Design, optimization, and applications of few-cycle Ti:Sapphire lasers

Abstract

Ti:Sapphire mode-locked lasers are a unique technology that enables a wide variety of applications. Owing to the ultrabroadband nature of the Ti:sapphire crystal and the invention of precisely engineered dispersion-compensating mirrors (DCMs), these lasers are now capable of generating stable pulse trains directly with octave-spanning spectrum, few-cycle pulse duration, and a desired repetition rate from a compact system. This paves the way to a new world of emerging applications ranging from the search of exoplanets, high-harmonic generation, to precision measurement Qualitatively, the key to the stable mode-locking of Ti:Sapphire lasers lies in the balance of various spatial and temporal nonlinear effects such as self-amplitude modulation(SAM), self-phase modulation(SPM), saturable absorption, self-focusing, gain-filtering, gain-guiding, and so on. However, since much shorter pulses and much higher intracavity intensities are often reached inside the laser gain medium, the spatiotemporal dynamics in such lasers are even more complicated as non-negligible multi-photon processes also come into play. Due to the strong coupling between these effects, performing a reliable analysis and optimization become extremely challenging. In this thesis we study the spatiotemporal dynamics of pulse evolution in the few-cycle regime and provide guidelines for designing and optimizing these lasers for repetition rate ranging from 85 MHz to 2 GHz. The essential background reviews as well as key concepts in KLM lasers will be given together with a demonstration of octave-spanning Ti:sapphire lasers with record-high repetition rate. A numerical model for simulating the full spatiotemporal dynamics is introduced. For an efficient numerical calculation, GPU accelerated computing techniques are adopted. With this model, many unique features that are observed from the experiments can be simulated for the first time. A novel type of output coupler called gain-matched output coupler is introduced which can greatly reduce the nonlinearity required for ultrabroadband mode-locking. Already at pump power levels close to the cw lasing threshold it is possible to initiate robust mode-locking and generate <8 fs output pulses from Ti:sapphire lasers with excellent beam quality operating in the center of its stability range. Moreover, the development of visible astro-combs based on fewcycle Ti:sapphire lasers will be discussed. This application is enabled by two promising technologies (broadband zero-GDD mirror sets and Cherenkov radiation in the few-cycle regime) which are developed to increase the repetition rate and spectral coverage of the laser systems operated in the few-cycle regime. Fiber-optic Cherenkov radiation in the few-cycle regime excited by sub-10fs Ti:sapphire pulses is studied. Through a dispersion-engineered PCF driven by a few-cycle pulse, the nonlinearity can produce highly efficient broadband frequency up-conversion to the visible wavelength range. Finally, we propose and demonstrate a new approach for broadband dispersion-free optical cavities using a zero-GDD mirror set; With the first zero-GDD mirror pair, the construction of a -40 GHz filtering cavity with 100 nm bandwidth for a green astro-comb (480-580 nm) was demonstrated. Finally, the thesis is concluded by discussing the practical issues related to the construction of a easy-to-operate, long-term stable few-cycle Ti:sapphire laser.