Gradient-index optical systems in holographic endoscopy

Small limiting apertures of endoscopes cause gross speckles when using coherent light sources in holographic endoscopy. Thus, for a high optical signal-to-noise ratio, imaging systems with high numerical apertures and low f/Nos. are required, which under given geometrical constraints can be provided in particular by gradient-index rod lenses. For additional speckle reduction the limiting aperture has to be enlarged within the given outer diameter of the instrument. This can be achieved by using the same gradient-index rod lens for both imaging and illumination, so that no separate illuminating fiber bundle is necessary. Holographic recording problems due to reflections of the illuminating object beam from the end faces can be solved by appropriate positioning and shaping of the rod as well as by holographic subtraction.