OPTICAL NOISE PERFORMANCE IN SMF-REACH AND SMF-REDUCED SLOPE FOR RAMAN FIBRE AMPLIFICATION USING DIFFERENT PUMPING TECHNIQUES
Signal amplification in optical fibres during transmission has become one of the techniques in improving capacity and reach in telecommunication networks. Fibre Raman amplifiers (FRAs) are currently being adopted in many long-haul signal transmission systems and passive optical networks (PONs), for metropolitan applications due to their ability to offer longer amplification spans and wider bandwidth. The fact that signal amplification is distributed in FRAs is the major advantage of their application. In this case the signal is amplified along the fibre during propagation, a fact that drastically reduces the system nonlinearities. Although FRAs are basically low noise amplifier, they are still subjected to several noise sources which limit the amplifier performance. In this work, two pumping techniques namely co-pumping and counter pumping were used both in the theoretical and experimental analysis of noise in FRAs. A low signal power of -10 dBm was used while pump power was either fixed or varied depending on a particular analysis. Two types of fibres, Single Mode Reach Fibre (SMF-Reach) and Single Mode Fibre Reduced Slope (SMF-RS) each of length 25 km and 50 km were used. It was found that the longer the fibre length, the higher was the on- off gain irrespective of the pumping technique used. The Co- pumping configuration provided a higher Raman onoff gain than counter pumping scheme. An on-off gain of 5.7 dB and 4.5 dB was achieved experimentally for co- and counter pumping schemes, respectively, for the 25 km SMF-Reach fibre. For a similar length of SMF-RS fibre, an on-off gain of 4.8 dB and 3.9 dB was achieved for co- and counter pumping schemes, respectively, for the 25 km fibre. For a 50 km fibre, an on-off gain of 6.6 dB and 5.1 dB was obtained for the two pump configurations respectively using SMF-Reach fibre. For a similar length of SMFRS fibre, an on-off gain of 5.3 dB and 4.3 dB was achieved for co- and counter pumping schemes, respectively. The Optical Signal to Noise Ratio (OSNR) also increased with an increase in on-off gain. An OSNR of 12.8 dB and 12.3 dB was achieved experimentally for co- and counter pumping schemes, respectively, for 25 km SMF-Reach. For 50 km fibre, an OSNR of 10.0 dB and 9.3 dB was recorded for the two pumping schemes respectively. OSNR was also observed to vary inversely with fibre length. The pump reflection power was noticed to vary inversely with gain and directly with fibre length irrespective of the pumping scheme applied. Co- pumping scheme had a better OSNR performance in both fibre types. Similarly, counter pumping scheme had a higher Noise Figure (NF) and pump reflection power. A NF of -2.2 dB and -1.9 dB was achieved experimentally for co- and counter pumping schemes, respectively, for the 25 km SMFReach fibre. For a 50 km fibre, a NF of -1.8 dB and -0.7 dB was obtained for the two pump configurations respectively using SMF-Reach fibre. NF was also observed to vary directly with fibre length. The noise analysis implies that SMF-Reach fibre is a suitable candidate in signal transmission due to its higher OSNR and better Raman amplification. The results also clearly indicate that, the co-pumping technique would highly be recommended due to its improved noise performance and would be preferred most in optical communication networks. The findings of this study are significant and give insight toward the optimization of fibre Raman amplifiers in long haul signal transmission.