Abstract: An interferometric fibre optic network includes a downlead optical fibre (12) and a pair of uplead optical fibres (14, 16). Sites for a plurality of interferometric sensor pairs (25) are arranged whereby each pair (25) is optically connected in use of the network to the downlead and uplead fibers (12, 14, 16) via a 3.times.3 fibre optic coupler (30) and respective further fibre optic couplers (32) on the uplead fibres (14, 16). The uplead fibres (14, 16) are multimode optical fibres, and each of the 3.times.3 couplers (30) is optically connected to the respective further couplers (32) by a single mode optical fibre (34). The further couplers (32) are asymmetric couplers having high tap-on coupling from the single mode optical fibre (34) to the respective uplead fibre (14, 16) but low tap-off coupling from the uplead fibre (14, 16).

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).