For many years, engineers
successfully integrated
optics and electronics
by using a standard silicon
CMOS process for
small-bandwidth structures
like photodetectors.
Advances in performance
and integration densities continue to energize
this niche, such as with the venerable
silicon photodiode in terms of functionality.
Photodiode advances may fall under the
radar a bit, but more visible progress can be
seen with ICs like detectors, sensors, LEDs,
lasers, and other devices that operate at relatively
low bandwidths. They serve barcode
scanners, printers, disk players, remotecontrol
devices, and other consumer applications.
Other markets include medical,
industrial, and construction applications.
Advanced Photonics, Cal Sensors, Opto
Sensors, Silicon Sensors, and UDT Sensors
all use silicon and other materials to produce photodetectors
and sensors. Some large firms like Agere, GE, Infineon,
Perkin-Elmer, and Siemens also make commercially available
solid-state photodetectors and sensors.
Products from Texas Advanced Optoelectronics Solutions
(TAOS) exemplify this trend. The company’s high-performance
optodetectors convert light to voltage, color, digital, and
frequency signals (see “Beyond Simple Photodiodes And Phototransistors”
at www.electronicdesign.com, Drill Deeper 18789).
The Position Sensitive Photomultiplier Array (PSMArray)
family of tiny solid-state light sensors from SensL represent the
first commercially available CMOS large-array, detector-based
silicon photomultiplier products. According to the company,
they surpass the performance of traditional photomultiplier
tubes (PMTs) and avalanche photodiodes (APDs).
The PSMArray is an arrayed version of SensL’s novel silicon
photomultiplier (SPM) pixels tiled together using flipchip
on glass techniques. It operates from 400 to 850 nm.
A MATERIALS CHALLENGE
For higher-bandwidth devices that
serve communications and computing
functions, though, the greater
challenge involves various compound
semiconductor materials. Nonetheless,
quite a few devices are spread
throughout the market, spearheading
record-breaking performance levels.
Also, many other products are in
development or on the cusp of introduction.
Companies cite progress
in producing higher-performance detectors, waveguides, modulators, lasers, switches, filters, couplers,
multiplexers, amplifiers, and other optoelectronic functions.
Device development mostly focuses on the use of silicon (Si),
germanium (Ge), and indium phosphide (InP). The holy grail for
high-bandwidth optoelectronics is to integrate these materials on a
standard CMOS silicon process, monolithically or in hybrid form.
Such integration would reduce manufacturing, packaging, and
testing costs. It also would increase reliability and improve performance
to levels practical with present and future communications
and computational needs. Whether it’s possible is unclear, since
large-volume applications for such devices don’t exist as of yet.
ON THE MARKET
A recent report by the Optoelectronics Industry Development
Association (OIDA) predicts robust growth for optoelectronic
components. The group expects steady expansion of the market,
both for optoelectronic components and the technologies they
will enable, with the overall industry doubling to $1.2 trillion
from 2007 to 2017 (Fig. 1).
One of the first complex silicon optoelectronics products to
reach commercialization, a highly accurate, multichannel model
2200 dynamic-gain equalizer module from Silicon Light
Machines, arrived in 2002. Based on MEMS and CMOS processing,
it consists of the patented Grating Light Valve (GLV) circuit,
a light engine, and an optical circulator.
An array of parallel aluminum-covered silicon-nitride (SiN2)
microribbons is suspended above an air gap. The dynamic ribbons
are then deflected up and down in response to an applied electrostatic
voltage, acting as variable optical attenuators in response to
a light signal. The module is being used in optical wave-division
multiplexing (WDM) communications, high-resolution displays,
and high-end high-definition video applications.
Manufacturing more complex and integrated optoelectronic
devices on a standard process like CMOS, however, is much more
challenging than making photodetectors. Silicon’s bandgap is too
large, compared to other materials, to effectively operate at the
infrared (IR) wavelengths used for optical communications and
needed for optical modulator circuits.
The CMOS integration
challenge hasn’t
stopped researchers
from forging ahead, though. Impressive developments have been achieved in
the lab for all-silicon devices with wide bandwidths. Last year,
Intel researchers announced the world’s fastest optical silicon
modulator, writing 30 Gbits of data into a light beam per second.
Three years ago, Intel demonstrated a 10-Gbit/s all-silicon
modulator. At one end of the modulator, light enters silicon diodes
that split the light beam into two beams that pass through the
diodes. Applying a voltage to the diodes shifts the light beams’
phase or position. This shifting encodes data into a binary form
of ones and zeros.
The 30-Gbit/s data rate is close to the 40-Gbit/s rate used in
modern communications systems. “By slightly altering the chemical
makeup of the diodes, we can expect 40-Gbit/s data rates to be
ready for commercialization by 2010,” says Intel research fellow
Mario Paniccia, director of Intel’s Silicon Photonics Research Lab.
“If you take 25 of these silicon lasers and direct them into an
array of 25 modulators, you can have a terabit of information all on
a piece of silicon the size of a fingernail,” adds Paniccia.
Researchers at IBM developed a photonic silicon modulator
that’s 100 to 1000 times faster than previous comparable modulators,
and they switch at 10 Gbits/s. The ultra-low-power RF power
modulator uses Mach-Zendor modulation (MZM) interferometry
and consists of 200-µm long p+-i-n+ active diode regions on
an effective chip area of about 0.12 µm2.
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