Several techniques for phase noise PSD measurement of continuous wave (CW) lasers to be used in coherent transmission systems are analyzed. Between them, we evaluate two novel techniques. The first employs a homodyne optical phase-locked loop, while the second uses a signal source analyzer. Experimental results obtained by these two methods are compared with classical linewidth measurement methods like self-heterodyne and Michelson interferometer. Limits and accuracy of each method are discussed. Furthermore, the comparison shows that, for coherent transmission system applications, only a subset of the analyzed methods is useful for laser phase noise characterization.
A novel subcarrier-based optical phase-locked loop (SC-OPLL 1 ), with off-the-shelf optical components, is presented and demonstrated. The method, based on a continuous-wave laser and optical subcarrier modulation using a standard LiNbO 3 Mach-Zehnder modulator, allows easier practical implementation than the previously proposed OPLL circuits based on laser direct modulation.
A short overview on Optical Phase locking techniques and a detailed description of the Phase Locking technique based on Sub-Carriers modulation is presented. Furthermore, a novel Single Side Sub-Carrier-based Optical Phase Locked Loop (SS-SC-OPLL), with off the shelf optical components, is also presented and experimentally demonstrated. Our new method, based on continuous wave semiconductor lasers and optical single side sub-carrier modulation using QPSK LiNbO(3) modulator, allows a practical implementation with better performance with respect to the previously proposed OPLL circuits, and permits an easy use in real time WDM signal coherent demodulation.
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