Cladding-pumped Raman fibre laser sources
Cladding-pumped Raman fibre laser sources
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发表时间：
2011-03
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通讯作者：
J. Ji
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作者：
J. Ji
在本论文中，我研究了包层泵浦拉曼光纤激光器和放大器。此类设备提供了一种产生拉曼增益的新颖方法，将非常成功的包层泵浦稀土掺杂光纤激光器的优点与受激拉曼散射的优点结合起来。它们不仅继承了传统光纤器件的大部分优点，例如灵活性、高效率、紧凑性和鲁棒性，而且还具有相对于传统光纤源的自身优势和独特特性，即波长灵活性和无需储能的近瞬时增益。包层泵浦拉曼光纤激光源利用双包层拉曼光纤作为增益介质。它们与稀土掺杂双包层光纤相似，只是纤芯没有激光离子掺杂。使用双包层光纤，低成本多模泵浦源的高功率输出可以转换为衍射极限信号光束，例如通过受激拉曼散射。因此，包层泵浦拉曼光纤激光源是一种亮度增强器。在本文的开头，我从理论上分析了限制此类器件亮度提升的各种因素。限制之一是不需要的第二斯托克斯一代，它限制了内包层和纤芯之间的面积比。通过设计一个带有W型核心的新DCRF，我成功地将这一限制放宽了近五倍。结合其他因素，即纤芯损伤阈值、走离、数值孔径和背景损耗，在具有正确泵浦源和参数的模型所示的此类器件中，设计的光纤可以实现超过 3500 的亮度增强。其次，我重点关注此类设备的转换效率。使用内包层与纤芯面积比约为 6 的精心设计的光纤作为双包层拉曼光纤，由具有近矩形脉冲形状的源泵浦。近乎矩形的脉冲是通过预脉冲整形从铒和镱共掺杂主光功率振荡器获得的。选择足够短的片段来减少背景损失和走离。在脉冲包层泵浦拉曼光纤放大器中，第一斯托克斯的最高峰值功率转换率为 75%，能量转换效率超过 60%。第三，我研究了功率可扩展性。从理论上讲，我分析了此类设备可实现的功率。在大多数情况下，核心尺寸是一个关键因素。如果核心足够大并且有足够的泵浦功率，则最终输出功率通过热透镜效应被限制在 24 kW 左右。在实验中，与同事合作，在 1116 nm 处演示了 100 W 包层泵浦拉曼光纤激光器。输出光束几乎受到衍射限制。它显示了此类设备的功率可扩展性的潜力，以及在稀土掺杂光纤提供的常规范围之外的波长下产生高功率衍射极限光源的能力。由于大纤芯尺寸是功率扩展的关键因素，因此引入了具有大模式面积的新型双包层拉曼光纤。实验证明它们的工作效率与以前的光纤一样。使用 Nd:YAG 激光器泵浦其中一根光纤，从而证明了具有良好光束质量的 1 mJ 拉曼光纤源。这表明双包层拉曼光纤为获得高亮度高能量光源提供了另一种途径。此外，基于包层泵浦拉曼光纤转换器，提出了一种简单有效的方法来产生超连续谱源。
In this thesis, I investigate cladding-pumped Raman fibre lasers and amplifiers. Such devices, offering a novel way to generate Raman gain, combine the advantages of the hugely successful cladding-pumped rare-earth doped fibre lasers with those of stimulated Raman scattering. They not only inherit most advantages of conventional fibre devices, such as flexibility, high efficiency, compactness, and robustness, but also provide their own advantages and distinct properties relative to conventional fibre sources, i.e., wavelength flexibility and nearly instantaneous gain without energy storage. Cladding-pumped Raman fibre laser sources utilise double-clad Raman fibres as the gain medium. These are similar to a rare-earth doped double-clad fibre except that there is no laserion doping of the core. With double-clad fibres, the high-power output from low-cost multimode pump sources can be converted into diffraction-limited signal beams, e.g., through stimulated Raman scattering. Thus, cladding-pumped Raman fibre laser sources are a kind of brightness enhancers. In the beginning of this thesis, I theoretically analyse various factors that limit the brightness enhancement of such devices. One of the limits is unwanted 2nd-Stokes generation, which restricts the area ratio between the inner cladding and core. By designing a new DCRF with a W-type core, I successfully relax this restriction by nearly five times. Combined with other factors, i.e., core damage threshold, walk-off, numerical aperture, and background loss, a brightness enhancement of more than 3500 for the designed fibre could be achieved in such devices shown by a model with right pump sources and parameters. Secondly, I focus on the conversion efficiency of such devices. A well-designed fibre with inner-cladding-to-core area ratio around six was used as a double-clad Raman fibre, pumped by a source with nearly rectangular pulse shapes. The nearly rectangular pulses were obtained from an erbium and ytterbium co-doped master optical power oscillator through prepulse shaping. A sufficiently short piece was chosen to reduce the background loss and walk-off. The highest peak power conversion into the 1st Stokes was 75% and the energy conversion efficiency was over 60% in a pulsed cladding-pumped Raman fibre amplifier. Thirdly, I study the power scalability. Theoretically, I analyse the achievable power of such devices. The core size turns out to be a critical factor in most cases. The ultimately output power is limited to around 24 kW by thermal lensing if the core is large enough and enough pump power available. Experimentally, in collaboration with co-workers, a 100 W claddingpumped Raman fibre laser was demonstrated at 1116 nm. The output beam was nearly diffraction-limited. It shows the potential of power scalability of such devices and the ability of generating high power diffraction-limited sources at wavelengths outside the conventional range that rare-earth doped fibres offer. Since a large core size is a critical factor for power scaling, new double-clad Raman fibres with large-mode areas were introduced. They were experimentally demonstrated to work as efficiently as the previous fibre. An Nd:YAG laser was used to pump one of these fibres, and a 1 mJ Raman fibre source with good beam quality was thus demonstrated. This shows that double-clad Raman fibres offer another approach to obtaining high-brightness high-energy sources. In addition, based on a cladding-pumped Raman fibre converter, a simple and efficient method was proposed to generate supercontinuum sources.