An alignment sensor for a lithographic apparatus is arranged and constructed to measure an alignment of a movable part of the lithographic apparatus in respect of a stationary part of the lithographic apparatus. The alignment sensor comprises a light source configured to generate a pulse train at a optical wavelength and a pulse repetition frequency, a non-linear optical element, arranged in an optical propagation path of the pulse train, the non-linear optical element configured to transform the pulse train at the optical wavelength into a transformed pulse train in an optical wavelength range, an optical imaging system configured to project the transformed pulse train onto an alignment mark comprising a diffraction grating; a detector to detect a diffraction pattern as diffracted by the diffraction grating, and a data processing device configured to derive alignment data from the detected diffraction pattern as detected by the detector.