The simplified and accurate analysis of in-line fiber filter is occurred by derive an easy theoritical formula for the refractive index of silica doped either GeO2 , P2O5 , F or B2O3 as a function of both the wavelength and the dopant ratio. At a specific filter cutoff wavelength (of either high pass filter, HPF, or low pass filter, LPF), the calculations of the dopant ratios of a two pairs materials (GeO2 and P2O5) and (F and B2O3) are done.
Two in-line fiber filters (HPF and LPF) are cascaded to make a bandpass filter.
These simplified formulas are applied for 8-channels demultiplexers. The accuracy of these simplified relations are very good agreement if it is compared with the original relations.
In-line fiber filter, Germanosilicate, Phosphosilicate, Borosilicate, Fluorosilicate, bandpass filter and demultiplexer