Optical amplifier systems point to high-capacity, all-optical network applications

Optical amplifier systems point to high-capacity, all-optical network applications

Advances in optical amplifier design and applications promise to broaden their use at data rates to 80 Gbits/sec in all-optical networks via dense WDM and optical channel routing

SERGE MELLE

PIRELLI CABLE CORP.

Optical amplifiers are expected to continue as key components in the development of all-optical networks. From first use as discrete amplification elements to key components in wavelength-division multiplexing, or WDM, networks, future optical amplifier systems are anticipated to continue to be integrated with other optical technologies to provide enhanced system modularity and network functionality. Key ongoing developments include dispersion compensation, wavelength routing and increases in channel count.

The continuing transition of telecommunications networks from electrical to optical systems is also rapidly accelerating the deployment of optical amplifier systems as a substitute for electrical regenerators. Optical system technology should continue to displace existing techniques as all-optical networks evolve. Such networks should contain the bandwidth and flexibility required to meet the expanding and changing network requirements needed to provision new communications services past the year 2001.

For example, as an enabling technology for dense WDM systems, optical amplifiers are currently being used to permit network capacity growth to 10 gigabits per second today, and are expected to expand network capacity to 40 Gbits/sec and beyond.

Combined with continuing developments in optical system functionality, such bandwidth upgrades provided by migration to all-optical transport networks are expected to cost-effectively support increasing network provisioning requirements for new service offerings.

Since their first deployment in digital fiber-optic networks during the early 1990s, optical amplifier systems have evolved considerably beyond their initial deployment as booster amplifiers. Optical amplifiers have found their way into line amplifier and preamplifier applications, further increasing transmission distances without the need for optical-electrical-optical regeneration. In addition, specially designed bidirectional line amplifiers operating over narrowband WDM networks serve to double network capacity over existing fiber plant. Wavelength multiplexing provides network capacity upgrades to 10 Gbits/sec, and with the planned introduction of 8- and 16-channel systems, will provide upgrades to 40 Gbits/sec and beyond in the near future.

The key component behind the deployment of viable optical amplifier systems is the erbium-doped fiber amplifier, or EDFA. Operating in the third low-loss window of optical fiber, centered around 1550 nanometers, EDFAs serve as building blocks to construct optical amplifier systems. The extremely broad 25,000-gigahert¥bandwidth of EDFAs makes them suitable for multiwavelength amplification of many spectral channels distributed across the entire erbium gain bandwidth.

Range of applications

Optical amplifiers are presently deployed in a variety of applications, including terrestrial point-to-point, linear, 2- and 4-fiber synchronous optical network, or Sonet, ring networks, point-to-point submarine coastal links (often called "festoons;" see Lightwave, November 1995, page 9), and in long-distance submarine links such as the 2100-kilometer Americas-1 and Columbus-2 systems in the Caribbean region.

Optical amplifier systems optimized for specific systems have been used either to extend network span distances or to increase network capacity. Typical system applications include:

Single-channel systems that increase the range of interoffice links and minimize or eliminate the need for intermediate signal amplification sites

Bidirectional amplifiers that are used in 1533- and 1557-nm narrowband WDM networks to minimize fiber usage by transmitting a complete data channel over one fiber

Dense WDM systems that increase network capacity four- or eight-fold over the existing fiber plant of nondispersion-shifted fiber.

Single-channel optical amplifier systems provide amplification for a single optical signal transmitted over a fiber pair. Depending on amplifier design and placement within a fiber-optic network, such optical amplifiers are typically used as booster, line or preamplifiers.

Booster amplifiers (also referred to as power amplifiers) are used directly after fiber-optic transmission system, or FOTS, transmitters to boost the optical signal power to higher levels (typically, 10 to 17 decibel relative to milliwatt), thus extending the loss-limited range of a typical fiber-optic system by an additional 65 to 100 km. Similarly, optical preamplifiers are used immediately preceding FOTS receivers to boost the power of faint optical signals and to extend the minimum sensitivity of receivers (typically to -37 dBm for a 2.488-Gbit/sec OC-48 system). The use of high-power booster amplifiers, either alone or combined with preamplifiers, can serve to eliminate the need for intermediate amplification sites for interoffice links to 230 km.

Single-channel line amplifiers replace conventional optical-electrical-optical regenerators (often referred to as 3R regens) by boosting the optical signal level at midspan locations along the fiber-optic route. With appropriate system design, several line amplifiers can be cascaded and combined with dispersion-compensating modules, booster and preamplifiers to extend the total loss-limited range of a fiber-optic system to 600 km or more. In addition, the greater section power budgets of optical amplifier systems increase amplifier site spacing an additional 125% compared to regens operating at 1310 nm.

The use of single-channel optical amplifier systems provides several advantages over the use of 3R regenerators, including:

Fewer amplifiers and amplifier shelter huts

Increased interoffice link distances

Bit-rate insensitivity for easy upgrades

Increased reliability and reduced maintenance.

Bidirectional line amplifiers extend the capability of single-channel systems by permitting the transmission of a complete OC-48 or 10-Gbit/sec OC-192 channel (transmit and receive signals) over a single fiber. Such systems are especially useful in 1:n linearly protected fiber networks as they permit minimum usage of available fibers for a given total network capacity (where "n" is the number of working channels for one protection channel).

For example, consider the mode of operation of a bidirectional line amplifier system. One channel is transmitted at 1533 nm, amplified by an EDFA, and then continues to the other end of the fiber network. A second signal at 1557 nm travels in the opposite direction (the "receive" signal). It is first routed by appropriate WDM couplers to the input of the EDFA, amplified, and then rerouted to continue along its original direction.

The use of bidirectional line amplifiers is cost-effective because of:

Shared amplifier cost in each direction

Low amplifier cost

Increased section power budgets

Improved reliability

Compatible with OC-48 and OC-192 transmission rates.

Dense WDM systems provide the capability for increasing network capacity beyond the bandwidths currently available with time-division multiplexing systems. By combining four or more optical channels at different wavelengths into one optical fiber, dense WDM systems immediately provide a cost-effective upgrade path for increasing network bandwidth to a capacity equivalent to an OC-192 rate of 10 Gbits/sec (see Lightwave, December 1995, page 42). Dense WDM systems are compatible for use in linear networks or in 2- and 4-fiber Sonet rings. The planned introduction of 8-channel WDM systems in 1996 is expected to increase network capacity to 20 Gbits/sec.

The cost-effective deployment of such systems is made possible through the use of optical amplifiers, whose broad amplification spectrum in the 1550-nm window permits them to simultaneously amplify several channels within the spectral window from 1530 to 1560 nm. Specially designed optical amplifiers are able to flatten the EDFA response curve to permit effective 8-channel WDM operation.

Some key advantages of dense WDM systems are:

Incremental capacity increase

Compatible with existing fiber base

Usage with existing OC-48 technology

Amplifiers compatible with higher bit rates

Larger amplifier spacing versus OC-192

Amplifier costs shared among many channels

Increased reliability.

Dispersion compensation built into optical line amplifiers is expected to permit the transmission of multiple OC-192 channels on standard, nondispersion-shifted fiber, WDM networks without the loss penalties associated with existing dispersion-compensating fiber. Built-in dispersion compensation is seen as providing an enabling technology for the optical multiplexing of four, eight or more OC-192 channels over one fiber pair. It should permit carriers to expand network bandwidth to 80 Gbits/sec or more over existing fiber plant.

Wavelength routing of optical channels at line amplifier sites should allow access to all optical channels carried in a WDM network. Passive wavelength routing capabilities available with dense WDM systems are expected to be expanded to include the ability to selectively add and drop any or all optical channels. Using such wavelength routers to add or drop traffic should enhance the ability to dynamically reconfigure network add/drop sites and topology, and lead to increased responsiveness in meeting changing customer demands.

Channel count increases in dense WDM systems planned for 1996 are projected to increase capacity by optically multiplexing as many as eight OC-48 channels on one fiber pair, thus providing a low-cost network upgrade path to 20 Gbits/sec. The costs for an 8-channel WDM system are expected to be 50% of the cost of deploying dual OC-192 systems when growing capacity to 20 Gbits/sec on a long-distance network.

Growth beyond 20 Gbits/sec can be accommodated in several ways using a combination of WDM and time-division multiplexing upgrades. In one scenario, additional OC-48 channels are added incrementally, leading to the deployment of 16- and eventually 32-channel WDM systems. In another scenario, the WDM channel bit rate is upgraded so that OC-48 channels are swapped out to OC-192 channels when additional capacity is required. In addition, wavelength channel count upgrades of bidirectional line amplifiers planned for introduction in 1996 are expected to permit the transmission of two OC-48 or OC-192 channels over a single fiber, for a total effective capacity of 20 Gbits/sec. u

Serge Melle is account development manager at Pirelli Telecom Systems Group, Lexington, SC.

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