Locking the carrier-envelope-offset frequency of an optical parametric oscillator without f-2f self-referencing.

The carrier-envelope-offset (CEO) frequency of pulses from a femtosecond optical parametric oscillator (OPO) was stabilized for 30 min without the need for f-2f self-referencing in either the OPO or its pump laser. Interference between the high-frequency modes of the pump supercontinuum and the modes of the non-phase-matched pump+signal sum-frequency-mixing pulses provided the beat signal used for locking. The -3 dB bandwidth of the locked CEO-frequency was measured as 1.1 kHz, and the cumulative phase error recorded over 1 s was 0.38 rad, representing 1-order-of-magnitude improvement in comparison to previous results.