Abstract: An intrusion detection system for sensing mechanical and acoustical vibrations, comprises a light source, a fiber optic acoustic transducer, a light intensity to current converter circuit, a display and an audio monitor. The light source is positioned at a point remote from an area to be protected and is coupled to the fiber optic acoustic transducer by a low loss fiber optic transmission line. Similarly, the light intensity to current converter circuit, the display and the audio monitor are located at a point remote from the area to be protected, and are coupled to the fiber optic acoustic transducer by a low loss fiber optic transmission line. When mechanical or acoustical vibrations impinge on the fiber optic acoustic transducer it modulates the intensity of the light beam generated by the light source, thereby causing the system to generate both a visual and audio indication that an intrusion is taking place. Since the system emits no radiation, (e.g. thermal, electrical, acoustical, electro-magnetic etc.