High-repetition-rate Yb-doped lasers for frequency comb generation
Schratwieser, Thomas Christian
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The research presented in this thesis addresses the development of directly diode pumped Yb3+:KY(WO4)2 and Yb:fibre lasers, operating at several-hundred-MHz repetition frequencies and investigates their suitability as the basis of high-efficiency, convenient, low-cost and moderate precision optical frequency combs. The design, construction, and characterisation of a 1042-nm 1-GHz Yb3+:KY(WO4)2 femtosecond laser is presented, achieving pulses with an average power output of 770 mW, bandwidths of 3.8 nm, and durations of 278 fs. The laser achieved an opticalto- optical conversion efficiency and slope efficiency of 61% and 69%, respectively, and the relative intensity noise was <0.1%. Spectral broadening of the output pulses in a photonic crystal fibre with a negative dispersion wavelength of 975 nm and a core diameter of 3.7 m resulted in a supercontinuum with a bandwidth of 400 nm, which was insufficient to enable f-2f referencing. A re-designed Yb:KYW laser was demonstrated, operating at a pulse repetition frequency of 666 MHz and producing pulses with reduced durations of 220 fs and increased bandwidths of 5 nm, while maintaining an average output power of >700 mW. Repetition-frequency locking was implemented on this laser and had the effect of reducing its relative intensity noise from 1.1% to 0.5%, with limitations on the locking stability being traced to cantilever-like vibrational modes of the mirrormount assemblies. A fully stabilised 1030-nm Yb:fibre frequency comb operating at a pulse repetition frequency of 375 MHz was developed. The comb spacing was referenced to a Rb-stabilised microwave synthesiser and the comb offset was stabilised by generating a supercontinuum containing a coherent component at 780.2 nm, which was heterodyned with a 87Rb-stabilised external cavity diode laser to produce a radiofrequency beat used to actuate the carrier-envelope offset frequency of the Yb:fibre laser. The two-sample frequency deviation of the locked comb was 235 kHz for an averaging time of 50 seconds, and the comb remained locked for over 60 minutes with a root mean squared deviation of 236 kHz.