In dense wavelength division multiplexing (DWDM) systems, the dense signals should be broken into two groups with large channel spacing firstly in the demultiplexing process. Therefore, the cost of the whole system can be reduced without upgrading the existing filter. The key device to realize this function of DWDM system is interleaver. In this paper, basing on the analysis of the working principle of micro-ring resonator filters and Mach-Zehnder (M-Z) interferometer, the effects of essential parameter changes on the output waveforms (amplitude and phase property) have been deeply discussed. On the basis of the phase properties of micro-ring resonator filters, we proposed a nested ring resonator (NRR) which has a unique phase characteristic. Through our research, we found that the clever combination of NRR and M-Z interferomert can achieve wideband filtering. In this paper, we proposed a new bandpass filter and an interleaver that consists of MZI side-coupled with a NRR.

The output function of the nested ring resonator coupled to M-Z interferometer has been deduced. The output of the NRR coupled to M-Z interferometer is simulated using the output function, The influence of key parameters on the bandwidth, center wavelength, ultra-narrow bandwidth, passband flatness, sidelobe depression and bandwidth ratio of the filter has been analyzed in this paper.