Characterized by ultrashort pulse and ultra-high peak power, high intense lasers provide unprecedented experimental possibilities and extreme physical conditions for human beings, thus enabling various applications in major scientific and technological frontier fields, such as advanced manufacturing, high-order harmonic generation, particle acceleration, and ultra-high-speed phenomenon research. However, owing to limitations by the thermo-optic effect, scaling the average power and repetition frequency of ultrafast intense lasers is challenging, and discrepancy by an order of magnitude exists between the current technologies and the relevant application requirements (Fig. 1). To achieve ultrafast intense lasers with higher average power, researchers have adopted multichannel fiber-laser coherent-beam combination (CBC) to generate ultrashort laser pulses with high peak and average power levels simultaneously. Specifically, owing to the merits of large surface-to-volume ratio and excellent thermal optical performance endowed by the unique waveguide structure of optical fiber, fiber lasers can realize a high conversion efficiency (electro-optical efficiency exceeding 30%) via a flexible and compact configuration, which is beneficial for implementing a multichannel CBC system. Nevertheless, demonstrating the CBC of ultrafast fiber lasers is extremely challenging owing to the requirement of high precision control of multidimensional parameters such as delay, phase, beam pointing, and polarization. Over the past 15 years, significant efforts have been devoted to address those issues, and significant progresses have been achieved regarding the average power and energy scaling of ultrafast fiber-laser CBC systems. Specifically, the development of time-domain CBC and multidimensional parameter control technologies for fiber lasers has further enhanced CBC, thus reflecting the integrated development of ultrafast intense lasers technology and fiber-laser CBC technology. Based on this perspective, the research progress and development status pertaining to the CBC of ultrafast fiber lasers domestically and internationally in recent years are comprehensively reviewed herein, and the development trend of domestic high-power ultrafast intense lasers based on CBC technology is prospected.

At the early stage of the CBC of ultrafast fiber lasers, the general scheme is similar to that of the continuous-wave counterparts, i.e., the spatial-domain CBC, which includes tiled- and filled-aperture configurations. Tiled-aperture CBC was first proposed for the International Coherent Amplification Network (ICAN) project (Fig. 2), which aims to realize a single-pulse energy of 10 J with a repetition rate of 10 kHz by combining ten thousands of fiber chirped pulse amplifiers (CPAs). Currently, researchers have realized 61 channels of tiled-aperture CBCs (Fig. 3), whereas the corresponding average power of a single channel is only 25 W. Additionally, the combing efficiency is intrinsically limited by the space duty cycle of the fiber-laser array (maximum of 67%). Meanwhile, filled-aperture CBC is based on the successive combination of two collimated beams achieved using spatial optical components, and the combining efficiency can reach 100% in ideal conditions (Fig. 5). Thus far, ultrafast lasers with an average power exceeding 10 kW has been realized through the filled-aperture CBC of 12 high-power CPAs (Fig. 6), and a higher power of 100 kW is anticipated.

For the time-domain CBC, the main schemes are divided pulse amplification (DPA) and pulse stacking assisted by passive resonators. In principle, DPA leverages polarization beam splitting and delay control to segment a pulse into several discrete subpulses, which are subsequently amplified (most likely in a single channel CPA) and then combined into a single pulse via the reverse process of pulse dividing for power and energy scaling (Fig. 8). The pulse stacking includes two representative techniques, i.e., the stack-and-dump enhancement cavity (Fig. 10) and the Gires–Tournois interferometer (Fig. 11), which can stack a pulse train into a single pulse to increase the pulse energy, although at the expense of decreased repetition rate and average power. In recent years, to realize high-power and -energy ultrafast lasers, researchers have developed the spatiotemporal CBC technique, which associates the advantages of spatial- and time-domain CBC. Currently, this technique has afforded a maximum average power of 700 W and a pulse energy of 32 mJ. Moreover, ultrafast lasers with hundreds of kilowatts of average power and tens of joules of pulse energy are believed to be compatible with such a scheme (Fig. 13).

The development of ultrafast fiber-laser CBC technologies in recent years has enabled the achievement of fiber lasers that exhibit high power, high energy, and narrow pulse widths simultaneously, thus enabling novel applications. Nonetheless, the discrepancy between related domestic and international research progresses is significant. However, ultrafast fiber-laser CBC is anticipated to develop rapidly in China, where solid research outcomes have been obtained concerning the relevant unit technologies.