Single-beam data encoding using a holographic angular multiplexing technique

We propose a novel configuration for angular multiplexing holographic encoding in which the signal beam and the reference beam are combined into a single beam. By using a spatial light modulator based on twisted nematic liquid crystals, the signal and the reference beams are modulated in amplitude mode and phase mode, respectively. The multiplexed interference patterns with the reference beams of different incident angles are recorded near the Fourier transform plane, and then the signals are selectively reconstructed by the corresponding reference beam. Both the simulation and the experiment of single-beam angular multiplexed holography are performed with consistent results. Compared with the traditional angular multiplexing holographic recording system, the single-beam configuration is more compact, easier to adjust, and less sensitive to the vibration of the environment. Therefore, it will be more attractive for potential applications in many fields, such as high-density signal recording and data encryption.     

Coaxial holographic encoding based on pure phase modulation

We describe a simple technique for coaxial holographic image recording and reconstruction, employing a spatial light modulator (SLM) modified in pure phase mode. In the image encoding system, both the reference beam in the outside part and the signal beam in the inside part are displayed by an SLM based on the twisted nematic LCD. For a binary image, the part with amplitude of "1" is modulated with random phase, while the part with amplitude of "0" is modulated with constant phase. After blocking the dc component of the spatial frequencies, a Fourier transform (FT) hologram is recorded with a uniform intensity distribution. The amplitude image is reconstructed by illuminating the reference beam onto the hologram, which is much simpler than existing phase modulated FT holography techniques. The technique of coaxial holographic image encoding and recovering with pure phase modulation is demonstrated theoretically and experimentally in this paper. As the holograms are recorded without the high-intensity dc component, the storage density with volume medium may be increased with the increase of dynamic range. Such a simple modulation method will have potential applications in areas such as holographic encryption and high-density disk storage systems.     

Polarization encoding and multiplexing of two-dimensional signals: application to image encryption

We discuss an optical system that encodes an input signal to a polarization state, using a spatial light modulator (SLM). Using two SLMs the optical system multiplexes two 2D signals in the polarization domain, and we demonstrate the multiplexing of two binary images. The encryption and decryption of two binary images using an XOR operation is also presented.     

Optical image encryption scheme with multiple light paths based on compressive ghost imaging

An optical image encryption method with multiple light paths is proposed based on compressive ghost imaging. In the encryption process, M random phase-only masks (POMs) are generated by means of logistic map algorithm, and these masks are then uploaded to the spatial light modulator (SLM). The collimated laser light is divided into several beams by beam splitters as it passes through the SLM, and the light beams illuminate the secret images, which are converted into sparse images by discrete wavelet transform beforehand. Thus, the secret images are simultaneously encrypted into intensity vectors by ghost imaging. The distances between the SLM and secret images vary and can be used as the main keys with original POM and the logistic map algorithm coefficient in the decryption process. In the proposed method, the storage space can be significantly decreased and the security of the system can be improved. The feasibility, security and robustness of the method are further analysed through computer simulations.     

Dual random phase encoding: a temporal approach for fiber optic applications

We analyze the dual random phase encoding technique in the temporal domain to evaluate its potential application for secure data transmission in fiber optic links. To take into account the optical fiber multiplexing capabilities, the noise content of the signal is restricted when multiple channels are transmitted over a single fiber optic link. We also discuss some mechanisms for producing encoded time-limited as well as bandwidth-limited signals and a comparison with another recently proposed technique is made. Numerical simulations have been carried out to analyze the system performance. The results indicate that this multiplexing encryption method could be a good alternative compared with other well-established methods.     

Collinear holography

A novel reading and writing technology for a holographic storage system called collinear holography is developed. With this method, two-dimensional page data can be recorded as volumetric holograms generated by a reference beam and a signal beam that are bundled on the same axis and irradiated on the recording medium through a single objective lens. The multiplex recording and reconstructing process is demonstrated, and it is presented that optical configuration and the dichroic media disk structure are suitable for a compact system. This method enables us to construct a small volumetric optical disk storage system with CD and DVD upper compatibility.     

Shift multiplexing with spherical reference waves

Shift multiplexing is a holographic storage method particularly suitable for the implementation of holographic disks. We characterize the performance of shift-multiplexed memories by using a spherical wave as the reference beam. We derive the shift selectivity, the cross talk, the exposure schedule, and the storage density of the method. We give experimental results to verify the theoretical predictions.     

Theoretical and experimental studies of hologram multiplexing that uses a random wave front generated by photorefractive beam fanning

A hologram multiplexing technique-that uses random wave fronts generated by photorefractive beam fanning is investigated. A storage photorefractive crystal generates various random wave fronts to be used as reference beams without the external diffusers such as ground glass and multimode optical fiber that are generally employed. We experimentally demonstrate hologram multiplexing with six images and show that the stored holograms can be selectively retrieved. We also simulate photorefractive beam fanning inside a BaTiO3 crystal, in particular regarding the correlation properties of the fanning beams for the first time to our knowledge, and reveal the conditions of incidence of an object beam and a reference beam required for suppressing image degradation, implementing low-cross-talk retrieval, and producing a large number of stored holograms.     

Encrypted optical storage with angular multiplexing

We present the first, to our knowledge, demonstration of an encrypted optical storage based on double-random phase encoding by using angular multiplexing in a photorefractive material. Original two-dimensional data are encrypted by use of two random phase codes located in the input and the Fourier planes and are then stored holographically in a LiNbO(3):Fe crystal. The retrieval of the original data can be achieved with a phase-conjugated readout scheme. We demonstrate the encryption and the decryption of multiple frames of two-dimensional digital data by using angular multiplexing. We also evaluate numerically the influence of the bandwidth of the optical system on the decrypted digital data. The bit error rate as a function of the optical system bandwidth is presented.     

Cross-talk analysis in a telecentric adaptive free-space optical relay based on a spatial light modulator

We present an analysis of the performance limit of an adaptive multichannel free-space optical interconnect based on a spatial light modulator (SLM). The SLM function is to provide an active alignment of the signal beam in the detector plane. A thorough cross-talk analysis based on the diffractive properties of an ideal SLM in an isoplanatic optical system is shown. We analyze the performance in terms of the bit-error rate (BER) due to cross talk between different channels in the optical interconnect for different alignment states and for different phase-modulation schemes.     

Security optical data storage in Fourier holograms

We have proposed and demonstrated a holographic security storage system that is implemented with a shift multiplexing technique. The security function of this storage system is achieved by using a microdiffuser (MD) for random phase encoding of the reference beams. The apparatus of random phase encoding in this system offers an additional and flexible function during the recording processes. The system can generate holographic security memory or nonsecurity holographic memory via using the MD or not. The storage capacity and the average signal-to-noise value of the security storage system are 16 bits/μm(2) and 3.5, respectively. Lateral shifting selectivity in this holographic security storage system is theoretically analyzed and experimentally investigated.     

一种改进的CORDIC实时生成加权系数方法

基于现场可编程门阵列实现多波束声呐频域波束形成算法时,通常需要使用坐标旋转数字计算(CoordinateRotation Digital Computer,CORDIC)方法实时生成加权系数,但波束数较多且为宽带信号时会消耗大量硬件资源。文章提出一种改进的基于CORDIC方法实时生成加权系数的方法。通过频点复用处理,多个波束组可共用同一个CORDIC IP核依次实时生成加权系数,降低了87.5%的CORDIC IP核资源消耗;通过引入二维乒乓、用后即抛思想,对存储加权系数的随机存取存储器(Random Access Memory,RAM)基于波束和频点复用进行乒乓处理,可降低97.9%的RAM系数存储空间占用。该改进方法通过多维度复用处理,占用少量RAM存储空间来节省大量CORDICIP核资源,均衡了RAM和CORDIC IP核资源的消耗,具有很好的工程应用价值。     

Peak-to-average power ratio reduction in OFDM using cyclically shifted phase sequences

Two approaches for reducing peak-to-average power ratio (PAPR) in orthogonal frequency division multiplexing (OFDM) are proposed that are relied on a set of cyclically shifted phase sequences (CSPS) and implemented using the time domain circular convolution. After multiplying CSPS with the frequency domain data, the signal candidates can be expressed as weighted sum of the circularly shifted OFDM time domain data in the first method, which is called CSPS method. In the second method, weighted coefficients for generating the signal candidates in CSPS method are optimally selected to improve its performance; thus, the second method is referred to as optimised CSPS (OCSPS) method. The performances of the CSPS and OCSPS methods are evaluated using simulated data and compared with those of selective mapping (SLM) and partial transmit sequences (PTS). The simulation results show that both the CSPS and OCSPS methods can reduce the PAPR effectively, and that the OCSPS performs even better than the CSPS. The OCSPS can achieve the same performance as compared to the PTS. A distinct feature of the proposed methods is that only one inverse discrete Fourier transform is needed, and thus, the candidates can be calculated in time domain directly.     

Spatioangular-multiplexing scheme for dense holographic storage

A novel multiplexing scheme for dense holographic storage in photorefractive crystals, spatioangular multiplexing, is described in detail. Compared with spatial multiplexing, spatioangular multiplexing increases the storage capacity by fully utilizing the volume of the storage medium. On the other hand, spatioangular multiplexing reduces the number of holograms overlapping any one hologram in a given volume and so increases the diffraction efficiency achievable as compared with angular multiplexing. Using this scheme, we succeeded in storing 756 high-resolution patterns in an Fe:LiNbO(3) crystal of volume 1 cm(3), with an average diffraction efficiency of 0.5%. This large database is designed for practical use in a novel associative-memory system, called a high-order feedback neural network.     

Recording schedule for partially coherent hologram multiplexing in a photorefractive medium

The exposure schedule for partially coherent hologram multiplexing, in which data pages are multiplexed by multiple signal beams and a single reference beam, is investigated in detail for the case of a pi/2 phase-shifted photorefractive medium. We found that the optimum recording schedule for partially coherent multiplexing cannot be determined by the classical recording schedule theory because of time-constant errors induced by partially coherent interaction between a reference beam and self-diffraction signal beams. To overcome the issue, we derive a modified recursion equation that accounts for the time-constant errors, and we also propose a novel iterative recording-schedule correction algorism for finding the optimum solution. In the calculation with hologram multiplicity of 30 and photorefractive coupling strength of 3.0, we could successfully obtain a flat diffraction-efficiency profile after the second recursion.     

Angle-multiplexed holographic data storage with minimum cross talk noise

The cross talk noise-to-signal ratio (NSR) of an angle-multiplexed holographic data storage system is studied, and we propose a method to determine the optimized multiplexing spacing with which the cross talk noise can be less than the conventional method. In our method, the optimization location at the image plane can be chosen arbitrarily, so the multiplexing of asymmetrical image patterns can be optimized. In particular, we investigate the 90° scheme and the transmission scheme angle multiplexing. For the 90° scheme, a holographic medium with a higher refractive index is recommended for cross talk-limited multiplexing. For the transmission scheme, a holographic medium with a lower refractive index is recommended for angular range-limited multiplexing. In addition, for the transmission scheme, a larger angle between the object arm and the reference arm results in less cross talk noise, whereas the highest storage density is achieved at a 45° angle.     

Ultrafast optical pulse digitization with unary spectrally encoded cross-phase modulation

A method to digitize the intensity of ultrashort laser pulses for high-speed optical signal processing is described. This digitization was based on the spectral broadening of a weak probe (carrier) pulse by a more intense pump (signal) pulse through the nonlinear optical process of cross-phase modulation (XPM). The signal pulse intensity was varied to generate different spectral widths that can be encoded into digital form. Using a 50-ps time-divided multiplexing pulse train with a waveguide splitter, combiner, and an array of fibers with variable lengths, a unary XPM encoding approach is demonstrated. The spectral encoding scheme can be used to achieve a 5-GHz sampling rate at a 16-level accuracy.     

Redundancy-free and BER-maintained selective mapping with partial phase-randomising sequences for peak-to-average power ratio reduction in OFDM systems

Selective mapping (SLM) is one of the most effective peak-to-average power ratio (PAPR) reduction techniques proposed for orthogonal frequency-division multiplexing systems. However, the fact that this technique requires the transmission of side information (SI), to enable the receiver to reverse the randomisation process before performing signal demodulation, can make it undesirable for certain applications as it degrades the system's bandwidth efficiency and, more significantly, the bit error rate (BER) performance. A modified SLM technique (MSLM) that can operate without the need for any dedicated SI for both channel-coded and uncoded scenarios while providing similar PAPR reduction performance as the ordinary SLM technique is proposed. It will be demonstrated that the proposed MSLM technique not only improves the system's bandwidth efficiency, but also achieves significantly better BER performance than the ordinary SLM technique.     

The impact of particle preparation methods and polymorphic stability of lipid excipients on protein distribution in microparticles

Objective: The present study aimed at elucidating the influence of polymorphic stability of lipid excipients on the physicochemical characters of different solid lipid microparticles (SLM), with the focus on the alteration of protein distribution in SLM.

Methods: Labeled lysozyme was incorporated into SLM prepared with different excipients, i.e. trimyristin (TG14), glyceryl distearate (GDS), and glyceryl monostearate (GMS), by water-oil-water (w/o/w) or solid-oil-water (s/o/w) method. The distribution of lysozyme in SLM and the release of the protein from SLM were evaluated by confocal laser scanning microscopy. The storage stability of SLM was characterized by HPLC, differential scanning calorimetry, X-ray powder diffraction, and scanning electron microscopy.

Results: Lysozyme was displayed as small scattered domains inside GDS and GMS SLM, whereas it was incorporated in the core of TG14 SLM formulated by the w/o/w method or evenly distributed in TG14 SLM prepared by the s/o/w method. Stability study at 37?°C revealed that only TG14 SLM made by the w/o/w method was able to maintain the lysozyme amount both on the particle surface and released from the SLM. Elevated storage temperature induced polymorphic transition of lipids in GDS and GMS SLM, which was, however, not remarkable for the TG14 SLM.

Conclusions: Lipid excipients and particle preparation methods were found to differently affect the lysozyme distribution in SLM, owning to varied storage stabilities of the lipids. The present study provides updated knowledge for rational development of lipid-based formulations for oral delivery of peptide or protein drugs.     

Efficient implementation of a spatial light modulator as a diffractive optical microlens array in a digital Shack-Hartmann wavefront sensor

A traditional Shack-Hartmann wavefront sensor (SHWS) uses a physical microlens array to sample the incoming wavefront into a number of segments and to measure the phase profile over the cross section of a given light beam. We customized a digital SHWS by encoding a spatial light modulator (SLM) with a diffractive optical lens (DOL) pattern to function as a diffractive optical microlens array. This SHWS can offer great flexibility for various applications. Through fast-Fourier-transform (FFT) analysis and experimental investigation, we studied three sampling methods to generate the digitized DOL pattern, and we compared the results. By analyzing the diffraction efficiency of the DOL and the microstructure of the SLM, we proposed three important strategies for the proper implementation of DOLs and DOL arrays with a SLM. Experiments demonstrated that these design rules were necessary and sufficient for generating an efficient DOL and DOL array with a SLM.