GB1396433A - Optical communication systems - Google Patents

Abstract

1396433 Modulating light POST OFFICE 2 Nov 1973 [24 Nov 1972] 54412/72 Heading H4F A section of a dielectric optical waveguide having a core and cladding is subjected to an electric field of an intensity which varies in a spatially undulating manner along the guide and produces a corresponding refractive index variation in the core, whereby optical energy in the core is transferred from a first mode of propagation to at least one other mode of propagation and/or a non-propagating mode. Transfer between modes is said to reduce the effects of group dispersion when using multimode guides in telecommunications, i.e. the phenomenon whereby pulses become distorted and lost due to different propagation velocities of different modes. Transfer to a non-propagating mode, in which light is radiated from the guide, is useful for intensity modulation, i.e. by varying the intensity of the electric field, the intensity of the light still propagating within the guide is controlled. In the latter case a single mode or multi-mode guide may be used. As particularly described, the field may be orthogonal to the waveguide axis and provided by a series of electrode strips tangential to the circumference of the guide, and, diametrically opposite, a second corresponding series of such strips or a tangential plate electrode. Alternatively, a field parallel to the guide axis may be provided by a series of electrode rings circumjacent the waveguide surface, alternately connected to opposite poles of a source of potential difference. The electrodes may be produced photo-lithographically, e.g. with a mask, or via laser interference fringes, or the parallel strips may be a wire grid. The Specification discusses criteria for determining the pitch of the spatially undulating field to effect transfer substantially between predetermined modes. It would appear possible for transfer to a non- propagating mode to occur directly or via intermediate (higher) propagating modes. If the pitch is strictly constant transfer to modes other than those predetermined is restricted, but by varying the pitch in a pseudo-random manner, i.e. between predetermined limits, other modes are coupled more efficiently, this being particularly useful when intensity modulation is required. As particularly described the waveguide may comprise a nitrobenzene core with soda-glass cladding, or a core of solid metanitroaniline with its C-axis parallel to the core axis in a flint glass cladding.