Characterisation of a Long Period Grating Refractive Index Profile through Correlation Analysis.

The theoretical analysis of long period gratings (LPG) is often limited by poor characterisation of the refractive index profile within the core of the fibre. LPGs are often manufactured with a known period (to provide particular wavelength loss bands) but with poorly characterised UV induced core refractive index change and, often, an inaccurate length. The model is further complicated by the potential for a d.c. component in the LPG core refractive index through the UV inscription and annealing processes. Presented is a novel technique for characterising the refractive index profile of a un-apodized LPG based on the correlation between theoretical models of the loss bands and their side lobes. It is demonstrated that, with a priori knowledge of the cladding refractive index and LPG period, the amplitude and length can be calculated with accuracies of better than 10-5 RIU and 10-4 m respectively. Results illustrating the excellent match between theoretical and experimental transmission spectra of an LPG are presented and the potential error based on the uncertainty of the LPG period is discussed.