Dissertation/Thesis Abstract

Energy Efficient Oxide Confined VCSELs for Optical Interconnects in Data Centers and Supercomputers
by Moser, Philip, Dr.Nat., Technische Universitaet Berlin (Germany), 2015, 202; 10697855
Abstract (Summary)

This dissertation provides the first systematic analysis of the dynamic energy efficiency of vertical-cavity surface-emitting lasers (VCSELs) for optical interconnects. Energy-efficient VCSELs are the key component to address the pressing ecological and economic issues of the exponentially growing energy consumption in data centers via energy-efficient optical interconnects. Energy-efficient data communication is one of the most important fields in “Green Photonics” enabling higher bit rates at significantly reduced energy consumption per bit. General rules of how to achieve energy-efficient data transmission with VCSELs are derived by the systematic investigation of the static and dynamic VCSEL properties of different oxide-confined VCSEL designs emitting at 850nm and 980nm. These rules are applicable to all oxide-confined VCSELs of any wavelength. The derived rules are verified via data transmission experiments leading to record energy-efficient error-free data transmission at room temperature of 56 fJ/bit at 25Gb/s. It is demonstrated that energy-efficient operation can also be achieved at high bit rates up to 40 Gb/s and across long multimode fiber transmission distances of up to 1000m at 25Gb/s, and over distances of 5m at 46 Gb/s at 85C and 50 Gb/s at 25C with record VCSEL modulation bandwidths of 24.7 and 23.0GHz at 25 and 85C, respectively. One main conclusion of this work is that the future required performance goals in terms of energy efficiency, bit rate, and temperature stability can be achieved with oxide-confined VCSELs. In order to achieve these goals simultaneously, the VCSELs must have small oxide-aperture diameters of 3-5 μm and must be operated at low current densities. Prior to this dissertation work the main focus of oxide-confined VCSEL research was on VCSELs with larger oxide-aperture diameters of 7-10 μm as these devices are currently employed in commercial optical interconnects, because these oxide-aperture diameters typically yield the largest modulation bandwidths. It has been the conventional wisdom that in order to meet the future bandwidth demands such large oxide-aperture diameter VCSELs will be used in future optical interconnects. In this dissertation it is demonstrated that such large oxide-aperture diameter VCSELs are not suited for energy-efficient operation especially not at the required high bit rates and at high ambient temperatures and low current densities. For those VCSELs trade-offs between energy efficiency and all other performance goals exist. This dissertation work demonstrates that VCSELs with smaller oxide-aperture diameters are more energy-efficient than similar VCSELs with larger oxide-aperture diameters. At the required low current densities for reliable commercial application small oxide-aperture diameter VCSELs are simultaneously faster and significantly more energy-efficient than similar VCSELs with larger oxide-aperture diameters and the modulation bandwidth and energy efficiency are more temperature stable as well. This paradigm change has stimulated the work of other groups on the energy efficiency of VCSELs with smaller oxide-aperture diameters. In order to investigate the suitability and potential for different VCSELs and VCSEL designs in different optical interconnect technologies, the modulation factor M is introduced. M is the equivalent to the spectral efficiency from Information Theory, relating the maximum channel capacity or bit rate of the system to the modulation bandwidth of the VCSEL. M is as a free parameter representing different optical interconnect systems technologies. By assuming certain M-factor values, the energy consumption per bit can be calculated from the measured intrinsic VCSEL properties for different VCSELs at given bit rates employed in different optical interconnect technologies. This new method allows optical interconnect designers on the systems level to include VCSELs into their models and predict the optimum optical interconnect technology operating at maximum energy efficiency and simultaneously fulfilling the performance goals required by the specific application.

Indexing (document details)
Advisor: Bimberg, Dieter
School: Technische Universitaet Berlin (Germany)
School Location: Germany
Source: DAI-C 81/1(E), Dissertation Abstracts International
Subjects: Electrical engineering, Computer Engineering
Keywords: Surface-emitting lasers
Publication Number: 10697855
ISBN: 9781392703984
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