Measuring the frequency of light with ultrashort pulses

Summary form only given. Sumary form only given. Femtosecond laser frequency comb techniques are revolutionizing the art of optical frequency metrology. The complex and highly specialized harmonic laser frequency chains of the past can now be replaced by compact and universal single-laser optical frequency synthesizers which produce a vast grid of precisely known reference frequencies spanning much of the visible and near infrared spectrum. The periodic pulse train of a mode-locked femtosecond laser behaves in the frequency domain as an evenly spaced comb of laser modes. A frequency comb spanning more than an octave can be produced by self-phase-modulation in a microstructured silica fiber with a small solid fiber core surrounded by air holes. The radiofrequency beat note between the second-harmonic of the red end of the comb spectrum and the blue end provides directly the frequency offset by which the comb lines are displaced from the integer harmonics of the repetition frequency. By locking the repetition rate and offset frequency to a cesium atomic clock, all mode frequencies become absolutely known. Performance characteristics and accuracy tests for several different femtosecond laser comb synthesizers are reported, and a number of recent precise spectroscopic frequency measurements are reviewed.