Design Of Efficient Coupling System For Pumping And Research On Key Technologies Of Fiber Lasers

Efficient coupling between semiconductor lasers and tailed fibers is very important in improving the fiber output power, pump absorption efficiency, and beam quality. This thesis is mainly devoted to the detailed theoretical and experimental researches on lensed fibers and graded index fibers (GIF), which play a great role in coupling system. Furthermore, three key element technologies for fiber lasers are involved:analysis on pump absorption efficiency of double-clad rare-earth-doped fiber, design of coupler used in high power fiber pigtailed semiconductor lasers, tunable erbium-doped fiber la-sers.The main achievements of this thesis are listed as follows:1.A mode overlapping integration model for coupling efficiency calculation is estab-lished by means of elliptical Gaussian beam approximation for stripe laser diode (LD). Based on the model, laser oscillating in butt-coupling between LD and single mode fi-ber (SMF) is analyzed, and on the contrary an efficient coupling is achieved by using lensed fiber. Wedged fibers and curved surface lensed fibers are fabricated by means of grinding technology and arc discharge technology, respectively.2.Propagation models for Gaussian beam in GIF are presented based on eigen mode function expansion and ABCD law, respectively. An optical system composed of com-pound lens and GIF is proposed for coupling between LD and SMF, and simultaneously another coupling system composed of telescope system and elliptical graded index fiber (EGIF) is proposed as comparison. Both of them are verified by propagation models mentioned above and finite difference beam propagation method (BPM), as a result, the coupling systems are efficient.3.An analysis on pump absorption of double-clad fiber (DCF) is made via 2D ray tracing method, however, it is qualitative.Consequently, a novel method is proposed to solve the calculation of cladding pumping, in which, two key theories are used:Com-plex refractive index fiber core and BPM. All the results show that the D-shape DCF and the graded index inner-clad DCF can provide a better absorption of pump light. By means of MCVD, two optimal designs of double-clad erbium-doped fiber are realized: D-shape and Hexagon-shape.4.Different coupling systems are designed for high power fiber pigtailed semicon-ductor lasers. By using a pure-silica-core fiber with low index coating, high coupling efficiency is obtained for a single fiber pigtailed semiconductor laser, and the same as a fiber bundle pigtailed semiconductor laser by a taper silica rod.5.A novel linear-cavity tunable erbium-doped fiber laser (EDFL) with a nar-row-linewidth (<0.01nm) and a high side mode suppression ratio (SMSR) (>50dB) lasing output is proposed and demonstrated. The laser is composed by a tunable fiber Bragg grating (TFBG) and an optical circulator (OC), the tuning range covers approxi-mately 7nm in C band. A novel swithcable dual-wavelength fiber laser using Panda type photosensitive polarization maintaining erbium-doped fiber (PMEDF) is proposed and experimentally demonstrated. By adjusting the PC in the cavity, the switching between two single-wavelength and dual-wavelength lasing outputs is achieved.