A type of an interleaver with tunable bandwidth ratio for a dense wavelength division multiplexing (DWDM) system is proposed, which integrates a nested ring resonator with a Mach–Zehnder interferometer. The merits of the interleaver are its structure simplicity and the ultralarge tunability on bandwidth ratio. Based on the coupled-mode theory, the effects of essential parameter changes on the output waveforms have been analyzed. According to the theoretical analysis and simulation results, the bandwidth ratio is more determined by the coupling efficiency while the bandwidth is controlled by the optical length of the ring resonator. It was found that the bandwidth ratio is adjustable and varies between 1.5 and 10.5. Therefore, the interleaver can provide important implications for the DWDM system.