Beam combining of fiber laser is an effective way to break the power limitation of single fiber laser and to achieve more powerful laser beams. In recent years, significant breakthrough has been achieved in power combining, spectral beam combining and coherent beam combining. In this paper, the recent progress of beam combining of fiber lasers is reviewed, the trend and the characteristics are summarized, and the challenges for further increasing the performance of laser beam combining systems are discussed.

All fiber signal combiner is widely used for power combining because of the advantage of compact structure. In 2013, IPG Photonics Corporation developed the world’s first 100 kW high-power fiber laser system, which was based on 90 fiber lasers with kW level output power. And such a 100 kW laser system was successfully applied to laser processing. In 2021, the 100 kW level fiber laser systems, which were power combined by using all fiber signal combiner, were reported by University of South China and Raycus Fiber Laser Technologies Corporation. In 2024, high-power fiber laser systems from 150 kW to 200 kW were reported by BWT Corporation, Han’s Laser Technology Industry Corporation, and Maxphotonics Corporation, respectively.

For spectral beam combining, most of the high power outputs were achieved by using reflective diffraction gratings or dichroic mirrors as the beam combination components. In 2016, Lockheed Martin achieved a fiber laser system with output power of >30 kW by spectral beam combining of 96 individual fiber lasers based on reflective diffraction grating. Using the similar method, 10 kW level spectral beam combining systems were developed by Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences and China Academy of Engineering Physics in the same year. In recent years, the output power of the diffraction grating based spectral beam combining system was increased rapidly. By spectral beam combining of 60 fiber lasers, output power of 150 kW was obtained by Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. By using dichroic mirrors, several fiber lasers can also be cascaded combined into one beam. 10 kW level output power was generated by spectral beam combining of 2‒3 fiber lasers based on dichroic mirrors by Nanjing University of Science and Technology, National University of Defense Technology and the 11th Research Institute of China Electronics Technology Group Corporation.

And then, the progress of coherent beam combining of nonlinear frequency conversion laser and ultrafast laser was reviewed in this paper. In some practical applications, lasers in ultraviolet, visible and mid-infrared bands are required. So, coherent beam combining was employed for power scaling of nonlinear frequency conversion lasers. For example, the average powers of 600 W at 520 nm (second harmonic) and 300 W at 347 nm (third harmonic) were generated from an eight-beam, sub-nanosecond fiber laser system by Osaka University. For another example, coherent combining of two mid-infrared difference frequency generators by active phase control was reported by the French Aerospace Lab. In recent years, great progress has also been made in coherent combining of ultrafast fiber lasers. In 2020, ultrafast pulses with 10.4 kW average output power based on coherent combining of 12 fiber amplifiers was reported by Friedrich Schiller University Jena, and coherent beam combining of 61 femtosecond fiber amplifiers in a tiled aperture configuration with combining efficiency of ∼50% was achieved by Institut Polytechnique de Paris. In 2023, by combining of 16 fiber amplifiers, the nearly Fourier transform limited pulses with an energy of 32 mJ and a duration of 158 fs were obtained by Friedrich Schiller University Jena. Besides, coherent beam combining technique has also been employed in versatile applications, such as laser guide star, laser interferometer gravitational-wave observatory (LIGO), laser communication and optical field manipulation.

Besides, those three beam combining technologies can be used in a hybrid way, therefore more powerful laser and much narrower pulse width could be expected. For example, coherent spectral combining technique was proposed to generate pulses with shorter pulse durations.

By using power, spectral and coherent combining technologies, the output power has been increased by a magnitude compared with that from a single fiber laser. And beam combining technologies have been moved from laboratory researches to practical applications, such as laser processing, laser communication and national defense. The developments of beam combining technologies have great significance in the history of high-power fiber laser, and demonstrate the possibility of obtaining higher output power and developing ultra-large scale fiber laser systems in the future. The current researches of beam combination of fiber lasers are basically concentrated in the near infrared band, and combining of lasers in ultraviolet, visible and mid-infrared bands will be an important direction of the next development. With the breakthrough of technologies such as deep learning in recent years, artificial intelligence has been used in the field of laser beam combining, and many outstanding research results have emerged. In the future, artificial intelligence can be further used in modulation signal optimization, combination element design, multi-parameter control, beam quality evaluation, and so on.