Development of Interferometric & Spectroscopic Techniques for High–resolution Spectral Measurements on Fibre Bragg Gratings Vol I of II

This thesis reports the development of a custom ˇ Czerny–Turner spectrometer, the SpectroBragg, which operates natively in the telecoms bands using an InGaAs photodiode array. The SpectroBragg’s photodiode array enables the monitoring of ∼ 70nm with ∼ 1 pm accuracy with a sampling rate of 30ms. The SpectroBragg is intended for use with fibre Bragg grating, FBG, sensors. The strain characterisation of a novel anisotropic FBG, inscribed in Corning SMF–28 telecoms fibre, via a two–photon process at 264 nm is also reported. The two–photon process is more efficient at FBG inscription and the FBGs are structurally anisotropic in isotropic fibre. These FBGs combine the characteristics of high–birefringence fibre at the FBG with the transmission characteristics of SMF– 28. Demodulation systems, that have polarisation–sensitivity, behave as polarisation analysers producing an intensity modulation dependent upon the state of polarisation, affecting high–accuracy phase sensitive wavelength interrogation schemes. In this thesis, two depolarisation approaches, a modified polarisation fixer system and a PDL balancing system, are examined as FBG signal depolarisers for anisotropic and isotropic FBG signals that are demodulated by the SpectroBragg spectrometer. The approaches are contrasted with the more traditional Lyot depolarisation system which is incorrectly specified. Rayleigh scattering provides the fundamental minimum to signal attenuation, consisting of two components: incoherently and coherently scattered radiation. The scattered radiation, within the coherence length of a source signal, adds coherently, modifying the guided mode, and for a phase sensitive demodulation system, is recovered as phase noise. In this thesis, the Hilbert transform technique, HTT, is used to analyse two– output, π–shifted interferograms from an all–fibre Michelson interferometer, to demodulate FBG signals, allowing for the recovery of interferograms not detectable with single–output interferograms. The Hilbert transformtechnique has been demonstrated to provide higher resolution wavelength determination than Fourier transform spectroscopy using shorter interferometer scans.