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Modulations imposed on a chaotic optical signal generated by a semiconductor laser can be suppressed by injecting the signal into another similar laser under conditions for chaos synchronization. This filter effect could be used to recover messages hidden in chaotic carriers for robust and secure communications. We use a numerical model to examine the filter properties and show that the filter can be described in terms of differences in characteristic transmission functions for imposed signal and chaotic carrier in the output of the synchronized laser. The filter effect is shown to be larger for lower frequencies and decreases as frequencies approach the relaxation oscillation frequency of the laser in the gigahertz regime, similar to the response of steady-state injection-locked lasers to small-signal modulation. The filter properties are confirmed in experiments using both single and multimode lasers.