3/2/98 Electronic Engineering Times 01 (SEE BOLD)
1998 WL 2383268
Electronic Engineering Times
Copyright 1998 CMP Publications Inc.
Monday, March 2, 1998
996
News
Advances in wavelength multiplexing, connector scheme extend fiber links beyond
the backbone
--
Denser optical nets push toward mainstream
Loring Wirbel
San Jose, Calif. - Optical networks took a leap forward at the Optical
Fibers Conference here last week when several vendors detailed a new
generation of wavelength-division multiplexing systems that make the
"dense" WDM implementations of the past seem sparse by comparison. WDM
systems with 50 wavelength-separated light channels are becoming
commonplace, and Lucent Bell Labs reported on multi-terabit systems with
up to 100 simultaneously operating channels.
The arrival of denser systems points to the commoditization of
lower-density WDM, as companies push the technology out from public
backbones to metropolitan-area networks (MANs) and eventually toward
private networks.
Pushing optical links even closer to the end user, an alliance of
opto-component makers, building on the Mini-MT ferrule standard
announced in early February, last week proposed an optic-fiber connector
module that fits the space constraints of an RJ-45 jack. The MT-RJ
format could make multimode and single-mode fiber lines as simple to
insert into computers and switches as telephone lines are now inserted
via RJ-11 connectors (see story, page 112).
Commercialization of practical WDM systems was a common theme at this
year's OFC. Bell Labs' late-paper presentations detailed not laboratory
curiosities but true systems that could be commercialized before year's
end, said Bell Labs team leader Yan Sun.
WDM market leader Ciena Corp. (Linthicum, Md.) demonstrated 40-channel
production WDM systems with 50-GHz channel spacing. Doug Green,
product-marketing director for transport systems at the company, said
double-density systems with 25-GHz spacing will soon be possible.
Pirelli Cables and Systems North America (Lexington, S.C.), meanwhile,
showed off a "hyper-dense" 64-channel system called WaveMux.
Suddenly super-dense WDM systems are being considered as more than a
way to reduce Sonet deployment costs or implement wavelength-based
add/drop multiplexers and cross-connects. Developers of the new breed of
terabit routers-newcomers such as Juniper Networks, Avici Systems and
Pluris Inc.-are approaching WDM system manufacturers with requests to
interface WDM products directly to routers with OC-48
(2.048-Gbit/second) ports. "New mega-router markets hold the potential
for being one of the fastest-growing areas for WDM system rollouts,"
said Ciena's Green.
The scramble to move to high system densities is sparking a second
wind of WDM interest among component manufacturers and EDA-tool
vendors. Passive-component manufacturers have argued the relative merits
of thin-film dielectric filters, glass-based Bragg gratings and
machined waveguides for cost-effectively reaching WDM densities of 32
channels and beyond.
Cascaded hybrid
Last week, startup Applied Fiber Optics Inc. (Fremont, Calif.) brought
out a coupler-based cascaded hybrid device for dense-WDM channelization,
mixing a Mach-Zender interferometer structure with a fuse-cascaded
stage. "Others had looked at Mach-Zender structures alone and concluded
they could never compete with Braggs or waveguides," said marketing
manager Diana Zankowsky. "We've come up with a fairly interesting
technique for a cascaded hybrid which, with any luck, will give us
several months' lead over any copiers."
Zankowsky said the approach "will easily scale to support 64 channels
and beyond."
The first EDA tools for WDM also were introduced at OFC. New elements
in the Advanced Design System, from Hewlett-Packard's Eesof group,
provide the first opportunities for internetworking engineers with
minimal photonics expertise to model WDM channels, simulating such
aspects as gain and channel separation, said communication- and
DSP-product manager Paul Washkewicz.
The WDM tool is only the first step in broadband-transmission and
photonic-network simulation, Washkewicz said. HP EEsof last week signed
a pact with Germany's BNeD Broadband Networks Design GmbH, specifying
the merging of BNeD optical library models with HP's simulation tools.
Demonstrations of the WDM simulator drew crowds, and Washkewicz
predicted that "once we add broader photonics support, we will have a
design capability that the typical EDA vendors aren't even talking about
yet."
Capacity on the cheap
WDM has been a key focus of OFC for at least five years, but the
arrival of startups like Ciena gave the market viability in the
mid-1990s. WDM has been touted as a way for carriers to postpone
deployment of higher-rate Sonet infrastructures. It allows optical
signals to be split by frequency, or color of light, thereby turning a
physical OC-12 (622-Mbit) optical pipe into an equivalent OC-48
(2.048-Gbit) or OC-192 (10-Gbit) channel without the installation of new
fiber.
Advances in WDM were expected to entail improvements in both optical
amps and the passive systems used to separate wavelengths. In the former
category, however, erbium-doped fiber amps (EDFAs) are proving a
dependable workhorse that's being used across WDM designs. Even Alcatel,
which planned to enhance standard EDFA architectures with the use of
fluoride lasers, announced last week that it's abandoning fluoride for
more standard lasers and commodity EDFA components.
That leaves channelization as a key source of differentiation.
Lucent/Bell Labs last week described a two-grating WDM system that
allows 10-Gbit/s sig-nals to be transported simul-taneously over 100
separate wavelengths, to distances of 400 km. The terabit rates
represent the capacity to carry the entire globe's Internet traffic over
one fiber cable.
While the demonstration project used an ultra-wideband optical amp,
Atul Srivistava of Bell Labs' photonic networks research department
emphasized that the amp marks an advancement of standard EDFA
technology. The real breakthrough, he said, was to combine standard
waveguide gratings with special long-period gratings, thereby flattening
the gain spectrum of the dual-band optical amp.
The approach "involves a new architecture, but the materials and
components were not exotic in any way," Srivistava said. "This could
easily be applied to the Lucent WaveStar systems."
Even commercial vendors realize the need to control the quality of
passive components. Ciena found, for example, that it could achieve the
dense channelization it desired only by developing the Bragg-grating
manufacturing system in-house. "We examined everything available in the
industry and concluded we had to stand by the tolerances we were
demanding, even if it meant creating our own manufacturing line for
gratings,"marketing director Dennis Bilter said.
There was consensus at the conference that the arrival of ultra-dense
WDM and the commoditization of lower-density systems means WDM will
eventually migrate to metropolitan-area and access networks. Ciena has
anticipated the trend by developing special systems for citywide rings,
called MultiWave Metro.
The drive to bring WDM downstream has become a primary goal of
Britain-based Bookham Technology Ltd., developer of a technology called
active silicon with integrated optical circuits (ASOC). Bookham founder
and president Andrew Rickman believes his company's integration of
waveguides on standard silicon holds promise for the eventual
integration of VCSEL arrays, transimpedance amps and even photonic
switching elements, though he said Bookham plans to partner with
optoelectric-design partners to achieve some high-integration WDM
components.
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