Optical Memory System Research Articles

Error and flow control in terabit intelligent optical backplanes

The design of a free-space optical backplane which supports error and flow control functions is described. Traditionally, these functions are implemented in custom high-speed electronic application specific integrated circuits, which are physically removed from the optical interconnect layer. In this paper, we consider migrating these functions directly into the optoelectronic layer, yielding an "intelligent optical backplane." Conventional error control protocols are infeasible with optical backplanes since they require excessive amounts of hardware. The design of an efficient error control protocol based upon a multidimensional parity check, along with the effective flow control protocol is proposed and analyzed. The key blocks of the protocol have been implemented in 0.8- and 0.5-/spl mu/m CMOS/SEED devices and are summarized. The protocols require significantly less hardware than alternative schemes, and smart pixel arrays supporting these protocols are scalable to higher bandwidths and lower latencies. A very large scale integration analysis indicates that using 2004 technology, a free-space backplane can potentially be clocked at 1 GHz and support 24 Tb/s of bandwidth. Finally, the proposed error control protocols should be useful in optical disks and holographic memory systems, which also perform error control on large two-dimensional arrays of optical bits.

Efficient photon echoes in optically thick media

We present the results of analytical and numerical calculations of the amplitude and temporal shape of photon echoes produced in optically thick absorbing media. In particular, we are interested in echo output signals that mimic the temporal shape of one of the input pulses (the data pulse). Echo recall efficiencies of temporally structured data pulses exceeding unity can be obtained in optically thick media while preserving the information content of the data pulse. We find that the relationship between the maximum echo efficiency and the Beer's length at which that maximum is attained is nearly linear. This linear relation holds well beyond the optimal absorption length predicted by linear theory for both two-pulse photon echoes and three-pulse stimulated photon echoes. These results have significance in the development of optical coherent transient memory and processing systems, where previous calculations predicted recall efficiencies of only a few percent.

Selection of electronic equipment for digital X-ray diagnostic scanners

1. For the purpose of mass-scale examination of lungs with scanning digital photoroentgenographs based on the roentgenologist's AWP, 17″ monitors are most suitable. Appropriate quality of paper copies of X-ray images can be attained using HP laser printers with resolution of 600 dpi. 2. Optical and magneto-optical memory systems are more appropriate for the purpose of storage of diagnostic information. Selection of the most appropriate memory device is determined by the type of the problem to be solved and by economic factors.

Requirements and constraints for the design of smart photodetector arrays for page-oriented optical memories

We discuss the feasibility of smart photodetector arrays (SPDAs) as the optoelectronic interface in page-oriented optical memory systems. In contrast to charge-coupled device arrays, SPDA's can combine fast parallel access with some processing capabilities, such as data decoding and error control. We develop a framework for SPDA design by defining their operational requirements and by delineating their scalability constraints with respect to pixel complexity, electrical power dissipation, and optical power limits. In the case of volume holographic memories, the low-diffraction efficiency of photorefractive crystals will ultimately determine page size, while for other storage technologies the primary constraint will be the circuit complexity. We also present our implementation of an experimental SPDA designed to perform parallel error correction on an entire page of data and satisfy the sustained data rates of parallel optical memories. Our prototype, fabricated in 0.35-/spl mu/m complementary metal-oxide-semiconductor, integrates a differential photoreceiver with a cluster error correction code capable of achieving corrected output data rates scalable to hundreds of gigabits-per-second in the current technology.

Multiplexing optical systems: Multicolor‐bifluorescent‐biredox photochromic mixtures

Optical memory systems that exhibit multiplexity and multifunctionality may allow high‐density storage and multiple addressing and readout. In a first step towards such systems, it is shown that two‐component mixtures of dithienylethene compounds (e.g., see Figure) can be switched reversibly between four different color states, using light as a trigger. The fluorescence of redox behavior increases the multiplexity (see front cover). magnified image

Parallel Error Correction using Spectral Reed-Solomon Codes

Parallel error correction is discussed for highly parallel and/or high speed serial optical memory and communication systems. Spectral Reed-Solomon decoders based on the Berlekamp Algorithm offer efficient parallel implementations. Parallel spectral decoders are compared with conventional serial decoders in terms of VLSI area, electrical power dissipation and maximum aggregate information rate. Design details of a (15,9) parallel spectral Reed-Solomon decoder are presented along with simulation and test results.

Optoelectronic-cache memory system architecture

We present an investigation of the architecture of an optoelectronic cache that can integrate terabit optical memories with the electronic caches associated with high-performance uniprocessors and multiprocessors. The use of optoelectronic-cache memories enables these terabit technologies to provide transparently low-latency secondary memory with frame sizes comparable with disk pages but with latencies that approach those of electronic secondary-cache memories. This enables the implementation of terabit memories with effective access times comparable with the cycle times of current microprocessors. The cache design is based on the use of a smart-pixel array and combines parallel free-space optical input-output to-and-from optical memory with conventional electronic communication to the processor caches. This cache and the optical memory system to which it will interface provide a large random-access memory space that has a lower overall latency than that of magnetic disks and disk arrays. In addition, as a consequence of the high-bandwidth parallel input-output capabilities of optical memories, fault service times for the optoelectronic cache are substantially less than those currently achievable with any rotational media.

Comparison of recording densities in three-dimensional optical storage systems: multilayered bit recording versus angularly multiplexed holographic recording

We compare the geometrical limits of the recording density that can be attained in three-dimensional optical memory systems that employ the multilayered bit-recording method and the angularly multiplexed holographic recording method. Both recording methods have the potential to overcome the recording density limitations in current optical storage systems. Using the Ewald sphere construction, we analyze the lateral and longitudinal bandwidths for each recording method. The respective recording densities of the two methods are also derived directly in the space domain and compared with each other. With the bit-recording method we found that the memory density increases inversely with the f-number of the recording lens. On the other hand, the density that can be achieved using holographic recording first increases with the f-number values, attains a maximum, and then decreases at larger f-number values. This implies that the bit-recording method yields larger memory densities when lenses of smaller f-numbers are used in the optical system.

Bacteriorhodopsin development approaching commercial goals

How has “bacteriorhodopsin” development been progressing over the last year? The protein is definitely proving itself worthy of serious consideration for use in miniaturized, three dimensional optical memory devices and systems, but realization of commercial 3-D holographic memories is still about three years away. On the nearer horizon, this promising bioelectronic medium could well become the material of choice for a number of non-destructive testing (NDT) applications using holographic interferometry as the fundamental operating principle. If all goes as the most aggressive investigators predict, real bacteriorhodopsin-based NDT systems could be commercially available in the next 2 years.

A Dual‐Mode Molecular Switching Device: Bisphenolic Diarylethenes with Integrated Photochromic and Electrochromic Properties

AbstractThe bisphenolic dithienylethene molecules 1a and 1b were synthesized in overall yields of 45% from 4‐bromoanisole and 44% from 2.6‐di‐tert‐butyl‐4‐iodophenol, respectively. The corresponding extended quinones 3a and 3b were also prepared. Photochemical studies showed that compounds 1 are photochromic; the open forms 1 could be converted with UV light of 312nm to the closed coloured forms 2 with photostationary states lying at essentially complete conversion (> 98%). The 1a‐2a system was found to exhibit good resistance to photofatigue and thermal stability for both photoisomers. Cyclic voltammetry studies involving the 2/3 couples showed that whereas 2b undergoes irreversible oxidation at + 0.85 V (vs. SCE in THF), the hydroquinone 2a is reversibly oxidized at an E1/2 of + 0.72V (in MeCN, quasi‐reversibly in THF at + 0.81 V); this reflects the differences in deprotonation behaviour of the generated QH2/2+ species. The large difference in oxidation potential between 1a and 2a allows the photochemical switching of redox properties. In a complementary fashion, redox switching of the photochromic properties within the 2a‐3a pair is possible since 3a is stable to visible light. Owing to this unique behaviour, the triad consisting of 1‐3a represents a novel molecular device with mutually regulating photo‐ and electrochromic behaviour. In addition, the ability to interconvert between the three stable states makes the system well‐suited as the basis for an optical memory system with multiple storage and nondestructive readout capacity through a write‐lock‐read‐unlock‐erase cycle.

Optical parallel-access shared memory system: analysis and experimental demonstration

An optical implementation of a parallel-access shared memory uses a single photorefractive crystal and can realize the set of memory modules in a digital shared memory computer. This implementation uses two arrays of sources that are individually coherent but mutually incoherent from region to region across each array, and it permits incoherent/coherent double angular multiplexing of data in the crystal. A complete instruction set for its memory access consists of four operations, READ, WRITE, SELECTIVE ERASE, and REFRESH, which can be applied to any memory module independent of (and parallel with) instructions to the other memory modules. In addition, a memory module can execute a sequence of READ operations simultaneously with the execution of a WRITE operation to accommodate differences efficiently in optical recording and readout times common to optical volume storage media. An experimental shared memory system demonstrates two memory modules, each consisting of up to two 5-bit data blocks, implemented in a single Fe:LiNbO(3) crystal. The projected performance of the optical parallel-access shared memory system is analyzed and compared with conventional page-addressed volume holographic memories.

Photon-echo amplification by an external-cavity amplifier

We demonstrate as much as 2.5 × 10(5) amplification of a photon-echo signal using a simple external-cavity amplifier. The method may be useful in regeneration of photon echo and memory refresh in a dynamic photon-echo optical memory system. We propose an image storage system that is capable of regeneration and amplification of phase-conjugate images in a feedback loop.

APPLICATIONS OF PHOTOREFRACTIVE VOLUME HOLOGRAPHY IN OPTICAL COMPUTING

After a brief review of the photorefractive effect, we describe three examples of the applications in optical computing using photorefractive volume holography. These are 3-dimensional optical storage, parallel optical matrix processor, and multiwavelength optical half-adder. For the 3-dimensional optical storage, we consider some fundamental issues in photorefractive memory, including the storage capacity and the dynamic range. We also discuss the photorefractive noise gratings formed during a multiple exposure schedule. These fundamental issues in photorefractive storage are important in understanding the potentials and limitations of optical memory systems. For parallel optical matrix processing, we propose and demonstrate a novel method of matrix-matrix application using grating degeneracy in photorefractive media in conjunction with incoherent laser source arrays. For multiwavelength optical half-adder, we describe photorefractive logic gates and their application in the summation of binary numbers.

Photochromism: Non-Linear Picosecond Kinetics and 3D Computer Memory

Abstract Spectroscopic and kinetic studies have made it possible to observe and assign all of the transient species and determine the formation and decay rates of some photochromic spiropyrans and naphthacenequinones. The non linear properties of these materials were utilized to design and operate a 3D optical memory system.

Surface Modifications on Charge-Transfer Complexes Using Scanning Probe Microscopy

ABSTRACTWe are exploring high density information storage systems based on organometallic materials. Silver and copper salts of tetracyanoquinodimethane (TCNQ) and its derivatives exhibit an electrical and optical field induced reversible switching phenomenon. We have demonstrated a field-induced, charge-transfer reaction driven by the electric field at the STM tip when the field generated by the STM exceeds the switching threshold of the organic charge-transfer complex. The phase transition induced by the STM tip appears as nanometer-sized domains on the metal-TCNQ and derivatives surface. We also have shown this phase transition occur by means of optical laser irradiation. This paper discusses our plans to combine our research results in optical switching with the scanning near-field optical microscope (NSOM) to develop a very high density optical memory system. In order toassess the feasibility of this, we performed a series of experiments aimed at determining the limitations of information storage using this class of organic charge transfer complexes.

Second Harmonic Generation of Bacteriorhodopsin and its Application for Three-Dimensional Optical Memory

ABSTRACTA novel three-dimensional optical memory system based on a light transducing protein, bacteriorhodopsin, is investigated. The system uses the nonlinear optical properties of bacteriorhodopsin to accomplish reading and writing operations. A nondestructive method of reading information in three-dimensional optical memory that uses second harmonic generation is demonstrated. This method has the advantage of fast speed, is nondestructive, and has the potential for parallel access.

High-Power, 790 nm, Eight-Beam AlGaAs Laser Array with a Monitoring Photodiode

A high-power, 790 nm, eight-beam individually addressable AlGaAs laser array on 50 µm centers with a monitoring photodiode has been developed for high-speed data transfer in optical memory systems. The maximum output power of each element is over 80 mW. When eight elements are simultaneously operated at 20 mW in APC (Automatic Power Control) mode using the one-beam monitoring control, the total light output variation is less than 7% at a heatsink temperature between 10 and 50°C.

Optical sheet memory system

AbstractThis paper proposes a system for information storage and delivery. It is based on the new concept where the information corresponding to 10 still pictures and approximately 3 min of sound are encoded digitally and printed on a postcard as a dot pattern to be read and restored. The system is called an optical postcard system. In this study, after the basic technologies for the realization of this system are investigated, the printed optical postcard and the reproducing system are constructed experimentally. As a result of there production experiment, the information recorded on the postcard is read and the original picture and sound signal are played back. This indicates that the information can be delivered by optical postcard. The proposed idea of an information storage/delivery system can be applied not only to postcard‐sized sheet, but also to sheets of various sizes. Also, there can be various applications which differ from the use of the traditional information media, e.g., the the use of the traditional information media, e.g., the zine.

High density recording by superresolution in an optical disk memory system

High density optical disk recording using an optical superresolution technique is proposed. By setting a shading band in the center of the collimating beam, the focused spot size on the recording medium is reduced to 80% compared with that of a conventional optical recording. The readout signal degradation affected by the focused spot sidelobe on the recording medium is suppressed by spatially extracting the main lobe region from the reflected back beam using a slit on a reimaged plane. The linear recording density has been improved to 1.2 times as high as that for a conventional optical recording using a normal resolution optical head.

Optical heteroassociative memory using spatial light rebroadcasters

A new device, a spatial light rebroadcaster, is described that can have optical disk resolution and high speed. Experimental results demonstrate resolution, speed, linearity, logic operation, and arithmetic computation. The device is suitable for optical computing, in particular for memory systems. Optical masking for controlling memory recall and 100 x 100 matrix-vector multiplication are demonstrated. A one pass optical heteroassociative memory system was assembled that uses an optical outer product formulation to store associated 32-bit vectors. Recall is achieved by optical matrix-vector multiplication. The results show the suitability of these devices for memory systems in optical computers.