Diode-pumped Nd : YAG lasers for generation of blue light by frequency doubling

Abstract: Quasi-three-level lasers in neodymium-doped crystals such asNd:YAG, Nd:YLF and Nd:YVO4have received a great deal of interest becausethey allow generation of blue light by frequency doubling. Forsolid-state blue laser sources, there exist numerousapplications as in high-density optical data storage, colourdisplays, submarine communication and biologicalapplications. Efficient lasing on quasi-three-level transitions at 900-950nm in Nd-doped crystals is considerably more difficult toachieve than on the stronger four-level transitions at 1-1.1µm. The problems with these quasi-three-level transitionsare a significant reabsorption loss at room temperature and avery small stimulated emission cross section. This requires atight focusing of the pump light, which is achieved byend-pumping with high-intensity diode lasers. n this thesis, progress in diode-pumped solid-state lasersfor generation of blue light by frequency doubling has beenmade. Nd:YAG lasers at the 946 nm transition have been builtand a maximum power of 7.0 W was obtained. By inserting a thinquartz etalon in the laser cavity, the938.5 nm laser linecould be selected. An output power of 3.9 W was obtained. By using nonlinear crystals, frequency-doubling of laserlight at both 946 nm and 938.5 nm by second harmonic generation(SHG) was achieved. SHG of the 946 nm transition gives bluelight at 473 nm. Efficient generation of blue light has beenachieved in periodically poled KTP, both in single-passextra-cavity and intracavity configurations. More than 0.5 Wwas obtained at 473 nm by intracavity doubling. Intracavity SHGof the 938.5 nm transition gave slightly more than 200 mW at469 nm. The influence of energy-transfer upconversion (ETU) is adetrimental effect in Nd-doped lasers. An analytical model hasbeen developed for continuous wave quasi-three-level lasersincluding the influence of ETU. The results of the generaloutput modelling are applied to a laser with Gaussian beams,and rigorous numerical calculations have been done to study theinfluence of ETU on threshold, output power, spatialdistribution of population-inversion density and fractionalthermal loading. The model is applied to a laser operating at946 nm in Nd:YAG, where thermal lensing and the dependency oflaser-beam size are investigated in particular. A simple modelfor the degradation of laser beam quality from a transversallyvarying saturated gain is also proposed, which is in very goodagreement with measurements of the laser in a plane-planecavity.