evolving optical fiber designs

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Evolving Optical Fiber DesignsKevin M. Able

Telecommunications Products DivisionComing IncorporatedComing,NY 14831

AbstractThe introduction of optical amplifiers early thisdecade, combined with the advancement oftransmitter and multiplexing technology, has allowedthe bundling of many transmission channels onto asingle fiber. This has occurred simultaneously withan increasing demand for information carryingcapacity driven by everything from deregulation to anexplosion in internet use. To meet these challenges,fiber manufacturers are evolving new fiber designs tofacilitate changing transmission techniques. Thispaper will discuss the driving forces behind new fiberdesigns, and examine some of the advances that havebeen made, including dispersion compensating fiber,non-zero dispersion shifted fiber, and large effectivearea fiber.

IntroductionThe workhorse of optical fibers since 1983 hasbeen the unshifted single-mode fiber, SMF.Optimized for operation at the 1310 nm wavelength,

it accounts for an installed base of tens of millions ofkilometers of fiber. Early installations operated at bitrates in the hundreds of megabits per second range,while today system upgrades over the same fiber areapproaching 10 Gb/s and beyond.In fact, standard single-mode fiber was provento be so capable, even for operation at 1550 nm(albeit at reduced distances and data rates), it becamea widespread belief that single-mode fiber hadunlimited bandwidth, and all that was required was achange in electronics to increase the information-carrying capacity. Several factors came together inthe early 1990s to change that view.requirements have historically doubled every 24 to 30months. The optical fiber that had been installedeasily met those needs. However, the first years ofthis decade have seen unparalleled growth in thetelecommunications ndustry. The lure of providing

As illustrated in Figure 1, bandwidth

true broadband service has begun to blur thedistinction between voice, video, and data providers.Broad deregulation and interactive services havecombined to strain the capacity of the worldscommunications infrastructure. In addition, the lackof sufficient fiber counts in early installations has leadto bottlenecks for some carriers who have literally runout of fiber in critical routes. As these carriersstruggle to add more capacity, new entrants into thehigh capacity transport business are constructing newroutes. This demand for bandwidth has taxed thecapabilities of unshifted fiber at a rate which can notbe met economically merely by installing more.Indeed, the demand for optical fiber has growndramatically, resulting in a reduced supplyworldwide.At about the same time, a key facilitator toincreasing fiber information carrying capacity wasintroduced commercially, the erbium doped opticalfiber amplifier (EDFA). Electrical signalregenerating and amplifying methods require signalconversion from optical to electrical and back tooptical, and must be designed for specific codingschemes and bit rates. EDFAs are all optical, andwill amplify whatever signal is input, regardless ofstructure or bit rate. A broad amplification bandmeans multiple wavelengths can be transmittedsimultaneously, effectively increasing availablebandwidth by factors of 8, 16,or even 32.

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Figure 1: Transmission data rates have doubledevery twoyears.

Because EDFAs operate in the 1550 nmwindow, they promised to be a key facilitator fordispersion shifted fiber (DSF), a technology whichComing had first introduced in 1985. By matchingthe source wavelength with the fiber zero dispersionwavelength, low attenuation, and opticalamplification,DSF seemed poised at last to meet therequirements for high data rate, long distanceapplications.

CCECE97 0-7803-3716-6 /97/$5.00 0 997 IEEE


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Nonlinear EffectsThe increased output power inherent in opticalamplifiers combined with the simultaneoustransmission of multiple wavelengths raised theimportance of phenomena which had until then beenof academic interest only. Under these newconditions, optical fiber exhibits a nonlinearresponse, and an entirely new set of issues arose tomake the fiber, for the first time, the limiting factor toincreased transmission capacity. In addition, a newvocabulary was added to the industry which includesterms such as self phase modulation, cross phasemodulation, modulation instability, and four wavemixing.The most troubling of the nonlinear effects isfour wave mixing. When multiple signals co-propagate, they mix to produce additional channelswhich can sap power from, and overlap with, theoriginal signals. Figure 2 illustrates this process forthree evenly spaced channels, h l ,b,nd b. hemixing components occur at hxyz= h,+ & - L.Because of the even spacing of the originalwavelengths in this example, some of these newlygenerated signals occur at the original channels.

Figure 2: Illustration of the generationof fourwave mixing components for three evenly spacedchannels.

The total number of mixing componentsgenerated, m, is calculated asm = 112 (N3-N2)

where N is the number of original channels. For athree channel system this means there arenineadditional signals to contend with. For an eightchannel system this number increases to 224.four wave mixing is to employ uneven channelspacing. However, while this is relatively straightforward for three channels, the task becomes

One obvious means of minimizing the impact of

significantly more complicated for a 32 channelsystem and 15,872 potential mixing components.The four wave mixilng process is most efficientat the zero dispersion wavelength, in direct conflictwith the need to keep fiber dispersion to a minimumto optimize transmission capability. Becausestandard dispersion shifted fiber has its zerodispersion wavelength within the operating band ofEDFAs, these conflicting requirements place limitson the capability of DSF for high data rate long haulnetworks utilizing wavelength division multiplexing.In response, a new category of optical fiber has beendeveloped; non-zero dispersion shifted fiber, NZ-DSF.Non-Zero Dispersion Shifted Fiber

The concept behind NZ-DSF is simple. Thezero dispersion wavelength of dispersion shifted fiberis further moved such that it resides outside theEDFAs operating gain band, effectively re-introducing a controlled amount of dispersion into thesystem. This is depicted in Figure 3, where thedispersion curves for Corning09 SMF/DSTMnd SMF-LSTMibers are compared.. The resulting dispersionfor the SMF-LS fiber is law enough to provide forlong routes, yet not so low that four wave mixingleads easily to system impairment.

SMF-DS,/

Figure 3: Diagram of th,e dispersioncharacteristics of non-zero dispersion shifted fibercompared with standard dispersion shifted fiberrelative to the operating gain band of an EDFA.

Using NZ-DSF, 8 x 10Gbds data rates over 360kilometers without compensation have beendemonstrated.'a single low loss, low dispersion fiber can have asignificant cost benefit over utilizing standard SMF.Regeneratorlamplifier spacing can be extended, and

The ability to combine many data channels onto


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there is no need to add additional equipment tocompensate for dispersion in current typical systems.It has been estimated that savings of as much as 30%to 50%can be realized on the cost of equipping thefiber.Dispersion Compensation

Notwithstanding the introduction of non-zerodispersion shifted fiber, a significant base of installedunshifted fiber already has been deployed. Toeffectively utilize this fiber in transmission systemsemploying optical amplification,a means of reducingthe accumulated dispersion resulting from 1550nmoperation over long distances is necessary. To meetthat need, manufacturers have introduced dispersioncompensating fiber (DCF). t 1550nm an unshiftedsingle-mode fiber will have dispersion on the order of+I7 ps/nm/km. Although this high dispersioneliminates four wave mixing as a concem, themaximum transmission distance for a given data rateis limited by chromatic dispersion. By the nature oftheir design, dispersion compensating fibers havehigh negative dispersion. When placed appropriatelywithin a system link, the large negative dispersionofDCF brings the overall dispersion for the link back tonearly zero, reversing the pulse spreading whichoccurred as the signal propagated.

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Figure 4: 10 Gbls upgradescenario for existingSMF installations.

This technique has allowed the use of unshiftedsingle-mode fiber at 10 Gbls over hundreds ofkilometers. A typical upgrade scenario is illustratedin Figure 4.Nevertheless, the need for increased bandwidthhas been steady, and the capabilitiesof dispersioncompensated standard fiber installations, and evenNZ-DSF fiber, eventually will be strained. To meetthe drive for even greater numbers of operatingchannels on new builds, several methods have beenproposed. The first utilizes a variation of dispersioncompensation. Generally referred to as dispersion

management, alternating lengths of positive andnegative dispersion fiber arecombined in a link in aplanned manner. In this way a finite local dispersionis maintained while the overall dispersion is limited toa near zero level. Recent experiments have showncapability reaching 32 channels at 10Gbls each over640kilo meter^.^ Although effective, the drawback tothis technique remains that careful planning isrequired to ensure a low overall dispersion.Restoration or other unanticipated reconfigurationcould impair the system unless sufficient margin isdesigned in. Clearly, the most desirable option is toinstall a fiber which can not only accommodatetodays range of WDM systems, but provide theflexibility for future upgrade.

Large Effective Area Fiber

By increasing the light carrying cross section ofthe fiber, the path average intensity can be loweredfor a given total power. The advantages to thisincrease in effective area include higher powerhandling capability, higher signal to noise ratio, lowerbit error ratio, longer amplifier spacing, and mostimportantly, higher information carrying capacity.Typical dispersion shifted fibers have effectiveareas of approximately 50 pm. Large effective areafibers with areas as large as 92 pm2 have beenre~or ted .~s shown in Figure 5 , an immediatebenefit to this increased effective area is a reductionin the amount of power funneled into four wavemixing components. This implies higher powerhandling capabilities, and consequently longeramplifier spacing. This is further illustrated in Figure6, which plots optical amplifier spacing as a functionof effective area.upgrade to very high bit rate, dense WDM. Only justrecently systems have begun to explore 10Gb/s datarates. Although the next step, 40 Gblslchannel, willbe some time in the future, NZ-DSF large effectivearea fiber will provide the platform capableofhandling these information rates over the entireEDFA gain band. Both of these benefits, reducedamplifier counts and increased spacing, and theability to easily upgrade to higher data rates, translatedirectly into reduced installation and operating costs.

Another advantage is the capability for future


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Figure 5: Four wave mixing component powercomparison between non-zero dispersion shiftedfiber and large effective area fiber. System modelresults for two channelswith 100 GH z spacingover 90 kilometers.

120 140Ac f f ( d ) loo80NZ-DSF 6o

Figure 6: Optical amplifier spacingasa functionof effective area.SummaryDeregulation, competition, and expanded services allhave played a part in the hunger for bandwidth. Acontinued increase in this requirement has beenresponsible for the development of cost effectivetechnologies to meet that demand. Foremost amongthese advancements has been the successfulsimultaneous deployment of optical amplifiers andwavelength division multiplexing. Moreover, themove to full utilization of the 1550nm operatingwindow has spurred activity to introduce otherenabling technologies. Dispersion compensatingfiber has facilitated a means of utilizing the vastinstalled base of standard single-mode fiber at bitrates of 10Gb/s/channel over distances of several

hundred kilometers. The development of non-zerodispersion shifted fiber far new builds provides theadvantage of low uncompensated dispersion, lowattenuation, and a reductilon in four wave mixingeffects. Next generation fibers must also be capableof minimizing the impact of nonlinear effects, and beable to accommodate the transport of hundreds ofinformation channels. Fibers with large effective areawill fill this need by providing a platform whichencompasses the flexibility to accommodate a rangeof wavelength plans, information carrying capacityfor now and into the future, and cost effectiveness.

V. da Silva et al., Error ree 8 x 10 G b h WDMtransmission over 360 km ofnon-zero dispersion-shifted fiber without dispersion management, Post-deadline paper presented ilt the Optical FiberCommunication Conference, Dallas, Texas, February1996.P. Palumbo, Bandwidth! eeds spur fiber diversity,Liphtwave, Nashua, NH: I?ennWell Publishing, pp. 1,November, 1996transmission ove r 640 km using broad band, gain-flattened erbium-doped silica fiber amplifiers, Post-deadline paper presented at the Optical FiberCommunication Conferens , Dallas, Texas, February

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A. Srivastava et al., 32 x 10G b h WDM

16-21, 1997.Y .Liu, Dispersion shifted large-effective-area fiberfor amplified high-capacity long-distance systems,Proceedings of the Optical Fiber CommunicationsConference, Dallas, Texas, February 16-21, 1997.

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