A new method of thermal diffusion bonding of different garnet crystals is proposed. Its main advantage is simplicity and low cost: not very stringent requirements to the quality of surface, muffle furnace without press is sufficient. The proposed method enables fabricating composites of YAG, Yb:YAG, Yb:GGG, and TGG crystals with an aperture up to 20 mm and optical contact whose mechanical strength is comparable with that of monocrystals and reflection coefficient at the boundary is close to the Fresnel one.
© 2014 Optical Society of America
Fabrication of composite laser elements is a promising way to improve their characteristics. For example, cladding allows suppressing parasitic generation in active elements (AE) . A widely used option is a disk AE with an undoped cap bonded to the end to reduce thermal distortions and the effect of amplified spontaneous emission (ASE) because of most of spontaneous radiation leave the pumping area without reflections from the crystal surfaces . A multilayer structure with doping variable along the direction of radiation enables controlling heat distribution inside the AE . The principle of microchip laser operation implies a composite AE structure . Bonding along the side surface permits increasing the optical element aperture. Creation of a composite consisting of two conformal media with close but different spectra allows increasing AE amplification band. By bonding a YAG crystal to the end of a TGG magneto-optical element (MOE) it is possible to reduce substantially thermally induced radiation distortions in the Faraday isolator at high average power of laser radiation due to high thermal conductivity of YAG.
Optical contact bonding is one of the simplest and most commonly used methods of producing composite elements. However, the strength of such a contact based on weak Van der Waals forces is insufficient for application in high-power laser systems. New techniques of providing strong contact in composite laser systems were developed in the recent years, including thermal diffusion bonding of optical elements [5,6] and surface chemical activation of optical elements . Chemical activation is the process whereby an ion flow or strong alkalis and acids act on sample surfaces after which they are joined together by optical contact. It is believed that the strength of the chemically activated contact may be comparable with the strength of the material . Thermal diffusion bonding, however, is a more unified process for various materials. Theoretically, it should provide maximum strong contact. Bonding is accomplished primarily in presses at temperatures exceeding 1500°С . Sometimes, to reduce requirements to bonded surface processing, a thin layer of quartz glass is sputtered on the surfaces . This layer melts at temperatures over 1200°С to form a layer wetting both surfaces that diffuses on further heating into the bulk of the bonded samples. Thermal diffusion bonding may also be performed after chemical activation. In this case, no press is needed at annealing .
In this paper we propose a new method of thermal diffusion bonding tested on a number of garnets. The method is easy to realize. It suffices to use a conventional muffle furnace with heating in an air atmosphere without compression at high temperatures. Details of the bonding technique are described in Sec. 2. The first tests of the bonded samples quality, materials for which the developed technology was used, as well as examples of their application are presented in Sec. 3. In the concluding section we consider the results obtained and plans for the future.
2. Description of bonding technique
The main goal of the research was thermal diffusion bonding of two yttrium aluminum garnet (YAG) plates. One of them was a ytterbium doped (Yb:YAG) plate with a thickness of 2 mm and a diameter from 10 to 20 mm. The second plate was 5 mm thick and also had a diameter from 10 to 20 mm (Fig. 1, а). Such AEs are used in disk lasers: an undoped plate permits to suppress amplified spontaneous emission, reduce maximum temperature of a doped plate, and prevent AE bending. The surfaces of the plates were polished up to 20-10 scratch-dig quality, ensuring flatness not worse than λ/4 and roughness not more than 10 А rms. Note for comparison that in the work  the crystal surfaces prepared for thermal diffusion bonding had flatness not worse than λ/20 and roughness of about 1 А rms. After polishing, the sides to be bonded were washed by ethyl alcohol and placed into a 30% solution of orthophosphoric acid for 30 min. Then, the surfaces were washed by alcohol and the plates were joined together to attain optical contact. As a result of such chemical processing an oxide layer is removed from the crystal surface and a very thin (several nanometers) layer consisting of phosphates and orthophosphoric acid residue is formed.
After chemical processing by orthophosphoric acid the samples underwent several stages of annealing. At the first stage, annealing was done at the temperature of ~300°С for 3 hours, with the samples pressed to each other by metal plates with a force of ~25 kg/cm2. As a result, the remaining moisture was evaporated from the bonded layer, the orthophosphoric acid transformed to fused phosphoric acid, and a primary contact withstanding high temperature gradient was formed. That enabled the next heating to higher temperatures without press. At the second stage, the crystals were annealed for two hours at the temperature of 800°С. At this stage the phosphoric acid is, presumably, transformed to phosphate glass. The final stage of annealing took 40 hours. The crystal was soaked in a conventional muffle furnace with air atmosphere at a temperature of 1200°C for 20 hours. During that period, the layer of phosphate glass diffused into the bulk of the bonded samples and the process of diffusion bonding was realized. Photographs of the obtained samples are presented in Fig. 1b. Note that the temperature of 1200°С used in our study is quite far from the melting temperature of YAG crystals (~1900°С). A similar heating regime was implemented in the work . By increasing the temperature it is possible to reduce the time of the final annealing stage dramatically. The bonding technique for the other garnets that will be considered in Sec. 3 is the same.
3. Characterization of the fabricated samples
3.1 Characterization of Yb:YAG/YAG composite elements
The quality of the contact in the fabricated samples was investigated by several methods. First, the coefficient of reflection from the bonded layer was measured after each annealing stage as a function of the transverse coordinate (Fig. 2). Upon the whole, losses on reflection from the layer were rather weak. The reflection reduced at each next stage of heating, and the distribution along the coordinate became increasingly more uniform.
In another series of experiments we compared mechanical strength of the contact at different stages of bonding by heating the Yb:YAG/YAG sandwich by diode pump pulses with a duration of 1 s at a wavelength of 940 nm. The crystal diameter was 15 mm, the thickness of the doped part was 1 mm, the thickness of the pure YAG was 5 mm, and the pump beam spot diameter was 6 mm. For increasing thermal load on the contact, the crystal was cooled in ambient air without additional heat sink. After the first stage of annealing (T~300°С) the sandwich was destroyed at the peak pump power (160 ± 20) W and after the second annealing stage (T ~800°С), at (170 ± 10) W. In both cases the sample simply split into two initial constituent parts. After annealing at a temperature of ~1200°С the sandwich was destroyed at the peak pump power (430 ± 10) W. The principal distinguishing feature is that in the latter case the crystal split into parts without a single chip along the bonding border (Fig. 3), which indicates that the mechanical strength of the obtained contact was higher than that of the crystal.
Chip sites across the bonding border were examined by means of an electron microscope with a resolution up to 1 nm. A few typical photos with different magnification are presented in Fig. 4. No cavities or inhomogeneities are observed at the border. There is no intermediate phosphate layer either, which means that it has diffused into the bulk of the bonded parts.
In addition to the experiments described above, we tested the obtained samples as AEs in different laser setups. Based on the fabricated composite Yb:YAG/YAG active elements we are currently developing a cryogenic pulse-periodic laser system. We have attained the energy over 120 mJ with 500 Hz pulse repetition rate at the output of the main amplifier . Thanks to effective suppression of amplified spontaneous emission, optical efficiency surpassing 30%, which is high for pulse lasers, has been demonstrated . AEs made of Yb:YAG/YAG are also used in cw disk lasers. The composite AE structure enabled achieving a rather high optical efficiency of 55% retaining a good quality of laser beam .
3.2 Characterization of composite elements made of other garnets
The possibility of using the described technique for bonding different optical media greatly expands its field of application. For example, thermal diffusion bonding of Yb:YAG and Yb:GGG crystals permits creating combined active elements with a wider amplification spectrum . These crystals have close thermo-optical parameters (including thermal expansion coefficients), but the melting temperature of Yb:GGG is a little lower than that of Yb:YAG. Several samples of such compound AEs were fabricated employing the developed technique. A 1 mm thick Yb:GGG disk was bonded to samples of composite Yb:YAG/YAG active elements from the side of the Yb:YAG crystal. In this way we created active elements possessing a relatively wide amplification band with suppression of the amplified spontaneous emission due to anti-ASE cup (Fig. 5).
The developed technique was also a useful tool for bonding YAG and TGG crystals with different thermal expansion coefficients (8.10−6K−1 and 9.7.10−6K−1, respectively) having an aperture of 10 mm. High thermal conductivity and absence of absorption in the YAG crystal in such a composite MOE allows providing heat sink through the YAG crystal and reducing thermally induced distortions in the TGG crystal broadly employed in Faraday isolators. For instance, in a cryogenic Faraday isolator at a radiation power >1 kW the depolarization was reduced by a factor of ~1.5 and the average MOE temperature by 40%. This looks promising for attaining the isolation degree of 25 dB, if a laser radiation of ~10 kW is used instead of 3.5 kW .
4. Conclusion and outlook
We have described a new method of thermal diffusion bonding for fabrication of composite laser elements. To provide a good contact between two bonded surfaces a very thin intermediate layer is used, similarly to the works . However, unlike the other works, this orthophosphoric acid layer is liquid at the first stage, and then it changes to a vitreous solid, ensuring a simple and strong contact at each stage of bonding. Hence, high quality polishing is not necessary; an ordinary muffle furnace with a temperature of 1200°С is sufficient for bonding, and there is no need to use complex expensive mechanisms to compress the bonded samples at high temperature. At the final stage of thermal diffusion bonding this intermediate layer diffuses into the bonded samples and with further heating it may fully diffuse from the sample’s bulk to the environmental space.
To date the described technology is employed for bonding YAG, GGG, and TGG crystals, including doped ones, with a diameter up to 20 mm in different combinations. It is worth mentioning that we have fabricated several composite AEs consisting of three disks: Yb:YAG, YAG, and Yb:GGG, each having two optical-quality contacts. The performed research has demonstrated that thermal diffusion bonding ensures a uniform (over the area) contact with the coefficient of reflection from the layer slightly differing from the Fresnel one (< 0.03%) and with mechanical strength comparable with that of monocrystals.
Absence of the need of strong compression at high temperature in the course of bonding is promising for successful bonding of larger aperture elements. We are currently preparing YAG and Yb:YAG crystals with a diameter of 40 mm (maximum size of YAG crystals may amount to 9 cm). We are also planning to expand the number of optical media for which the developed technique of thermal diffusion bonding may be applied.
This work was funded by a Mega-grant of Government of the Russian Federation No. 14.B25.31.0024, and RFBR grant No.13-02-97119.
References and links
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