Abstract: A passive transponder for use in an interrogator-responder communication system is disclosed, including a first signal receiving element having a variable reflection property, a code signal generator for supplying a code signal, a modulating device responsive to the code signal for varying the signal reflection property of the first signal receiving element, and a power supply for supplying direct-current operating power to the code signal generator and to the modulating device, the power supply including a second signal receiving element for receiving a second electromagnetic signal having a second frequency different from the first frequency, and an a-c to d-c converter for converting the second signal to direct-current power.

Abstract: In a telecommunications system employing modulated optical signals, a highway is formed from optical fibres and a source of coherent light or other electromagnetic radiation of an adjacent frequency band is connected to one end of the highway. One or more modulators are connected to the highway at intermediate positions along its length, the or each modulator being arranged to periodically vary the optical path length of the highway. A demodulator is connected to the other end of the highway.