Near- and mid-IR fibre grating devices and applications

Abstract

This thesis presents a detailed research work on the fabrication, characterisation and applications of optical fibre grating devices with operation wavelengths cover from near- to mid- infrared (IR) range. One of the major contributions described in this thesis is the systematic investigation on the structures, fabrication methods and spectral, thermal, strain and surrounding refractive index (SRI) characteristics of near-IR fibre gratings including fibre Bragg gratings (FBGs), chirped fibre Bragg grating (CFBGs) and long period grating (LPGs). For some applications, such as special engineering sensors and high power fibre laser, the fibre gratings have been fabricated on different novel fibres (metal coated fibre and large mode field fibre), respectively. Another important contribution from the studies is experimental investigation on 45º tilted fibre gratings (45°-TFGs) and excessively tilted fibre gratings (ex-TFGs), and their applications. 45º-TFGs with high polarisation dependent loss (PDL) in single mode and polarisation maintaining (PM) fibres have been fabricated. The 45°-TFG has been employed as in fibre polariser to obtain the single polarised laser, which has been further developed as transverse loading sensor achieving high sensitivities. Furthermore, all fibre Lyot filter with narrow bandwidth (26 pm), constructed by two 45º-TFGs with 100m long cavity in PM fibre has been demonstrated. For ex-TFGs, SRI sensor based on a surface modified 81°-TFG, showing capability to detect glucose concentration with relatively high RI sensitivity (~168nm/RIU). Finally, an all-fibre loading sensor based on a hybrid 45° and 81° TFG structure has been demonstrated. Finally, I have fabricated fibre gratings into mid-IR 2μm range. The mid-IR FBGs have been evaluated for thermal and strain response, revealing higher temperature sensitivities than that in near-IR range. The mid-IR LPGs have been investigated for the thermal and refractive index sensitivities, also showing significant enhancement. The 45°-TFGs in mid-IR have been investigated for their PDL characteristics. The mid-IR FBGs and 45°-TFGs have been employed in Tm-doped fibre laser cavity to realize multi-wavelength continued wave (CW) and single polarisation operation.