Raman Amplifiers Extend Transmission Distances at High Speeds

Using stimulated Raman scattering to provide distributed gain over a span of optical fiber, Lucent researchers transmitted data at 40 Gb/s over 100 km spans without repeaters.

By: Kristin Lewotsky

As bandwidth demand spirals higher and higher, optical researchers are searching for to achieve higher data rates at increased channel density. Using a prototype hybrid system that combined erbium-doped fiber amplifiers (EDFAs) with distributed Raman amplification, researchers from Lucent Technologies' Bell Labs (Murray Hill, NJ) demonstrated 1.6 Tb/s aggregate data rate—40 Gb/s over 40 spectral channels. The innovative amplification scheme allowed the group to expand the distance between repeaters in the system, send data over four 100-km-long spans rather than the conventional 80-km spans.

Raman amplification
The key to the success of the project was Raman amplification. Based on stimulated Raman scattering, the technique involves a frequency-conversion process in which light traveling down a fiber interacts with the vibrating molecules in the silica material. The interaction triggers a spectral shift that transfers the energy from the shorter-wavelength pump beam to the longer-wavelength signal beam. The Raman effect is wavelength dependent—pumping at a higher frequencies yields gain at relatively higher frequencies as well.

Raman amplification takes place along the length of the transmission fiber itself, rather than in a discrete amplifier/repeater component spliced into the network—in other words, the transmitting fiber itself becomes the gain medium. Unlike EDFAs, the gain process does not require doped fiber, but rather can take place in various conventional types. "You have to launch the pump wavelength at an appropriate power, but you do not have to do anything in particular to the fiber," explains Charlie Roxlo, director of advanced optical networking at Lucent Technologies (Murray Hill, NJ). "The fiber is the amplification."

Nevertheless, the strength of the effect does depend to some degree on the details of the fiber design. In the case of the Lucent experiment, researchers used True Wave reduced-slope fiber, which according to Roxlo appeared to be particularly well-suited to the project.

Longer spans, higher data rates
Theoretically, Raman amplification could eliminate the need for EDFAs, but such an approach ignores the primary benefits of the hybrid system. "An erbium amplifier is still a very low noise, practical, cost-effective amplifier," says Roxlo. "What the Raman technique allows us to do is up the data rate through those amps and up the density of channels so that we can get much higher capacity."

In conventional systems consisting of transmission spans interspersed with repeaters, the signal-to-noise ratio (SNR) drops along the length of the span, typically by 25 dB. As the data rate increases, so does the per-span loss. "When you go from 10 Gb/s in a channel to 40 Gb/s," says Roxlo, "you need four times the power. By the time you get to the end of an 80 km span, the signal has already disappeared into the noise." The advantage to Raman amplification is that it does not merely add more gain at the repeater site, but instead increases signal power across the length of the span, providing sufficient gain to allow the signal to reach the repeater before fading into the noise.

Increased channel spacing
More effective amplification allows system designers to decrease launch power, which has important implications for dense wavelength division multiplexing (DWDM). The level of four-wave mixing between channels in a DWDM system increases as the square of the channel density, but also as the square of the signal launch power. The launch power dependency limits the ability of system designers to achieve longer span lengths or accommodate higher data rates by increasing launch power.

Raman amplification eliminates the need for higher launch powers; quite the opposite, in fact. "We can use the extra gain from the Raman effect to lower launch power," explains Roxlo. "If you double the channel density and half the launch power, you can still transmit successfully." Wavelength division multiplexing is a modular technique—double the channel density yields double the data transmission.

Demonstration
In the Lucent demonstration, the group began with a 40 Gb/s signal around 1550 nm, then pumped the transmission fiber from the opposite direction with a pump beam around 1450 nm. The Raman effect thus began to contribute gain at the point in the transmission span at which the SNR began to drop.

Successful implementation of Raman amplification is not as easy as pumping a fiber with a short-wavelength, counter-propagating high-power beam, Roxlo notes. "There are a lot of control aspects and system engineering aspects that need to be taken into account," he says, declining to part with details. Although it does not appear feasible to create a Raman amplifier product per se, it is likely that it will appear in systems-level solutions at some point in the future.

For now, continuing research will focus on doubling the capacity again by using the longer-wavelength L-band range.