Comparison between optical SCM systems for multimedia applications

Multimedia services require high quality both of the signal and of the transmission systems. High performance must be achieved both in broadcasting services and in interactive ones, whose market is currently increasing throughout the world. There are several network architectures able to deliver high quality multimedia services: among these HFC systems are widely used. For example, in Italy SOCRATE network carries broadband services in the access network. Since these networks have to carry both analog and digital channels, the requirements for network design are critical, mainly because the transmission level for the analog signals are much higher than those of the digital ones. This leads to severe intermodulation effects, which mainly affect the digital channels. Therefore, the design of these networks require simple but accurate models to evaluate the distortion effects. Therefore, a comparison is presented concerning the use of semiconductor laser or intensity external modulator at the transmitter of hybrid AM/QAM subcarrier multiplexed optical transmission systems. In the first case several effects are considered, which give rise to different distortion phenomena: intrinsic dynamic distortions due to interaction of photons and electrons in semiconductor laser, clipping impulsive noise, AM-FM conversion effect due to the joint action of semiconductor laser chirping and fibre chromatic dispersion and nonlinear distortions due to Kerr effect. In the case of external intensity modulator, the distortion noise due to its intrinsic nonlinear response is analyzed. The effect due to optical fibre multiple reflections, which give rise to an interferometric noise affecting the performance of both systems, is considered, as well. The analysis provides useful information about the design and the performance evaluation of SCM based optical access networks.