Maximizing the temperature insensitivity, energy efficiency, and bit rate of 980-nm VCSELs via small oxide-aperture diameters and photon lifetime tuning

Abstract

ABSTRACT Energy-efficient oxide-confined vertical-cavity surface-emitting lasers (VCSELs) emitting at 980 nm, particularly well suited for optical interconnects operating at up to 85°C are presented. The modulation bandwidth f 3dB of our VCSELs increases at low currents with increasing temperature up to 23 GHz at 85°C . The impact of cavity photon lifetime and oxide-aperture diameter on the energy efficiency, temperature stability, and static and dynamic properties of our VCSELs are analyzed. Error-free 40 Gb/s operation at 85°C with an energy-to-data ratio below 100 fJ/bit and a current density close to 10 kA/cm 2 is predicted based on small signal modulation experiments. Keywords: Green photonics, energy efficiency, modulation, photon lifetime, vertical-cavity surface-emitting laser, oxide-aperture. 1. INTRODUCTION 1.1 The need for energy-effici ent optical interconnects The rapid growth of global Internet traffic since the 1980s is predicted to continue following an exponential growth rate for the next several decades [1,2], leading to a corresponding exponential growth in energy consumption, most noticeably in data centers but also for high performance computers. The in crease in energy consumption cannot proceed unlimited. The total energy consumption of the Internet is growing faster than the predicted growth in global energy production. It has been predicted that based on present technologies by circa 2023 the energy consumption of the Internet will exceed the available energy from electrical power sources. Crossing this borderline would result in an Internet or bandwidth stasis, cost per bit explosion, communication bottleneck, or perhaps a decline in ubiquitous connectivity and data reliability [2]. Thus to enable an ecologically and economically feasible Internet beyond circa 2023, new technologies must not only provide larger bandwidth, but must simultaneously operate in a dramatically improved energy-efficient way. Information transfer between and within servers, and hardware cooling currently consume the largest fraction of the energy in data centers. In supercomput ers similar problems with data input and output, and cooling abound. Clearly energy-efficient and temperature-stable high-speed optical interconnects (OIs) depend on advanced key enabling photonics component technologies. Significant progress has been made, especially in the last three years, in increasing the erro r-free bit rate and the energy efficiency of vertical-cavity surface-emitting lasers (VCSELs) operating at 850, 980, and 1060 nm. These GaAs-based infrared VCSELs are low-cost, reliable, and compact light-sources that enable present short-reach OIs across multimode optical fibers in data centers and petaflops-scale supercomputers. In this paper we demonstrate the impact of the oxide-aperture diameter and the cavity photon lifetime on the band width, energy efficiency, and temperature stability of state-of-the-art 980-nm VCSELs and show that a comparably small oxide-aperture diameter of 4-5 µm enables simultaneously energy-efficient and temperature- stable VCSEL operation at large bandwidth. The design principles presented here apply equally well for VCSELs emitting at 850-nm, 1060-nm, and other wavelengths.