|Title||Femtosecond Frequency Combs for Optical Clocks and Timing Transfer|
|Year of Publication||2007|
The rapid development of femtosecond optical frequency combs over the last decade has brought together ultrastable phase control of both cw and mode-locked lasers and ultrafast time-domain applications. Frequency-domain laser stabilization techniques applied to the ultrashort-pulse trains emitted by a mode-locked laser result in a level of optical phase control previously achievable only for radio frequencies and microwaves. I present our work extending such control to mode-locked lasers for both timing and frequency stabilization applications of optical frequency combs.
I first present a microwave technique for synchronizing two independent modelocked lasers at a level of timing precision less than the duration of an optical cycle, below 1 fs of residual rms timing jitter. Using these synchronized pulses, simultaneous sum- and difference-frequency generation of 400-nm and tunable mid-infrared fs pulses is demonstrated, opening the door for broadband coherent control of atomic and molecular systems.
For frequency metrology, I report on an offset-free clockwork for an optical clock based on the 3.39-μm transition in methane. The clockwork’s simplicity leads to a robust and reliable table-sized optical frequency reference with instability approaching a few parts in 1014. Then I describe a directly-octave-spanning, self-referenced Ti:sapphire laser employed as the robustly-running phase-coherent clockwork for an 87Sr optical lattice clock. The optical comb distributes the 2-s coherence time of the 698-nm ultrastable clock laser to its modes spanning the visible and near-IR spectrum, and is therefore simultaneously used as a hub for measuring absolute frequencies or frequency ratios between the Sr clock and other remotely-located microwave and optical atomic standards.
Finally, I report on the transfer of ultrastable frequency references, both microwave and optical, through 10-km-scale optical fiber links. Actively stabilizing the optical phase delay of such a fiber link, we are able to transfer a cw optical frequency standard with a transfer instability of 6×10−18 at 1 s, more than two orders of magnitude lower than reported for any fiber link of similar length. Phase coherence between ends of the fiber link is preserved at the mHz linewidth level, and the transfer phase noise corresponds to less than 80 attoseconds of rms timing jitter integrated from 10 mHz to 30 MHz.