in applying optical switches to packetswitching. “Hero experiment” teams re-ported they had stretched terabit trans-mission across thousands of kilometers.Some technical session attendees werehard pressed to decide between the manyinteresting papers and tutorials being pre-sented simultaneously.
The bubble burstsIn retrospect, the business euphoria atOFC 2001 was less a reflection of econom-ic reality than a scene reminiscent of a Sat-urday morning cartoon. The telecom mar-ket had charged ahead so fast that no onenoticed when the industry actually racedbeyond the edge of the cliff: the force ofgravity didn't take hold until after OFC2001, when many companies looked downand realized they were treading on air. Thebig question now is which of yesterday’shigh flyers will remain flat as pancakes atthe base of the cliff, and which will bounceback and zoom off, like cartoon heroes.
To be sure, signs of trouble were al-ready visible at last year's OFC. A numberof companies scaled back their travel plansbefore the meeting. Some exhibitors failedto display any readily identifiable prod-ucts: their press kits and Web sites devotedmore space to investors and managementthan to technology and applications. Al-
OFCJeff Hecht
T he Optical Fiber CommunicationConference & Exposition (OFC)weathered the burst of the
telecommunications bubble much betterthan did many telecom companies. Ex-hibit sales were up, while attendance wasdown only slightly, to 33,000. The 13%drop from the record high registered atOFC 2001 was minor compared to jobcuts in the 50% range at some telecomcompanies and to stocks that plungedover the course of the past year to a frac-tion of their peak values. The total of1,204 paid exhibitors represented ahealthy increase over last year's 970, al-though some empty booths were scat-tered around the sprawling AnaheimConvention Center, and some exhibitingcompanies appeared to have scaled downtheir displays.
Technology roars aheadThe best news from OFC 2002 was contin-uing advances on the technology frontier.Developers reported new types of “holey”fibers with strong nonlinear properties,pointing to potential uses in optical com-ponents. Advances in amplification tech-nology were showcased in experimentsdemonstrating that optical parametricamplification could be useful in fibers. Sci-entists addressed the challenges inherent
After the Bubble
though the “dot.coms” were alreadyplunging in March 2001, much of thetelecommunications industry was in astate of denial, convinced that the fact thattheir product offerings consisted of hard-ware would keep them from evaporatinglike an e-commerce Web site.
The implosion of the telecom bubblemade the past year a tough one for manycompanies. The OSA Executive Forumthat preceded this year's OFC was aptly ti-tled “Managing Through Cycles.” JohnRyan, chief analyst of market research firmRHK, Inc., said in his opening talk that“the drunken sailor years” of 1999-2001had left North American “wireline” tele-phone companies with a $49 billion hang-over in excess capital investment. Duringthe bubble, Ryan said, cumulative annualgrowth in capital investment soared from13% averaged over the four precedingyears to an unsustainable 42%. Thenspending levels plummeted: from $92 bil-lion in 2000 to an estimated $48 billionthis year.
Both the boom and the bust were con-centrated in North America. Normally,Ryan said, North American sales are 40%of the global total, but during the bubble,the fraction peaked at 60%. This year, heexpects North American market share todrop to a little over 30% of global capitalexpenditures. “Full recovery is at least ayear away,” Ryan predicted, although hesaid he remains optimistic about the longterm. “Unlike the concept of selling dogfood on the Internet, telecom isn't goingaway.”
In the meantime, vast sums of moneyand a number of companies have gone upin smoke, with more expected to follow.On March 24, The New York Times report-ed that “$1.4 trillion in investor wealth hasevaporated” from the telecom bubble thatpeaked at $2 trillion. Many firms left withlittle stock equity are struggling to copewith high debt loads. In addition, The NewYork Times reported, securities regulatorsare investigating transactions betweensome companies, which they suspect “mayhave been designed to pad inadequate rev-enue.”
Ryan, who told the group that in hisopinion “bankruptcies are necessary,” saidindustry consolidation is proceeding—al-beit slowly. Many investors who oncethrew money at anything optical are stillreluctant to give up on their investments.In addition, participants in the Executive
Forum agreed, mergers can't solve everyproblem. “When you put two rocks to-gether, they don't float any better thanthey did separately,” said David Hardwick,CEO of Confluent Photonics and chair ofthe OSA Corporate Associates Committee.
Empty booths at OFC 2002 may havebeen the unmarked graves of some of thevictims. The curtains on one booth bore asign reading: “Optenia ceased to exist onMarch 1. However, the technology liveson. To learn more about it, contact...” An-other company sent out frantic e-mailsjust before the show, trying to sell its pre-paid booth at half price. Fewer freebiesand scaled-down booths were other signsof tight budgets.
Yet most observers shared Ryan's con-viction the industry would bounce back.New exhibitors offering well-definedproducts were plentiful: innovationsranged from packaging components andsealants to molded micro-optics, advancedtunable lasers, and waveguide optics.
Holey fibersA number of technical sessions focused onthe evolution of “holey,” or microstruc-tured, optical fibers, made by assemblingrods and tubes of uniform compositionand drawing them into fibers with holesrunning along their length. The mi-crostructures create photonic bandgaps—regions where light of certain wavelengthscannot propagate—which can bearranged to tailor light propagation in thefiber.
Now that the basic concept has been es-tablished, developers of the technique areexploring the unusual transmission prop-erties they can produce and their potentialapplications. In a tutorial, Tanya Monro ofthe University of Southampton describedhow holey fibers could be made highlynonlinear for applications including opti-cal parametric amplifiers, Raman amplifi-cation, optical switching, and soliton gen-eration. Dispersion tailoring can be ap-
plied to dispersion compensation.In the post-deadline session, Kim P.
Hansen of Crystal Fibre A/S in Birkerod,Denmark, reported the first microstruc-tured fiber with zero dispersion at 1550 nm, a development which Hansenreported can enhance nonlinear effects inthat band. The polarization-maintainingfiber had a nonlinear coefficient of20 (W-km)-1 20, about double that ofstandard nonlinear fibers. It has a 2.3-�mcore and can be spliced to conventionalfiber with loss of only 0.3 dB. Hansen saidthe combination could lead to nonlinearapplications like optical regeneration andclock recovery.
In another post-deadline paper, Monroand nine Southampton colleagues de-scribed a new single-mode holey fiber witheffective nonlinearity of 550 (W-km)-1.They produced a 16-mm preform by ex-truding a commercial high-lead contentglass with a refractive index of 1.80 at 1.53 �m. Then they drew it down in twostages to a fiber with 120-�m outer diame-ter. The result, Monro said, is the first hol-ey fiber made from a compound glass, andwith nonlinearity more than 15 timeshigher than the highest reported for anyother microstructured fiber. Such highnonlinearity is essential to limit the lengthof nonlinear fiber components to a scale ofmeters.
Three regular-session papers from agroup at Murray Hill, New Jersey-basedOFS, the former Optical Fiber Solutionsdivision of Lucent Technologies, describedtunable components based on holeyfibers. Charles Kerbage and colleaguestuned fiber properties by inserting fluidswith high or low refractive index to fillpart of the holes running the length of thefiber. When one end of a hole was sealed,heating the air on that side pushed the flu-id plug back and forth by capillary action,affecting light coupling along the fiber. Tofacilitate tuning, other optical propertiesalong the fiber also varied. In one case, along-period grating was written in thefiber core, so that moving the liquidcaused it to interact with different parts ofthe grating. In a second variation, the liq-uid was moved in and out of a taperedpart of the fiber. These movementschanged the wavelength filtered out in thelong-period grating, or the attenuation inthe taper.
Kerbage's group also demonstratedsimilar tunability when they filled air holes
OFC 2002 REVIEW
20 Optics & Photonics News ■ May 2002
Most observers agree with John Ryan's
prediction that the industry will bounce back.
with a special polymer in which the refrac-tive index depends strongly on tempera-ture. Heating the material from near roomtemperature to 120°C reduced attenuationof a tapered fiber from 30 dB to 0.8 dB. Inanother paper, R. L. Bise, Kerbage, and fourOFS colleagues filled holes with a liquidwith temperature-sensitive refractive indexnear 1.8, and varied transmission by tun-ing between room temperature and 150°C.
Fiber-optical parametric amplifiersAlthough optical parametric amplifiershave been around a long time, they at-tracted little interest from the fiber-opticcommunity until the recent developmentof highly nonlinear optical fibers.
Parametric amplification is a nonlinearprocess that, like four-wave mixing, in-volves the interaction of light waves at dif-ferent frequencies. In a parametric ampli-fier, the interaction occurs through the in-tensity dependence of the refractive indexof silicon. An intense pump beam in thefiber interacts with a signal at nearby fre-quency, generating one side band at thesignal frequency and a second side band,the “idler,” at a frequency as far from thepump as the signal is, but on the other sideof the pump. As in four-wave mixing,phase-matching increases the efficiency ofthe process.
In an invited paper, Jonas Hansryd ofCENiX, Inc., and the Chalmers Universityof Technology, said optical parametricamplifiers offer the theoretical potential ofnoiseless amplification. Gain slope, meas-ured in decibels per watt per kilometer offiber, is usually higher for parametric am-plification than Raman gain. Highly non-linear conventional fibers had gain slopesof 28 for Raman amplification and 100 forparametric amplification. For the highernonlinearities possible with holey fibers,Hansryd cited gain slopes of 440 for para-metric amplification compared to 68 forRaman amplification. Like Raman ampli-fication, parametric amplification has afemtosecond response. Researchers havereported parametric gain bandwidths to200 nm, considerably wider than Ramangain in silica. Potential applications in-clude wavelength conversion from signalto “idler,” amplification outside the er-bium-fiber gain window, and optical time-domain multiplexing.
In a post-deadline paper, Stojan Radicof Bell Labs and colleagues from both Bell
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May 2002 ■ Optics & Photonics News 21
and OFS reported a new approach to fiberparametric amplification that they saypromises more practical devices. Insteadof pumping with a single source at a wave-length in the anomalous dispersion re-gion, they pump with two sources, oneabove and the other below the fiber's zero-dispersion wavelength. This approach al-lows efficient parametric amplificationwithout assembling segments of differentfiber types to produce the right dispersionprofile. They reported flat 40-dB gainacross a 22-nm band. Advantages include“an increase in degrees of freedom in gainsynthesis, circumvention of Brillouinthreshold, and more efficient pump-signalconversion,” Radic concluded.
Optical packet switchingThis year's OFC also saw system develop-ers start to address a serious mismatch be-tween optical-switch capabilities and therequirements of modern networks. To-day's optical switches make optical con-nections from one terminal to another, aprocess called circuit switching. Circuitswitching is standard in the telephone net-work, where it creates connections that lastthe duration of a phone call.
The problem is that most of the growthin telecommunications traffic is on the In-ternet, which operates differently. Internetdata is organized into blocks called pack-ets, each of which is assigned a “header”that specifies where the packet is to be de-livered. Packet switching is done by elec-tronic devices called “routers,” which readthe header, analyze the best way to routethe packet, and then direct the packetalong that route. Routers must analyze theheader information and store the packetwhile deciding what to do with it.
The need for both processing powerand memory makes packet switchingmore complex than circuit switching.While circuit switching can be performedoptically or electronically, today's routersrequire electronics for processing andstorage. Some observers doubt near-termprospects for optical routing. “Practicaloptical cache memory does not exist, so itis not possible to make an optical packetswitch,” Julie E. Fouquet of Agilent Tech-nologies said in a tutorial on waveguideswitching. Yet that has not discouraged a number of laboratories from workingon optical approaches to the routingproblem.
Developers generally envision opticalrouting on a separate network “layer” thanelectronic routing, so packets would re-quire both optical and electronic headers.In this case, the electronic headers wouldbe packaged together with the electronicpacket into an optical packet, with a sepa-rate optical header (often called an opticallabel) added. The optical labels can be sep-arated from the encapsulated packet intime or wavelength. Bit-serial transmis-sion puts the optical information in an op-tical header, separated from the packet bya buffer interval or guard band. Alterna-tively, optical subcarrier multiplexing putsthe optical label at a different wavelength,where it can be separated optically. Whenthe signal reaches an optical router, the op-tical label is converted to electronic formand processed electronically. In a type ofprocessing called “all-optical label swap-ping,” the optical packet passes through afiber delay line set to delay the light for aslong as the electronics need to decide onoptical routing. Meanwhile, the optical la-bel is erased, a new one is added, and thepacket is switched to a different wave-length, David Blumenthal of the Universi-ty of California at Santa Barbara, said inan invited talk. Wavelength-demultiplex-ing optics then route the optical packet tothe desired destination.
In a post-deadline paper, Blumenthal'sgroup reported what they called “severaldramatic improvements” in all-optical la-bel swapping. They transmitted variable-length packets at 80 Gbit/s, with optical la-bels at 10 Gbit/s, which they used to directoptical conversion of the packet to a newwavelength and generate a new optical la-bel. They demonstrated the process ontwo successive hops, showing that opticalrouting can be performed in series.
Others are investigating ways to adaptpacket switching to optics rather than op-tics to packet switching. Instead of chop-ping data into packets, Bishwaroop Gan-guly of the MIT Lincoln Laboratory sentbrief flows of optically circuit-switcheddata between two points.“A flow-switchedtransaction bypasses electronic routersalong its path,” reducing router load, hesaid. The switching can be done withcheap, readily available optics, and givesusers the guaranteed quality of service thatcomes with circuit switching. Schedulingbrief connection intervals allocates systemresources efficiently, which serves the samefunction as packet switching.
Hero experiments and record low lossLast year's OFC hero experiments setrecords for total bit rate through compara-tively short lengths of fiber. The hero ex-periments in this year's post-deadline ses-sion concentrated instead on the morepractical goal of sending terabits per sec-ond through the thousands of kilometersneeded for long-haul transmission. Ateam from Tyco Telecommunicationstransmitted 256 10-Gbit/s channels, a totalof 2.56 Tbit/s, through 11,000 km of fiberusing hybrid Raman/erbium-fiber ampli-fiers with continuous bandwidth of80 nm. Peak-to-peak variation across theentire range was 7.5 dB, corresponding toequalization of channel power to an aver-age of 0.028 dB per amplifier.
A group at Lucent Technologies' BellLabs sent an equivalent load—sixty-four40 Gbit/s channels—through 4,000 km ofnon-zero dispersion-shifted fiber, claim-ing a record for 40 Gbit/s capacity timesdistance. A key step in the process was useof a new format called return-to-zero dif-ferential-phase-shifted-keyed modulation,in which every bit interval has an opticalpulse, with data encoded as a phase shiftof either 0° or 180° between successive in-tervals.
The most surprising record to fall wasone of the most venerable, for lowest fiberattenuation. Dating to 1986, when H.Kanamori and nine colleagues reached0.154 dB/km, it had begun to look like aphysical limit for solid-silica fiber. At thepost-deadline session, Katusya Nagayamaand nine co-workers from Sumitomoclaimed a new record of 0.151 dB/km at1568 nm. Their fiber had a pure silica core,with first and second claddings dopedwith fluorine. The Sumitomo group’smeasurements showed that Rayleigh scat-tering accounted for 0.128 dB/km of lossat the minimum loss wavelength, with in-frared loss of 0.014 dB/km, hydroxyl-ionabsorption loss of 0.004 dB/km and im-perfection loss of 0.004 dB/km. Total at-tenuation was below 0.160 dB/km acrossan impressively broad range, from 1520 to1606 nm. The researchers said their fibercould extend transmission distance 30%beyond that of conventional germanium-doped single-mode fiber.
Jeff Hecht is a contributing editor for Laser Focus World,a correspondent for New Scientist, and the author ofCity of Light:The Story of Fiber Optics and UnderstandingFiber Optics. E-mail [email protected].
