1.4 GHz femtosecond comb generation by Fabry-Perot filtering of optical parametric oscillator frequency comb

Laser frequency combs based on optical parametric oscillators provide widely tunable sources of regularly spaced frequencies from the visible to the mid-infrared, with potential applications in precision metrology, spectroscopy and astronomy. Reported is the extension of the comb mode spacing into the GHz domain by using a Fabry-Pérot cavity, the length of which was electronically stabilised to a harmonic of the original pulse repetition frequency via a dither-locking technique. A 1500 nm femtosecond optical parametric oscillator frequency comb, the repetition frequency and carrier-envelope offset frequency of which were locked to 280 MHz and 10 MHz, respectively, was filtered by a high-finesse cavity to generate a pulse sequence at 1.4 GHz, corresponding to a factor of five increase in the mode spacing of the comb.