Q-switched fibre laser sources for distributed sensing applications
This thesis examines pulsed fibre sources for distributed sensing applications. A number of Q-switched fibre laser sources optimised for high peak powers, narrow linewidth and short pulse duration are described. The source specifications were dictated by the requirements of Raman and Brillouin distributed sensing systems. The spatial resolution of distributed sensors is related to the pulse width whereas the range is dependent on the power launched into the sensing fibre. Brillouin distributed sensors also require that the source linewidth is less than 10 GHz, the separation between the Rayleigh and Brillouin backscattered light. This constraint on laser linewidth leads to coherent Rayleigh noise on the Rayleigh backscattered trace. This noise can be reduced by a technique of frequency shift averaging. A Q-switched laser incorporating this technique was developed, which resulted in a Brillouin distributed temperature sensor with a temperature resolution of 1.4°C and a spatial resolution of 10 metres over a range of 6.5km. The development of high power Q-switched fibre lasers leads to the possibility of generating Raman shifted pulses at wavelengths of 1.64-1.65µm. Interest in this wavelength region stems from the increase in sensitivity to fibre micro-bend losses at these higher wavelengths and the ability to monitor the fibre whilst carrying out live data transmission. A diode pumped, pulsed source at 1.64µm producing 8 Watt, 10ns pulses through a process of Raman generation was demonstrated. Q-switched laser technology was also used to increase the dynamic range of 1.65µm OTDR. The technique utilised delayed Raman amplification of the 1.65µm signal pulse by a co-propagating 1.53µm pump pulse. Amplification occurs when the two pulses overlap. The position of the overlap is determined by the initial delay between the pulses and the fibre dispersion. An increase in dynamic range of 17.5dB has been observed and the 1.65µm OTDR range was extended to in excess of 100km.