Abstract
We perform an extensive numerical analysis of Raman-Assisted Fibre Optical Parametric Amplifiers (RA-FOPA) in the context of WDM QPSK signal amplification. A detailed comparison of the conventional FOPA and RA-FOPA is reported and the important advantages offered by the Raman pumping are clarified. We assess the impact of pump power ratios, channel count, and highly nonlinear fibre (HNLF) length on crosstalk levels at different amplifier gains. We show that for a fixed 200 m HNLF length, maximum crosstalk can be reduced by up to 7 dB when amplifying 10x58Gb/s QPSK signals at 20 dB net-gain using a Raman pump of 37 dBm and parametric pump of 28.5 dBm in comparison to a standard single-pump FOPA using 33.4 dBm pump power. It is shown that a significant reduction in four-wave mixing crosstalk is also obtained by reducing the highly nonlinear fibre interaction length. The trend is shown to be generally valid for different net-gain conditions and channel grid size. Crosstalk levels are additionally shown to strongly depend on the Raman/parametric pump power ratio, with a reduction in crosstalk seen for increased Raman pump power contribution.
Highlights
The need for higher capacity optical communications systems appears evident as worldwide demand for data continues to surge with ever more data-hungry multimedia applications and E-services appearing [1]
We show that the RamanAssisted Fibre Optic Parametric Amplifier (RA-FOPA) offers significantly reduced four-wave mixing (FWM) crosstalk compared with the conventional FOPA over all conditions whilst providing gain levels which would not be achievable using purely discrete Raman gain without encountering extreme problems of signal corruption and noise from double Rayleigh backscattering [5]
We extend our previous work [3] by numerically characterising a RA-FOPA using the key WDM metrics of signal gain and FWM crosstalk power at both maximum and minimum wavelengths of the amplified spectrum
Summary
The need for higher capacity optical communications systems appears evident as worldwide demand for data continues to surge with ever more data-hungry multimedia applications and E-services appearing [1]. FOPA performance has been shown to strongly depend on the quality of parametric pump [9] and to suffer from the generation of unwanted FWM crosstalk components when amplifying WDM signals [10]. This remains a major limitation to the prospect of using FOPAs in telecoms applications. The Raman pump provides direct signal amplification through Raman scattering and indirect signal gain through amplification of the parametric pump This approach can potentially widen the amplification bandwidth, increase overall gain and improve performance in comparison with the conventional FOPA. The Raman pump power would most likely just be increased to a suitable level before double Rayleigh effects start to dominate for that fibre
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