Study Of High Power Double-clad Ytterbium-doped Fiber Laser

Due to the rapid development in recent year, high-power fiber laser has gotten far away from the conception of low energy equipment and limited field application. All the improvement should owe to the improvement in high power and high brightness semiconductor pump lasers and the emergence of cladding pump double-clad fiber's technology.Act as gain medium of fiber laser, fiber has some notable features such as good cooling quality and high energy-transfer ratio, which is higher than 60%. This kind of high-power equipment can be widely used in industry, laser hitting, military, medicine and DWDM communication system. For these Ytterbium-doped fiber laser that can work at the wavelength range from 1060nm to 1200nm, they have been the main competitor with Nd:YAG laser in industry. Fiber laser is the laser that use fiber as matrix and doped some activated ion as working media, or the laser that take advantage of the non-linear effect of fiber.As a brand new technology, fiber laser has been widely investigated owing to its excellent quality in technology. 135W single mode laser was reported in 2002. Many research groups in china also involved this field. Shanghai Insititute of Optic and Mechanics repoted 4.9W output of high power double-clad Ytterbium-doped fiber laser, and Tsinghua University reported 10.8 w output of high power double-clad Ytterbium-doped fiber laser. All the resonantors they adopted was realized by adding Media mirror outside of the fiber. This method faced much difficulty such as high loss, complicated structure, and inconvenience to modify the beam path. In the paper, we report a new method by adding two fiber grating at two edge of the active fiber to act as the laser resonantors. We did research on high power double-clad Yb3+ doped fiber laser pumped by high power fiber-coupled semiconductor laser module, and got 10.8 w limited diffraction single mode output of fiber laser. The wavelength is 1100.5nm and the bandwidth is 0.54nm. 1. Method and equipment.As described in Fig.1, we use tapered fiber to achieve the low loss connection between fiber. Whose transfer ratio is up to 90% according the experimental measurement. The diameter of double-clad fiber and Yb3+ doped core is 7um. The inner-cladding is quasi-circular structure. (The interfere pattern in fiber surface was showed in fig1.) The length of double-clad fiber is 20 meters. Collimate lens was put on the output side of fiber, and the collimate beam will be analyzed by spectrum analysis instrument and power meter.Fiber laser 's resonantor was based the reflection of fiber grating. The application of fiber grating make fiber laser simpler and systemize its structure. The primary reflection wavelength of fiber grating resonantor we constructed is 1100 nm, its transmission ratio for pump beams(970nm)is 99%. The reflection ratio of fiber grating, which acts as back reflection len, was 100% for pump beams. Since there is 4% Fresnel reflection in the output side of fiber grating, The fiber grating work as a narrow-bandwidth wave filter rather than a len. 2.Result and discussion:In our experiment, the diameter of high-power semiconductor laser fiber model's output fiber is 400 um. NA is 0.22. Diameter of the inner-cladding is 210um. NA between inner-cladding and outer-cladding is 0.45. the shape of inner-cladding is quasi-circular. Quasi-circular and other asymmetry shape were verified to be high coupling efficiency in both experiment and theory. That method also is propitious to make the whole structure composed by fiber. So we selected tapered fiber: the diameter of input side is 400 um, NA is 0.22. The diameter of output side of fiber is 200um, which is a little bit smaller than the inner layer to guarantee lower loss of connection. The length of tapered fiber is 2.5m, and total length is 3.5m. pump fiber-coupled module output fiber,taper fiber, and double-clad fiber was connected by large core fuse-joint instrument. Some proper ultraviolet pastern materials was use to pack. We apply circular water cooling...