By adjusting the coating parameters to vary the refractive index of the thin film material, we are able to fine tune the bandwidth of a narrow bandpass filter to an arbitrary value. The relation between the varied index Deltan and the maximum arbitrary bandwidth was analyzed. A 4-skip-0 bandpass filter for a 100 GHz DWDM system was designed and fabricated. In addition, the relation between the tolerance of the index and the bandwidth was also analyzed to avoid broadening or narrowing the bandwidth. The final results showed that the arbitrary bandwidth met the requirements very well.