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A silicon-based variable optical attenuator (VOA) is used to overlay optical control data on an underlying high-speed optical data channel. We demonstrate experimentally the addition of a 10.7-MHz (5 Kb/s) control subcarrier with <1-dB penalty to a high-speed channel (500 Mb/s) and quantify the penalty versus control data rate. The modest modulation bandwidth of the VOA enables control subcarriers at frequencies sufficiently high to avoid gain saturation-induced crosstalk in erbium amplifiers, while using the same devices required for channel gain equalization or power control. At OC-192 data rates, we predict that control-channel data rates of 10 Kb/s can be tolerated with <0.5-dB penalty.