Towards ultra-fast and ultra-broadband all-fibre laser systems for the mid-infrared

The ongoing research and development of laser sources pushes the spectral boundaries of their operation wavelengths. Located on the long-wavelength side of the electromagnetic spectrum, the mid-infrared (IR) region has attracted considerable attention due to its unique overlap with the strongest rotational-vibrational absorption lines of many common molecules, as well as with the strongest absorption peaks of liquid water. These unique features of the mid-IR spectral region have found numerous applications, ranging from remote gas sensing to ablative surgery of soft and hard tissue. However, ultrafast fibre lasers and ultra-broadband fibre sources operating in the mid-IR region are at a very early development stage, despite the advantages fibre-based systems offer in general, associated with their unique stability and high potential for power scaling. The main aim of this work was therefore to pave the way for the development of ultrafast and broadband all-fibre mid-IR fibre lasers and to fabricate the required in-fibre components. This goal was achieved by exploring three major topics. The main outcome was the construction of a novel ultrafast Ho³⁺, Pr³⁺ doped 3 μm class ZBLAN fibre laser system with a record-high peak power of 41 kW and ultrashort pulse duration. The developed laser system has the capability to benefit applications that require long-distance atmospheric propagation as well as applications associated with the precise ablation of biological tissue due to the overlap of the emission wavelength with the liquid water absorption maximum and the reduced atmospheric losses, associated with the water vapor and CO2 absorption, compared to existing Er³⁺ doped ZBLAN fibre lasers. The second major achievement presented in this Thesis was the direct femtosecond laser inscription of chirped fibre Bragg gratings into silica and into ZBLAN fibres via a newly developed technique. The inscription of chirped fibre Bragg gratings in ZBLAN fibre has never been reported before. The inscription of chirped fibre Bragg gratings is an important step towards an all-fibre mid-IR laser design as it enables intracavity dispersion management as well as chirped pulse amplification of mid-IR laser pulses. An all-fibre design is highly desirable due to its extraordinary robustness and versatility. The last key result was the demonstration of supercontinuum generation in chalcogenide and fluoride glass fibres, pumped by the developed ultrafast holmium [III] laser. This supercontinuum source demonstrates the high potential of the holmium [III] fibre laser for the generation of ultra-broadband optical radiation in the mid-infrared and thus represents an important milestone for the remote gas sensing applications.