Versatile femtosecond optical parametric oscillator frequency combs for metrology
Abstract
This thesis addresses the development of broadly tunable, high repetition rate frequency combs
in the mid-IR region. A novel PPKTP crystal design was used to provide phasematching for
parametric oscillation and simultaneously give efficient pump+idler sum-frequency generation
(SFG). This innovation enabled a fully stabilized idler comb from a 333-MHz femtosecond
optical parametric oscillator to be generated in which the carrier envelope offset frequency
fCEO together with the repetition frequency fREP were stabilised. This OPO platform was
then extended to demonstrate, via harmonic pumping, a fully stabilized 1-GHz OPO frequency
comb from a 333-MHz pump laser. Next, an alternative route to a 1-GHz OPO comb was
investigated by synchronously pumping an OPO directly with a 1-GHz Ti:sapphire laser. Here
the comb was fully stabilized for the signal, idler and pump pulses by using a narrow linewidth
CW diode laser developed for the project and whose design is also presented. A further increase
in the comb mode spacing was performed with a Fabry-Pérot cavity. A stabilised cavity was
used to filter 1.5 m signal pulses from a 333-MHz repetition rate OPO frequency comb to yield
a 10-GHz comb. The length of the Fabry-Pérot cavity was dither locked to a single-frequency
ECDL and later on directly to the OPO frequency comb. Finally the 333-MHz OPO comb
was demonstrated in an optical frequency metrology experiment. The frequency comb mode
number and the absolute frequency of a narrow-linewidth CW laser were measured and the
performance of the OPO comb was found to be comparable to that of a commercial fibre laser
comb used as a benchmark in the experiment.