Direct link between microwave and optical frequencies with a 300 THz femtosecond laser comb

We demonstrate a great simplification in the long-standing problem of measu ring optical frequencies in terms of the cesium primary standard. An air-si lica microstructure optical fiber broadens the frequency comb of a femtosec ond laser to span the optical octave from 1064 to 532 nm, enabling us to me asure the 282 THz frequency of an iodine-stabilized Nd:YAG laser directly i n terms of the microwave frequency that controls the comb spacing. Addition al measurements of established optical frequencies at 633 and 778 nm using the same femtosecond comb confirm the accepted uncertainties for these stan dards.