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Dynamics and long-term stability of single and multi-longitudinal mode 473-nm diode-pumped Nd:YAG/KNbO3 lasers

Diode pumped frequency doubled Nd :YAG microchip lasers should become an alternative to air-cooled argon ion lasers. The main issues to be solved are the long-term stabilization of the single frequency operation and the power control of the multi-frequency operation. These questions are mostly related to the laser dynamics. In this paper, we present an accurate modelling of the laser dynamics, including quenching processes, non uniform pumping, partial overlap of optical signals and excited-state populations, hole burning and type-I frequency doubling. We theoretically predict that even in multimode operation, only one mode is oscillating at a time, with a mode hoping at a rate of about 50 kHz. This behavior, quite different from the well known dynamics of intra-cavity type-II frequency doubled lasers (green noise) is experimentally confirmed. Diode pumped frequency doubled Nd:YAG 473 nm lasers based on a simple linear cavity are built and exhibit high output power (90+ mW) and record slope efficiency (45% with respect to absorbed power). The understanding of its 2-mode operation allowed us to stabilize the average output power. A similar laser was operated on a single frequency. No wavelength drift could be measured and no mode hopping was observed over 24 hours.