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Ytterbium (Yb3+)-doped fiber lasers can offer high-power output with great brightness and excellent beam quality, and have been widely used in many application areas such as material industry, 3D printing, remote free-space communication and optoelectronic countermeasure. Especially in industrial processing field, the requirement for high-power fiber lasers keeps increasing year by year. In this field, the traditional fiber lasers usually deliver output beam with Gaussian intensity. However, according to the practical application experience, the high-power fibers laser with ring beam output reveals unique advantages in industrial processing such as higher initial rate of absorption, faster welding speed, deeper fusion penetration and wider fusion width, and can be used for high-quality welding of mild steel, stainless steel, titanium, red copper and brass. In this letter, we demonstrated a monolithic high-power all-fiberized laser oscillator with power scaling up to 5 kW which had output beam with ring intensity distribution and its output characteristics were carefully studies. The schematic diagram of this fiber laser was shown in Fig. 1. It employed the bidirectional-pump scheme which adopted two pump/signal combiners (PSCs) to coupling the pump energy into the fiber oscillator. One high-reflection fiber Bragg grating (HR FBG) and one output-coupling FBG (OC FBG) constituted the two ends of the oscillator. The gain fiber was a piece of 25/400 μm large-mode-area Yb-doped fiber (LMA YDF). A fiber QBH was fused with the OC FBG for laser output and a cladding pump stripper (CPS) was adopted in between them for removing residual pump power. The backward laser was collected using a useless-laser garbage. By properly adjusting the pump power of forward and backward directions to suppress the transverse mode instability (TMI) and stimulated Raman scattering (SRS), as shown in Fig. 2(a), the maximum output power reached 5080 W and the corresponding pump-to-signal conversion efficiency was calculated to be 68%. Fig. 2(b) was the measured signal spectrum under 5080 W. The central wavelength was located at 1080 nm with a FWHM of 3.6 nm. The intensity of SRS light was 37 dB lower than the 1080 nm signal light. The time-domain characteristics at the maximum output power and the corresponding fast Fournier transformation (FFT) were shown in Fig. 2(c), revealing that no TMI was observed. The beam quality (M2) at the maximum output power was measured to be 2.483 in X direction and 2.514 in Y direction, and was shown in Fig. 2(d). The insert in Fig. 2(d) was the beam intensity distribution at the focal point, which indicated that the signal beam was a standard ring beam and the intensity in annular region was 1.5 times as that in center region. In addition to the tested performance listed above, the fiber laser oscillator worked continuously for one hour and the recorded output power was shown in Fig. 2(e). The figure reveals that the output power was very steady and no obvious power fluctuation was observed.
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