Using a combination of fiber types and distributed amplification, Jake Bromage and others at Lucent Technologies (Holmdel, NJ) demonstrated transmission of 40 channels of NRZ data, each at 10 Gbit/s, over 600 km in the S-band from 1488 through 1518 nm.1 Such a system could open up use of the S-band, in addition to C- and L-bands, thus increasing the bandwidth of the fiber.
The S-band (1480 to 1530 nm) lies within the low-loss window of silica fiber. It has not been developed as much as the C-band, in which WDM systems were first developed; or the L-band, which can also be pumped by erbium-doped fiber amplifiers (EDFAs). But to use this band for DWDM, researchers must first find a way around problems with nonlinear effects in silica fiber and must develop an amplifier.
The recirculating loop experiment uses a prototype fiber and a mix of distributed and discrete Raman amplifiers. The nonzero dispersion-shifted fiber typically used for WDM systems will not work for WDM in the S-band because the dispersion is too low at these wavelengths. If the channels do not disperse enough, the optical energy is high enough to cause nonlinear effects, including four-wave mixing.
The researchers used a prototype fiber being developed by Lucent that has the zero-dispersion wavelength shifted to below 1400 nm (rather than the typical 1450 nm). The TrueWave fiber has enough dispersion to allow DWDM transmission without incurring penalties from four-wave mixing.
EDFAs don't provide gain in the S-band. Developers use one of two amplification methods: thulium-doped fiber amplifiers (TDFAs) or Raman amplifiers. Thulium-doped fiber provides gain from 1450 to 1480 nm, but this range can be shifted to longer wavelengths to cover the S-band. While not as efficient as EDFAs thus far, TDFAs have been reported with efficiencies of around 40%, which is far better than Raman amplifiers, which have efficiencies of around 10%. (A number of papers at OFC discussed TDFAs.)
Bromage's group chose to work with Raman amplifiers. One advantage of these devices is that they can provide gain at any wavelength, depending on the pump laser wavelength. (The gain in doped-fiber amplifiers depends on materials properties.) Because the amplification does not require a special fiber, "you can have amplification through the length of the transmission fiber," explains Bromage.
Raman amplifiers offer distributed amplification that permits system performance not possible using discrete amplifiers alone. In the experiment the group added some discrete amplification in the dispersion compensation fiber to simultaneously compensate for span dispersion and residual loss.
In an all-Raman recirculating loop experiment, the researchers transmitted 40 x 10 Gbit/s channels over 6 x 100 km (see figure). The system achieved a bit-rate-distance product of 240 (Tbit/s)/km, which is an order of magnitude higher than previously published results for S-band transmission.
Bromage is careful not to take credit for the design of either the fiber or amplifiers. "We combined several ingredients previously reported," he said, noting their results are from the experimental system the group built. Bromage adds of the group's work on Raman amplifiers, "We've built on at least 10 years of work."
For more information contact Jake Bromage at email@example.com.
- J. Bromage et al., OFC 2001 Postdeadline paper 4.