Comparative optical properties and laser performance at 2.012µm of Tm:YAG bulk crystals and planar waveguides grown by liquid phase epitaxy

Bulk Tm:YAG is well known as a broad band laser source emitting near 2µm, for lidar and medical applications. However, it acts as a quasi-three level laser and so exhibits a significant population in the lower laser level at room temperature leading to a relatively high threshold pump intensity requirement. As in the fibers, the confinement of the pump and laser beams in the optical waveguides leads to lower thresholds and higher gains than in bulk crystals, but always presents extra-propagation losses. For the quasi-three level systems, these propagation losses become negligible compared to the absorption losses at the emission wavelength, as it has been demonstrated in low loss YAG epitaxial waveguides doped with Nd, Yb and Er, which exhibit high efficiencies and low thresholds. In this work, Tm:YAG layers with various concentrations (4 to 10%) have been grown by liquid phase epitaxy in a way similar to Yb:YAG. Codoping with gallium and lutecium allows the adjustment of both the refractive index and the lattice parameter of the- films. A drawback of these substitutions is that spectroscopic properties can be significantly affected, as it has already been demonstrated in Nd:YAG waveguides. Room temperature absorption and emission spectra have then been measured in codoped Ga, Lu, Tm:YAG waveguides, with several gallium doping levels. The 2µm laser performance of these different Ga, Lu, Tm:YAG waveguides, with various active epilayer thicknesses and lengths, have been investigated using two plane mirrors butted directly to the end faces of the waveguides to form the resonator cavity. Both diode array and Ti:sapphire pumping have been used. The thresholds and slope efficiencies observed in epilayers are respectively lower and comparable with best results obtained in Tm:YAG bulk crystals, despite the fact that these waveguides are only planar. Etching of channel waveguides is now in progress, which should lower thresholds and increase laser performance, due to a better confinement and overlap of pump and laser beams