Contributing Editor, MEMS Investor Journal
Optical MEMS have long been a goal of forward-thinking electronics innovators, but these technologies have had a rocky development road. Lately, however, the big money in semiconductor research – IBM and Intel – have reported significant successes in using the traditional CMOS toolkits to micromachine optical structures. Related work is also being done at Hewlett-Packard, Alcatel-Lucent and other research labs worldwide. Waveguides, gratings, resonators, modulators, and other tiny mechanical structures can effectively sculpt light for communication purposes when shrunk to sizes that correspond to the wavelength of light being manipulated. And with the semiconductor powerhouses behind these efforts, optical MEMS may finally be on the threshold of mass commercialization.
The one major success story in optical MEMS –Texas Instrument's digital light processor (DLP) – uses micron-sized mechanical mirrors to re-direct light from LEDs or lasers into the raster patterns that create visible displays. However, the new wave of optical MEMS structures is being crafted at finer scales in order to directly manipulate the light itself. By modulating, filtering, multi- and demulti-plexing infrared light beams, the information of the future will travel via light instead of the electrons used by devices today.
Intel demonstrated a 50-Gb/s optical transmitter and receiver chips using silicon waveguides that it plans to scale up to terabit-per-second speeds.
For several years now, both IBM and Intel have been filling their photonic toolbox with various demonstration devices that utilize micromachined silicon structures to manipulate light. Intel's first reported success was in 2004, when it developed a "transistor-like" photonic modulator that encoded data onto a light beam. Intel's silicon modulator split a beam of light into two separate streams, one whose phase was shifted with an electrical charge, causing the light to "blink" when the two beams were re-combined, effectively encoding the on and off pattern of light as serial bits traveling at one gigahertz. The following year (2005) Intel demonstrated a Raman laser by etching a silicon waveguide that amplified light from an external laser to demonstrate lasing on a silicon chip. Then, in 2006, the company showed how it could bring that external laser on-chip by bonding a flake of indium phosphide to its silicon waveguide thereby creating its hybrid silicon laser. In 2007, Intel showed how it micromachined on-chip silicon waveguides that could modulate optical signals by electrically encoding high-speed data streams onto light beams. And in 2008, Intel showed it could also decode those optical bit streams with its silicon-based avalanche photodetector.
Arrays of 40-gigabit-per-second (Gbps) germanium avalanche photodetectors complete what IBM calls its nanophotonic toolkit.
Putting all of these pieces together last week, Intel showed a 50 Gbps optical communications link that leverages the manufacturability of silicon MEMS to herald the migration from electronic- to photonic-computing. By enabling light to be encoded with data, transmitted, received and decoded back into an electrical signal, optical MEMS has finally been delivered in a platform that could be commercialized within the next few years.
IBM has put a light-emitting nanotube (LEN) inside an optical waveguide to achieve directional surface emission, wavelength selectivity and the potential for ultrahigh efficiency silicon chips.
IBM has likewise been accumulating the micromachining expertise to encode, communicate and decode optical signals using photonic structures on silicon chips. In 2005, IBM showed its first optical waveguides on silicon chips by demonstrating that it could electrically alter the effective index of refraction, theoretically enabling tunable optical delay-line chips, optical buffers, high-extinction optical switches and highly efficient wavelength converters. Then, in 2006, IBM reported that it had crafted the world's fastest optical delay line in CMOS, besting the previous world's record in silicon photonics (set by Alcatel-Lucent's Bell Laboratories) with an optical ring resonator just six microns in diameter. In 2007, IBM reported success as coaxing nanotubes into lasing on silicon chips and in 2008 demonstrated its light-emitting nanotube inside an silicon waveguide thereby achieving directed surface emission and wavelength selectivity. Earlier this year IBM announced it too had cleared the last hurtle to silicon photonics – demonstrating a CMOS-compatible germanium avalanche photodetector as well as a graphene photodetector, both of which were shown capable of decoding optical signals. And last week, IBM demonstrated that it too could put all the pieces together, as Intel has done, by showing CMOS-based waveguides, gratings and mixers micromachined into a silicon optical amplifier.
Scanning electron microscope cross-sectional image of the silicon (Si) waveguide core, the silicon dioxide (SiO2) and silicon oxynitride (SiOxNy) cladding layers.
As IBM, Intel, HP, Alcatel-Lucent and more than a dozen other labs worldwide perfect their silicon photonics toolkits over the next five years, look for optical MEMS to begin harnessing these silicon photonic devices to craft all the necessary components to enable a new “computing with light” paradigm. Initially, optical MEMS will merely replace current coaxial connections between systems with photonic links. Next will come optical interconnects to replace printed-circuit board-to-board copper-wire busses, then optical connections between cores on the same board, and eventually optical MEMS connections between arithmetic-logic units on the same chip.
Copyright 2010 MEMS Investor Journal, Inc.
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