Optoelectronic Modulators for Optical Interconnects

Optical interconnects have been widely studied as a solution to the electrical interconnect bottleneck foreseen in computing systems.  The mature technology of silicon CMOS electronics is well-established for high-speed information processing, while optical systems excel at information transmission.   Future computing systems are likely to incorporate electronic components communicating along an optical channel that requires optoelectronic devices to convert signals from the electronic into the optical domain and vice versa.

Electroabsorption modulators designed for this application must be compatible with  both  the  electrical  and  optical  systems.    This  dissertation  will  begin  with  a discussion of the requirements for an optoelectronic modulator design.   In particular, I will describe the advantages and challenges of 2D arrays of surface-normal modulators that operate over a wide wavelength range with a low voltage drive.

Two  surface-normal  modulator  architectures  will  be  presented.    First,  I  will outline the design, fabrication and integration of an asymmetric Fabry-Perot AlGaAs/GaAs modulator.   Following a post-integration cavity tuning step, this device achieved a contrast ratio of 3 dB over a wavelength range from 847 nm to 852 nm using a voltage drive of only 1 V.  The second device, a novel design called the quasi-waveguide angled-facet electroabsorption modulator (QWAFEM), was simulated and fabricated in the InGaAsP/InP material system. An experimental contrast ratio of 3 dB over a 16 nm wavelength range near 1510 nm was measured for a voltage drive of only 0.8 V.  To the best of our knowledge, no other reported low-voltage surface-normal modulator offers 3 dB of contrast ratio over such a wide wavelength range around 1.5 µm.  Improvements to the QWAFEM design were simulated and a brief discussion of the advantages and practical challenges of such devices precedes the conclusion.

INTRODUCTION

As computer technology improves, the information processing power increases with each generation.  Advances in design and fabrication of individual computer chips enable devices to operate faster, to consume less electrical power, and to perform more complex functions.    Primarily,  this  is  achieved  by  shrinking  the  size  scale  of  the transistors and using a lower voltage swing to denote the digital bits.  Though this plan of shrinking the transistor size is projected to continue to improve the performance of the chips, the Semiconductor Industry Association has predicted that the performance of the overall system in the upcoming years will be limited by the metallic wires that connect the chips to each other . Even though the processors and memory may get faster and more efficient, the ability of these chips to communicate with each other will become impaired by the imperfections of the electrical interconnections. Predictions of when electrical interconnects will become the performance-limiting factor of computers vary somewhat, but most predict the necessity of addressing the problem between 2009 and Several researchers have investigated these fundamental limits of electrical interconnects and have proposed various solutions .  Perhaps, advanced electronic architectures will be sufficient to address these problems for the upcoming future. In  parallel  to  these  all-electrical  approaches,  many  research  efforts  have  been focused on replacing the wires with optical links.   For fundamental physical reasons, optical interconnects offer many advantages over electrical interconnects for high speed links .   However, practical concerns, such as manufacturability, design complexity, reliability, and cost, must also be taken into account in considering whether optical interconnects will penetrate the marketplace

The vision of optical interconnects consists of many powerful electronic information processing modules communicating with each other and with the internet’s optical network via optical channel links.  This design would utilize the strengths of both technologies: electronics for information processing and optics for communications. Currently, optical networks for long-distance telecommunications (“long-haul”) are widespread and carry most of the voice and data traffic.  Demand for internet bandwidth has  increased  at  a  steady  rate  in  the  last  several  years,  despite  fluctuations  in  the economic markets. The network providers have responded by installing optical systems to replace wide-area and metro-area networks.  This demand trend is likely to continue, driving optical networks to shorter and shorter distances.

Electrical signals that are targeted for an off-chip destination in an optical- interconnect system must be converted into the optical domain for transmission and then back into an electrical signal at the receiver chip.  Many devices have been proposed for these electrical-to-optical (EO) and optical-to-electrical (OE) converters.   The focus of this dissertation is the design of semiconductor optoelectronic modulators optimized as EO transmitter devices for optical interconnects.