An improved visual display system for a helmet produces two- and three-dimensional images by use of the limited domain of spatial light modulation (SLM) values. One especially notable improvement incorporated into the design of this system is a simplification in the design of dynamic holographic optical element (HOE) switches. This system can be expected to afford immediate and calculable benefits to the United States space program; similar benefits for the military and private industry are also anticipated.

This system generates small volumes of three-dimensional images, satisfies the size constraints of space-related applications, and offers significant improvements over pre-existing holographic helmet displays. Visual display systems that render two- and three-dimensional images for space applications offer a major advantage over pre-existing systems: it is possible to develop distinct and specifiable phase relationships between the elements of either two- or three-dimensional arrays. As an extension of the work that produced the present system, a more efficient means of storing holographic images was created. This system is a major improvement over prior means for storing holographic images, in that it significantly refines holographic helmet displays and the information that can be generated from them.

Holographic images are used most often in helmet visual displays. In the design of the present system, the required optical interconnections are made when SLMs direct coherent light beams into the light patterns needed for applications. In this way, optical reciprocity, which enables a single receiver to obtain light from a reprogrammable array of source locations, is achieved. Optical reciprocity takes advantage of the complex nature of SLMs and enables the use of continuously variable modulators with coupled phase and amplitude characteristics. One application involves focusing of light from a single source placed at arbitrary locations in a three-dimensional array, enabling spatial addressing of nonlinear optical storage media; a second application involves the creation of two-dimensional arrays of spots for optical interconnections.

In broadband communications, signals with very large bandwidths are often transported on light carriers. The connections between sources and destinations must be reconfigurable so that light can be distributed to the intended destinations. Although a great deal of work has already been done on switching light signals, the utility of SLMs as switches has been limited by conflicts among requirements and capabilities that pertain to speed and complex values: A fast SLM is often restricted to a binary set rather than a continuum; a continuously variable SLM is restricted to, at most, the curvilinear subset of a complex unit disk. In the design of the present system, the limited domain of SLM values is maximized, creating a dynamic HOE switch with a design simpler than that of any such switch constructed previously.

To simplify the switch design, the researcher who developed the present system first had to simplify some computation-intensive annealing techniques. He devised an innovative method of optimizing a metric, the value of which is related monotonically to a measure of the quality with which an HOE performs an optical switch function. By doing this, he discovered that HOEs can be computed for the curvilinear continuum of complex values typical of readily available SLMs, but this was only a single improvement. He also examined conventional HOE computation techniques to discover ways of improving these as well.

Since conventional HOE computation techniques are often iterative, problems can arise. To ameliorate any problems, the researcher developed a means of directing light from a single source to many receivers. This made it possible to specify and optimally realize the intensity and phase of light sent to receivers. The receivers can be detectors that convert light into other forms (electrical signals or photographs), locations in a photo-addressed nonlinear optical information storage device, or ducts that can transmit more light.

The optical setup devised for the receivers is simple: An SLM is disposed in a beam of light, where its complex characteristics are used to change an arriving light wave into an approximation of an ideal departing light wave. The single departing light wave is separated into three light waves that converge on different locations that need not be coplanar. The incoming beam of light is directed to a pattern of three spots. A collimating lens is not necessary but can be used if convenient.

After simplifying the setup, the researcher next determined the optimum set of values of transmittance. For a given value of k, the ideal values of the light-wave phasor were optically represented on a curvilinear SLM domain by the Euclidean closest values. (For this purpose, the SLM domain was represented as a curvilinear continuum; it was not restricted to a binary set.) Next for a given value of k, the ideal values of the light-wave phasor were represented on a discrete SLM domain by the Euclidean closest values. Although the SLM is a ternary (three-valued) domain, the ideal values are the same as the nonrestricted binary values.

This work was done by Richard D. Juday of Johnson Space Center. This invention has been patented by NASA (U.S. Patent No. 5,768,242). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

the Patent Counsel
Johnson Space Center
(281) 483-0837.

Refer to MSC-22746.

Photonics Tech Briefs Magazine

This article first appeared in the February, 2001 issue of Photonics Tech Briefs Magazine.

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