Fibre Bragg Grating Based Optical Sensors Accuracy Improvement

Fibre optic based sensors are becoming very successful tool for accurate and perturbation insensitive sensing of many non-electric and electric values. Favorite fibre sensoric structure capable of point and semi-distributed sensing of all effects influencing physical dimensions or refractive index of a fibre waveguide is the Fibre Bragg Grating. An interrogation of the grating reflectance spectrum is the important part of the fibre grating based sensorial system that can significantly affect the facilities of the measuring system and influence the applicability of the technology. The article analyses the spectrum scanning type of the grating sensor interrogation. We expose two main impacts that influence the accuracy of the measurement of the fibre grating spectral wavelength shift that bears the measured value. Both of them are connected with the quantity of the Grating Quality factor that we have defined for this purpose. The first impact refers to the way of scanning the wavelength spectrum of the sensorial grating and shows that the optimal measuring results are obtained when using the scanning filter wavelength spectrum width equal to the wavelength spectrum width of the sensorial fibre Bragg grating. The second impact lies in finding the grating design assuring the minimal size of the noise grating linewidth, also defined in this work. The work shows how the application of the specific apodization schemes to the fibre Bragg grating design can improve the noise grating linewidtg. Analysis of two suitable apodization profiles shows that the specific apodization of gratings can significantly improve the measuring accuracy of the scanning spectral interrogation of the grating based optical sensors.