Digital Pre-compensation of Chromatic Dispersion in QPSK high speed telecom systems

Abstract: Chromatic dispersion (CD) is one of the most significant impairments in optical fiber communication systems. Since expensive and complex optical components are required to mitigate CD in the optical domain high-speed digital signal processing techniques are becoming an alternative to compensate electronically both non-linear and linear optical fiber degradations in the transmitter or receiver. This thesis investigates a new electronic dispersion compensation technique based on signal predistortion using an electro-optic modulator driven by signals previously filtered by a linear Finite Impulse Response filter. Moreover, since transmitter and local oscillator lasers phase noise has usually been assessedindependently without regard to the effect of chromatic dispersion on the phase noise in the system performance, a comparative study between pre- and postcompensation of chromatic dispersion influence on equalization enhanced phase noise (EEPN) in coherent multilevel systems is carried out. For that purpose, carrier phase estimation is implemented by a one-tap normalized least-meansquare filter. Simulations of chromatic dispersion equalization in the transmitter demonstrate that a 56-Gbit/s QPSK coherent system is able to compensate large amounts of fiber chromatic dispersion using a predistorting linear finite impulse response filter. Concerning impact of chromatic dispersion compensation on equalization enhanced phase noise, simulation results show for postcompensation scheme the local oscillator phase noise limits the EEPN influence in the system. However, when the CD equalization is performed in the transmitter, the transmitter laser phase noise is the limiting factor that determines the EEPN effect in the transmission system. Most of those constraints may be mitigated by performing CD compensation in optical domain in such a way that the EEPN influence could be neglected.