Significance Fiber laser phased arrays have been applied in multiple regions, such as adaptive correction of wavefront phase distortion, dynamic beam steering technology, laser coherent beam combination and remote detection imaging, etc.. Among them, great progress has been achieved in dynamic beam steering owing to its tiling structures, organizable sub-apertures and available high power laser of laser coherent beam combination, which means it could be beneficial for flexible deflection, fast implementation of access addressing long-distance transmission. To further improve the performances of the whole scanning system and spatial scanning characteristics, this review discussed effective strategies provided by laser optical phased arrays, such as accurate regulation of array units, diverse array designs, flexible and free phase controlling methods, constant update of phase controlling algorithm and broad coverage of spectrum. These related advantages promote the development towards the direction of non-mechanical, large-angle, high precision and strong anti-interference.
Progress With the development of coherent beam combination, high power and more channels of system could be available in practice. Owing to these outstanding results, more possibility could be realized in dynamic beam steering, which was recognized to be an important application based on coherent beam combination. Since 2005, DARPA have tried to break through limitation of scanning speed with purely electronic-controlled laser phased arrays, which is caused by disadvantages of mechanical scanning device. In the same year, they put forward 'APPLE' system configured by distributed sub-apertures, which could be used for adaptive wavefront compensation and adaptive phase-locking controlled by stochastic parallel gradient descent algorithm (Fig.1). This novel structure provides strategies for flexible, non-mechanical and fast addressing beam steering. In 2013, DARPA kept on carrying out the project of SWEEPER, which aimed to realize 10 W laser scanning within a wide field of view up to 45 degrees. Recently, Civan Lasers develop multiple dynamic beam laser solutions to output power controlling, beam shaping and light field pattern customization (Fig.2). Moreover, their products could be applied in cutting, welding, metal additive manufacturing and drilling, such as OPA 6 laser and OPA 12 laser. It is worth noting that these device could work at high power. Interiorly, Xidian university have developed multiple beam steering system for realizing fast beam deflection with LiNbO3 phase shifters in 2021. Institute of Optoelectronic Technology, Chinese Academy of Sciences have achieved great progress in continuous beam steering with the combination of piezoceramics phase-modulators and adaptive fiber optical collimators in the same year (Fig.9). To further judge the effects of beam steering, some indexes have been given including scanning precision, range, speed, energy concentration and effect of sidelobes. Although the works were still limited in dynamic beam steering based on coherent beam combination, it showed potential in large scientific devices with high power, expending functions and large array integration.
Conclusions and Prospects Coherent beam combination possess great potential in information transmission, defense security, laser processing and material technology, etc.. There is still wide space in developing dynamic beam steering based on fiber laser phased arrays. Firstly, the dynamic instantaneous tracking scanning is hopefully realized by such platforms with large array areas and relatively high power. Secondly, scanning ability should be focused on wide field of view, high precision, large active range to achieve the sufficient details in the full region. Lastly, more light-weight device is required to be adaptive in all kinds of environment. Although great progress has been made in multiple indexes of coherent beam combination, large-scale application is still a challenge in dynamic beam steering.
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