PTB News 3/2000 (554 kB) PTB News 3/2000, English edition, Issue December 2000Lasers stabilised to atomic or ionic transitions may eventually lead to optical atomic clocks of unprecedented precision. PTB has developed an optical frequency measurement system that allows one to relate any optical frequency to the frequency of the primary standard of time, the Caesium atomic clock, with atomic clock accuracy.
The novel scheme is based on a femtosecond Titanium: Sapphire laser. It has been used for phase-coherent measurement of the frequency of an optical frequency standard with atomic clock accuracy. The use of such a single oscillator substantially simplifies the complex, bulky harmonic frequency chain employed so far at PTB and elsewhere for this purpose (see PTBnews 96.1). In the frequency domain, the pulse sequence of a femtosecond laser corresponds to a comb of frequencies which are separated by the pulse repetition frequency. The width of this comb which is given by the width of a single pulse is spectrally broadened in a novel type of optical microstructure fiber to cover the entire visible and near infra-red range. The frequency of any one of the comb lines can be obtained by measuring the line separation, i.e., the pulse repetition rate, and a second frequency that gives the position of the entire comb with respect to the frequency origin.
As a first application, the frequency of the Ca stabilised laser used as one of the most accurate realisations of the metre was re-determined. The result was found to agree with previous measurements. PTB now has a dense grid of millions of accurately known reference frequencies which can be used as a universal, self-referenced frequency ruler throughout the entire visible and near infrared spectrum.