Precision spectroscopy with femtosecond light pulses

Summary form only given. The reciprocal relationship between time and frequency implies that a single short laser pulse has a broad spectrum. A pulse circulating inside a laser cavity or the periodic pulse train if a mode-locked laser, however, can be described as a coherent superposition of discrete laser modes. This frequency-domain comb of modes is now providing an elegant and universal solution to the long challenging problem of how to measure the frequency of light. In a Kerr-lens mode-locked Ti:sapphire laser, each mode is injection locked by modulation sidebands of the other modes via a nonlinear refractive index. Any mode must follow this collective dictate and oscillate in precise lock-step, or it will suffer high round trip losses in a properly designed cavity. We have demonstrated experimentally that the teeth of such a frequency comb are precisely equally spaced to within a few parts in 10/sup 18/, and that their spacing is exactly given by the pulse repetition rate.