| In a distributed Brillouin sensor system, it is crucial to keep the pulse energy uniform for constant signal to noise ratio. This means that the variable DC leakage (pulse base) through the electro-optic modulator must be locked. In this thesis I examine two different methods of locking the pulse base level and look at the advantages and disadvantages of each locking method. It is found that the two locking methods, one based on a lock-in amplifier and the other using proportional-integral-derivative control, both have applications in which they excel at locking the pulse base. Also, a technique to simultaneously lock the pulse base, top, and width is developed and tested.;Also, studies of the Brillouin linewidth under cw pump and Stokes waves are done in order to confirm the validity of a new definition of the threshold power in Brillouin fiber amplifiers which involve both input pump and input Stokes waves - all previous threshold definitions took into account only the input pump power.;Finally, some interesting lineshape characteristics such as spectral hole burning and side-lobes on the Brillouin spectrum are observed for high power 2 ns Stokes pulses and their origins explained qualitatively. The evolution of these features with increasing pump power is investigated. The effect of laser linewidth and fiber length on these features is also considered.;In the field of structural health monitoring, it is often advantageous to monitor the dynamic behaviour of a structure in real-time. The traditional distributed Brillouin sensor does not allow for this dynamic measurement due to the need to sweep the frequency difference between the two lasers and subsequent averaging of waveforms. For the first time to our knowledge, a real-time vibration sensor based on polarization-state perturbations in stimulated Brillouin scattering instead of resonant frequency mismatching monitoring of the Brillouin spectrum has been proposed. The long measurement time of traditional distributed Brillouin sensors is avoided by eliminating the frequency sweep of the pump and Stokes lasers and instead locking them at a single beat frequency corresponding to the static strain of the structure in which the fiber is embedded. This unique sensor allows measurement of vibration frequencies along a sensing fiber as shown in laboratory experiments and also the detection of impact waves from passing vehicles in field tests in which the sensor was embedded in the concrete pavement of a highway. A 20 ns pulse width with potential spatial resolution of 2 m was used over a sensing length of 300 m. |
