Wavelength-division multiplexing is rapidly moving to 10-Gbit/s modulation per wavelength. Increasing the number of wavelengths, however, makes reduction of the signal-to-noise ratio (SNR) and equalization of the optical amplifier gain important. Using Raman amplification in long-haul transmission has been shown to improve SNR, but the technique requires high pump power. Raman amplifiers also exhibit low gain compression and can cause performance of the system to become unstable. Researchers at Alcatel Submarine Networks (Clichy, France) have found that combining a 980-nm-pumped erbium-doped fiber amplifier (EDFA) with a Raman amplifier gives both an improvement in SNR and the high gain compression and output power common to EDFAs.
In their experiment, the researchers looked at 32 wavelengths with an amplifier output power of +10 dBm. To test a hybrid amplifier system, researchers built a 501-km-long recirculating loop composed of optical amplifiers spaced 45 km apart with a 10.5-dB span loss (see figure). When turned on, the Raman stage provides 100 mW of pump power. With a 4-dB Raman gain, the researchers saw 1.3-dB improvement. When taking into account an improvement of 0.2 dB without Raman gain while increasing output power from +10 to +11 dBm, and considering a 0.4-dB insertion loss from the 1450-nm multiplexer, the net improvement with the hybrid amplifier was 0.7 dB. For more information, contact Olivier Gautheron at olivier. firstname.lastname@example.org.