4. MEMPLEX-HDXTM
An Advanced Holographic Material

MEMPLEX-HDXTM
An Advanced Holographic Material

The field of holography was developed in 1947 by Dennis Gabor in an effort to improve resolution in a microscope. Practical holography required the development of a more coherent light source which occurred with the development of the laser in 1960.

To write a hologram, a photosensitive material is used to record the interference pattern made by two interfering laser beams resulting in a material having different refractive indexes. The obvious early choice was to use a photographic emulsion similar to ordinary photographic film. Many display holograms are still made using these types of materials since they are very light sensitive and can be made to large sizes.

Since the emulsion layers tend to be quite thin, it was quite interesting to observe what occurred when light interference patterns were recorded in a "thick" material (a few mm or more). There are primarily two effects: (1) more information can be stored per unit of surface area and (2) recording and reading beam geometries become more narrowly defined.

MEMPLEX-HDXTM was developed in the late 90's by Laser Photonics Technology, Inc. as part of a U.S. Air Force project to develop ways of storing image information in a compact format. MEMPLEX-HDXTM has since been licensed to HT.

In data storage applications, an image is recorded in a volume of material defined by the size of the intersecting laser beams. The same volume of material has the capacity to record many more images, but to be distinguishable from the others, the write (and hence the read) laser beams must be angularly or wavelength different. This multiplexing strategy permits hundreds of images to be stored in the same physical space and read out by carefully adjusting the angles of the read laser beam.

The storage capacity of modern hard drives and the maturity of this technology makes the routine use of holography not very practical.

However, holographic data storage is unique and superior in two respects: (1) information can be read out in parallel and very fast and (2) stored information can be optically interrogated without ever reading the images into a binary format. The inherent speed and "all-optical" image processing make this technology a very powerful tool in high speed information processing.


How Are the Images Stored?

Two interfering laser beams are recorded in MEMPLEX. The complicated patterns of light are recorded much like photographic emulsions operate. Since MEMPLEX is relatively thick (several mm), a large number of images (hundreds) can be recorded in the same physical space using a principle known as "angular multiplexing." This method is illustrated below.
 


 

The laser has its beam split into two equal parts by a beamsplitter (BS1) and the two beams are directed to the MEMPLEX sample. One of the beams passes through a photographic negative (or equivalent) and carries a "shadowgram" of an image (a circle in the first case) and the other beam (reference beam) contains no image. The interference of these beams is permanently recorded in a small volume of the material.

Repositioning M1 permits successive images (star, square) to be written to the same volume of MEMPLEX material. While all these images are recorded in the same physical space, they are said to be "angularly multiplexed."


Retrieving An Image

Stored images are retrieved by illuminating the holograms with one of the writing beams and taking a photograph of the diffracted light - a replica of the stored image. A CCD camera captures the entire image at once - a 1K x1K camera captures a minimum of 1 Mega pixel (no gray scale) of information in one read operation. Retrieving information at this speed clearly outperforms retrieval of data from hard drives and effectively moves the "bottleneck" from the memory system to the CPU.

Optical Correlation Technology

Images can be identified or matched by an optical correlation process that is extremely fast, all-optical, and is accomplished in two steps:

A. Images are recorded in Hybrid Technologies' MEMPLEX material.
B. A "test image is compared to the library of stored images and their "degree" of "sameness" is measured by the strength of a simple optical signal.



How Are The Correlations Performed?

On the left side of the figure below, the reading beam passes through a photographic negative (or equivalent) that imparts the image of the circle. This beam interrogates all the three stored images simultaneously. Some light is "harmlessly" transmitted, but there are three distinct beams of different intensity directed in paths equivalent to the beams used to record each of the three images. These diffracted beam intensities are proportional to how similar the images are.

The simple images illustrated here could be replaced by fingerprints, faces, etc.

Taking Optical Correlation Into Fourier Space
A More Powerful Technique

The ability to optically work in Fourier space permits complicated queries to be made on stored images in real time (with no computational time required). The previous illustrations of optical correlation require that the images be of the same size and orientation. It is possible to function this way with fingerprints and other relatively simple shapes, but is too large an obstacle for many potentially useful applications.

The advantages of this technique can be illustrated in an actual military example:

Reconnaissance images obtained from satellites can provide a wealth of data that is actually beyond the capability of human operators to read and analyze it in a timely fashion. Let us assume than 10,000 images are routinely captured in Iraq targeting key roadways, "hot" sites, regions near military bases, etc. Let us assume that intelligence exists that a cement truck is going to be used to bomb a building in the Baghdad area. The reconnaissance photos are stored in a MEMPLEX-HDXTM and each of these images is queried in Fourier space using the next level of optical correlation technology. A top-view image of a typical cement truck is used as query input.

The correlation signal (a strength of the diffraction intensity) for an image containing a truck will be significantly higher than for one that does not contain a cement truck. Once a short candidate image list is obtained (say 2% of the images that show the strongest correlation signal), the images can be further probed to reject those that provided a false positive result. In the end, there are a small number of images that have been selected for further (human) investigation. The time required to perform this operation (query of 10000 images and developing a list of potential target images) is less than a second due to the inherent parallelism of the proposed optical correlation technology. Correlations with other candidate images (crowds, trucks, etc.) can be used to further gain intelligence and/or confidence.

Summary

HT is the manufacturer of a proprietary and patented material (MEMPLEX) which is at the heart of a technology known as optical correlation.

This technology can analyze images in parallel and virtually instantaneously making it possible to analyze very large databases quickly and efficiently.

There are many opportunities to use this technology in the military and intelligence communities.

HT has teamed together with experts in optical design, pattern recognition, and systems engineering to advance this technology and is looking for the opportunity to develop and demonstrate practical systems.

 

Please let us know how we can assist you in your applications.