Phase-coherent synthesis of optical frequencies and waveforms

Precision phase control of an ultrawide-bandwidth optical-frequency comb has produced remarkable and unexpected progress in both areas of optical-frequency metrology and ultrafast optics. A frequency comb (with 100 MHz spacing) spanning an entire optical octave (>300 THz) has been produced, corresponding to millions of marks on a frequency “ruler” that are stable at the Hz level. The precision comb has been used to establish a simple optical clock based on an optical transition of iodine molecules, providing an rf clock signal with a frequency stability comparable to that of an optical standard, and which is superior to almost all conventional rf sources. To realize a high-power cw optical frequency synthesizer, a separate, widely tunable single-frequency cw laser has been employed to randomly access the stabilized optical comb and lock to any desired comb component. Carrier-envelope phase stabilization of few-cycle optical pulses has recently been realized. This advance in femtosecond technology is important for both extreme non-linear optics and optical-frequency metrology. With two independent femtosecond lasers, we have not only synchronized their relative pulse timing at the femtosecond level, but have also phase-locked their carrier frequencies, thus establishing phase coherence between the two lasers. By coherently stitching the optical bandwidth together, a “synthesized” pulse has been generated with its 2nd-order autocorrelation signal displaying a shorter width than those of the two “parent” lasers.