The impact of optical modulation schemes on fibre non-linearities in D-WDM systems

This thesis investigates alternative optical modulation schemes and their impact on the non-linear effects in a dense WDM optical network. In detail, two alternative modulation schemes, duobinary and dicode modulation are extensively studied in the context of optical transmission and their application in WDM optical networks. Two-level and three-level duobinary modulation schemes are applied to a simple optical system in order to explore the level of the reduction of Stimulated Brillouin Scattering (SBS) effects relative to binary transmission. The two duobinary schemes are compared to each other and to binary transmission and the techniques are assessed in terms of system penalties. Two-level duobinary suppresses SBS effects, giving an eye-opening improvement of 0.25 dB to 1.2 dB relative to binary transmission over a range of input power levels. An experimental study follows, where duobinary modulation is applied in a dense WDM system. The modulation scheme successfully suppresses the Four Wave Mixing products by a maximum of 3 dB. The suppression is maintained over a range of channel spacings. An investigation of the impact of fibre dispersion on Four Wave Mixing products under binary, duobinary and dicode modulation in a three channel dense WDM system is then performed through simulation. The interchannel spacing is varied from a maximum of 100 GHz to a minimum of 37.5 GHz, and the optical power applied to the fibre is also varied over a typical range of values. Results guide the choice of modulation schemes over a range of operating conditions. A basic study on the dispersion tolerance of dicode modulation, relative to normal binary transmission is also performed. Dicode modulation does not offer any dispersion compensation relative to standard binary transmission. A strengths, weaknesses, opportunities and threats analysis identifies the merits and drawbacks of using duobinary and dicode modulation in dense WDM systems. The thesis comes to a conclusion by summarising the main results and proposing areas where further work maybe fruitful.