Preform Rare-Earth Profiler (PREP)

在过去的十年中，高功率光纤放大器和激光器得到了迅速发展，并成功地商业化，用于许多工业应用，如切割，焊接和标记。工业光纤激光器业务目前价值超过8亿美元/年，复合年增长率约为13%，是不同激光技术中最高的。这一成功的主要贡献者之一是显著改进的稀土掺杂光纤制造技术。高性能光纤依赖于折射率改性掺杂剂的受控掺入，以及提供增益的稀土离子。高效率、高平均和/或峰值功率的工业光纤激光器和放大器总是使用具有复杂折射率和稀土分布的大模面积光纤。在大多数情况下，同时使用许多不同的掺杂剂，以控制折射率和增益分布，并通过它控制光纤模态和模态微分增益。附加的掺杂剂也用于减少非线性效应，例如受激布里渊散射和拉曼散射，以及其他寄生效应，例如光暗化。各种掺杂剂具有不同的尺寸、迁移率和扩散速率，因此，所得的折射率分布通常与纤芯内的稀土分布大不相同，并且不能从另一个推断出一个。此外，取决于制造技术，掺杂剂的分布沿光纤预制件沿着是不均匀的，使得拉制的光纤性能可变和“不均匀”。特别地，在最通用和广泛使用的制造技术中，已知改进的化学气相沉积（MCVD）制造技术具有较差的可重复性和沿着预成型件长度的较大可变性。这显著地损害了纤维产率并增加了纤维成本。此外，甚至更重要的是，目前还没有关于纤维“适用性”的可靠信息。其适用性只能在完整的光纤激光器已经构建并经过全面测试之后才能进行测试和量化，这大大增加了光纤激光器模块的周转时间，产量和成本。因此，需要在纤维拉伸过程中早期识别不合适的预成型件或预成型件的部分并将其丢弃。另一个要求最近出现在光纤电信领域。在过去的几年里，多模电信系统的研究出现了强劲的复苏，空分复用（SDM）有望解决预测即将到来的电信容量紧缩。SDM系统的成功开发完全依赖于高性能多模光纤放大器的开发，该放大器具有精心优化的稀土（铒或钍）分布，用于模式增益均衡。同样，详细和准确的知识，活性掺杂剂分布在光纤横截面沿着整个预制件长度是至关重要的成功演示这种改变游戏规则的方法，以增加单光纤传输容量。该提案的主要目的是开发广泛适用的非破坏性表征技术，用于准确和详细地确定纤维预制件中的活性掺杂剂分布，并在预制件被拉制成纤维之前提供所需的可靠信息。这种预制件表征技术预计将对先进的高功率光纤激光器系统以及目前研究的SDM电信系统的性能和成本产生重大影响，并提高英国制造基础的竞争力，并加强英国在这些领域的尖端研究活动。

Over the last decade, high power fibre amplifiers and lasers have been rapidly developed and successfully commercialized for a number of industrial applications such as cutting, welding, and marking. The industrial fibre laser business is currently worth over $800M/year, with compound annual growth rate of about 13% - the highest among the different laser technologies. One of the main contributors to this success has been the significantly improved rare-earth doped fibre fabrication technologies. High performance fibres rely on controlled incorporation of refractive index modifying dopants, as well as, gain providing rare-earth ions. High efficiency, high average and/or peak power industrial fibre lasers and amplifiers invariably use large-mode area fibres with complex refractive indices and rare-earth distributions. In most cases, a number of different dopants are used simultaneously in order to control the refractive index and gain distribution, and through it the fibre modality and modal differential gain. Additional dopants are also used to reduce nonlinear effects, such as Stimulated Brillouin and Raman Scattering, and other parasitic effects, such as photodarkening. The various dopants have different sizes, mobility, and diffusion rates and, as a consequence, the resulting refractive index profiles can in general be much different to rare earth distributions within the core, and one cannot be inferred from the other. In addition, depending on the fabrication technique, the distribution of dopants is not uniform along the fibre preform, rendering the drawn fibre performance variable and "patchy". In particular, Modified Chemical Vapour Deposition (MCVD) fabrication technique, among the most versatile and widely used fabrication techniques, is known to suffer from poor repeatability and large variability along the preform length. This compromises significantly the fibre yield and increases the fibre cost. In addition, and even more importantly, currently there is no reliable information regarding the "fitness-for-purpose" of fibre in advance. Its suitability can only be tested and quantified after a full fibre laser has been built and thoroughly tested, adding considerably to the fibre laser module turn-around time, yield and cost. So there is a need for unsuitable preforms or parts of preform to be identified early in the fibre drawing process and be discarded. Another requirement has lately appeared in the fibre telecom area. Over the last few years there has been a strong resurgence in multimode telecom systems research, with spatial-division multiplexing (SDM) promising to solve the predicted forthcoming telecom capacity crunch. Successful development of SDM systems relies exclusively on the development of high performance multimode fibre amplifiers with carefully optimized rare-earth (Erbium or Thulium) profiles for modal gain equalization. Again, detailed and accurate knowledge of the active dopant distribution over the fibre cross-section along the entire preform length is critical for successful demonstrations of this game-changing approach to single-fibre transmission capacity increase. The main aim of this proposal is to develop widely applicable, non-destructive characterization techniques for the accurate and detailed determination of active-dopant distribution in fibre preforms and provide the required reliable information well before the preforms are drawn into fibres. Such preform characterization techniques are expected to have a big impact on the performance and cost of advanced high-power fibre laser systems, as well as, currently researched SDM telecom systems, and increase the competiveness of the UK manufacturing basis as well as enhance the UK cutting-edge research activities in these areas.