 A common path interferometer is a class of interferometer in which the reference beam and sample beam travel along the same path . Fig . 4 illustrates the Sagnac interferometer , the fibre optic gyroscope , the point diffraction interferometer , and the lateral shearing interferometer . Other examples of common path interferometer include the Zernike phase contrast microscope , Fresnel 's biprism , the zero @-@ area Sagnac , and the scatterplate interferometer . 
