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Abstract
Based on the self-developed non-photodarkening large mode field gain fiber and the 976 nm wavelength-locked high-power and high-brightness pump source, and using the secondary fiber power combining technology, a high-performance 100kW fiber laser in China was built, realizing high-order mode and non-linear effect suppression. The maximum output power of the laser can reach 101.65 kW, the center wavelength is 1080 ± 5 nm, the spectral bandwidth is (3dB) 5-8 nm, the output fiber core diameter is 400µm, the beam quality BPP is 19.28 mm*mrad, and the laser power instability is ±1.1%. Its laser non-destructive cladding stripping technology, distortion-free taper technology, inclined multi-die beam combining technology and circular inner cladding modification design have all reached the international advanced level.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Continuous fiber lasers have great research value and application prospects in the military [1], scientific research [2], and industry [3] due to their high energy and good beam quality. High power, high beam quality, and industrialization are three key indicators to measure the level of fiber lasers. Using large mode field gain fibers and optimizing the uniformity of fiber doped particles can suppress photodarkening and improve laser efficiency and power [4–7]. Reducing the number of laser paths and controlling the damage of the fiber surface can obtain high-quality beams [8,9]. In China, in 2019, Xiaolong Chen and others built a nationally produced 10 kW fiber laser by using self-developed gain fibers and passive components, using double-end pumping technology, with a maximum output power of 10.14 kW, and the light-to-light conversion efficiency of the main amplifier stage is 87.8% and the slope efficiency is 89.2% [10]. In 2020, Maxphotonics CO.,Ltd. developed a 40 kW multi-mode continuous laser, which achieved high beam quality output with a core diameter of 100 µm on the basis of a single module of 6 kW, promoting the rapid development of the laser equipment industry towards ultra high power [11]. In foreign countries, in 2009, IPG Photonics Corporation of the United States took the lead in using cascaded pumping to obtain a single-mode fiber laser with a power of 10 kW [12], and in 2013, they reported a multi-mode coupled fiber laser with a power of 120 kW, which can operate in continuous or modulated mode. It is still the highest output industrial fiber laser in the world, operating over a dynamic range of 10% to full power with no change in beam divergence or beam profile [13].
Recently, the laser jointly developed by the University of South China and Wuhan Raycus Fiber Laser Technology Co., Ltd. took only 6 months from the start of the project to its successful development. It is the first 100kW industrial fiber laser with the highest power in China, and it is also the second most powerful industrial laser in the world, as shown in Fig. 1.
Based on the self-developed non-photodarkening large mode field gain fiber and the 976 nm wavelength-locked high-power and high-brightness pump source, the team constructed a fiber laser system using the secondary fiber power combining technology. Figure 2 (a) after the diode-pumped laser is coupled through a double-clad Yb-doped fiber with a core diameter of 20.1 µm and a cladding diameter of 400.6 µm, the output power is greater than 3kW of single-mode laser, and the output fiber length is greater than 3 meters. The Raman suppression ratio is greater than 30dB. Then Fig. 2(b) uses a 7×1 power combiner with a first-order beam combining 20/400µm (NA = 0.064) fiber input and 100/400µm (NA = 0.14) fiber output for a 3kW single-mode laser to achieve a laser output greater than 20kW; In Fig. 2(c), a 7×1 power combiner with a two-stage combining 100/400µm fiber input and 400/460µm (NA = 0.22) fiber output for the 20kW laser is used. Among them, 2 beam combining modules will be used for further expansion, and finally achieve a multi-mode laser output greater than 100kW.
2. Three innovative technologies
Design and development of large mode field gain fiber without photodarkening; Design and development of 976nm wavelength-locked high-power and high-brightness pump source; Design and development of a laser with a combined beam output power of more than 100kW; Design and development of a fiber combiner with beam quality M2<3, These four technologies are internationally recognized problems. In addition to the development of the above 100kw fiber laser, we have solved three other international problems, namely circular modified large mode field double-clad Yb-doped fiber, high-power and high-brightness pump source and fiber laser beam combiner.
2.1 Circular modified large mode field double-clad Yb-doped fiber
In the fabrication process of gain fiber, the four key technologies of photodarkening effect, high-order mode and nonlinear suppression, preform preparation and low-refractive index coating aging have always been international problems. Our team established a model to characterize the effective calibration photo-darkening rate for large-mode field gain fibers of 20/400 µm and above. Figure 3(a) uses a double exponential function to test the light-dark effect induced by the test. Effective curve fitting of the curve to obtain the additional loss aatt, time parameter τ and other related characteristic parameters that characterize light and dark, for the first time in the world to achieve effective analysis and accurate calibration of photo-darkening performance, Internationally, it requires at least 1000 hours of aging to obtain photodarkening data, we can get the data within 20 minutes. The three major waveguide design technologies invented at the same time, Fig. 3(b), by matching the distribution of ytterbium ions in different mode field distributions, suppressing high-order mode gain and nonlinear effects, and realizing single-mode laser output of more than 6kW in 20/400µm active fiber; Large-scale preform preparation technology, Fig. 3(c) the slanting efficiency of 20/400µm active optical fibers was measured to achieve uniformity of large lengths and batch consistency. A method for improving low-refractive index coatings with additives was proposed, the aging of 20/400m active optical fiber coatings was compared in Fig. 3(d), our effect is the best, solving the problem of poor aging of low-refractive index coatings and not meeting the long-term use of high-power fiber lasers.
Fig. 3. (a)Photodarkening at 6xxnm wavelength produces additional loss over time. (b) Ytterbium ion distribution in different mode fields. (c) Measured graph of 20/400µm active fiber oblique efficiency. (d) Aging comparison chart of 20/400µm active optical fiber coating
In order to improve the absorption coefficient of the pump light of the gain fiber cladding, the team used the technology of embedding fluorine-doped units into the inner cladding of the fiber, and prepared a circular 20/400 µm Yb-doped double cladding with four fluorine-doped units embedded in it. The prepared circular 20/400µm Yb-doped double-clad fiber has a cladding pump absorption of 0.60dB/m at a wavelength of 915nm, which is 1.50 times that of the conventional octagonal 20/400µm Yb-doped double-clad fiber and the performance index exceeds the traditional octagonal structure Yb-doped fiber. According to the optical fiber preform test method, the diameter and refractive index distribution of the large-size optical fiber preform are tested first, and the core rod is sleeved with a suitable quartz sleeve in the complete system to achieve the designed core-to-pack ratio (1/20). Then, four holes are symmetrically drilled in the inner cladding region of the preform behind the sleeve, and F-doped low-refractive-index quartz rods are respectively inserted into the holes. Finally, at a temperature of 2050∼2250°C, the preform assembly was drawn into a 20/400µm optical fiber with a circular double cladding by a drawing tower, and the improved low-refractive index coating was applied. The cured coating can provide a cladding-pumped NA of 0.46.
2.2 High power and high brightness pump source
The spectrum of the semiconductor laser pump source varies greatly with temperature and operating current, and the spectral linewidth is also wider, which limits its wide application in pumping fiber lasers and other fields. Therefore, stabilizing the emission wavelength and compressing the spectral linewidth of semiconductor lasers has become an important subject of research.
Figure 4(a) is a fiber-coupled 710 W 976 nm wavelength-locking semiconductor laser developed by the team. It is realized by 30 single-tube chips through space, polarization beam combining and wavelength locking. The technology for the coupling of the whole device is based on the self-developed inclined multi-die polarization beam combining technology. As shown in Fig. 4(b), multiple laser diodes in the device are fixed on different inclined stepped surfaces with high unevenness. When the laser diode emits light with different heights, the light is collimated by the fast-axis collimating lens FAC and the slow-axis collimating lens SAC and then becomes a parallel beam, which is turned by the mirror and integrated in one area. After passing through the collimating lens, focus enters the fiber. Using the new VBG wavelength locking process, it can lock the wavelength at a temperature of 15°C to 35°C under the condition of full current. The wave locking capability can reach FWHM≤1nm at 15A∼30A, the output power is ≥700W, and the brightness is 21MW/ cm2·sr.
Fig. 4. (a) Fiber-coupled 710W 976 nm wavelength-locked diode laser. (b) Inclined Multi-die Polarization Beam Combination.
2.3 Fiber laser beam combiner
The team established the theoretical model of the fiber laser power combiner and the mode field distribution of the output beam, According to the relationship between the input mode and the output mode, this will guide the optimization of various parameters of the 10,000-watt 3×1 fiber combiner, reduce the number of channels, and improve the beam quality. Using the new technology of laser etching fiber cladding, a 10,000-watt fiber combiner with beam quality M2<3 was prepared. The 3 input fibers are 25/400µm GDF fibers. After removing the coating layer, the cladding layer is etched to about 35µm, and then spliced with 3500W single-module output fibers with 25/400µm GDF fibers. The output quality of the 3500W single-mode fiber laser was M2X = 1.22, M2Y = 1.27. The output 50/400µm GDF fiber is spliced with the 50/400µm QBH fiber interface, and the measured beam combining slant efficiency is shown in Fig. 5(a), reaching 99.5%. The measured beam quality is shown in Fig. 5(b), M2 = 2.93. The arm’s withstand power is 3.5kW and the output power is 10.1kW.
3. Comparison of technical indicators
After comprehensive comparison and analysis of domestic and foreign literature, there is no report of beam quality M2<3 of a combined 10,000-watt fiber laser, Table. 1 compares this project with similar international products or experiments [13–18]. The main indicators of the laboratory demonstration are compared, all of which are based on the 7×1 power beam combiner to achieve laser output. Through comparison, it is concluded that the output performance indicators of this project are comparable to those of IPG in the United States.
Table 1. Comparison of main indicators of international similar products or laboratory demonstrations
4. Summary
In conclusion, we pioneered a circular modified large mode field double-clad Yb-doped fiber without photodarkening, invented a high-brightness and high-power semiconductor laser pump source, and solved the problem of high beam quality 100kW fiber laser power combining. The fiber laser jointly developed by the University of South China and Wuhan Raycus Fiber Laser Technology Co., Ltd. has an output power of up to 100kW, beam quality BPP is 19.28 mm*mrad, the electro-optical conversion efficiency is greater than 40%, and the slope efficiency is 83.52%. It can be used in nuclear power plants in the future. High-end applications such as welding of thick-walled pipelines in nuclear facilities [19], dismantling of nuclear facilities in radioactive environments [20], and curing glass of high-level waste liquids.
Funding
Wuhan Municipal Science and Technology Major Project (2021012002023424).
Acknowledgments
We thank Wuhan Raycus Fiber Laser Technology Co., Ltd.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this Letter are not publicly available at this time but may be obtained from the authors upon reasonable request.
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