Pump-beam M2 factor approximation for design of diode fiber-coupled end-pumped lasers

A model for optimization of fiber-coupled diode-laser end-pumped solid-state lasers, which describes the pump beam by its M-2 factor, is proposed. Using the mode-overlap integrals of a four-level laser, numerical investigations were carried out to obtain the pump beam optimum focusing conditions that ensure a minimum threshold pump power and a maximum mode-matching efficiency at a given laser beam spot size. For the purpose of design simplicity the numerical data were fitted with analytical functions of laser mode volume, pump beam M-2 factor, and active medium absorption efficiency at the pump wavelength. With a simple output-to-input power relation, these formulas provide a straightforward procedure to design the optimum laser resonator and the coupling optics and to determine the expected output power. To show the model utility a Nd:YAG medium, end-pumped with fiber-coupled diode-laser systems of different M-2 factors, was considered. A good agreement between experiments and theory was obtained. (C) 1999 Society of Photo-Optical instrumentation Engineers. [S0091-3286(99)00711-4].