Abstract: The or each strained quantum well layer of a quantum confined Stark effect modulator is provided with a substructure of substructure layers not all having the same lattice constant. The thickness and composition of these substructure layers may be arranged to produce a differential strain that is asymmetric with respect to the mid-plane of the quantum well and so skews the hole wavefunctions for heavy-holes, HH1, and light-holes, LH1, in opposite directions. This enables the choice of composition designed to provide substantial matching of the E1-HH1 and E1-LH1 Stark shifts for one particular polarity of applied field, thereby providing a modulation facility that is substantially polarisation insensitive. Alternatively, the thickness and composition of the layers may be chosen to produce a symmetrical strain profile in which the same effect is provided, but for both polarities of applied field.