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_ceo and repetition rate f_rep, referenced to the Cs atomic frequency standard. By using two wide-band digital phase-detectors they realize a phase-coherent link between f_rep and f_ceo with the relation f_ceo=f_AOM-5/6f_rep≡0, where f_AOM=5/6f_rep is the phase-locked driving frequency of an acousto-optic modulator in a self-referencing interferometer and f_rep=100 MHz. For the simultaneous control of f_rep and f_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_rep=100 MHz with f_ceo=0. In their optical frequency synthesizer, the frequency of the nth component (f_n) is given exactly by the simple relation f_n=nf_rep, enabling them to use the fs laser comb as a frequency ruler in the optical frequency metrology.