Characterisation and modelling of non-collinearly phase-matched femtosecond optical parametric oscillators based on KTA and operating beyond 4 micrometers

We describe the design and operation of a critically phase-matched femtosecond optical parametric oscillator based on KTA, By employing a small pump-signal noncollinear angle, tuning of the idler to beyond 4 mu m is achieved using a Ti:sapphire pump laser, A Gaussian-beam model is described which can be used to identify the optimal noncollinear phase-matching geometry, Idler and signal pulses are characterized fully both temporally and spectrally and interferometric autocorrelation data at 3.5 mu m showing idler pulses of only eight optical cycles duration are presented.