Eric S. Maniloff

Maximized photorefractive holographic storage

Recording angularly multiplexed holograms in photorefractive media requires an understanding of both the recording and erasure dynamics. In this paper a coupled‐wave analysis is used to describe both the recording and erasure processes. This analysis has been applied to the recording of multiplexed holograms, resulting in a procedure to record maximum diffraction efficiency holograms. Experimental results confirming the theory for both the dynamics of a single exposure and the efficiencies of multiple exposures are presented. Using an uncoupled recording wave analysis, an expression for the dependence of the diffraction efficiency on the number of exposures in the case of equal record/erase time constants is presented. Approximate results are presented for the case of unequal time constants. This serves to set an upper limit on the diffraction efficiencies for a given saturation index of refraction modulation.

Dynamic holographic interconnects using static holograms

In this paper, a dynamic information routing architecture, which uses fixed volume holograms accessed by a ferroelectric liquid crystal spatial light modulator (SLM), is presented. The interconnects are there- fore reconfigurable at the SLM frame rate. The design of two routing networks is presented, and experimental results are given for each net- work. Since multiple-exposure holographic recording is necessary to im- plement these networks, results demonstrating maximum diffraction ef-

Procedure for recording multiple-exposure holograms with equal diffraction efficiency in photorefractive media

We report experimental results that demonstrate for the first time, to our knowledge, more than 20 holograms stored and recalled with equal diffraction efficiency in photorefractive LiNbO(3). A physical explanation for this new recording procedure is presented, and a comparison is made with previous exposure methods that shows an order-of-magnitude increase in diffraction efficiency with the order of exposure for optically erasable holograms. The procedure holds for recording times much less than the saturation exposure time.

Recording of 6000 holograms by use of spectral hole burning.

Experiments verifying a new method of storing spectral hole-burning holograms, which yields reduced cross talk as compared with standard spectral hole-burning holograms, have been conducted. Results demonstrating the reduced width of this type of hologram in both frequency and the applied electric-field dimension are presented. Analytic solutions for the spectral width and diffraction efficiency of these holograms are presented. Using this exposure technique, we have recorded 6000 holograms in a single spectral hole-burning sample.

Effects of scattering on the dynamics of holographic recording and erasure in photorefractive lithium niobate

Absorption and scattering, or fanning, have a pronounced effect on the observed dynamics of the photorefractive effect. In this article these effects are included in the coupled wave equations for both recording and erasure, and a comparison is made with experimental observations. It is shown that a single recording and erasure time constant is necessary to fit the experimental measurements, whereas separate time constants would be required if absorption and scattering were neglected.

Frequency and phase swept holograms in spectral hole-burning materials.

A new hologram type in spectral hole-burning systems is presented. During exposure, the frequency of narrow-band laser light is swept over a spectral range that corresponds to a few homogeneous linewidths of the spectrally selective recording material. Simultaneously the phase of the hologram is adjusted as a function of frequency-the phase sweep function. Because of the phase-reconstructing properties of holography, this recording technique programs the sample as a spectral amplitude and phase filter. We call this hologram type frequency and phase swept (FPS) holograms. Their properties and applications are summarized, and a straightforward theory is presented that describes all the diffraction phenomena observed to date. Thin FPS holograms show strongly asymmetric diffraction into conjugated diffraction orders, which is an unusual behavior for thin transmission holograms. Investigations demonstrate the advantages of FPS holograms with respect to conventional cw recording techniques in freq ncymultiplexed data storage. By choosing appropriate phase sweep functions, various features of holographic data storage can be optimized. Examples for cross-talk reduction, highest diffraction efficiency, and maximal readout stability are demonstrated. The properties of these FPS hologram types are deduced from theoretical considerations and confirmed by experiments.

Spectral Data Storage Using Rare-Earth-Doped Crystals

Conventional optical data-storage techniques, such as magneto-optic disks and CD-ROMs, record a single bit of information at each particular substrate location. In order to produce the gigabyte-class storage substrates demanded by todays computers using such conventional technologies, access to tens of billions of individual material locations is required. This brute-force approach to optical data storage has produced impressive results. However, there is increasing interest in methods for more efficiently accessing storage materials. One approach is to record multiple bits at a single storage-material location. This can be accomplished by multiplexing the bits spectrally, using differing optical frequencies to record data bits. It has been realized for over 20 years that when certain materials are cooled to appropriate temperatures, typically below 20 K, the possibility of spectrally multiplexing large numbers of bits in a single material location arises. Although this approach, known as spectral hole-burning, has been proposed as a data-storage mechanism, to date it has primarily been used as a tool to study material properties. Rare-earth-doped crystals have been demonstrated to have properties that lend themselves to a variety of different spectral hole-burning-based data-storage applications. In this article, we will review the principles of spectral hole-burning, discuss some specific material systems in which spectral hole-burning is of particular interest, and describe methods for producing high-capacity, high-data-rate spectral memories. Spectral hole-burning, and spectral memories based on spectral hole-burning, depend on a material property referred to as inhomogeneous absorption line broadening. Materials exhibiting this property contain active atoms or molecules that individually respond to (absorb) very specific frequencies of light, but the collective response of all of the materials active atoms or molecules covers a spectral region that is broad compared with the response of a particular active atom or molecule. Inhomogeneous absorption line broadening is caused by local variations in the structure of the host, which in turn lead to variations in the electronic levels of the active atoms or molecules. The absorption linewidth of an individual absorber is referred to as the homogeneous linewidth Γ h , and the absorption width of a collection of inhomogeneously broadened absorption centers is referred to as the inhomogeneous linewidth Γ i . Application of monochromatic light to such a material has the effect of exciting only a very small subset of active absorbing atoms—those residing in the illuminated spatial volume within a homogeneous width of the exciting lights specific frequency. If the frequency of the imposed light is shifted, a different subset of active absorbing atoms in the illuminated volume responds.

Data compression in frequency-selective materials using frequency-swept excitation pulses.

We demonstrate the temporal compression of photon echoes in frequency-selective materials by application of frequency-swept excitation pulses. Experimental results in Pr(3+):Y(2)SiO(5) for two- and three-pulse photon echoes are presented and compared with theory. A possible application to temporal reduction of optical data streams is shown.

Power broadening of the spectral hole width in an optically thick sample

Under the assumption of negligible absorption, spectral holes are known to broaden in frequency proportional to the square root of the recording intensity. In this paper the standard expression for power broadening is compared with more general results for the case of non-zero absorption. Analytic results are derived for the upper and lower limits of the hole width, corresponding to the cases of low and high absorption, respectively. These results are compared with numerical solutions, and used to fit experimental data for an optically thick sample of chlorin in PVB, in a temperature range of 450 mK to 3 K. The temperature dependence and zero kelvin value of the dephasing time, T2, are evaluated.

Signal-to-noise limitations on the number of channels in holographic interconnection networks

Volume holograms stored in a photorefractive crystal have been used as routing templates for an optical communication switch. These holograms are recorded and read out by using coded object beams. A spatial light modulator is used to modulate the object beam with different addresses, such that the coded signal is routed from an input channel to a desired output channel. In this paper the maximum number of channels that can be addressed and hence the maximum number of equal-diffraction-efficiency holograms that can be stored in the photorefractive crystal for a given bit error rate are presented.