Optical frequency metrology with femtosecond laser pulses

The possibilities of frequency combs generated by femtosecond lasers in ultra high precision spectroscopy on atomic hydrogen at the 10/sup -14 /accuracy level are explored. Fundamental tests of the fs frequency comb technique down to the 10/sup -21 /level are done. Further applications arise in the time domain where now the phase slip between carrier and envelope of such pulses can be controlled.