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 photoniccrystal fibre (PCF) with a Ge-doped core is numerically investigatedby using the beam propagation method (BPM). It is shown that theinterstitial air holes (IAHs) can make Bragg resonance wavelengthλ_B shift a little towards short wavelengths andincrease λ_B-λ₁ (the wavelength spacingbetween the main peak with Bragg resonance wavelength λ_B and the first side peak with wavelength λ₁ and thecoupling coefficient к of Bragg resonance. Moreover, whenthe ratio of air hole diameter (d) to pitch (Λ),d/Λ, is small, IAHs can suppress the cladding moderesonance. When d/Λ is large, IAHs increase the numberof mode that could strongly interact with the fundamental mode. Bycomparing the transmission spectral characteristics of PCF-basedfibre Bragg grating (FBG) with IAHs with those without IAHs at thesame air-filling fraction, it is clarified that the change oftransmission spectral characteristics of PCF-based FBG with IAHs isnot due to a simple change in air-filling fraction. It is alsoclosely related to the distribution of interstitial air holes.