Molecular Iodine Clock

We demonstrate a simple optical clock based on an optical transition of iodine molecules, providing a frequency stability superior to most rf sources. Combined with a femtosecond-laser-based optical comb to provide the phase coherent clock mechanism linking the optical and microwave spectra, we derive an rf clock signal of comparable stability over an extended period. Measurements suggest the stability ( 5×10^-14 at 1 s) of the cw laser locked on the iodine transition is transferred to every comb component throughout the optical octave bandwidth (from 532 to 1064 nm) with a precision of 3.5×10^-15. Characterization of the performance of the optical clock shows (in-)stability below 3×10^-13 at 1 s (currently limited by the microwave sources), and 4.6×10^-13 over one year.