Femtosecond combs for optical frequency metrology

Abstract

This thesis is dedicated to femtosecond combs as a tool for optical frequency metrology and as an integral part of an optical clock. After an overview of optical frequency measurement techniques, the design of two frequency combs based on mode-locked femtosecond lasers as they were at the beginning of my project is described. The first comb is based on an Er:fibre laser operating at a central wavelength of 1550 nm with a repetition rate of 100 MHz. The second is a Ti:sapphire-laser-based comb operating at a central wavelength of 810 nm with a repetition rate of 87 MHz. Improvements to the original design of the Ti:sapphire comb are detailed in the next chapter. A novel f-to-2f self-referencing scheme based on a pair of Wollaston prisms and employing a PPKTP crystal for SHG results in up to 20 dB enhancement of the signal to noise ratio in the carrier-envelope offset frequency beat signal f0 and in up to 15 dB lower phase noise in the f0 beat signal compared to a Michelson interferometer based system. Next, the factors influencing the stability and accuracy of the microwave reference signal and the performance of two synthesisers used for the stabilisation of the frequency combs were investigated. It is shown that stability of the maser reference signal is reduced by the distribution system by factor of 1.5. A fractional frequency change of 4.1(0.7) × 10−16 (K/h)−1 was measured for the better of the two synthesisers (an IFR 2023A) indicating that for accurate frequency measurements the synthesiser signal should be monitored to enable systematic frequency corrections to be made. Finally, an absolute frequency measurement of the electric quadrupole clock transition in a frequency standard based on a single 171Yb+ trapped ion is described. The result f = 688 358 979 309 310 ± 9 Hz agrees with an independent measurement made by the PTB group within the uncertainty of the measurements.