Optical clockworks and the measurement of laser frequencies with amode-locked frequency comb

摘要

Femtosecond laser-frequency comb techniques are vastly simplifyingnthe measurement and synthesis of optical frequencies. A singlenmode-locked femtosecond laser, with its spectrum broadened by self-phasenmodulation in a microstructured or tapered nonlinear fiber, can producenmillions of sharp laser lines in a precise evenly spaced grid spanningnmuch of the visible and near-infrared spectrum. The absolute frequencynof each line is determined by two observable radio-frequency signals.nThe pulse repetition rate gives the spacing of the comb lines and thenrate at which the phase of the lightwave slips, relative to thenintensity envelope from pulse to pulse determines the offset frequencynby which each line is displaced from a precise integral multiple of thenrepetition frequency. This offset frequency can be measured most easilynif the comb spans more than an optical octave so that one can observe anradio frequency beat note between the second harmonic of the infraredncomb lines with the corresponding comb lines at the blue end. Such annoptical-frequency synthesizer makes optical oscillations readilyncountable and provides the long-awaited compact optical clockwork for annall-optical clock