Diode-bar-pumped planar waveguide lasers

Diode-bar lasers have been used with great success to pump high power solid state lasers in numerous different geometries; nevertheless it remains a challenge to use the highly asymmetric diode-bar output efficiently. We describe an approach to this problem in which the solid-state gain medium is fabricated as a planar waveguide, into which the high-aspect-ratio emission from the diode-bar can be coupled using a simple optical system. This is a geometry which should in principle handle heat dissipation well. We have investigated the performance of a planar Nd-doped YAG waveguide pumped by a 20-W 807-nm diode-bar. A high-quality, low propagation loss guide is essential for such a device; in this experiment we used a liquid-phase-epitaxy-grown structure. The 5-mm long waveguide had an 80-μm-thick 1.5-at.% Nd:YAG core sandwiched between substrate and protective cladding layer of undoped YAG. The Nd content of the core raised its refractive index by 4.8×10^-4 relative to substrate and cladding, creating an optical waveguide with a numerical aperture of 0.06. Pump radiation from the diode-bar was coupled into the waveguide using a cylindrical lens system which produced a line focus at the input face of the waveguide with measured beam radii of 10 μm and ~1 mm along the guided and non-guided axes respectively. A monolithic laser cavity was formed by dielectric coatings applied to the plane-parallel end-faces of the waveguide