Image signal degradation of computational ghost imaging is an important issue in atmospheric turbulence, especially for the images in laboratory safety monitoring. However, the relevant research under non-Kolmogorov turbulence model has been rarely reported. To address this issue, we proposed an efficient image signal processing and transmission scheme tailored for low-sampling conditions to enhance image signal denoising for applications like educational laboratory safety monitoring. Our approach employed computational ghost imaging enhanced by a modified sequence on a Hadamard orthogonal basis. Then we simulated turbulence using a non-Kolmogorov spectrum mode, which a phase screen approach realized. We applied a bucket detector to capture light intensities reflected or scattered from the target and reconstructed the target image from captured signals with the computational ghost imaging algorithms. The orthogonally modified Hadamard matrix patterns with a compress ratio were applied to reduce noise and enhance image quality. Simulations show significant image quality improvements over traditional schemes.
Ghost imaging (GI) is a technology of nonlocal optical imaging, which is widely used in the field of information encryption. However, there are some inherent shortcomings in the encryption process, such as imperfect security and low imaging precision. To solve these problems, we propose a camouflaged encryption mechanism based on sparse decomposition of principal component orthogonal basis and ghost imaging (CSP-GI). Unlike the traditional encryption method, there are two-level keys and ciphertexts. The information of the secret image is hidden in the camouflaged image to improve the security of the system. The use of the sparse decomposition of principal component orthogonal basis expands the information content of encrypted transmission, improves the accuracy of reconstructed image, and reduces the reconstruction time. The simulation and experimental results show that the proposed method not only has good security and robustness but also reduces reconstruction time.
Optical hierarchical sorting has attracted significant attention in recent years. The existing approaches use either complex numerical calculation or computer-aided experimental tools for optical hierarchical sorting. We proposed a method to perform hierarchical sorting, which is computationally simple and does not need computer aid. In particular, we employed a focused optical vortices array (FOVA), which is generated and focused by a spiral phase plate array (SPPA) and a microlens array, respectively. We designed different heights for the spiral phase plate in different columns of the SPPA. This enabled different columns of the FOVA to carry different topological charges and consequently possess different capture capabilities. To realize hierarchical sorting, we exploited the properties of FOVA by deploying it in a microfluidic chamber containing particles of various sizes. The four columns of the FOVA formed four corresponding capture regions in the flow area of the particles. From our theoretical analysis and numerical results, we observed that particle sizes in the range of 1 to 582 nm could be sorted. Our approach provides a theoretical framework that can be readily employed in experiments for optical hierarchical sorting.
Multistaircase spiral phase plates (SPPs) are more commonly used to generate an optical vortex, as compared to ideal continuous surface SPPs. However, due to the complexities and difficulties involved in the manufacturing of the multistaircase SPPs, the number of the staircases M should not be high and should be sufficient to guarantee a similarity between the M staircase situation (considering an intrinsic topological charge l) and the ideal situation. Therefore, a Fraunhofer diffraction analysis model is proposed to quantitatively and quantificationally solve the diffraction field of the vortex generated by multistaircase SPPs. A finite hypergeometric series summation is applied to solve the diffraction fields of the vortices with different parameters, under the conditions of uniform and Gaussian incident beams. The simulation results show that the summation of the first certain terms of the Fourier expansions can appropriately approximate the diffraction field, and M is positively related with l to approach the ideal situations. Thus, the proposed model can provide a reference for designing and setting the parameters of multistaircase SPPs.
KEYWORDS: Modulation, Free space optics, Turbulence, Free space optical communications, Binary data, Signal to noise ratio, Telecommunications, Wireless communications, Fiber optic communications, Energy efficiency
A 4×8 overlapping amplitude and pulse position modulation scheme under non-Kolmogorov turbulent Gamma-Gamma Channel Free-Space Optical Communication system is proposed and simulated. Then the bit-error-rate performance is simulated for further research and application.
A theoretical model is proposed to analyze the time-domain spectral phase en/decoding and evaluate the unconditional security of the optical code-based secure optical communication systems with DPSK modulation. The confidentiality of the systems with and without bit-by-bit code scrambling technique is investigated. It mathematically proves that the system without code scrambling lacks confidentiality and the system employing code scrambling can realize unconditionally secure transmission. Furthermore, encoding only within the central band of the spectrum is sufficient for perfect secrecy, and the secrecy rate can be potentially improved. It allows the system to operate in the burst mode for code generation and encoding and thus have an idle time for rearm.
Theoretical analysis as well as system simulation of a NRZ/RZ (NRZ and duty cycle 33% 50% 66% RZ) shaped APPM
(2×4 and 4×4) modulated scheme for gamma-gamma distribution turbulent free-space optical communication is done.
Results show lower BER in RZ scheme especially with duty cycle 33%, however the NRZ scheme has higher frequency
efficiency. So both BER performance and frequency efficiency can be synthetically considered to choose a proper
scheme according to simulation results.
T-APPM is combined of TCM (trellis-coded modulation) and APPM (Amplitude-Pulse-position modulation) and
has broad application prospects in space optical communication. Set partitioning in standard T-APPM algorithm has
the optimal performance in a multi-carrier system, but whether this method has the optimal performance in APPM
which is a single-carrier system is unknown. To solve this problem, we first research the atmospheric channel model
with weak turbulence; then a modified T-APPM algorithm was proposed, compared to the standard T-APPM
algorithm, modified algorithm uses Gray code mapping instead of set partitioning mapping; finally, simulate the two
algorithms with Monte-Carlo method. Simulation results showed that, when bit error rate at 10-4, the modified
T-APPM algorithm achieved 0.4dB in SNR, effectively improve the system error performance.
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