Heterodyne optical phase-locked loop by confocal Fabry-Periotcavity coupled AlGaAs lasers

Two external confocal Fabry-Perot cavity-coupled 0.83-μm AlGaAs lasers phase-locked to one another by a heterodyne optical phase-locked loop are discussed. The phase error under phase-locking conditions was measured using the square root of the Allan variance. The result confirmed that the heterodyne signal was a full replica of the spectrum of the reference RF signal within the loop bandwidth. The estimated phase-error variance was 0.02 rad², i.e. ≃8.1°