Flat-top narrow-band spectral response obtained from cascaded resonant grating reflection filters

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Abstract
Cascaded identical resonant grating reflection filters are shown to exhibit flattened spectral responses when the individual filter elements are cascaded π out of phase. Off resonance, the net response of a cascaded arrangement is given approximately by the sum of the individual filter responses. Cascading filters π out of phase thus result in a reduction in the off-resonance reflection levels and correspondingly an increase in the spectral bandpass ratio. The spectral bandpass ratio is a figure of merit used to gauge the flatness of a response and is defined as the ratio of the linewidth at an efficiency of 90% to the linewidth at an efficiency of 10%. Cascading two and three filters in this manner results in respective increases in the spectral bandpass ratio of three times and more than five times that of a single filter.