Electro-optic frequency combs generated via direct digital synthesis applied to sub-Doppler spectroscopy.

Direct digital synthesis in concert with an electro-optic phase modulator was employed to generate optical frequency combs with tooth spacings as low as 100 Hz. These combs were utilized to probe electromagnetically induced transparency (EIT) and hyperfine pumping in potassium vapor cells. Long-term coherent averaging was demonstrated with performance similar to that achieved with a vastly more expensive arbitrary waveform generator. From the potassium EIT transition we were able to determine the ground state hyperfine splitting with a fit uncertainty of 80 Hz. Importantly, because of the mutual coherence between the control and probe beams, which originate from a single laser, features with linewidths several orders-of-magnitude narrower than the laser linewidth could be observed in a multiplexed fashion. This approach removes the need for slow scanning of a traditional cw laser or mode-locked-laser-based optical frequency comb.