A compact sensor apparatus having an illumination beam, a beam shaping system, a polarization modulation system, a beam projection system, and a signal detection system. The beam shaping system is configured to shape an illumination beam generated from the illumination system and generate a flat top beam spot of the illumination beam over a wavelength range from 400 nm to 2000 nm. The polarization modulation system is configured to provide tenability of linear polarization state of the illumination beam. The beam projection system is configured to project the flat top beam spot toward a target, such as an alignment mark on a substrate. The signal detection system is configured to collect a signal beam comprising diffraction order sub-beams generated from the target, and measure a characteristic (e.g., overlay) of the target based on the signal beam.