Femtosecond point-by-point fiber Bragg grating interrogation in multimode fibers using spatial mode demultiplexer

Fiber Bragg Grating sensors dedicated to harsh environments are increasingly inscribed in silica single-mode fiber with femtosecond lasers and the point-by-point technique. However, the recent advent of spatial mode multiplexers able to separate optical signals from the various modes propagating in multimode fibers, enables the interrogation of fiber optic sensors manufactured within them while discriminating between those different modes. This paper presents an investigation that was conducted in order to evaluate potential advantages of interrogating FBG sensors inscribed in multimode fiber by using spatial mode multiplexing. Different interrogation setups are used here in order to interrogate sensors with and without the spatial demultiplexer. Potential application for strain and temperature discrimination is studied and a curvature sensor with a novel principle of operation and based on Bragg Gratings formed by low-volume microvoids is presented as well.