Phase-mismatched optical parametric oscillators based on aperiodic quasi-phase-matched crystals.

We present that femtosecond optical parametric oscillators (OPOs) based on aperiodic quasi-phase-matched (AQPM) crystals can be configured to operate in the "phase-mismatched" region. The phase-mismatch-induced frequency chirp is mainly introduced by the subcrystal that first interacts with pump pulses. By optimizing the intracavity group velocity dispersion and by selecting the direction of crystals, OPOs based on AQPM crystals could support the generation of wavelength-tunable, transform-limited pulses in an all-normal-dispersion cavity. In a preliminary experiment, transform-limited optical pulses with a tuning range over 1520-1660 nm were obtained by simply altering the cavity length. This scheme represents a unique and concise scheme of producing widely tunable, chirp-free femtosecond optical pulses.