A New Method for Achieving High-Energy Mode-Locked Lasers

Through the development in ultrashort high energy laser pulses, advances in ultrafast sciences have so far resulted in two Nobel prizes: the observation of transition states of molecules and frequency metrology. More are expected to come when the pulse duration is decreased and the pulse energy is increased fiirther. High-energy ultrafast lasers also find applications in precision machining and health care.

Ultrashoit high-energy light pulse sources are instrumental in the advance of ultrafast science. Fiber lasers offer major practical advantages over solid state lasers because light is contained in a waveguide, so careful alignment of an optical cavity is not required. The potential of fiber has motivated research for nearly two decades, but short-pulse fiber lasers have very little impact compared to solid-state lasers because of pulse energy and duration limitations. New insights into pulse-propagation physics in the past few years have provided order-of-magnitude increases in the pulse energy and peak power from femtosecond fiber lasers. It is now realistic to design short-pulse fiber devices that compete directly with existing solid state lasers in energy performance while offering major practical advantages and substantially reduced cost. At high cavity energy, however, either the mode locking mechanism used to produce short pulses breaks down or the pulse splits into multiple pulses. This so-called multi-pulsing instability (MP1) imposes a fundamental limitation on the energy of a single mode-locked pulse. Theory and experiments also suggested that near the MPI, both periodic and chaotic behavior could be observed as operating states of the laser cavity for a narrow range of parameter space. Recently, we proposed an iterative technique that provides a simple geometrical description of the entire multi-pulsing transition behavior as a function of increasing cavity energy. The model captures all the key features observed in experiment, including the periodic and chaotic mode-locking regions, and more important it provides valuable insight into laser cavity engineering that can

maximize fiber laser performance, i.e. to increase the mode-locked pulse energy of fiber lasers.

Although for most laser configurations, it is not difficult to simulate the laser dynamics numerically, extracting analytic results remains mathematically difficult because of the discrete and periodic nature of the most laser configurations. Reducing the governing laser dynamics to a single field variable that can be described by a master equation remains important because theoretical results can be used to characterize the mode-locking dynamics. Another valuable approach to study laser dynamics is to use low-dimensional reduction on the master equation. In this project, we propose to use the insight gained from the iterative geometric description of the pulses dynamic to connect the master equations, reduced dynamics, and iterative models together in order to guide the experiments to produce ultrafast high energy pulse from fiber lasers. We will investigate one of the most commercially successful mode-locked fiber laser configurations which uses a passive polarizer and waveplates for mode-locking. We will also study other passive mode-locking mechanisms such as using the nonlinear optical loop mirror as the mode-locker. The main objective is to produce ultrashort pulses from fiber lasers with energy comparable or even

higher than that from solid state lasers.