A closed-loop servo control based on balancing the gain of two probing frequencies is proposed for real-time Brillouin optical time-domain analysis (BOTDA) without post-processing. With the most basic BOTDA hardware setup, the system can perform measurement in 150 ms and track a sudden Brillouin frequency shift (BFS) change in excess of 300 MHz (corresponding to a temperature change of more than 250°C) over ∼5 km of fiber with a spatial resolution of 2 m. Moreover, the feedback loop is independent of the loss experienced by the probe and pump, with no requirement on the BFS uniformity along the fiber. All these advantages make the proposed system suitable for field applications in harsh environments.

A simplified model describing the polarisation characteristics of spun fibres is proposed, aiming at determining how close to a circularly birefringent medium such a fibre is. This is of crucial importance regarding the interest of such a medium for magneto-optic sensing using optical fibres, mostly exploited in fibre optics current sensors (FOCS). The spun fibre is modelled as a stack of thin linearly birefringent plates, each experiencing a slight incremental rotation. The eigenvalues and eigenvectors of the full stack of plates are determined, enabling us to assess how far a given spun fibre is from an ideal circularly birefringent medium for any spinning rate and linear birefringence. Experimental validation is carried out to compare the model results with real fibres.