Amplifier and multiplexing technologies expand network capacity

Dec. 1, 1995

Amplifier and multiplexing technologies expand network capacity

Integrated optical amplifier/wavelength-division multiplexing systems offer a four-channel capacity alternative to deploying new fiber or an OC-192 system in short- and long-range networks

serge melle, fahri diner and christoph p. pfistner

Pirelli cable corp.

Commercial second-generation optical amplifier systems integrated with high-channel-count wavelength-division multiplexing, or WDM, technology arm network planners seeking additional fiber-optic network capacity with a cost-effective option. Compared to future time-division multiplexing, or TDM, systems, such as forthcoming OC-192 versions at 10 gigabits per second, combined optical amplifier/WDM systems save expenses on new and existing long-distance fiber-optic networks of more than 50 kilometers and short-range links of greater than 5 km. More importantly, they are compatible with both the large installed base of singlemode fiber and the emerging high-bit-rate systems.

The explosion in demand for telecommunications services has led to a parallel increase in network capacity requirements. Increasing capacity can be achieved by deploying additional fiber cable, which may be time- and cost-prohibitive; increasing the transmission system bit rate, which is limited by delays in the introduction of new OC-192 systems; or wavelength-division multiplexing more channels over existing fiber routes. Given these choices, network planners face the daunting task of selecting the most cost-effective technology that also permits capacity growth into the future.

Fortunately, turnkey optical amplifier systems operating in the 1550-nanometer region are now commercially available. First-generation systems consisting of stand-alone booster amplifiers, line amplifiers and preamplifiers served to overcome fiber loss through signal gain and were primarily used in long-distance terrestrial and undersea routes.

However, second-generation optical amplifier systems combined with integrated WDM technology provide immediate solutions to network congestion problems. By amplifying and multiplexing several channels over existing fiber-optic cables, these latest systems offer a cost-effective means for increasing network capacity in four areas:

Fiber capacity-exhausted long-distance routes

New long-distance routes

Fiber capacity-exhausted short-range links (5 to 60 km)

Routes requiring incremental capacity upgrades in the future.

Optical amplifier/WDM systems also offer compatibility with systems at the OC-48 rate of 2.5-Gbit/second operation over existing fiber plant. Effective maximization of available fiber plant capacity is important in today`s markets, where a temporary worldwide fiber shortage places demands on network planners considering new fiber deployment (see Lightwave, July 1995, page 1).

Technology overview

Optical amplifier systems used in telecommunications networks apply erbium-doped fiber amplifier technology to overcome the optical signal power loss caused by fiber attenuation. Connected inline within a fiber-optic network, these systems use a pump laser to energize a length of erbium-doped fiber, which then acts as a gain medium to amplify input signals from 1530 to 1560 nm. Amplification thus extends the distance over which the signal can propagate along the fiber while meeting minimum input power levels at the receiver.

Commercial optical fiber amplifier systems offer several advantages over older optical-electrical-optical regenerator technology. Perhaps the most important for increasing network capacity is the broad spectral amplification range. This wide range serves as the enabling technology for developing cost-effective dense multi-wavelength systems operating in the 1550-nm region.

Wavelength-division multiplexing consists of transmitting several optical channels over the same fiber. By selecting different wavelengths for each channel, several signals can simultaneously be spatially overlapped and propagated. Appropriate passive fiber-optic multiplexing and demultiplexing components combine and subsequently extract the individual channels at each end of the fiber span.

Until recently, two-channel wavelength-division multiplexing was used with existing line terminal equipment to combine either 1310- and 1550-nm or 1533- and 1557-nm systems over a common fiber, effectively doubling the network capacity over existing fiber plant. However, in the former system, the capabilities of the 1550-nm system are under-engineered because fiber loss at 1550 nm is 30% to 40% less than that at 1310 nm. In addition, total capacity is limited to two channels for both systems. Moreover, signal amplification in conventional regenerator-based systems must be performed separately on each channel, thus eliminating the potential cost savings from using shared amplification equipment. Although WDM counts of greater than two channels are theoretically possible, such systems without optical fiber amplifiers would be cost-prohibitive.

By contrast, dense wavelength-division multiplexing combines several wavelength channels (typically four or more) with narrow spectral spacings of 1 to 7 nm within the 1550-nm wavelength region. The complementary multi-wavelength amplification capabilities of erbium-doped optical amplifiers permit amplifier system costs to be amortized over the total number of channels and eliminate the need for multiple WDM splitters at each amplification site for connection to the fiber plant. The gain spectrum of erbium matches the low-loss wavelength of the third telecommunications window (1550 nm) of optical fibers, thereby maximizing achievable transmission distances.

Second-generation optical amplifier/WDM systems include bidirectional line amplifiers and multichannel systems for either long- or short-range applications.

Bidirectional optical line amplifiers simultaneously amplify two counter-propagating signals traveling over the same fiber, thereby doubling the network capacity over the existing fiber plant. Typically operating at wavelengths of 1533 and 1557 nm, these systems have been adopted by long-distance carriers in linearly protected point-to-point networks. Immediate cost reductions are achieved by sharing the investment of the amplifiers over both channels, while future capacity needs are ensured through compatibility with OC-192 systems (see Lightwave, May 1995, page 1).

For high-capacity applications, multi-wavelength systems can be used to increase total capacity to 10 Gbits/sec or more over a single fiber pair, the highest speed commercially available today. A typical four-channel WDM system converts four 1310- or 1550-nm signals at 2.488 Gbits/sec to the specific system operating wavelengths, which are then multiplexed over a common fiber. These systems are especially cost-effective in two- and four-fiber synchronous optical network, or Sonet, rings or linear spans where fiber plant capacity is exhausted.

Depending on the link length, a variety of booster amplifiers, line amplifiers and preamplifiers is used to amplify the combined signals traveling along the fiber. Demultiplexing components at the receiver end separate each wavelength channel for the line terminal receivers. These systems furnish equal signal gain at each wavelength, filter out accumulated noise and maintain acceptable channel crosstalk isolation. In addition, externally modulated laser transmitters are used to reduce laser chirp and permit operation over singlemode fiber links to 550 km (350 miles) without requiring external dispersion compensation. This capability permits multi-wavelength operation over the large installed base of non-zero dispersion-shifted fiber.

Because of non-linear effects in optical fibers, the use of dense WDM technology is not possible on all types of fiber, however. The main requirement is the need to move the zero-dispersion point of the fiber outside the operating range of the WDM system. Fortunately, the installed base of non-zero dispersion-shifted fiber is an excellent transmission medium compatible with WDM systems. But future fiber plant deployments should consider the use of non-zero dispersion-shifted fiber if compatibility with dense WDM systems at bit rates greater than OC-48 is required.

Cost benefits

High-capacity WDM systems are generally installed in long-distance, interexchange carrier and corporate networks. Used in two- and four-fiber ring or linear networks, multichannel WDM systems offer cost benefits to network designers seeking to expand capacity on existing routes or to deploy new routes. Moreover, significant cost savings are also achieved on fiber capacity-constrained short-range links.

In fiber capacity-exhausted long-distance routes, deployment of an optical amplifier/WDM system should be compared to laying or leasing a new cable route with additional line terminal equipment or upgrading to higher-bit-rate time-division multiplexing systems, such as OC-192.

Cost-comparison studies reveal that increasing network capacity through implementation of wavelength-division multiplexing or deployment of OC-192 for links greater than 50 km (30 miles) is expensive. For OC-192 technology, however, the expected mid-1996 limited availability of these systems must be traded off against immediate capacity and service provisioning requirements. Furthermore, the low dispersion tolerance of OC-192 systems essentially limits their use to dispersion-shifted fiber networks or requires the addition of dispersion-compensation devices. For existing networks, the reduced gain budgets of OC-192 systems may prove incompatible with the regenerator hut infrastructure currently in place. These issues potentially limit the suitability of higher-bit-rate-system upgrades as a viable means of increasing capacity over existing non-zero dispersion-shifted fiber networks.

On new routes, optical amplifier systems are more cost-effective than conventional regenerator-based systems. The increased span gain of optical amplifiers reduces the total number of controlled environment vaults required to house signal amplification equipment. In addition, the reduced power consumption of optical amplifiers permits the use of vaults with reduced battery power. Optical amplifiers typically cost 50% less than pre-fabricated regenerator huts, further compounding cost savings.

In fact, studies indicate that total equipment and infrastructure costs per DS-3 line at 44.74 Mbits/sec using optical amplifier/WDM systems in a 500-km network are 62% less than those of regenerator-based systems. Furthermore, WDM reduces fiber-count requirements, which translates into more savings through the use of lower-fiber-count cables or by leasing excess capacity to other customers. These costs compare favorably to expected OC-192 costs, which are estimated to be 2.5 to 3 times those of OC-48 systems, when expected deployment commences in mid-1996.

In a major shift from classical network design, integrated optical amplifier/ WDM systems also prove to be a cost-effective capacity upgrade path for short-haul applications. Similar to multichannel long-range systems, short-range optical amplifier versions do not require line amplifiers or preamplifiers, and use a multi-wavelength booster amplifier to compensate for WDM component insertion losses to maximize the span gain budget.

Typically designed for operation to 60 km (35 miles), optical amplifier/WDM systems are rarely dispersion-limited and can use low-cost directly modulated transmitters. Application examples include intra-city wide area networks, high-volume business services, connections to long-distance carrier points of presence, and in the near future, the trunking and distribution of residential interactive broadband services and digital television.

In many urban or suburban areas where fiber networks were first deployed, total capacity over existing low-fiber-count cable appears inadequate for future service demands. In this case, the cost of a cable overbuild should be compared to increasing network capacity by using wavelength-division multiplexing. This approach seems cost-effective in metropolitan areas, where the potential lack of available duct space for new fiber pulls could require expensive and disruptive digging and laying of new conduits. The time required for such construction projects would also impact service provisioning plans in high-growth areas.

Because fiber-optic cable deployment costs vary depending upon location and available infrastructure, three network types for the following situations have been investigated--metropolitan areas with available duct space, suburban areas where armored fiber is trench buried and metropolitan areas requiring new conduits.

Studies show that deploying a WDM system achieves cost parity with deploying a new fiber-optic cable network for links greater than 30 to 40 km. In the case of conduit installation in a metropolitan area, the deployment of a WDM system is considerably less expensive for links longer than 5 km. These results expose the myth that optical amplifier/WDM technologies are reserved for long-distance applications.

Recent product introductions and continued research and development of optical amplifier/WDM systems promise to expand the flexibility and capacity of fiber-optic networks. One such product is an optical drop/insert unit for use in WDM-based networks. Used in a multi-wavelength system, this unit provides a capability unavailable in OC-192 systems by permitting one wavelength channel to be selectively tapped out and subsequently re-inserted from an intermediate network station equipped with an add/drop multiplexer. The unit serves as an exit/entrance access point on a multi-wavelength network, where one channel at a given wavelength carries local traffic, and other channels carry "express" traffic.

By providing multiple and repeatable access points to a long-distance fiber-optic network, the drop/insert unit allows all-optical routing of cross-country and local traffic and furnishes cost savings compared to having to demultiplex all channels at each add/drop multiplexer site. In addition, it permits existing long-distance networks to bypass low traffic density areas while allowing future network architecture reconfiguration as customer demands change. u

Serge Melle and Christoph P. Pfistner are account development managers, and Fahri Diner is product line manager at Pirelli Telecom Systems Group, Lexington, SC.