Efficient intracavity frequency doubling of a passively modelocked diode-pumped Nd:LSB laser

Abstract

We report for the first time a stable and simple method for intracavity frequency doubling of a solid-state laser passively modelocked by using a low-finesse antiresonant Fabry-Perot saturable absorber (A-FPSA [1]). With a Neodymium-Lanthanum-Scandium-Borate laser (Nd:LSB) as gain material and a beta-Barium-Borate (β-BBO) nonlinear crystal with type I phase matching, we achieved a conversion efficiency of 48% resulting in 190 mW output power of TEM00 green light at modest pump powers of 1.2 W. Intracavity frequency doubling is favourable for the low to intermediate power regime, where the typical peak intensities of the output beam are not sufficient to obtain high single pass nonlinear frequency conversion efficiencies. But random noise fluctuations in cw intracavity requency doubled lasers, caused by nonlinear coupling between the laser modes and referred to as the green problem [2], made these lasers inappropriate for many applications. This problem can be overcome with a ring laser approach, where 780 mW of green light at a pump power of 4 W could be shown [3], by the use of additional intracavity elements, which make the laser rather complex and critical to operate, or with the use of an external resonator, where up to 200 mW of green light were produced at a diode pump power of 1 W, but which requires electronic frequency locking elements [4], Another possibility to obtain stable output are modelocking techniques, which lock all longitudinal modes together in phase. We describe an all solid-state, completely passive approach for intracavity frequency doubling of a solid-state laser, which is stable, non critical to align, and does not require any electronic control elements.