Amplitude-only Spatial Light Modulator Research Articles

Enhancing efficiency of complex field encoding for amplitude-only spatial light modulator based on a neural network.

The widespread adoption of artificial neural networks for hologram synthesis can be attributed to their ability to improve image quality and reduce computational costs. In this study, we propose an alternative use of artificial neural networks to improve the optical efficiency of complex field encoding. The neural encoding significantly enhances the efficiency of amplitude-only SLMs, resulting in 2.4-fold optical efficiency enhancement with negligible image quality degradation compared to the Burch encoding method. Notably, the experimental results demonstrate that the neural encoding method has even higher image quality, providing an approximately 2.5 dB enhancement in a peak signal-to-noise ratio. The neural encoding method offers promise in mitigating a fundamental challenge of conventional amplitude-only holograms, namely its low efficiency.

High-fidelity temporally-corrected transmission through dynamic smoke via pixel-to-plane data encoding.

We propose a new approach for high-fidelity free-space optical data transmission through dynamic smoke using a series of 2D arrays of random numbers as information carriers. Data to be transmitted in dynamic smoke environment is first encoded into a series of 2D arrays of random numbers. Then, the generated 2D arrays of random numbers and the fixed reference pattern are alternately embedded into amplitude-only spatial light modulator, and are illuminated to propagate through dynamic smoke in free space. Real-time optical thickness (OT) is calculated to describe temporal change of the properties of optical wave in dynamic smoke environment, and transmission noise and errors caused by dynamic smoke are temporally suppressed or corrected. Optical experiments are conducted to analyze the proposed method using different experimental parameters in various scenarios. Experimental results fully verify feasibility and effectiveness of the proposed method. It is experimentally demonstrated that irregular analog signals can always be retrieved with high fidelity at the receiving end by using the proposed method, when average optical thickness (AOT) is lower than 2.5. The proposed method also shows high robustness against dynamic smoke with different concentrations. The proposed method could open up an avenue for high-fidelity free-space optical data transmission through dynamic smoke.

Direct generation of 2D arrays of random numbers for high-fidelity optical ghost diffraction and information transmission through scattering media

In this paper, we propose a new approach for high-fidelity optical ghost diffraction and information transmission through scattering media in free space using a series of 2D arrays of random numbers as information carriers. A series of 2D arrays of random numbers are directly generated to encode the ghosts, i.e., original signals or images. The generated 2D arrays of random numbers are sequentially embedded into amplitude-only spatial light modulator (SLM), and are illuminated to propagate through scattering media in free space. Experimental results show feasibility and effectiveness of the proposed method, and different types of ghosts (e.g., analog signals and grayscale images) can be encoded and retrieved with high fidelity. In addition, comparisons using different algorithm parameters and different experimental parameters (e.g., different diffraction distances and wavelengths) are conducted to analyze and illustrate performance of the proposed method. It is found that the proposed method possesses high robustness against scattering media, and can realize high-fidelity ghost diffraction and optical information transmission. The proposed high-fidelity optical ghost diffraction could open up an avenue for many applications, e.g., free-space optical transmission through scattering media. @ 2022.

Physically-secured high-fidelity free-space optical data transmission through scattering media using dynamic scaling factors.

In this paper, we propose a method of physically-secured high-fidelity free-space optical data transmission through scattering media using physically- and dynamically-generated scaling factors. Optical channel characteristics are explored, and scaling factors are physically and dynamically generated to serve as security keys in the developed free-space optical data transmission system. The generated dynamic scaling factors provide a security layer for free-space optical data transmission. To the best of our knowledge, it is the first time to physically and dynamically generate scaling factors in free-space optical data transmission system to realize data encryption. The scaling factors existing in free-space optical data transmission channel are physically and dynamically controlled by using two optical devices, i.e., variable beam attenuator (VBA) and amplitude-only spatial light modulator (SLM). Nonlinear and dynamic variation of scaling factors is realized in different free-space wave propagation environments. It is experimentally demonstrated that high security can be guaranteed in the developed physically-secured high-fidelity free-space optical data transmission system, since one random scaling factor is physically and dynamically generated for the transmission of each signal pixel value. In addition, the proposed physically-secured free-space optical data transmission scheme is robust to noise and scattering, and high-fidelity signals are retrieved at the receiving end. The proposed method could open up a new research perspective for the secured free-space optical data transmission.

Physically-enhanced ghost encoding.

In this Letter, we propose a physically enhanced ghost encoding scheme that is realized by exploring optical channel characteristics, i.e., physically and dynamically generated scaling factors. It is found that scaling factors can be physically and dynamically generated to serve as security keys in a ghost encoding scheme, dramatically enlarging the key space and enhancing the security of optical ghost encoding schemes. To the best of our knowledge, this is the first time that dynamic scaling factors have been controlled in the optical path to realize physically enhanced ghost encoding. In addition to the illumination patterns used in optical ghost encoding schemes, the proposed method applies a variable beam attenuator and an amplitude-only spatial light modulator (SLM) to physically generate dynamic scaling factors as keys. Nonlinear variation of scaling factors is achieved in different free-space wave-propagation environments in the proposed method. A series of optical experiments are conducted to verify the feasibility and effectiveness of the proposed physically enhanced ghost encoding scheme. The proposed method could open up new research perspectives in optical ghost encoding.

Arbitrary Control of Polarization and Intensity Profiles of Diffraction-Attenuation-Resistant Beams along the Propagation Direction

We report on the theory and experimental generation of a class of diffraction-attenuation-resistant beams with state of polarization (SoP) and intensity that can be controlled on demand along the propagation direction. This is achieved by a suitable superposition of Bessel beams, whose parameters are systematically chosen based on closed-form analytic expressions provided by the Frozen Waves (FWs) method. Using an amplitude-only spatial light modulator, we experimentally demonstrate three scenarios. In the first, the SoP of a horizontally polarized beam evolves to radial polarization and is then changed to vertical polarization, with the beam intensity held constant. In the second, we simultaneously control the SoP and the longitudinal intensity profile, which was chosen such that the beam's central ring can be switched-off over predefined space regions, thus generating multiple foci with different SoP and at different intensity levels along the propagation. Finally, the ability to control the SoP while overcoming attenuation inside lossy fluids is shown experimentally for the first time in the literature (to the best of our knowledge). Therefore, we envision our proposed method to be of great interest for many applications, such as optical tweezers, atom guiding, material processing, microscopy, and optical communications.

Iterative Fourier Transform Algorithm Based on the Segmentation of Target Image for a High-Speed Binary Spatial Light Modulator

A digital micro-mirror device (DMD) has the potential to modulate an incident wave with high speed, and the application for holographic display has been studied by many researchers. However, the quality of reconstructed image isn't good in comparison with that from a gray-scale amplitude-only hologram since it is a binary amplitude-only spatial light modulator (SLM). In this paper, we suggest a method generating a set of binary holograms to improve the quality of the reconstructed image. Here, we are concerned with the case for which the object plane is positioned at the Fourier domain of the plane of the SLM. In this case, any point in the Fourier plane is related to all points in the hologram. So there is a chance to generate a set of binary holograms illuminated by incident wave with constant optical power. Moreover, we find an interesting fact that the quality of reconstructed image is improved when the spatial frequency bandwidth of the binary hologram is limited. Therefore, we propose an iterative segmentation algorithm generating a set of binary holograms that are designed to be illuminated by the wave with constant optical power. The feasibility of our method is experimentally confirmed with a DMD.

Speckle-free digital holographic recording of a diffusely reflecting object

We demonstrate holographic recording without speckle noise using the digital holographic technique called optical scanning holography (OSH). First, we record a complex hologram of a diffusely reflecting (DR) object using OSH. The incoherent mode of OSH makes it possible to record the complex hologram without speckle noise. Second, we convert the complex hologram to an off-axis real hologram digitally and finally we reconstruct the real hologram using an amplitude-only spatial light modulator (SLM) without twin-image noise and speckle noise. To the best of our knowledge, this is the first time demonstrating digital holographic recording of a DR object without speckle noise.

An active coronagraph using a liquid crystal array for exoplanet imaging: principle and testing

High-contrast imaging coronagraphs, used for the detection of exoplanets, have always adopted passive coronagraph optical components. It is therefore impossible to actively optimize the coronagraphs to achieve their best performance. To solve this problem, we propose a novel high-contrast imaging coronagraph which combines a liquid crystal array (LCA) for active pupil apodization and a deformable mirror (DM) for phase correction. The LCA we use is an amplitude-only spatial light modulator. The LCA is well calibrated and compensates for its amplitude non-uniformity and nonlinear intensity responsivity. We measured the imaging contrasts of the coronagraph system with the LCA only and without the DM deployed. Imaging contrasts of 10-4 and 10-5 can be reached at an inner working angular distance of 2.5 and 5λ/D, respectively. A simulation shows that the phase errors on the coronagraph pupil limit the contrast performance. The contrast could be further improved if a DM is deployed to correct the phase errors induced by the LCA and coronagraph optics.

Three-dimensional display of a horizontal-parallax-only hologram

We propose a three-dimensional (3D) holographic display by converting an optically recorded complex full-parallax (FP) hologram to an off-axis horizontal-parallax-only (HPO) hologram. First, we record the complex FP hologram of an object using optical scanning holography. We then convert the complex FP hologram to an off-axis HPO hologram through fringe-matched Gaussian low-pass filtering and with the introduction of an off-axis reference. Finally, we reconstruct the off-axis HPO hologram optically using an amplitude-only spatial light modulator. Until now, only computer-generated HPO holograms have been displayed optically. To the best of our knowledge, this is the first demonstration of a 3D display of an optically recorded HPO hologram.

Reconstructing unknown wavefronts by use of diffraction-intensity pattern subtraction

We propose an easy method for unknown wavefront reconstruction by a typical Fourier transform system based on an amplitude-only spatial light modulator (SLM). The Fourier transform of the diffraction intensity pattern on the backfocal plane is the autocorrelation of the wavefront-transmitted SLM. Two intensity patterns were recorded when we set the transmittance of one corner pixel on the SLM as 0 and 1, separately, under the condition that the transmittances of the other pixels equal 1. The complex amplitude of the wavefront-transmitted SLM can be extracted from the Fourier transform of the diffraction intensity pattern subtraction without complicated calculations. The simulation results certifying our theories are also given. Our method provides us with an easy way for detecting the wavefront with high speed and high quality in real time without the reference wave.

Adaptive correction to the speckle correlation fringes by using a twisted-nematic liquid-crystal display

An innovative technique for reducing speckle noise and improving the intensity profile of the speckle correlation fringes is presented. The method is based on reducing the range of the modulation intensity values of the speckle interference pattern. After the fringe pattern is corrected adaptively at each pixel, a simple morphological filtering of the fringes is sufficient to obtain smoothed fringes. The concept is presented both analytically and by simulation by using computer-generated speckle patterns. The experimental verification is performed by using an amplitude-only spatial light modulator (SLM) in a conventional electronic speckle pattern interferometry setup. The optical arrangement for tuning a commercially available LCD array for amplitude-only behavior is described. The method of feedback to the LCD SLM to modulate the intensity of the reference beam in order to reduce the modulation intensity values is explained, and the resulting fringe pattern and increase in the signal-to-noise ratio are discussed.

Decrypting binary phase patterns by amplitude

A method for decrypting a binary phase-only mask using an amplitude-only spatial light modulator key is proposed. The system is based on a double passage through a generalized phase contrast setup. A binary amplitude key is converted to phase for decrypting a binary phase-only encrypted pattern on the first passage. The decrypted phase pattern is subsequently converted to an intensity pattern on the second passage. A compact dual-path system, applicable in reflection geometry, is suggested for this decryption operation.