Gong Ma-Li, Pang Lu, Tian Jiading, Huang Yu-Sheng, Yi Yong-Qing, Wang Xue-Jiao, Wang Ze-Hui, Yan Ping, Li Dan, Xiao Qirong, and Pan Rong
In recent years, high power fiber laser has received great attention, leading to wide applications in numerous fields such as industry, biology and relevant research. Nevertheless, the output power of typical diode pumped fiber laser is limited by the thermal effect and brightness of pump source. Owing to the low quantum deficit, the tandem pumping employing ytterbium-doped fiber lasers (YDFLs) as the pumping source can effectively reduce the thermal issue and achieve high power output. With the much lower absorption coefficient at 1018 nm than at 976 nm, longer gain fiber is necessary in tandem pumped configuration to sufficiently absorb pump light, which in turn induces a more severe nonlinear effect such as the stimulated Raman scattering, bringing in more challenges in laser configuration design. In this paper, we demonstrate an all-fiber laser under master oscillator power amplifier configuration based on tandem pumping with domestic gain fiber produced by China Electronics Technology Group Corporation No. 46 Research Institute. The diameters of the core and inner cladding of the Yb3+ doped double cladding fiber are 25 m and 250 m, respectively. The modified chemical vapor deposition method with gas-solution co-doping method is adopted so that the fiber has a more uniform distribution of Yb ions, larger absorption cross section and higher absorption coefficient (0.41 dB/m@1018 nm). In the amplifier stage, a 40-m-long gain fiber is pumped by fourteen 1018 nm fiber lasers with a maximum total output power of 3511 W. A 67.8 W 1080 nm seed is amplified to 3079 W with a corresponding slope efficiency of 85.9%. The beam quality factor M2 is measured to be 2.14. In addition, no stimulated Raman scattering is found in output spectrum and the 3 dB band width of output laser is measured to be 1.4 nm. To the best of our knowledge, this marks the highest result ever reported for tandem pumping based on domestic gain fiber. Taking stimulated Raman scattering into account, the rate equations are built to calculate the properties and power evolution in the fiber amplifier. The numerical results are in good agreement with the experiment results. Besides, based on heat conduction equation, heat power density in the fiber core is analyzed, showing that the tandem pumping has great advantages in heat management and a huge potential to reach a higher power compared with the method of direct pumping. The theoretical and experimental results show that with ever-maturing fiber manufacturing technology, domestic fiber is capable of withstanding laser power as high as 3 kilowatts. Meanwhile, domestic fiber may achieve a higher output power by increasing the pump power, optimizing the gain fiber length and improving the cooling condition.