Femtosecond Lasers for Optical Frequency Measurement

Equidistance modes in the frequency domain can represent femtosecond laser pulses in the time domain. By stabilizing two parameters of these equidistance modes, namely, the repetition rate and the offset frequency, these modes can be called an Optical Frequency Comb (OFC). Optical frequency measurement was a complicated task before introducing the first OFC. However, the certain procedures have to be followed to assure accurate measurement, such as the determination of the mode number and the sources of uncertainty. This review article will present an overview of femtosecond lasers in the time and frequency domains. In addition, the principle of optical frequency measurement using OFCs is explained. For example, an optical frequency measurement using an OFC on the hyperfine components of the two-photon transition 5S_1/2 - 5D_3/2 at 778 nm of a rubidium atom is demonstrated measurement and the uncertainty is calculated. Several other accurate transitions can be measured using OFC.