Large-Signal Equivalent Circuit for Datacom VCSELs

Abstract

Increasing the baud rate in optical interconnects (OIs) will require the use of more sophisticated driver and receiver electronics. This will help overcome the stagnated bandwidth of the Vertical-Cavity Surface-Emitting Laser (VCSEL) and the pin-photodetector. Next generation OIs operating at single lane rates of 50+ Gbaud will therefore require careful co-optimization of the electronics and the optoelectronics. To facilitate this work there is a need of an accurate equivalent circuit for the optoelectronic components, functioning over a broad drive current and ambient temperature range. The VCSEL is the most important and complex to model due to its nonlinear behavior and strongly varying characteristics with drive current and ambient temperature. For this purpose, a large-signal equivalent circuit dedicated for high-speed datacom VCSELs has been developed and is presented here. The distributed electrical parasitics in the device layout are carefully considered, the intrinsic speed limitation from carrier transport effects in the Separate-Confinement-Heterostructure (SCH) and the carrierphoton interaction in the Quantum Wells (QWs) are included, and self-heating effects in the device are monitored. The circuit is purposely based on physical instead of empirical models so that it can provide usable feedback to VCSEL designers. For circuit demonstration, it is implemented in Keysight's Advanced Design System (ADS) software and thereafter applied to replicate the performance of a state-of-the-art 28-GHz-bandwidth VCSEL at different temperatures and drive currents. Comparison is made between simulated and measured steady-state characteristics, small-signal behavior, and large-signal response under 28 Gbaud On-Off-Keying (OOK) and Pulse-Amplitude-Modulation 4 (PAM4) modulation, showing good agreement.

Performance Analyses of Photonic-Crystal Surface-Emitting Laser: Toward High-Speed Optical Communication

“…Nevertheless, for the conventional VCSEL structures, the lack of optical confinement factor in emission direction has become an obstacle, hindering the further development of modulation speed in recent years. The confinement factor of a high-speed VCSEL remains below 4% [47], which severely limits the improvement capability of a VCSEL bandwidth.…”

Performance Analyses of Photonic-Crystal Surface-Emitting Laser: Toward High-Speed Optical Communication

“…Nevertheless, for the conventional VCSEL structures, the lack of optical confinement factor in emission direction has become an obstacle, hindering the further development of modulation speed in recent years. The confinement factor of a high-speed VCSEL remains below 4% [30], which severely limits the improvement capability of a VCSEL bandwidth.…”

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

“…Therefore, our comprehensive physics-based large-signal equivalent circuit model for datacom VCSELs, previously presented in [3], where it was proven to be able to reproduce the large-signal output waveforms over a broad range of bias currents, ambient temperatures and modulation formats, is in this work extended to also include the intensity noise properties in the VCSEL output. A detailed theoretical description of the noise implementation is here provided, which is based on that each internal physical process of carrier transport and recombination, and photon generation and loss etc., is noisy by nature.…”

“…This paper is organized as follows. In Section II the noise extension in our established large-signal VCSEL equivalent circuit model [3] is presented in detail, and in Section III this updated circuit model is demonstrated, applying the same VCSEL as in [3]. Simulations of VCSEL RIN spectra are first compared to measurements.…”