Control scheme for accurately optimizing (and also automatically stabilizing) the interferometer phase bias of Symmetric-Mach-Zehnder (SMZ)-type ultrafast all-optical switches is proposed. In this control scheme, a weak cw light is used as a supervisory input light and its spectral power ratio at the switch output is used as a bipolar error signal. Our experimental result at 168-Gb/s 16:1 demultiplexing with a hybrid-integrated SMZ switch indicates the feasibility and the sensitivity of this control scheme.