| The key techniques for high-power all-fiber lasers include diode pump source technique, double-clad fiber technique, fiber coupling technique, fiber grating technique and terminal output technique. The fiber coupling technique as one of the core technique, which aims to couple pump light or signal light to a double-clad fiber efficiently. In general, there are two main fiber coupling technique: tapered fiber bundles endcoupling fiber technique and side-coupling fiber technique. However, the fiber couplers for high-power fiber lasers mostly imported. Therefore, developing high power fiber laser based on China-made device is vital important for our country. In this article, we will focus on several subjects: the mode field characteristics of several special waveguide fibers, end-coupling technique and side-coupling technique. The main contents and results of this article includes are as follows: 1. The mode field distribution and beam transmission characteristics of bend fibers, tapered fibers and thermally expanded core(TEC) fibers were analyzed using beam propagating method(BPM), and a hundred watts mode field adapter(MFA) was fabricated by TEC technique. Firstly, the distortion of the mode field in the bending large mode area(LMA) fiber were investigated. The results showed that high-order fiber modes were sensitive to fiber bending; a large fiber bend radius and small refractive index difference can lead to serious mode field distortion. Secondly, the mode field of tapered single-mode fiber(SMF) and multi-mode fiber were analyzed. The results showed that SMF can obtain high transmission efficiency as long as the adiabatic tapering conditions was satisfied. However, for multi-mode fiber, the critical affecting factor of the transmission efficiency was taper ratio. Finally, a high power MFA developed by TEC technique was investigated. The mode field characteristics of the TEC LMA fiber was analyzed. Comparison to the SMF, we found that the mode field diameter(MFD) of LMA fiber increasing rapidly than SMF. The MFDs of the SMF and LMA fiber could be matched each other by TEC technique. In addition, analysis results show that even the MFDs matching each, the transmission loss still exist. We used the SMF-6/125μm and LMA-15/130μm fiber fabricated a hundred watts MFA with transmission efficiency of 74% and the maximum handle power as high as 100 W. The MFA offers a superior performance than the commercial MFA in market. In addition, thermally expanded core technique can also be used for making fiber couplers. 2. Based on TEC technique, a novel method called altnative fiber method for large clad-core ratio of the LMA fibers was applied to combine single-mode laser beams, and high power N×1 pump combiners and signal combiners were developed. Firstly, the N×1 end-pumped fiber combiner was investigated theoretically and experimentally. Based on Vytran GPX Series glass processing system, we fabricated the 3×1, 7×1, 19×1 and 61×1 high-power end-pumped fiber combiners by TFB technique. Secondly, the N×1 single-mode signal beam combiner was investigated theoretically and experimentally. We analyzed the mode field distribution of the 7×1 signal beam combiner. Especially, the relationship between the transmission efficiency and the spatial transient in the combiner was investigated. Seven 600 W single-mode fiber lasers were combined by a 7×1 signal beam combiner, the total combined power up to 4k W. 3. The side-pumped fiber coupling technique was investigated and high-power(N+1)×1 side-pumping couplers were developed and a distributed side-pumped, ytterbium-doped all-fiber amplifier was built. Firstly, the fused-tapered side-pumped fiber coupling technique was studied theoretically and experimentally. A(2+1)×1 fused-tapered side-pumped coupler with a coupling efficiency of 94% and maximum handle power of 400 W was fabrticated; Secondly, the angle-polished side-pump fiber coupling technique was investigated. A(1+1)×1 angle-polished side-pumped fiber coupler was fabricated by fused splicing angle-polished multi-mode fibers and 20/400μm double-clad Yb-doped LMA fiber together. The coupling efficiency of this side-pumped coupler up to 97%, the leakage ratio of signal light less than 2% and the handle power as high as 140 W. Thirdly, we analyzed the performance of a single sidepumped coupler as well as the distribution of cascade stage couplers impact on the efficiency of fiber amplifier. 4. Using self-developed N×1 pump couplers, we built a kilowatt Yb-doped fiber laser with master oscillator power amplifier(MOPA) structure. Thermal management and the beam quality of fiber laser was investigated. The output power of 1.02 k W and the beam quality M2 = 1.07 were obtained. Also, a prototype was producted and applied in cutting metals. In addition, a distributed side-pumped, Yb-doped all-fiber master oscillator power amplifier was built using self-developed(N+1)×1 side-pumped couplers. In experiment, the amplifer was pumped by 8 cascade stage side-pumped couplers and the continuous and single mode laser of 1080 nm wavelength, 303 W maximum output power is obtained.
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