Optical phase locking among femtosecond subharmonic pulses

We stabilized the relative carrier-envelope phase slip among the pump pulse and its subharmonic signal and idler pulses in a femtosecond optical parametric oscillator, resulting in long-term phasecoherence among the pulses. The stabilized beat signal corresponding to the relative carrier-envelope phase slip among subharmonic pulses had an accumulated phase error of 0.24 rad in the 1-mHz-1-MHz region. The fluctuation of the beat frequency measured by a 1-s-averaged counter was less than 1 mHz in a 1480-s measurement.