Abstract

Multi-wavelength Raman pumping can be used to generate flat Raman gain over an amplification band approachiwg some 100 nm in the 1.5 µm wavelength band,^1,2 to actively flatten the gain of Erbium-doped fiber amplifiers without the need for lossy gain equalizers in a hybrid amplifier configuration,³ or to compensate for the gain tilt due to Raman crosstalk among WDM channels.⁴ In conventional Raman systems, the Raman pump lasers operate continuously at fixed, prede termined wavelengths. Due to pump-to-pump Raman interactions, the shorter wavelength pumps transfer a significant amount of energy to the longer wavelength pumps, leading to much higher power demands on the short wavelength pumps than would be required without such interactions.⁵ Further, four-wave mixing between the pumps can produce strong unwanted lines in the signal band.⁶ A way to (i) avoid four-wave mixing, (ii) have no pump-to-pump Raman interactions, and (iii) become more flexible with respect to dynamic and adaptive wavelength and power reconfiguration of the individual Raman pumps was conceived by Mollenauer, Mamyshev, and Grant:⁷ Instead of having a number of single-frequency lasers, a single, but tunable laser is employed as a pump source in a backward-pumped Raman amplification scheme. The tunable laser is repetitively swept according to some wavelength pattern, with the time spent at a particular wavelength dictating the amount of Raman gain arising from that wavelength. If the pattern repetition rate is chosen high enough, the (counter-propagating) signal does not experience any significant temporal gain variations, but sees the composite Raman gain of all pump wavelengths making up the sweep pattern, in analogy to a multi-wavelength, continuous Raman system.

© 2002 Optical Society of America