FBG-DETECT

FBG-DETECT project aims to demonstrate a new interrogation system for fiber Bragg grating (FBG) optical sensors dedicated to acoustic waves measurements for structural health monitoring (SHM). For such a system, the key devices are tunable liquid-crystal (LC) photodiodes (PD) which have been recently demonstrated by using an original approach based on the integration of polymer-based LC microcells onto III-V semiconductor photodetectors (InGaAs/InP). A reliable and reproducible spectral tuning of these PDs have been achieved over 80 nm around 1550 nm with a low applied voltage (8V). The FBG-DETECT’s multidisciplinary consortium proposes to exploit the unique properties of these devices to track the spectral position of FBGs at low frequency (drift due to the deformation of the structure to be monitored and/or to large thermal variations), while the acoustic wave measurements will be conducted at high frequency by convolution of the transfer function of the respective spectra of the PDs and the FBG sensors. This will greatly simplify the analysis chain and enable multiplexed measurements on a single line. To meet this system requirements, it is required to improve the devices performances in terms of sensitivity and linewidth, but also to develop a new fabrication process. Indeed, the hybrid technology used until now does not allow an assembly and individual control of each PD into a multiplexed system. To solve these issues, we will optimize the device design and tackle this challenge by demonstrating the first monolithic LC-based PD. In particular, we will develop an innovative “bottom-up” nano-structuring method to pre-align LC molecules inside a III-V microcell. The fabricated devices will be evaluated for ultrasonic measurement, first into a single-channel configuration (hybrid generation), then into a multichannel one (monolithic generation), leading to the final design of a low-cost optical instrumentation system for acoustic measurement in harsh environment.