Study On Ytterbium-Doped Single-clad/Double-clad Fibers And The Fiber Lasers

The dissertation focuses on the Yb³⁺-doped single clad/double clad fibers and the fiber lasers, includes the detailed descriptions on the following subjects: 1. Study and manufacture of Yb³⁺-doped single-clad/double-clad fibers (1) By using MCVD process, liquid doping method and optical machining technique, we manufacture Yb³⁺-doped single-clad/double-clad fibers (DCF) firstly in China. The inner cladding of the Yb³⁺-doped DCF has big geometrical dimension and large numerical aperture. The doping concentration can reach as high as 3640ppm (weight ratio). There are three kinds of inner-clad shape of DCF: circular (CDCF), square (SDCF) and rectangular (RDCF), and the NA of inner-clad is 0.323-0.360. (2) We put forward designing principle of DCF. By taking several effective measures, we obtain Yb³⁺-doped fiber having high concentration, high quality, low propagating losses at 1060nm(laser) in fiber's core and 976nm(pump light) in inner cladding. (3) According to the standard measuring methods suggested by ITU-T, the following parameters are measured: the geometric dimensions and refractive index distributions of optical fiber performs; the geometric dimensions, optical propagation parameters and absorption coefficient at 976nm of single-clad /double-clad fibers. By using the technique of stripping clad modes, we measure the absorption coefficient about pump light in the core of DCF directly. By comparing with the data obtained from single-clad fiber, its relative error is less than 5%. (4) Using ICP-AES method, Yb³⁺ concentration is measured. Through experiments, we determine the linear relation between the concentration of immersion liquid and Yb³⁺ concentration: one gram of Yb2O3 in one liter of water can dope 767.8ppm Yb³⁺. Yb³⁺ concentration has linear relationship with the absorption coefficient at 976nm: 1ppm Yb³⁺ can produce 4.11dB/km absorption. 2. On the basis of fundamental principle of fiber laser and new model, we derive the representation of effective absorption coefficient and explain the characteristics of DCF: its absorption coefficient changes with fiber's length and other factors. Using ray optics, the light propagation characteristics in CDCF, RDCF and SDCF are analyzed. At the same time, the pump absorption efficiencies are also discussed. 3. Experimental studies on Yb³⁺-doped single-clad fiber lasers (1) By comparing two kinds of pumping scheme, in which the wavelength are 976nm and 915nm respectively, we demonstrate that using 976nm pump source can lead to higher gain in the long wavelength region of 1030nm. Through adjusting the central wavelength of 980 LD toward short wavelength as long as 2nm, the pump power of 976nm is improved as high as 10mW. (2) We manufacture Fabry-Perot cavity Yb3+-doped fiber (YDF) lasers with FBG selecting frequency. The optimum YDF length is 9m, the launched pump threshold power is 30.0mW, the maximum laser power is 8.8mW and the slope efficiency with respect to launched pump power is 70.4%. The laser wavelength is 1060nm and the FWHM is about 0.4nm. (3) We manufacture ring cavity Yb3+-doped fiber lasers with FBG selecting frequency. The optimum YDF length is 12m, the launched pump threshold power is 30.2mW, the maximum laser power is 7.5mW and the slope efficiency with respect to launched pump power is 61.7%. The laser wavelength is 1060nm and the FWHM is about 0.2nm, which is 10% of the FWHM of the laser with mirror selecting frequency. The optimum coupling ratio of the output coupler is also discussed. 4. Experimental studies on Yb3+-doped double-clad fiber lasers (1) We design key components of DCF lasers. It is demonstrated that front-cavity mirror can improve the laser spectrum, increase the output power and the slope efficiency of laser and decrease the threshold . (2) Using CDCF with different length to make fiber lasers. It is found that there is an optimum fiber length in Yb3+-doped DCF laser and laser wavelength has linear relation with the length of DCF, changing ratio is 0.41nm/m. The relation between the laser wavelength and the fiber length is derived theoretically. (3) Using 20m long CDCF, SDCF and RDCF, the fiber lasers with different inner-clad shapes are made. By comparison, we find that the RDCF laser has the highest pump absorption efficiency, the lower launched pump threshold power (34.6mW), the narrowest FWHM (less than 2nm) and the highest slope efficiency (77%). (4) Using a group of back-cavity mirrors with different transmission ratio to constitute RDCF lasers, the changing regularities of some parameters (such as laser power, slope efficiency) with transmission ratio are studied. We obtain following conclusions: back-cavity mirror with a higher transmission ratio is better; back-cavity mirror can control laser wavelength; the relationship between slope efficiency and transmission ratio is in accordance with exponential function, the maximum slope efficiency is about 60%.