Overview

• development and operation of standards of time and frequency and coordination of them with other world standards;

• development of optical frequency standards supporting wavelength and length metrology;

• provision of time and frequency services to the United States; and

• basic and applied research in support of future standards, dissemination services, and measurement methods.

  The work supporting length metrology derives from the dependence of the definition of the meter on the realization of the second. This work contributes to a larger program in the Precision Engineering Division (MEL) which has primary responsibility for length and its dissemination.

Femtosecond comb generator used to measure optical frequency: This picture shows schematically how a femtosecond comb generator is used to measure optical frequency. The output of the mode-locked femtosecond laser, with its repetition rate referenced to the cesium frequency standard, is input to the system along with the outputs of the optical frequencies to be measured, in this case spectral lines in mercury ions and neutral calcium. With the repetition rate locked to the cesium reference, the frequencies of all the modes of the comb are known. The signals go through a short piece of special non-linear optical fiber, which broadens the comb to cover more than a full octave. The beat frequencies between elements of the comb and the frequencies f₁ and f₂, then give the offsets of these frequencies from these comb elements, which are integer multiples of the cesium frequency.