Spectral Control of Functional Fiber Sources
Time: Thu 2020-09-24 10.00
Subject area: Physics, Optics and Photonics
Doctoral student: Robert Lindberg , Laserfysik, Laserfysik
Opponent: Professor Stefano Taccheo, Politecnico di Torino, Torino, Italien
Supervisor: Professor Valdas Pasiskevicius, Laserfysik
This thesis is based on five projects aimed at spectral control of fiber-based laser architectures. Two of these concern electrically wavelength-tunable laser sources, namely a fiber amplifier system for CO2-monitoring and a novel tunable laser design for flexible pulsed operation. The remaining three projects are devoted to ultrafast systems; with the development of a testbed cavity for ultrafast fiber laser optimization and a numerical model for analyzing ultrafast fiber-based amplification, as well as a study of length optimization of anomalous dispersion fibers for supercontinuum generation.
The first electrically tunable source, i.e. the CO2-monitoring system, was based on a narrow-band laser, with a tunable wavelength at 1572±1.5 nm., that was boosted to watt-level powers in a single fiber amplifier. This enabled the first demonstration of a new technique for atmospheric CO2-monitoring, which alleviates the laser requirements for such applications.
The novel cavity design only comprised three standard components and had a tuning range of 35 nm. Despite the simple layout, it also supported versatile and controllable time-multiplexed multi-wavelength operation.
Spectral control not only entails operating wavelength but also achievable bandwidths, which is crucial in ultrafast systems —as it dictates attainable pulse durations. The testbed cavity, incorporating an intra-cavity compressor for dispersion control, was thus developed to facilitate characterization of key parameters to reach target performance. This enabled the preparation of a saturable absorber that could support stable pulses in three different regimes, with pulses of 7–31 nm bandwidths and durations of a few picoseconds down to hundreds of femtoseconds.
Stable operation of such lasers is typically obtained at low powers. They are thus often paired with amplifiers to reach power requirements. Fiber amplifiers are notably adapted for high repetition-rate sources, due to geometrically favorable heat-dissipation. Yet, high intensities and long interaction lengths require attention to nonlinear and dispersive effects. A numerical model accounting for these during amplification was thus developed. The accurately reproduced experimental results demonstrate the model’s utility, i.e. enabling studies of various design aspects, in system development.
The achievable peak powers from such systems make them ideal for nonlinear interactions to extend the spectral coverage, e.g. supercontinuum generation in adapted fibers with anomalous dispersion —where light propagates faster for shorter wavelengths. Yet, such fibers are known for fluctuations owing to the onset of intricate nonlinear and dispersive dynamics. The last study in this thesis did however find that length optimization of such fibers is a viable technique for improving the stability, while still retaining broad spectra.