Research On The Key Technology And Output Stability Of High Power Single-mode Fiber Laser

With the developing of fiber lasers,the all-fiber lasers have been applied broadly in medicine,automobile manufacturing,laser ranging,and laser medical treatment,etc.by their outstanding characteristics such as small size,high transform efficiency and superior reliability etc.It shows that the all-fiber laser has a great potential applications and commercial value for researching.The key technology and output stability of high power single-mode all-fiber laser are studied in the thesis,and the main contents are as follows:(1)The characteristics of fiber components of the all-fiber laser such as the large-mode-area(LMA)fiber,fiber Bragg grating(FBG),fiber combiner,fiber cladding mode stripper(CMS)and pump lasers are introduced and analyzed.The theoretical model of the fiber laser is simulated according to the rate equation theory and the heat equation of the fiber,and the output characteristics of the fiber laser are analyzed and discussed with different parameters.The high power fiber laser can be designed according to the simulated results.(2)The output characteristics of the high power ytterbium-doped single-mode all-fiber laser are measured and analyzed experimentally,and the slope efficiencies of the fiber lasers are compared by pumping with wavelengths of 915 nm and 976 nm.In the experiments,the spectral width of the laser can be broadened with the output power increasing.When the spectral width exceeds the bandwidth of the HR FBG,the backward laser power will increase rapidly,and the corresponding forward output power will decrease.Therefore,the influence of the FBGs with different reflective bandwidths in a bi-directional pumped Yb-doped all-fiber laser oscillator is studied experimentally.The signal power and backward power increased linearly well with the increasing pump power by using the HR FBG with 3 run bandwidth,and more than 1.5 kW signal power with near diffraction-limited beam quality(M2 ?1.11)is obtained.The slope efficiency of the fiber laser is about 82.3%.Moreover,the single-directional pumped all-fiber laser is studied by high power domestic pump lasers,and more than 2 kW output power with perfect beam quality(M2 =1.16)is obtained experimentally.The research achievement proves that the technology of high power fiber laser oscillator in our works has reached the intra-industry advanced level.(3)The high power single-mode all-fiber amplifier system is designed and studied experimentally,the stimulated Raman scattering(SRS)effect and mode instability(MI)phenomenon which are limited the power scaling up of the fiber amplifier are analyzed theoretically.The high power all-fiber amplifiers with single-directional and bi-directional pumping schemes are studied experimentally,and more than 3 kW signal power with good beam quality are obtained.The MI phenomenon of the output beam is observed and analyzed by comparing different coil radii of the gain fiber in the same fiber amplifier according to the characteristic of the MI,and the high-order-modes in the fiber can be suppressed by decreasing the fiber bend radius.We has demonstrated the MI power threshold increases obviously with reduction of the coil radius,and the output stability of the all-fiber amplifier can be improved effectively.The maximum output power of the fiber amplifier is about 3.7kW.A cladding-pumped Yb-doped all-fiber amplifier by bi-directional pumping scheme has been demonstrated experimentally.Different seed powers were injected to the fiber amplifier,and we observed the SRS light with different threshold powers.The results of the experiments show that the SRS threshold power is inversely proportional to the injected seed power,and it increases obviously with lower injected signal power with the fixed fiber type and length of the fiber amplifier.More than 3.5 kW signal power with good beam quality(M2 =1.28)of the all-fiber amplifier is obtained by using a suitable seed power injected in the experiments,and the slope efficiency is about 84.4%.The world-class technology of the high power fiber amplifier has been demonstrated in our works.