In ophthalmology diagnosis, high-fidelity fundus images are essential for disease diagnosis and intervention. However, many real-world clinical conditions may degrade the quality of the acquired images and thus affect clinical diagnostic accuracy. Traditional convolutional neural network-based retinal fundus image enhancement methods cannot always capture long-range dependencies, which reduces the overall visual quality of images, especially for real retinal fundus images. Furthermore, existing enhancement methods often fail to fully utilize low-resolution structural detail information, which potentially leads to inaccurate pivotal fundus vessel topology or capillary details. In this paper, we propose a novel Structure-Aware Transformer-based attention fusion Network (SAT-Net) for low-quality retinal fundus image enhancement. First, we introduce a Transformer-based attention fusion module which incorporates window-based self-attention and channel self-attention to capture global spatial dependencies and emphasize important feature channels simultaneously. This fusion significantly improves the overall perceptual quality of the image by enhancing both the local details and the uniformity of the non-vessel background regions. Second, we introduce a cross-quality knowledge distillation technique, which bridges the quality gap between high-quality and low-quality fundus images. By designing a high-performing teacher network to guide a lightweight student network, the student network enables to capture detailed features from low-quality fundus images, further preserving critical diagnostic information and fine topology structures. Moreover, we design a structure-aware multi-scale loss function by using a trainable subnetwork to obtain the edge structure from different scales to better constrain pivotal fundus vessel structure and capillary details. Comprehensive quantitative and qualitative experiments on both synthetic and real fundus image datasets robustly validate that our proposed SAT-Net outperforms other state-of-the-art methods for fundus image enhancement. In addition, extensive comparative experiments on both the vessel segmentation and Optic Disc/Cup detection tasks further validate the effectiveness and superiority of our proposed method.

Based on information-theoretic security, physical-layer security in an optical code division multiple access (OCDMA) system is analyzed. For the first time, the security leakage factor is employed to evaluate the physical-layer security level, and the safe receiving distance is used to measure the security transmission range. By establishing the wiretap channel model of a coherent OCDMA system, the influences of the extraction location, the extraction ratio, the number of active users, and the length of the code on the physical-layer security are analyzed quantitatively. The physical-layer security and reliability of the coherent OCDMA system are evaluated under the premise of satisfying the legitimate user’s bit error rate and the security leakage factor. The simulation results show that the number of users must be in a certain interval in order to meet the legitimate user’s reliability and security. Furthermore, the security performance of the coherent OCDMA-based wiretap channel can be improved by increasing the code length.

We theoretically demonstrate that the spontaneous emission rate (SER) of quantum emitter is greatly enhanced in the proximity of single or double layers of black phosphorus (BP). The maximum enhancement factor is on the order of 10⁶, about 3 times larger than that of graphene, due to the large photonic density of states near BP. We also study how the transition frequency, the BP doping level, the distance between emitter and BP, the existence of substrate underneath BP, and the orientation of transition dipole moment influence the SER. With properly designed parameters, both high SER of emitter and low propagation losses of BP surface plasmon polaritons can be achieved in the quantum system, making it a promising candidate for mid-infrared single-photon generation.

In this paper, 10 Gb/s free space optical (FSO) wiretap channel using optical code division multiple access (OCDMA) time-diversity reception is designed and demonstrated for the first time. Reliability and security are investigated under weak and strong turbulence conditions. Two-dimensional optical encoder and decoder are constructed by wavelength selective switches and optical delay lines, and one-dimensional optical encoder and decoder are constructed by couplers and tunable optical delay lines. Experimental results show that reliability and security can be enhanced simultaneously using OCDMA time-diversity reception. At the received power of 2.58dBm in strong turbulence, compared to non-diversity system, bit error rate (BER) of legitimate user in OCDMA time-diversity system is reduced from $3.9\times 10^{-7} to $6.73 \times 10^{-8}$ . Furthermore, the reliability improvement is more obvious when the correlation between the two signals of OCDMA time-diversity is smaller. The secrecy capacity can be increased from 0.546 bit/symbol to 0.665 bit/symbol in strong turbulence when received power is 2.28dBm. On the other hand, the secrecy capacity of strong turbulence is about 15% higher than that of weak turbulence.

In the context of current communication systems, there is an urgent demand for more efficient and higher-speed optical signal processing technologies. Researchers are actively exploring new materials and devices to harness nonlinear optical phenomena, seeking advancements in this field. Nonlinear carbon materials, especially promising 2D materials, have garnered attention for their potential interaction with light and have become integral to the development of all-optical signal processing devices. This study focuses on utilizing a photonic device based on a nonlinear Au/CB composite material for optical Kerr switching. The application of Au/CB as a nonlinear material in the Kerr switch represents a noteworthy advancement, demonstrating its capability to modulate optical signals. By appropriately applying a pump light, the study achieves optical Kerr switching with an extinction ratio of approximately 15 dB in the fully off state of the signal light carrying a 10 GHz analog signal, marking a pioneering achievement in the field to the best of our knowledge. The experimental results, encompassing extinction ratios, signal control, and stability, not only validate the feasibility of this technology but also underscore its potential applicability within optical communication systems. The successful modulation and control of a 10 GHz analog signal showcase the practicality and effectiveness of the Au/CB-based optical Kerr switch. This progress contributes to the continuous evolution of optical Kerr switching, a crucial component in modern optical communication systems. Therefore, we believe that the Au/CB-based optical Kerr switch is an exceptionally promising and stable all-optical signal processing device. As the contemporary communication landscape evolves, the integration of this technology holds the potential to enhance the efficiency and speed of optical signal processing.

Due to the effect of atmospheric turbulence, the transmission reliability and physical-layer security of free space optical (FSO) communication system are facing many challenges. In this paper, for the first time, we design and investigate a 10 Gb/s reconfigurable FSO wiretap channel based on spatial diversity and optical code division multiple access (OCDMA). Reconfigurable two-dimensional optical encoder and decoder are constructed by wavelength selective switch and optical delay lines, and reconfigurable one-dimensional optical encoder and decoder are constructed by couplers and tunable optical delay lines. Under three different turbulence conditions, bit error rates (BER) of legitimate user and eavesdropper are measured. It can be seen that under turbulence effects, BER of legitimate user in spatial-diversity is lower than that of non-diversity system. By the rapid code reconfiguration of legitimate users, the eavesdropper can only use unmatched decoder. Thus, the eavesdropping methods of code interception and brute-force searching can be avoided. Experimental results show that this scheme can ensure the reliable transmission of legitimate users, and the eavesdropper can't crack the information when the extraction ratio is 1%. Hence, reliability and physical-layer security can be enhanced simultaneously.