We report on the use of π-shifted interferometry to extend the distances over which signals from Fibre Bragg
gratings, FBGs, can be effectively demodulated. The Hilbert transform technique is successfully applied to
demodulate differential π-shifted interferograms recovered after transmission over a 20km span of fibre. However,
this technique applied in standard way does not overcome the signal-to-noise-ratio/degradation due to Rayleigh
scattering over the 20 km.
We report strain characterization of birefringent fiber Bragg gratings fabricated in hydrogen-free photosensitive Ge/B
codoped PS1250/1500 fiber by high-intensity femtosecond 264 nm pulses. These anisotropic FBGs demonstrate high
PDL value of 8 dB but, unlike the FBGs inscribed in PM fibers, show shape preservation under strain of [0, 700 &mgr;&Vegr;] by
interrogation based on broad-band source and optical spectrum analyzer.