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Second-order nonlinear interactions offer many properties advantageous to ultrafast laser sources. We are developing these devices to enable lasers and frequency combs in new regimes of operation. A key concept for this work has been nonlinearity management intracavity.
For high repetition rate frequency combs, their high power per comb line is attractive for high-speed and high-sensitivity molecular spectroscopy. We have introduced a new type of nonlinear device to address the long-standing Q-switching damage problem which had limited the repetition rate achievable from SESAM-modelocked femtosecond solid-state lasers. We use adiabatic excitation of cascaded quadratic nonlinearities, implemented via two-dimensionally quasi-phase-matching gratings, to achieve a large self-defocusing nonlinearity intracavity with low loss. This offers energy-efficient soliton formation for stable modelocking, and also intensity clamping via self-defocusing (preventing SESAM damage prior to stable modelocking). We have developed a repetition-rate stabilized >10 GHz 100-fs modelocked laser with this approach. Combined with highly nonlinear waveguides, supercontinuum generation, f-2f interferometry, and mid-infrared generation are within reach.
For high power lasers, thin-disk laser (TDL) oscillators offer a compelling route towards high-energy pulses for industrial or scientific applications, since they bypass the complexity of conventional laser-amplifier systems. However, the highest power TDL oscillators have been operated in vacuum chambers because the nonlinearity of air destabilizes the pulse formation. We have explored intracavity nonlinearities to address such challenges. We demonstrate nonlinear-mirror modelocking, and nonlinear-cancellation via cascaded quadratic nonlinearities, recently obtaining >200 W from a SESAM-modelocked TDL operated in ambient air. Scaling towards kW average powers will be discussed.
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