Optical fibers containing fiber Bragg gratings (FBGs) are extensively used for remote sensing. Temperature or mechanical strain on a remotely placed FBG grating in a long fiber changes the FBG’s period and thus shifts its center wavelength; this shift can be as small as 1 pm for some very sensitive applications. Multiple FBGs along the fiber allow sensing at many different points. However, the spectrometer or swept-laser-based wavelength interrogators used to sense these wavelength shifts are complex and expensive.
A simple, integrated system for wavelength interrogation of FBG sensors has been developed by a researchers at the U. of Alberta (Edmonton, AB, Canada). In their approach, they used a tapered waveguide clad with omnidirectional Bragg reflector claddings to serve as bandpass filters. The dispersion within the tapered waveguides themselves causes lights of different wavelengths to be coupled through claddings at different positions along the waveguide. An image sensor detects the light’s position shift from a particular FBG sensor when its wavelength changes. A single tapered waveguide can be used for interrogating multiple FBGs along a fiber, and multiple tapered waveguides, fabricated closely side-by-side, can interrogate multiple FBGs from multiple fibers, all using the same image sensor. The experimental device was clad by silicon/silica (Si/SiO2)-based Bragg mirrors and was imaged by a microscope onto an indium gallium arsenide (InGaAs)-based infrared camera. Wavelength shifts down to 10 pm were resolved with a peak-detection algorithm. Reference: C. Potts et al., Opt. Lett. (2014); http://dx.doi.org/10.1364/ OL.39.005941.
