High spatial resolution physical and chemical sensing based on BOFDA

Brillouin fiber sensors permit to perform distributed temperature and strain measurements along an optical fiber through the changes of the Brillouin Frequency Shift (BFS). In conventional Brillouin schemes operating in the time-domain, the BFS is detected by analyzing the interaction between a pulsed pump wave, and a counter-propagating probe wave. The duration of the pump pulse determines the spatial resolution, which however is limited to about 1m due to phonon lifetime. Alternatively, Brillouin optical frequency-domain analysis (BOFDA) and Brillouin optical correlation-domain analysis (BOCDA) configurations provide cm-scale, or even mm-scale distributed sensing capabilities. In particular, BOFDA sensors make use of a continuous wave (CW) pump wave with superimposed a small-signal modulation. The analysis consists in determining, by use of a vector network analyzer (VNA), the amplitude and phase of the corresponding modulation induced on the probe wave intensity, over a discrete number of modulation frequencies. In BOFDA sensors, spatial resolution can be improved down to the cm-range or mm-range, thanks to the pre-activation of the acoustic wave involved in the scattering process. In this work, we show that mm-scale spatial resolution can be exploited, in a BOFDA configuration, to perform both physical (temperature) and chemical (refractive index) sensing. For the latter, a side-polished fiber was used in order to make the BFS sensitive to the surrounding refractive index (SRI). A sensitivity of the BFS to the SRI as large as 293 MHz/RIU at = 1.40 is demonstrated experimentally and validated numerically.