32 Optics & Photonics News ■ March 20011047-6938/01/03/0032/4-$0015.00 © Optical Society of America

The first single-channel, EDFA-basedsystems yielded a significant increase incapacity. Once EDFA technology becameestablished, other insights and inventionsenabled rapid growth. The first DWDMtechniques made it possible to use moreand more of the available optical band-width in the fiber.2 Later, dispersion man-agement was introduced along with newfiber types, innovations which together al-lowed for DWDM channels to be more-densely packed.3 Error-correcting codeswere introduced at the electronics end ofthe optical transmitters and receivers,which allowed for an increase in systemmargin.4 Finally, the appearance of newtransmission formats allowed the bit rateper channel to increase from 2.5 to 10Gbit/s.5

The upper trace in Fig. 2 shows the to-tal capacity of testbed demonstrations

March 2001 ■ Optics & Photonics News 33

NEAL S. BERGANO AND HOWARD KIDORF

Cable NetworksGlobal Undersea

Both the planning and the construc-tion of optical telecommunicationsnetworks are now being conducted

on a global scale. Phase one of the TyComGlobal Network (TGN) will use 250,000km of cable (and over 2,000,000 km of op-tical fiber) to connect the majority of theearth’s population centers with primary ca-ble landings in the Atlantic, Pacific, andMediterranean oceans (see Fig. 1). Thetransmission capacity of these next-genera-tion links will far exceed 1 Tbit/s, a levelonce considered exclusively the realm oflaboratory experiments. This remarkableachievement has resulted from the applica-tion of advanced transmission techniques,

Optical fiber networks span the globe with Tbit/sdata transmission capacity.

Global Undersea

Figure 2. Transmission capacity of laboratory experiments and installed systems and the years in whichthey were demonstrated/installed.

Figure 1. The TyCom global network.

developed over the course of the past twodecades, to expand the capacity of light-wave systems. These techniques include de-velopments in both dense-wavelength divi-sion multiplexing (DWDM) transmissionequipment and undersea transmissionlines. A new generation of terminal equip-ment has allowed DWDM channels to beplaced closer than ever before, resulting inrecord-setting spectral efficiency. In under-sea transmission lines, optical amplifierswith increased optical bandwidth and newgenerations of transmission fibers permithigher data rates over longer distances. Inthis article we review the technology thatwill make these systems possible.1

Capacity marches on At the end of the 1980s, there were ques-tions about the capacity that could ulti-mately be achieved by long-haul lightwavesystems. Although there was agreementthat the intrinsic capacity of optical fiberwas very large, the capacity of transmis-sion systems based on digital regeneratorswas known to be limited. This limitation isa result of the capacity bottleneck in theelectro-optic regenerators in which opticalsignals were converted to electrical signals,processed with integrated circuits, andthen retransmitted optically. The key de-velopment was the introduction of the er-bium doped fiber amplifier (EDFA).

Cap

acity

per

fibe

r pa

ir,G

bit/

s

Year Commencing Service

Laboratory Results

Installed Systems

1988 1990 1992 1994 1996 1998 2000 2002

1000

100

10

1

0.1

TPC-5) used (255,239) Reed SolomonFEC codes to achieve about 5 dB codinggain. More recent undersea systems, suchas Yellow, 360Atlantic, SAm, and the Ty-Com Global Network, will make use ofsecond generation FEC codes with severaldB of additional coding gain.6

Another area in which undersea termi-nal transmission equipment differs fromthat used for terrestrial applications is op-tical modulation techniques. Terrestrialsystems operating at 10 Gbit/s typicallyuse ordinary non-return-to zero (NRZ)signaling. Research has shown that the re-turn-to-zero (RZ) format, combined withphase modulation, which is synchronouswith the data rate,7 performs significantlybetter than ordinary NRZ on longtransoceanic systems. The pulse format,known as chirped-return-to-zero (CRZ),5

achieves its performance improvement bytolerating higher levels of fiber nonlinear-ity and dispersion.

An important part of the undersea ca-ble network’s terminal is the power feedequipment used to supply electrical powerto the optical amplifiers located in the un-dersea repeaters. Undersea systems use DCpower conductors in the same cable carry-ing the optical fibers to supply power tothe undersea repeaters. The repeaters arepowered by running a constant current ofabout 1 Amp through the power conduc-tor. Power regulators in each repeater sup-ply local power for each amplificationstage. A transoceanic length cable typicallyrequires several thousand volts at the endsto maintain this constant current.

A maintenance system is used to iden-tify the location of faults and/or degradedcomponents by monitoring the underseaequipment from the shore terminals. Fig-ure 4 shows a diagram of a typical amplifi-

er pair located in an undersea system. Anoptical monitoring signal is reflected andcoupled back into the fiber in the reversedirection at a low optical power. The SNRof this low level signal is enhanced usingsignal correlation techniques to providedata on repeater gain, gain tilt, and spanattenuation. The repeater design is alsocompatible with the use of coherent opti-cal time domain reflectometer techniquesto identify the location of a cable cut orother fault between repeaters.

Undersea cable transmission terminalequipment, like land-based systems, hasprogressed in terms of bit rate per fiber.Initial amplified undersea systems in-stalled for service in 1995–96 used a singlewavelength operating at 5 Gbit/s. The next

generation dropped back to 2.5 Gbit/s perwavelength to allow DWDM operation,increasing the total capacity per fiber pairto 20-40 Gbit/s. Recently deployed systemsuse 10 Gbit/s per wavelength. Underseasystems in the planning phase may have asmany as 96 wavelengths on each of asmany as eight fiber pairs. When otherpieces of terminal equipment are consid-ered, this could easily lead to a situation inwhich a thousand or more network ele-ments require monitoring in a single cablestation. Centralized operations supportcapabilities are required to efficientlymonitor and coordinate maintenance ofmodern undersea cable systems.

Growth of bandwidthTransoceanic cable networks with globalreach and multi-terabit capacity are beingbuilt. The total optical bandwidth that canbe equalized over long distances, the spec-tral efficiency that can be achieved, and theability to electrically power the underseaplant, will dictate the ultimate capacity ofnext generation systems. As Fig. 2 clearlyshows, the demonstrated capacity of light-wave systems has grown at a fast rate.Within the next few years, installedtransoceanic systems will fill the conven-tional gain band in erbium doped fiberamplifiers; thus, new amplifiers with largeroptical bandwidths will be needed to con-tinue the capacity growth.

References1. N. Bergano,“Undersea fiberoptic cable systems:

high-tech telecommunications tempered by a cen-tury of ocean cable experience,” Optics & Photon-ics News, 11 (3), p. 20-5 (March 2000).

2. H.Taga, et al.,“Over 4,500 km IM-DD-2-channelWDM transmission experiments at 5 Gbits/s using138 in-line Er-doped fiber amplifiers,” OFC/IOOC’93, postdeadline paper PD4, San Jose, CA, Feb.1993.

3. F. Forghieri, R.W.Tkach, and A.R. Chraplyvy,“Fibernonlinearities and their impact on transmission sys-tems,” Chapter 8 in Optical Fiber Telecommunica-tions IIIA, ed. Ivan P. Kaminow and Thomas L. Koch,Academic Press 1997.

4. J. L. Pamart, et al.,“Forward error correction in a 5Gb/s 6400 km EDFA based system,” Electron. Lett.Feb. 17, 30 (4).

5. N. S. Bergano, et al.,“320 Gb/s WDM Transmission(64x5 Gb/s) over 7,200 km using large mode fiberspans and chirped return-to-zero signals,” OFC ’98,paper PD12, San Jose CA, Feb. 1998.

6. C. R. Davidson, et al.,“1800 Gb/s transmission ofone hundred and eighty 10 Gb/s WDM channelsover 7,000 km using full C-band EDFAs,” paperPD23, OFC 2000, Baltimore, Maryland, March 2000.

7. US Patent #5,526,162 ,“Synchronous Polarizationand Phase Modulation for Improved Performance ofOptical Transmission Systems.”

Neal S. Bergano and Howard Kidorf are with TyComLaboratories in Eatontown,New Jersey.Neal Berganocan be reached at nbergano@tycomltd.com.

UNDERSEA CABLE NETWORKS

March 2001 ■ Optics & Photonics News 35

performed by TyCom Laboratories overthe past decade. The data show an increasein capacity that has been doubling eachyear. The growth in the transmission ca-pacity of installed undersea cable systemshas been accompanied by a reduction inthe lag between laboratory experimentsand deployment in the field: in the early1990s it generally took about five years fora technology demonstrated in the lab toreach the field. Today, it takes only two tothree years.

Today’s generationof undersea networksThe base of installed undersea fiber opticnetworks can be classified as fourth gener-ation undersea optical transmission tech-nology. (Previous generations were regen-erative 1.3 μm transmission, regenerative1.55 μm transmission, and optically am-plified single channel transmission, usual-ly with 1.48 μm pump lasers.) The currentgeneration of networks is characterized bya number of attributes, including:

• undersea repeater technology thatsupports transmission on up to eightfiber pairs using EDFAs pumped withredundant 980-nm laser pumps;

• gain equalization distributed along thefiber path that has resulted in systemssupporting up to 64 wavelengths, eachof which will operate at 10 Gbit/sacross the Atlantic (this year) and Pa-cific (in 2002). Over shorter distances,systems will support 96 wavelengths in2002; thus, each fiber pair will have the

potential of about 1 Tbit/s, giving aneight-fiber-pair cable a maximum ca-pacity of 8 Tbit/s;

• fiber embedded in the undersea cablethat is engineered to give low end-to-end dispersion and reduce impair-ments caused by the fiber’s nonlinearindex of refraction. The “dispersionmap” uses a mix of positive- and nega-tive-dispersion fibers, along with a mixof fiber effective areas;

• high-performance terminal equipmentthat is specifically designed for trans-mitting and receiving DWDM carrierchannels for undersea transmission —included in this equipment are featuressuch as forward error correction(FEC), synchronous optical phasemodulation, signal preemphasis anddispersion compensation tailored foreach channel;

• when needed, branching units locatedoff the continental shelf support fiberrouting by splitting fiber connectivitybetween the main undersea fiber optictrunk cable and a branch cable termi-nating at a landing site along the cableroute. Branching units that use wave-length selective filtering can be used tosplit the capacity between the maintrunk cable and the branch cable;

• protection against failures using equip-ment redundancy, facility protection(span, ring, or mesh switching), or acombination of both. Many of the net-working features are created by usingstandard terrestrial add–drop multi-plexing equipment configured for the

required application, such as self-heal-ing rings. Increasingly, optical cross-connects (both opaque and transpar-ent) and routers are being planned tofulfill the role of protection;

• a transmission topology managed by aproprietary element management sys-tem overlaid by a network manage-ment system (NMS) providing config-uration, fault, performance, and secu-rity management features.

Developments in transmissionterminal technologyTerminal equipment technology hasplayed a critical role in the rapidly increas-ing capacity of undersea systems in thepast and will continue to do so in the fu-ture. The terminals include equipment tocondition digital data for transmission un-dersea, power feed equipment to provideDC power to the undersea equipment, linemonitoring devices to ascertain the loca-tion of undersea cable cuts and otherfaults, and maintenance equipment tosupport the rest.

Undersea terminal transmission equip-ment generally differs significantly in sys-tem length from equipment for terrestrialapplications. Typical transoceanic cablelengths are 6,000 km to 7,500 km in theAtlantic Ocean, and 8,000 km to 10,000km in the Pacific Ocean. Even with low-loss fiber, amplifier noise figures near thetheoretical optimum, and relatively mod-est repeater spacing (which minimizes to-tal noise), a high level of amplified sponta-neous emission noise is necessarily presentat the receiver. Potential signal distortioncaused by the fiber’s nonlinear index com-bined with chromatic dispersion createsan upper limit on the optical power thatcan be launched from each amplifier at agiven wavelength.

Transmission impairments caused bylow SNR are overcome with forward error correction coding in the terminals.FEC is a digital processing technique inwhich additional, redundant bits areadded to the customer information bits atthe transmitter, and used in turn at the re-ceiver to identify and correct transmissionerrors. FEC allows operation of the trans-mission line at a lower SNR. In theory,FEC is useful in both terrestrial and un-dersea systems, but it has taken longer forFEC to be implemented in land-based net-works. The first transoceanic undersea op-tically amplified systems deployed usingoptical amplifiers in 1996 (TAT 12/13 and

UNDERSEA CABLE NETWORKS

34 Optics & Photonics News ■ March 2001

Figure 3. Diagram of a typical undersea cable link.The undersea equipment consists of cable sectionsjoined by repeaters, which house the erbium doped fiber amplifiers.The terminal equipment grooms the“terrestrial grade” signals for transmission across the ocean.

Figure 4. Block diagram of amplifier pair showing loopback arrangement for separate maintenance signals.

BranchingUnit

Repeater

Terminal

Cable

LineTerminatingEquipment

Undersea NetworkManagementEquipment

Power FeedEquipment

HighPerformanceOpticalEquipment

Traffic

WDMCoupler

3 dBCoupler

ErbiumDoped Fiber

ErbiumDoped Fiber

Isolator

Isolator

980 nmLaser Pump Unit

Loop-backCouplerModule