Embodiments of mechanisms of an optical inspection system for inspecting an object are provided. The optical inspection system includes a light source emitting a coherent beam having a first width, and a beam expander increasing the first width to a second width. The optical inspection system also includes an polaroid module adjacent to the beam expander and polarizing the coherent beam. The object generates an inspection beam with an interference pattern by reflecting the polarized coherent beam. The optical inspection system further includes an image module capturing the inspection beam.