Passband shapes that minimize the insertion loss and bandwidth of coupled-resonator bandpass filters

• DOI: 10.1364/opticaopen.23917578
• Publication Date: 2023-08-10T09:03:57Z
• Authors: Xinchang Zhang; Dingning Li; MANUJ KUMAR SINGH; Milos Popovic
• Keywords: Passband; Narrowband; Decoupling (probability); Q factor

Numerous emerging applications of photonic integrated circuits are calling for extremely narrowband and/or low-insertion-loss bandpass filters. Both properties fundamentally limited by resonator losses, i.e. the intrinsic quality factor constituent resonant cavities. However, choice inter-cavity and bus couplings establishes tradeoffs between these two passband shape, which have been little explored. Using widely-used second-order system as example, we show that new classes filter shapes provide lowest insertion loss narrowest bandwidth a given Q. They often yield an uncommon, asymmetric coupling configuration, introduce straddle ``exceptional point'' in pole-zero configuration complex-frequency plane. A normalized design conclusions based on temporal coupled mode theory (CMT) model presented, including tool to apply results. These results may benefit loss-sensitive filtering such quantum-correlated photon pair sources RF-photonic circuits.