Increasing bandwidth requirements have posed significant challenges for traditional access networks. It is difficult for intensity modulation/direct detection to meet the power budget and flexibility requirements of the next-generation passive optical network (PON) at 100G and beyond considering the new requirements. This is driving researchers to develop novel optical access technologies. Low-cost, wide-coverage, and high-flexibility coherent PON is emerging as a strong contender in the competition. In this article, we will review technologies that reduce the complexity of coherent PON (CPON), enabling it to meet the commercial requirements. Also, advanced algorithms and architectures that can enhance system coverage and flexibility are also discussed.

In this Letter, we propose and experimentally demonstrate a lens-free wavefront shaping method that utilizes synchronized signal block beam alignment and a genetic algorithm (SSBGA) for a diffuse non-line-of-sight (NLOS) visible light communication (VLC) system. The proposed method effectively controls the position and mobility of visible light beams by partitioning spatial light modulator pixels and manipulating beams to converge at distinct spatial positions, thereby enhancing wavefront shaping efficiency, which achieves a significant 23.9 dB optical power enhancement at +2mm offset, surpassing the lens-based continuous sequence (CS) scheme by 21.7 dB. At +40° angle, the improvement reaches up to 11.8 dB and 16.8 dB compared to the results with and without lens-based CS, respectively. A maximum rate of 5.16 Gbps is successfully achieved using bit-power loading discrete multi-tone (DMT) modulation and the proposed SSBGA in an NLOS VLC system, which outperforms the lens-based CS by 1.07 Gbps and obtains a power saving of 55.6% during the transmission at 4 Gbps. To the best of our knowledge, this is the first time that high-speed communication has been realized in an NLOS VLC system without a lens.

We propose an encryption technique for underwater visible light communication (UVLC) based on chaotic phase scrambling (PS) and conjugate frequency hopping (CFH). The technique is experimentally tested using an 8-level pulse amplitude modulation (PAM-8) and a 1.2 m underwater link. The security key of the phase scrambling code is generated according to a logistic map, and the frequency hopping is achieved by adding the same zero frequency points to the signal spectrum. The maximum transmission rate of 2.1 Gbit/s is measured with bit-error-rate (BER) below 7% the hard-decision forward error correction (HD-FEC) threshold of 3.8×10-3.

粒子喷溅是熔石英后表面激光损伤过程的重要现象，其实验观测及动力学参数获取一直存在困难。为实现后表面损伤粒子喷溅动力学特征获取，搭建了时间分辨双帧阴影成像系统，并开发了基于分层Voronoi图和弹性势能模型的粒子识别与匹配算法。结合图像配准和去噪步骤，该方法能够克服双帧阴影成像技术固有的背景噪声严重和原点失配的缺点，在6 μs及之后的延迟图像中能实现100%正确检测与匹配。与人工处理对比，该方法能以像素精度同时获取粒子的尺寸、速度大小及飞行方向，且数据处理时间缩短了约20倍。

Spontaneous optical emission properties of laser-produced plasma during laser damage events at input and exit surfaces of fused silica were retrieved and compared. We show that plasma at the input surface is much larger in size and exhibits significantly higher electron number density and excitation temperature, even when smaller laser energy was used. This effect was attributed to the stronger laser–plasma coupling at the input surface. In addition, a strong continuum background containing three peaks at 1.3 eV, 1.9 eV, and 2.2 eV was observed at the exit surface, and possible origins for this effect are also discussed.

Particle ejection is an important process during laser-induced exit surface damage in fused silica. Huge quantities of ejected particles, large ejection velocity, and long ejection duration make this phenomenon difficult to be directly observed. An in situ two-frame shadowgraphy system combined with a digital particle recognition algorithm was employed to capture the transient ejecting images and obtain the particle parameters. The experimental system is based on the principle of polarization splitting and can capture two images at each damage event. By combining multiple similar damage events at different time delays, the timeline of ejecting evolution can be obtained. Particle recognition is achieved by an adaptively regularized kernel-based fuzzy C-means algorithm based on a grey wolf optimizer. This algorithm overcomes the shortcoming of the adaptively regularized kernel-based fuzzy C-means algorithm easily falling into the local optimum and can resist strong image noises, including diffraction pattern, laser speckle, and motion artifact. This system is able to capture particles ejected after 600 ns with a time resolution of 6 ns and spatial resolution better than 5 μm under the particle recognition accuracy of 100%.

This tutorial focuses on devices and technologies that are part of laser-based visible light communication (VLC) systems. Laser-based VLC systems have advantages over their light-emitting-diode-based counterparts, including having high transmission speed and long transmission distance. We summarize terminologies related to laser-based solid-state lighting and VLC, and further review the advances in device design and performance. The high-speed modulation characteristics of laser diodes and superluminescent diodes and the on-chip integration of optoelectronic components in the visible color regime, such as the high-speed integrated photodetector, are introduced. The modulation technology for laser-based white light communication systems and the challenges for future development are then discussed.

A model that considers both thermal expansion and thermo-optical effects is developed to investigate the transmission variation of optical coatings when they are exposed to an intense laser beam. Our results indicate that a higher gradient of the transmission spectrum curve at a certain wavelength leads to a more evident variation of the coating transmission. Three customized HfO2–SiO2 multilayer coatings with different transmission spectra are used to measure the transmitted power under the irradiation of a 1080 nm continuous-fiber laser. Excellent agreement is found between the experimental result and the theoretical prediction. Our result is helpful for the improvement of such devices in the application of high-power laser systems.