Patent GB-1490652-A (GB)
Title: System for multiplexing a communication channel constituted by an optical fibre
IPC: H04J14/00
Filed: 1975-01-16
Abstract: 1490652 Photo-electric communication apparatus THOMSON - CSF 16 Jan 1975 [18 Jan 1974] 1998/75 Heading G1A Multi-plex communication apparatus comprises: a plurality of sources comprising diodes for emitting light at different wavelengths; at least one optical fibre; a plurality of photo-detectors; means independent of the photo-detectors for selecting said wavelengths; modulating means applied to the sources; and demodulating means applied to the detectors. Modulators C 10  ... C 19 , Fig. 1, controlled by signals applied to terminals V 10  ... V 1  9 , apply pulse modulation to LEDs E 10  ... E 1  9  (e.g. Aldoped Ga-As diodes), arranged to radiate in respective spaced narrow bands of wavelengths in the range 0À88-0À97 Ám. The radiation, collimated by a lens L 10 , is reflected by a mirror M 10  to pass through other mirrors M  11  ... M 19  each having a selective reflection band corresponding to the wavelength of the collimated radiation from a respective one of the other sources E 1  1  ... E 19 . The radiation thus multi-plexed is focused by a lens 101 on the input face 11 of an optical fibre 1 which transmits the light to a complementary arrangement comprising a collimating lens 102, mirrors M 20  ... M 29 , lenses L 20  ... L 29 , PIN silicon photodiodes R 20  ... R 29  and demodulators C 21  ... C 29 . The transmitting and receiving elements are so ordered that the light in any channel undergoes the same number of passages through, and reflections by, mirrors, regardless of its position. The wavelength selection may alternatively be done by filters with beam-splitting mirrors or, as in the embodiment of Fig. 3, with dispersive elements. Light from each of sources E i , E j  arranged in the focal plane of a lens L 1  arrives as a parallel beam aimed at a respective angle of incidence at the face of a dispersive element 30 (e.g. a prism or diffraction grating), the respective wavelengths and incidence angles being so chosen that the light emerges from the element 30 as a single multi-coloured parallel beam which is then focused by a bus L2 in the input face 11 of the optical fibre 1. A complementary arrangement at a receiving station (not shown) sorts the light into its various wavelengths and . focuses it on to respective detectors in the focal plane of a lens. Alternatively, Fig. 4 (not shown), the steps of collimation, dispersal and focusing may be combined by a hologram in a thick medium (40) at the transmitter and a similar element at the receiver. The multiple mirror arrangement of Fig. 1 may be replaced by a thick slab of glass (51, 52), Fig. 5, (not shown) having parallel partially-transmitting faces. A light beam inclined to the surface undergoes multiple internal reflections, some of the light emerging at each point of reflection to pass through an appropriate hand-pass filter (T 1  ... T 6 ). Similarly light from each of respective sources (E 2 , E 3 ) undergoes two or more internal reflections to combine with light from a source (E 1 ) undergoing transmission through the slab. At least one source and its modulator may be exchanged for a corresponding detector and its demodulator to provide a combined transmitting and receiving station at each end of the fibre, Figs. 5 and 6, (not shown). The transmission path may comprise a bundle of fibres.