Optical encryption in spatially-incoherent light using two LC SLMs for both information input and encryption element imaging

We show the feasibility of two new programmable diffractive optical elements (DOE). On one hand, we demonstrate the realization of programmable apodizers. With the term apodizer we refer to non-uniform amplitude filters used to modify the point-spread function (PSF) of an optical system. On the other hand, we show the simultaneous realization of a Fresnel lens and an amplitude filter in a single DOE: the programmable amplitude apodized Fresnel lens (PAAFL). Two different modulation regimes are required to generate these DOEs: amplitude-only regime for the programmable apodizer and phase-only regime for the PAAFL. We show that a twisted-nematic liquid crystal spatial light modulator (TN-LCSLM) inserted between two wave plates and two polarizers is able to provide both modulation regimes. Different types of amplitude filters, such as axial hyperresolving, transverse apodizing and transverse hyperresolving have been implemented both as programmable apodizers and as PAAFLs. We provide experimental results for the performance of the two new DOEs. The agreement with the numerical results is excellent, thus demonstrating the feasibility of our proposal.

Liquid crystal spatial light modulators are used in a wide range of modern problems in science and technology. These modulators are used to control the amplitude, phase, and direction of propagation of coherent optical radiation in optical information processing systems. However, the influence of the characteristics of the temporal dynamics of liquid crystal spatial light modulators on the performance of information optical systems, including diffractive neural networks, has not been sufficiently studied. The article presents the results of a study of the temporal dynamics of phase modulation of the liquid crystal spatial light modulator SLM-200 (Santec, Japan). Computer-synthesized binary phase diffractive optical elements were used in the experiments, and the characteristics of the temporal dynamics of the optical modulator were measured: 125 ms is the rise time of the diffraction efficiency when displaying diffractive optical elements on the screen; 61.9 ms is the decay time when switching frames. With these characteristics, it is possible to ensure the formation of a variable optical field at a frame display frequency of 2 Hz with an interference level of –17.1 dB. Increasing the frame display frequency leads to the appearance of unavoidable interframe interference, which in turn limits the effective performance of the information system. The results obtained can be useful in designing high-performance optical information processing systems and diffraction neural networks

Liquid crystal spatial light modulator (LCSLM) realizing equal-intensity multiple beams often has some features, i.e., phase valley between two adjacent pixels, flyback region when phase decreases immediately from 2π to 0, and inevitable backplane curvature, which are different from those of most conventional diffractive optical elements (DOEs), such as static DOEs. For optimal intensity uniformity, equal-intensity multi-beam generation must be considered for these artifacts. We present a tunable-grating method in which the intensity uniformity can be improved by considering the LCSLM artifacts. For instance, tuning phase modulation depth of the grating, called isosceles triangle multilevel phase grating (ITMPG), can be used not only to improve the intensity uniformity, but also to fast steer four beams with narrow beamwidths, determined by the same effective aperture of ITMPG. Improved intensity uniformity and high relative diffraction efficiency are demonstrated through experiments with phase-only LCSLM.

Synthesis and reconstruction of computer generated holograms by a double pass technique on a twisted nematic-based liquid crystal spatial light modulator

Optical encryption is an actively developing field of science. The majority of encryption techniques use coherent illumination and suffer from speckle noise, which severely limits their applicability. The spatially incoherent encryption technique does not have this drawback, but its effectiveness is dependent on the Fourier spectrum properties of the image to be encrypted. The application of a quick response (QR) code in the capacity of a data container solves this problem, and the embedded error correction code also enables errorless decryption. The optical encryption of digital information in the form of QR codes using spatially incoherent illumination was implemented experimentally. The encryption is based on the optical convolution of the image to be encrypted with the kinoform point spread function, which serves as an encryption key. Two liquid crystal spatial light modulators were used in the experimental setup for the QR code and the kinoform imaging, respectively. The quality of the encryption and decryption was analyzed in relation to the QR code size. Decryption was conducted digitally. The successful decryption of encrypted QR codes of up to 129 × 129 pixels was demonstrated. A comparison with the coherent QR code encryption technique showed that the proposed technique has a signal-to-noise ratio that is at least two times higher.

A versatile, scalable wavefront control approach based upon proven liquid crystal (LC) spatial light modulator (SLM) technology was extended for potential use in high-energy near-infrared laser applications. The reflective LC SLM module demonstrated has a two-inch diameter active aperture with 812 pixels. Using an ultra-low absorption transparent conductor in the LC SLM, a high laser damage threshold was demonstrated. Novel dual frequency liquid crystal materials and addressing schemes were implemented to achieve fast switching speed (<1ms at 1.31 microns). Combining this LCSLM with a novel wavefront sensing method, a closed loop wavefront controller is being demonstrated. Compared to conventional deformable mirrors, this non-mechanical wavefront control approach offers substantial improvements in speed (bandwidth), resolution, power consumption and system weight/volume.

Recent advances in the shaping of ultrafast optical waveforms using liquid crystal (LC) spatial light modulators (SLM) are presented. Two LC SLMs are used in a novel arrangement to produce programmable waveforms with specified time-dependent amplitude and temporal phase profiles with the greatest fidelity and complexity to date. The apparatus is also used to demonstrate the generation of an ultrafast waveform with a programmable time-dependent polarization profile. A general theoretical result that describes the space-time electric field profile of waveforms shaped by the spectral filtering of spatially separated frequency components is also presented. The main result is that diffraction gives rise to a translational spatial shift in the electric field profile that varies linearly with time along the shaped waveform.

A simulation platform is set up to study the ship-borne laser communications. A liquid crystal spatial light modulator is used as a non-mechanical beam steering control device to simulate beam motions of moving platforms. Because of its light weight, low power and compact form, the liquid crystal spatial light modulator is superior to its mechanical counterparts. The theory of beam steering is introduced firstly. And then the performance of the simulation platform is researched and evaluated. The steering angles range from -2.89° to 2.89° with the control precision of ~ 0.02°. The simulated sinusoidal frequency of the spatial light modulator can reach 4 Hz in maximum. Finally, the beam motions of the ship platforms with different natural rolling periods are simulated based on the platform. The correlation coefficients between the theoretical and the simulated motion curves are greater than 0.9805. Results show that it is feasible to realize the beam motions simulation of specific moving platforms using the liquid crystal spatial light modulator.

It is proposed in this paper to use phase only liquid crystal spatial light modulators to realize non-mechanical lateral and axial confocal microscopic laser scanning. With a phase only liquid crystal spatial light modulator used as a scanner to realize laser beam deflection, a confocal microscopic lateral beam scanning system is designed. A zoom illuminating lens is formed by incorporating a liquid crystal spatial light modulator along the confocal illumination light path, and thus the focus of the objective lens is axially shifted to realize the axial non-mechanical scanning. The theoretical analyses indicate that phase only liquid crystal spatial light modulators can be used to realize non-mechanical 3D confocal microscopic laser scanning.

Holographic stereograms are an important way of generating 3-D images from photographs of real objects or of computer-generated 3-D objects. We have made holographic stereograms using several commercially available liquid crystal spatial light modulators, which have been modified in some cases, to produce transmission masks for a video image. The spatial light modulator is used to form the image in producing holographic stereograms. The image source has been either a video camera (or VCR) or the video signal from a computer graphics terminal. The use of the liquid crystal spatial light modulator eliminates the photographic intermediary in the process of forming the holographic stereograms. This technique speeds the process of forming holographic stereograms, increases automation of the process, and reduces the time required in creating the hologram. The major limitations of this technique are the contrast and resolution of the spatial light modulators which are commercially available and of low cost. Since the spatial light modulator technology is rapidly advancing, better performance in both of these areas is expected. Therefore, we believe that this technique offers an attractive means of producing holographic stereograms, especially from computer graphics, in situations where a short cycle time is important.

With the rapid development of computer network technology, information security has attracted increasing attention. Due to the characteristics of multi-dimensional operation and parallel processing capability, optical image encryption techniques have been receiving more and more attention. Since the well-known double random phase encoding technique was proposed, many other methods based on optical information processing means such as the use of optical transform, interference, and polarized light encoding, have been proposed for optical image encryption. However, recent researches have demonstrated that traditional optical encryption techniques are symmetric cryptosystems, in which decryption keys are identical to encryption keys and they have been found to be vulnerable to different types of attacks, such as known plaintext and chosen plaintext attacks. To overcome this shortcoming, asymmetric cryptosystems based on nonlinear phase-truncation techniques and phase retrieval algorithm have been proposed. Asymmetric cryptosystem is a cryptographic system in which encryption keys are different from decryption keys. The encryption keys are used as public keys which are disseminated widely, and the decryption keys are used as private keys which are known only to the authorized users. So, asymmetric cryptosystem can offer a higher-level security than symmetric cryptosystem. However, asymmetric cryptosystems based on phase retrieval algorithms require a lot of computational time, and asymmetric cryptosystems based on phase-truncated Fourier transforms have been found to be vulnerable to special attack. Therefore, in this paper, a novel asymmetric image encryption method is proposed by using the gyrator transform and vector operation. The original image is encrypted into two phase masks with vector operation. One is a random phase mask and the other is a phase mask related to the original image. In the encryption process, the random phase mask is used as a phase key and the other phase mask is transformed by gyrator transform. The transform result is performed by Fourier transform after being modulated by a phase distribution. The ciphertext is the amplitude of the above result. Compared with previous encryption schemes, the suggested method has two advantages. Firstly, we have proposed a new asymmetric encryption method based on the gyrator transform and vector operation. The decryption process is different from the encryption process. The gyrator transform and Fourier transform are used in the encryption process, while only the inverse operation of Fourier transform is employed in the decryption process. In addition, the decryption keys produced in the encryption process are different from the encryption keys. Therefore, the proposed scheme has high resistance against the conventional attacks. Secondly, the encrypted result is real-valued, which is convenient for display, transmission and storage.Numerical simulations illustrate the feasibility and effectiveness of the proposed encryption scheme.

Liquid crystal spatial light modulators for precise dynamic manipulation of coherent light fields, used in diffractive optoelectronic optical data processing systems, are considered. This paper presents the results of a study of the temporal dynamics of the HoloEye PLUTO-2 VIS-016 liquid crystal spatial light modulator for analysis of light fields rate modulation. Experiments using binary phase computer generated holograms and binary focusing phase diffractive optical elements were conducted. Based on experimental data, the time characteristics of the modulator response were determined. It was found that when the rise time of the diffraction efficiency was 146 ms after the hologram displaying onto the SLM, and when switching to a new hologram, the decay time was 97 ms. These results allowed the dynamic generation of an alternating holograms at a refresh rate of 2 Hz with an interference level of –16 dB. Increasing the frequency of fringe pattern updates increases the level of interframe noise in the generated holograms, and when updated at the specification frequency, the generated distributions cannot be separated. Determining the actual frame rate based on the rise and decay times of the diffraction efficiency makes it possible to correctly calculate the minimum operating time of an information optical system containing a liquid crystal spatial light modulator.

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In this paper we report multiple optical trapping of microscopic dielectric particles using diffractive optical elements implemented on twisted nematic liquid crystal spatial light modulators. The particles are trapped in arrays disposed in plane or in volume and can be moved independently in x-y-z by changing the configuration of the diffractive optical element. We show also multiple trapping using Laguerre-Gaussian and Gaussian beams simultaneously. The orbital angular momentum of the Laguerre-Gaussian beam is transferred to the particle, making it to move on a circular trajectory defined by the intensity pattern specific to this beam. We use sample cells built with two microscope slides separated by 120 µm with a sticky tape. The space between the two slides is filled with 2 µm diameter silica spheres diluted in water (concentration 0.026% wt). We show that optical trapping is also possible in a small glass capillary with a diameter of 100 µm.

To correct wave front distortion, a phased only Liquid Crystal Spatial Light Modulator (LCSLM) is used. LCSLM can modulate the phase of the incident beam by changing the electric field applied to the transparent electrodes of a liquid crystal cell. The voltage change causes the refractive index of the liquid crystal change, so a phase shift produces. The distortion of a wave front can be measured by using a wave front interferometer. Since each pixel of SLM may be addressed and controlled independently, so proper voltage is supplied to each pixel to compensate the phase corresponding to the distortion. Before wave front correction, the LCSLM is tested and corrected by employing a Twyman-Green interferometer, which shows the relationship between the applied voltage and the phase value of each pixel. Experiment result shows that after error compensation of LCSLM, the wave front distortion correction accuracy reaches 0.06 λ.