High resolution fibre Bragg grating sensor demodulation using a diffraction grating spectrometer and CCD detection

In-fibre written Bragg gratings (FBGs) have been shown to offer many advantages over conventional interferometric and polarimetric sensors in particular for applications requiring embedded sensors. In certain implementations, robust readout systems are necessary requiring immunity to vibration and the ability to operate at changing temperatures and pressure levels. These requirements may sometimes rule out the use of established demodulation schemes, or the use of passive optical fibers that are inherently susceptible to temperature fluctuations. Another important parameter of signal processing techniques is the possibility of addressing more than one single sensor. In this article, a passive WDM multiplexed technique for high resolution detection of the wavelength of peak reflection from FBGs is reported. The approach is based on overcoming the low spectral resolution associated with inexpensive grating and linear array CCD detector combinations, which is typically 0.1 nm. Initial results obtained in our laboratories indicate that a resolution of 1 pm can easily be achieved. We also report a demodulation technique, based on curve fitting algorithms, that uses short broadband gratings and that can be used with low levels of light at the detector