Exploring novel host glass materials to enhance the performance of fibre lasers in the middle-infrared and visible spectral bands

<p dir="ltr">Fibre lasers have become a versatile tool in an ever-increasing number of applications in industry, medicine, and science. Especially notable their advances in near-infrared spectral band, where multi-kW power level systems have enabled precise material processing, while low propagation loss in passive silicate glass fibres have allowed the establishment of a worldwide-scale optical communication network. Some of the applications, however, require the wavelengths going beyond the near-IR. For instance, in the middle-infrared region, strong water absorption bands can be targeted for biological tissue surgery, while various molecular fingerprints would allow the development of sensing and security tools. In the visible spectral band, laser sources open the way for the creation of high-contrast displays with a wide colour gamut. Certain visible wavelengths are employed for retinal procedures, skin treatment, and fluorescence imaging. </p><p dir="ltr">Advancing fibre laser sources in both of these spectral bands is exacerbated by a unique set of problems, related to the host glass. In middle-infrared, the increasing propagation loss of the most common fluorozirconate glass and its poor thermomechanical properties limit the applications and further power scaling beyond 3.8 μm. Going the opposite way, short-wavelength pumping requirements promote the photodarkening process, and the high level of scattering loss constrain the use of the most developed silicate glass for visible fibre lasers development. This study attempts to address the issues of both the mid-IR and visible sides of the spectrum. The results show that fluoroindate glass at its current stage of development with its wider transmission window and low background loss is a suitable alternative for a common ZBLAN to push the operating limit beyond 4 μm. In the corresponding Chapter, several methods of mitigating the photodarkening in silicate glass were discussed, including the changing of the host glass composition and nanoparticle-doping. These concepts have been tested using silica-based glass fibers doped with rare-earth ions (Sm³⁺, Dy³⁺), that enable direct generation of visible laser light. </p><p dir="ltr">In general, the research aims to deepen the understanding of existing challenges associated with host glass materials used in the mid-IR and visible spectral bands, while also outlining future steps to expand the boundaries of fibre laser technology.</p>