Absolute Optical Frequency Measurements with a Fractional Frequency Uncertainty at 1 × 10 -15

We report the technical details specific to our recent measurements of the optical frequency of the mercury single-ion frequency standard in terms of the SI second as realized by the NIST-F1 cesium fountain clock. In these measurements the total fractional uncertainty is ap 10 -15 , limited by the statistical measurement uncertainty. In this paper we will address the techniques employed for the optical-to-microwave comparison itself, which had an estimated fractional uncertainty of ap 3 times 10 -16 , limited by the stability of the electronics used for the comparison