The main aim of this study was to investigate the anomalous linewidth behaviour of the DFB fibre lasers, as the observed linewidths of these lasers remain far above their predicted theoretical limit based on the Schawlow-Townes linewidth formula. Narrow linewidth, single-frequency fibre lasers are attractive sources for optical coherent communication, wavelength division multiplexing, optical sensors and spectroscopy, as they have kilohertz linewidths, direct compatibility with the fibre network, wavelength tunability and are simple to fabricate. Another aim of the study was to extend the operating wavelength of the fibre DFB lasers around 2 μm, this research was driven by the number of possible applications in areas such as remote gas sensing, laser imaging detection and ranging (LIDAR) and medicine. in the fibre, a nonradiative transition in the 2 μm region and also the large intrinsic losses of the silica fibre associated with the 2.1 μm wavelength.