Computer generated holography

Computer Generated Holography (CGH) is the method of digitally generating holographic interference patterns. A holographic image can be generated e.g. by digitally computing a holographic interference pattern and printing it onto a mask or film for subsequent illumination by suitable coherent light source. Alternatively, the holographic image can be brought to life by a holographic 3D display (a display which operates on the basis of interference of coherent light), bypassing the need of having to fabricate a "hardcopy" of the holographic interference pattern each time. Consequently, in recent times the term "computer generated holography" is increasingly being used to denote the whole process chain of synthetically preparing holographic light wavefronts suitable for observation. [Cite
author=Ch. Slinger, C. Cameron, M. Stanley
title =Computer-Generated Holography as a Generic Display Technology
journal=Computer
publisher=IEEE
date=Aug. 2005]

Computer generated holograms have the advantage that the objects which one wants to show do not have to possess any physical reality at all (completely synthetic hologram generation). On the other hand, if holographic data of existing objects is generated optically, but digitally recorded and processed, and brought to display subsequently, this is termed CGH as well. Ultimately, computer generated holography might serve all the roles of current computer generated imagery: holographic computer displays for a wide range of applications from CAD to gaming, holographic video and TV programs, automotive and communication applications (cell phone displays) and many more.

Overview

Holography is a technique originally invented by Hungarian physicist Dennis Gabor (1900-1979) to improve the resolving power on electron microscopes. An object is illuminated with a coherent (usually monochromatic) light beam; the scattered light is brought to interference with a reference beam of the same source, recording the interference pattern. CGH as defined in the introduction has broadly three tasks:
# Computation of the virtual scattered wavefront
# Encoding the wavefront data, preparing it for display
# Reconstruction: Modulating the interference pattern onto a coherent light beam by technological means, to transport it to the user observing the hologram.Note that it is not always justified to make a strict distinction between these steps; however it helps the discussion to structure it in this way.

Wavefront computation

Wavefront calculations are computationally very intensive; even with modern mathematical techniques and high-end computing equipment, real-time computation is tricky. There are many different methods for calculating the interference pattern for a CGH.

Ray tracing method

Ray tracing is perhaps the simplest method of computer generated holography to visualize. Essentially, the path length difference between the distance a virtual "reference beam" and a virtual "object beam" have to travel is calculated; this will give the relative phase of the scattered object beam.

Fourier transform method

In a Fourier Transform hologram the reconstruction of the image occurs in the far field. This is usually achieved by using the Fourier transforming properties of a positive lens for reconstruction. So there are two steps in this process: computing the light field in the far observer plane, and then Fourier transforming this field back to the lens plane.Instead of the Fourier transform, one might also utilize the Fresnel transform to obtain near field holograms.

Interference pattern encoding

Once it is known how the scattered wavefront of the object looks like or how it may be computed, it must be fixed on a spatial light modulator (SLM), abusing this term to include not only LCD displays or similar devices, but also films and masks. Basically, there are different types of SLMs available: Pure phase modulators (retarding the illuminating wave), pure amplitude modulators (blocking the illumination light), and SLMs which have the capability of combined phase/amplitude modulation [Cite book
author=W. Lauterborn, T. Kurz
title=Coherent Optics
publisher=Springer
edition=2nd edition
date=2002] .

In the case of pure phase or amplitude modulation, clearly quality losses are unavoidable. Early forms of pure amplitude holograms were simply printed in black and white, meaning that the amplitude had to be encoded with one bit of depth only [Cite journal
author=B. R. Brown, A. W. Lohmann
title=Complex spatial filtering with binary masks
journal=Appl. Opt.
volume=5
pages=967ff
date=1966
publisher=OSA
url=http://www.opticsinfobase.org/abstract.cfm?URI=ao-5-6-967] .Similarly, the kinoform is a pure-phase encoding invented at IBM in the early days of CGH [Cite journal
author=L. B. Lesem, P. M. Hirsch, J. A. Jordan, Jr.
title=The Kinoform: A New Wavefront Reconstruction Device
journal=Journal of Research and Development
volume=13
number=2
pages=150
date=1969
publisher=IBM
url=http://domino.watson.ibm.com/tchjr/journalindex.nsf/0b9bc46ed06cbac1852565e6006fe1a0/ecd8ca89a481416c85256bfa00684019?OpenDocument] .Even if a fully complex phase/amplitude modulation would be ideal, a pure phase or pure amplitude solution is normally preferred because it is much easier to implement technologically.

Reconstruction

The third (technical) issue is beam modulation and actual wavefront reconstruction. Masks may be printed, resulting often in a grained pattern structure since most printers can make only dots (although very small ones). Films may be developed by laser exposure. Holographic displays are currently yet a challenge (as of 2008), although successful prototypes have been built. An ideal display for computer generated holograms would consist of pixels smaller than a wavelength of light with adjustable phase and brightness. Such displays have been called phased array optics [cite book
last = Wowk B
authorlink = Brian Wowk
chapter = Phased Array Optics
title = Molecular Speculations on Global Abundance
editor = BC Crandall
publisher = MIT Press
date= 1996
pages = 147-160
isbn = 0262032376
url = http://www.phased-array.com/1996-Book-Chapter.html
accessdate = 2007-02-18] . Further progress in nanotechnology is required to build them.

Available CGH devices

Currently, several companies and university departments are researching on the field of CGH devices:

* [http://alumni.media.mit.edu/~lucente/holo/holovideo.html MIT Media Lab] has developed the "Holovideo" CGH display
* SeeReal Technologies have prototyped a CGH display

References