and Web service providers of the
world. Copper cables would be too
big and heavy to serve as patch cables
in today’s data centers and, at transmission speeds approaching 10 Gbps,
provide too much distortion as well.
The need for speed has prompted
data center interconnections to evolve
from “box-to-box” to the circuit
boards within rack-mounted components. Researchers are now pursuing
chip-to-chip connections on individual boards. Such technology is on the
verge of being used in supercomput-ers, which drive the high end of the
datacom world.
“People even talk about optics on
the chips, of course,” Coldren said.
“There’s a vector here in the comput-
ing arena that’s pushing data into
the box.”
Silicon, of course, is compatible
with the silicon in microproces-
sors, and the dividing line between
photonics and electronics has moved
closer to the chips in circuit boards.
Most efforts to create silicon photon-
ics devices still use an external laser
with a modulator on the silicon chip,
although a joint UCSB-Intel team
has created a hybrid silicon evanes-
cent laser in a silicon-on-insulator
waveguide. Such devices, though,
probably won’t replace VCSELs in the
data center.
These modern laser modules incorporate a tunable laser with a semiconductor optical amplifier on an InP chip.
Tunability and packaging
In communications, lasers are usually
incorporated into a transponder or transceiver, said Christopher
Doerr, distinguished technical staff member in the Bell Laboratories division of Alcatel-Lucent. Such a device translates electrical signals into optical signals and vice versa and also includes
receiving equipment. Manufacturers have developed specialized
packaging types, such as the transmitter optical sub-assembly,
or TOSA, which integrates a laser with pre-aligned lenses, amplifiers and standardized connectors.
Since the advent of WDM, interest in tunable lasers has
been growing at both the datacom and long-distance ends of
the communications spectrum, said Jens Buus, a consultant
with Gayton Photonics in Gayton, England. “It’s becoming a
very diverse market,” he said.
Tunable lasers must meet all the power, accuracy and noise
specifications of fixed-wavelength lasers, with the added ability
to have their wavelength adjusted either just prior to installa-
32 | OPN Optics & Photonics News
Syntune AB (Kista, Sweden)
tion or during operation using micro-electro-mechanical systems (MEMS).
According to Coldren, the original
idea behind tunable communications
lasers was to use them as universal
spare parts for the fixed-frequency
lasers in a system.
“Today, every part in the rack is a
tunable laser,” Coldren said. To his
knowledge, he added, no one puts
fixed-frequency lasers in their WDM
systems anymore. They don’t want
to have their racks filled with fixed-frequency lasers that must be replaced
by a very specific and difficult-to-find
part if one of them should fail. The
universal sources don’t cost any more
than fixed-frequency devices in the
end, because all the costs are in
the packaging.
Tunable lasers couldn’t really take
off before the price difference between
them and the fixed-wavelength lasers
had shrunk, Buus said. The growth
in the market for tunable lasers took
four or five years longer than originally anticipated. However, last year
it exceeded 100,000 units. Although
long-term frequency stability of tunable lasers was originally a concern,
people trust them now, Buus said.
Future communications systems will use the tunable lasers as a
wavelength-selective switch as well
as a source, according to Coldren.
By changing the wavelength, the
system will send the information to
different customers.
“The excitement, I think, is photonic integration in the
long haul,” Coldren said. His research group at UCSD studies
photonic integration circuits (PICs) for universal transceivers
and other applications.
Each PIC starts with a widely tunable laser that can be set
to any of the wavelengths in the WDM spectrum. Packaged
on the same chip as that universal light source is an advanced
modulator. The chip then needs to be wavelength-division-multiplexed with other, similar PICs.
Active optical cables
Datacom companies are making networking even easier for
data-center companies by attaching optical transceivers permanently to the ends of fiber cables, thus making active optical
cables (AOCs).
