High-power fiber laser: 15 years of software and hardware development from 10,000 watts breakthrough to technological leap

In the fourth article of this column, the author mentions that in 2004, at least four different units in the world published the research results of kW-class fiber lasers. At that time, the research work of high-power fiber lasers in China had just started [1]. Searching CNKI, the more representative research results include researchers from the Shanghai Institute of Optics and Mechanics of the Chinese Academy of Sciences and Tsinghua University who have achieved (several) ten-watt lasers based on ytterbium-doped fibers [2,3], and a joint research group of the Shanghai Institute of Optics and Mechanics of the Chinese Academy of Sciences and Shanghai Jiao Tong University based on fiber laser frequency doubling to generate 59mW green light [4]; In addition, in 2003, researchers from the Xi'an Institute of Optics and Mechanics of the Chinese Academy of Sciences realized a 400 mW laser based on cladding-pumped erbium-ytterbium co-doped fibers [5], among others. It can be seen that there was a clear gap between China's research achievements and the advanced level at that time.

In fact, in the following years, the gap did not narrow, but on the contrary, there was a tendency to widen. On the one hand, although many units have achieved kW-level power output around 2007, the "all-fiber structure" mentioned in the fourth article of this column has not received enough attention at that time, and the single-fiber power has not been further improved. On the other hand, the output power of single-fiber lasers in the world continues to increase, and by 2009, a 10 kW single-mode laser has been successfully developed. In a sense, the 10 kW single-mode laser has the value of "milestone" and "wind vane", which makes the domestic counterparts basically form a consensus on how to realize the technical scheme of single-fiber high-power high-beam quality laser, although some units have carried out research on all-fiber structure fiber lasers and achieved preliminary results [1]. Since then, China's high-power fiber laser research ideas have gradually become clear, achieved rapid development, and have been widely used. For example, the 20 kW power output of a single fiber is achieved based on ytterbium-doped fibers [6-10], the power output of a single fiber with high beam quality is 10 kW [11], the kW single-mode green output is achieved based on optical fiber frequency conversion [12], and the power output of 300W is achieved based on erbium-ytterbium co-doped fibers [13], etc. The realization of the above important achievements is inseparable from the support of pump source, laser fiber, passive components, basic theory and design software, which are the basic software and hardware for the development of high-power fiber lasers. Combined with the author's own research experience during his reading and work, this paper tries to review the development process of basic software and hardware in the past 15 years with a brief review of the development of basic software and hardware in the 1μm band, and briefly analyzes the current situation faced by the research and development of high-power fiber lasers.

In terms of pump sources

There are usually two options for the development of high-power ytterbium-doped fiber lasers: one is to directly use semiconductor lasers (typical wavelengths are 915 nm, 975 nm, etc., unified into 9xx nm; It can be called "direct pumping"), and the other is a fiber laser based on a diode laser that generates a short-wavelength band (typical wavelength is 1018 nm) and then uses a fiber laser to pump it (can be called "cascade pumping"). Around 2010, there were not many options for high-power and high-performance pigtail output diode lasers in China, and many of them were in the trial stage, and the maximum power of the 9xx nm band semiconductor laser output by the 100μm diameter pigtail (numerical aperture of about 0.22) based on single-tube chips and bars was about 50 W and 70 W respectively. In 2010, at the International Fiber Laser Symposium held in Russia, the maximum power of the 9xx nm band semiconductor laser output by the international counterparts based on the 100 μm diameter pigtail (numerical aperture less than 0.15) of a single-tube chip has reached 140 W, and the brightness is several times different. In terms of 1018 nm fiber laser used in cascade pumping, a single beam of 300 W laser has been reported internationally. Due to the lack of understanding of laser physics and the insufficient supply of supporting fiber gratings, the relevant research work was basically not started, and there were no reports of commercial lasers in related bands at that time. In the past 15 years, China's research on pump sources has achieved rapid development, and the power of 9xx nm semiconductor laser output by commercial 100μm diameter pigtail has exceeded 200 W. By using spectral synthesis, the output power of two (or more) bands of laser synthesis can be multiplied. The output power of a single fiber laser with a high brightness of 1018 nm has reached the order of kW [14]. In terms of pumping capacity, it has been able to support 10 kW direct pumping and 20 kW cascade pumping solutions.

Laser fibers

Although around 2007, many domestic units achieved single-fiber kW-level power output, and from the published literature, some of the results were based on domestic optical fibers, but due to the limitations of preparation process and fusion splicing methods, they were not directly used in all-fiber structures, that is, based on spatial structures. According to literature reports, the output power of all-fiber structure lasers supported by domestic optical fibers at that time was in the 100-watt range. In the following 10 years, China has carried out a comprehensive layout in laser fibers, and the results of kW have been continuously reported. Around 2020, breakthroughs were made in high-power ytterbium-doped laser fibers, and fibers developed by several units supported 10 kW high-power output [15-18]. In addition to conventional double-clad ytterbium-doped fibers, researchers have also made innovative achievements in partially doped fibers, low-numerical aperture fibers, triple-clad fibers, pump-gain integrated fibers, gully fibers, microstructured fibers, and new fibers that support high-power laser transmission [19]. In addition, linearly polarized fiber lasers have achieved a power output of 5 kW [20].

Basic theoretical aspects

In the early research stage, Chinese researchers mainly focused on "tracking" and "reproduction" in the basic theory of high-power fiber lasers, and combined the relatively mature rate equations, heat conduction equations, and mode coupling equations with the lasers (amplifiers) to be developed to guide parameter selection, structural design and system integration. With the meticulous and in-depth research, researchers are gradually faced with the situation that there is no mature theoretical model, and it is necessary to devote great efforts to research, the more typical ones are the mode instability effect, optical fiber fuse (optical discharge) phenomenon, photon darkening phenomenon, etc., in recent years, Chinese researchers have also made a lot of innovative achievements, and some effective methods to suppress the mode instability effect and photon darkening phenomenon have been obtained. It is worth noting that, in addition to the high-power continuous-wave fiber laser that is concerned in this paper, many of the research of Chinese researchers in spatiotemporal mode-locking and ultrafast laser dynamics are in a state of "parallel running" or even "leading".

Design software

According to the author's memory, in the early research stage, the commercial software that can be used to design high-power fiber lasers mainly includes Liekki Application Designer (LAD) and RP Fiber Power, among which LAD was the agent in China no later than 2006. The operation interface of LAD is similar to Labview and Simulink, which is relatively easy to use, but the version is relatively old. RP FiberPower includes a rich library of components that can be calculated for a wide variety of laser types and parameters, but requires "programming" to do things like "programming" (e.g., entering system types and parameters in an input box or code editor). Since the 13th Five-Year Plan, especially in recent years, with the support of national scientific research projects, and through the joint efforts of multidisciplinary researchers in optical engineering and software engineering, SeeFiberLaser, a software for the simulation of high-power ytterbium-doped fiber lasers, SFTool, a fiber laser toolset software that can perform simple calculations of fiber laser-related parameters, and SeeNano for parameter and structure design of doped/non-doped fibers have been successfully developed[27]. It can provide professional tools for peers in the field of high-power fiber lasers around the world. It is worth noting that although it is easy to realize the "connection" between various components at the software level, in the actual research and development process, the steps of stripping, cutting, splicing, and re-coating of the coating layer of the optical fiber require professional tools to be executed, and the research and development of these professional tools themselves ("maker of tools") contains profound knowledge.

With the support of the above-mentioned software and hardware, the development of high-power continuous-wave ytterbium-doped fiber lasers in China has achieved rapid development, and the output power of conventional single-fiber lasers has been basically consistent with the international advanced level. Judging from the published literature, some subdivisions, such as high-power single-frequency/narrow-linewidth ytterbium-doped fiber lasers [28,29] and broadband ytterbium-doped fiber lasers [30], have been at the highest value. Due to the improvement of hardware performance, high power