Application of a communication signal, RF (radio frequency) or microwave signal to a modified optical fiber for modulating or switching-off the optical signal propagating within the optical fiber. The passive cladding material of an ordinary optical fiber is removed from a small region and the fiber is coated with one or more layers of coated materials including a layer of electrooptic material surrounding the fiber core. A microwave cavity is positioned around the modified fiber where the optical fiber is passing through the cavity in the direction of the cavity main axis to provide a modulating microwave electromagnetic field applied to the electrooptic material. A communication signal applied to the microwave cavity will result in optical modulation so that the modulated intensity and phase of the optical signal at the receiving end of the fiber can be detected, and demodulation of the signal can be achieved.