High Refractive Index Bonds

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.