Femtosecond laser-written fibre Bragg gratings for future all-fibre mid-infrared laser systems
The mid-infrared (mid-IR) wavelength regime in the electromagnetic spectrum has invited extensive attention in today’s world due to its unique overlap with the vibrational molecular resonances of atmospheric gases and liquid water. This extraordinary feature of the mid-IR spectral region has found great interest in various applications such as remote gas sensing, material processing, ablative surgery of soft and hard tissues. Therefore, lasers emitting at mid-IR wavelengths have received significant attention in ongoing research and development. Fibre lasers are a powerful tool to generate light at mid-IR wavelengths. However, mid-IR fibre laser development remains challenging, mainly due to the unavailability of fibre coupled optical components to create all-fibre laser cavities, which severely limits their practical applications.
This thesis investigates the feasibility of using femtosecond laser (fs) pulses to inscribe in-fibre components such as uniform fibre Bragg gratings (FBGs), tilted fibre Bragg gratings (TFBGs) and chirped fibre Bragg gratings (CFBGs) which can add a new perspective for mid-IR all-fibre lasers. Here we explore the potential of fs laser beams to directly inscribe type-I fibre gratings into the core of passive and active soft-glass fibres. We investigated and compared various fabrication methods and techniques to tailor the FBG properties, therefore, we could develop highly reflective and low loss gratings in the mid-IR regime. Temperature stabilization of the gratings via annealing was also performed to enable the development of high power, high efficiency, alignment-free all-fibre laser systems in the mid-IR region.
In this thesis we have demonstrated, the first in-fibre polarizer for the mid-IR region and the first linearly-polarized all-fibre mid-IR laser system by using a 45° TFBG. Another major achievement presented in this thesis is the demonstration of an in-band pumped grating stabilized Fabry-Perot laser cavity generating up to 1.06 W at 3.15 µm wavelength using dysprosium-doped fibre with a record high slope efficiency of 73%. Later, we also experimentally demonstrated the first stable mode-locking from an Er3+ doped fluoride fibre laser cavity using the novel two-dimensional saturable absorber materials PtSe2 and MXene operating near 2.8 µm wavelength. This linear cavity contained a high reflective CFBG to provide wavelength stability.
Our results presented in this thesis demonstrate the feasibility of developing potential in-fibre components for the mid-IR wavelength regime thereby paving way to the development of robust high-power, alignment-free, all-fiber mid-IR laser systems.