Chromatic Dispersion Monitoring Method Based on Semiconductor Optical Amplifier Spectral Shift Effect in 40 Gb/s Optical Communication Systems

The optical signals degrade as travel down the optical link due to the optical fiber properties such as chromatic dispersion, polarization mode dispersion, polarization dependent loss, polarization dependent gain and various fiber nonlinear effects, which are considered as limitations in the high-speed, lang-haul fiber communication systems. Where chromatic dispersion causes different wavelengths to travel at different group velocities in single-mode transmission fiber and it has become a major source of transmission degradation due to the continuing increase of the bit rate and distance in high-speed long-distance optical communication systems (Kaminow et al., 2008). In the long-haul optical fiber communication systems or optical fiber communication networks, the accumulated chromatic dispersion is managed by creating a dispersion “map”, in which the designer of a transmission optical fiber link alternates elements that produce positive and the negative chromatic dispersion. In this dispersion “map”, the dispersion has some nonzero value at each point along the optical fiber link, the degradations, from nonlinear effects such as four-wave-mixing (FWM) and cross phase modulation (XPM), are effectively eliminated, but the total accumulated dispersion is near to zero at the end of the optical fiber link (Kaminow & Li, 2002). It seems that there need not other dispersion compensation techniques any more. Unfortunately, chromatic dispersion changes with dynamic optical fiber network reconfiguration and variation with environmental conditions such as temperature in practice (Agrawal, 2002). This dynamical action causes dynamical residual chromatic dispersion in a dynamical fiber link. In addition, signal tolerance to accumulated chromatic dispersion diminishes as the square of the bit rate. Therefore, 40Gbit/s signals are 16 times more sensitive to chromatic dispersion than 10Gbit/s signals. The signal tolerance to chromatic dispersion is restricted about 50ps/nm in single channel speed 40Gbit/s fiber communication systems, so there requires more carefully chromatic dispersion management. The residual chromatic dispersion of a dynamical fiber link can easily extend the tolerance in those high speed fiber communication systems, they need more precise and dynamical monitoring and compensation methods (Pan, 2003). Many novel and effective dispersion compensation methods have been proposed (Kaminow & Li, 2002) (Pan, 2003), and they are included, dispersion compensating fibers, linear-chirped fiber Bragg gratings etc., for fixed dispersion compensation. The dispersion compensating fibers have negative dispersion, which can compress the extended signal pulses due to the 8