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

在过去的四十年里，光纤彻底改变了电信，并使我们今天所知的互联网成为可能。传感是使用光纤的另一个领域，例如用于监测工程结构（例如，沿着桥梁、隧道等应变和振动），或者为先进仪器提供光，例如下一代显微镜，其可以看到传统仪器不可见的材料/组织特性。光纤还通过产生和传输能够焊接和切割的强激光来引领制造业的一场革命。然而，光通过玻璃传播的传统光纤无法应对如此高的功率，这是由于高光强引起的非线性效应和材料损伤。玻璃的吸收也限制了在可见光和近/中红外线方面纤维技术的开发。这些缺点正在由下一代光纤解决，即所谓的空芯光纤，通过中心孔引导光，从而避免显著的光-玻璃相互作用。这些光纤中的光是通过围绕中心孔建造的特殊设计的玻璃微结构来引导的。最近，这种微结构的设计和制造得到了显着改进，现在出现的空芯纤维在几乎所有方面都超过了传统纤维的性能。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，空气在制造或后续处理过程中进入光纤。虽然光与空气的相互作用明显小于与玻璃的相互作用，但这种相互作用仍然施加了明显的限制。一个例子是在诸如1300 nm（由于水蒸气）或中红外（大气气体的吸收）的波长处的吸收。另一示例涉及高功率脉冲（例如，如基于激光的焊接所需要的），其中与空气的非线性光学相互作用导致显著的光束失真。最终的解决方案是抽空光纤芯，从而消除空气与光的相互作用。初步计算表明，由于纤芯直径较小（通常约为0.03 mm），排空较长长度的空芯光纤（公里）将花费不切实际的时间（年）。还没有开发出表征气体压力或气体含量沿纤维长度的沿着的技术。如果没有这样的测量，就很难监测疏散过程，或者验证描述疏散过程的模型。该项目致力于研究，理论和实验，技术，以准确地表征（剩余）空气压力沿沿着一段空芯光纤。随后，我们将研究几种解决方案，以便在长距离内可靠地排空它们并密封它们，同时实现光进出的低损耗耦合。最后，我们将展示这些改进的空芯光纤将如何实现下一代应用，目标是三个选定的领域：1）电信，其中疏散将实现大波长范围内的通信，在给定时间内可以传输的数据量增加数倍。2)高功率激光脉冲不仅可用于焊接/钻孔/采矿，还可用于生物医学成像，我们预计，通过抽真空的空芯光纤传输的功率将是空气填充光纤的100 - 1000倍，是目前玻璃芯光纤的100万倍。3)中红外光的透射（"分子指纹区域"）和在远程碳氢化合物分析中的应用的示范，感兴趣的，例如，威尔斯的油井中。真空空芯光纤将提供任何其他光纤技术所无法比拟的上级性能，从紫外线一直到中红外线，为科学、技术和应用开辟了新的机遇。

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.