Objective Since the development of Yb-doped fiber laser, increasing the output of a single fiber has been one of the most important directions. The output of a single fiber has now been able to attain 10 kW-level or above. However, these results are mainly laboratory reports, and few fibers have been adopted in practical applications because the coating material of these double cladding fibers age quickly due to the injected high pump power and large amount of quantum defect heating. To minimize the impact of high pump power on coating materials, triple-clad Yb-doped fibers, with an addition of fluorine-doped fused silica between silica and low-index coating, have been suggested. For a given fiber, high brightness of the tandem-pumping can not only reduce the quantum defect heating, but also further increase the pump injection and output-power scaling. To achieve a more reliable and higher laser output, a highly Yb-doped triple-clad fiber used for tandem-pumping is proposed.
Methods The Yb-doped fiber preform has been prepared by modified chemical vapor deposition (MCVD) in combination with sol-gel solution-doping method to improve the Yb-doped concentration without clusters. The doped preform was overcladded and shaped via grinding in an octagonal shape to form the pump cladding. The preform was further surrounded by the second highly fluorine-doped synthetic fused silica cladding with a depressed refractive index to form the out cladding. Then, the preform was drawn and coated as conventional double cladding fiber to obtain the triple-clad fiber (Fig. 1). The fiber was characterized by refractive index profiling, electron probe microanalysis (EPMA), loss, and absorption. Finally, the fiber laser performance was characterized.
Results and Discussions The fiber is 50/350/400 μm core-clad diameter, 0.06/0.22/0.46 NA, and approximately 0.6 dB/m inner cladding absorption at 1018 nm. The EPMA results showed that the homemade triple-clad fiber has a much higher Yb-doped concentration than the commonly used 20/400 μm double cladding fiber (Fig. 2). The reduced core numerical aperture was achieved by higher fluorine-doped silica, and laser performance of the fiber was demonstrated by an all-fiber master oscillator power amplifier at 1080 nm. Figure 3 shows the experimental setup; the seed laser is about 600 W, generated via a 20/400 μm Yb fiber laser oscillator. The triple-clad fiber was used as a gain medium in the power amplifying stage, and the pump sources are 1018 nm fiber lasers. When about 11606 W pump power was absorbed, up to 9010 W laser output with a slope efficiency of 80.5% was achieved (Fig. 4). Further power scaling was limited by the unabsorbed helical light in the fluorine-doped fused silica cladding, which is round-shaped. Thus, in the high-power laser experiment, only the forward pump was on. In future studies, we will improve the design of fiber laser setup and reduce the pump power in the fluorine-doped silica to further increase the fiber laser output.
Conclusions A maximum 9010 W laser output was achieved by the 1018 nm tandem-pumping using the homemade 50/350/400 μm large-mode-area Yb-doped triple-clad fiber. This is an important progress in the field of high-power fiber laser materials. Compared with double cladding fiber, the triple-clad fiber design, which deliveries most of the pump light inside the fluorine-doped fused silica, can greatly minimize the aging effect of the pump light on the coating material. This is of great significance for the long-term operation reliability of high-power laser fiber.
