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In the existing binocular stereo-matching methods for a speckle pattern, the pixel-by-pixel correlation weight calculation across the entire global disparity range is time-consuming. To address this issue, this paper presents a fast stereo-matching method for a speckle pattern based on grid and gradient constraints. The grid pattern is projected onto the measured object. The grid features are extracted to form the local disparity constraints, which narrow the search ranges. Moreover, the image gradient is used in selecting the pixels involved in the correlation weight calculation, which reduces the redundant computations. 3D reconstruction experiments of paper and plaster models were employed to evaluate the matching accuracy and speed of the proposed method. Furthermore, the experiments of measuring displacement were designed to validate the accuracy of our method. In contrast to the existing methods, our approach increased the matching speed by 66% on average while ensuring the matching accuracy. [ABSTRACT FROM AUTHOR]

This paper presents a novel reconstruction method based on multi-refraction to remedy the non-linear refraction, which recovers 3D geometry from 2D images. To simplify calibration, this paper prove that air calibration can replace underwater calibration. Finally, a multi-line rotating structured is designed to improve the measurement efficiency without additional equipment. The results of the clean water experiment show that the measurement accuracy is 1.91 mm within the 1000–2000 mm depth of field. The qualitative analysis demonstrates our method can reconstruct the surface topography of underwater U and N letter boards. In different Nephelometric Turbidity Unit (NTU) (as NTU = 0.156 mg/ml) environment, a circular plate with a diameter of 220 mm is measured, the shape reconstruction error (whether the reconstructed shape is a circular, square, or other) is 1.74 mm; the diameter measurement result is 218 mm, so the size error of the circular plate is 2 mm. [ABSTRACT FROM AUTHOR]

This paper uses a numerical simulation method to evaluate the propagation performance of the partially coherent flat-topped vortex hollow (FTVH) beam and the Gaussian Schell-model (GSM) beam through atmospheric turbulence along slant links. The atmospheric turbulence is simulated by random phase screens, and the performance of the beam models is evaluated by using scintillation index and signal-to-noise (SNR) on the receiver plane with various simulation conditions and parameters. This paper has shown that there is an optimal coherence length for both the partially coherent FTVH beam and the GSM beam. In different parameter settings, the partially coherent FTVH beam has the best propagation performance at N = 1 M = 1. Compared to the GSM beam, the partially coherent FTVH beam obtains an obvious improvement in SNR. The results show that the partially coherent FTVH beam is able to improve the communication efficiency of an FSOC system. [ABSTRACT FROM AUTHOR]

The periodic misalignment of the wrapping phase and fringe order based on the phase-shift + coding method is known to occur easily, leading to jump errors in absolute phase recovery. This paper proposes a two-phase unwrapping method based on shifting-phase coding. This approach encodes the fringe order information and the wrapping phase into the four-step phase shift mode. It generates four composite fringe patterns to directly resolve the wrapping phase and fringe order, thus improving the measurement speed by reducing the number of fringe projections. The interleaved wrapping phases are then constructed by a sequence that changes the initial wrapping phase. The two-phase unwrapping becomes the absolute phase, so the phase error is avoided and the measurement accuracy is improved. The proposed strategy's effectiveness is proved by the comparison experiments of different methods. Similarly, the suggested method's reliability is verified by the measurement experiments of several discontinuous objects. [ABSTRACT FROM AUTHOR]

This paper provides a concise overview of hybrid Free Space Optical (FSO)/Radio Frequency (RF) systems operating under fog and smog channels. With the rapid growth of wireless communication and the emergence of the Internet of Things (IoT) have brought about unprecedented connectivity and transformed various aspects of our daily lives. However, certain environmental conditions pose significant challenges to wireless communication systems like FSO and RF technologies. Fog and smog with its dense moisture-laden atmosphere and presence of particulate matter cause absorption and scattering of light, leading to reduced link quality and limited range for FSO communication system. Whereas, RF systems are although less affected by fog/smog as compared to rain but still have inherent limitations in terms of bandwidth and capacity. To overcome these challenges and ensure reliable communication, researchers have turned their attention to hybrid FSO/RF systems which can combine the strengths of FSO and RF technologies and mitigate the limitations of each technology. This work summarizes and explores the hybrid system architecture and machine learning techniques used to enhance the performance and adaptability of wireless communication systems. Additionally, we present the existing experimental research on hybrid FSO/RF systems, performance evaluation metrics, and future research directions to pave the way for further advancement. Abbreviation FSO: Free Space Optical; RF: Radio Frequency; IoT: Internet-of-Things; OS: Operating Systems; OWC: Optical Wireless Communication; LOS: Line-of-Sight; LPF: Low Pass Filter; AM: Amplitude Modulation; FM: Frequency Modulation; PSK:Phase Shift key; QAM: Quadrature Amplitude Modulation; OFDM: OrthogonalFrequency Division Multiplexing; LF: Low frequency; M-L: Medium frequency; UHF:Ultra High Frequency; SHF: Super High Frequency; EHF: Extremely High frequency; PM: Phase Modulation; QPSK: Quadrature Phase Shift Key; DSSS: Direct SequenceSpread Spectrum; FHSS: Frequency Hopping Spread Spectrum; LAN: Local AreaAccess; D/A: Digital to Analog; LO: Local Oscillator; PA: Power amplifier; A/D:Analog to Digital; LNA: Low Noise Amplifier; FEC: Forward error correction; APC: Automatic Power Control; RSSI: Received Signal Strength Indicator; ML: Machine Learning; RL: Reinforcement Learning; DL: Deep Learning; LR:Logistic Regression; DT: Decision Tree; SVM: Support Vector Machine; KNN:K-Means Neighbour; PCA: Principal Component Analysis; QL: Q-Learning; DQN:Deep Q-NEtwork; R-F: Random Forest; GB: Gradient Boosting; NA: NaiveBayes; ANN: Artificial Neural Network; DTA: Decision Tree with AdaBoost; MIMO: Multiple-Input Multiple-Output; GEO: Geostationary Orbit; NIR: NearInfra-Red; VLC: Visible Light Communication; OP: Optical Communication; BER: Bit Error Rate; ECC: Ergodic Channel Capacity; SNR: Signal-to-NoiseRatio; AO: Adaptive Optics; ATP: Acquisition- Tracking and Pointing; LEO:Low-Altitude Earth Orbit; UAVs: Unmanned Aerial Vehicles; NTN: Non-TerrestrialNetwork; RSSI: Received Signal Strength Indicator; FSPL: Free Space Path Loss; ITU: International Telecommunication Union; GANs: Generative AdversarialNetworks [ABSTRACT FROM AUTHOR]

Optical image encryption technique has become increasingly important. Recently, double-encryption is introduced to enhance security. However, most coherent double-cryptosystems suffer from artefact noise, while the incoherent double-cryptosystem is not stable enough. This paper proposes a method for image double-encryption using speckle decorrelation properties in spatial and spectral domains. The plaintext is disguised as several different fake plaintexts that are separately encrypted through rotated random phase masks under incoherent illumination with different wavelengths. Since the speckle decorrelation properties in spatial and spectral domains, the plaintexts contained in the detected speckle images are multiplexed in a ciphertext. The security is enhanced since correct decryption can be achieved only when all the keys with spatial and spectral domains information are obtained. Simulations, experimental demonstrations and attack resistance tests are implemented with a simple incoherent cryptosystem that is easy to achieve in practical applications. [ABSTRACT FROM AUTHOR]

In multimode interferometers, the interaction between several modes brings a high level of complexity to the interpretation of its light patterns. With recent advances, it is possible to selectively excite only a couple of modes inside the device. This paper presents an analytical method for determining the phase difference between two propagating modes in the multimode interferometer, in an effort to simplify the usage and expand the range of applications for this type of optical devices. [ABSTRACT FROM AUTHOR]

In a recent work, a technique was proposed to recover the absolute phase maps of fringe patterns with two or three selected fringe wavelengths with the aim to improve anti-error capability. To further increase the anti-error capability associated with the phase unwrapping operation, multiple fringe patterns with different wavelengths may also be used. In this paper, we present a general rule to select the wavelengths and the strategy to increase phase error tolerance bound. Based on this rule, it is concluded that to maximize the error tolerance, four-wavelength method should be selected, where the maximum upper bound of the tolerable phase error is π/6. To minimize the required number of fringe patterns, three-wavelength method with three-step PSP should be selected, that is exactly the case the previous paper has demonstrated. These theoretical conclusions are verified by experimental results. [ABSTRACT FROM AUTHOR]

Dark-field microscopic imaging system has high spatial resolution, but small image field. When an optical surface with a large-diameter is tested, the image stitching is necessary to obtain the full-aperture detection results. In this paper, Harris corner detection algorithm is used to achieve the full aperture testing result by stitching multiple detection images. According to the Shannon sampling theorem, how the size of the overlapping area influences the stitching results is analysed in detail. Combined with the spatial scale of surface defects, a method for determining the size of the overlapping area is given. The standard scratch patterns are used to simulate the stitching process. On this basis, an actual stitching processing is carried out on the detection images of surface scratches of four different spatial scales. Both the simulation and experimental results show that it is reasonable to use the scale of the smallest defect to determine the number of sampling points in the stitching area. [ABSTRACT FROM AUTHOR]

In this paper, a high-performance metamaterial sensor with a square split-ring resonator and a double circular split-ring resonator is proposed, which can simultaneously excite broad-band and narrow-band electromagnetically induced transparency-like (EIT-like) effects in the terahertz band. First, the mechanisms of the dual-band EIT-like effects have been revealed by the analyses of the transmission spectra and surface current distributions in the metamaterial. Then, the influences of structural parameters on the dual-band EIT-like are discussed further. Finally, the metamaterial is very sensitive to the change of the refractive index of the surrounding media, and the sensitivity of the dual-band EIT-like resonances can reach 150 GHz/RIU and 237.5 GHz/RIU and the quality factor values are 1.67 and 7.92, respectively. The proposed metamaterial broadens the pathway for the excitation of multi-band EIT-like effects and offers promising applications in multi-frequency sensors, filters and many other aspects. [ABSTRACT FROM AUTHOR]

At present, the co-phasing detection algorithms for segmented mirrors struggle to balance the measurement range and accuracy. This paper proposes a coarse co-phasing measurement method using the incoherent properties of a broadband (400–800 nm) light source combined with the incoherence of two semicircular diffraction patterns of visible light as a template to detect the piston error, and a narrowband algorithm for fine co-phasing measurement. The diversity factors between template patterns and actual diffraction patterns can be calculated by the cross-correlation algorithm. This proposed method addresses long measurement time caused by low target flow rate and is characterized by wide measurement range, high precision and high energy utilization. The method is theoretically researched and simulated and an experimental system which was constructed to verify the effectiveness of this method achieves 15 nm detection accuracy, and the measurement range using the proposed method can match the measurement range of the actuator. [ABSTRACT FROM AUTHOR]

In this paper, we propose a full-Stokes image reconstruction (FIR) network to reconstruct polarization images from a snapshot of random modulated image. The Stokes-Mueller matrix theory demonstrates that a image captured by conventional polarization imaging system is a linear superposition of the four Stokes vectors, thus image decomposition methods can be used for full-Stokes image reconstruction. The network is constructed based on a cycled conditional adversarial network, which is trained utilizing the physical mapping relationships between the Stokes vectors and the modulated images. Simulations and experiments are conducted to demonstrate the network's wavelength independence and modulation independence, proving the effectiveness of proposed FIRnet. To some extent, our method alleviates the contradiction between measurement time and accuracy of polarization, promoting the application of compact snapshot full-Stokes imaging. [ABSTRACT FROM AUTHOR]

This paper describes the development of a common path off-axis quantitative phase microscope (QPM), enabling high-speed quantitative measurements of sub-cellular dynamics in a live cell. The proposed 4f optical system consists of a polarizer, lenses, calcite crystal, pinhole and analyser, attached to the output port of an optical microscope converting it into a temporally stable QPM. The temporal phase stability of this optical configuration is measured to be equal to ∼20 mrad without a vibration isolation table. The utility of the microscope is demonstrated by conducting experiments on a polystyrene sphere (dia. 12 μm) and human red blood cells (RBCs). Furthermore, the developed system is implemented to study temporal membrane fluctuation in human RBCs over ∼30s and mouse embryonic fibroblast cell dynamics over 2 hours. Therefore, the system offers an energy-efficient and simple solution for generating object and reference beams for common-path QPM, making it an attractive alternative to existing approaches. [ABSTRACT FROM AUTHOR]

Blood oxygen content plays an important role in clinical diagnosis and treatment. However, interference signals will be introduced in the process of the signal acquisition, which will affect the accuracy of measurement results. Although the filter can eliminate most of the interference, there will still be a part of the residual interference signal. In order to eliminate these residual interference, this paper proposes a signal denoising method based on ensemble empirical mode decomposition (EEMD), lifting wavelet transform (LWT) and singular spectrum analysis (SSA). Firstly, the measured signal is decomposed by EEMD to obtain multiple modal components (IMF). The high-frequency components are denoised by LWT, and then reconstructed with the low-frequency components. Finally, the SSA is carried out on the reconstructed signal, the appropriate components are selected to reconstruct the signal to obtain the final denoised signal. After testing the EEMD-LWT-SSA method with simulated and measured signals, it can be seen that the method can effectively suppress the residual noise in signals and improve the accuracy of data. [ABSTRACT FROM AUTHOR]

Transmitting a vortex beam (VB) with orbital angular momentum encounters significant aberrations due to atmospheric turbulence (AT) in free-space optical communication systems. Historically, research has focused on adaptive optics (AO) to correct distorted VB intensity distributions by generating phase compensation screens. In this paper, we introduce an image-translation-based machine learning (ML) framework using generative networks to directly produce corrected VB intensity profiles, thus mitigating AT-induced aberrations. Our network was trained with numerical simulation datasets to learn the mapping relationship between distorted and pristine intensity distributions. Subsequently, we evaluated its generalization capabilities using datasets with varied AT conditions and propagation distances. The trained network consistently generated corrected intensity profiles closely matching the desired outcomes, significantly reducing the required training data. Our methodology effectively compensates for distorted VB intensity, achieving a remarkable 40 dB enhancement compared to the distorted profiles. [ABSTRACT FROM AUTHOR]

In this paper, an encryption scheme, based on a double deterministic phase mask and the Yang-Gu algorithm, is proposed. It consists of double random phase encoding and the Yang-Gu algorithm in Fresnel and Fourier transform, respectively. The random phase masks are replaced with deterministic phase masks that are constructed using chaotic logistic and cubic maps. The encrypted image obtained by the scheme in one-way binary modulation is real and more convenient for storage and transmission. The validation of the proposed cryptosystem is carried out using numerical simulations on greyscale images of Man, DICOM and CUH binary images. The proposed scheme is investigated for its robustness through statistical analysis using histograms and correlation plots. The scheme is also tested for its resistance against occlusion, noise and cryptographic attacks. The proposed cryptosystem is found to be highly sensitive to its private key and Fresnel parameters and therefore secure. [ABSTRACT FROM AUTHOR]

Under uneven lighting conditions, power inspection (PI) images captured by drones exhibit low contrast, lost details and severe noise, which affects the efficiency of monitoring. In response to these issues, this paper proposes a new method for enhancing PI images based on discrete fractional wavelet transform (DFRWT). Firstly, according to the image entropy feature, an appropriate fractional order of DFRWT is selected to decompose the PI image. Secondly, we introduce a low-frequency fusion enhancement method based on complex number domain. It employs different mapping functions to enhance the brightness and contrast of PI image, then the results are combined to reconstruct a new low-frequency component. Here, an improved quantum-behaved particle swarm optimization (QPSO) algorithm is proposed to determine the optimal parameters of the functions, preventing over-enhancement. Finally, an adaptive threshold function is applied to handle noise and edge coefficients in high-frequency components. Using 5 quality metrics to evaluate 8 existing algorithms on 9 datasets, the results indicate that the proposed algorithm can effectively enhance the clarity of PI images, performing excellently in noise suppression and edge detail enhancement. [ABSTRACT FROM AUTHOR]

A low-bias operational modified partially depleted absorber photodetector (MPDA-PD) is proposed in this paper. By adopting gradual doping following Gaussian distribution and inserting a buffer layer, the bandwidth-efficiency product and the saturation characteristics of the traditional partially depleted absorber photodetector (PDA-PD) in the low-bias range (0∼1 V) are optimized. When the thickness of the absorption layer is 1200 nm, the 3-dB bandwidth of the MPDA-PD with the diameter of 8 µm reaches 59.1 GHz at 0.6 V. The bandwidth-efficiency product of the MPDA-PD reaches 33.98 GHz, which is 40.6% higher than the traditional PDA-PD in the reference. Meanwhile, the direct-current (DC) saturation current of 121.1 mA and RF output power of −12.2 dBm at 50 GHz are obtained. [ABSTRACT FROM AUTHOR]

In digital holography (DH), the key parameters in the optical path structure, such as diffraction distance, optical path difference and deflection angle, affect greatly the image quality of holographic reconstruction. Thus, it is crucial to determine the appropriate values of parameters. In this paper, the influence of these parameters on the quality of reconstructed images is deeply analysed through the DH principle and numerical simulation. A method to optimize automatically the parameters in the optical path is proposed based on an improved particle swarm optimization algorithm. An experimental setup for reflective off-axis DH is constructed, and coin and optical resolution plate are taken as test samples to perform the experimental research. Experimental results demonstrate that the proposed method can not only effectively enhance the quality of reconstructed images in the light of five indicators but also reduce the complexity and difficulty of adjusting the optical path parameters in the experiments. [ABSTRACT FROM AUTHOR]

Current studies show that binary fringe patterns can be optimized in the phase or intensity domain. The problem with the former is the poor sensitivity to defocusing levels and the problem with the latter is that phase errors can not be restricted efficiently. In this paper, a binary fringe pattern optimization technology, combining intensity similarity and phase similarity, is proposed to improve fringe pattern structure similarity and defocusing robustness. Moreover, a binary patch optimization framework is also proposed to solve the time-consuming problem. Simulation and experiment results show that the proposed optimization can produce binary fringe patterns with high phase quality and defocusing robustness. [ABSTRACT FROM AUTHOR]

Phase measurement profilometry (PMP) is widely used to reconstruct three-dimensional (3D) shape of objects. However, errors are introduced during standard phase unwrapping process, which considerably brings down measurement accuracy and lowers method reliability. This paper proposes a novel method to identify the errors, and subsequently correct these errors through erecting adaptive local windows. Under proposal, two robust algorithms are developed to correct these errors in initial rudimentary correction as well as in subsequent refined error removal. The adaptive local window is designed as such that the fringe order corresponding to each period of the wrapping phase is independent of each other. Therefore, it avoids the error transmission in the correction process. Experiments are carried out on absolute phase error correction and 3D point cloud reconstruction. Experimental results verified the developed method in terms of reducing the stratification of 3D point cloud and improving the quality of 3D measurement. [ABSTRACT FROM AUTHOR]

Traditional multi-view feature extraction methods based on manifold learning frequently overlook the similarity sequence between samples, failing to capture the intrinsic manifold structure of raw nonlinear samples and restricting the recognition performance of multi-view learning. In this paper, we propose a novel similarity-sequenced multi-view discriminant feature extraction method, called Similarity -sequenced Multi-view Discriminant Correlation Analysis (SMDCA), which explicitly considers the sample sequences based on similarity. The method constructs similarity-sequenced discriminant scatters for preserving the sequence structure of within-class samples and develops between-class correlations with the similarity-sequence structure information for further constraining intrinsic manifold structure of cross-view samples. SMDCA can also simultaneously extract low-dimensional sequence features with well-discriminative power from multiple views. Extensive experiments exhibit that SMDCA can provide higher recognition accuracy and stronger robustness in image recognition tasks. [ABSTRACT FROM AUTHOR]

The binary defocusing has been extensively studied in the three-dimensional measurement. But if the projector is slightly defocused, the binary fringes after defocusing still contain high-order harmonics compared with the ideal sinusoidal fringes, so the phase error caused by the nonlinear response is not negligible. In this paper, two models are proposed to calculate and compensate phase error, which include a double-precision iterative compensation model (DPICM) and a dual-domain iterative compensation model (DDICM). These two models obtain accurate phase errors by fusing the phases at different precision and domains. DPICM is composed of the double precision method and the improved iterative algorithm, and DDICM consists of the dual-domain method and the improved iterative algorithm. This improved iterative algorithm is used to compensate phase error, which can improve phase accuracy. DPICM and DDICM reduce the RMS error by 25.5% and 13.5% respectively. [ABSTRACT FROM AUTHOR]

Recently, metal halide perovskite quantum dots (MHPQDs) have attracted considerable interest in sensing activities due to their excellent optoelectronic properties and high surface activity. Here, aiming for heavy metal detection, we proposed an ultrasensitive turn-off fluorescent probe based on MAPbBr3 MHPQDs for the measurement of copper ions in an organic solvent. A low limit of detection (LOD) of 5.6nM was achieved and the test range was recorded from 0nM to 100nM. In order to achieve this high sensitivity, a thin film layer of electrospun polystyrene(PS) nanofibre was introduced for the structural support of MHPQDs, by which the sensitivity increased two orders of magnitude. The quenching mechanism is also discussed in the paper. We believe the electron transfer effect between Cu2+ and perovskite QDs made the main contribution for turn off sensing in our prototype device. [ABSTRACT FROM AUTHOR]

To improve the accuracy of phase recovery in quantitative phase imaging, a novel two-stage synthetic denoising method is proposed in this paper. In this method, to avoid the loss of effective detail information, the mode of fixed attenuation parameter setting is changed into two stages of self-adaptation setting: the first attenuation parameter is defined as the convolution of filtered pixels in the window of interest, the second attenuation parameter is defined as the noise standard deviation of each pixel after filtering in the first stage. Moreover, the Pearson coefficient expression is redefined to get a more accurate similarity evaluation, and the algorithm's operational efficiency has been improved using cross-correlation functions and fast Fourier transform. The comparative results of simulations and experiments show that this method is more effective in reducing speckle noise in quantitative phase imaging and enhancing phase recovery accuracy, providing a new digital image denoizing method for efficient and accurate imaging. [ABSTRACT FROM AUTHOR]

The classification of White blood cells (WBCs) plays an important role. However, the traditional method of blood smear analysis is laborious. This paper presented a classification method of WBCs based on hyperspectral images and Deep learning (DL). The U-net network was proposed to extract spectral features of WBCs region of interest (ROI) under the pseudo-color images with specific hyperspectral images (420.8, 557.2 and 667.4 nm). For spectral features and the pseudo-colour images of hyperspectral data, attention-aided one and two-dimensional convolutional neural networks were applied to establish WBCs classification models, respectively. The overall average accuracy can reach 94.20% and 92.60%, respectively. A fusion network was constructed to make full use of the spectral and image spatial features, and its classification accuracy reached 96.20%. In terms of overall average accuracy, the fusion network model was the optimal, but for individual types of WBCs, each network had its own unique advantages. [ABSTRACT FROM AUTHOR]

Infrared image systems often generate stripe noise because of the non-uniformity of the focal plane array, which significantly reduces the visual quality of the image. To solve this problem, we proposed an effective single-frame denoising method in this paper. First, the wavelet function extracts the approximate and vertical components of the original image containing stripe noise. Then, the approximation component is denoised by parameter estimation, and the vertical component is denoised by guided filtering. Finally, wavelet reconstruction is performed to realize the denoising process of the original image. This method avoids the loss of details of other components of the image and achieves an excellent denoising effect. The experimental results on the public datasets indicate that our proposed method can effectively eliminate the stripe noise of infrared images compared with some advanced methods. [ABSTRACT FROM AUTHOR]

In this paper, we consider a cavity magnomechanical system composed of a microwave (MW) cavity, magnon, and phonon modes, where the magnon is coupled, respectively, to the MW cavity and phonon through both the magnetic-dipole and magnomechanical interactions. We demonstrate that the flux-driven Josephson parametric amplifier with a nonlinear gain is crucial in creating the squeezed states of the magnon and phonon modes. We show that, by the cavity-magnon state-swap interaction, the magnons can be completed in a squeezed state, and that the phonons can also be squeezed by driving the magnons through a strong red-detuned MW field. Essentially, our system reveals a squeezing transfer from cavity photons to magnons and then to phonons. The influence of physical and environmental parameters on the degree of squeezing is widely explored. [ABSTRACT FROM AUTHOR]

Optical coherence tomography (OCT) is an emerging optical imaging modality with high resolution and non-invasive, which plays an important role in applications such as material detection and disease diagnosis, especially for ophthalmic retinal diseases such as age-related macular degeneration, diabetic macular edema and choroidal neovascularization. However, since OCT utilizes the coherent interference of light, the generated image is inevitably affected by speckle noise, which blurs the structural information of the image such as layer structure and lesion point, and the low-quality OCT image makes its subsequent application become difficult. To solve this problem, an OCT image denoising fusion based on discrete wavelet transform and spatial domain feature weighting is proposed in this paper. Extensibility experiments show that the proposed algorithm can better remove noise and retain its precise structural information compared with several state-of-the-art OCT image denoising algorithms. [ABSTRACT FROM AUTHOR]

Topography measurement is a pivotal application in quantitative phase imaging using digital holographic microscopy. In this paper, a reflection-type digital holographic microscope is set up to reconstruct the phase image of the sample with step characteristics, and a noise suppression approach based on an iterative adaptive bilateral filter is proposed to reduce phase coherent noise. The proposed denoising approach considers the phase difference in the neighbourhood to better identify and preserve step characteristics. Experimental results demonstrated the effectiveness of the proposed method. [ABSTRACT FROM AUTHOR]

This study considers the propagation of spatial soliton in nematic liquid crystals in the presence of damping in the nonlocal regime. The importance of diffractive radiation shed for an undisturbed propagation of nematicon has been analysed in this paper using variational and numerical methods. When the damping coefficient is constant, a large value of diffractive radiation height is required to counteract the effect of damping. If the damping coefficient varies with the propagation distance, the diffractive radiation has an important role in reducing the damping effect. In the case of the periodic damping profile, even larger diffractive radiation is needed to overcome the effect of damping as compared to the constant damping profiles. But a low value of diffractive radiation is enough to overcome the hyperbolic damping. [ABSTRACT FROM AUTHOR]

This paper proposes a multiple-input multiple-output (MIMO) terrestrial-satellite laser communication uplink system based on optical spatial modulation with medium-based modulation (OSM-MBM) joint modulation. The Málaga(M) distribution is used to describe the combined effects of near-ground turbulent light intensity scintillation, beam wander and angle of arrival fluctuation. The closed expression of the bit error rate (BER) of the OSM-MBM joint modulation MIMO terrestrial-satellite laser communication uplink is derived. The simulation analyzes the influence of transmitting radius, receiving aperture, beam divergence, zenith angle and signal-to-noise ratio on the system error performance, and compares it with traditional modulation, media-based modulation and optical space modulation. Finally, the experiment is verified by Monte Carlo. This research provides a theoretical reference for the engineering realization of the modulation technology for the MIMO terrestrial-satellite laser communication uplink system. [ABSTRACT FROM AUTHOR]

In general, the reflectors of off-axis three-mirror system have large apertures and large differences in surface shape, which means it is difficult to use one set of null compensators to test all the aspheric mirrors in the off-axis three-mirror system because of the limitation of the dynamic range of the testing equipment. In this paper, we propose a surface shape test method that combines a refractive-zoom lens group and a Spatial Light Modulator (SLM) as a null compensator to test all the aspheric surfaces of one off-axis three-mirror system. The proposed method is verified by simulating the null test process of three aspheric mirrors of one off-axis three-mirror system, with the consideration of the fabrication and alignment errors. Simulation results show that the probability that residual wave aberration is within 0.1λ RMS is 50% for the primary mirror and 40% for the secondary mirror and 80% for the tertiary mirror. [ABSTRACT FROM AUTHOR]

In this paper, we propose a new network to achieve low-sampling computational ghost imaging. The proposed neural network that combines ResNeXt, eHoloNet and spatial attention mechanism is efficient for object reconstruction just based on EMNIST training, which is much simpler and time-saving. The generalization of the neural network is verified with multi-slits as well as complex object. Both simulated and experimental results show that the proposed network can give an effective reconstruction at 0.7% sampling rate. The neural network in this work is of great significance to ghost imaging in a wider application scenarios, such as real-time imaging and dynamic detection of motion object. [ABSTRACT FROM AUTHOR]

Digital holographic microscopy is a new type of quantitative phase measurement tool. When a coherent light source is used for illumination, the phase coherent noise seriously affects the imaging quality and measurement accuracy. A method of phase coherent noise reduction based on adaptive non-convex sparse regularization is proposed in this paper. This denoising problem is considered as a sparse regularization model, which uses a non-convex penalty called the generalized minimax-concave; the penalty can lead to a more accurate estimation of the phase image amplitude and reduce most of the noise. Meanwhile, the threshold of the algorithm to solve this problem is guided by the edge credibility map adaptively. In edge regions, the threshold value is small so that edge details are preserved; however, in other regions, the threshold value is big so that the phase coherent noise is removed. The experimental results confirmed the validity of the proposed method. [ABSTRACT FROM AUTHOR]

In this paper, we quantitatively distinguish electromagnetically induced transparency (EIT) and Autler–Townes splitting (ATS) in a photonic crystal nanobeam cavity (PCNC)-waveguide coupled system. Akaike's information criterion (AIC) is applied to calculate the two effects' respective weights in the simulated transmission spectra. EIT and ATS are two phenomena that both have a transparent window in the transmission caused by different mechanisms in the atomic system. It's difficult to distinguish these two phenomena directly from the spectrum. AIC provides a measurement distinguishing the two effects in the atomic, plasmonic, optomechanical system, etc. Based on the temporal coupled-mode theory, the theoretical model of the structure is established, and the transmission formula is deduced. Distance between the PCNC and the waveguide is varied to further explore the factors affecting the weights. The study shows great potential in precisely and quantitatively evaluating the interference of fields in future optical on-chip integration design. [ABSTRACT FROM AUTHOR]

The concept of a near superposition [Othman A, Yevick D. Int. J. Theor. Phys. 2018; 57:2293–2308] has recently been introduced as the superposition of two semi-identical states and has, in particular, been realized with coherent states. This paper provides a higher-order generalization of this procedure and, in particular, examines the 'derivative limit' for which the phases of all the lower-order states are identical and equal to a particular constant. In the derivative limit, the near coherent state superposition is transformed after successive derivative operations into a high-order Derivative Coherent State (DCS). The DCS is accordingly generated by constructing and subsequently displacing a certain stationary seed state formed by superimposing only even or odd Fock states. The mathematical and statistical properties of the DCS display several behaviours and non-classical features that are pronounced for high-order states. These non-classical properties include the Q-Mandel, higher-order antibunching, linear squeezing, and amplitude squared squeezing. [ABSTRACT FROM AUTHOR]

Scattering-medium-based optical image encryption provides an intrinsically tamper-resistant physical key. In this paper, we utilize a scattering medium with ideal optical properties as the physical key. The plaintext is transformed into four noise-like holograms with the help of the four-step phase-shifting digital holography. Low-frequency components of the four holograms are extracted and combined into a new image by the lifting wavelet transform (LWT). Then the combined image is scrambled by the nonlinear 3D chaos-based cyclic shift. In decryption, the digital optical phase conjugation (DOPC) technique is utilized to extract plaintext. Numerical simulations prove the feasibility of the proposed method, and the security analysis shows that our method can resist different attacks. [ABSTRACT FROM AUTHOR]

Salinity gradients (SGs), temperature gradients (TGs), and different-sized air bubbles affect the optical signal propagating in the UWOC link. These effects contribute to beam wandering, spot dancing, and intensity fluctuations in the received optical signal. In this paper, experimental analysis has been carried out to estimate the effect of SG, TG and air bubbles on the underwater optical signal. The beam wandering, mean and variance deviation, displacement of weighted centroid, the statistical distribution of received intensities and link outage probability have been estimated in this work. It has been found that the beam wandering effects follow a Gaussian mixture model (GMM). It is also observed that in the presence of air bubbles in the SG, the chances of link outage increase from 48% to 80% and it increases from 7% to 28% in the TG conditions. Furthermore, the experimental and theoretical modelled outage probabilities are in close agreement. [ABSTRACT FROM AUTHOR]

In this paper, we report on the number of nanorods changing in the structural and optical properties of manganese oxide pyramidal nanostructures with a square cross-section deposited on the glass substrate. The structural and optical properties of the manganese films were investigated. The X-ray diffraction (XRD) analysis showed manganese oxide pyramidal nanostructures with 10, 9, and 8 nanorods that have Mn3O4, Mn3O4, and Mn2O3 phases, respectively. The optical measurement for manganese oxide thin films showed the transmittance and the energy band gap in the ultraviolet and visible regions. The transmittance decreases with increasing the number of nanorods. In addition, the optical band gap increases from 2.64 eV to 2.88 eV with increasing the number of nanorods. [ABSTRACT FROM AUTHOR]

In order to reduce the amount of data transmission and key consumption, a double-image compression and encryption scheme based on chaotic system and real-preserving multi-order discrete fractional Fourier transform (RPMODFrFT) is proposed in this paper. It can compress and encrypt two images simultaneously, and the measurement matrix of compression sensing is controlled by 2D Logistic-Sine-Coupling map (2D-LSCM). After consolidating two compressed images into one image, the image is further encrypted by scrambling and 2D-LSCM controlled RPMODFrFT operation. At last, the bitwise XOR operation using 2D-LSCM with the initial value related to the plain image is carried out. The simulation results show that the proposed encryption-compression scheme has high security and robustness through a lot of attack analysis, i.e. differential attack analysis (NPCR > 99.6022% and UACI >33.5658%), entropy values (average 7.99710), statistical attack (correlation coefficient values are almost 0), noise attack and cropping attack. [ABSTRACT FROM AUTHOR]

In the optical coherence tomography system, the mismatch in dispersion between the two interference arms will reduce the axial resolution of the system. Therefore, dispersion compensation is required. However, the current depth-resolved dispersion compensation method uses a fixed window function to extract signals at different depths, which has phase calculation error and reduces the accuracy of dispersion compensation. In this paper, we optimized the rectangular window function for the signal interception. According to the functional relationship between the phase and the wave number in the presence of dispersion, the second-order fit to the obtained phase is performed and the standard error of the fit result is used as the criterion. The optimized rectangular window is used to intercept the signal and perform FFT, thereby effectively reducing the phase error caused by the window function. And we accurately extracted the dispersion coefficients of samples to improve the accuracy of dispersion compensation. [ABSTRACT FROM AUTHOR]

At present, there is always a potential threat in the process of information transmission. As a way to protect data security, image hiding has attracted extensive attention. Current image hiding algorithms have insufficient resistance to deep leaning based steganalysis algorithms and relatively low hiding capacity. This paper presents an image hiding algorithm based on a generative feedback residual network (GFR-Net), which hides multiple color secret images in a single color carrier image. First, several secret images and a carrier image were fed into the image hiding network, in which the secret images were embedded into the carrier image, resulting in an output of container image. A recovery network also based on GFR-Net was designed to reconstruct the secret images from the container. The extensive experiments for hiding normal and encrypted images show that the proposed image hiding model has a good performance in terms of payload and security. [ABSTRACT FROM AUTHOR]

Using a photoconductive antenna to generate terahertz (THz) waves is a common method, and the material of the antenna is usually low-temperature gallium arsenide (LT-GaAs). A LT-GaAs epitaxial wafer with four layers structure is prepared in this paper, and then the LT-GaAs thin film is obtained by etching the aluminium arsenide (AlAs) with dilute hydrochloric acid and transferring it to the silicon wafer to make a thin film photoconductive antenna. The performance of the antenna is evaluated using an asynchronous optical sampling (AOS) terahertz time-domain spectroscopy (THz-TDS) system with variable repetition frequency. The system uses the spectral coherence method of two lasers to obtain trigger signals and generates THz signals with high signal-to-noise ratio. This also confirms that the system has the advantages of high spectral resolution and fast detection speed. It has incomparable advantages over the traditional THz-TDS system in the environment requiring rapid measurement. [ABSTRACT FROM AUTHOR]

A critical area of research is studying the performance of semiconductor optical amplifiers (SOAs) at high temperatures. In this paper, the performance of all-optical exclusive-OR (XOR) logic gate is investigated at high operating temperatures (TOP) using different SOAs technologies such as conventional SOAs, carrier reservoir (CR)-SOAs, reflective SOAs (RSOAs), and photonic crystal (PC)-SOAs, for the first time to our knowledge. The time-dependent differential equations of SOAs, CR-SOA, RSOAs, and PC-SOAs at different operating data rates have been solved. The performance of the XOR logic gate is evaluated by calculating the quality factors and plotting the eye diagrams for each amplifier. Besides, the dependence of the key operating parameters on the TOP is examined and assessed for each scheme. The results showed that the RSOAs perform better at high temperatures than other proposed SOA technologies. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the focusing properties and focal shift of a partially coherent vortex cosine-hyperbolic-Gaussian beam (PCvChGB) passing through a lens system. Based on the extended Collins formula, the analytical expression for a PCvChGB propagating through a thin lens is derived in detail. By using the obtained formula, the influences of the spatial coherence length, the Fresnel number, and the initial beam parameters on the focusing properties and the focal shift of a PCvChGB are discussed numerically. It is shown that the average intensity distribution in the focal region and the focal shift of the focused PCvChGB are strongly affected by the above factors. The obtained results could be beneficial for applications of vortex cosine-hyperbolic-Gaussian beam (vChGB) beams in beam shaping and atom optics. [ABSTRACT FROM AUTHOR]

The paper discusses the atmospheric turbulence effect on modulating retro-reflector (MRR) optical communication systems with theoretical analysis and experimental verification. The transmission model is proposed by the phase screen method. Based on this model, the scintillation indices (SI) are calculated, and the aperture averaging effect of the MRR terminal is analyzed. Furthermore, a test bed with 506 m transmission link is established. In the experiment, the SI values of the MRR link and the direct link are obtained and compared. Experimental data agree well with the numerical simulation results and verify the theoretical model. The theoretical results are useful for analyzing the influence of turbulence on MRR systems. [ABSTRACT FROM AUTHOR]

A rapid 3D measurement method based on three-channel projection is proposed in this paper. Project a composite color fringe map and an all-white map to the object to be measured. Firstly, crosstalk correction is performed on the composite color fringe pattern collected by the camera then the reflectivity of fringe image is corrected by all-white image. Then, the background normalization principle is used to solve the spectrum aliasing problem when the Fourier transform obtains a truncated phase. Finally, the three-frequency heterodyne method is used for phase unwrapping to obtain the continuous phase and 3D shape. Compared with the traditional color fringe projection method, this method not only eliminates the spectrum aliasing problem but also solves the uneven fringe reflectivity problem when measuring color objects using an all-white map. This method only requires two images to achieve measurement of 3D color dynamic objects. [ABSTRACT FROM AUTHOR]

We discussed evolution characteristics of optical vortices with the cross phase in free space and found that both splitting and rotating can occur for vortices with circular symmetry due to cross phase. The rotation is bidirectional and the rotational direction will reverse at some certain distance. The physical mechanism behind the splitting and rotating of vortices is considered to be the effective anisotropic diffraction, which results from the cross phase. We also investigated the dynamics of superimposed pattern with two astigmatic vortices of opposite cross phase. The rotating of vortices always happens, however, the splitting only occurs for nonzero net cross phase coefficient. The results in the paper provide us with a theoretical method to control the dynamics of vortices by adjusting the cross phase, then may find potential applications in the field of optical tweezers. [ABSTRACT FROM AUTHOR]

The high quality mixed white light is widely used in some fields of illumination of machine vision and optical projection. Optical design can improve white light production using RGB LEDs mixture method. In this paper, a kind of optical design method based on double-layer random microlens array is proposed for better white lighting. The design effect is verified by some comparisons with the double-layer non-random microlens array system. Experimental results show that the normalized standard deviation of white point shift is decreased from 0.05 to 0.01, the average colour difference is decreased from 44 to 2.5, and the illumination uniformity is increased from 0.66 to 0.82. The proposed design method can eliminate the incomplete overlap of rays and colour shadow effectively, and the proposed optical system design method can solve the problems of white point shift, colour difference, uniformity in mixed white light illumination. [ABSTRACT FROM AUTHOR]