Phase-coherent frequency measurements of the calcium- and ytterbium-optical frequency standards with a Kerr-lens modelocked femtosecond laser

Abstract

Summary form only given. Recent progress in femtosecond pulse generation and the advent of microstructure optical fibers have substantially facilitated optical frequency measurements. These schemes start with the highly periodic pulse train of a Kerr-lens modelocked laser which corresponds in the frequency domain to a comb-like frequency spectrum of equidistant lines. The spectral span of this comb is determined by the inverse duration of an individual pulse while the spacing between the lines is determined by the pulse repetition frequency. The fast, spectrally far-reaching Kerr-lens modelocking mechanism enforces a tight coupling of the optical phases of the individual lines. As a result, the frequency of any of these lines is given by an integer multiple of the pulse repetition frequency f/sub rep/ and a frequency /spl nu//sub ceo/ which accounts for the offset of the entire comb with respect to the frequency origin. The spectrum from the femtosecond laser reaching from around 700 nm to 900 nm is further broadened in a microstructure fiber by self-phase modulation to about 500 nm to 1200 nm. Whereas counting of the repetition rate or one of its harmonics is straightforward, the frequency /spl nu//sub ceo/ requires to count the beat between a number of modes around 550 nm and a number of frequency-doubled modes at 1100 nm. An unknown frequency /spl nu//sub opt/ of an external optical signal is measured by counting the frequency of the beat-note /spl nu//sub opt/-/spl nu//sub comb/ between this signal and a suitable. comb line /spl nu//sub comb/. Thus, by refering the three radiofrequencies f/sub rep/, /spl nu//sub ceo/ /spl nu//sub opt/-/spl nu//sub comb/ to the primary cesium standard, the frequency of the optical signal can precisely be measured. We report the first application of this method to phase-coherent measurements of two different optical frequency standards operating in the PTB: cold-atom-based Ca standard and the single-ion ytterbium standard.