Raman Amplification in Storage Area Networking
by Per B. Hansen - Director, Business Development, Optical Solutions ADVA Optical Networking - Friday, 22 September 2006.
Similarly, there are important innovations taking place in the area of optical-network amplification. Again, the technological developments are simultaneously reducing enterprise costs and delivering enhanced service benefits.

Achieving Longer Amplifier Spacing

Optical networks support data transmission over thousands of kilometers, but amplification is necessary along the fiber path. Enterprises have typically deployed in-line Erbium-doped fiber amplifiers in their networks for this job. But locating Erbium-doped amplifiers along an optical link introduces security concerns; plus, there are CAPEX, EFI&T and OAM expenses incurred with each hut that must be located to house an amplifier.

Eliminating – or, at least, reducing the number of – amplifier sites needed to drive application traffic across an optical link would not only cut an enterprise’s costs, it also would greatly enhance the enterprise’s disaster-recovery and business-continuity capabilities (Figure 1). By enabling longer fiber spans, the enterprise would be able to locate primary and backup data centers farther apart and still support bandwidth-intensive, real-time services such as synchronous data mirroring. The survivability of the data mirror in the event of a business-threatening event would be increased.

Figure 1: Hybrid Raman/Erbium Amplification. Increasing span lengths allows network owners to minimize or even entirely eliminate in-line amplifiers and the facilities that house them.

The supportable distances have gradually expanded, delivering increasingly greater protection against more severe events.

A New Role for a Proven Technology

Service providers have relied on all-optical Raman amplification for years, especially in submarine systems. Today, the technology has matured to the point that enterprises, too, can cost-effectively employ Raman amplification. And the benefits are particularly compelling in the context of synchronous storage applications.

In Raman amplification, the fiber is used as the medium for amplification. An optical beam – or, “pump wave” – is launched at an appropriate wavelength into the fiber. The data signal propagating across that wavelength is amplified as the optical beam releases its energy. In this technique, amplification is distributed over the length of the fiber strand, as opposed to being limited to discrete amplifier sites in the case of Erbium-based amplification (Figure 2).

Figure 2: Raman amplification in transmission fiber.

Pump-induced Raman scattering provides amplification of the data-carrying signal in new and already-installed transmission fiber, boosting performance beyond that achievable with conventional Erbium amplifier technology.

With Raman amplification employed, synchronous storage applications of speeds up to 10Gbit/s can be supported across distances of 200 km. Because it relies on the optical fiber as a medium for amplification, Raman amplification does not require doping of the fiber. This means that Raman amplification can be performed on an enterprise’s or managed service provider’s already-installed base of single-mode fiber. And eliminating the amplifier sites necessary in Erbium-based amplification slashes costs, improves transmission characteristics, abstracts away security concerns and increases survivability of a data mirror by allowing longer optical links between storage data vaults.

Amplification also can be based on a combination of Raman scattering and in-line Erbium-doped amplifiers. High-performance, hybrid Raman/Erbium amplifiers are emerging that enable asynchronous storage applications to be supported over even longer distances. Fewer amplifier huts are needed than if Erbium-based amplification alone was used, and the enterprise’s CAPEX, EFI&T and OAM costs are cut accordingly.



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