We present a contactless optical sensor based on the laser-self-mixing effect for real-time measurement of linear and transverse displacements of a moving stage. The sensor is able to measure linear displacements of up to 400 mm along the main optical axis while simultaneously estimating straightness and flatness deviations up to 1 mm. The sensor exploits two identical coplanar nonparallel self-mixing interferometers and requires only one reference plane. The reduction in the number of optical elements allowed by the self-mixing configuration and the intrinsic stiffness of the adopted geometry result in a compact, low-cost, and easy-to-align setup.