Optically induced light scattering and beam distortion in iron-doped lithium niobate

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Abstract
In using lithium niobate crystals for optical applications such as the storing of phase holograms, a potentially serious problem is the progressive development of optically-induced beam distortion and diffuse scattering. Experimental observations are reported on these effects using a single laser beam. It was found that the beam is eventually reduced to a small fraction of its initial magnitude. A ray tracing calculation confirms that the observed distortion of the high intensity part of the beam is consistent with the lens action of the observed pattern of induced refractive index change due to a single beam. This distortion effect is distinguished from the scattering process which produces lobes along the c-axis and is strongly angular sensitive. It is clear that the observed scattering is due to parasitic gratings. These are produced by interference between undeflected light and light which has been scattered by some triggering process. The model we propose for the scattering process is as follows. Due to bulk and surface inhomogeneities, a speckle pattern develops within the crystal. The high intensity parts of the speckle pattern produce sharp index variations which act as lenses to decollimate the light and cause it to interfere with other parts of the beam. As a result, parasitic gratings will be produced which cause the observed scattering pattern. Such a model explains the observed experimental results as well as other results reported in the literature.