Low-Cost Wideband Interrogation System for Fiber Optic Sensors

Abstract

The interrogation of optical fiber sensors (OFS) often relies on complex devices such as Optical Spectrum Analyzers (OSA) which are expensive with low portability and mainly suited to laboratory measurements or dedicated interrogation systems with limited spectral range. An interrogation unit was designed and fabricated using a photodetector combined with a Micro Electro-mechanical System and a Fabry-Pérot Interferometer (MEMS-FPI) working as a tunable filter with a response in the range 1350 to 1650nm. Deconvolution techniques were applied to mitigate the effect of the broadband response of the tunable filter on the measured signal. The performance of the unit was validated with the interrogation of Long Period Fiber Gratings (LPFG) as temperature, refractive index (RI) and relative humidity (RH) sensors. For the temperature a sensitivity of 0.135 ± 0.007nm/°C was obtained, which showed a 4.9% relative error when compared to the same measurement with an OSA. For the RI a sensitivity of 147 ± 11 nm/RIU was obtained which showed a relative error lower than 1% when compared to the OSA. For the humidity, sensitivities of 0.742 ± 0.005 nm/%RH and 0.056 ± 0.006 nm/%RH were obtained, with errors of 2.75% and 6.67% respectively, when compared to a commercial dedicated interrogation system. The low relative error obtained when compared to commercial alternatives, shows the potential of the system to be used in real-time applications that require portability, low-cost, energy efficiency and capacity for integration in dedicated systems.