Abstract: An optical sensor includes elongate light transmission means such as an optical fibre (13). A lens (16) of at least nearly half-pitch length is positioned with one of its focal planes substantially at a mouth (13a) of the optical fibre to receive and focus a diverging cone of light emergent therefrom. A reflector (20) is disposed substantially at another focal plane of the lens to reflect the focused light back to the mouth of the fibre. Detection means (14) in the path of the light between the mouth and the reflector is arranged to influence the light in response to an external condition in a manner whereby the reflected light received back at the fibre mouth may be monitored to detect the influence and thereby the condition. An optical fibre network has an incident fibre (40) and a detection fibre (42) coupled to plural optical components (60) via respective optical fibres (62) associated with the components and one or more optical fibre couplers (64, 68, 69).

Abstract: An asymmetric multi-mode fibre optic coupler is assembled from two or more multi-mode optical fibres of different core and cladding diameters but substantially equal cladding refractive indices. The fibres are selected so as to have substantially equal effective refractive indices for their respective highest-order cladding modes, at least in the fabricated coupler.