Pulsed fiber laser at 1 μm wavelength region
Date of Issue2017-08-23
School of Electrical and Electronic Engineering
Thanks to its plentiful merits, such as high efficiency, vibration proof and maintenance free, high power Ytterbium-doped pulsed fiber lasers (YDPFLs) at 1 µm wavelength region have seen rapid development driven by the broad industrial applications over the past decades. However, there is still a lot of room for innovation and the performance enhancement for the YDPFLs is valuable. This thesis focuses on the analysis of high power YDPFL at 1 µm wavelength region. Firstly, the theoretical modeling of YDPFLs and design of pulsed pump YDPLs with adjustable pulse duration and pulse repetition rate are investigated. Large amount of stored energy is the basis for high power YDPFLs. The more energy stored in the laser cavity, the more available gain for signal pulse laser injection. At the same time, the more inter-pulse Amplified Spontaneous Emission (ASE). When the ASE is not properly controlled, undesired laser treatment outcomes or self-lasing may occur. Pulsed pump is a superior avenue to address this issue. By modeling and simulation an ytterbium-doped double-clad fiber amplifier (YDFA), the most crucial factors of pulsed pump, pump power (PP) and pump duration (tp), have been investigated. For YDPFLs with adjustable pulse duration and pulse repetition rate, I proposed a new pulsed pump method. By using this pulsed pump scheme, the performance of the YDPFL is improved without additional pump or cost. Secondly, high average power and high peak power YDPFLs have been constructed and studied. High power YDPFLs have progressed rapidly and been widely utilized for material processing. When laser-material interaction occurs, the energy of the laser pulse whose peak power is higher than the ablation threshold power of the material contributes to the final desired treatment outcomes. The rest of the laser pulse energy is wasted. In order to characterize this, a new parameter, Usable Pulse Energy Ratio (UPER), is proposed. Through this parameter, whether the workpiece can be laser processed or not, how much the pulse energy is wasted, and the width of Heat-Affected Zone (HAZ) after laser treatment can be perceived qualitatively. Then a high average power YDPFL with output power of 160 W has been designed. The YDPFL operating with four pulse duration (80 ns, 56 ns, 34 ns and 10 ns) has been studied. The optical-to-optical conversion efficiency of ~ 67% for the main amplifier and the 4σ beam quality (M2) of 1.7 is obtained. The limiting factors for high power YDPFLs (such as ASE, Stimulated Raman Scattering (SRS) and Stimulated Brillouin Scattering (SBS)) are cautiously studied and addressed. A high peak power YDPFL with adjustable pulse duration from 350 ps to hundreds of ns seeded by an superluminescent light emitting diode (SLED) is also demonstrated. The monolithic YDPFL is constructed with MOPA configuration which consists of three cascade amplifiers and the 350 ps laser pulses are generated through gain switch. Finally, laser applications can not only verify the design of the laser, but also provide the design requirements for the laser. Instead of using expensive and bulky ultra-short lasers, high power nanosecond YDPFL is applied to the blackening of a bulk Al alloy substrate with an alumina surface application. The laser induced porous nanostructures/microstructure (NSs/MSs) layer is formed and located beneath the alumina surface of the BSBs. This is the first time to reveal the porous NSs/MSs formed beneath the alumina surface by a nanosecond YDPFL to the best of our knowledge and the NSs/MSs layer is suggested to be responsible for the structural color. From the application point of view, the properties of the output laser of an YDPFL and the blackening effect are directly related. This in turn emphasizes the strong bond between the application and the laser design.