Improvement Scheme for Directly Modulated Fiber Optical CATV System Performances

Abstract

In an optical CATV system, the signal was directly or externally modulated with lightwave before communicates. Directly modulating signal with LD is an economic method, but the transmission distance and performance are significantly limited by laser chirping issues. In another hand, the externally modulation schemes have been proofed to provide better outcomes by eliminating the laser chirping issue. Nevertheless, an expensive externally modulated transmitter is required causing an increased capital expenditures. In order to provide an economic structure with advanced transmission performance as in external modulation system, direct modulation method is often combined with other techniques or components to compose higher performance and lower cost CATV system. The split-band techniques for example have been proofed as a powerful assistant for direct modulation CATV systems. In this structure, the full channel loading is shared by additional LDs causing a wider optical linewidth to eliminate the SBS degradation. In addition, by increasing the wavelength numbers, major parts of CSO distortions from each transmission band are automatically removed from each channel. These outstanding techniques and impressive outcomes are consequently analyzed and discussed as parts of this chapter. In parallel with the effective of the SBS degradation, the CNR is in direct proportion to the optical power, as well as the second-order harmonic distortion-to-carrier ratio (HD2/C) and the third-order inter-modulation distortion-to-carrier ratio (IMD3/C) values are in inverse proportion to the laser resonance frequency. Improving the utilized LD characteristic is therefore another useful methodology to upgrade the optical CATV system. External lightinjection and optoelectronic feedback techniques in particularly have been experimentally proofed as efficient methods to enhance laser output power and laser resonance frequency (Lee et al., 2007; Lu et al., 2008). As a result, the CNR value will be proportionally increased with the enhanced LD intensity, as well as the HD2/C and IMD3/C will be proportionally reduced with the improved laser resonance frequency. The reduced HD2/C and IMD3/C values are subsequently leading to an improvement in CSO/CTB values. Following with the modifications of modulation schemes and lightwave characteristics, compensating fiber dispersions is another research direction to promote fiber optical CATV systems. Numbers of compensation methodologies such as eliminating parts of optical dispersion by optical filter and cascading systems with negative dispersion fibers are