Efficient single-pass resonantly-pumped Ho:YAG laser

Abstract

ABSTRACT A simple scheme for efficient generation of two micron laser radiation is reported. Using a thulium fibre laser to pump a Q-switched Ho:YAG laser 31.7 W of average output power is achieved with an M 2 of 1.5 (an optical-to-optical conversion efficiency of 67% in terms of absorbed pump power) at a wavelength of 2.1 µm. A single-pass pump geometry is used, eliminating the risk of feedback of unabsorbed light into the fibre laser. Keywords: Thulium fibre laser, Ho:YAG solid-state laser 1. INTRODUCTION Efficient solid-state lasers in the eye-safe two micron regime are required for a number of applications, including remote sensing, coherent LIDAR (LIght Detection and Ranging), medical applications, and as a pump source for generating mid-IR laser light via nonlinear frequency conversion. Holmium lasers operating at 2.1 µ m are commonly preferred over thulium lasers because of their longer laser wavelength, resulting in higher atmospheric transmission and lower absorption in certain nonlinear crystals (e.g. zinc germanium phosphide (ZGP) for mid-IR light generation). Moreover, since the holmium upper level typically has a longer lifetime than that of thulium, in addition to a stronger emission cross section, holmium-doped laser materials have a saturation intensity that is typically an order of magnitude lower, allowing higher peak power generation when operating the laser in Q-switched mode. However, as commercially available laser diodes suitable for direct pumping of holmium ions have much lower brightness and efficiency in comparison with other standard diode-laser wavelengths, alternate routes, such as using thulium as a sensitizer, or thulium laser resonant (in-band) pumping of holmium, are considered to be the only practical routes to achieve high laser efficiency with high average powers. Holmium-based lasers ha ve been demonstrated to be very flexible sources of two micron radiation, operating efficiently in the cw, Q-switched and mode-locked regimes. The highest reported powers and efficiencies have been demonstrated using a thulium fibre laser to resonantly pump the holmium laser in an end-pumped geometry. 42 W from a Ho:YAG laser pumped by 68 W of thulium fibre laser output, at an optical conversion efficiency of 62%, has recently been reported by Lippert et al.