Precise phase control of ultra-wide-bandwidth optical frequency combs has produced remarkable and unexpected progress in precision metrology and ultrafast science. The combination of the ability to do completely arbitrary, optical, waveform synthesis with recently developed optical pulse measurement techniques is analogous to the development of oscilloscopes and waveform generators in the early 20th century. The development of ultra-stable optical frequency standards into optical atomic clocks and optical frequency synthesizers again complement and rival the similar technologies developed in the radio frequency domain.
A frequency comb spanning an entire optical octave (> 300 THz) has been produced corresponding to millions of marks on a frequency “ruler” stable at the Hz level. Optical frequency measurements can now be carried out with vastly improved simplicity and accuracy. We have used the comb to establish a simple optical clock based on an optical transition of iodine molecules. The clock provides an rf clock signal with a frequency stability comparable to that of an optical standard and superior to almost all conventional rf sources. A CW optical frequency synthesizer has also been demonstrated, with the capability of setting a CW laser frequency at a precisely pre-determined value and continuously tuning the laser frequency with rf precision.
For the ultrafast science, carrier-envelope phase stabilization of few cycle optical pulses has recently been realized. We have now demonstrated stabilization of the pulse-to-pulse and the absolute carrier-envelope phase to a level of milli-radians and tens of milli-radians, respectively, paving the groundwork for synthesizing electric fields with known amplitude and phase at optical frequencies. Working with two independent femtosecond lasers operating at different wavelength regions, we can coherently stitch together their optical bandwidths, leading to a synthesized optical pulse. The simultaneous control of timing jitter (repetition rate) and carrier-envelope phase has already been applied to diverse fields such as optical amplifier and nonlinear-optics based spectroscopy and imaging. In short, we now appear to have all the experimental tools required for complete control over coherent light, including the ability to generate pulses with arbitrary shape, and precisely controlled frequency and phase, and to synthesize coherent light from multiple sources.