Point-diffraction interferometer by electro-optic effect in lithium niobate crystals

We propose a new point-diffraction interferometer (PDI) based on a pinhole filter made by a z-cut lithium niobate (LN) crystal. A thin aluminium layer with a circular opening is fabricated on the surface of the crystal by conventional photolithography and subsequent aluminium deposition and lift-off. This aluminium layer acts both as electrode and as pinhole filter on the exit face of the crystal, while a uniform planar aluminium layer is deposited on the opposite face. When a voltage is applied across the z-axis of the crystal, the refractive index changes everywhere in the crystal except in a small portion underneath the area of the pinhole. Therefore, the applied voltage causes an uniform phase shift over the aberrated wavefront while leaving unaffected the diffracted reference beam passing through the pinhole. The interference taking place behind the sample produces an interference fringe pattern containing the information on the aberrated wavefront. Four phase shifted images of the fringe pattern are acquired and processed by means of the Carre algorithm to retrieve the aberrated wavefront. The proposed PDI arrangement has several important advantages over the other PDI configurations. The technological processes are very simple, and the phase-shift operation can be applied at very high speed limited only by the minimum acquisition time of the camera device. In fact, the electro-optic effect can be induced onto LN with bandwidths up to several GHz. Moreover, LN is transparent in very wide spectral range from 400 nm to 5500 nm, thus being useful in numerous applications.