Beam synthesis for high-power fiber lasers: progress, trends, and prospects

1 Introduction
Laser beam synthesis has been proposed and widely studied and applied in order to overcome the challenges encountered in improving the performance of single laser beams. As early as the 60s of the 20th century, Mr. Liu Songhao [1] pointed out in the article "The Development Status of Lasers": "In terms of device structure, in order to increase the output energy of a single rod-shaped working substance, in addition to increasing the length and diameter of the rod and increasing the energy density of the excitation light source, a multi-light source excitation device can be used. In order to increase
With the addition of output energy, dozens or even hundreds of devices can be coupled to form a so-called light maser array. The use of a maser array not only has the potential to greatly increase the output energy of the device, but also reduces the divergence angle of the output beam
Few. This approach has the potential to be one of the ways to develop high-energy devices. "The results of the literature survey show that the research process of laser beam synthesis is almost synchronous with that of lasers [2]
。 As stated in Ref. [1], "The implementation of the light maser array is very difficult, and many complex scientific and technical problems must be solved. ”
Since the beginning of the 21st century, fiber laser has been fully developed. With the overlapping factors such as the modularization of fiber lasers, the superior performance of fiber devices, and the rapid development of information technology, important progress has been made in laser beam synthesis technology with fiber lasers as typical units [3-12], which has become a scientific frontier and key research direction in the field of lasers, and has been an important topic of international conferences such as Photonics West and Advanced Fiber Laser.
The domestic research results are also very fruitful, with scientific and technological journals successively publishing special albums [13-14], comprehensive academic conferences setting up special seminars [15], and beam synthesis gradually realizing the empowerment of laser systems [16]. There are many types of lasers that can be synthesized and technical solutions for synthesis [17-23]. Ref. [24] provides a comprehensive analysis of the progress of laser beam synthesis from 2011 to 2020, covering all laser types. Ref. [25] focuses on the progress of fiber laser coherent synthesis. In this paper, we comprehensively review the research progress of various synthesis technologies in recent years from multiple perspectives such as power synthesis, spectral synthesis, coherent synthesis and composite synthesis, analyze the development trend, summarize the research experience, and refine the latest trends, so as to provide reference for scientific research, teaching and application personnel in the field of fiber laser and beam synthesis.

2 Power synthesis
Power synthesis is the most common laser synthesis method [26], which can generally be divided into two categories: space power synthesis and all-fiber power synthesis. Among them, space synthesis generally refers to the control of the optical axis of each laser beam to make it pass
Free transmission or focusing and other methods to achieve spot coincidence at the target. Their common feature is that the beam quality is reduced while increasing the power [27]. The pigtail coupled diode laser, which is commonly used in the development of fiber lasers, mostly adopts the method of spatial synthesis in its internal structure. For fiber lasers, most of the reports on spatial synthesis have focused on the development of high-power fiber laser systems [28]. In recent years, there have been few reports on technology.
In contrast, all-fiber power synthesis has been a hot topic in laser synthesis in recent years, and its typical structure is shown in Figure 1 [29]. As early as 2013, IPG Photonics reported that the world's first 100 kW high-power fiber laser system was realized based on the power synthesis of 90 kW lasers, which was successfully applied to the field of laser processing [30]. Soon after, a high-power fiber laser system in the 120 kW class was reported. The key to all-fiber power synthesis is the low-insertion-loss, high-power adaptive power combiner, which IPG Photonics' homepage envisions for a 500 kW power output [31]. 

With the improvement of traction and power combiner performance required by applications, in recent years, several units have realized 100 kW fiber laser systems based on all-fiber power synthesis. In 2021, the University of South China and Ruike Gong
The company reported the first 100 kW fiber laser system in China [32-33]; In 2024, Kaplin, Han's, and Chuangxin have successively reported high-power fiber laser systems ranging from 150 kW to 200 kW [29,34-36]. As long as the power beam combiner has sufficient "brightness redundancy" (i.e., the product of the diameter of the output pigtail and the numerical aperture is greater than the sum of the diameter and numerical aperture of all input fibers), then there is great potential to achieve low insertion loss and high power acceptance. Of course, the product of the diameter of the output pigtail and the numerical aperture also determines the beam quality of the output laser, which determines the application scenario and application effect.

In addition to continuing to increase the output power, there are three trends worth paying attention to in the power synthesis of all-fiber structures. The first is the development of ultra-high power fiber laser systems of 100 kW (or more) based on all-fiber power synthesis, which not only drives technological progress in the direction of laser devices and laser technology, but also promotes the development of advanced optoelectronic measurement [37]. For example, the 150 kW fiber laser system reported in Ref. [35] has been criticized by researchers because the output power exceeds the range of common calorimetry-based laser power meters
The innovative use of optical pressure-based power measurement methods [38] provides a solution for direct measurement of higher power lasers. The second is the quality (brightness) of the laser beam synthesized by power. As mentioned above, if the product of the diameter of the output pigtail and the numerical aperture is large enough, then ultra-high power output can be achieved
But the quality of the output laser beam will deteriorate. The author has noticed that in 2009, IPG Photonics announced a project to achieve an output power of more than 50 kW and a beam quality of M2 through multi-laser all-fiber power synthesis<4. Although the specific progress of the above projects has not been seen in the follow-up, the advanced nature can be felt from the technical indicators. The third is through a specially designed beam combiner, supplemented by a laser Key technologies related to coherent synthesis [39] (e.g., phase control [25]) are possible.To further improve the beam quality of the system output, for example, Ref. [40] has achieved a highly stable near-single-mode 10,000-watt laser output, and the mode control based on photonic lanterns [41], which has attracted much attention in recent years, is essentially in this category.

3 Spectral synthesis
Spectral synthesis is actually the inverse process of dispersion, and can be achieved by using elements such as prisms, dichroic mirrors, volume Bragg gratings, and diffraction gratings [19,42]. Based on the analysis of the published literature, combined with the performance characteristics of the components, most of the reflective diffraction gratings that have achieved high power output have been adopted
Or a dichroic mirror as a synthetic element.
In terms of reflective diffraction gratings, spectral synthesis based on reflective diffraction gratings has been comprehensively described in Ref. [42]. Among them, the more representative work in the world is Lockheed Martin in 2012
The synthesis of 12 beams of 300 W laser spectra [43] and the synthesis of 96 beams of 300 W laser spectrum reported in 2016 (see Fig. 2) [44], the integrated system has been validated. Domestically, the China Academy of Engineering Physics and the Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences (hereinafter referred to as Science China
Shanghai Institute of Optics and Mechanics) has a strong representativeness. In 2015, the research group of China Academy of Engineering Physics realized the spectral synthesis of 5 kW narrow linewidth fiber lasers, with a maximum output power of more than 5 kW.
The synthesis efficiency reached 91.2%[45]; In 2016, the research group of the Chinese Academy of Engineering Physics and the Shanghai Institute of Optics and Mechanics of the Chinese Academy of Sciences both achieved a spectral synthesis of 10 kW output [46-47]. In recent years, the Shanghai Institute of Optics and Mechanics, Chinese Academy of Sciences, has continued to carry out research on high-power spectral synthesis systems, as pointed out in Ref. [42].
"From 2018 to the present, the increase from 40 kW for 32 routes to 150 kW for 60 routes has been realized." It is worth noting that based on spectral synthesis to achieve power improvement, most of the preliminary research in China is oriented to fiber excitation with continuous wave operation In fact, foreign research groups have long used it to improve the performance of pulsed lasers [48-49]. In recent years, domestic researchers have also carried out relevant work and made progress. In 2021, the research group of the 11th Research Institute of China Electronics Technology Group Corporation (hereinafter referred to as the 11th Institute of China Electronics Technology Group) reported 4 roads 100 W class pulsed laser spectroscopy [50], with an average power of 360 W, The laser with a peak power of 122 kW and a pulse energy of 0.36 mJ has a synthesis efficiency of 90%, and the output beam quality at maximum power is M2<2.

In terms of fiber laser spectral synthesis based on single or multiple dichroic mirrors, many units at home and abroad have carried out research work in recent years due to its relatively loose requirements for laser linewidth. Internationally, the University of Michigan, USA
and the University of Jena in Germany were among the first to carry out related research. In 2008, a research group at the University of Michigan used bichroic mirrors to synthesize a three-channel pulsed fiber laser, with an average power of 52 W and a pulse energy of 1.9 mJ [51]. In 2009, a research group at the University of Jena used dichroic mirrors to synthesize four pulsed fiber lasers with an average laser power of 208 W and a pulse energy of 6.3 mJ [52]. In 2019, the research group of the Institute of Optics and Mechanics of the Fraunhofer-Gesellschaft realized the spectral synthesis of three laser channels based on dichroic mirrors, with the wavelengths of the three lasers being 1050 nm, 1070 nm and 1090 nm, respectively.

The synthesis power is 5.5 kW and the synthesis efficiency is 94% [53].
In China, the research group of Nanjing University of Science and Technology used bichroic mirrors in 2017 to realize the synthesis of two wide-spectrum fiber lasers, with a single-channel laser power of 5.3 kW and 4.9 kW, respectively, and a synthesis laser power of 10.12 kW and a synthesis efficiency of 98.9%. Since the synthesis unit itself is not a single-mode laser, the synthesized laser beam is also multi-mode [54]. In 2022, the research team of the National University of Defense Technology synthesized an 8 kW near-single-mode fiber excitation based on a broad-spectrum laser bichroic mirror
Light apparatus [55]. In 2023, the 11 research groups of CETC Group achieved the spectral synthesis of three high-power narrow linewidth fiber lasers, obtaining a 9.65 kW laser output, a synthesis efficiency of 92%, and a beam mass of 1.7, and a schematic diagram of its spectral synthesis structure is given in Figure 3 [56], and in the same year, the research group of the National University of Defense Technology achieved a 10 kW near-single-mode laser output based on the spectral synthesis of two broadband fiber laser amplifiers, with a synthesis efficiency of about 98.3% and a beam quality factor of about 1.29 [57]. In the spectral synthesis system, the pointing error between the unit beams (or the tilt phase difference) will affect the quality of the synthesized beam due to the influence of environmental jitter, specular thermal effect, and gas thermal effect of the transmission channel. The research team at the National University of Defense Technology applied active control technology to a high-power two-color mirror spectral synthesis system, using a random parallel gradient descent algorithm to control the piezoelectric deflection stage, lock the tilt of the two lasers, and achieve a stable near-single-mode laser with an output power of 8.3 kW [58].

There are four trends in the development of fiber laser spectroscopic synthesis that are worth paying attention to. The first is the choice of technical solutions. At present, researchers have achieved high power output by using reflective diffraction gratings or dichroic mirrors as synthesis elements, and the output power of reflective diffraction gratings is higher. In contrast, the use of reflective diffraction gratings requires the unit involved in the synthesis to be a narrow linewidth laser, which generally needs to be specially developed. The use of bichroic mirrors has no special requirements for the laser line width involved in the synthesis, and the shelf products can basically meet it. On the other hand, from the perspective of "available bandwidth", the number of channels involved in the synthesis of the laser decreases when a wide linewidth laser is used. It can be seen that the spectral synthesis using the reflective diffraction grating has been reported with more than 90 beams, while the number of beams is still in the single digits for the spectral synthesis using dichroic mirrors. The second is the expansion of the laser band. eye
Previously, most of the research on fiber laser spectral synthesis focused on the 1 μm band, but in fact, the principle of application to other wavelength bands is the same. For example, for a fiber laser in the 2 μm band, the average output power in this band is the highest
The kW class [59-60] has not been able to make further breakthroughs for more than ten years due to factors such as laser fiber technology, pump brightness, and thermal management. The research groups of the University of Central Florida in the United States, Birken University in Turkey, and the Hannover Laser Center in Germany have made preliminary achievements in the synthesis of laser spectroscopy in the 2 μm band
step results [23]. On the other hand, breakthroughs have been made in recent years in the narrow linewidth fiber laser in the 2 μm band [61-62], and if breakthroughs are made in related synthetic components, it is expected that high-power and high-beam quality 2 μm-band fiber lasers in the multi-kW range can be rapidly realized. The third is the use of bandwidth and edge wavelength high-power lasers
R&D. Unlike power synthesis and coherent synthesis, which will be described later, spectral synthesis has the concept of "usable bandwidth", which depends on the operating bandwidth of the synthesized element and the high bandwidth within which the laser participating in the synthesis achieves
The ability to output power. Taking a 1 μm band laser as an example, it is relatively easy to achieve high power output in the 1060~1090 nm band based on ytterbium-doped fibers, and the working bandwidth range of the synthesis element can be greater than 30 nm [63], so a larger number of lasers can be accommodated to participate in the synthesis by effectively overcoming the small net gain and achieving high power output at the edge wavelength [64]. In recent years, researchers have achieved high power output in edge bands such as 1030 and 1130 nm [65-67], which provides a reference for further improving the performance of spectral synthesis systems. Fourth, the application of the pulse system. Due to the characteristics of the gain medium, fiber lasers face great technical challenges in achieving high beam quality and high-energy pulsed laser output. The results of existing studies at home and abroad show that spectral synthesis can be applied to the pulse system. If it is not an ultrashort pulse laser (such as a nanosecond laser), then the number of synthetic beams is also considerable from the perspective of "available bandwidth". Therefore, it is technically possible to obtain a high-beam quality and high-energy pulsed fiber laser with tens of millijoules by spectral synthesis.

4 Coherent synthesis
Compared with spectral synthesis and power synthesis, the composition of laser coherent synthesis system is relatively complex and involves more key technologies, but the potential (ability) in power calibration amplification and output light field control is relatively greater and potential.More application scenarios. A schematic diagram of the coherent synthesis structure based on active phase control is shown in Figure 4. There have been many reviews on fiber laser coherent synthesis, and this paper only focuses on fiber frequency conversion laser coherent synthesis and ultrafast light.There are three aspects: fiber laser coherent synthesis and application.

4.1 Frequency conversion laser coherent synthesis
The nonlinear frequency conversion based on fiber laser is an important technical solution to obtain laser output in special bands such as ultraviolet, visible light, and mid-infrared. In order to produce nonlinear effects in crystals, pump lasers are required to have a higher degree of proficiency.High brightness. The use of coherent synthesis technology to produce high-brightness pump laser can effectively improve the conversion efficiency of variable frequency laser. In 2017, the research team of the Lithuanian Center for Physical Science and Technology coherently synthesized four nanosecond pulsed fiber lasers with a center wavelength of 1064 nm, and used LBO crystals to double the frequency to obtain an average power of 29 W at 532 nm
laser, with a conversion efficiency of 51% [68]. In 2019, the same method was used to obtain a variable frequency laser with an average power of 97 W, a pulse width of 240 ps, and a center wavelength of 519 nm, with a conversion efficiency of 49% [69]. In the same year, the University of Paris-Saclay and other institutions carried out coherent synthesis of three semiconductor lasers, and output a 976 nm single-frequency laser with a power of 12.9 W, and then realized the second harmonic transformation through a periodically polarized lithium niobate (PPLN) crystal to obtain a 2 W 488 nm laser output [70].

In order to further improve the brightness of the variable frequency laser, coherent synthesis of multiple variable frequency lasers can also be performed. In 2014, a research team at the French Center for Space Research used two PPLN crystals to multiply the frequency of two 1064 nm fiber lasers, and then realized the coherent synthesis of two 532 nm lasers by phase control of the two fiber lasers, with phase residuals better than λ/30 [71]. In 2015, the same method was used to achieve coherent synthesis of two 775 nm lasers with phase residuals better than λ/20 [72]. In 2017, the research group realized the coherent synthesis of two 3.4 μm variable frequency lasers: two fiber lasers with a center wavelength of 1.06 μm were pump lasers, and each pump laser amplified the power to about 100 W through a Yb-doped fiber amplifier. The central wavelength of the signal light is 1550 nm, and the power is amplified to about 10 W by an Er-doped fiber amplifier, which is then divided into two channels by a beamsplitter. Two PPLN crystals were used as optical parametric oscillation (OPO) crystals to generate a 3.4 μm variable frequency laser. By locking the phase of one of the pump lasers, two channels are realized.Coherent synthesis of a 3.4 μm frequency-converted laser [73]. In 2019, the research group paired:.The system was optimized, and the phase control residuals were better than λ/28 [74].

In practical application, the above methods can also be combined, firstly, the pump light is synthesized to improve the brightness of the pump light, and then the laser after frequency conversion is further synthesized. In 2017, a research group from Osaka University in Japan synthesized four laser beams into one beam through coherent synthesis and polarization synthesis to produce a 1040 nm pump laser with a repetition rate of 10 MHz, a pulse width of 285 ps, and an average power of kW. Then, two of these pump laser modules are used to pump two sets of LBO crystals separately to produce a variable frequency laser. Finally, a 520 nm laser output with an average power of 600 W (second harmonic) and a 347 nm laser output (third harmonic) with an average power of 300 W were obtained by polarization coherent synthesis of two frequency-conversion lasers [75]. In 2024, the research group at Tianjin University obtained a fundamental frequency light with an average power of 238 W through coherent synthesis, with a power of 210 W and a pulse width of 230 fs after a transmission diffraction grating compressor. Then, a femtosecond laser with an average power of 128 W, a center wavelength of 517 nm, and a pulse width of 216 fs was successfully output in a 2 mm thick LBO crystal [76].

4.2 Ultrafast fiber laser coherent synthesis
Driven by applications such as higher harmonic generation, advanced manufacturing, and particle acceleration, the coherent synthesis of ultrafast fiber lasers has made significant progress in recent years. Compared with CW lasers, the ultra-wide spectral characteristics of ultrafast lasers make their coherence length very short (generally in the order of tens of microns), resulting in extremely high delay control accuracy in the synthesis process, and the nonlinear phase shift accumulated in the process of power increase will also affect the synthesis efficiency. In order to solve the above problems, the coherent synthesis of multiple ultrafast fiber lasers generally requires high-precision control of time delay, phase, and beam direction at the same time to achieve high synthesis efficiency. At present, the University of Jena in Germany has achieved ultrafast fiber laser synthesis output with an average power of up to 10.4 kW [77] and a single pulse energy of 32 mJ [78], respectively, and the Ecole Polytechnique de Paris has achieved efficient coherent synthesis of 61 ultrafast fiber lasers with a synthesis efficiency of about 50% [79].
There are many domestic and foreign literatures that have reviewed this research direction from different perspectives [80-83]. Generally speaking, ultrafast fiber laser coherent synthesis can be divided into three forms: spatial, time-domain, and spatio-temporal coherent synthesis. Among them, the technical scheme of airspace coherent synthesis can be divided into filling aperture and tiling
There are two types of apertures, which mainly increase the average power of ultrafast lasers by synthesizing the output of multiple fiber amplifiers. At present, researchers have achieved an average power transfer of more than 10,000 watts using a synthetic scheme with filled pore size
However, the scheme involves a complex spatial coupling structure, which makes it extremely challenging to further expand the number of synthesis paths to obtain higher power [77]; Although the power achieved by the tiled aperture synthesis scheme is still in the kW level, there is a large room for expansion in the number of synthesis channels and single channel power, and there will be in the future
It is expected to achieve an average power output of tens of thousands of watts [79]. In addition, researchers in the near
In recent years, he has also developed a coherent synthesis scheme for filled aperture airspace based on multi-core active optical fibers, and the relevant work is most representative of the research group at the University of Jena. In 2023, the research group will use rod-shaped 49-core optical fibers and segmented mirror arrays
The kW-level ultrafast laser synthesis output was obtained [84]. The coherent synthesis scheme based on multi-core active fibers combines the high efficiency of conventional filled-aperture synthesis with the compactness of tiled aperture synthesis, and relevant theoretical studies have shown that ultrafast lasers with an average power of 10 kW or pulse energy of hundreds of mJ can be achieved [85].

In terms of time-domain coherent synthesis, there are two main technical schemes: pulse amplification and pulse stacking. Among them, the split-pulse amplification technology divides the laser seed source into discrete sub-pulses in the time domain through polarization beam splitting and delay control, so as to greatly alleviate the nonlinear effect in the process of fiber amplification. The amplified sub-pulse sequences are further combined into a single pulse by time-delay control and polarization beam combination, which can increase the power and energy of ultrafast lasers [86-87]. The pulse stacking technology uses a passive cavity to directly synthesize a high repetition rate laser pulse sequence in the time domain, increasing the energy of a single pulse while reducing the repetition rate, that is, "power for energy". At present, researchers have developed the stacking and cavity emptying technique and the Gires Tournois interferometer technique, which has achieved time-domain synthesis of nearly 100 pulses and obtained a femtosecond laser output with an energy of 10 mJ [88-91].

In order to further obtain higher ultrafast laser power and energy, researchers have developed spatiotemporal coherent synthesis technology by combining spatial and temporal coherent synthesis. An ultrafast laser with a maximum average power of 700 W and a single pulse energy of 32 mJ was achieved by using coherent synthesis of filled aperture space and time-domain fractional amplification [92-93]. In addition, researchers from the research group at the University of Jena, based on the technology of space-time coherent synthesis, proposed the realization of hundreds of kilowatts of power and A detailed protocol for a tens of joules energy ultrafast laser [88] is shown in Figure 5.

The research on ultrafast fiber laser coherent synthesis in China started late. In 2014, a research group at the National University of Defense Technology realized the coherent synthesis of airspace-filled aperture with four picosecond fiber lasers [94]. In the past 10 years, research groups from Tianjin University, East China Normal University, Institute of Physics of Chinese Academy of Sciences, China Academy of Engineering Physics, Peking University, Huazhong University of Science and Technology, National University of Defense Technology and other units have made achievements in ultrafast fiber laser coherent synthesis
Significant progress [95-96]. In 2023, a research team from the Institute of Physics of the Chinese Academy of Sciences obtained an ultrafast laser with a single pulse energy of 1 mJ based on passive space-time coherent synthesis [97]; In the same year, a research group at Peking University realized an ultrafast laser with an energy of 1.2 mJ based on time-domain fractional pulse amplification [98], and a joint research group at East China Normal University and National University of Defense Technology achieved a high average power output for the coherent synthesis of two ultrafast lasers [99-100]. In 2024, the research group of the National University of Defense Technology will realize the coherent synthesis of ultrafast laser in the 2 μm band for the first time in China [101], and cooperate with the research group of East China Normal University to achieve high-efficiency coherent synthesis of 8-channel ultrafast fiber lasers [102]. The research group at Huazhong University of Science and Technology obtained an ultrafast laser output of more than 400 W based on the coherent synthesis of two filled aperture [103].

4.3 Application
Fiber laser coherent synthesis has been used in many occasions, for example, in fields with high requirements for laser time coherence, such as sodium guide stars, coherent radar, and gravitational wave detection. Limited by stimulated Brillouin scattering
The output power of a single fiber laser is often difficult to meet its application requirements. As early as 2010, researchers at the European Southern Observatory (ESO) performed coherent synthesis of a 1178 nm single-frequency fiber Raman amplifier output, followed by frequency doubling, to obtain a 589 nm sodium guide star laser with a power of 50 W [104]. The research team of the French Space Research Center carried out coherent synthesis of two single-frequency nanosecond pulses, and obtained a single-frequency pulsed laser with a peak power of 100 W and a pulse width of 240 ns, which improved the accuracy of the wind radar and range [105]. The University of the Côte d'Azur in France has completed a long-term, low-noise coherent synthesis experiment of two 40 W single-frequency fiber lasers, demonstrating the feasibility of fiber laser coherent synthesis as a light source for the Laser Interferometric Gravitational Wave Observatory (LIGO) [106].
Compared with microwave communication, laser communication has the advantages of bandwidth, good directivity and strong anti-electromagnetic interference ability. The array beam is used to emit light signals and coherently synthesize at the target, which can effectively compensate for the influence of atmospheric turbulence. Changchun University of Science and Technology, Institute of Optoelectronic Technology, Chinese Academy of Sciences, University of Paris-Saclay and other units have carried out relevant research [107-109]. In 2020, the research group of the Institute of Optoelectronic Technology, Chinese Academy of Sciences. The atmospheric transmission of 19 fiber lasers was realized, and the power in the barrel was increased by about 8.27 times after the system was closed-loop [108]. In 2024, a research group at Ben-Gurion University in the Negev, Israel, based on 32-channel laser coherent synthesis, realized laser communication at a distance of 10 km [110]. In the same year, the research team of the National University of Defense Technology found that the construction of orbital angular momentum (OAM) coherent arrays can be effective Improving optical communication capabilities [111]. In addition, multi-aperture signals are connected It can also alleviate the effects of atmospheric turbulence through digital synthesis or optical synthesis [112-114]. In 2023, a research team from the Institute of Optoelectronic Technology of the Chinese Academy of Sciences used 19 units for beam reception, and in the 2.1 km atmospheric transmission experiment, the receiving power of the system was increased by about 12 times after the system was closed-loop [115].

By constructing an array beam so that the phase and polarization direction of each laser are the same, the brightness of the array beam can be improved, and a new type of structured light field can be generated. For example, by manipulating the phase of the light field, a vortex beam with a spiral wavefront structure can be generated [116-117]. If the duty cycle of the array beam is changed, the array vortex beam can also be obtained [118]. In 2021, a research team at the Sorbonne University in France controlled the phase of a 61-channel femtosecond laser to output a special light field such as a vortex beam [119]. In 2023, the research team at the National University of Defense Technology used a six-channel laser array to achieve a vortex beam output of 1.5 kW [120]. In addition to phase manipulation, the polarization state of the light field can also be manipulated to generate vector beams with singular focusing characteristics [121-123]. In 2024, the research team of the National University of Defense Technology will control the polarization direction of the 6-channel laser to produce nullity
Column vector beam [124]. In addition, amplitude, phase, and polarization can be controlled in multiple dimensions at the same time, resulting in a structured light field with more characteristics such as diffraction transmission. In 2024, the research team of the National University of Defense Technology will obtain a perfect vortex beam by adjusting the tilt and phase of each laser [125]. The construction of light fields with special diffraction characteristics and far-field spot morphology in the above methods has a wide application prospect in the fields of particle manipulation and industrial processing. For example, in industrial processing, if the light source
The output spot shape can be dynamically switched, which can improve the speed and efficiency of processing. The Israeli company Civan has developed a 100 kW fiber laser coherent synthesis light source for industrial processing applications [126] and released basic information on 100 kW high-energy system products [127].

4.4 Research trends
In recent years, the research on fiber laser coherent synthesis has shown the following trends. First, the use of artificial intelligence technology has greatly improved the control ability of coherent synthesis systems [128-130]. Research groups in the United States, Italy, Japan, China, and other countries have introduced artificial intelligence technology and applied it to coherent synthesis phase control, which has achieved remarkable results and has become an important direction in the field of AI-enabled lasers [131]. Second, the scale of laser synthesis continues to expand, and it is expected to help the research and development of large scientific devices and new big science. Development of the device. Thanks to the improvement of hardware performance and the breakthrough of software technology, coherent synthesis at the scale of 100 and 1,000 beams has been preliminarily verified [132-133], and some large laser scientific devices under development have proposed to use coherent synthesis to break through the performance limitations of single lasers [134-135]. At present, it seems that with the maturity of unit development technology and the reduction of manufacturing costs, the development of new large-scale scientific devices based on fiber laser coherent synthesis is also feasible [136-138]. Third, there is a trend of in-depth integration of science and education. Laser coherent synthesis involves the basic principles of interference and diffraction, and is the teaching of physical optics and other courses. Excellent material. For a long time, due to the experimental cost, danger, technology maturity and other reasons, the research results of fiber laser coherent synthesis have not entered the classroom. In recent years, with the maturity of theories and computers and networks
With the advancement of technology, researchers have developed virtual laboratories that can intuitively understand the nature of coherent synthesis through virtual experiments [139]. In addition, the study of coherent synthesis involves the knowledge of physics, optical engineering, electronic science and technology, control science and engineering, and other disciplines. A good platform for students to be able to interdisciplinarily [140].

5 Composite Synthesis
In recent years, the parallel development of multiple beam synthesis methods, and the integration of two or more synthesis techniques to further improve the performance of synthetic lasers have also become the direction of attention of researchers, which has been preliminarily introduced in Ref. [23]. In composite synthesis, coherent polarization synthesis and coherent spectroscopy synthesis are more representative. For coherent polarization synthesis, Lockheed Martin Corporation of the United States [141], the University of Jena of Germany [142], the Institute of Optoelectronic Technology of the Chinese Academy of Sciences [143], the University of Shanghai for Science and Technology [144], and the National University of Defense Science and Technology [145]
The number of synthetic pathways reached 16 [146], but there are not many research results reported recently.

Coherent spectroscopic synthesis has been a hot topic in recent years. Different from the continuum system narrow linewidth laser spectral synthesis based on dispersive elements, the coherent spectroscopic synthesis is mainly aimed at the broad-spectrum ultrafast laser, and the synthesis scheme is mainly based on the space-filled aperture synthesis. Specifically, the lasers involved in the synthesis partially overlap the spectrum, and the interference effect of the overlapping part is used to realize the synthesis of more than two ultrafast lasers, so as to obtain a wider spectrum than a single laser. In the process of increasing power and energy through chirped pulse amplification, ultrafast fiber lasers are affected by the gain narrowing effect, resulting in a spectral width of less than 10 nm and difficulty in compressing the pulse width to less than 200 fs [147-148]. The coherent spectroscopic synthesis scheme can further increase the laser power and energy, and realize the broadening of the laser spectrum and the compression of the pulse width. The research group at École Polytechnique in France is based on a two-way ultrafast. A laser with a spectral width of 19 nm and a pulse width of 130 fs was obtained by fiber laser coherent spectroscopy [149]. The research group at the University of Michigan in the United States reported the results of compressing the laser pulse width to 1/3~1/2 using the 3-channel synthesis of laser 51, Issue 19/2024/October 2024/China [150]. In 2019, a research group at Tianjin University synthesized two ultrafast lasers with nonlinear spectrum broadening, and realized an ultrafast laser with compression of 8 fs [151]. In 2023, the research group of Lawrence Berkeley National Laboratory, USA A laser with a spectral width of 80 nm and a pulse width of 42 fs was realized by introducing pulse shaping into a chirped pulse amplifier by synthesizing three ultrafast laser coherent spectroscopy [152]. At present, the unit has established relevant scientific research projects, and plans to combine spatio-temporal coherent synthesis and coherent spectroscopic synthesis to compress the pulse width while achieving a significant increase in laser energy, and obtain a single pulse energy of A 200 mJ ultrafast laser with a peak power of TW provides a driving light source for cutting-edge applications such as particle acceleration [153], as shown in Figure 6. In addition, the ultrafast laser coherent spectroscopy synthesis scheme has also been validated on solid-state laser systems, and the researchers obtained the pulse width of the compressed subperiod based on the coherent spectroscopy synthesis of two solid-state lasers [154-156].

 6 Analysis of research characteristics
Through the research trend and trend of fiber laser beam synthesis introduced above in recent years, and an in-depth analysis of the published literature, it can be seen that in addition to its own theoretical breakthroughs and technological progress, related fields have also appeared
Here are a few features:
The first is the expansion of the object-oriented approach of beam synthesis. Historically, none
In terms of coherent synthesis or spectral synthesis, semiconductor lasers were the first to be studied in depth [157-158]. Since the beginning of the 21st century, with the development of fiber lasers, the academic popularity of fiber laser beam synthesis is higher than that of semiconductors [159]. The reason for this is that although the semiconductor lasers used to pump high-power fiber lasers are all beam synthesized, some are used
Spatial synthesis, some by polarization, some by spectral synthesis [160], and some by a variety of methods, but this may be the reason why the R&D units are mostly high-tech enterprises (or other reasons), and there are relatively few academic papers published. In recent years, the development of fiber laser beam synthesis has led to the progress of synthesis technology, and key technologies such as coherent synthesis and spectral synthesis have been maturing, and semiconductor laser coherent synthesis and special wavelength semiconductor laser spectral synthesis have also become the objects of academic attention [161-166], which is expected to further accelerate the development of semiconductor lasers.

The second is the popularization of beam synthesis research methods. In the research process of fiber laser beam synthesis, researchers continue to propose new methods, or introduce research methods from other fields into the field of beam synthesis. For example, the stochastic parallel gradient descent algorithm was first applied to optical imaging and adaptive optics [167-168], and after being applied to the direction of coherent synthesis [169-171], researchers gradually achieved good results, supporting tens of thousands of watts of power output [172] and hundred-beam scale coherent synthesis [173]. The research group of the National University of Defense Technology further applied it to the mode decomposition of multi-mode laser, and also achieved good results [174-175]; The research group at Nanjing University of Science and Technology improved the relevant algorithms to further improve the effect of pattern decomposition [176]. In addition, in recent years, stochastic parallel gradient descent algorithms have also been successfully applied in the fields of communication, remote sensing, measurement, and laser development [177-180].
The third is driven by the demand for beam synthesis systems. For partially coherent synthesis and spectral synthesis systems, if the laser involved in the synthesis is a narrow linewidth laser, it will help to improve the synthesis efficiency and output beam quality. Affected by factors such as nonlinear effect and mode instability effect, it is more difficult to increase the output power of narrow linewidth fiber laser than conventional excitation
light [181-182]; Nevertheless, in recent years, the high power has been narrow, driven by demand. The development of linewidth lasers [183-185]: the power of near-single-mode randomly polarized lasers has reached 7 kW, and the output power of near-single-mode linearized lasers has reached 5 kW [185], and the output power limit of the corresponding non-narrow linewidth fiber lasers has almost been reached, taking into account the type of fiber used. In particular, it is worth pointing out that a considerable number of the power records of high-power narrow linewidth fiber lasers have been achieved by Chinese scientific researchers. The R&D and performance of gratings used in high-power spectral synthesis continue to improve, and the demand is also one of the important reasons, and researchers vividly call it "near-infrared strong laser".
interweaving with reflective holographic planar diffraction gratings" [186].

7 Lookout
Due to factors such as mode instability, nonlinearity, and physical properties of gain materials, the output power of single-fiber lasers is limited [187-189]. Taking the continuous wave operation system as an example, the theoretical analysis of researchers and the experimental research in the past 10 years have shown that it is difficult for a single-fiber laser to achieve a 10,000-watt high beam quality output [190-192]. With beam synthesis technology, power synthesis, spectral synthesis and coherent synthesis have all achieved an increase in the output power of a single fiber laser, and the power output of 100 kW has been achieved. This is an event of great significance in the history of the development of high-power fiber lasers (even high-power lasers), which fully demonstrates the possibility of obtaining higher output power and developing ultra-large-scale fiber laser systems in the future.
In Ref. [16,159], the author has proposed that with the development of technology, beam synthesis has moved from laboratory research to practical application, and gradually realized the "empowerment" of lasers. For fiber lasers, the current
"Empowerment" is basically concentrated in the near-infrared band, and there are few related studies on the visible and mid-infrared bands [193-196]. In fact, in practical applications, the absolute value of the power requirements of the above bands is not high compared to the near-infrared band, and it may be possible to meet the requirements by combining several beams and (counting) ten watts of power. However, components such as couplers, beam combiners, and tuning are often used in synthesis systems. Fabricators and synthetic components are still in the R&D or exploration stage, which is the next step to solve.

With the breakthrough of deep learning and other technologies in recent years, artificial intelligence has been widely used in the research of natural sciences and high-tech fields [197], and many outstanding fields have emerged in the field of fiber laser and beam synthesis. Research results [198-200]. In the future, in the research and development process of narrow linewidth and high power fiber laser for coherent synthesis and spectral synthesis [201-202], the design of spectral synthesis components, the multi-parameter intelligent control of coherent synthesis system/optical axis intelligent control of spectral synthesis system, and the quality intelligence of synthetic beams
[203] and other aspects, AI will be very useful. How to better combine the knowledge of intelligent science and technology disciplines will be the key to the research and development of higher performance and high-power beam synthesis systems.