Abstract

Multi-wavelength pump lasers have been used to demonstrate broadband Raman amplifiers with bandwidth extending beyond 90 nm.¹ These amplifiers exhibit significant Raman interactions between the pump lasers such that shorter pump wavelengths transfer energy to longer pump wavelengths. Raman pumps may be designed, concentrating most lasers at shorter wavelengths in order to maximise the gain whilst maintaining a flat gain spectrum. Whilst the gain spectrum is flattened, the noise performance is not. Shorter wavelength channels perform worse due to increased spontaneous emission as signals approach pump wavelengths, greater fibre attenuation for both shorter wavelength pumps and signals, stimulated Raman scattering between signal channels² and importantly, energy transfer between pumps.³ This energy transfer is analogous to second order Raman pumping⁴ and allows longer wavelength pumps to penetrate deeper into the transmission fibre, whilst shorter wavelength pumps provide their gain near the end. This results in a tilt in the effective noise figure. In addition, when using multiple pump wavelengths which lie close to the transmission fibre’s zero dispersion wavelength, four-wave mixing products are produced that may fall within the signal bandwidth.⁵ We demonstrate a novel, counter-pumped Raman pump unit where pump wavelengths may be modulated and temporally separated within the transmission fibre. This allows pump-to-pump Raman interactions to be minimised and four-wave mixing of pump light to be eliminated.

© 2002 Optical Society of America