Laser module as alignment source, metrology system, and lithographic apparatus

An improved light laser module is disclosed. The laser module can include a laser source (310) configured to generate laser light, and an infinite impulse response filter configured to reduce coherence effect of the laser light by decorrelating phase of components of the laser light. The infinite impulse response filter can include a plurality of optic couplers (410,420,430,440,450) to form a plurality of optical propagation loops with different optical path lengths (L1,L2,L3), respectively. The laser module can further include an acoustic-optic modulator arranged in an optical propagation loop, and configured to shift optical carrier frequency, such that an output of the laser module has a widen spectral to further reduce coherence effect.