Simultaneous measurement of thermal lens and temperature map in ytterbium-doped fluoride crystals

We report on the simultaneous characterization of temperature map and thermal lensing in Yb:CaF2 and Yb:SrF2 crystals under high-power pumping with/without laser operation. This in situ measurement would allow proper designs of high-power cavities. ©2008 Optical Society of America OCIS codes: 140.5680 Rare earth, solid state laser, 140.3480 Laser, diode-pumped 1Introduction Interest in Yb-doped materials is still growing, owing to the increasing availability of high power InGaAs laser diodes emitting around 980nm. Ytterbium has very attractive properties including a weak quantum defect, and no problem of concentration quenching. Fluoride crystals, having the cubic Calcium fluoride structure, are well-known to be transparent in a large wavelength region (from IR to VUV) and mix the advantages of the oxide crystals and glass. In particular, they feature broad absorption and emission bands. Fluoride ytterbium-doped crystals are therefore very promising for the development of solid-state lasers and amplifiers[1,2]. Here, we present the experimental assessment of both the temperature map and the thermal lens (TL) properties of Yb:CaF2 and Yb:SrF2 under high power pumping, with or without laser operation. 2Experimental set up The experimental setup is shown in Fig.1. The 5 mm 3-at % Yb:CaF2 or 3-at % Yb:SrF2 is directly pumped with a 120 W 400 μm core fiber-coupled laser diode (LIMO) emitting at 980 nm. The probe beam, generated by a fibercoupled LED emitting at 660 nm, is collimated and superposed with the pump beam. The crystal is imaged through a magnifying system composed of two lenses to a Shack-Hartmann wave-front sensor (HASO64, Imagine Optics). A bandpass filter at 660 nm is placed in front of the analyzer to remove the parasitic beams. With pump off, a reference wavefront is recorded to cancel all static aberrations from the optical elements and the cold crystal. The reference is subtracted from the measured wavefront when the pump is on. Simultaneously, an 8μm-12μm high resolution thermal camera with a germanium objective is used to measure the temperature map of one facet of the crystal. Fig. 1. (a) Experimental setup and (b) zoom of the laser cavity design. PM = plane mirror and IR = interferential filter. ha l-0 08 08 70 9, v er si on 1 6 Ap r 2 01 3 Author manuscript, published in "Advanced Solid-State Photonics, Nara : Japan (2008)"