Electronic Distortion Compensation in Optical Differentially Modulated Systems
Examensarbete för masterexamen
A rapid research and development has been going on in the telecom industry since a few decades, which increases the demand of high data rate transmission systems. This increasing demand of growing data markets, forced the optical industry to find out new ways to improve the optical system capacity. The capacity of an optical system can be enhanced by applying two techniques, first by increasing the bandwidth of transmission system and secondly by increasing the modulation levels of the transmission system. Due to the fiber channel impairments such as chromatic dispersion (CD) and polarization model dispersion (PMD), the OOK modulation format was capable of transmission rates up to 10 Gbps. The second possible solution to increase the transmission rate in presence of channel impairments was the multilevel modulation formats. As a result of these efforts many modulation formats such as m-QAM, m-QPSK and m-DPSK were reported for optical fiber systems. In optical fiber channels the CD and PMD are the causes of the optical pulse broadening which put the limitation on transmission distance and spectral efficiency of multilevel modulation formats. For coherent and non coherent optical systems dispersion effects are very critical for the system performance. To solve the performance degradation due to CD and PMD, different types of compensation techniques were introduced for coherent and non-coherent direct detection systems. Due to these compensation techniques the transmission distance of optical systems can be enhanced many times. The aim of this thesis work is to observe and mitigate the CD effects on non-coherent direct detection differentially phase modulated DPSK and DQPSK systems. The non coherent differential phase modulated systems modulate the information on differential phase. In the m-DPSK receiver, the optical signal passes through an optical delay line interferometer and a balanced detector. During the process of the detection the phase field information lost. It is however possible to recover the received phase field information in the electrical domain by digital signal processing (DSP). By using DSP and exploiting the orthogonal fields of non coherent direct detection DPSK receivers, the complex optical received field can be reconstructed in the electrical domain. It is then possible that the reconstructed field can be compensated by the electronic dispersion compensation (EDC) method. In EDC, numeric compensation or the inverse CD transfer function is used for compensation. After mathematical modeling and simulation, and by evaluating the obtained results, we demonstrated that in non coherent differentially phase modulated DPSK and DQPSK systems received optical field can be reconstructed in the electrical domain by DSP and reconstructed optical signal can be compensated by such numerical compensation techniques.
Informations- och kommunikationsteknik , Fotonik , Information & Communication Technology , Photonics