Article about silicon optics by David Geer

Page 1

Computer

Published by the IEEE Computer Society

T E C H N O L O G Y N E W S

endors of computer-, net-

working-, and telecommu-

nications-related equip-

ment have long faced a

dilemma. Much of their

equipment is based on silicon and

copper wiring, which are inexpen-

sive to use but offer limited data

rates. Thus, as microprocessor and

networking speeds have increased,

the speed of communications within

chips, between chips or circuit

boards, within LANs, or even along

the “last mile” from ISPs to cus-

tomers has not kept pace.

An option is to use optical con-

nections for these communications.

“Any signal processing you can do

with electronics you can do faster

with light,” said Alfred University

professor Alexis Clare.

However, optics has been expensive

and, therefore, not suitable for any

but the largest networking operations

with the most traffic and the biggest

potential for return on investment.

Thus, optics has been used largely in

settings such as telecommunications

vendors’ backbone networks.

Optical connections have been im-

practical for communications on or

between chips because the shorter

distances involved yield fewer band-

width improvements, which don’t

justify the expense, said University

of Rochester distinguished professor

Philippe Fauchet.

Now, though, vendors are attempt-

ing to combine the best of both worlds

and offer silicon optics, which uses

complementary metal-oxide semicon-

ductor (CMOS) technology to fabri-

cate optical components on silicon.

This approach would speed up

traditionally silicon-based systems.

It could also reduce the cost of opti-

cal equipment and bring optical sys-

tems within reach of more users,

including companies and service

providers with networking opera-

tions smaller than those of large

telecommunications providers.

Combining silicon and optics is a

complex process. However, some

vendors are already selling some less

complex silicon-optical components.

Meanwhile, scientists are making

progress on more complicated com-

ponents. For example, Intel and

UCLA researchers have each devel-

oped prototype silicon lasers.

Nonetheless, obstacles remain, so

while some components are already

available, it may be years before sil-

icon optics can be widely and reli-

ably used commercially.

SILICON AND OPTICS

Transistors, diodes, and most

logic-control components in today’s

computers are silicon-based. Silicon

is found in sand, which is plentiful

and relatively inexpensive to process,

explained Clare.

Numerous fabrication plants have

worked with silicon for many years.

This enables high-volume produc-

tion and makes it easy for manufac-

turers to use the material and

integrate components. Moreover, as

a semiconductor, silicon is power-ef-

ficient and generates relatively low

amounts of heat.

However, silicon, working with

copper wiring, creates the binary

ones and zeros of data via electronic

pulses, so factors such as electrical

resistance and capacitance limit the

technology’s bandwidth.

Optics generates binary ones and

zeros via a modulated laser beam.

This makes the technology particu-

larly fast and able to handle large

quantities of data.

Typical electronic components

offer maximum data rates of 1 Gbit

per second, according to University

of Oklahoma professor Patrick J.

McCann. Proponents say silicon op-

tical components, on the other hand,

could offer rates as high as 40 Gbps

during the next few years.

In addition, light beams used in op-

tical transmissions can be split into

multiple communications channels

that can be multiplexed onto a single

link, thereby offering very high data

capacities, according to Fauchet.

Manufacturers frequently use ex-

otic materials like gallium arsenide

and indium phosphide with optical

components to maximize perfor-

mance and add special properties,

such as minimal light loss during

transmissions. However, these alloys

are expensive. “Indium, for example,

is a limited resource that must be

mined,” noted University of Mary-

land professor Thomas E. Murphy.

Silicon Optics

Aims to Combine

the Best of

Both Worlds

David Geer

Page 2

June 2006

ing the continuous laser beam’s [in-

tensity] in an external device (the

modulator). No one has succeeded

yet in building a silicon laser that

can be directly driven by an electri-

cal current.”

Manufacturers—such as IBM,

Intel, Kotura, and Luxtera—are cur-

rently developing new types of mod-

ulators that can convert electrical

signals to optical.

Amplifiers compensate for the loss

of light that occurs during the trans-

mission process by producing more

photons and thereby strengthening

signals.

Individual optical filters let only a

single set of signals pass onto specific

wavelengths within a laser beam.

This separates signals into discrete

groups of transmissions.

Waveguides direct the light as it

passes through open spaces within a

system, leading it around corners or

along paths other than a straight

line, noted Harold Hosack, director

of interconnect and packaging sci-

ence for Semiconductor Research,

a research-management consortium.

Optical switches move data be-

tween light paths, sending data

streams to their proper recipients.

In addition, using exotic materials

in manufacturing is difficult, time-

consuming, and costly. For example,

Clare explained, manufacturers

often must use specialized equip-

ment that deposits the materials as

vapors or a liquid, layer by layer.

WHEN SILCON MEETS OPTICS

US Air Force researchers pio-

neered silicon optoelectronics in the

mid 1980s for sophisticated com-

munications and signal processing,

said Richard Soref, research scien-

tist with the Air Force Research

Laboratory’s Sensors Directorate.

Since then, a variety of universities

and companies, such as Bookham

and Intel, have worked on silicon

optics.

Key issues

In most current silicon-optical de-

vices, such as waveguides and filters,

the silicon is used only as a passive

medium through which light can be

transmitted. These devices do not

tackle the more fundamental problem

of converting signals from optical to

electronic and vice versa, said the

University of Maryland’s Murphy.

This critical capability maximizes the

use of optical technology and in-

creases performance.

Many of today’s systems use pho-

todetectors that convert signals only

from optical to electronic.

Another key obstacle to achieving

true silicon optics has been the lack

of a silicon-based laser, noted

McCann. Semiconductor physics has

limited the ability to build such a laser.

The issue is bandgap: the energy dif-

ference between a material’s conduc-

tive and nonconductive state. Direct

bandgap semiconductors such as gal-

lium arsenide and other material used

in optics are efficient at emitting light,

while indirect bandgap semiconduc-

tors such as silicon are not.

Some researchers are looking into

techniques such as introducing other

materials that will make the silicon

transmit light more efficiently or

adding dopants that themselves

transmit light effectively, according

to University of Surrey professor

Graham Reed.

Meanwhile, scientists continue

looking for other ways to provide

silicon optics.

Silicon optical components

In silicon optics, manufacturers

build both the optical and electronic

components on a single silicon chip,

explained UCLA professor Bahram

Jalali. This reduces mass-manufac-

turing costs and simplifies packaging.

Vendors generally design silicon-

optical components to fit on a sili-

con substrate as part of a standard

CMOS manufacturing process, ac-

cording to Arlon Martin, vice pres-

ident for sales and marketing for

silicon-optics vendor Kotura.

The principal optical component

is the laser, shown in Figure 1, which

acts as the system’s light source.

“When an electrical current passes

through a semiconductor laser, it

emits a coherent beam of light (pho-

tons),” explained the University of

Maryland’s Murphy. “Information

can be encoded onto the optical

signal in two ways: by simply turn-

ing on and off the electrical current

that drives the laser or by modulat-

Waveguides devices

Photodetectors

Passive

align-

ment

CMOS

Source: Intel

1. Light source

2. Guide light

3. Modulation

4. Photodetection

5. Low-cost assembly

6. Intelligence

Figure 1.Intel is working with six building blocks for producing a silicon-optical

transceiver.(1) A laser produces the light that will eventually carry the data.(2)

Waveguides guide the light across the chip along the proper route.Splitters divide

the light into separate beams to carry multiple signal sets.(3) A modulator adds the

signal to the light beam at high rates.(4) A recipient’s photodetector decodes the

encoded light and turns the optical signal back into an electrical signal for

processing.(5) Passive alignment precisely but inexpensively aligns optical elements.

(6) Chip-based intelligence processes encoding and decoding.

Page 3

Computer

T E C H N O L O G Y N E W S

icon lasers. Manufacturers have had

trouble beaming lasers onto a chip

without causing heat, alignment,

and other problems.

Intel’s approach introduces pho-

tons onto a chip via a laser. It then

uses transistor-like devices to re-

move the buildup of electrons that

the light source creates, explained

Mario Paniccia, the company’s re-

search director. If not removed, the

electron cloud would absorb light

and interfere with the generation of

the continuous laser beam that is

necessary for proper modulation

and data transmission, he said.

Intel expects to have silicon lasers

ready for commercial use by the end

of this decade,” noted Victor Krutul,

the company’s director for enter-

prise-initiatives technology manage-

ment.

Replacing components

in optical systems

In addition to replacing compo-

nents in silicon-based systems to

make them faster, silicon optics can

also replace traditional optical com-

ponents in optical systems to make

them more affordable.

For example, Kotura is using sili-

con optics to make arrays of variable

optical attenuators, which incremen-

tally adjust the power of the signal

passing through an optical system,

according to the company’s Martin.

The VOAs are made of a silicon

chip with parallel waveguides.

Applying an electronic current

across the waveguide attenuates the

light, Martin explained.

Using silicon makes the manufac-

turing process less expensive and im-

proves the VOAs’ performance.

Manufacturers such as Intel are

also using silicon optics to make

modulators for optical systems.

USING SILICON OPTICS

Proponents see four primary ways

in which silicon optics could be used

in the near future.

LANs

Silicon-optical components could

replace existing transceivers in 1- and

10-Gbit Ethernet LANs. Because

LANs typically include many nodes

with many connections, the use of

silicon optics could improve overall

communications enough to justify

the expense, UCLA’s Jalali explained.

Equipment vendors are interested

in building silicon optics for LANs

in part because many corporations

use these networks and thus repre-

sent a lucrative market.

Communications

among components

While microprocessors have got-

ten faster, overall computer-perfor-

mance increases have been limited

because of slower communications

via data buses and along copper

wiring between chips and circuit

boards.

Using optical communications in-

stead of electrical wiring would be

faster, and silicon optics could keep

the implementation affordable.

On-chip communications

Silicon-optical interconnects could

replace copper wiring on chips, ac-

cording to Semiconductor Research’s

Hosack. A single chip would contain

most of the basic optical components

except for the light source.

Short, high-density interconnects

should be left in copper wiring be-

cause the necessary optical compo-

nents would be too large and require

too much power, noted Hosack.

Research into on-chip silicon op-

tics is ongoing. “Prototype wave-

guides and detectors for optical

interconnects are available,” Hosack

said. “However, the necessary exten-

sive studies of reliability and devel-

opment of cost-effective manufac-

turing methods have not been done.”

In addition, researchers have only

just begun developing prototype sil-

Silicon optics uses

CMOS technology

to fabricate optical

components on silicon.

STILL TO OVERCOME

Lasers use significant electricity,

which generates heat. This is a prob-

lem for chip-based silicon optics be-

cause manufacturers try to reduce

processors’ power usage and heat

generation.

Manufacturers must precisely

align optical elements such as wave-

guides and lasers so that light trans-

fers precisely from one component

to another. “If they are misaligned,

then light misses the receiving device

and is lost,” explained Reed. Proper

alignment can be a challenge during

the mass production of silicon-opti-

cal components.

Some silicon-optical elements,

such as waveguides, are relatively

large and thus occupy considerable

real estate on chips, particularly as

processors shrink in size. This causes

design challenges

In addition, replacing selected

electrical wires with optical inter-

connects can cause the loss of some

of the desired performance advan-

tage because of the time it takes to

transform electrical signals to opti-

cal signals and vice versa, said

Semiconductor Research Corp.’s

Hosack.

There can also be loss of light that

the material in waveguides absorbs.

Moreover, Reed said, waveguides

that interact with one another can

generate noise that interferes with

signal detection.

roponents say that during the

next few years, silicon-optical

components will shrink in size

and become more efficient, which

will encourage more widespread use.

And, they add, manufacturers and re-

searchers will demonstrate increas-

ingly complex integration of optical

technology with electronics.

Meanwhile, usage could increase

as more researchers and companies

work on silicon optics.

According to UCLA’s Jalali, the

greatest impact will be in enabling

low-cost optical transceivers for

communications over short dis-

Page 4

June 2006

tances such as those in interchip and

intrachip interconnects.

Because so many chips are made

and sold worldwide each year, the

biggest potential market for silicon

optics is on-chip interconnects, pre-

dicted Richard Wawrzyniak, an ana-

lyst at Semico Research, a semi-

conductor-market research firm.

Complex commercial silicon-op-

tical devices and applications will hit

the market within two years, Reed

said.

In the future, manufacturers could

develop ways to use silicon optics on

backplanes and to connect comput-

ers and peripherals. Also, noted

Reed, ISPs could use silicon optics to

cover the last-mile connection be-

tween their facilities and their cus-

tomers. This would be much faster

than DSL- or cable-based broad-

band, he said.

Wawrzyniak predicted that rev-

enue from silicon optics will increase

from $10 million this year to $1.8

billion by 2010.

“There will be an explosion of in-

terest in silicon photonics in the next

few years,” said Reed. “Maybe in 10

years, your laptop will rely on sili-

con photonics as much as it relies on

silicon electronics today. Silicon is

simply an excellent material for

mass production.” ■

David Geer is a freelance technology

journalist based in Ashtabula, Ohio. Con-

tact him at david@geercom.com.

Editor: Lee Garber, Computer,

l.garber@computer.org

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