Atmospheric effects including absorption and scattering, and turbulence could introduce signal power loss and severe mode crosstalk for the orbital angular momentum (OAM)-based free-space optical communication (FSOC). Therefore, it is of great significance to simultaneously increase signal power and mitigate mode crosstalk. In this paper, for the OAM beam from a coherent laser array with a discrete vortex (CLA-DV) based on coherent beam combining, we investigate its propagation characteristics by employing theoretical derivation and the random phase screens simulation in atmospheric propagation, respectively. The probability density and OAM spectrum are given and compared for CLA-DV and Gaussian vortex beam. The results demonstrate that the Gaussian vortex beam exhibits smaller mode crosstalk under weak atmospheric turbulence conditions, while CLA-DV shows a good performance on crosstalk mitigation for strong atmospheric turbulence conditions in long-distance links. Furthermore, with a specially designed radial phase-locked Gaussian laser array composed of two orthogonal polarized coherent laser arrays carrying different OAM states, a scheme of optical communication system possessing simultaneously polarization-division multiplexing and OAM multiplexing is proposed. The normalized energy weight matrices of all 16 non-zeroth-order OAM modes are numerically calculated. To verify the feasibility of the proposed scheme, the performance of an eight-bit grayscale Lena image facing various atmosphere turbulences is evaluated. The quality of transmitted images becomes worse with the turbulence strength and transmission distance increase, which is confirmed by the trend of average optical signal error rates. This work will provide theoretical insight for improving the performance of OAM-based FSOC under scattering conditions.

The predecessor of China Optics was China Optics and Applied Optics Digest, which was founded in 1985. At that time, it was the only retrieval journal in the field of optics in China. At the end of 2008, China Optics and Applied Optics Digest was renamed 《中国光学》的前身是1985年创办的《中国光学与应用光学文摘》，当时是国内光学领域唯一的检索刊物。2008年底《中国光学与应用光学文摘》转型更名为《中国光学与应用光学》期刊，刊载内容突破了文摘的限制，正式成为学术刊物。

In this work, we demonstrate a three-stage all-fiber master oscillator power amplifier (MOPA) system emitting over 1 W of the linearly polarized narrow-linewidth pulse output power with 10 kHz pulse repetition frequency and 400 ns pulse width without the amplified spontaneous emission (ASE) at ~1.5 µm. Initially, we analyze the effect of the ASE on the first derivative of backscatter intensity via a contrast experiment. The experiment results show that the noise intensity with angle of inclination is affected by the ASE of the amplifier, thereby decreasing the signal-to-noise ratio of the output power. The laser is characterized by a very low or no ASE level of the output signal when acquiring the first derivative of backscatter intensity with low noise. This condition provides a reference for improving the properties of the laser used for cloud detection.

In this study, we presented a 1550 nm, high-power, high-efficiency random fiber laser. A method, utilizing the single-mode erbium-ytterbium co-doped fiber with proper length and the highly reflective fiber Bragg grating with wide reflection bandwidth, is used to surmount the generation of Yb-ASE and low slope efficiency. More than 10 W output power is achieved, with a slope effi-ciency of 36.7% and single transverse mode output. The random fiber laser stably operates without significant amplitude fluctuation under maximum power, and which can provide a high-performance light source for a variety of applications.