Abstract: An optical fibre is fixed against a first face of a prism. A coherent beam of optical radiation is directed at the prism such that a portion B propagates directly to the first face while a second portion propagates via total internal reflection at a second face. The interference of the two portions creates a refractive index grating in the fibre at a wavelength longer than that of the radiation. The method provides a stable and simple method of sidewriting of waveguide gratings.

Abstract: An optical waveguide in which the rate of change with wavelength of the reciprocal of the difference between the effective refractive indices of a fundamental wavelength mode and a second harmonic wavelength mode is zero at a first wavelength, and there is an internal periodic grating providing phase matching in the wavelength region of the first wavelength.

Abstract: A method of structurally modifying a silica monomode optical fiber (4) by launching optical power into the fiber (4) from an Nd:YAG laser operating at 1.064 .mu.m and raising the temperature of a portion of the fiber (4) by bringing a metal film (12) into contact with a polished half-coupler block (6) to absorb energy from the laser (2). A structural modification of the fiber is initiated which propagates towards the laser (2) sustained by the optical power. It provides a means of decommissioning an optical fiber. The modified optical fiber (4) has regular periodic damage sites which can be used to form a diffraction grating. The method may also be used as an optical power limiter to protect optical networks from too high optical power inputs.

Abstract: A polarization insensitive optical freqency mixer comprising an optical fiber coupler block (2), a silica fibre (6) with a region whose cladding (4) has been polished off close to the core (8) on which is rotatably mounted interdigitated electrodes (12) spaced from the coupler block (2) by a polymer film spacer (12). The electrodes (12) induce a spatially periodic electric field within the core (4) which reverses every half period which provides frequency doubling of light passing through the fibre (6) over the whole grating length. The electrodes are dimensioned to provide polarization insensitized frequency mixing by providing that the electric field components within the optical waveguide are substantially equal in two mutually orthogonal directions transverse the waveguide.

Abstract: An optical fibre has a coating of, for example, liquid crystal polymer which causes temperature-dependent stress-induced changes in the optical fibre such as to counteract temperature-induced changes in the transmission delay of the fibre.

Abstract: An optical switch, e.g. a blocking switch or a coupler switch, has a liquid crystal control element which, in at least one of its control configurations, has a splayed molecular orientation. In a preferred embodiment the liquid crystal is surrounded by chemical control surfaces, e.g. by homeotropically coated control surfaces.

Abstract: An optical fibre Michelson interferometer has mirrors of highly reflective coatings deposited on the ends of the fibres forming the free arms of the interferometer. The interferometer preferably comprises single mode fibres, and is operated by a frequency swept laser.The interferometer has applications in the optical fibre interferometer sensing field.