Silicon Photonic Tunable CROW Filter With Ultra-Narrow Bandwidth for Carrier-Extracted Self-Coherent Detection

The growing demand for high-capacity short-reach optical interconnects in datacenters and 5G fronthaul networks imposes stringent requirements on spectral efficiency, footprint and power consumption. In such applications, silicon photonic (SiP) self-coherent detection emerges as an attractive solution. As an advanced SiP device, microring resonator (MRR) also gains a significant attention due to its design flexibility and wavelength selectivity, heightening its application potential in optical communication systems such as self-coherent detection systems. In this work, we demonstrate a silicon photonic tunable ultra-narrow-bandwidth (UNB) coupled resonator optical waveguide (CROW) filer based on second-order add-drop microring resonators. Under the minimal insertion loss condition for the drop port, we optimize the coupling coefficients to achieve an UNB CROW filter with a low insertion loss. Significantly, the CROW filter has been demonstrated with a 1.26-GHz 3-dB bandwidth, 4.62-GHz 20-dB bandwidth, approximately 4-dB insertion loss and 60-dB extinction ratio (ER) based on a 1-μm-wide ridge waveguide which has a weaker sidewall scattering compared with a strip waveguide. Due to its ultra-narrow bandwidth and high ER, it can provide a good rejection of signal components for a self-coherent signal, thereby a pure optical carrier can be extracted to realize coherent detection without an additional local oscillator (LO). Applied in the carrier-extracted self-coherent (CESC) detection system, achieving a 168.3 Gb/s transmission rate over 100-km standard single mode fiber (SSMF), the performance of this CROW filter is well verified.