Multiplex communications – Wide area network – Packet switching
Patent
1982-06-14
1985-03-12
Sikes, William L.
Multiplex communications
Wide area network
Packet switching
350 9619, 350619, 370 1, G02B 514
Patent
active
045041113
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method of multiplexing data-transmission channels using light waves transmitted along a single step-index optical fibre and an optical device for carrying out this method.
Hereafter a step-index optical fibre is to be understood to mean a fibre whose "core" material has a substantially uniform index of refraction for light, which core is surrounded by a cladding whose index of refraction is also substantially uniform but lower than that of the core. It is known that any ray whose angle with the fibre axis is smaller than the aperture angle of the fibre (which is equal to half the acceptance angle), which aperture angle is a function of the values of the index of refraction of the core and the cladding material respectively, is propagated along the fibre by a number of successive total reflections at the core/cladding interface, the angle of the ray with the fibre axis after each reflection being maintained during propagation.
It is known to increase the number of transmission channels in such an optical fibre by using a light wave of a different wavelength for each channel. Such a method necessitates a spectral filtration of the light, both when it is injected into the fibre and when it is received at the fibre outputs. By virtue of its principle, this method does not restrict the number of modes of propagation generated in the fibre, so that it does not lead to an improvement of the transmission pass-band.
The method in accordance with the invention has several advantages in this respect. The light used has the same wavelength for all the multiplexing channels. Moreover, by virtue of its principle, the number of modes of propagation in each channel is limited, so that the transmission band in each channel is extended.
The method is based on the conservation of the angle that each light ray makes with the fibre axis as it propagates in the fibre. The invention envisages the formation of transmission channels inside the fibre by means of light rays which are grouped in accordance with the values of their angles with the fibre axis, each channel being provided with means for injecting light at the fibre input and means for receiving light at the output, both means being specific of the values of said angle.
For each transmission channel the value of the angle of the rays lies between two values which are comparatively close to each other, so that in any point of the fibre the rays corresponding to each transmission channel are situated between two cones whose common apex is formed by said point and which are rotationally symmetrical about a common axis parallel to the fibre axis, the respective values of their apex angles being one of said two comparatively close values. In any point of the fibre the pairs of cones corresponding to each transmission channel form a nested set, the apex-angle values relating to each pair being selected in such a way that said pairs of cones are juxtaposed or, preferably, separated from each other by spaces which are void of active radiation.
For carrying out this method it is necessary to simultaneously inject in a differentiated manner luminous fluxes at any point of the input surface of the fibre in the angular apertures situated between the two cones corresponding to different transmission channels and simultaneously and, at the fibre output, of simultaneously and in a differentiated manner receiving said luminous fluxes from said different channels in the same apertures.
To this end, the device for carrying out the method in accordance with the invention, in addition to a step-index fibre, comprises a multi-axial optical system at each end of said fibre, which system comprises optical elements each having a single axis, the number of axes being equal to the number of transmission channels in the fibre, the optical elements forming, in the object space, as many conjugate images of the fibre end as there are axes. Each optical element has an annular pupil which is centred relative to the fibre axis, which pupil determines the effective aperture por
REFERENCES:
patent: 4050782 (1977-09-01), Uchida et al.
Electro Optic System Design, vol. 12, No. 6, Jun. 1980 (Chicago, U.S.) "Fresnel Lenses as Fiber-Optic Demultiplexers," p. 20.
Sikes William L.
Tamoshunas Algy
U.S. Philips Corporation
Ullah Akm E.
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