Abstract
The benefits of distributed Raman amplification in telecommunications networks have been widely reported.1,2 The main limitations of this technique arise from the phenomena of single and double pass Rayleigh back-scattering of amplified spontaneous emission (ASE) and optical signals in the transmission fiber.3 These impairments grow super-linearly with increasing distributed gain, imposing limits on the maximum allowable distributed gain in a system. One can improve the equivalent noise figure (NF) associated with this maximum allowed gain by use of a second Raman pump with a photon energy two Stoke’s shift above the signal photon energy (so called second order Raman pumping). This technique was first proposed with the second order pump co-propagating with the signals, in order to provide Raman gain at the beginning of a fiber link without the noise transfer problems associated with first order co-propagating Raman pumps.4 Subsequently a second technique of second order Raman pumping was proposed whereby both the first and second order pumps counter-propagate with respect to the signal.5 There is a certain interaction length associated with the transfer of power from the second to first order pump which pushes the distributed gain further back into the fiber, thus improving the effective noise figure. Such a scheme is more appropriate than forward propagating second order pumping when span lengths are long, or when long distance unrepeatered transmission is sought. In addition, this configuration implements Raman amplification in the unsaturated gain regime, and as such will be more immune to transient effects.
© 2002 Optical Society of America
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