at 1-5°C / hour to complete the heat treatment and annealing of the crystal optical glue pre-bonded parts.

[0029] The heat treatment temperature is related to the physical and chemical properties of the crystal itself (the melting point of MF2 crystal is about 1400°C). The lower heat treatment temperature proposed in the present invention can achieve the bonding effect and effectively avoid the crystal oxidation problem during the heat treatment process. If the heat treatment temperature is too low, it will lead to low bonding strength; if the heat treatment temperature is too high, it will lead to large thermal stress on the bonding surface and crystal oxidation problems. In addition, the heating / cooling rate should not be too fast, otherwise thermal stress will be formed in the crystal, resulting in bonding failure.

[0030] In summary, the preparation method disclosed in the present invention performs heat treatment on fluoride crystals under air conditions, while effectively avoiding oxidation of bonded crystals, and the heat treatment and annealing of bonded crystals are completed in one step, thereby improving bonding efficiency. The RE:MF2 / MF2 crystal bonding element obtained by the preparation method provided by the present invention has high bonding strength and good optical uniformity. While conventional fluoride crystal heat treatment often needs to be carried out in a vacuum or a special atmosphere to prevent crystal oxidation, the method proposed in the present invention performs heat treatment under air, which can be completed using an ordinary muffle furnace, has low requirements on equipment, low heat treatment cost, and high efficiency. The bonding strength of the rare earth-doped alkaline earth fluoride / alkaline earth fluoride bonded crystal obtained by the preparation method provided by the present invention is comparable to that of a pure matrix crystal.

[0031] The following further examples are given to illustrate the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention and cannot be construed as limiting the scope of protection of the present invention. Some non-essential improvements and adjustments made by those skilled in the art based on the above content of the present invention all belong to the scope of protection of the present invention. The specific process parameters of the following examples are also only examples within a suitable range, that is, those skilled in the art can make selections within a suitable range through the description of this article, and are not limited to the specific values ​​​​exemplified below. If not specifically stated, the technical means used in the embodiments are conventional means well known to those skilled in the art.

[0032] Example 1

[0033] The method for preparing the rare earth doped alkaline earth fluoride / alkaline earth fluoride bonded crystal provided in this embodiment comprises the following steps: (1) Using an internal cutting machine, Er:SrF2 crystal and pure SrF2 crystal are cut into square columnar crystal blocks of 6 mm×6 mm×11 mm, and the pre-bonded end faces are coarsely ground and finely ground using W50 and W20 CeO2 grinding liquids respectively, and the length of the crystal block is ground to 10 mm; then the bonding surface is polished using diamond grinding liquid until the end surface roughness is better than 1 nm and the flatness is better than λ / 10; the polished Er:SrF2 crystal and pure SrF2 crystal pre-bonded parts are ultrasonically cleaned in deionized water for 1 hour, and then immersed in 5 wt.% NaOH ethanol solution for 30 minutes, washed with deionized water, and then immersed in 12 wt.% HCl aqueous solution for 30 minutes, and finally washed with ethanol and dried to obtain pre-bonded parts; (2) placing the Er:SrF2 crystal and the pure SrF2 crystal pre-bond on the optical adhesive body, and placing them together on a temperature-controlled heating plate at 100°C for heating for 15 minutes, and then applying a pressure of 0.5MPa to the crystal to make the bonding surfaces closely contact and form optical adhesive, thereby obtaining a crystal optical adhesive pre-bond; (3) Fix the Er:SrF2 / SrF2 crystal after photoresist with a stainless steel clamp, apply a pressure of 0.2MPa on the vertical bonding surface through a spring, place the whole thing in a cylindrical alumina crucible, and embed the Er:SrF2 / SrF2 crystal with SrF2 powder with a particle size of less than 1mm, and cover the crucible with a lid; place the crucible in a muffle furnace, install the insulation layer, close the furnace lid, and heat the crucible to 500℃ at a heating rate of 10℃ / hour, and keep The temperature was set at 400 °C for 24 hours and then cooled to room temperature at a rate of 5 °C / hour to complete the heat treatment and annealing of the crystal optical resin pre-bonded parts; the Er:SrF2 / SrF2 bonded crystal was coarsely ground and finely ground using W50 and W20 CeO2 grinding liquids respectively, and the cross section of the bonded crystal was ground to 5mm×5mm to obtain an Er:SrF2 / SrF2 bonded crystal with a size of 5mm×5mm×20mm and a length of 10mm for the Er:SrF2 and SrF2 parts respectively.

[0034] Figure 1 This is a photo of the crystal optical adhesive pre-bonded part after the heat treatment in Example 1. It can be seen from the figure that the bonding surface is well bonded, there is no edge debonding problem, and the crystal is not obviously oxidized.

[0035] Figure 2 This is a photo of the Er:SrF2 / SrF2 bonded crystal element prepared after shaping in Example 1. It can be seen from the figure that the crystal bonding quality is good, the bonding surface interface is clear, there are no pores and debonding problems in the entire bonding surface; the bonding strength is high, and the sample does not debond after grinding and polishing.

[0036] Comparative Example 1

[0037] The preparation method of the rare earth doped alkaline earth fluoride / alkaline earth fluoride bonded crystal provided in this comparative example refers to the example, with the main difference being that in step (3), the heat treatment temperature is 700°C.

[0038] Figure 3 This is a photo of the Er:SrF2 / SrF2 bonded crystal element obtained in Comparative Example 1. It can be seen from the figure that there is obvious stress near the bonding interface, which is caused by the excessively high heat treatment temperature.

[0039] Although the content of the present invention has been described in detail through the above preferred embodiments, it should be appreciated that the above description should not be considered as a limitation of the present invention. After reading the above content, it will be apparent to those skilled in the art that various modifications and substitutions of the present invention will occur. Therefore, the protection scope of the present invention should be limited by the appended claims.

Claims

1. A method for preparing rare earth doped alkaline earth fluoride / alkaline earth fluoride bonded crystals, characterized in that: The preparation method comprises the following steps: (1) polishing and cleaning the bonding surface to prepare a pre-bonded piece of rare earth-doped alkaline earth fluoride RE:MF2 crystal and alkaline earth fluoride MF2 crystal; (2) placing the pre-bonded parts of RE:MF2 crystal and MF2 crystal on the optical adhesive body with the bonding surfaces facing each other, heating and applying pressure to make the bonding surfaces of the two crystals closely contact each other, thereby obtaining the crystal optical adhesive pre-bonded parts; (3) Fix the crystal photoresist pre-bonded part and apply a pressure of 0.2-5 MPa again perpendicular to the bonding surface through a stainless steel spring, embed the crystal photoresist pre-bonded part with MF2 crystal powder, and heat treat it at 400-650° C. in a muffle furnace to obtain the rare earth-doped alkaline earth fluoride / alkaline earth fluoride bonded crystal.

2. The preparation method according to claim 1, characterized in that: In step (1), the alkaline earth fluoride MF2 includes CaF2, SrF2, BaF2, and PbF2, and the added rare earth element RE includes at least one of La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Sc, and Y.

3. The preparation method according to claim 1 or 2, characterized in that: In step (1), the pre-bonding surface is a square cross-section with a side length of ≤20 mm or a circular cross-section with a diameter of ≤Φ20 mm; preferably, the roughness of the polished bonding surface is less than 1 nm and the flatness is less than λ / 10.

4. The preparation method according to any one of claims 1 to 3, characterized in that In step (1), the cleaning process includes: first ultrasonic cleaning in deionized water for 1-2 hours, then soaking in a 3-7wt.% NaOH ethanol solution for 15-60 minutes, then cleaning with deionized water and then soaking in a 10-15wt.% HCl aqueous solution for 10-60 minutes, and finally cleaning with ethanol and drying; Preferably, the concentration of the NaOH ethanol solution is 4-5wt.%, and the soaking time is 30 minutes; the concentration of the HCl aqueous solution is 10-13wt.%, and the soaking time is 30 minutes.

5. The preparation method according to any one of claims 1 to 4, characterized in that: In step (2), the heating temperature is 100-110° C., and the heating time is 10-30 minutes, preferably 10-15 minutes.

6. The preparation method according to any one of claims 1 to 5, characterized in that: In step (2), the direction of applying pressure is perpendicular to the bonding surface, and the applied pressure is 0.2-10 MPa, preferably 0.5-1 MPa.

7. The preparation method according to any one of claims 1 to 6, characterized in that In step (3), the particle size of the MF2 crystal powder used for pre-bonding of the embedded crystal optical glue is less than 1 mm.

8. The preparation method according to any one of claims 1 to 7, characterized in that: In step (3), the heat treatment process is: first heat to 400-650°C at a heating rate of 2-10°C / hour, keep warm for 10-30 hours, and then cool to room temperature at 1-5°C / hour to complete the heat treatment and annealing of the crystal optical resin pre-bonded parts.

9. A rare earth-doped alkaline earth fluoride / alkaline earth fluoride bonded crystal obtained by the preparation method according to any one of claims 1 to 8.

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