Coupled resonator optical waveguides (CROWs) have broad applications in optical communications, optical computing, and quantum optics. They can flexibly manipulate spectra and generate slow light to enhance light-matter interactions. Their operation wavelength range is characterized by the 3-dB bandwidth, which equals the product of free spectral range (FSR) and the inverse of finesse (ℱ). However, the bandwidth has been constrained by a fundamental tradeoff between these two key parameters: achieving a large FSR tends to weaken the evanescent coupling and reduce ℱ-1. This seesaw tradeoff severely limits the bandwidth. Previous attempts to alleviate this tradeoff remained within the evanescent coupling framework, leaving FSR and ℱ still mutually dependent.
Recently, a collaborative research team led by scientists from Sun Yat-sen University and Shanghai Jiao Tong University has turned to topological nanophotonics for a solution. In their recent work published in PhotoniX, they introduce topological couplers into CROWs and successfully decouple ℱ from FSR, leading to a significant improvement of 3-dB bandwidth.
The key innovation is replacing conventional evanescent couplers with topological couplers. Protected by topology and symmetry, the topological couplers exhibit a robust coupling ratio that is independent of coupling length. When the cavity size is reduced to enlarge the FSR, the coupling ratio and ℱ remain almost unchanged. The two parameters are thus decoupled and the bandwidth constraint is alleviated. In the experiment, the topological CROW achieved an FSR of ~32 nm and a broad bandwidth of 9.0 nm around 1550 nm. This bandwidth represents a significant improvement compared with conventional microring CROWs.
To further demonstrate the functionality of their approach, the researchers cascade topological CROWs to construct a two-channel add-drop filter. The filter exhibits tolerance against dimensional errors, which are common in fabrication. Even with errors of ±10 nm, the 3-dB bandwidth of all transmission peaks remains above 4.7 nm. The team goes a step further by performing high-speed data transmission experiment. Both drop ports successfully support data transmission at 170 Gb/s, laying the foundation for their application in wavelength division multiplexing.
This work exploits topological principles in nanophotonics. By converting topological protection and symmetry analysis into concrete device design methods, the team has established a paradigm for overcoming intrinsic tradeoffs in on-chip photonic devices. The demonstrated topological photonic devices provide promising applications for on-chip filtering, nonlinear optical processing, and enhanced light-matter interactions—paving the way for next-generation high-performance photonic integrated circuits.
