High-Extinction Photonic Filters by Cascaded Mach–Zehnder Interferometer-Coupled Resonators

In this study, we demonstrate high-extinction stop-band photonic filters based on Mach–Zehnder interferometer (MZI)-coupled silicon nitride (Si₃N₄) resonators fabricated using I-line lithography technology. Leveraging the low-loss silicon nitride waveguide, our approach enables the creation of stable, high-performance filters suitable for applications in quantum and nonlinear photonics. With destructive interference at the feedback loop, photonic filters with an extinction ratio of 35 dB are demonstrated with four cascaded MZI-coupled resonators. This cascading design not only enhances the filter’s extinction but also improves its spectral sharpness, providing a more selective stop-band profile. Experimental results agree well with the theoretical results, showing linear scaling of extinction ratios with the number of cascaded MZI-coupled resonators. The scalability of this architecture opens the possibility for further integration and optimization in complex photonic circuits, where high extinction ratios and precise wavelength selectivity are critical for advanced signal processing and quantum information applications.