Optical Database Management

We are working on a novel implementation of ODBM. A system schematic is shown below. The heart of the system is a POHM (Page Oriented Holographic Memory). This optical memory device permits storage of vast quantities of information with parallel recall.Any queries are performed in parallel by an optical correlator system.

The amount of data currently stored in databases is so large, that it is exceeding the ability of conventional computers to perform all but the most basic queries. The inherent parallelism of optics lends itself to very high speed database searching . Spatial Light Modulators (SLMs) limit the system bandwidth to less than the speed of the electronic subsystem, since pixels are addressed in series. Our approach uses optical storage, a POHM, to store and retrieve the information. We can select and retrieve a whole page of information in one system cycle, which is determined by the speed of the Acousto-Optic (AO) cell. This leads to potential data throughput rates of more than 10¹⁰ Bytes/sec. Since we do not alter the search criteria as often as the pages of the database we search, we can use a conventional SLM to introduce filter masks.

The overall system concept is shown in the Figure below. The user enters the query into the computer. The computer then generates the appropriate filter mask (FFT) and loads it into the Optical Pattern Recognizer (OPR). It then proceeds to page through the POHM. The ASIC connected to the CCD array then determines matches. If the query is requires more than one OPR operation, then the page numbers and locations are stored in the cache memory. The Opto-Electronic (OE) RAM is then filled with matches before going on to the next stage of OPR.

The OPR component dictates the overall optical system design. The data are stored in ternary (-1,0,1) encoded form in POHM. A code with a length of 20 bits shown in the figure to the left. It occupies an array of 5x8 array. The Fourier plane filter masks are binary (-1,1). The filters themselves can be generated electronically (by FFT) or by a separate optical FT processor.

The fact that the output of the OERAM is also ternary poses no difficulty since the regions of value 0 are common to all codes. To store the data in the OERAM, we need a source of coherent light to interfere with the imaged (though upside-down and reversed) input page. This allows the detector to be integrated on the same component as the electronic RAM and the SLM, which uses the same beam for readout.

The electronic RAM must be of sufficient size to minimize the number of times the Fourier transform filter will need to be changed. What is sufficient? This is a function of the density of expected "hits"/page and the overall number of pages. There must be a compromise between the amount of RAM and cycling the filters into the FT SLM, since this is to be a general purpose search engine.

The overall optical layout of the system is shown below. The heart of the system is a POHM, addressed by an AO cell. The chosen page then is sent to the Fourier Transform (FT) plane SLM for correlation. After a second FT the correlation signal is detected by a CCD array.

The Application Specific Integrated Circuit (ASIC) then determines which signals are above threshold. The front end computer then addresses, the shift SLM (also in the FT plane). This SLM will select the data lines to be imaged to the OERAM. The OERAM detects them and stores them in electronic format.

In the next stage(s) of the query, these pages will be recalled from the OERAM, again Fourier transformed and correlated against a new filter. The electronic cache keeps track of which lines are to be deleted (electronically) from the OERAM for successive stages of the query.

The FT lenses are of a new six-element design that reduces the length of an optical FT well below 1f. In this architecture these compact FT systems are combined with folded light paths, to obtain a very compact optical system.

Some more information

Download ODBM Manuscript (.pdf)

Download OSA Meeting ODBM Vuegraphs (.pdf)

Download OSA Meeting ODBM Coding Poster (.pdf)

This work was supported by SEA-DOE DE-FG05-94ER25229 and the SSDC FIND Center DASG60-95-2-0001.