Digital signal processing techniques used in conjunction with phase-diversity coherent optical receivers are
presented in this paper. These techniques enable coherent detection and mitigation of optical impairments introduced
in transmission. The main advantage of this approach is that the optical impairments can be handled in the digital
domain instead of analog or optical domains, thus reducing various hardware requirements (e.g. dispersion compensating fibers). Dispersion compensation and carrier phase estimation techniques are discussed in detail.
The impact of chromatic dispersion on the complex electric field of an optical phase-coded duobinary signal is investigated through numerical simulation. The evolution of the optical field is most clearly represented by the optical constellation diagram at varying transmission distances. Dispersion causes distortion and rotation of the optical constellation, leading to eye closure of the received signal. When direct detection is employed rapid eye closure starts after approximately 213km. In this letter, the use of coherent detection is suggested to extend this transmission distance. A decision directed phase locked loop is suggested to establish the feasibility of use of coherent detection for this modulation format. Employing coherent detection increases this dispersion limit by 70km.