Aim

This project aims to exploit the longitudinal chromatic aberration present in a basic implementation of a metalens in order to produce an ultra-compact chromatic confocal sensor which aims to supersede established technologies based around bulky and difficult to fabricate diffractive optical elements.

Details

Chromatic confocal probes are well-established in industry for the measurement of position and film thickness due to their ability to maintain a well-controlled lateral resolution across the whole measurement range. This arises from the fact that the longitudinal chromatic aberration (LCA) yields an optimised focal spot size at any given axial measurement position (see fig. 1).

This project utilises the LCA arising as a consequence of a metalens design that is optimised at only a single illumination wavelength to realise a chromatic confocal probe that allows the location of a scattering object along the optical axis to be determined from the wavelength of light that is most strongly returned into the instrument.

By combining this ultra-compact front end probe with an ultra-compact wavelength detecting element a distance sensing instrument can be created that can be deployed in a range of applications that were previously unfeasible. This project also covers the initial deployment multi-metasurface elements within the programme, by utilising two metasurfaces to tune the amount of LCA generated by the system, and thus the measurement range of the instrument.

A proof of principle system has been demonstrated using an all-dielectric metalens (MS) based on cylindrical pillars of GaN on an Al2O3 substrate. The metalens requires a collimated input which requires an additional collimating element (RC2) to operate. The measurement target (mirror M2) was translated axially in increments of 1 um over a 300 µm range, establishing a positional uncertainty of approximately 0.162 µm. This uncertainty estimate includes the contributions of any motion errors associated with the motorised translation stage (see fig. 3).

Ongoing work

The requirement for a collimating element adds complexity to the probe and reduces potential for miniaturisation. Therefore, a second metalens has been designed and fabricated which takes uncollimated light directly from the input fibre and allows the removal of the additional collimating element (RC2).

Additionally, it is useful to be able to tune the axial measurement range of CCS probes in order to apply them to a variety of measurement scenarios. Tuning the axial measurement range means changing the amount of LCA generated by the metalens. To facilitate this, a metalens doublet has been designed where the first element collimates the beam from the input fibre, while the second focuses. We are investigating the interplay between the naturally occurring LCA in both elements in order to provide a ‘tuneable’ LCA which can allow for a targeted measurement range.

An outstanding issue for devising a low-cost and compact chromatic confocal measurement system is the requirement for spectral detection. Conventional spectrometer technology based on mirrors/gratings are expensive and complex assemblies with limited potential for miniaturisation. We have established two approaches to address this issue: a) a metasurface-based spectrometer; b) a scattering spectrometer based on a scattering media created using a femtosecond laser to inscribe nanovoids in fused silica. The latter technology has been developed in conjunction with collaborator Dr Martynas Beresnas from the University of Southampton [2].

References

[1] J.H.T. Chan et al., “An ultra-compact metasurface-based chromatic confocal sensor, ” CIRP Annals, Volume 72(1), pp. 465-468, 2023.

[2] P. Falak et al., “An Ultracompact Metasurface and Specklemeter-Based Chromatic Confocal Sensor,” in IEEE Transactions on Instrumentation and Measurement, vol. 73, pp. 1-8, 2024.