4 kW Near-Diffraction-Limit Laser Output Basedon Low-Numerical Aperture Confined-Doped Long-Tapered Fiber

Objective High-power ytterbium-doped fiber lasers (YDFLs) have good beam quality, high conversion efficiency, high reliability, and good compactness, making them widely usable in industrial processing, military, and national defense fields. The output power of YDFLs has been unprecedentedly improved with the development of double-clad fiber, laser diodes, and passive devices. This unprecedented progress is hampered by nonlinear effects, such as stimulated Raman scattering (SRS) and transverse mode instability (TMI). The most effective and fundamental way to suppress these two effects is to optimize the structure design of active fiber. We propose a low-numerical aperture confined-doped long-tapered (LCT) fiber that combines the advantages of low numerical aperture (NA), Yb3+ ions restricted doping, and longitudinal tapered design and can theoretically suppress TMI and SRS effects simultaneously. Methods The LCT fiber is proposed and successfully fabricated using modified chemical vapor deposition (MCVD) in conjunction with a solution doping technique (SDT). The LCT fiber has a core NA of 0.05 and a gain dopant doping diameter ratio of 77%, with a core/cladding diameter of 25/400 tan at both ends and 37.5/600 ttm in the middle. A bidirectional-pumped master oscillator power amplifier (MOPA) system verifies the laser performance of the LCT fiber. Results and Discussions A laser output of 4.188 kW was obtained with a slope efficiency of 82. 8% (Fig. 3). The intensity of the Raman Stocks light was 18 dB lower than that of the signal laser at 4.188 kW output power, and the M-2 factor was about 1.3, maintaining a single-mode output. Further optimization will focus on improving the pump absorption and effective mode area of this fiber to mitigate SRS. Conclusions We present a novel low-NA (0.05) confined-doped long-tapered fiber fabricated using the MCVD process in conjunction with SDT. The Yb-ions doping diameter ratio is similar to 77%, and the middle section core/cladding diameter is 37.5/600 mu m, tapering to 25/400 mu m at both ends. In the bidirectional pumping MOPA configuration, a 4.188 kW laser is obtained with a slope efficiency of 82.8 Yo. The M-2 factor is about 1.3 at 4.188 kW output power, maintaining a single-mode output. The results above reveal that using low-NA confined-doped long-tapered Yb-doped fiber to achieve high power output with high brightness is a promising prospect.