Abstract

Fiber Raman amplifiers (FRAs) can provide broadband and low-noise-figure amplification, and the gain can be achieved at any wavelength. It has been proved that FRA is a useful technique to extend the span lengths and capacity of fiber-optic transmission systems. In particular, lumped fiber Raman amplifiers (LRAs), based on dispersion compensating fibers, have been considered in long-haul WDM transmission systems. However, noise can limit the performance of LRAs having a gain of more than 15 dB. Noise degrades the optical signal-to-noise ratio (OSNR) resulting in receiver sensitivity penalty and thus the amplifier gain is limited to some extent for a single LRA. Highly nonlinear fiber (HNLF) is a fiber with high nonlinearity to generate Raman gain efficiently and can be used as a gain medium for lumped Raman amplifiers. In this thesis, we will investigate Raman gain and noise characteristics of LRAs using HNLFs as a gain medium. It is shown that both the signal and the amplified spontaneous emission induced multiple-path interferences are suppressed in LRAs with HNLF, thus for the same Raman gain a better noise performance can be achieved independently of co-, counter- and bi- directional pumping methods, compared to LRAs with DCF gain medium. Moreover, the effect of Rayleigh scattering coefficient on LRA's noise performance improvement is also investigated. To achieve an accurate modeling, a theoretical model, which includes effects of multiple-path interference (MPI), anti-Stokes, and Rayleigh scattering, is used, and a new Raman gain coefficient scaling method is also employed in this modeling.