Self-injection Locking Laser Enabling Robust 800 Gbps/λ Coherent Lite Transmission

The rapid expansion of online video streaming, cloud computing, and the Internet of Things (IoT) has led to a substantial increase in the volume of data processed within data centers (DCs). The self-homodyne coherent (SHC) transmission scheme emerges as a promising solution for next-generation data center interconnection (DCI) due to its ability to eliminate carrier recovery algorithms. Without the need for such algorithms, the performance of SHC systems primarily hinges on phase noise, which arises from the laser linewidth and the fiber length mismatch between the signal path and the local oscillator (LO) path. In SHC systems based on distributed feedback (DFB) lasers with MHz-level linewidths, the fiber length mismatch must be limited to mere centimeters to maintain communication quality. However, these systems are highly susceptible to phase noise induced by mechanical vibrations, current fluctuations, ambient temperature changes, and other environmental factors. In this study, we design and demonstrate a hybrid integral self-injection locking (SIL) laser with an exceptionally narrow intrinsic linewidth of 42 Hz, achieving a 2035-fold reduction compared to the free-running state. The integral linewidth was measured using the delayed self-heterodyne method with a 50 km delay fiber and found to be 15 kHz. The SIL mechanism effectively suppresses spontaneous emission, reducing shot noise and consequently lowering the relative intensity noise (RIN), which remains below - 140 dBc/Hz beyond 0.1 MHz. Leveraging this SIL laser, we successfully implemented an SHC transmission system capable of achieving net data rates of up to 800 Gbps per wavelength using 120-GBaud DP-16QAM signals, excluding the 20% forward error correction (FEC) overhead. Remarkably, the SHC system maintains reliability even under severe noise conditions caused by a 40 km fiber length mismatch. The proposed Si3N4 micro-ring is based on wafer-level processes, alleviating the limits on design, cost, and fabrication. Hence, the laser shows great potential for prototyping and large-scale production for data center interconnection.