An optical interconnection network comprises an optical bus (60) formed by a D-fibre (62) embedded in a thermoplastic substrate (64) with the flat of the D-fibre (62) flush with the top surface of the substrate. A module (70) similarly constructed is dimensioned to be a push fit in a wall structure (65) formed on the substrate (64) with the fibres (72) and (62) in a position to evanescently couple optical signals from one fibre to the other.