Abstract: An optical fibre structure usable in amplifying optical signals, comprising a length of optical fibre (1) which includes two uniformally spaced monomode optical cores (2,4) located within a common cladding (6) so as to provide two optical guidance paths, the optical characteristics of at least the cores being different such as to give the two guidance paths different propagation constants the values of which coincide for a predetermined coupling wavelength, one of the cores including a fluorescent material capable of producing stimulated emission. There is also disclosed a method of amplifying an optical signal using such an optical fibre structure, comprising arranging for the wavelength (.lambda..sub.s) of the optical signal, the fluorescent wavelength of said material in said one core, and said predetermined coupling wavelength all to be substantially the same, launching optical pump energy having a different wavelength (.lambda..sub.

Abstract: An attenuating device 10 for use with signal carrying optical fibres 14 comprises a length 18 of optical fibre coiled within a casing 16. The optical fibre length 18 is of higher attenuation per unit length than the optical fibres 14 and the device comprises means, for example optical fibre tails 22,24 fusion spliced at 20,22 to the opposite ends of the length 18 for optically connecting those ends to the fibres 14.

Abstract: An optical fibre sensor 11 has two monomode cores 13,14 arranged in a common cladding 15 to allow cross-talk of optical signals between the cores as a function of strain, temperature and optical wavelength. Optical signals are generated by a generator 16 at two wavelengths, at one of which cross-talk between the cores 13,14 is temperature independent. The generated optical signals are injected into the element 11 via an optical fibre lead 18 having a single monomode core 19 which is connected to one of the cores 13 of the element 11 at one end thereof and after cross-talk between the cores, optical signals are extracted from the element, for example via the same lead 18, and processed to determine temperature and strain at the element.