Experimental and Theoretical Investigation of Dysprosium Doped Fibre Lasers

Dysprosium (Dy3+), a trivalent rare-earth ion, doped ZBLAN fibre lasers offer emissions in the mid-infrared (mid-IR) and visible spectral regions, which have a wide array of important applications in various fields such as defence, sensing, material processing and medicine. As a laser-active ion, Dy3+ has several distinct and unique features that are convenient for mid-IR and visible fibre lasers, which have been largely overlooked. Over the last few years, Dy3+ has emerged as a viable candidate for mid-IR fibre lasers offering a wide spectral emission region (from 2.6{3.4 µm) at around 3 µm than other alternative rare-earth dopants. To access the full emission bandwidth of the 3 µm emission of Dy3+-doped mid-IR fibre lasers, a near-infrared (near-IR) pumping wavelength is required. However, all previous near-IR pumped Dy3+-doped mid-IR fibre lasers' performance has been limited by the detrimental pump excited state absorption (ESA). In this thesis, we have utilised two novel ESA-free pump wavelengths (0.8 µm and 0.9 µm) that enabled us to demonstrate a more efficient Dy3+-doped 3 µm fibre laser than existing near-IR pumped systems. Subsequent exploration of Dy3+ potential as a laser-active ion opens up opportunities for visible laser design. The Dy3+ offers two strong visible emissions in the blue (peak at around 480 nm) and yellow (peak at around 573 nm) spectral ranges. These visible emissions can be directly excited by the 450 nm pumping wavelength, which sets the Stokes limit to 94% and 79% for the 480 nm and 573 nm lasing wavelengths, respectively. Notably, a Dy3+-doped fibre laser offers yellow emission, which is rare and extremely difficult to access from a compact, efficient and cost-effective primary source. Moreover, the convenient availability of cost-effective high-power laser diodes at the pump wavelength and high theoretical efficiency limit suggests a real possibility of highly efficient and high-power visible fibre lasers, which demands a detailed investigation. Therefore, we present a detailed experimental and theoretical investigation of visible fibre lasers with a major focus on yellow emission. Experimentally, we measure the yellow laser output power with a maximum slope efficiency of 33%, which is less than half of the Stokes limit (79%). Subsequently, we investigate the potential causes of the low experimental slope efficiency and find contributions from the background loss of the fiber and ESA of the intracavity yellow light. We also provide possible solutions to reduce the impact of yellow light ESA on lasing performance, which are then verified by numerical simulations. Following this, we demonstrate a gain-switched Dy3+-doped yellow fibre laser for the first time (to the best of our knowledge). From the gain-switching Dy3+-doped fibre laser, we measure a maximum average yellow output power of 8 mW at a repetition rate of 1000 Hz with a corresponding pulse energy and pulse duration of 8 _J and 1.5 _s, respectively. We also investigate the power scaling potential of a yellow laser using a high concentration (4 mol.%) double-clad fibre and identify the challenges of a ZBLAN-based system. To mitigate the challenges of a ZBLAN-based system, we propose a new route to develop a compact, efficient, and high-power yellow fibre laser using a high-phonon energy glass host (e.g. silicate) for Dy3+.