A passive, remote sensor including an optical subsystem for receiving electromagnetic radiation scattered off a surface. A detector is responsive to electromagnetic radiation received by the optical subsystem and generates a composite signal comprising a DC current component and an AC current component. A filter extracts the DC current component from the composite signal. A light emitting device is driven by the DC current component. A detector is responsive to the light emitting device and is connected to the first detector to subtract at least a portion of the DC component from the composite signal producing a modified signal. An amplifier is responsive to the modified signal detects vibrations of the surface to be detected.