Four-level phase pair encoding and decoding with single interferometric phase retrieval for holographic data storage

We propose four-level phase pair encoding and decoding with single interferometric phase retrieval for holographic data storage. Inherent with phase pair encoding, phase shifting is generated by assigning a certain phase difference between two pixels of the phase pair. Multiple phase shifting operations are not required. In addition, a phase-readout reference beam can be a plane beam with an arbitrary phase in our method because phase shifting can be encoded on the phase-only spatial light modulator easily and accurately. Therefore, our method can not only increase the data transfer rate, but also improve the robustness of the holographic data storage system.Although the code rate of our method needs to be sacrificed by half, the code rate is still twice that of amplitude code when four-level phase encoding is used. We demonstrated experimentally that there is only a 1 × 10~(-2) order of bit error rate before error correcting, which is acceptable. We believe our method will further advance the phase-modulated holographic data storage technique.     

Non-interferometric phase retrieval for collinear phase-modulated holographic data storage

Optical Review - Because of its simple optical system setup and robust noise tolerance, non-interferometric phase retrieval is an important technique for phase-modulated holographic data storage....     

Special phase mask and related data format for page-based holographic data storage systems

Random phase masks in object and reference beam of page-based holographic storage systems suppress the DC-peak and improve the overlap of both beams inside the storage material. Furthermore, they allow for a narrow shift-selectivity. In a holographic setup the phase mask has to be introduced at a conjugate image plane of the spatial light modulator (SLM), if it is not fixed directly on the SLM itself. A binary phase mask with cells generating 0 and π phase shifts has to be aligned very accurately with respect to the SLM pixels, otherwise image artifacts disturb the received data page. We present a phase mask, where the phase cells have the size of a data block which consists of a rectangular set of SLM pixels. Additionally, the corresponding data page has no data at the position of phase jumps and thus relaxes the alignment tolerance significantly.     

Selective phase masking to reduce material saturation in holographic data storage systems

Emerging networks and applications require enormous data storage. Holographic techniques promise high-capacity storage, given resolution of a few remaining technical issues. In this paper, we propose a technique to overcome one such issue: mitigation of large magnitude peaks in the stored image that cause material saturation resulting in readout errors. We consider the use of ternary data symbols, with modulation in amplitude and phase, and use a phase mask during the encoding stage to reduce the probability of large peaks arising in the stored Fourier domain image. An appropriate mask is selected from a predefined set of pseudo-random masks by computing the Fourier transform of the raw data array as well as the data array multiplied by each mask. The data array or masked array with the lowest Fourier domain peak values is recorded. On readout, the recorded array is multiplied by the mask used during recording to recover the original data array. Simulations are presented that demonstrate the benefit of this approach, and provide insight into the appropriate number of phase masks to use in high capacity holographic data storage systems.     

Gray-scale data pages for digital holographic data storage

The prospects for gray-scale (or multilevel) digital holographic data storage are theoretically and experimentally investigated. A simple signal-to-noise ratio (SNR) partitioning argument shows that when SNR scales as 1 over the number of holograms squared, five gray levels (log(2) 5bits/pixel) would be expected to result in a 15% capacity increase over binary data pages. However, the additional signal-dependent noise sources present in practical systems create a baseline SNR that reduces both the optimal number of gray levels and the resulting gain in capacity. To implement gray-scale recording experimentally, we adapt the predistortion technique previously developed for binary page-oriented memories [Opt. Lett. 23, 289 (1998)]. Several new block-based modulation codes for decoding gray-scale data pages are introduced. User capacity is evaluated by an experimental technique using LiNbO(3) :Fe in the 90 degrees geometry. Experimental results show that a balanced modulation code with three gray levels provides a 30% increase in capacity (as well as a 30% increase in readout rate) over local binary thresholding.     

Pixel-matched holographic data storage with megabit pages

Digital data-page holograms consisting of 1024 x 1024 arrays of binary pixels have been stored and subsequently retrieved with an optical exposure consistent with a data rate 1 Gbit /s. Each input pixel was precisely registered with a single detector pixel, and a raw bit-error rate as low as 2.4 x 10(-6) was demonstrated with global-threshold detection. To our knowledge, this is the first demonstration of the often-cited goal of holographic data storage of megabit data pages and a gigabit-per-second data rate.     

Modulation coding for pixel-matched holographic data storage

We describe a digital holographic storage system for the study of noise sources and the evaluation of modulation and error-correction codes. A precision zoom lens and Fourier transform optics provide pixel-to-pixel matching between any input spatial light modulator and output CCD array over magnifications from 0.8 to 3. Holograms are angle multiplexed in LiNbO(3):Fe by use of the 90 degrees geometry, and reconstructions are detected with a 60-frame/s CCD camera. Modulation codes developed on this platform permit image transmission down to signal levels of ~2000 photons per ON camera pixel, at raw bit-error rates (BER's) of better than 10(-5). Using an 8-12-pixel modulation code, we have stored and retrieved 1200 holograms (each with 45,600 user bits) without error, for a raw BER of <2x10(-8).     

Associative data search in phase-encoded volume holographic storage systems

We present a technique that enables true associative data search in phase-encoded volume holographic storage systems. The technique overcomes crucial shortcomings related to the only two methods proposed for associative searches in phase-encoded systems so far. An additional interferometric readout during content addressing is utilized to ascertain the cross-correlations between an input information and all data pages that are recorded by superposition in one location of the storage media. We present experimental investigations and thoroughly discuss the reliability of the technique. Under realistic conditions the inevitable normalization procedure, used to determine absolute correlation values, as well as the probability of small correlation values crucially affect the capabilities of associative search in phase-encoded holographic storage systems. PACS  42.30.-d; 42.40.Pa; 42.79.Hp     

Compensation for pixel-mismatch errors in shift-multiplexed holographic data storage

To read out correct data from shift-multiplexed holographic data storage, we propose a novel method of pixel matching by use of a tiny plane-parallel refraction plate placed in the path of converging readout beams. Tilting the refraction plate produces a shift in the lateral position of the readout beam incident upon the holographic storage disk, leading to compensation for pixel-mismatch errors by an improvement in bit-error rate of 3 orders of magnitude.     

Compensation for pixel misregistration in volume holographic data storage

We describe what we believe to be a novel postprocessing algorithm for compensating for misregistrations between a detector array and the coherent image of a pixelated two-dimensional data page. A lookup table of baseline local offsets is combined with the dynamically measured global offset of the received data page, producing an estimate of the total lateral shift of each small block of pixels. A serial algorithm then reallocates the appropriate portion of the signal detected by each pixel to its neighbors, accounting for both the linear and the quadratic contributions introduced by coherent illumination of square-law detectors. This procedure can relax the tight constraints on page registration, optical distortion, and material shrinkage that currently hamper page-oriented holographic storage systems. Experimental results from a pixel-matched 1-Mpixel volume holographic system are presented, showing an increase in position tolerance (for a raw bit-error rate <10(-3)) from +/-16% to +/-40% of the pixel pitch.     

Phase modulated gray-scale data pages for digital holographic data storage

We propose a method for gray level holographic digital data storage by using three-gray levels in the phase mode. Gray level data pages are displayed on a SLM operating in the phase mode to obtain a homogenized Fourier spectrum by suppressing the extremely high intensity DC component in the Fourier spectrum of conventional amplitude based binary/gray level data pages. Holographic interferometry has been used to recover the gray level amplitude data page from phase data page. Numerical simulation results are presented for three-gray level data pages. Fourier plane homogeneity, bit-error-rate, storage density, phase modulation error of the SLM, and misalignment tolerances are investigated through computer modeling. A comparison of the present method with the amplitude-modulated gray level case with and without using a phase mask in conjunction with the data page is carried out. An experimental demonstration of the proposed three-gray level phase data page method is also presented.     

Multifrequency acousto-optic page composers for holographic data storage

A new concept for one- and two-dimensional acousto-optic page composers to be used in holographic memories is proposed and experimentally demonstrated. The concept is based on multifrequency operation and compensation of the Doppler shift of the optical frequencies. Acousto-optic page composers without the usual exposure time limitation are in this way realized.     

Ruggedized digital holographic data storage with fast access

Recent investigations in holographic mass memory systems have produced proof-of-concept demonstrations that have highlighted their potential for providing unprecedented capacity, data transfer rates and fast random access performance (Heanue et al. 1994; Hong et al. 1995; Psaltis and Mok 1995; Bernal et al. 1996; McMichael et al. 1996). The exploratory nature of most such investigations has been largely confined to benchtop experiments which have ignored the practical constraints of packaging and environmental concerns. We have embarked on an effort to demonstrate the holographic mass memory concept by developing a compact prototype system geared for avionics and similar applications which demand the following features (mostly interdependent factors): (1) solid state design (no moving parts), (2) fast data seek time, (3) robust with respect to environmental factors (temperature, vibration, shock). In this paper, we report on the development and demonstration of two systems, one with 100 MB and the other with more than 1 GB of storage capacity. Both systems feature solid state design with the addressing mechanism realized with acoustooptic deflectors that are capable of better than 50 sec data seek time. Since the basic designs for the two systems are similar, we describe only the larger system in detail. The operation of the smaller system has been demonstrated in various environments including hand-held operation and thermal/mechanical shock; a photograph of the smaller system is provided, as well as actual digital data retrieved from the same system.     

Orthogonal-reference-pattern-modulated shift multiplexing for collinear holographic data storage

A novel hybrid shift multiplexing method for collinear holographic data storage (CHDS) by using orthogonal reference patterns (RPs) is proposed, analyzed, and demonstrated. For this method, holograms are multiplexed by not only shifting the media but also using different RPs. Compared with the traditional method, the shift pitch for the hybrid method is substantially reduced because of the selectivity introduced by different RPs. The interpage cross talk due to Bragg mismatch and degeneracy for multiplexing holograms in the same volume by using orthogonal RPs is also attenuated by utilizing the shift selectivity of the hologram. A 1.5 μm shift pitch is experimentally achieved by using three amplitude RPs in a system that would be 4.5 μm with only one RP. This new method offers an alternative to significantly increase the data density and transfer rate of the CHDS system given that the media has ideal properties.     

Information capacity of 3-D holographic data storage

Capacities for angular and wavelength multiplexed holographic data storage systems are considered. Limitations due to the spatial resolution of the recording and reconstructing fields and to the holographic recording process are derived.     

Correlator-aided alignment of phase key in encrypted holographic storage systems

An encrypted holographic scheme with a compact alignment system for a phase key is proposed. Alignment for a phase key is performed by a holographic correlator. A filter in the correlator system contains the phase information used for encryption. Thus, it can be used to check the authorization of the phase key. Owing to the shift-invariant property of the holographic correlator, the location of the correlation peak is highly related to the alignment of the phase key. Therefore, the phase key can be repositioned on the desired location by searching for the specific position of the correlation peak. With the help of the correlator, alignment of the phase key can be done in 1 min. Experimental results using a correlator system in support of our proposed idea are demonstrated as well.     

Experimental implementation of multiplexing/demultiplexing in digital images using virtual phase conjugation for holographic data storage

We experimentally implemented and proved the concept of multiplexing and demultiplexing in digital images using virtual phase conjugation, which was proposed in our previous study. In the experiment, we concluded that two digital images multiplexed in a single signal beam are recorded in a holographic medium, and these images are independently and successfully reproduced. In this method, the digital images are multiplexed by superimposing them on a complex amplitude, and not using volume hologram’s multiplexing. Thus, the exposure amount in the holographic medium is constant regardless of the number of multiplexing of digital images, and the method has great potential for achieving high recording density.     

像素1:1匹配的晶体全息存储系统中像素位置偏移的补偿算法

在体全息存储系统的数据读出过程中,为了提高读出速度,需要做到SLM与CCD像素的1∶1匹配。但在实现1∶1匹配的过程中,如何降低误码率是一个难点。利用傅里叶光学成像理论,推导出了在4f系统中单个SLM像素成像的光强分布,并由此定量地分析了像素位置偏移对读出误码率的影响。提出了针对像素偏移的数值补偿算法,并获得了较理想的实验结果。     

二芳烯材料在重复擦写体全息存储中的应用研究

二芳烯材料是一种性能优异的光致色变材料,具有很好的抗疲劳性能,一般能重复擦写上万次,而不损失其性能。通过体全息光栅记录实验、角度复用体全息存储实验、重复擦写及强光记弱光读等实验,对二芳烯材料在重复擦写体全息存储中的应用进行了系统研究。由于二芳烯材料具有很高的折射率调制度(Δn=1.15×10-3)、可多次重复擦写、强光记弱光读的准非易失等性能,在可重复擦写的体全息存储中具有广阔的应用前景。     

Multiplexed phase-conjugate holographic data storage with a buffer hologram

We describe and demonstrate a volume holographic storage system in which a phase-conjugate object beam is reconstructed by the same reference beam that was used for recording. An intermediate hologram is used as a temporary buffer, recorded with its own reference beam and the data-bearing object beam. Reading this buffer hologram with the phase conjugate of its reference beam reconstructs the phase conjugate of the object beam, which can then be recorded into the desired volume hologram for long-term storage. This method combines the immunity to lens aberrations provided by phase-conjugate readout with the simplicity of using the same multiplexed reference beam for both recording and readout. Only a single pair of phase-conjugate reference beams is required. Experimental results are shown with a single LiNbO(3):Fe crystal used as both buffer and storage holograms and a self-pumped phase-conjugate mirror in BaTiO(3) that provides the pair of phase-conjugate reference beams.