Research On Key Technologies Of Large-Mode-Area Double-Clad Fiber Side-Pump Combiner

High power fiber lasers have an important effect on fiber communication, industrial processing and military fields and have entered a period of rapid development. For high power fiber lasers, there are two types of pumping methods for coupling light into large-mode-area double-clad fiber, including end-face pump combiner and side-pump combiner. For end-face pump combiner, pump light can cause damage to the conjunction of double-clad fiber. Meanwhile, when high power fiber laser is used for industrial processing, the reflective light, which is in the machining district, can cause damage to pump source. An approach to overcome this problem is side-pumping. Multi-pump lights are coupled into double-clad fiber at different fusion spots, and high power fiber lasers can be easily realized. However, it is difficult to realize side-pump combiner compared with end-face pump combiner. Parameters of side-pump combiner, such as the signal insertion loss of 0.5dB and the pump efficiency of 90%, need to be improved, so that side-pump combiner can satisfy the cascaded combiners for high power fiber lasers.According to the parameters of pump combiner, which is in high power fiber laser, the large-mode double-clad fiber side-pump combiner with high coupling efficiency, low signal insertion loss and low pump transmission loss is developed. Firstly, the asymmetric coupling theory of double waveguide is analyzed and simulated. According to the asymmetric coupling theory, multi-functional fiber tapering and fusing systems are developed. Then, the side-fusing technology of pump combiner is designed; the parameters of side-pump combiner are tested, and the packaging of pump combiner is designed. Lastly, the large-mode double-clad fiber side-pump combiners and high power fundamental-mode fiber lasers with cascaded combiners of bi-directional pumping and MOPA are developed. The main research contents are as follow.1. According to the incomplete coupling theory of double waveguide directional combiner, the coupling coefficients equations and the power equations of high-power asymmetric pump combiner are further derived and the simulation results are obtained as well. The conclusions are following: The power of pump fiber and main fiber changes periodically. When the taper angle of pump fiber is between 1° and 1.5°, the coupling efficiency of more than 97% is obtained. The local variation of the coupling length has little effect on the coupling efficiency.2. Multi-functional fiber tapering and fusing systems are developed, and the side-fusing technology of pump combiner is designed. Generally, pump fiber is tapered and twisted in order to fuse with main fiber. Pump fiber can cause tension and stress to main fiber. When pump fiber is fused with main fiber, the signal insertion loss is great. In this paper, pump fiber is fixed with main fiber via optical adhesive and fused together, causing no tension and stress to main fiber. The signal insertion loss of pump combiner is less than 0.04 dB.3. The packaging of side-pump combiner is designed, including coating, optical adhesive and quartz substrate. The distribution of pump light, which is in the cladding, the coating, the optical adhesive and the quartz substrate, are simulated via Tracepro. The thickness, length and refractive index of the optical adhesive have impact on the pump light, which is stripped by the optical adhesive.4. The parameters of pump combiner are tested, including pump power, pump efficiency, signal insertion loss, additional loss, splitting ratio and directionality. The technologies which have effect on the parameters of pump combiner are analyzed, including fusing time, torch position, torch height, volume of hydrogen and oxygen, moisture of hydrogen and oxygen, stretching speed, stretching length, coaxiality of clamps and moisture of environment. Lastly, the stability of pump combiner is tested, including lifetime testing, vibration testing, environment testing.5. High power fundamental-mode fiber lasers with cascaded combiners are developed. Firstly, 906 W fundamental-mode fiber laser of bi-directional pump is developed and the stability of the fiber laser is tested. According to the testing results, the pump combiner has the anti-reflection property and can protect pump source from the damage of reflective light. Meanwhile, 890 W fundamental-mode fiber laser of MOPA is developed.