Coherent combining of fiber lasers by active phase control is an effective way to break through the power limit of a single fiber laser and achieve higher output power while maintaining good beam quality. Based on the research progress in China and abroad, this paper introduces the representative achievements in the past 20 years made by the coherent beam combination research group in National University of Defense Technology and presents the prospect of coherent beam combining (CBC) of fiber lasers.

We present our representative achievements in CBC of fiber lasers in this paper, which are organized as follows.

First, the high power key components for CBC were designed and manufactured. Various types of fiber amplifiers have achieved power breakthroughs. For example, a 500 W level single-frequency fiber amplifier, 7 kW level narrow line-width fiber amplifier, and 500 W femtosecond fiber amplifier were obtained. High-power phase modulators based on piezoelectric ceramics were developed. We also designed two kinds of high power adaptive fiber-optics collimators (AFOC), which were based on flexible hinges and piezoelectric bimorph actuators respectively.

Second, the active phase control of fiber lasers was studied. Various phase control methods were deeply researched, including the stochastic parallel gradient descent (SPGD) algorithm, dithering technique, heterodyne interference measurement technique, and deep learning algorithm. Some innovative phase control techniques were proposed to increase the control bandwidth, such as the single dithering technique, orthogonal dithering technique, and cascaded phase control technique.

Third, we also studied the high precision control of other optical parameters for CBC, including optical path difference control, tilt-tip control, and defocus aberration control. For example, we proposed an all-fiber optical path difference adaptive control method and simultaneously controlled phase and optical path in coherent combing of broadband light sources based on spectral filtering. In addition, a collimator was designed for defocus aberration compensation.

Fourth, beam combination techniques were demonstrated. Beam combination can be classified into tiled aperture and filled aperture. In the aspect of tiled aperture, a series of beam combination methods with high fill factor were designed and developed. For example, we proposed a coherent fiber-optics-array collimator that was mainly composed of a single unitary collimating lens and a prism. We also proposed a novel scheme of fiber collimator based on rod lens, which had good application prospects in the CBC of a large number of fiber lasers. In the aspect of filled aperture, we experimentally testified coherent polarization beam combining (CPBC) of eight low power fiber lasers, and 5.02 kW output power was obtained by CPBC of four fiber lasers with combining efficiency of 93.8% and beam quality of M²<1.3.

Fifth, based on the enabling technology mentioned above, a number of experimental systems were built. For high power fiber laser CBC systems, 1.08 kW output power was obtained by coherent combing of nine fiber lasers in 2011; CBC of a seven-channel fiber laser array with 7.1 kW overall output power was reported in 2020, and 21.6 kW was generated by CBC of 19 fiber lasers in 2021. For a large number of fiber laser CBC systems, phase locking of 32, 60, and 107 fiber lasers was realized by using the SPGD algorithm in 2014, 2019, and 2020 respectively. Based on the heterodyne interference measurement technique, efficient phase compensation of 397 and 1027 laser channels were realized in 2022 and 2023 respectively. For the pulsed fiber laser CBC system, 1.2 kW average power was generated by the coherent combining of seven nanosecond fiber amplifiers array in 2013; CPBC of two-femtosecond fiber lasers was realized with 313 W average power in 2018, and CPBC of two ultrafast laser channels was realized based on fiber stretcher and SPGD algorithm in 2022. For target-in-the-loop CBC systems, CBC of a fiber laser array with nine channels and 100 W level was reported in 2013, and atmospheric turbulence compensation was realized over a 1 km level propagation path for a six-channel fiber laser array based on target-in-the-loop CBC in 2018. In addition, CBC of fiber lasers with special wavelengths such as 1018 nm and 2 μm has also been achieved.

Sixth, the novel compact internal sensing phase locking techniques were presented. By using those techniques, the phase noises in the laser channels can be detected and compensated for before the lasers form the laser array. Based on spatial structure, internal phase locking of 12 fiber lasers was realized, and 1.5 kW output power was generated by CBC of three fiber lasers. Based on an all-fiber network, methods to compensate for π‑ambiguity between channels were proposed, and CBC of three fiber lasers was experimentally verified.

Seventh, CBC technique was employed for light field control, and special light fields such as vortex beams and vectorial beams were generated. For example, by CBC of six fiber lasers, a vortex beam with an output power of more than 1.5 kW has been generated.

Our group has researched CBC for nearly 20 years. Some representative results have been achieved. Artificial intelligence and light field control have been integrated with CBC. Some innovative breakthroughs have also been made in interdisciplinarity. The scientific research results have been continuously added to undergraduate and graduate courses such as Physical Optics and Advanced High Energy Laser Technology. A large number of graduate students have become the backbone force of scientific research. In the future, we will focus on the development of science and technology, student education, and talent cultivation integrally and make unremitting efforts to produce innovative results in this field.