We design three kinds of photonic crystal fibres (PCF) with two zero-dispersion wavelengths (ZDWs) using the improved full vector index method (FVIM) and finite-difference frequency domain (FDFD) techniques.Based on these designed fibres,the effect of fibre structure,pump power and wavelength on the modulation instability (MI) gain in the anomalous dispersion region close to the second ZDW of the PCFs is comprehensively analysed in this paper.The analytical results show that an optimal MI gain can be obtained when the optimal pump wavelength (1530 nm) is slightly shorter than the second ZDW (1538 nm) and the optimal pump power is 250 W.Importantly,the total MI gain bandwidth has been increased to 260 nm for the first time,so far as we know,for an optimally-designed fibre with Λ=1.4 nm and d/Λ=0.676,and the gain profile became much smoother.The optimal pump wavelength relies on the second ZDW of the PCF whereas the optimal pump power depends on the corporate operation of the optimal fibre structure and optimal pump wavelength,which is important in designing the most appropriate PCF to attain higher broadband and gain amplification.
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