Orthogonal control of femtosecond mode-locked laser having zero carrier-offset frequency with three-axis PZT

The authors demonstrate an optical frequency synthesizer based on a femtosecond (fs) mode-locked Ti:sapphire (Ti:s) laser by simultaneously stabilizing the carrier-offset frequency f/sub ceo/ and repetition rate f/sub rep/, referenced to the Cs atomic frequency standard. By using two wide-band digital phase-detectors they realize a phase-coherent link between f/sub rep/ and f/sub ceo/ with the relation f/sub ceo/=f/sub AOM/-5/6f/sub rep//spl equiv/0, where f/sub AOM/=5/6f/sub rep/ is the phase-locked driving frequency of an acousto-optic modulator in a self-referencing interferometer and f/sub rep/=100 MHz. For the simultaneous control of f/sub rep/ and f/sub ceo/, an orthogonalization scheme using a three-axis piezoelectric transducer is introduced by which the end mirror of fs laser is controlled. As a result, the authors could stabilize all components of the fs laser comb at once with an equal frequency separation f/sub rep/=100 MHz with f/sub ceo/=0. In their optical frequency synthesizer, the frequency of the nth component (f/sub n/) is given exactly by the simple relation f/sub n/=nf/sub rep/, enabling them to use the fs laser comb as a frequency ruler in the optical frequency metrology.