NONLINEAR OPTICAL INTERFEROMETERS.

Abstract

This dissertation is an experimental study of a novel type of interferometry based on the generation of Second Harmonic (SH) light. In this work interferometers are described in which an interference pattern arises when two SH waves are superimposed. These waves come from doubling the fundamental frequency of a laser by means of non-linear crystals. Three interferometers are described that have different applications according to their sensitivity to detect wavefront distortions. One interferometer has low sensitivity and is useful in the contouring of refractive objects that produce large wavefront distortions of the order of hundreds of visible wavelengths. The other two interferometers have high sensitivity and one of them is capable of detecting wavefront distortions as small as 1/20 of wavelength. Special emphasis is placed on development of the first interferometer which is a real-time, common-path, self-referencing interferometer that yields interferograms in the visible. The interferometer is based on the fact that a SH wavefront generated under PM conditions is a faithful replica of the laser wavefront. The two interfering SH wavefronts are produced one before and the other after the object under study, and by virtue of its chromatism, they are very slightly different. Consequently, very low density fringes are produced upon their superposition. In this application, noncritically phase matched crystals perform best, and we have found that Y-cut LiNbO₃ crystals configured for surface acousto-optic applications are very convenient. The conversion efficiencies are very low (of the order of 10⁻⁵) consequently optical damage to the LiNbO₃ due to the SH is no problem. The crystals are phase matched by controlling their temperature and are used with a repetitively pulsed Nd:Yag laser operating at 1.06 μm. The chief limitations of this interferometer come from practical considerations in imaging objects with high spatial frequencies that reduce the contrast of the interferograms. The high sensitivity interferometers make use of the changes of phase and amplitude induced in the SH wavefront by the phase mismatch of angle tuned crystals to provide information. The interferometers are directly sensitive to small wavefront tilts and do not require additional reference wavefronts.