A heuristic model for mode-locked fiber lasers

Abstract

Offering the advantages of compact, portability, and low cost, fiber lasers have many applications. Mode-locked fiber laser is applied in many fields such as optical sensing, and super-continuum generation. Mode-locking requires balance of various effects such as gain, loss, dispersion and nonlinearity. An in depth understanding of how the various effects interact in a laser cavity would help the laser design. Laser modeling can be used to analyze laser dynamics. With given parameters, a good laser model can give a prediction from simulation results which match the experimental observations and give the guidance on choosing the parameters of a laser. A good model does not always mean it contains all the effects in a laser system or it can give results close to experimental results. On the contrary, it can be a simple heuristic model. A heuristic model might not accurately match experiment data but helps to check whether one (or some) effect(s) is (are) responsible for an observed behavior in a laser cavity and give a good understanding on the working mechanism of lasers. The theoretical works based on a simple heuristic model where only the most intrinsic factors, e.g. gain and loss, are contained have been proposed. The works point out that even only containing fundamental effects, various complex states, i.e. multi-pulse generation, chaos, still exist. One should note that in a heuristic model, the choice of physical effects to be included is crucial and not easy. In our former model, pulse shaping, i.e. dispersion and nonlinearity, was considered not influence the pulse energy and it only contains gain and loss terms. However, such an assumption is not proper when a high peak pulse is lasing in the laser. Dispersion and nonlinearity have obvious impacts on the pulse, which further impacts the gain loss dynamics. In this thesis, we proposed an extension of our former model by taking into account the effects of pulse shaping. The model is applied to study the mode-locked fiber laser. In this thesis, we investigate both single- and multi-channel mode-locked fiber lasers. Obtaining a high energy (high peak and narrow pulse width) pulse in single-channel mode-locking is attractive. But complex states such as multi-pulsing, periodic and chaotic states also exist in the laser. Mechanisms of multi-pulsing, periodic and chaotic states, which are thought to be factors that limit the pulse energy, are investigated and the guidance on obtaining high energy short pulse is given. Multi-channel mode-locking is also studied. The impacts of the devices in a laser cavity on the mode-locked pulse are studied. We found the energies of the mode-locked pulses in different channels can be different. The light interaction through pulse shaping and the impact of the spectral filter are found to be crucial roles for this phenomenon.