Holography Based Optical Super Resolution

Abstract

This thesis presents techniques on optical superresolution based on holographic recording. Optical 4f imaging system has been discussed to describe the superresolution techniques. An object transparency placed in the 4f system is illuminated with different plane waves where different spectral parts are allowed to pass through the limiting aperture placed at the Fourier plane of the 4f imaging system. A reference beam is added to the image plane and a CCD is placed to record the object hologram. The recorded holograms are processed using computer in Fourier domain. In this way, different spectral parts are retrieved and joined together to synthesize the super-resolved spectrum, which is inverse Fourier transformed to obtain super- resolved image of the input object. The designing of illumination system is the main part of this thesis. This thesis has presented four techniques of super resolution. In all the four techniques, different illuminators have been described. In the first technique, a planar array of fibers arranged in the form of a matrix, is described where each fiber is used once at a time to illuminate the object with one plane wave. The input object is therefore illuminated with a large number of plane waves from different directions to pass different spectral contents of the object through the optical system. The number of holograms recorded is equal to the number of fibers used to illuminate the object. The post processing of the recorded holograms gives the super resolved image. In the second technique, a simplified illumination array of fibers is used at the expense of object in-plane rotations with angular step of /2 for each set of fibers. The object is illuminated sequentially by fibers and spectral parts of the objects are recorded in the vi form of holograms. The recorded holograms are processed to obtain super resolved image of the input object. In the third technique, Spatial Light Modulator (SLM) is used as illumination source to redirect the higher spatial frequencies of the object spectrum into the pass band. Initially a diverging lens function is assigned to the SLM, which sequentially illuminates the object with large number of beams from different directions. As a result, two types of holograms: with object and without object are recorded at the CCD plane. All these holograms are processed further to obtain the phase and amplitude of object, which constitute the super resolved image. In the fourth technique, the object is illuminated simultaneously with multiple beams produced by SLM. Different object spectral parts simultaneously pass through the pass band. In this technique, the reference beam is eliminated and the image is recorded as a result of mutual interference of illuminating beams. A set of constant phases is also assigned to illuminating beams to retrieve the correlation terms during post processing. The recorded images are processed further to obtain the super resolved image.