Efficient detection and control of the carrier-envelope offset frequency in a self-referencing optical frequency synthesizer

An efficient means of isolating and detecting the carrier-envelope offset frequency signal in an optical frequency synthesizer is demonstrated. The technique uses spliced and connectorized fiber for comb broadening, a periodically poled KTP crystal for doubling 1064-nm light in the supercontinuum, and a laser-line filter at 532 nm for comb-section selection. The technique produces an offset frequency with a 40-dB signal-to-noise ratio in a resolution bandwidth of 100 kHz with as little as 55 GW/cm² of peak pulse intensity inside the fiber. The strong signal-to-noise ratio helps realize an offset frequency signal with frequency instability of 1 mHz at 1 s when controlled through feedback to the optical power driving the femtosecond laser. PACS 06.30.Ft; 06.20.Fn; 42.62.-b; 42.60.-v