PHASE-SHIFTING COMMON-PATH INTERFEROMETER USING A LIQUID-CRYSTAL PHASE MODULATOR

A common path phase shifting interferometer is proposed using a liquid-crystal cell as a phase modulator. In the proposed common path interferometer, the specular component of the object light, separated in the Fourier transform plane, plays the role of a reference wave for the diffracted component of the object. A homogeneous-type nematic liquid-crystal cell is used as a phase only modulator, to introduce an arbitrary phase modulation between the specular and the diffracted components to achieve phase shifting interferometry. This makes quantitative measurements possible as well as qualitative observations for phase objects like in a Zernike's phase contrast microscope. Finally, experiments were carried out to show the validity of the proposed method. Experimental results show that the present method is quite stable against mechanical shocks and air fluctuation owing to its common path arrangement. The accuracy of the present method was experimentally verified to be better than 1/30 of wavelength by comparing the results with those obtained by the heterodyne interferometer.