Effect of interstitial air holes on Bragg gratings in photonic crystal fibre with a Ge-doped core

The effect of interstitial air holes on Bragg gratings in photonic crystal fibre (PCF) with a Ge-doped core is numerically investigated by using the beam propagation method (BPM). It is shown that the interstitial air holes (IAHs) can make Bragg resonance wavelength \lambda _\rm B shift a little towards short wavelengths and increase \lambda _\rm B-\lambda _1 (the wavelength spacing between the main peak with Bragg resonance wavelength \lambda _\rm B and the first side peak with wavelength \lambda _1) and the coupling coefficient \kappa of Bragg resonance. Moreover, when the ratio of air hole diameter (d) to pitch (\it\Lambda), d/\it\Lambda, is small, IAHs can suppress the cladding mode resonance. When d/\it\Lambda is large, IAHs increase the number of mode that could strongly interact with the fundamental mode. By
comparing the transmission spectral characteristics of PCF-based fibre Bragg grating (FBG) with IAHs with those without IAHs at the same air-filling fraction, it is clarified that the change of transmission spectral characteristics of PCF-based FBG with IAHs is
not due to a simple change in air-filling fraction. It is also closely related to the distribution of interstitial air holes.