Large-Signal Equivalent Circuit for Datacom VCSELs – Including Intensity Noise

Abstract

Optical interconnects (OIs) continue to require more and more sophisticated driver and receiver electronics as higher baud rates are pushed in datacom, mainly due to the bandwidth stagnation of the optoelectronic components such as the vertical-cavity surface-emitting laser (VCSEL) in the transmitter as well as the photodetector (PD) in the receiver. Another important focus is maintaining high energy efficiency for the OIs. For both cases, a reliable equivalent circuit model for high-speed VCSELs is required for link optimization. This work is an extension of our previously presented large-signal VCSEL equivalent circuit model, where noise is added to the physical processes that the VCSEL model is based on. Thus, a new step is taken in the strive of developing an even more accurate physics-based large-signal VCSEL equivalent circuit model for datacom applications. Following a detailed description of the VCSEL noise modelling, a presentation is given on simulated results of VCSEL relative intensity noise (RIN) spectrum and eye diagrams under 28 Gbaud on-off keying (OOK) and pulse-amplitude 4 (PAM4) modulation, and that are compared with corresponding measurement results. Good agreement is found over a wide range of VCSEL driving conditions and temperatures. A 28-GHz-bandwidth VCSEL is applied for the demonstration, and the presented noise extended VCSEL equivalent circuit model was implemented in Verilog-A, and the simulations were performed using Cadence Spectre.