RU2560438C1 - Method of parts connection of optical element out of garnet crystals - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: such composite optical elements are widely used in lasers and other optical devices. Method includes polishing of the connected surface of parts, their alignment and heating, at that the connected part surfaces are treated by the solution of orthophosphoric acid in alcohol, and optical contact is provided, then the connected parts at the atmosphere pressure are heated to temperature below melt point of the connected parts. Heating is performed by at least two stages with holding at the first stage for at least three hours at 300°C, and with holding at second stage for at least twenty hours at temperature 1200°C. The developed method excludes expensive operation of the intermediate layer spraying between the connected garnet crystals, and excludes in the finished product this intermediate layer. Besides, the invention ensures manufacturing of the composite optical element with rather uniform contact, minimum losses and strength compared with strength of the material.

EFFECT: easy manufacturing of the composite optical elements by this method ensure making of the assemblages with large aperture.

5 cl, 4 dwg, 4 ex

Description

The invention relates to the field of manufacturing an optical element by combining several crystals of garnets. Such composite optical elements are widely used in lasers and other optical devices. So the use of cladding allows you to suppress spurious generation in active elements (AE) [T. Gonçalvès-Novo, D. Albach, B. Vincent, Μ. Arzakantsyan, and J. - S. Chanteloup, "14 J / 2 Hz Yb3 +: YAG diode pumped solid state laser chain", Optics Express, Vol. 21, No. 1, 855, 2013]. Widely used are disk AEs with an unalloyed part welded to the end to reduce thermal distortions and weaken the effect of amplified spontaneous emission [O.L. Vadimova, I.V. Mukhin, I.I. Kuznetsov, O.V. Palashov, Ε.Α. Perevezentsev, Ε.Α. Khazanov, "Calculation of the gain coefficient in cryogenically cooled Yb: YAG disks at high heat generation rates", Quantum Electronics, 43 (3), 201-206, 2013]. The creation of a multilayer structure with doping variable along the radiation direction allows controlling the distribution of heat inside the AE [M. Azrakantsyan, D. Albach, N. Ananyan, V. Gevorgyan, and J. - C. Chanteloup, "Yb3 +: YAG crystal growth with controlled doping distribution", Optical Materials Express, Vol. 2, No. 1, 20, 2012]. The principle of operation of a laser microchip implies a monolithic composite AE structure [Ying Cheng, Jun Dong, and Yingying Ren, "Enhanced performance of Cr, Yb: YAG microchip laser by bonding Yb: YAG crystal", Optics Express, Vol. 20, Issue 22, pp. 24803-24812 (2012)]. Splicing along the side surface allows you to increase the aperture of the optical element. The creation of a composite optical element based on two media with close but different spectra allows one to increase the gain band of AE. By welding a YAG crystal to the end face of a magneto-optical element (MOE) of TGG, it is possible to significantly reduce the thermally induced distortion of radiation in a Faraday isolator at high average transmitted radiation power due to the high thermal conductivity of YAG.

Optical contact is one of the simplest and most common methods for creating composite optical elements. However, the strength of such a contact, based on weak van der Waals forces, is insufficient for use in high-power laser systems. Recently, new methods for organizing strong contact in composite laser elements have appeared. Among them, methods based on thermal diffusion welding of parts of an optical element can be distinguished [HC Lee, PL Browlie, N.E. Meissner and E.S. Rea, "Diffusion bonded composites of YAG single crystals", SPIE vol. 1624 Laser-Induced Damage in Optical Materials, 1991; Akira Sugiyama, Hiroyasu Fukuyama, Tsuneo Sasuga, Takashi Arisawa, and Hiroshi Takuma, "Direct bonding of Ti: sapphire laser crystals" April 20, 1998, Vol. 37, No. 12, APPLIED OPTICS] and methods using chemical activation of the surfaces of optical component parts [Traggis, Ν., Claussen, Ν., "Epoxy Free Bonding for High Performance Lasers", 11th Annual Directed Energy Symposium Proceedings, Directed Energy Professional Society (2008) ]. Chemical activation is carried out by exposure to the surface of the parts of the optical element with strong alkalis and acids or ion flow, after which the treated parts are placed on the optical contact. It is believed that the strength of the chemically activated contact can be comparable with the strength of the material itself. However, thermal diffusion welding is a more unified process for various materials and, in theory, should provide the most durable contact. The process of thermal diffusion welding is mainly carried out in presses when heated to temperatures above 1500 ° C and a high pressure of hundreds of kg / cm ² [HC Lee, PL Browlie, N.E. Meissner and E.S. Rea, "Diffusion bonded composites of YAG single crystals", SPIE vol. 1624 Laser-Induced Damage in Optical Materials, 1991]. Sometimes, to reduce the requirements for processing welded surfaces, a thin layer of silica glass is sprayed on them [SN Bagayev, A.A. Kaminskii, Yu. L. Kopylov, I.Μ. Kotelyanskii, VB Kravchenko, "Simple method to join YAG ceramics and crystals", Optical Materials 34 (2012) 951-954]. This layer of silica glass is melted at temperatures above 1200 ° C with the formation of a layer wetting both surfaces, which diffuses into the volume of the welded parts upon further heating. Similarly, thermal diffusion welding can be carried out after the chemical activation of the surfaces to be welded, which eliminates the use of a press during annealing [A. Sugiyama, Η. Fukuyama, T. Sasuga, T. Arisawa, and Η. Takuma "Direct bonding of Tksapphire laser crystals", Applied Optics, Vol. 37, No. 12, 20, 1998].

Of the known technical solutions, the closest to the proposed invention is a method for connecting parts of refractory oxides, in particular from garnet crystals, presented in patent RU 2477342 (IPC С30В 33/06, 29/16, 29/22, publ. 03/10/2013) . The prototype method includes polishing the joined surfaces of the parts, their combination and heating. A feature of the prototype method is that one of the joined parts is applied (usually sprayed) with a layer of material (usually quartz glass) forming a solid solution with at least one of the materials of the connected parts. Further heating of the bonded parts is carried out to a temperature above the temperature of formation of a solid solution (until a liquid solution is formed), which allows two parts to be connected through an intermediate layer of the deposited material (liquid solution). As a result of subsequent annealing, a layer with a continuous change in the chemical composition is formed at the boundary of the connected components (parts), and the boundary itself is “washed out”. The method described above was used in the prototype, for example, to connect parts of yttrium-aluminum oxide with the structure of garnet, aluminum oxide, germanium oxide, etc.

The main disadvantage of the prototype method is the presence of an intermediate layer between the connected parts in the finished optical element. This leads to the occurrence of losses of transmitted radiation on this layer, to a deterioration in the thermal and mechanical contact between the parts to be joined in comparison with the traditional diffusion welding method. Another disadvantage of the prototype is the need to use a sufficiently long and expensive spraying process of the said intermediate layer.

The problem to which the present invention is directed, is to develop a method for connecting parts of an optical element from garnet crystals, which allows to exclude the presence of an intermediate layer between connected parts and, accordingly, to exclude from the method of connecting parts an expensive and lengthy spraying process of this intermediate layer.

The technical result in the developed method is achieved in that the developed method for connecting parts of an optical element from garnet crystals, like the prototype method, includes polishing the surfaces of the parts to be joined, their combination and heating.

New in the developed method is that the joined surfaces of the parts are treated with a solution of phosphoric acid in alcohol and placed on an optical contact, after which the connected parts are heated to atmospheric pressure below the melting point of the connected parts, and heating is carried out in at least two stages with exposure in the first stage for at least three hours at a temperature of about 300 ° C and with exposure in the second stage for at least twenty hours at a temperature of about 1200 ° C.

In the first particular case of the implementation of the developed method, it is advisable to select the parts to be joined from alloyed yttrium-aluminum garnet and undoped yttrium-aluminum garnet.

In the second particular case of the implementation of the developed method, it is advisable to select the connected parts from gallium-gadolinium garnet.

In the third particular case of the implementation of the developed method, it is advisable to select the connected parts from gallium-gadolinium garnet and yttrium-aluminum garnet.

In the fourth particular case of the implementation of the developed method, it is advisable to select the parts to be connected from terbium-gallium garnet and yttrium-aluminum garnet.

The invention is illustrated by the following figures.

In FIG. 1 presents a photograph obtained by the claimed method of optical elements.

In FIG. Figure 2 shows the dependence of the measured residual reflection of laser radiation on the boundary between two connected parts in an optical element on the transverse coordinate of this element.

In FIG. Figure 3 shows a photograph of a composite optical element after exposure to destructive laser radiation.

In FIG. 4 presents three micrographs of the boundary between two parts of a composite optical element manufactured by the claimed method.

The developed method for connecting parts of an optical element from garnet crystals in accordance with paragraph 1 of the formula is as follows.

Initially, as in the prototype method, polished parts to be connected from garnet crystals of the manufactured optical element are polished to the required purity class and required flatness. Then, in accordance with the proposed method, the joined surfaces of the parts are treated with a solution of phosphoric acid in alcohol. As a result, after washing the optical parts, a very thin (several nanometers) amorphous (liquid) layer of phosphates forms on their surface. After preliminary joining of the parts when matching at room temperature, this layer provides reliable optical contact between the parts due to the Van der Waals attractive forces.

Parts placed on an optical contact pass through at least two stages of heating (annealing). Annealing of optical parts means not only their smooth heating to the selected temperature, but also subsequent exposure of the optical parts at a given temperature and their smooth cooling to room temperature. At the first stage, with a slight heating to a temperature of about 300 ° C and holding at this temperature for at least three hours, the aforementioned thin amorphous (liquid) layer of phosphates is converted into a solid phase (phosphate glass). Such a very thin layer “sticks together” the connected parts of the optical element, filling in with itself all the polishing irregularities. At the second stage, upon further heating of the manufactured optical element and holding it for at least twenty hours at high temperature (about 1200 degrees Celsius), this thinnest layer (several nanometers) of phosphate glass diffuses completely into the volume of the connected parts, and the materials of the parts themselves also diffuse into each other. Due to the very small (several nanometers) of the initial thickness of the said connecting layer in the composite fiber optic element made from garnet crystals, no intermediate layer, unlike the prototype, remains (see figures 1 and 4), which allows us to solve the problem.

Thus, the developed method for joining parts of an optical element eliminates the costly and time consuming operation of sputtering the intermediate layer between the optical parts and eliminates this intermediate layer in the finished product.

After connecting the parts of the optical element from garnet crystals, the contact quality can be checked in several ways. In each manufactured composite optical element, the reflection coefficient of radiation from the boundary of the connected parts is measured depending on the transverse coordinate. An example of the results of such a measurement for a sample with a diameter of 15 mm is shown in FIG. 2. On the whole, rather weak losses due to reflection from the boundary of the connected parts can be noted. When the reflection value is more than 0.1% (0.001 along the y axis), it is considered that the quality of the connection of the optical elements is unsatisfactory.

Two other measurements are performed only for some samples of composite optical elements from the series, since they are associated with the destruction of the fabricated optical elements. In one of the measurements, the mechanical strength of the contact parts of a composite optical element is studied by heating it with laser radiation. The power of laser radiation is increased until the destruction of the composite optical element. If the destruction of the optical element occurs along the plane of connection of the optical parts, it is believed that the series of manufactured composite optical elements does not satisfy the requirements for contact strength. If the sample of the optical element is destroyed as a whole (see Fig. 3), the series is considered successful. Also, sometimes the homogeneity of the composite optical element compound is checked using an electron microscope. For this, the sample of the optical element is destroyed with a chip across the junction of the two parts of the composite optical element, and micrographs of this compound are made at various scales. If the connection does not contain foreign matter or an intermediate layer, as shown in FIG. 4, a series of manufactured optical elements is considered successful.

Three micrographs of the portion of the boundary between the two parts of the composite optical element in FIG. 4 are presented in increments of 5 μm, 0.1 μV and 10 nm. The microphotographs shown show that there are no extraneous inclusions and an intermediate layer at the interface of the compound.

The following are specific examples of the implementation of the developed method in accordance with paragraphs 2, 3, 4 and 5 of the formula.

Example 1. The method of joining two parts of garnet crystals: a thin plate of yttrium-aluminum garnet doped with ytterbium (Yb: YAG), and a thick plate of undoped yttrium-aluminum garnet (YAG). The thickness of the Yb: YAG plate is 2 mm and the diameter is up to 20 mm. The thickness of the second plate (YAG) is 5 mm, and the diameter is also up to 20 mm. Active optical elements made by joining such plates are used in disk lasers: the undoped part allows one to suppress amplified spontaneous emission, reduce the maximum temperature of the doped part, and prevent the active element from bending. The surfaces of the plates are polished up to the 2nd class of cleanliness, while the flatness is no worse than λ / 8, and the micro roughness is not more than 10Å (for comparison, it can be noted that in [Hoya optics: N.S. Lee, PL Brownlie, N.E. . Meissner, ES Rea, "Diffusion bonded composites of YAG single crystals", SPIE Vol. 1924 Laser-Induced Damage in Optical Materials. 1991] for the implementation of thermal diffusion welding, crystal surfaces were prepared with a plane no worse than λ / 20 and microroughness on level 1Å). After polishing, the joined sides of the plates are washed with ethanol and placed in a 30% phosphoric acid solution for 30 minutes. Then the surfaces are washed with alcohol again and the plates are placed on the optical contact. This treatment allows you to remove the oxide layer from the surface of the parts from garnet crystals and forms a very thin layer (several nanometers) of phosphates and phosphoric acid residues. The parts placed on the optical contact pass through several stages of heating (annealing). The heating and cooling rates at each of the annealing stages do not exceed 2 degrees per minute. At the first stage, heating (annealing) is carried out at a temperature of about 300 ° C for 3 hours. As a result of this effect, moisture residues are evaporated from the contact area, the amorphous (liquid) layer of phosphates is transformed into a “glazed” phase, due to which a primary contact is formed that can withstand large temperature gradients. This makes it possible to conduct subsequent heating to higher temperatures without the use of a press. In the second stage of heating, the crystals are annealed for two hours at a temperature of 800 ° C. Presumably, at this stage, phosphates are completely converted to phosphate glass. The final stage of annealing lasts 40 hours. The manufactured crystalline optical element is heated for 2 hours at a rate of 2 degrees per minute in a conventional muffle furnace with an air atmosphere to a temperature of 1200 ° C and kept at this temperature for 20 hours. After that, the temperature of the crystalline optical element is gradually reduced at the same speed. Moreover, as established by the authors, the thinnest layer (several nanometers) of phosphate glass diffuses into the volume of the parts to be welded, and the diffusion welding process is implemented. Photographs of the obtained samples of composite optical elements are presented in FIG. 1. With increasing temperature, the time of the final annealing stage can be significantly reduced. For other types of grenades, the welding technology is identical.

Below are other examples of the manufacture of composite optical elements from garnet crystals in accordance with other special cases of the implementation of the proposed method, as claimed in paragraphs 3-5 of the formula.

Example 2. The method of connecting two parts from the same crystals of gallium-gadolinium garnet. Pre-treatment of parts, as well as the method of joining parts are close to the processing described above in example 1. However, a distinctive feature of this joining method is that the melting point of gallium-gadolinium garnet is lower than the melting temperature of yttrium-aluminum garnet. This allows you to reduce the duration of the last stage of annealing to 10 hours at a temperature of 1200 ° C.

Example 3. The method of joining two parts from different crystals of garnets: gallium-gadolinium garnet and yttrium-aluminum garnet. Preliminary processing of parts, as well as the method of joining parts, are close to the processing described above in Example 1. However, the distinctive feature is that the thermal expansions of these two media are different. This leads to the need to reduce temperature gradients and heating rate of manufactured samples of optical elements. Compared with example 1, the heating and cooling rate of the samples is reduced by half to 1 degree per minute.

Example 4. The method of joining two parts from different crystals of garnets: terbium-gallium garnet and yttrium-aluminum garnet. Preliminary processing of parts, as well as a method of connecting parts are close to the processing described above in example 1. However, a distinctive feature of this method is that the thermal expansion of these two environments is different. This leads to the need to reduce the temperature gradients and the heating rate of the parts in the same way as in example 3. Compared to example 1, the heating and cooling rate of the parts is reduced by half to 1 degree per minute.

Thus, the proposed method for connecting parts of an optical element from garnet crystals makes it possible to produce a composite optical element with a very uniform contact, minimal losses and with a strength comparable to the strength of the material itself. The simplicity of manufacturing composite optical elements in this way allows you to create assemblies with a large aperture, demonstrated the ability to connect the developed method of plates from Yb: YAG and YAG 40 mm aperture.

Claims (5)

1. A method of connecting parts of an optical element from garnet crystals, comprising polishing the surfaces of the parts to be joined, combining them and heating, characterized in that the surfaces of the parts to be joined are treated with a solution of phosphoric acid in alcohol and placed on the optical contact, after which the connected parts are heated to atmospheric pressure to temperatures below the melting temperature of the parts to be joined, and heating is carried out in at least two stages with exposure at the first stage for at least three hours at a temperature e about 300 ° C and exposure in the second step is not less than twenty hours at a temperature of about 1200 ° C. 2. The method according to p. 1, characterized in that the connected parts are made of alloyed yttrium-aluminum garnet and undoped yttrium-aluminum garnet. 3. The method according to p. 1, characterized in that the connected parts are made of gallium-gadolinium garnet. 4. The method according to p. 1, characterized in that the connected parts are made of gallium-gadolinium garnet and yttrium-aluminum garnet. 5. The method according to p. 1, characterized in that the connected parts are made of terbium-gallium garnet and yttrium-aluminum garnet.

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