We demonstrate a novel approach for coherently detecting microwave-modulated optical phase through interferometric phase detection and an optical phase-locked loop (OPLL). An asymmetric interferometer converts phase modulation into intensity modulation for direct detection. The optical phase in the interferometer is modulated by the detected signal through an OPLL. Balanced detection is achieved by biasing the interferometer at its quadrature point to cancel common-mode intensity noise and even-order nonlinear distortions. The concept was validated experimentally at a frequency of 25.7 MHz, limited by the large loop delay associated with discrete optical components. A numerical simulation is used to project system operation beyond the limitations of the experiment, showing that the linearity performance of the interferometric phase detector may be significantly improved by signal feedback.
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