Fourier Transform Spectroscopic Demodulation of Fibre Bragg Grating Arrays

The application of interferometric techniques to the measurement of the thermal and strain induced shift in the resonant reflected wavelengths from an in-fibre Bragg grating array is reported in this thesis. High power issues relating to the reliability, and subsequently to the interrogation techniques, of fibre Bragg gratings inscribed with an infrared femtosecond laser using the point-by-point writing method are reported. The study has revealed the presence of broad spectrum power losses. When high powers are used, even at wavelengths far removed from the Bragg condition, these losses produce an increase in the fibre temperature due to absorption in the coating. The principal interest of the work is in the application of Fourier Transform Spectroscopy and Hilbert Transform Processing techniques to the calibration of interferometric delay and to provide simultaneous, high resolution measurement of all gratings in an array. These approaches are applied to interferograms captured using customised interferometric configurations. The interferometric delay is scanned by mechanical means. Calibration is based on recovery of the temporal phase vectors of the interferograms fromwhich non-uniformdelay sampling is corrected for using the temporal phase vector obtained from a reference interferogram. This thesis demonstrates the efficacy of the Hilbert transformprocessing approach for long-scan delay calibration (1.2 ns delay). A referencing systembased on amultiple transverse mode beam, allowing co-propagation with the measurand beam through the demodulating interferometer, is also demonstrated to provide identical Fourier transformspectralmeasurements. The Hilbert transform approach to grating interrogation is also applied to an allfibre interferometric configuration to provide∼3 pmwavelength resolution. TheHilbert transformtechnique basesmeasurement on the ratio of reference andmeasurand temporal phase vectors, providing measurement of the mean reflected wavelength of the gratings. A scheme where the reference interferogram for the all-fibre interferometer is generated by a fibre Bragg grating is also evaluated