Highly transparent Yb doped YAG single crystals with diameter more than 90mm have been grown by Bagdasarov method also known as Horizontal Direct Crystallization.
© 2012 OSA
Trivalent Ytterbium ion (Yb3+) doped crystals, characterized by lower thermal load and higher capacity for laser energy storage, when compared with Nd3+ doped matrices, have attracted much attention and are among the most promising materials for diode-pumped solid state lasers. Yttrium aluminum garnet Y3Al5O12 (YAG) is a very interesting host for Yb3+ in high average power DPSSL applications due to its high thermal conductivity, small quantum defect, absence of excited state absorption and up-conversion losses, a relatively long radiative lifetime of the upper laser level (optimizing pump laser diodes capital cost investment). Moreover, YAG is relatively easy to grow over a wide range of doping concentrations and through several distinct techniques, such as Czochralski, Temperature Gradient and Bagdasarov techniques (also called Horizontal Directed Crystallization). Requirements in large diameter of high quality laser crystals are indeed extremely relevant for laser programs like the European High Power laser Energy Research facility (HiPER) dedicated to demonstrate the feasibility of laser driven fusion as a future energy source . Engineering high energy / high average power laser system like the HiPER laser program requires facing challenges not previously addressed by other multiple 100kJ-class laser systems like the National Ignition Facility (NIF) or Laser Mega Joule (LMJ). Indeed, HiPER’s goal being to demonstrate Inertial Fusion Energy (IFE) production through Shock Ignition in a mode of operation similar to a reactor of a real IFE power plant. Such an operation requires a minimum repetition rate of ~10 Hz. Considering the cost of electricity  analysis and maturity of the available technology, it is very likely that such multiple ~10 kJ/~10 kW beam line facility will rely on diode pumped lasers. In addition to common issues with LMJ/NIF facilities, a high repetition rate operation brings thermal management on the forefront of key laser issues . Current laser gain media used on such large laser facilities rely mainly on glass which cannot be used for HiPER. Therefore large size crystals or ceramics are required. This work is dedicated to the description of the first 90mm diameter crystal. Investigations were performed to verify doping distribution, optical and crystalline quality. These studies were performed at the crystal growth department of Lazerayin Tekchnika CSC, Yerevan, Armenia and at the Laboratoire d'Utilisation des Lasers Intenses (LULI), Ecole Polytechnique, Palaiseau, France in the framework of the Lucia program .
2. Yb:YAG crystallization by Bagdasarov method
Yb3+:YAG single crystals are mainly grown by Radio frequency-heated Czochralski (Cz) method in an inert nitrogen or an oxygenized atmosphere (typically N2 + 2Vol%O2). However, there are some disadvantages using the Cz method to grow Yb3+:YAG crystals. Defects such as cores and light-scattering particles easily appear in Cz-grown Yb3+:YAG crystals. Large weight loss of iridium crucible frequently occurs during Czochralski growth process, especially in oxygenized atmosphere [5,6]. Another method for growing Yb3+ doped YAG crystals is the so-called temperature gradient technique (TGT). This method is widely used for large YAG growth. The largest YAG crystals obtained by this method are reported by G.Zhao and etc [5,6]. and have the size of 75 and 76mm. Another critical negative factor to be mentioned is the presence of inhomogeneous doping distribution in Cz and TGT grown crystals which is discussed in . A method which we used to obtain large size crystals was proposed and developed by Bagdassarov and called Horizontal Direct Crystallization (HDC) . Sapfir-2MG furnaces (greatly modified from the ones described in ) at the crystal growth department of Lazerayin Tekchnika CSC, Yerevan, Armenia were used to grow large size and low doped Yb3+:YAG single crystals. These furnaces offer the possibility to control growth parameters, like crucible speed and heater temperature, during the crystallization phase itself. We can therefore control the desired doping distribution throughout the whole growth process, i.e. during the preparatory phase , and later by controlling several key parameters during the crystallization phase itself. Such flexibility gives us the ability also to obtain a controlled variably doped boule [9,10]. Description of the principle of operation of the furnace designed by Bagdasarov can be found in [8,10]. We used boat shape Molybdenum crucibles, where the crystal seed is located at its bow. The width of the crucible is limited by the volume of the heater, but the length can vary within a wide range. Since the regular width of boat shape crucibles is not more than 70mm, for this task, we especially prepared large crucibles with more than 100mm in width. This requested also the modification of the heaters. Figure 1 shows the regular and “wide” crucibles.
The depth of the crucibles used for these experiments were limited to 40 mm. The YAG seed placed at the crucible bow (Fig. 2 ) is oriented along the requested preferential crystallographic axis, in our case the  direction perpendicular to the large face of the disk shown on Fig. 2.
High purity oxide powders for Y2O3 (X99.999%), Yb2O3 (X99.999%) and Al2O3 (X99.95%) are weighed out in appropriate mole ratios for 0.4 at.% concentration and placed in the crucible. Physical and chemical processes taking place during the Horizontal Direct Crystallization process (such as heat and mass transfer) are described in [8,11].
3. Absorption and quality measurements
3.1 Distribution of Yb ions
From the grown boule, a 92 mm crystal with the thickness of 8.1mm was extracted (Fig. 2 left). The yellow color is due to the presence of Ce ions in the boule. The furnace used to obtain this crystal was previously used for Cerium and Neodymium co-doped YAG growth which explains the presence of Cerium ions. The absorption spectrum in the central part of the crystal has been recorded with a Cary 500 Spectrometer (Fig. 3 right). Using these spectral data and Lambert’s law T = exp(-Nsz), where T is the transmission, N the number of absorbing atoms, the absorption cross-section for 1 at.% and z the thickness, we obtain a doping concentration equal to 0.41 ± 0.02at % of Yb3+.
On the absorption spectrum, we can easily see Yb3+ as well as Ce3+ characteristic absorption peaks. To define the homogeneity of Yb3+ ions distribution, careful measurements were performed. Transmission spectrum near Yb3+ absorption region was registered for 6 different points (every 60° of rotation around cylindrical axis) 5mm away from the periphery of the crystal and concentration was defined for each point with the method described above. Figure 4 shows the difference in absorption spectrum for these points near 1030nm wavelength and calculated doping concentrations,
The difference between maximum and minimum values is about 0.04 at%, i.e. approximately 10% variation around the nominal value of 0.4 at%. Considering that the current requirements on LUCIA laser system is 10% of doping variation within the pump region which is only central part of the crystal, we can consider that the obtained crystal satisfies this homogeneity criteria. Also Energy Dispersive X-ray spectroscopy (EDX) measurements were performed in HTSCLab (High Temperature Superconductivity) IPR NAN, Armenia in order to verify element composition. No presence of impurities were observed (the Ce concentration is too low). Trivalent Yb ions concentration value obtained was 0.39 ± 0.07at% which is in accordance with previously obtained values. X-Ray rocking curve measurements reveal a well defined single crystal structure.
3.2 Birefringence behavior
Birefringence behavior of the extracted crystal was verified by observation under crossed polarizers for visible region (Fig. 5 ).
As it can be observed, the stresses are mainly concentrated near the periphery of the crystal which is very close to the molybdenum crucible and crystal interface. Further Verification was performed in infrared region in order to describe quantitatively the birefringence. A laser beam was sent through the crystal and the analyzer, then the intensity of transmitted light was registered for different rotation angles of the analyzer. Three cases are illustrated on Fig. 6 : laser light free propagation (without crystal), laser light travels through the center of the crystal (free of stresses part) and near the periphery (the most stressed point). With the help of this simple experiment it was not possible to measure any birefringence in infrared region even though in the visible spectrum some strains were clearly observed. This is due to the fact that stress induced depolarization can be interpreted as a local wave plate with a phase difference Δφ. The transmission in Fig. 5 is registered with a camera in auto-exposure acquisition mode, so quantitative values cannot be deduced out of this picture. Δφ is inversely proportional to the measurement wavelength. And, since the depolarization losses scales with sin2(Δφ), the observed transmission drastically decreases when using longer wavelengths, like 1064nm for the second experiment . Consequently measurements in the infrared could not reveal any measurable difference.
Microscopy observation has shown no bubbles or scattering particles, whereas no scattering of He-Ne laser beam travelling through the sample could be detected.
4. Conclusion and outlook
To our knowledge the largest Yb3+:YAG single crystal have been successfully obtained with Bagdasarov technique. The doping was estimated and quality measurements were performed illustrating the laser grade quality of obtained crystal. The method can now be considered as established and several boules with 100x100x40mm sizes were successfully produced. Three high quality YAG crystals with diameter of 77mm and thickness of 10mm (Fig. 7 ) were extracted out of these boules and will be used in LUCIA cryogenic amplifier in near future.
The authors gratefully acknowledge the support of the MŠMT, Ministry of Education, Youth and Sports of the Czech Republic, the Délégation Générale à l’Armement of the Ministry of Defense of France and Ministry of Defence of Armenia in supporting this work.
References and links
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