EFFECTS OF CAVITY DETUNING ON THE PULSE CHARACTERISTICS OF A FEMTOSECOND SYNCHRONOUSLY PUMPED OPTICAL PARAMETRIC OSCILLATOR

We report spectral and temporal characteristics of lambda similar to 1.5 mu m, nominally 150-fs signal pulses from a KTP-based high-repetition-rate optical parametric oscillator (OPO) synchronously pumped by a 90-fs, 85-MHz mode-locked Ti:sapphire laser. This OPO can be operated with cavity detunings as large as +/-2 mu m, although stably for only +/-1.3 mu m. When the OPO is operated without dispersion compensation at average powers <20 mW the spectrum is singly peaked, with a width of similar to 20 nm; for output powers between 20 and 150 mW self-phase modulation induces a double-peak structure in the spectrum and broadens it to 50 nm. Because of time-dependent gain effects in the OPO, the spectrum is asymmetric, and the asymmetry changes dramatically with detuning, reversing when the detuning changes from +1.3 to -1.3 mu m. Cross-correlation measurements between pump and signal pulses reveal asymmetric temporal pulse shapes consistent with the observed spectra. For optimum power, which we use to define the zero-detuning condition, the pulse has a trailing edge steeper than its leading edge. The operation of the OPO for nonzero detuning can be understood in terms of group-velocity dispersion effects, whereby shifts in the signal spectrum compensate for as much as 50% of the time lag induced at each round trip between the pump and the signal pulses. (C) 1995 Optical Society of America