Abstract: To improve the filtering performance of conventional Mach-Zehnder interferometer filters，a Mach-Zehnder interferometer filter assisted by ″8″ shaped microring resonator was designed. According to the microring resonance theory，the waveguide theory，and the Mersen formula，the transfer function of the filter output was derived，and the output spectrum of the filter was simulated in MATLAB. By setting appropriate structural parameters，the output spectrum with obvious roll drop along the flat top edge was obtained at the output end of the filter. Using the principle of thermal expansion of SOI material，the effective refractive index of SOI material is changed by adjusting the temperature，so that the filter has tunability.

Key words: ring resonator, Mach-Zehnder interferometer, signal flow graph, optical device, tunable filter