An input device includes an enclosure defining a three-dimensional input space and one or more self-mixing interferometry sensors coupled to the enclosure and configured to produce a self-mixing interferometry signal resulting from reflection of backscatter of emitted light by a body part in the three-dimensional input space. In various examples, movement of the body part may be determined using the self-mixing interferometry signal, which may in turn be used to determine an input. In some examples, a body part displacement or a body part speed and an absolute distance to the body part may be determined using the self-mixing interferometry signal and used to determine an input. In a number of examples, multiple self-mixing interferometry sensors may be used and the movement may be determined by analyzing differences between the respective produced self-mixing interferometry signals.