Femtosecond optical parametric oscillators (OPOs) are now recognized to be highly attractive sources for the generation of ultrashort pulses, tunable from the ultraviolet to the infrared. Since the early 1990s, the advent of quasi-phase matching (QPM) technique and availability of high quality periodically poled crystals such as PPLN and PPKTP has revolutionized the generation of femtosecond OPO, because they allow any combination of wavelength within the transparency range of the material to be phase-matched by tailoring the nonlinear crystal, while utilizing the material’s highest noncollinear coefficients. However, the introduction of QPM did not remove the restrictions on crystal length caused by the group-velocity (GV) mismatch between the pump and the generated signal/idler waves in conventional collinear phase-matching geometry. Therefore a thin crystal is commonly used in ultrashort OPOs, which produces low parametric gain and high operational threshold. This also indicates that pump sources such as Ti:Sapphire laser are required to generate the required high pump powers. If an effective way can be found to increase the interaction length, thereby reducing the operational threshold, pumping a femtosecond OPO with compact low power femtosecond sources (for instance diode pumped Cr:LiSAF) would be feasible and most welcome in many practical applications.

We propose a novel approach to achieve simultaneous QPM and GV matching for femtosecond OPOs.This theory is applied to numerically study noncollinear QPM femtosecond OPO in PPLN and PPKTP. It is demonstrated that, by a proper choice of the noncollinear angles and grating periods, the small-signal gain can be enhanced by orders of magnitude, while the operational threshold is significantly reduced.

Based on the results we derived, with a suitable choice of noncollinear angle and corresponding grating period according to the desired signal wavelength, we expect improved performance and ease of operation relative to the collinear geometry.

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