MODELLING OF OPTICAL FIBRE SENSOR BASED ON STIMULATEDBRILLOUIN SCATTERING

Abstract

Distributed fibre optic sensing presents unique features that have no match in conventional sensing techniques. The ability to measure temperatures and strain at thousands of points along a single mode fibre is particularly interesting for monitoring of elongated structures such as pipelines, flow lines, oil wells and landslides. Brillouin Scattering in optical fibre is the result of the interaction between acoustic wave, a pump wave and a Stokes wave. In this research work investigations were carried out using VPI software for simulations. The probe wave was pulsed and launched into an optical fibre while the pump wave was launched through the other end of the fibre. The frequency difference between the two laser sources was set at 10.5-11GHz.This was used to investigate the existence of Stimulated brillouin scattering in an optical fibre and the parameters affecting the backscattered power. The possibility of using these parameters to model a fibre optic sensor was done. Investigation showed that when the pump and stokes (probe) wave counter propagate in the fibre there is transfer of energy between the pump and stokes wave resulting in the pump and stokes waves being depleted and amplified respectively as they travel along the fibre. Thus the Brillouin gain peaks at Brillouin frequency. Further investigations show that the backscattered power was low for input power below5 dBm but increased rapidly above it and saturated above input power of 10 dBm for different fibre lengths. The effect of Polarization Mode Dispersion on Stimulated brillouin scattering was found to decrease the signal power over time, this was due to differential group dispersion impairing the interaction between the pump and probe wave. Further results showed that power reduces with increase in temperature, the frequency shift (Brillouin shift) is directly proportional to temperature, and this was used to map out the temperature change along the fibre. The designed sensor can be used in civil structural monitoring of pipelines (i.e. leakage and intrusion), bridges, dams and railways for disaster prevention.