Femtosecond optical parametric oscillator frequency combs for coherent pulse synthesis
McCracken, Richard A.
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Coherent pulse synthesis takes as its objective the piecewise assembly of a sequence of identical broadband pulses from two or more mutually-coherent sequences of narrowband pulses. The requirements for pulse synthesis are that the parent pulses share the same repetition frequency, are phase coherent and have low mutual timing jitter over the required observation time. The work carried out in this thesis explored the requirements for broadband coherent pulse synthesis between the multiple visible outputs of a synchronously pumped femtosecond optical parametric oscillator. A femtosecond Ti:sapphire laser was characterised and used to pump a PPKTP-based OPO that produced a number of second-harmonic and sum-frequency mixing outputs across the visible region. Using a novel lock-to-zero CEO stabilisation technique, broadband phase coherence was established between all the pulses on the optical bench, producing the broadest zero-offset frequency comb to date. Employing a common optical path for all the pulses provided common-mode rejection of noise, ensuring less than 150 attoseconds of timing jitter between the pulses over a 1 second observation window. The parent pulses were compressed and their relative delays altered in a quasi-common path prism delay line, allowing pulse synthesis at a desired reference plane.