Significance Coherent beam combining (CBC) of lasers is an effective technical approach for scaling the power of a laser while maintaining good beam quality. In 2009, Northrop Grumman Aerospace Systems demonstrated the world's first 100 kW solid-state laser system, in which seven 15 kW master oscillator-power amplifier laser chains were coherently combined. It was an important milestone in the history of laser development. In the past 10 years (2011—2020), CBC has developed rapidly, and many representative results have been produced. In this paper, the research progress of coherent laser beam combining in the past decade is reviewed.
Progress First, the output power of fiber, solid-state, and semiconductor lasers has increased significantly, which has provided high-power combinable laser elements for CBC systems. For example, the output power levels of single-frequency and narrow-line-width fiber lasers have reached 500 W and 4 kW, respectively; the average power of femtosecond fiber lasers has exceeded 1 kW; and Nd∶YAG and Yb∶YAG solid-state lasers have both generated over 20 kW in output power.
Second, enabling technologies for coherent laser beam combining have been developed, including phase control, tip-tilt control, polarization control, optical path difference control, high-order aberration control, and aperture?filling. More than 100 fiber lasers have been phase-locked based on active phase control technology. High-power adaptive fiber optic collimators have been designed that can be used for tip-tilt control of fiber lasers with kilowatt-level output power. Active polarization control of a kilowatt-level fiber amplifier has been realized, which has been employed to increase the combination efficiency of CBC systems. High-precision optical path difference real-time control systems have been designed and used for CBC of broad-spectrum and ultrafast lasers. Mode control technologies and adaptive optics methods have been employed for flexible mode manipulation. Aperture-filling technologies, such as microlens arrays, diffractive optical elements, and polarization beam combiners, have also been developed to increase efficiency of combination.
Third, representative results of the coherent combining of various kinds of lasers have been produced. For semiconductor lasers, coherent combining of 218 elements, with total output power of 38.5 W, has been realized. For solid-state lasers, pulse energy of 15.3 J has been obtained by the coherent combing of 6 solid-state lasers, and peak power of 3.7 GW has been obtained by the coherent combining of 2 ultrafast Yb∶YAG lasers. For fiber lasers, the number of laser channels has been increased to 107, and 16-kW output power has been obtained by the coherent combining of 32 fiber lasers. For ultrafast lasers, 61-fs fiber lasers have been coherently combined, and 10.4 kW average power has been generated by the coherent combining of 12 fs fiber lasers. In order to increase pulse energy, divided-pulse amplification and coherent temporal pulse-stacking technologies have been developed. Based on divided-pulse amplification, femtosecond pulses with energy of 23 mJ have been generated; using temporal pulse-stacking technology, 81 pulses were coherently combined to be one pulse with 10 mJ energy.
Fourth, coherent laser beam combining has been employed in versatile applications. For nonlinear frequency conversion applications, high pump brightness is required. A 600 W, 520 nm laser (second harmonic) and a 300 W, 347 nm laser (third harmonic) were obtained based on 1040 nm with kilowatt output power generated through a CBC system. Similarly, by coherently combining narrow-line-width lasers, sufficient output power was obtained for applications such as laser guide star and laser radar. By controlling the optical parameters of a coherent laser array, structural light fields with special spatial distribution can be obtained, such as vortex beams carrying orbital angular momentum. In recent years, research plans for large scientific facilities based on coherent laser beam combining have been proposed. For example, LIGO needs a low-noise single-frequency laser with hundreds of watts of output power; researchers have demonstrated the feasibility of obtaining such a laser source through coherent combining of fiber lasers. The International Coherent Amplification Network project has been proposed to provide a laser source for the next generation of particle accelerators.
Prospects In future work, increasing the number of coherent laser array elements and scaling the power of single-channel lasers will still be the development tendency. Lasers can be extended to almost arbitrary gain medium and wavelength band (visible light, mid-infrared, and even terahertz). Moreover, with the rapid development of computing technology and computational power, artificial intelligence techniques may be used for CBC-enabling technologies such as phase control and tip-tilt control. In the case of multiple-parameter control for massive laser arrays, high-power laser phased arrays will be realized.
