Raman amplifier with improved system performance

Summary form only given. We describe the predominant sources of noise for various amplifier configurations. In addition, we present the results of a successful 2.5 Gbit/s systems test of a two-stage Raman amplifier with net small signal gains of 29 dB and output powers exceeding + 15 dBm. Although Raman amplifiers do not suffer from increased noise figure due to incomplete population inversion, the passive loss of the amplifier does contribute to the noise figure through increased signal-spontaneous beating. Nonetheless, we have constructed a Raman amplifier with a noise figure contribution from signal-spontaneous beating that is less than 5 dB. A second source of noise in Raman amplifiers is the coupling of intensity fluctuations in the pump light to the signal. The fundamental cause of this noise is the lack of a long upper-state lifetime to buffer the Raman gain from fluctuations in the pump intensity. We present results demonstrating that the transit time through the amplifier can be used to average gain fluctuations if a strictly counterpropagating-pump geometry is employed. Lastly, double Rayleigh backscattering also contributes to the noise figure of the amplifier but can be reduced by decreasing fiber lengths and by constructing multistage amplifiers. Based on the above considerations, the two stage Raman amplifier with a counterpropagating pump was constructed. The requisite pump light was generated by a cascaded Raman laser operating at 1240 nm that was pumped by a cladding-pumped neodymium laser operating at 1060 nm. A 2.5 Gbit/s bit-error-rate test of this amplifier was performed.