Abstract: Optical measurement apparatus, for example for the measurement of displacement, comprises a radiation source 2 coupled to an optical fibre 1 connected to a sensing head 12 comprising a Y-coupler 8 supplying an optic fibre tip 5 and a parallel path incorporating a delay 10 and terminating in a tip 11. A lens 4 of appropriate focal length is positioned between tip 5 and a reflector 3 to focus light onto reflector 3 so that the amount of light reflected back to tip 5 is a function of the position of the reflector. A signal detector 6 is also coupled to fibre to detect and measure the reflected light. Tip 11 is positioned at the focus of lens 4 so that the light reflected back to tip 11 is independent of the position of reflector 3. The ratio of the two reflected signals detected in detector 6 is calculated to give the position of reflector 3 with compensation for the effects of deterioration of the parts of the apparatus in which there is a common path for the two quantities of light.

Abstract: A telecommunication system has a laser-energized multimode optical fibre highway. Data is entered on to the highway by means of a series of modulators each operating at a different subcarrier frequency. At the far end of the highway demodulation is carried out be a quadrant photodetector which applies each of its outputs to a respective set of channels. Each channel in a set has a band pass filter centered on a subcarrier frequency followed by a non-linear device. The signals from all the channels of different sets carrying the same subcarrier frequency are combined and the signals passed through band pass filters centered on the second harmonic of the subcarrier frequency.