Laser action along and near the optic axis of a holmium-doped KY(WO 4 ) 2 crystal
Résumé
We demonstrate the first (to our knowledge) quasi-three-level conical refraction laser operating at 2 μm, with 1.6 W of output power at 2074 nm, using a holmium-doped KY
WO 4 2 crystal. A maximum slope efficiency of 52% has been achieved, along the optic axis with respect to the absorbed pump power. Furthermore, lasing operation around the optic axis has been performed. In this case, a maximum output power of 3 W has been reached, with a slope efficiency better than 70%, which are the best performances ever reported on this material. Studies of conical refraction (CR) have recently revived. When a Gaussian light beam propagates along the optic axis of a biaxial crystal, under certain conditions a two ring-shaped beam can be observed, at the exit face of the crystal. Several publications describe this effect, theo-retically and experimentally [1–4]. Recent availability of long biaxial crystals, with good optical quality, facili-tates CR studies. Therefore, several applications of this effect have recently been investigated, such as optical tweezers [5], particle trapping [6], beam shaping [7–11], and microscopy [12]. There are also some publications about laser operation along an optic axis of a biaxial crys-tal, the first publication being [13]. Using a short (3 mm long) biaxial crystal of a CR-cut ytterbium-doped KGd
WO 4 2 (KGW), the output polarization of their laser was modified, by translating the output coupler mirror. The researchers obtained a slope efficiency of 60% along the CR axis, which is similar to the value obtained for a classical N g -cut orientation. However, the threshold was 2× higher for CR orientation. A few years later, [14,15] presented a four-level conical refraction laser, made with neodymium-doped KGW. The researchers obtained both a Gaussian and CR output beam. They reported a slope efficiency close to the quantum defect limit [14], and no influence of any thermal effect over the measurement range (up to 3 W at 1067 nm) was observed. The thermo-optic properties of neodymium-doped KGW were studied in [16]. The researchers show the anisotropy of the thermo-optic properties, along this propagation direc-tion with a short crystal (1 mm long). However, the effect of CR inside the cavity, changing the light distribution in-side the crystal, is still not well-known. In this Letter, we report the first experimental results (to our knowledge) of a holmium-doped KY
WO 4 2 (KYW) CR laser, showing laser action along the CR axis. Measurements near the optic axis have also been per-formed, in order to be able to compare CR and Gaussian beam intracavity laser performance.
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