Optical single-sideband modulation with tunable optical carrier to sideband ratio in radio over fiber systems

Radio over fiber (RoF) has been considered as a promising technology for the distribution of the future high capacity wireless signals such as ultra-wideband and millimeter-wave signals. In RoF systems, optical subcarrier modulation (SCM) is commonly used, such as optical double sideband (ODSB) modulation, and optical single sideband (OSSB) modulation. However, ODSB modulation will introduce RF signal power fading due to the fiber chromatic dispersion. To evade this problem, conventional optical single sideband (OSSB) modulation was proposed and demonstrated. Unfortunately in the conventional OSSB, the modulation efficiency, defined as optical carrier-to-sideband ratio (OCSR) in optical power, is extremely low for small modulation index, i.e. low driven voltage used. On the other hand, when high voltage is applied to the modulator, i.e. high modulation index used, the nonlinear distortion will be induced due to the nonlinearities of the optical modulator response, which will degrade the receiver sensitivity and lead to a poor bit error rate (BER) and spurious free dynamic range (SFDR). Eventually, this will increase RoF system cost and render it impractical. So far, suppressing the OCSR has become a popular way to improve the modulation efficiency. Lots of related techniques have been proposed and demonstrated. The objective of this thesis is to develop a simple modulation technology that OSSB modulation and tunable OCSR are obtained simultaneously only by adjusting the direct current (DC) bias of two parallel non-ideal Mach-Zehnder Modulators (MZMs). In this thesis, the theoretical analyses to prove the functions of this modulation technique are given out first. Then by comparing the theoretical analyses with simulation results, it is found that the results are well matched. Here the impacts of imperfect situations are concerned: bias voltage drift, phase imbalance of the two parallel MZMs, mismatch of splitting and combining coupler's factors, MZM extinction ratio imbalance, inter-modulation distortions (IMDs), high order distortions and chromatic dispersion. The functions and reliability of this modulation technique are deeply discussed. Furthermore, some methods to compensate the impacts due to the non-ideal situations are also considered. Finally, the experimental results are presented to validate the proposed technique