Method of measuring relative movement in two dimensions of an object and an optical input device using a single self-mixing laser

An optical input device for measuring relative movement between an object (15) and a sensor unit comprising a laser device (3, 5) having a laser cavity for emitting a measuring beam (13, 17) and a respective radiation-sensitive detector (4, 6) for generating a measurement signal representative of changes in the operation of the laser device (3, 5) as a result of measuring beam radiation re-entering the laser cavity. A sensor unit is provided for measuring relative movement along each measuring axis in an action plane, and the resultant measurement signal from one or each of the sensor units is used to determine distance and/or movement of the input device and the object (15) relative to each other along a measuring axis transverse to the action plane by summing the offset frequency of a rising and falling slope of the measurement signal.