Frequency-shifted feedback was recently used to synchronize the beat note between the two modes of a dual-frequency laser with the radiofrequency emitted by a local microwave oscillator. This technique allows one to obtain an optically-carried RF clock signal that can find applications in microwave photonics. Two regimes of synchronization are then found [1]. When the frequency difference Δν between the two oscillators is smaller than a frequency fA determined by the feedback amount, the two oscillators are phase-locked, leading to a constant relative phase φ. Then, a regime of frequency locking without phase locking appears. This regime, also called bounded phase [2,3], extends the frequency synchronization beyond fA up to a frequency fB. Finally, when Δν is larger than fB, the two oscillators are not synchronized, resulting in an unbounded drifting phase. Here, we experimentally characterize the three regimes and compare the results to numerical simulations.
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