An optical element (3) is mechanically retained within a slot of (2) in a rigid substrate (1), e.g. of a ceramic or crystalline material, by contact with the sides of the slot. Where the difference between a dimension of the optical element and the slot width at ambient temperature is a very small and positive, it may be removed by heating the substrate and/or cooling the element (differential thermal expansion) to allow insertion of the element. Then the temperature difference is removed so that the element is mechanically retained under compression by the slot sides. Alternatively, where the element may be directly inserted so that it is in contact with or immediately adjacent the slot wall, the material of the substrate and/or the optical element is the oxidized in the region of contact so as to increase the dimension of the oxidized material and mechanically retain the element. Either technique is applicable to parallel sides slots, but the element may be additionally or alternatively mechanically retained by shaping the edges of the slot. The fibre (3) shown may be butted against a second fibre, or coupled to a guide or other element within the substrate.