Abstract
Narrow linewidth fibre lasers are used in applications such as optical fibre sensing, spectroscopy, and optical communication. To build such fibre lasers, it is important that the erbium doped fibre laser theory be understood from the outset by modelling laser-using rate and propagation equations. An erbium doped ring cavity fibre laser model is shown in our design configuration and is compared with the output from different coupling ratios, and different lengths of gain medium. It was observed that the slope efficiency of the fibre laser in our model, is connected to the coupling ratio and fibre length. This proved to be very important in optimisation of the fibre laser where power budgeting was considered with respect to losses incurred by components used in the configuration, and the length of the gain medium. In optimisation of the coupling ratio, the amplitude of the output power was balanced for two wavelengths reflected from 1530.8nm and 1550.08nm. When pump power of 100mW was used, a balanced output power of 3.4mW was obtained for a wavelength of 1530.18nm, and 3.1mW for a wavelength of 1550.08nm. The balanced output power at a coupling ratio of 95% was obtained, as well as an OSNR of 61dB for the laser outptut spectrum for each wavelength. The fibre laser pump power was characterised to an output power of 100mW. A tunable filter was incorporated in the configuration to select the wavelengths. The linewidths were also measured. Prior to measuring the linewidths, the stability of the fibre laser was monitored for 2 hours at intervals of 5 minutes at a 95% coupling ratio, where the two output power were balanced. A fluctuation of 0.177dB and 0.169dB at wavelength 1550.08nm and 1530.18nm respectively, were obtained. Using a saturable absorber (SA), the ouput power showed no visible fluctuation for either wavelength. The linewidth of the fibre laser without a SA was measured, and outcomes of 27.7kHz and 28.3kHz for 1530.18nm and 1550.08nm respectively, were indicated. With the SA, linewidths of aproximately 25.3kHz and 21.1kHz for 1530.18nm and 1550.08nm were obtained. This showed an approximate 9% and 25% reduction in the optical linewidths for output wavelengths of 1530.18nm and 1550.08nm, respectively.