An optical system includes a collimated light source, a beam splitter, two mirrors and two lenses, a focus lens, and a detector. An initial light beam is generated by the light source and then separated by the beam splitter into a first light beam and a second light beam. The two mirrors respectively direct the first and second light beams on a sample with symmetrical paths and the two lenses focus the first and second light beam on the sample respectively. The first and second light beams are reflected from the sample and along the counterpart paths to the beam splitter. An interfered light beam is then generated by combining the reflected first and second light beams, and focused by a focus lens on a detector. A Dove prism can be configured between one mirror and one lens of the two for contrast enhancement. It can produce the photon combination with same of direction in this setup to enhance contrast.