Efficiency signal conversion parameter to evaluate astigmatic femtosecond-optical parametric oscillator cavities

In this work, we define the efficiency signal conversion numerical parameter, V_eff, useful to evaluate the operation efficiency of femtosecond-Optical Parametric Oscillator (fs-OPO) cavities considering the astigmatism effect. For the validation of the V_eff, we have performed experimental measurements. We present different high efficiency home-made singly resonant fs-OPO cavities, with signal tuneability from 1.1 µm to 1.6 µm based on a 0.5 mm Periodically Poled Lithium Niobate doped with MgO (MgO:PPLN) crystal. We have also defined the pump energy threshold per crystal unit length, ζ_p,th. Pump threshold, achieved by following the V_eff, was 142 mW at 810 nm, and ζ_p,th = 2.10 nJ/mm, the lowest value, in comparison with other studies. The V_eff is based on an ABCD matrix Gaussian beam propagation method, which calculates the mode coupling between the pump and signal beams along the crystal under different cavity configurations taking into account the astigmatism. The model was compared and tested with 3 different experimental singly resonant fs-OPO ring cavity configurations that we have defined as single-folded, two-folded, and direct-pump cavity.