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Research On Key Element Technologies Of High-power Fiber Lasers

Posted on:2009-07-02Degree:DoctorType:Dissertation
Country:ChinaCandidate:F ZhangFull Text:PDF
GTID:1118360272484604Subject:Communication and Information System
Abstract/Summary:PDF Full Text Request
High power rare-earth-doped double-clad fiber lasers have some unique advantages including excellent beam quality, small volume and weight. They will be widely used in optical fiber communications, medical, industrial processing and military. This thesis is mainly devoted to the detailed theoretical and experimental researches on three key element technologies of high-power rare-earth-doped double-clad fiber lasers: beam shaping techniques for laser diode bars, side pumping methods by using sub-wavelength diffraction gratings and global design of high-power rare-earth-doped fiber laser systems. And the main achievements of this thesis are listed as follows:1. A non-sequential optical model of the laser diode bars is established and based on this model, a novel collimator for collimating the fast and slow axis of laser diode bars simultaneously is designed by ZEMAX and fabricated by using a precise three-dimensional control actuator to erode the side of a D-type fiber. This collimator consists of a D-type fiber with a series of longitudinal cylindrical lens which has the same refractivity as fiber and acts as the slow axis collimator. Meanwhile, the D-type fiber is also able to collimate the fast axis with its transverse D-type structure.2. A taper fused silica rod is proposed for beam shaping of laser diode bars, whose fast axis have been collimated. This beam shaping element is fabricated by drawing directly the pure fused silica rod from 10mm (diameter) to 200um. It is able to concentrate the output beam of laser diode bars from 10mm×1.1mm to 200umx200um and the concentration efficiency achieves 68%.3. Two simple non-imaging beam concentrators for the laser diode bars are presented. They consist of the tapered SiO2-rod with slanted and curved surfaces. The principle and applicability of these devices are numerically investigated by ZEMAX and experimentally illustrated for the laser diode bars. The slanted and curved surfaces of the SiO2-rod provide the necessary anamorphic transformations where coupling between the two transverse directions may exist to increase the brightness in one transverse direction and compensate for angular and spatial fluctuations in the input beam to produce a uniform output beam. Results show that a concentrated rectangular output beam spot with uniform intensity distribution at the slow and fast axis is obtained at the output facet of the rod and the beam size is only 1.3mm><0.8mm.4. Diversiform sub-wavelength diffraction gratings are presented for side pumping double-clad fiber laser. Doubly-grooved metal reflection gratings and double-groove and two-depth dielectric transmission gratings are introduced and applied to side coupling of pump lights. Meanwhile, multi-level binary metal diffraction gratings, composite diffraction gratings with a dielectric-metal-dielectric (DMD) sandwich structure and asymmetric dual-side-gratings are presented to achieve polarization-independent, high power handling and robustness.5. A global design method-genetic algorithm (GA) is proposed to optimize the critical parameters of high-power rare-earth-doped fiber laser systems for the first time. Six critical parameters of Yb3+-doped double-clad fiber lasers are optimized by a micro-GA written by ourselves. Besides, niche hybrid GA based on fitness sharing and elite replacement is introduced and applied to optimize Nd3+-Yb3+ co-doped double-clad fiber lasers and results show that dividing equally input pump power among 808nm (Nd3+) and 940nm (Yb3+) is not an optimal choice and the pump power of Nd3+ ions should be kept around 10%~13.78% of the total pump power. Three optimal schemes are obtained by niche hybrid GA and the highest slope efficiency of the laser system is able to reach 80.1%.
Keywords/Search Tags:High-power Rare-earth-doped Fiber Laser, Laser Diode Bar, Beam Shaping, Side pumping, Sub-wavelength Diffraction Grating
PDF Full Text Request
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