The analysis on the traditional asymmetric Mach-Zehnder interferometer (AMZI) optical filter based on two 3 dB directional couplers (DCs) shows that by adding an additional nonlinear phase generated by phase-generating coupler (PGC) to the original phase difference of the AMZI, its non-periodic frequency response can be modified, and a strictly periodic spectrum can be obtained. A novel structure of the AMZI filter using two PGCs before and after the AMZI region is proposed. With the needed free spectrum range (FSR) of 20 nm, the design and optimization of the device are performed using polymer SU-8 as the core and PMMA-GMA as the buffer. Though the insertion loss (IL) gets larger than that of the traditional AMZI filter, the FSR is nearly uniform as the expected period of 20 nm.