Characteristics of cascaded FILM filters in wavelength-division-multiplexing system

The optical signal along the light-path in the optical network will traverse a cascade of optical filters. The equivalent pass-band of the cascaded filters decreases in comparison with a single filter, and it is required that the light sources should be much more stable. In this paper, we investigated the cascaded characteristics of FILM filters that are widely used in WDM systems due to their flat pass-band and low insertion loss. Both the influence of amplitude response and the influence of phase response are considered together. Firstly, we give two equations of the pass-band characters and the phase response of FILM filters, and then calculate the variation of the cascaded FILM filters' bandwidth versus the number of the cascaded filters. We evaluated the influence of the filters' phase nonlinearity in more detail. The simulation results imply that the frequency range of the cascaded FILM filters under the condition of zero power penalty decreases, and the power penalty curve caused by phase nonlinearity fluctuates more drastically than that without the consideration of the phase response.