A method for detecting motion direction of an object (4) comprises the steps of: laser output light (Ll) is generated, using a semiconductor laser (2) having a thermal response frequency (fr); - the laser is driven with rectangularly modulated DC current (I) having a modulation frequency higher than said thermal response frequency (fr) and preferably higher than twice said thermal response frequency (fr), such as to triangularly modulate the wavelength of the laser output light; the laser output light is directed to the object; - a portion of reflected light (L3) is allowed to interfere with light (LO) within the laser; a portion of the laser light is used as measuring beam (5); the frequency spectrum of the measuring beam (5) is analyzed in conjunction with the modulated laser current in order to determine the direction of movement of the object (4).
