An anti-theft device for monitoring integrity of a tether cable and connection between a tether cable and a security sensor. An optical echo chamber is formed at the interface of the coupling between the tether cable and the security sensor. An optical transmitter and an optical sensor are disposed within cable. The optical transmitter is configured to emit an optical signal into the echo chamber. The optical sensor is configured to detect a reflection of the optical signal within the chamber. If the cable is either cut or disconnected, the optical sensor will fail to detect the reflected signal, thereby triggering an alarm.