We are developing a high density, high speed optical memory, using rare-earth doped hole- burning materials. These materials are theoretically capable of achieving storage densities of 1000 Gigabits/cm3 at input/output (I/O) rates of several gigabits/sec. One remarkable attribute of this storage concept is that both the temporal and spatial information encoded on a laser beam can be stored. Because both the temporal and spatial information can be stored, digital data can be recorded serially as a data or packet stream as well as holographically using the page format. During the past year we have achieved a breakthrough in demonstrating random access holographic data storage at high frame rates. Five hundred holograms were stored and retrieved with very good fidelity at 30 Hz (video rate). Each holographic image with 512 multiplied by 488 pixels could be randomly accessed during the storage and retrieval process. This frame rate is the highest demonstrated frame rate for any optical technique. This breakthrough was achieved through the invention of a memory architecture that allows multiple holographic frames to be stored without any mechanical beam scanning. In this new architecture multiplexing of 500 holograms was achieved by stepping the laser frequency over a range covered by high speed acousto-optic modulators (AOM).
We are developing a high density, high speed optical memory, using rare-earth doped hole- burning materials. These materials are theoretically capable of achieving storage densities of 1000 Gigabits/cm3 at input/output (I/O) rates of several gigabits/sec. One remarkable attribute of this storage concept is that both the temporal and spatial information encoded on a laser beam can be stored. Because both the temporal and spatial information can be stored, digital data can be recorded serially as a data or packet stream as well as holographically using the page format. During the past year we have achieved a breakthrough in demonstrating random access holographic data storage at high frame rates. Five hundred holograms were stored and retrieved with very good fidelity at 30 Hz (video rate). Each holographic image with 512 multiplied by 488 pixels could be randomly accessed during the storage and retrieval process. This frame rate is the highest demonstrated frame rate for any optical technique. This breakthrough was achieved through the invention of a memory architecture that allows multiple holographic frames to be stored without any mechanical beam scanning. In this new architecture multiplexing of 500 holograms was achieved by stepping the laser frequency over a range covered by high speed acousto-optic modulators (AOM).
One of the unique and perhaps the most attractive features of an optical memory is its ability to store and retrieve information in a bit-parallel format.1-3 Recently, we have proposed a new scheme for parallel data storage in TDOM.4,5 This scheme was proven to be practical and can be implemented with existing technologies. In a proof-of-principle experiment, four wavelength-multiplexed single-page volume spectral holograms, generated with black-and-white transparencies, were successfully stored in a single spatial location at a speed of approximately 23 μs/frame. The experimental results project system's input/output bandwidth to exceed 40 Gbps, two orders of magnitude faster than a semiconductor cache memory.
The objective of this research was to quantitatively evaluate the concept of time-domain optical memory (TDOM) using the hole-burning mechanism. The TDOM concept offers both high speed and high memory density. The advantage of TDOM over traditional semiconductor memory is its inherent parallelism, which allows data to be stored and retrieved from the memory at much higher speed. The practical success of the proposed memory, therefore, depends critically on-among other issues-the storage, retrieval, and processing of images. In addition, new data storage techniques are needed to implement a practical memory. The last three years of this research program have focused on (1) development of new phase modulation techniques to enhance the storage, retrieval, and integrity of the stored information; (2) experimental studies of image storage and phase conjugate retrieval; (3) Image processing; and (4) practical holographic storage of images by using the hybrid time-frequency approach. Specifically, SRI demonstrated data encryption by phase modulating the read and write pulses. Image correlation and convolution was demonstrated and its implication for pattern recognition examined. Finally, this laboratory demonstrated high-fidelity random access and retrieval using holographic storage in Pr3+: YA103 with a single-frequency ring laser. These experiments have clearly demonstrated the practical viability ofmore » TDOM with existing technology. In the next stage of the research, the performance Issues such as bit-error-rates, signal-to-noise ratios, and laser stability need to be addressed. Stimulated photon echoes, Optical memory, Image processing, Pattern recognition, Phase conjugation.« less
We describe a new approach to develop a high-speed, high-density random access cache memory. This new technique is based on a novel approach using, in essence, a hybrid of the time domain stimulated echo (SE) concept and the frequency domain scheme. It has been demonstrated that the rare-earth doped crystals, can at low temperatures store large amounts of information for up to 24 hours using SE. The basic approach we have developed is the partitioning of the absorption frequency domain into smaller bins, so that each frequency bin stores a smaller portion of information independently. The bins are distinguishable by their different absorption frequencies and they are accessed by changing the laser frequency (color). However, information is still stored and retrieved using the time-domain pulse sequency used in SE. Specifically, with the laser frequency set at the particular absorption of a frequency bin, the laser is externally modulated by an acousto-optic modulator to produce amplitude modulated pulses representing the digital data, the write pulse, and the read pulse, during the write and retrieve operation, respectively. The advantages of this approach are that memory can be stored in space, time, and frequency domain, allowing us to tailor a flexible memory architecture to match those of the computational processor.
An optical storage technique based on time-domain holography for the rapid recording and readout of page-formatted digital data is demonstrated. Storage of 356 kilobits of data was achieved at a single spatial location in a rare-earth–doped crystal. The digital data were recorded and accurately retrieved at a peak rate of 300 megabits per second without the use of error-correcting codes. The system's raw bit error rate is about 10 − 7 . This low bit error rate was achieved by a detection scheme for extraction of binary data. These results have implications for dynamic optical memory.
Optical data storage has been interested in several decades because of the potential of fast access time and high-density storage. Recently volume holographic data storage technique has drawn much interest as new materials and improved techniques have developed.1 However, the large of hologram storage is restricted by low diffraction efficiency and mainly focused to read only memory because of the material's characteristic difficult to rewrite in short time interval. Another optical memory technique (photon echo optical memory)2 based on time-domain has developed since 1980's and made big progress for the area of real time image processing and dynamic random access memory. In this technique, data pulses interfere with timely separated write pulse (reference), and their interference pattern is written as population gratings. A read pulse (reference) retrieves each data pulse information as a four-wave mixing process. The write and read pulse separations are restricted by phase and population decay times of the material, respectively, and storage density is determined by the ratio of inhomogeneous to homogeneous linewidths. Even though photon echo technique has advantages of fast access time (≥ ns) and high-density (≥ Mbits/laser spot), its operating temperature is kept extremely low around liquid helium temperature. This is because that homogeneous decay time rapidly shortens as temperature increases due to phonon interactions, i.e., the ratio of inhomogeneous to homogeneous decay rates is ~ 1 at liquid nitrogen temperature (77 K) in a rare-earth doped solid, so that restricts its applications.
One of the most attractive features of an optical memory is its ability to write and read data in a bit-parallel format, giving rise to theoretically very high (in excess of 1 Gbps) data transfer rates. However, such a potential has not been demonstrated experimentally because of various inherent technical difficulties associated with existing optical storage techniques. In a photorefractive or a persistent spectral hole-burning (PSHB) memory, for example, the time required to record one page varies from a fraction of a second to several seconds.1-3 For a page containing 1000 × 1000 bits of data, it translates to a bandwidth of approximately 1 Mbps, substantially slower than any existing semiconductor memories.