Holography

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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.

Overview

Holography was invented in 1948 by Hungarian physicist Dennis Gabor (1900-1979), for which he received the Nobel Prize in physics in 1971. He received patent GB685286 on the invention. The discovery was an unexpected result of research into improving electron microscopes at the British Thomson-Houston Company in Rugby, England, but the field did not really advance until the invention of the laser in 1960.

The first holograms which recorded 3D objects were made by Emmett Leith and Juris Upatnieks in Michigan, USA in 1963 and by Yuri Denisyuk in the Soviet Union.

Several types of holograms have been made. The very first holograms were "transmission holograms", which were viewed by shining laser light through them. A later refinement, the "rainbow transmission" hologram allowed viewing by white light and is commonly seen today on credit cards as a security feature and on product packaging. These versions of the rainbow transmission holograms incorporate a reflective foil backing which provides the light from "behind" to reconstruct their imagery. Another kind of common hologram is the true "white-light reflection hologram" which is made in such a way that the image is reconstructed naturally using light on the same side of the hologram as the viewer.

One of the most promising recent advances in the short history of holography has been the mass production of low-cost solid-state lasers - typically used by the millions in DVD recorders and other applications, but sometimes also useful for holography. These cheap, compact, solid-state lasers can compete well with the large, expensive gas lasers previously required to make holograms, and are already helping to make holography much more accessible to low-budget researchers, artists, and dedicated hobbyists.

Technical description

The difference between holography and photography is best understood by considering what a Black & White (B&W) photograph actually is: it is a point-to-point recording of the intensity of light rays that make up an image. Each point on the photograph records just one thing, the intensity (i.e. the square of the amplitude of the electric field) of the light wave that illuminates that particular point. In the case of a colour photograph, slightly more information is recorded (in effect the image is recorded three times viewed through three different colour filters), which allows a limited reconstruction of the wavelength of the light, and thus its colour.

However, the light which makes up a real scene is not only specified by its amplitude and wavelength, but also by its phase. In a photograph, the phase of the light from the original scene is lost. In a hologram, both the amplitude and the phase of the light (usually at one particular wavelength) are recorded. When reconstructed, the resulting light field is identical to that which emanated from the original scene, giving a perfect three-dimensional image (albeit, in most cases, a monochromatic one, though colour holograms are possible).

File:Holography-record.png

Holographic recording process

To produce a recording of the phase of the light wave at each point in an image, holography uses a reference beam which is combined with the light from the scene or object (the object beam). Optical interference between the reference beam and the object beam, due to the superposition of the light waves, produces a series of intensity fringes that can be recorded on standard photographic film. These fringes form a type of diffraction grating on the film, which is called the hologram.

File:Holography-reconstruct.png

Holographic reconstruction process

Once the film is processed, if illuminated once again with the reference beam, diffraction from the fringe pattern on the film reconstructs the original object beam in both intensity and phase (except for rainbow holograms where the depth information is encoded entirely in the zoneplate angle). Because both the phase and intensity are reproduced, the image appears three-dimensional; the viewer can move their viewpoint and see the image rotate exactly as the original object would.

Because of the need for interference between the reference and object beams, holography typically uses a laser in production. The light from the laser is split into two beams, one forming the reference beam, and one illuminating the object to form the object beam. A laser is used because the coherence of the beams allows interference to take place, although early holograms were made before the invention of the laser, and used other (much less convenient) coherent light sources such as mercury-arc lamps.

The coherence length of the beam determines the maximum depth the image can have. A laser will typically have a coherence length of several meters, ample for a deep hologram. Small pen laser pointers tend to have a smaller coherence length and were considered too small to do holography. That has been shown to be incorrect, and people have successfully made small holograms with laser pens. Large analogue holograms cannot be made with laser pens due to their lower power (typically 1mW to 5mW). Digital holography does not suffer from this problem.

Real-Time Holography

Template:Sectionclean The discussion above describes "conventional" holography, whereas the steps of recording, developing and reconstruction are performed independently or at different times in sequence. Also, 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 unchangable.


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 a material with properties which allow continuous updating of the hologram making the hologram dynamic so that the image information which records the hologram can change and the reconstructed output can also track, or change, simultaneously.


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 will "track" changes in the interference pattern formed by the recording beams. 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.


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).


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.


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."


References: 1. Scientific American, December 1985, "Phase Conjugation," by Vladimir Shkunov and Boris Zel'dovich. 2. Scientific American, January 1986, "Applications of Optical Phase Conjugation," by David M. Pepper. 3. Scientific American, March 1987, "Optical Neural Computers," by Demetri Psaltis and Yaser S. Abu-Mostafa. 4. Scientific American, October 1990, "The Photorefractive Effect," by David M. Pepper, Jack Feinberg, and Nicolai V. Kukhtarev. 5. Scientific American, November 1995, "Holographic Memories," by Demetri Psaltis and Fai Mok.

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.

Holography in art

Salvador Dalí claims to have been the first to employ holography artistically. He was certainly the first and most notorious Catalan surrealist to do so, but the 1972 New York exhibit of Dalí holograms had been preceded by "the first holographic art exhibition [which] was held at the Cranbrook Academy of Art in Michigan in 1968. The second took place at the Finch College gallery in New York in 1970 and attracted national media attention." (source: http://www.holophile.com/history.htm ). (A vastly entertaining account of a 1973 Dalí holography project (with Alice Cooper as subject) can be found at: http://www.alicecoopertrivia.pwp.blueyonder.co.uk/people/p-dali.php)

An interesting new development is the holography of Yves Gentet. Using a new emulsion, the quality of the holograms has increased dramatically. Examples of it can be seen at [1]. The artistic possibilities are evident, however the technology is new and still mostly undiscovered by the art world.

Holographic data storage

See the main article at holographic memory.

Holography can be applied to a variety of uses other than recording images. Holographic data storage is a technique that can store information at high density inside crystals (à la HAL 9000) or photopolymers. As current storage techniques such as DVD reach the upper limit of possible data density (due to the diffraction limited size of the writing beams), holographic storage has the potential to become the next generation of storage media. The advantage of this type of data storage is that the volume of the recording media is used instead of just the surface.

Using currently available SLM's can produce about 1000 different images a second at 1024 X 1024 bit resolution. With the right type of media (probably polymers rather than something like LiNbO3), this would result in about 1 Gigabit per second writing speed. Read speeds can surpass this and experts believe 1 Terabit per second readout is possible.

In 2005, the company Optware has produced a 120 mm disc that uses holographic surface to store data to a possible 1TB (terabyte). See Holographic Versatile Disc, for more information.


Other applications of holograms include metrology and optical computing.

External links

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