Cause of LO2–beat relative-phase drift during OPL operation

Ascertain the underlying mechanism responsible for the observed drift of the relative phase between the second local oscillator (LO2) and the master–slave beat signal in the optical-phase-locking feedforward architecture, which persists even when the optical phase-locked loop is active; specifically, determine whether the drift arises from near-mode-hopping behavior in the semiconductor slave laser or from other sources.

Background

The method adds a feedforward stage to a conventional optical phase-locked loop (OPLL), requiring a stable demodulation quadrature achieved by actively controlling the phase of a second local oscillator (LO2) relative to the master–slave beat. During operation, the authors observed a drift of the LO2–beat relative phase even when the OPLL was engaged, motivating the inclusion of an auxiliary stabilization loop.

The authors explicitly state that the exact cause of this phase drift is unclear and suggest it may be related to near-mode-hopping behavior in the semiconductor slave laser. Identifying the cause would inform strategies to mitigate or eliminate the drift, improving long-term robustness without additional stabilization.