A major development to improve high power laser chains, is to achieve both higher yields and better tunability. For future high power lasers that operates by diode-pumping at high rate, several materials likely to meet this double requirement (single crystals, glass and ceramics doped) are studied.
Among these materials, calcium fluoride (CaF2) doped by Yb3+ cristals, like those manufactured and studied by the "Materials and Instrumentation Laser (MIL)" group of the CIMAP Laboratory, are one of the most serious candidates. Under its single crystal form and doped with trivalent ytterbium ions (Yb3+), it has remarkable properties: broad absorption and emission bands, high radiative life time, good transition cross-sections and excellent thermo-mechanical properties.
These characteristics combine in one single material the benefits of glasses and of some single crystals. They enable to obtain or to amplify short pulses and / or to have a large tunability in wavelength like in glassy materials, with very good yields, and high damage thresholds, comparable to those of crystals. Finally, a very important point for a large-scale development, it has been widely demonstrated for the purpose of UV photolithography that it is possible to produce non-doped crystals of great size and excellent quality. With an adaptation of growing conditions, it is quite possible to manufacture crystals doped with 20, even 30 cm in diameter for amplifiers of high power laser chains.
In a recent article published in "Laser Physics", the MIL team shows that it is possible to further improve the performance of these crystals by lowering their temperature. It is indeed becoming more common to lower the temperature of laser crystals of high power laser chains (as for example for Ti-sapphire and YAG: Yb lasers). This allows to both strengthen the crystal against the flux damages, by a better evacuation of the heat, and lower the thresholds by increasing the absorption of pump radiation, this also increasing the laser gain. For 3-level systems, like Yb3+ doped crystals, higher improvements can be expected in low temperature operation as these systems becomes like a 4-level one, whose performance is much better. It is shown that for CaF2: Yb3+, cooling to the liquid nitrogen temperature allows reducing the threshold of laser operation and increasing its efficiency by more than 15% (as compared to room temperature), and achieving a power yield (luminous power emitted / absorbed light power) of more than 80%.
Cylindrical crystals (∅ 15 mm x 20 mm) polished with parallel faces prepared in the CIMAP for laser amplification tests on the POLARIS (European Project petawatt laser (1015 W), Univ. Jena, Germany).
For these results, the CIMAP-MIL team is sought today by several research groups including the "Laser Particle Acceleration Group" of FZ Dresden-Rossendorf for the study and development of crystals of several centimeters in diameter, by an evolution of the technique developed in Caen. These crystals will first be tested in place of fluorophosphate glasses used in the laser POLARIS. If successful, it is envisaged to develop a petawatt system (1015 W) based on complete CaF2: Yb3+.
Yb3+ doped (Ca,Sr,Ba)F2 for high power laser applications,
J.L. Doualan, P. Camy, A. Benayad, V. Ménard, R. Moncorgé, J. Boudeile, F. Druon, F. Balembois, P. George,
Laser Physics® (international journal), sous presse.
CW and tunable laser operation of Yb3+ doped CaF2,
V. Petit, J.L. Doualan, P. Camy, V. Ménard and R. Moncorgé, Appl. Phys. B 78 (2004) 681.
High power diode-pumped Yb:CaF2 femtosecond laser,
A. Lucca, G. Debourg, M. Jacquemet, F. Druon, F. Balembois, P. Georges, P. Camy, J. L. Doualan, and R. Moncorgé , Opt. Lett. 29 (2004) 2767.
CW and tunable laser operation of Yb3+ in Nd:Yb:CaF2,
V. Petit, P. Camy, J.L. Doualan, R. Moncorgé, Appl. Phys. Lett. 88 (2006) 051111.
Comparative spectroscopic and laser properties of Yb3+-doped CaF2, SrF2 and BaF2 single crystals
P. Camy, J.L. Doualan, A. Benayad, M. von Edlinger, V. Ménard and R. Moncorgé, Appl. Phys. B 89 (2007) 539.
Thermal behaviour of ytterbium-doped fluorite crystals under high power pumping,
J. Boudeile, J. Didierjean, P. Camy, J. L. Doualan, A. Benayad, V. Ménard, R. Moncorgé, F. Druon, F. Balembois, and P. Georges, Opt. Expr. 16 (2008) 10098.
Spectroscopy of Yb3+ :CaF2: from isolated centers to clusters,
V. Petit, P. Camy, J-L. Doualan, X. Portier, and R. Moncorgé, Phys. Rev. B 78(8) (2008) 085131.
Terawatt diode-pumped Yb:CaF2 laser,
M. Siebold, M. Hornung, R. Boedefeld, S. Podleska, S. Klingebiel, C. Wandt, F. Krausz, S. Karsch, R. Uecker, A. Jochmann, J. Hein, and M. C. Kaluza , Opt. Lett. 33(23) (2008) 2770,
Femtosecond pulses reach terawatt power via Yb:CaF2,
Valerie C. Coffey, Laser Focus World, 44 dec. 2008.