Simultaneous measurement of relative humidity and temperature based on forward Brillouin scattering in polyimide-overlaid fiber

A polyimide-overlaid single mode fiber is used to realize simultaneous measurement of relative humidity (RH) and temperature based on the forward Brillouin scattering (FBS) in the fiber. The FBS driven by the optomechanical interactions in the fiber gives rise to the transverse acoustic waves that propagate transversally or circumferentially, which bounce at the silica-polyimide boundaries. The properties of the reflected acoustic waves reveal the boundary conditions and thus can be used for quantifying variations of the environmental parameters. Both the RH and temperature variations change the acoustic properties of the polyimide fiber, but result in different responses manifested in the reflected acoustic waves. By monitoring the 3-dB linewidth and center frequency of a selected resonant peak in the FBS spectrum, RH and temperature variations can be extracted. Experimental results show that the proposed sensor is able to realize RH measurements with RHdependent center frequency shift and linewidth variation coefficients of 6.32 kHz/%RH and 2.56 kHz/%RH, respectively. In the simultaneous sensing tests, RH and temperature are successfully discriminated with small standard deviations of 1.651%RH and 0.279 degrees C, respectively. Such a novel and simple sensing technique for simultaneous measurements will pave a way for developing cost-effective, reliable and practical humidity sensing systems.