Transform-limited pulse generation in phase-mismatched optical parametric oscillators

The relation between the phase-mismatch and the pulse characteristics of ultrafast optical parametric oscillators (OPOs) is demonstrated. We show that a non-zero phase-mismatch in a nonlinear crystal introduces a frequency up-chirp or down-chirp in the resonating pulses of OPOs. Based on this, we propose a new scheme called 'phasemismatched OPOs', which is distinguished by using the phase-mismatch in the nonlinear crystal as a phasemodulating mechanism: transform-limited femtosecond optical pulses could be generated due to the interplay between the phase-mismatch and the intracavity group-velocity-dispersion effects. Experimentally, from an OPO with an optimized group-velocity dispersion and phase-mismatch, we obtain transform-limited femtosecond pulses at 1570 nm with an average power of 2.1 W, and subsequent generation of a coherent supercontinuum covering 1070-2225 nm. Phase-mismatched OPOs enables the direct generation of transform-limited optical pulses with duration from picoseconds to sub-50 fs, at arbitrary wavelengths, and having great potential for power-scaling, which would benefit many applications including nonlinear frequency conversion, supercontinuum generation and multi-photon imaging.