GB1488253A - Telecommunication system - Google Patents

Abstract

1488253 Optical signal transmission DEN DAVIES and S A KINGLEY 19 Dec 1974 [19 Dec 1973] 58847/73 Heading G1A [Also in Division G2] A telecommunication system comprises: a source of coherent electromagnetic radiation; a fibre-optic path; means located at intermediate positions on the path for varying the optical length of the path; and demodulating means located at the end of the path. The system, Fig. 3. A laser or solid-state source 22 sends coherent radiation of visible or near-visible frequency via an optical fibre 20, along which are positioned one or more modulators 26 (28) arranged to vary the optical length of the fibre 20 in response to direct signals or modulated sub-carrier-frequency signals from a data unit 23(24). The output of the fibre 20, phase-modulated in response to the sub-carrier or direct signals, is combined in a beam-splitter 32 with a steady coherent output, at a different optical frequency, from a voltage-tunable laser 36, to give an input to a photo-detector 34 in the form of radiation amplitude-modulated at the heterodyne frequency. A half-wave plate 30 in the path of the optical fibre output is adjusted so that the polarization plane of the signal light coincides, at the photo-detector input, with that of the light from the local laser 36. A heterodyne-frequency signal component in the photo-detector output is demodulated by a FM receiver 38 tuned to the heterodyne frequency, and integrated to convert the frequency-discriminator output to a correct representation of the phase modulation. The demodulated signals are applied to a demultiplexer 40; and a D.C. component representing the mean optical heterodyne frequency is amplified at 42 and applied to the control element 44 of the voltage-tunable laser 36 as automatic frequency control. Manual frequency control 46 may be provided for setting-up the system when the FM receiver output has nil D.C. component. Modulators (a) A portion of the optical fibre 10, Fig. 1 is wrapped once or more in tension round a modulator comprising a hollow Piezo-electric cylinder 11 (e.g. of lead zirconate titanate) coated on the outside and in the bore with conductive films 12,13. Hoop strains in the cylinder 11, responsive to voltages applied between the electrodes 12, 13, communicate themselves to the fibre portion 10 as changes of length. (b) A Piezo-electric bar 15, Fig. 2, having end electrodes 17, is attached to a portion of the fibre 14 with adhesive 16. (c) A portion of fibre 80, Fig. 6, having protec- tive plastics cladding 82, is surrounded by a closefitting cylindrical film of Piezo-electric material having coaxial film electrodes 84, 88. Peizo-electric hoop strains in the cylinder are transmitted through the plastics cladding 82 as radial strains which effectively vary the optical path length of the fibre 80. (d) In a body 96, Fig. 8, of elliptical crosssection, a Piezo-electric cylinder 98, 100 at one focal axis applies radial stresses to a portion of fibre 94 at the other focal axis, by internal reflections of acoustic waves. The body 96, which may be of metal, has high acoustic impedance, and is sufficiently lossy to damp resonances due to repeated reflections, without severely attenuating the waves. Alternatively the body may be of parabolic cross-section with a portion of the fibre at the focal axis and a plane Piezo-electric transducer arranged transverse to the parabolic axis. Alternative system A single laser 52, Fig. 5 (not shown) provides the main signal carrier and also, via a Bragg cell (58) coupled to an oscillator (72) a shifted reference frequency which is transmitted, via either an optical delay line (71) or another optical fibre (not shown) routed alongside the signal fibre (50) to the beamsplitter (62) which combines this radiation with the main signal radiation. The heterodyne frequency, which thus has a mean value equal to that of the oscillator (72) becomes, after detection by the photo-detector (52) the main carrier frequency of the input to the FM receiver (74). The delay due to the line (71) or length of optical fibre is arranged to differ from that due to the main signal fibre (50), by an interval which is shorter than the coherence time of the laser (52). The delay line (71) may itself be a coiled optical fibre.