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By implanting nonreciprocal phase shifts (NPS) into a ring-coupled Mach-Zehnder interferometer (RCMZI) structure, wavelength-selective isolation can be obtained over a wideband range. Different transmission lineshapes and structures of the nonreciprocal RCMZI were examined to meet the requirements of multiwavelength usage. It was demonstrated that a multiwavelength isolation with wideband and high-peak isolation could be achieved with just a reciprocal ring-coupled arm and a nonreciprocal linear arm in a MZI structure. For 5% ring loss per loop, the ring power coupling ratio kappa has a tolerance of 0.06 to keep the 30-dB isolation band wider than 0.3 × FSR. Also, an isolator over C-band was designed to verify the feasibility and discuss the wavelength-dependence issues.