A low-numerical-aperture (NA) concept enables large-mode-area fiber with better single-mode operation ability, which is beneficial for transverse mode instability and nonlinear effects suppression. In this contribution, we reported a high-power fiber amplifier based on a piece of self-developed large-mode-area low-NA fiber with a core NA of 0.049 and a core/inner cladding diameter of 25/400 µm. The influence of the pump wavelength and fiber length on the power scaling potential of the fiber amplifier is systematically investigated. As a result, an output of 4.80 kW and a beam quality factor of ∼1.33 were finally obtained, which is the highest output power ever reported in a fiber amplifier exploiting the low-NA fiber. The results reveal that low-NA fibers have superiority in power scaling and beam quality maintenance at high power levels.

High-power ultrafast laser amplification based on a non-polarization maintaining fiber chirped pulse amplifier is demonstrated. The active polarization control technology based on the root-mean-square propagation (RMS-prop) algorithm is employed to guarantee a linearly polarized output from the system. A maximum output power of 402.3 W at a repetition rate of 80 MHz is realized with a polarization extinction ratio (PER) of > 11.4 dB. In addition, the reliable operation of the system is verified by examining the stability and noise properties of the amplified laser. The M² factor of the laser beam at the highest output power is measured to be less than 1.15, indicating a diffraction-limited beam quality. Finally, the amplified laser pulse is temporally compressed to 755 fs with a highest average power of 273.8 W. This is the first time, to the best of our knowledge, that the active polarization control technology was introduced into the high-power ultrafast fiber amplifier.

Orbital-angular-momentum (OAM) multiplexing technology offers a significant dimension to enlarge communication capacity in free-space optical links. The coherent beam combining (CBC) system can simultaneously realize OAM multiplexing and achieve high-power laser output, providing substantial advantages for long-distance communication. Herein, we present an integrated CBC system for free-space optical links based on OAM multiplexing and demultiplexing technologies for the first time, to the best of our knowledge. A method to achieve flexible OAM multiplexing and efficient demultiplexing based on the CBC system is proposed and demonstrated both theoretically and experimentally. The experimental results exhibit a low bit error rate of 0.47% and a high recognition precision of 98.58% throughout the entire data transmission process. By employing such an ingenious strategy, this work holds promising prospects for enriching ultra-long-distance structured light communication in the future.

The high-power mode-programmable orbital angular momentum (OAM) beam has attracted significant attention in a wide range of applications, such as long-distance optical communication, nonlinear frequency conversion, and beam shaping. Coherent beam combining (CBC) of an optical phased array (OPA) can offer a promising solution for both generating the high-power OAM beam and rapidly switching the OAM modes. However, achieving real-time phase noise locking and formation of desired phase structures in a high-power CBC system faces significant challenges. Here, an internal phase-sensing technique was utilized to generate the high-power OAM beam, which effectively mitigated thermal effects and eliminated the need for large optical devices. An OPA with six elements was employed for experimental demonstration. The first effective generation of over 1.5 kW mode-programmable OAM beam in a continuous-wave domain was presented. Moreover, the results demonstrated that the generated OAM beam could be modulated with multiple dimensions. The topological charge can be switched in real time from -1 to -2. Notably, this OAM beam emitter could function as an OAM beam copier by easily transforming a single OAM beam into an OAM beam array. More importantly, a comprehensive analysis was conducted on power scaling, mode switching speed, and expansion of OAM modes. Additionally, the system’s compact design enabled it to function as a packageable OAM beam emitter. Owing to the advantages of having high power and programmable modes with multiple dimension modulation in phase structures and intensity distribution, this work can pave the way for producing high-power structured light beams and advancing their applications.