An optical sub-assembly is disclosed, having a mating end adapted to receive a mating fibre, and a back end provided with a converging lens 18 establishing an expanded beam interface 43 with an optical device 41. The use of an expanded beam interface provides greatly improved stability, and reduces the sensitivity to slight misalignments The optical sub-assembly comprises a receptacle for which the manufacturer thereof has full control over the optical interfaces and refractive index boundaries. The receptacle and the optical device may be manufactured separately.
