Holography

Holography (from the Greek, Όλος-holos whole + γραφή-graphe writing) is the science of producing holograms, an advanced form of photography that allows an image to be recorded in three dimensions. The technique of holography can also be used to optically store and retrieve information. Holograms are common in science-fiction, most notably Star Trek, Star Wars, and Red Dwarf.

Real-Time Holography

The discussion above describes "conventional" holography, that is, the steps of recording, developing and reconstruction are performed independently, or, at different times in sequence. Moreover, the recording medium for conventional holography is typically film, so that the hologram is permanently formed. This static recording notion is, in fact, similar to a photograph, which is also a permanent recording of image information. Thus, once formed, the hologram is "fixed," or, in essence, is unchangeable.

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Beyond conventional holography, there exists a technique whereby all the steps used to form a hologram are performed simultaneously in a material that can be refreshed. That is, the steps of recording, developing and reconstruction all take place at the same time (not sequentially). Moreover, the material used for this novel hologram is not film, but, instead, a material whose properties allow one to, in effect, continuously update the hologram. Thus, in essence, the hologram is dynamic, so that the image information which records the hologram can change, and, hence, the reconstructed output also tracks, or change, simultaneously.

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Such a dynamic hologram is called a "real-time hologram." The material that replaces film must also be capable of changing in response to a varying set of recording beams and input image information. Examples of such materials are referred to as "nonlinear optical materials," and can be realized using a variety of media such as photorefractive crystals, atomic vapors and gases, semiconductors (including "quantum wells"), plasmas and, even liquids. In this case, the local absorption and/or phase in the nonlinear material will be "exposed," and, moreover, will "track" changes in the interference pattern formed by the recording beams. That is, as the interference pattern changes, the local absorption and/or phase pattern in the material will also change and replace the original pattern. Beyond these "passive" materials, the dynamic media can also be in the form of "active" electro-optical devices, such as spatial light modulators (SLMs). In this case, the pixelated image-bearing input port serves as the "dynamic recording material," whereas the pixelated output of the device (e.g., the output display, or projection port) functions as the effective holographic reconstruction port. Currently, SLMs involve the use of liquid crystal layers as well as micro-electrical mechanical (MEMS) technologies as the pixelated image-bearing output (projection) port. The pattern imposed onto the input port of the SLM will give rise to a corresponding output pattern, as read out by the reconstruction beam. By virtue of the SLM, the output, or reconstruction, beam will be spatially encoded as a corresponding amplitude, phase or polarization pixelated mapping of the input image.

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The speed, or, frame-rate, of such real-time media - that is, the number of independent holograms that can be formed, erased, updated and reconstructed by this process - can be in the range of many seconds to picoseconds of faster. In the case of high-definition (about one million resolvable pixels) high-speed video-rate information (about 1 msec frame rate), this implies an effective optical processing rate of a gigahertz (GHz). In the case of an advanced spatial light modulator (with a frame-rate in the microsecond range), the effective computational rate of a real-time holographic processor can exceed a terahertz (THz).

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In the jargon of nonlinear optics, this operation that involves the simultaneous recording and reconstruction of a hologram in a material is referred to as "degenerate four-wave mixing" ("DFWM"). This follows, since there are four optical beams that interact to form the real-time hologram: a pair of recording beams, a readout beam, and, the resultant output, or reconstructed beam. The search for novel nonlinear optical materials for real-time holography is a very active area of present-day research.

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Potential applications of such real-time holograms include phase-conjugate mirrors ("time-reversal" of light), optical cache memories, image processing (pattern recognition of time-varying images), and optical computing, among others (see below). As an example, in an outdoor laser communication system across an atmospheric path, one must compensate for atmospheric turbulence (the phenomenon that gives rise to the "twinkling" of starlight as well as beam wander) to enable a high-quality optical channel to exist. That is, without such atmospheric compensation, the optical receiver at the end of the link cannot distinguish between useful data transmission (such as modulation of the laser beam) and that of the random "twinkling" of the laser beam as it propagates from the sender to the receiver. By using a real-time hologram to form a "phase-conjugate" mirror at one or both ends of the link, the effects of atmospheric turbulence can be "undone" ("untwinkling the starlight"), resulting in an optical channel without random noise. Hence, the optical link, even across an atmospheric path, will behave as if the link is established in the vacuum of space, where the stars do not twinkle. In one example, a phase-conjugate mirror with a modulation capability at one end of the optical link, can be used to simultaneously compensate for propagation distortions and encode information (data) to be beamed to the other end of the link. This device is referred to as a "retro-modulator."

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References:

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1. Scientific American, December 1985, "Phase Conjugation," by Vladimir Shkunov and Boris Zel'dovich.

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2. Scientific American, January 1986, "Applications of Optical Phase Conjugation," by David M. Pepper.

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3. Scientific American, March 1987, "Optical Neural Computers," by Demetri Psaltis and Yaser S. Abu-Mostafa.

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4. Scientific American, October 1990, "The Photorefractive Effect," by David M. Pepper, Jack Feinberg, and Nicolai V. Kukhtarev.

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5. Scientific American, November 1995, "Holographic Memories," by Demetri Psaltis and Fai Mok.

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Digital holography

An alternate method to record holograms is to use a digital device like a CCD camera instead of a conventional photographic film. This approach is often called digital holography. In this case, the reconstruction process can be carried out by digital processing of the recorded hologram by a standard computer. A 3D image of the object can later be visualized on the computer screen.

Related Topics:
CCD - Computer

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