Spectral analysis of the TEC fiber Bragg grating

We have fabricated Bragg gratings by using thermally expanded core fiber (TEC-fiber) and their transmission spectrums have been analyzed. The measured spectrums show double lobe structure reflecting the effective index variation along the fiber axis and also the characteristic peak splitting caused by the grating pair formed over the expanded section, which implies that the effective index profile of the TEC-fiber along the axis could be deduced from the spectral response of the inscribed Bragg grating.