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A novel subcarrier-based optical phase-locked loop (SC-OPLL®), 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 LiNbO3 Mach-Zehnder modulator, allows easier practical implementation than the previously proposed OPLL circuits based on laser direct modulation.

Introduction: Optical phase-locked loops (OPLLs) for homodyne or heterodyne coherent detection received a great deal of attention at the beginning of the 1990s [1, 2] in order to increase the receiver sensitivity, but they never found practical applications, first because of their complexity, secondly due to the introduction of EDFAs, that greatly leveraged sensitivity issues. In the near future, however, coherent detection may come again to the forefront in scenarios such as: multilevel optical phase modulation (N-PSK), dispersion compensation in the electrical domain, ultra-dense WDM, fast reconfigurable optical networks, optical sensors, microwave photonics, etc. The previously proposed OPLLs were based on fast direct laser frequency tuning, and required complex/expensive optoelectronic devices, such as solid-state lasers [1], wide-bandwidth electrical PLL circuits and piezoelectrically controlled narrow optical filters [2], multisection DFBs and external cavity lasers with fast direct laser frequency tuning [1], an acoustic optical modulator with limited tunable frequency range ($10 MHz), and an additional fast frequency control unit for the external cavity laser in order to increase the frequency range (several hundred MHz)[3].