EVacuAted OptiCal Fibres for Ultimate UV-to-Infrared Light TransMission (VACUUM)

在过去的40年里，光纤使电信发生了革命性的变化，并使我们今天所知的互联网成为可能。传感是另一个使用光纤的领域，例如用于监测工程结构(例如，桥梁、隧道等沿线的应变和振动)，或者用于为先进仪器提供光，例如可以看到用传统仪器看不见的材料/组织属性的下一代显微镜。光纤还通过产生和传输能够焊接和切割的强激光，在制造业中引领着一场革命。然而，由于高光强引起的非线性效应和材料损伤的开始，传统光纤无法应对如此高功率的光通过玻璃。玻璃吸收也限制了可见光和近/中红外纤维技术的开发。这些缺点正在被下一代光纤解决，这种光纤是所谓的中空光纤，它引导光通过中心孔，从而避免了显著的光-玻璃相互作用。这些纤维中的光被引导，这要归功于围绕中心孔建造的特殊设计的玻璃微结构。最近，这种微结构的设计和制造得到了显著改进，现在出现了中空纤维，其性能几乎在所有方面都超过了传统纤维。在这些新型光纤中，光通过纤芯传播；在大多数情况下，纤芯包含空气，在制造或后续处理过程中空气进入光纤。尽管光与空气的相互作用比与玻璃的相互作用要小得多，但这种相互作用仍然施加了明显的限制。一个例子是波长为1300 nm的吸收(由于水蒸气)或中红外吸收(大气气体的吸收)。另一个例子涉及高功率脉冲的传输(例如，根据基于激光的焊接所需)，其中与空气的非线性光学相互作用导致显著的光束失真。最终的解决方案将是疏散光纤芯，从而消除空气与光线的相互作用。初步计算表明，由于纤芯直径较小(通常约0.03毫米)，疏散长长度的中空纤维(公里)将需要不现实的长时间(数年)。表征沿纤维长度的气体压力或含量的技术也尚未开发出来。如果没有这样的测量，就很难监测疏散过程，也很难验证描述疏散过程的模型。该项目致力于从理论和实验上研究准确表征一段中空纤维上的(残余)气压的技术。随后，我们将研究几种解决方案，以可靠地长时间疏散它们，并在实现光输入和输出的低损耗耦合的同时对它们进行密封。最后，我们将展示这些改进的空芯光纤将如何支持下一代应用，目标是三个选定的领域：1)电信，疏散将实现大波长范围内的通信，使给定时间内可以传输的数据量增加数倍。2)用于焊接/钻孔/采矿的高功率激光脉冲，以及生物医学成像，我们预计通过抽空的空芯光纤可以提供的功率比充气的光纤大100-1000倍，比今天的玻璃芯光纤高出100万倍。3)中红外光(“分子指纹区域”)的传输和在远程碳氢化合物分析中的应用演示，例如在油井中。真空中芯光纤将提供比任何其他光纤技术更优越的性能，从一路在紫外光中引导到中红外，为科学、技术和应用打开新的机遇。

Over the last four decades, optical fibres have revolutionised telecommunications and enabled Internet as we know it today. Sensing is another area where optical fibres are used, for example for monitoring engineering structures (e.g., strain and vibration along bridges, tunnels, etc.), or to deliver light for advanced instruments such as next-generation microscopes that can see material/tissue properties that are invisible with traditional instruments. Optical fibres are also leading a revolution in manufacturing by generating and delivering intense laser light capable of welding and cutting.However, conventional fibres, where light propagates through glass, cannot cope with such high powers due to the onset of nonlinear effects and material damage caused by the high light intensities. Glass absorption also limits the exploitation of fibre technologies in the visible and near/mid-infrared. These shortcomings are being addressed by the next generation of optical fibres, so-called hollow-core fibres that guide light through a central hole, thus avoiding significant light-glass interaction. Light in these fibres is guided thanks to a specially engineered glass microstructure built around a central hole. Recently, the design and manufacturing of this microstructure has been improved significantly and hollow-core fibres are now emerging with properties that surpass those of traditional fibres in almost every regard. In these novel fibres, light propagates through the core; in most cases the core contains air which enters the fibre during fabrication or onward handling. Although light interacts with air significantly less than with glass, this interaction nevertheless still imposes appreciable limitations. One example is absorption at wavelengths such as 1300 nm (due to water vapour) or in the mid-infrared (absorption of atmospheric gases). Another example relates to the transmission of high-power pulses (e.g., as needed for laser based welding) where nonlinear optical interactions with the air result in significant beam distortions. The ultimate solution would be to evacuate the fibre core, thereby eliminating the air-light interaction. Preliminary calculations show that evacuating a long length of hollow-core fibre (kilometres) would take impractically long (years) due to the small core diameter (typically ~0.03 mm). Techniques to characterize the gas pressure or content along the fibre length have also not been developed yet. Without such measurements, it is difficult to monitor the evacuation process, or to validate models that describe the evacuation process. This project is dedicated to investigating, theoretically and experimentally, techniques to accurately characterize the (residual) air pressure along a length of hollow-core fibre. Subsequently, we will research several solutions to reliably evacuate them over long lengths and to seal them while enabling low loss coupling of light in and out. Finally, we will demonstrate how these improved hollow-core fibres will enable next-generation applications, targeting three selected areas:1) telecommunications, where evacuation will enable communication over a large wavelength range, increasing several times how much data can be transmitted over a given time. 2) high-power laser pulses for welding/drilling/mining, but also bio-medical imaging, where we expect up to 100-1000 times larger powers to be deliverable through the evacuated hollow-core fibres as compared to air-filled ones and up to one million times more than with today's glass-core fibres. 3) transmission of mid-infrared light ("molecular fingerprint region") and demonstration of applications in remote hydrocarbon analysis, of interest, e.g., in oil wells. Evacuated hollow-core fibres will offer superior performance to any other fibre technology, ranging from guiding in the UV all the way to mid-infrared, opening new opportunities in science, technology, and applications.