Abstract: Fault location in an optical signal transmission network in which a main optical fibre is divided into a plurality of branch circuits, is simplified. Optical time domain reflectrometry (OTDR) equipment transmits an output signal along the main fibre, and for each branch circuit the signal is caused to be returned to the OTDR equipment from the branch to be modified in a manner unique to that branch.

Abstract: Fault location apparatus is disclosed for use in an optical signal transmission network which comprises a main optical fibre having a plurality of branch fibres, and in which each final branch fibre of a chain of branches ends in a terminal unit. The fault location apparatus comprises OTDR equipment connected to the network so as to transmit an output signal through the main fibre and the branch fibres, and a filter incorporated in each of the final branch fibres. The filter is capable of reflecting back towards the OTDR equipment at least one wavelength in the OTDR band, but not traffic wavelengths. The OTDR equipment is operative to make measurements on the mirror or "virtual" image of the path from the terminal unit incorporating the filter returning a signal to the equipment.

Abstract: An integrated optic waveguide coupler having an interaction length throughout which two waveguides lie parallel and in contact with each other, in which the interaction length incorporates at least one straight section and, adjoining the straight section, there is provided at least one additional interaction length having a predetermined curvature. The curvature and the length of the interaction length together minimize wavelength sensitivity.