High Refractive Index Bonds

不存在用于将由高折射率（RFI）材料制成的透镜结合到用于捕获光频率信号的光电二极管的表面的明显且简单的方法。任何气隙都会限制光学视场，除非找到直接且光学有效的粘合技术，否则会浪费在更大角度上捕获光的潜力。对于折射率较低的光学器件，可以使用粘合剂，但由于现有粘合剂的最大RFI低于2.0，因此无法用于RFI较高的透镜。如果使用，则在粘合剂和透镜的界面处将存在显著的背反射，并且高RFI光学器件内部的可用入射角将受到限制。将光学器件直接3D打印到表面上是不可行的，因为目前没有RFI大于2的树脂。Tethir希望找到并验证一种可行的方法，将光电二极管粘合到高RFI透镜的出射孔径，粘合区域直径在0.5 - 1.5范围内。实现高耦合效率的最可能的方法要求接触表面非常干净和平坦（<1 nm表面粗糙度）。需要测量透镜出射表面的表面粗糙度，并且由于没有标准光电二极管供应商公布关于其光电二极管表面粗糙度的信息，因此也需要测量该表面粗糙度。根据这些测量的结果，可以探索改善表面粗糙度和后续清洁的方法。目前还不可行实施直接键合，但测量光电探测器表面的技术，如原子力显微镜（AFM）将确定它是否是可行的未来。还可以检查一些粘合剂粘合的触点，以便X射线计算机断层扫描可以调查粘合表面的尺寸和完整性。这项调查的成功结果将使Tethir能够评估未来设计中光学表面的粘合方式，并为NPL提供有价值的评估，以了解其各种工具如何最好地应用于光通信表面的表征。

There is no obvious and simple method for bonding a lens made of a high Refractive Index (RFI) material to the surface of photodiodes that are used to capture optical frequency signals. Any air gap unavoidably constrains the optical field of view and the potential to capture light across much larger angles is wasted unless a direct and optically efficient bonding technique can be found.For lower refractive index optics, adhesives can be used but since those available have a maximum RFI below 2.0, none can be used for lenses with a higher RFI. If used, there would be significant back reflections at the interface of the adhesive and lens and usable incidence angles inside the high RFI optics would be limited. 3D printing of optics directly onto the surface is not feasible because there are currently no resins with an RFI greater than 2\.Tethir wishes to find and validate a workable approach to bonding photodiodes to the exit aperture of high RFI lenses with the bonding area diameter in the range 0.5 - 3 mm but it lacks the resources needed to carry out the investigations.The most likely methods for achieving high coupling efficiency require the surfaces in contact to be extremely clean and flat (< 1nm surface roughness). The surface roughness of the lens exit surface will need to be measured and because none of the standard photodiode suppliers publish information on the roughness of their photodiode surfaces, this too will need to be measured. Depending on the results of these measurements, methods for improving surface roughness and subsequent cleaning can be explored.It is not yet feasible to implement direct bonding but measurements of photodetector surfaces with techniques such as Atomic Force Microscopy (AFM) will establish whether it is feasible in future. It will also be possible to examine some adhesive bonded contacts so that X-ray Computed Tomography can investigate the dimensions and integrity of bonded surfaces.A successful outcome to this investigation will allow Tethir to assess how optical surfaces should be bonded in future designs and provide NPL with valuable assessments of how its various tools can best be applied to the characterisation of surfaces for optical communications.