Fig. 1 illustrates the signal processing principle of closedloop OVS [19]. The photoelectric detector converts the light signal into electric signal for further processing. The closedloop error signal is modulated into high frequency carrier wave for suppressing the 1/f noise in the detecting circuit. A digital demodulation scheme is designed to extract the closed-loop
ves passing and retracing the OPM is τ =3 .6 μs, which determines the minimum closed-loop period