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    lectures lectures Document Transcript

    • Optical Holography1-Introduction:In (1948) Dennis Gabor outlind a two steplensless imaging process. It is radically anew technique of photographing the objects and is known as wave frontreconstruction. The technique is also called holography. The word holography isformed by combining parts of two Greek words: holos, meaning "whole", andgraphein meaning "to write". Thus holography means writing the complete image.Holography is actually are cording of interference pattern formed between twobeams of coherent light coming from the same source. In this process both theamplitude and phase components of light wave are recorded on a light sensitivemedium such as a photographic plate. The recording is known as a hologram.Holography required an intense coherent light source. Laser was not availablewhen Gabor formulated the idea of holography. Holographic technique became apractical proposition only after the invention of lasers. Leith and Upatnicksprepared laser holograms for the first time.2-Principle of holography:Holography is a two-step process. First step is the recording of hologram where theobject is transformed into a photographic record and the second step is thereconstruction in which the hologram is transformed into the image. Unlike in theconventional photography, lens is not required in either of the steps. A hologram isthe result of interference occurring between two waves, an object beam which isthe light scattered off the object and a coherent background, the reference beam,which is the light reaching the photographic plate directly. In Gaboss originalexperiments, the reference beam and object beams were coaxial. Further advance 1
    • was made by Leith and Upatnieks, who used the reference beam at an offset angle. That made possible the recording of holograms of three-dimensional objects.2-1-Recording of the hologram:In the off-axis arrangement a broad laser beam is divided into two beams, namely a reference beam and object beam by a beam splitter. The reference beam goes directly to the photographic plate. The second beam of light is directed onto the object to be photographed. Each point of the object scatters the incident light and acts as the source of spherical waves. Part of the light, scattered by the object,travels towards the photographic plate. At the photographic plate the innumerable spherical waves from the object combine with the plane light wave from the reference beam. The sets of light waves are coherent because they are from the same laser. They interfere and from interference fringes on the plane of thephotographic plate. These interference fringes are a series of zone-plate like rings, but these rings are also superimposed, making a complex pattern of lines andswirls. The developed negative of these interference fringe-patterns is a hologram. Thus, the hologram does not contain a distinct image of the object but carries a 2
    • record of both the intensity and the relative phase of the light waves at each point.Figure (1): Hologram recording: the interference pattern produced by the reference wave the object wave is recorded2-2-Reconstruction of the image:Whenever required, the object can be viewed. Forreconstruction of the image, thehologram is illuminated by a parallel beam of light from the laser. Most of the lightpasses straight through, but the complex of line fringes acts as an elaboratediffraction grating. Light is diffracted at a fairly wide angle. The diffracted raysfrom two images: a virtual image and a real image. The virtual image appears at thelocation formerly occupied by the object and is sometimes called as the true image.The real image is formed in front of the hologram. Since the light rays pass throughthe point where the real image is it can be photographed. The virtual image of thehologram is only for viewing. Observer can move to different positions and look 3
    • around the image to the same extent that he would be able to, were he lookingdirectly at the real object. This type of hologram is known as a transmission hologram since the image is seen by looking through it.The original configuration adopted by Gabor for recording hologram was a coaxialarrangement, as illustrated in figure (2). In this arrangement the real image islocated in front of the virtual image and is inconvenient for viewing orphotographing. The advantage of the off-axisconfiguration is that the two images are separate.The fundamental difference between a hologram and anordinary photograph is likethis. In a photograph the information is stored in an orderly fashion: each point inthe object relates to a conjugate point in the image. In a hologram there is no suchrelationship; light from every object point goes to the entire hologram. This hastwo main advantages 1-As the observer movessideways viewing the hologram, the image is seen in three dimensions2-If the hologram were shattered or cut into small pieces, each fragment would still reconstruct the whole object, not just part of the object. 4
    • Figure: (2)Image reconstruction: light diffracted by the hologram reconstructs the object wave.3-Applications:3-1-Holographic optical elementsA hologram can be used to transform an optical wave front in the same manner asa lens. In addition, computer generated holograms can produce a wave fronthaving any arbitrary shape. As a result, holographic optical elements (HOEs) can perform unique functions and have been used in several specialized applications.A major advantage of HOEs over conventional optical elements is that theirfunction is independent of substrate geometry. In addition, since they can beproduced on thin substrates, they are quite light, even for large apertures. Anotheradvantage is that several holograms can be recorded in the same layer, so thatspatially overlapping elements are possible. Finally, HOEs provide the possibility 5
    • of correcting system aberrations in a single element, so that separate corrector elements are not required.The recording material for a HOE must have high resolution, good stability, highdiffraction efficiency and low scattering. Photoresists and dichromated gelatin are,at present, the most widely used materials. Photopolymers are an attractive alternative.3-2-holographic neural networks:Holographic neural networks are attractive because they offer large storagecapacity as well as parallel access and processing capabilities during both the learning and reading phases.In a holographic neural network, neurons are represented by the pixels on a spatiallight modulator. The brightness of a pixel corresponds to the activation level of theneuron. If a pair of pixels are illuminated with a coherent beam, a volumehologram can be formed in a suitable recording material. If, subsequently, one ofthe original two beams is used to address the hologram, the other beam isreconstructed with an efficiency that represents the weight between these neurons.With a photorefractive recording material, a process of learning can be implemented by increasing, or decreasing, the weights selectively.3-3-Acoustic holographyIt is easy to produce coherent sound waves. Sound waves readily propagate insolids. Therefore, a three dimensional acoustical hologram of an opaque object canbe made. By viewing such hologram in visible light the internal structure of theobject can be observed. Such techniques will be highly useful in the fields ofmedicine and technology. In one of the techniques, two submerged coherent sound 6
    • generators emit the reference and the signal, scattered by an object, respectively. On a calm surface of water, these two contributions produce ripples. The ripple pattern is the hologram. The pattern may be photographed and then reconstructed. As sound waves can propagate through dense liquids and solids, acoustical holography has an advantage in locating underwater submarines etc and internal body organs show in figure(3).Figure (3) 7