Objectives: Coherent combining of fiber lasers is an effective approach to pass through the power limitation of single channel fiber laser and achieve high output power, and is also an important foundation for the traditional high energy laser system developing towards laser phased array high energy fiber laser system. The system configuration of high power fiber laser coherent combining is introduced, and the main study object and the key technique are pointed out. The research status and development tendency of fiber laser coherent combining are presented. The main technique challenges for fiber laser coherent combining are analyzed.

Methods: By analyzing the method and research status of the key techniques of the fiber laser coherent combining, including linear-polarization narrow-linewidth fiber amplifier, aperture filling and multi-beam comprehensive control (phase control, optical path difference control, tilt/tip control and polarization control), the influences of the key techniques on the effect of the fiber laser coherent combining is clear and definite. Through analyzing of the development trends and techniques level of fiber laser coherent combining, we point out the technical challenges of the coherent combining.

Results: Linear-polarization narrow-linewidth fiber amplifier is the foundation of fiber laser coherent combining with high power output, the highest power of the coherent combinable linear-polarization narrow-linewidth fiber amplifier is 1.89kW, and the highest power of the non-polarization maintaining narrow-linewidth fiber amplifier is 3kW. Aperture filling is the key to obtain high energy concentration degree, the key for divided-aperture coherent combining is to improve the fill factor of the splicing beams; the common-aperture coherent combining employing the method including diffractive optical element, coherent polarization beam combining, and optical waveguide to achieve laser with single aperture output. Multi-beam comprehensive control is the key point for achieve coherent combining with stable and good beam quality. In coherent combing, optical axis, tilt, polarization, optical path difference and the phase difference of each beam should be effective controlled. With the development of the techniques, coherent combining that takes the long-range transmission and adaptive atmospheric compensation into account, laser phased array obtained by divided-aperture coherent combining and cutting-edge application oriented pulse coherent amplification network are the trends of the fiber laser coherent combining. In 2015, the highest power of divided-aperture coherent combining was achieved by MIT, with 44 kW laser output power by 42 channels of fiber lasers. The highest output power of common-aperture coherent combining was obtained by AFRL in 2016, with the output power of 4.9 kW. From the development of the key techniques, fiber laser coherent combining face a lot of challenges. Firstly, the primary challenge faced is how to improve the output power of linear-polarization narrow-linewidth amplifiers, how to improve the beam quality, how to improve the polarization degree and how to compress the laser linewidth, by comprehensively suppressing the mode instability and nonlinear effect. Secondly, how to improve the beam fill factor of the divided aperture coherent combining, and how to simultaneously improve the peak power and the main-lobe power proportion, is a big technical challenge; how to improve the efficiency for the common-aperture coherent combining is also a big challenge should be considered. Finally, How to realize the decoupling control of these parameters in the large-scale array coherent combining system, how to improve the control bandwidth and system stability for the multi-parameter control of coherent combining, especially the coherent combining’s control bandwidth for long-range transmission, is a critical technical challenge for the coherent from laboratory to the real application.

Conclusions: Coherent combining of fiber laser is not only an effective technical scheme for obtaining higher output power, but also the foundation of fiber laser phased array. Many institutes worldwide have conducted a lot of studies on various key technologies of coherent combining. Divided-aperture coherent combining is the foundation of laser phased array, and has good application prospects in asteroid defense and space debris cleanup. Common-aperture combining is the inevitable route to achieve the pulse CAN, and has broad scope of applications in the frontier and basic areas like laser particle accelerator. Although significant progresses have been achieved on the various key technologies of coherent combining and 44 kW laser output has been obtained, the current output power still cannot satisfy the requirements of many practical applications. To become practical, the coherent combining needs to overcome a lot of challenges in basic theories, key techniques and engineering implementations, with respect to improvement of single-channel comprehensive performance, improvement of fill factor and efficiency, large-scale array comprehensive control and large-scale array system integration. In order to meet the future application’s requirements, it’s necessary to take in-depth investigation on the various key technologies, and to make breakthroughs with regard to the various technical challenges, thus to lay foundations for applying the fiber laser coherent combining to the basic & frontier areas.