We describe an optical sensor for process monitoring of Nd:YAG laser welding. This sensor detects the broadband radiation produced by the welding process, dividing it into broad spectral bands (designated UV/visible and IR). Fourier analysis is used to investigate an oscillatory intensity modulation of the optical signals, which is believed to arise from a combination of keyhole and weld-pool oscillations. The spectral content of the oscillations may be used to detect a fully open welding keyhole and to determine the work-piece thickness under this welding regime. These oscillations have also been utilized in the development of a tracking technique which detects the misalignment from an overlap welding seam and excessive gaps in the lap joint.