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Garcia-Garcia, A. (2009) Optical phase synchronization in coherent optical beamformers for phased array receive antennas. Master's thesis, University of Twente.
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Optical phase synchronization is an important issue in coherent optical beamforming systems. In this kind of architectures, the optical carrier produced by a common laser is split and then modulated by the RF signals coming from different antenna elements, delayed as desired, and combined in couplers. At the combining points it is fundamental that the optical carriers on the two branches are in phase, so that constructive interference can occur at the detector, resulting in maximized optical power and, as a consequence, maximized RF signal power. The issue of optical phase synchronization becomes particularly sensitive when dealing with hybrid setups, where the integrated optical chips are connected by means of optical fibers. In this case, a number of causes concur to de-synchronize the optical phases, originating destructive interference and dramatically reducing the output signal-to-noise ratio of the system. In this thesis, several solutions to this problem are proposed and analyzed. The total power feedback loop technique is then chosen as the most suitable approach, and shown to be a valid solution to synchronize the optical phases. Several feedback algorithms are analyzed and simulated in their pros and cons, and then an optimum hybrid algorithm is chosen to allow, at the same time, local maxima avoidance and fast tracking against phase drifts. The performance of this solution was first simulated, optimized in its parameters and finally implemented and tested in the real system. This work proved that, by means of the designed feedback loop, the optical output power is successfully stabilized at the maximum value despite the drifting in the hybrid setup parameters.
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