Ho:YLF is an attractive laser material for 2 µm high energy sources since it has a much longer upper laser level lifetime (~14 ms) and higher emission cross section than Ho:YAG. In addition, the very weak thermal lens on the polarisation helps to deliver diffraction limited beams even under intense end-pumping. However, Ho:YLF has a somewhat stronger quasi-three-level nature, which implies that in order to reach transparency at the 2065 nm line, 22% of the Ho ions need to be pumped into the upper laser level (at room temperature), but it already reaches transparency at the 1940 nm pump wavelength with only 56% of the Ho ions in the upper laser level. In addition, the pump absorption cross section at 1940 nm is relatively low and strongly polarised.
Reference:
Bollig, C, Strauss, HJ and Esser, MJD. 2009. Compact fibre-laser-pumped Ho:YLF oscillator-amplifier system. Conference on Lasers and Electro-Optics (CLEO) and Quantum Electronics and Laser Science Conference (QELS), Munich, Germany, 14-19 June 2009, pp 1
Bollig, C., Strauss, H. J., Esser, M., Koen, W., Schellhorn, M., Preussler, D., ... Botha, L. (2009). Compact fibre-laser-pumped Ho:YLF oscillator-amplifier system. IEEE. http://hdl.handle.net/10204/3663
Bollig, C, Hencharl J Strauss, MJD Esser, W Koen, M Schellhorn, D Preussler, K Nyangaza, Cobus Jacobs, EH Bernhardi, and LR Botha. "Compact fibre-laser-pumped Ho:YLF oscillator-amplifier system." (2009): http://hdl.handle.net/10204/3663
Bollig C, Strauss HJ, Esser M, Koen W, Schellhorn M, Preussler D, et al, Compact fibre-laser-pumped Ho:YLF oscillator-amplifier system; IEEE; 2009. http://hdl.handle.net/10204/3663 .
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