Silicon Photonics
Faster way of delivering data and information
1. between computers => fiber optics
2. within a computer and between chips => silicon phtonics
Increasing challenges to deliver more data faster have promoted two ways of research in
computer science.
1. macrochip: interconnecting microchips electrically by stacking them edge to edge on a big
silicon wafer, making larger chips on a single wafer of silicon
2. silicon photonics: optical networking by connecting chips via laser beam
Usually electric wires connect computers and chips. Even within a chip, there are so many wires
connecting semiconductor transistors. The most serious drawback in using wires is their electrical
resistance. An object's electrical resistance is a function of both its physical geometry and the
resistivity of the material it is made from.
where
"l" is the length
"A" is the cross sectional area, and
"ρ" is the resistivity of the material
The resistivity depends on the temperature and kinds of material. In general, electrical resistivity of
metals increases with temperature, while the resistivity of semiconductors decreases with
increasing temperature. In both cases, electron-phonon interactions can play a key role. At high
temperatures, the resistance of a metal increases linearly with temperature. This table shows the
resistivity and temperature coefficient of various materials at 20 °C (68 °F).
Material
Resistivity (Ω-m) at 20 °C
Coefficient*
Silver
1.59×10−8
.0038
Copper
1.72×10−8
.0039
Gold
2.44×10−8
.0034
Gold is more ductile than copper or silver, and has less resistance at a high temperature. This is
why gold is used as micro wires in a semiconductor chip. However, in a motherboard which has a
lot of chips connected with each other, copper wires are used.
These metal wires generate
additional currents and heats leading to less energy efficient operating.
It also causes
bottlenecks because electrical current is slower than light speed.
Photonics is the technology of signal processing, transmission and detection where the signal is
carried by photons (light) and it is already heavily used in photonic devices such as lasers,
waveguides or optical fibers. Optical technology has always suffered from its reputation for being
an expensive solution, due to its use of exotic materials and expensive manufacturing processes.
This prompted research into using more common materials, such as silicon, for the fabrication of
photonic components, hence the name silicon photonics.
Although fiber-optic communication is a well-established technology for information transmission,
the challenge for silicon photonics is to manufacture low-cost information processing components.
Rather than building an entirely new industrial infrastructure from scratch, the goal here is to
develop silicon photonic devices manufactured using standard CMOS techniques. A recent review
paper takes a look at the state of silicon photonics and identifies the challenges that remain on
the path to commercialization.
The ultimate goal of this project is to develop a technology for on-chip integration of ultracompact nanophotonic circuits for manipulating the light signals, similar to the way electrical
signals are manipulated in computer chips.
[Futuristic silicon chip with monolithically integrated photonic and electronic circuits]
This hypothetic chip performs all-optical routing of multiple N optical channels each supporting
10Gbps data stream. N channels are first demultiplexed in WDM (wavelength-division
multiplexing) photonic circuit, then rearranged and switched in optical cross-connect OXC module,
and multiplexed back into another fiber with new headers in WDM multiplexer. Data packets are
buffered in optical delay line if necessary. Channels are monitored with integrated Ge
photodetector PD. CMOS logical circuits (VLSI) monitor the performance. Electrical pads are
connecting the optoelectronic chip to other chips on a board via electrical signals. (Image: IBM)
In order to "siliconize" photonics, there are six main areas or building blocks for investigation.
These include generating the light, selectively guiding and transporting it within the silicon,
encoding light, detecting light, packaging the devices and finally, intelligently controlling all of
these photonic functions. Intel is working to address these areas, and this research has produced
a few recent success stories, including the first continuous-wave silicon laser and the first gigabit
speed silicon modulator.
low-cost all-silicon Raman laser
First GHz Silicon Optical Modulator
Optical modulators are used to encode a high-quality data signal onto an optical beam,
effectively by turning the beam on and off rapidly to create ones and zeros. Before the year 2004,
no one had built an optical modulator from silicon that was faster than about 20 MHz. In 2005,
Intel researchers further demonstrated that this silicon modulator is capable of transmitting data
up to 10 gigabits per second (Gbps).
Silicon photonics:
(1) The thin lines on the
surface of this chip are
waveguides,
carved
into
channels
silicon
that
direct light.
(2) Five indium-phosphidebased lasers mounted to a
die.
(3)
Luxtera
researchers
modified a standard piece
of
semiconductor
test
equipment so that it can
test
optical
as
well
as
electrical functions.
(4)
This
device
couples
fiber to Luxtera’s siliconbased chip.
Credit: Luxtera