This conference presentation, “Precision displacement laser interferometry” was recorded for the Optics and Photonics for Advanced Dimensional Metrology II conference at SPIE Photonics Europe 2022.

Automatic control of the geometry of manufactured components is a cornerstone in the Industry 4.0 transfer that still constitutes an elusive goal. There are however some full-field optical techniques that in combination with optimized software have started to make the transition from the measurement room out to the production line and are able to generate real-time measures of sufficient quality. In this presentation two such techniques are reviewed together with challenges that still remains to be solved before a full implementation is possible. Short-range photogrammetry is a technique that utilizes the explosive development of high-resolution detectors to measure shape from differences in perspective observed in two or more cameras. As a non-interferometric technique, it relies on the presence of structure on the surface of the component that can be identified from different views. With present day technology it has been shown that it is possible to reach a precision of roughly 10-4 of the measurement volume in serial production. Another technique that has the potential to make the same transition is multiplexed multi-wavelength holography. Historically phase wrapping has been seen as an obstacle to apply dual-wavelength holography on generally shaped objects, which has pushed the technique towards wavelength scanning or coherence-gating and scanning. However, new approaches in software have made multiplexed multi-wavelength holography attractive for use also in an industrial environment, which is demonstrated by the use of a specially designed three-wavelength pulsed laser system. The precision achievable is comparable or slightly better than for photogrammetry and the robustness is sufficient. Despite the fundamental differences in technology both techniques require for its realization the same software steps to generate absolute measures, which include object identification, alignment, virtual fixturing and calculation of absolute measures. In the presentation the role of each of these steps are described in some detail.

This work presents a realistic, wave-optical signal modelling tool for chromatic confocal systems that employ partially incoherent illumination. In such systems, the partially incoherent illumination is obtained either via intrinsically partially coherent light sources or by propagating the light from a spatially coherent source through a multimode optical fibre of appropriate length and core diameter. The practicality of our modelling approach has besides the coherent illumination likewise been demonstrated for systems that employ incoherent illumination. Where the confocal signal was obtained simply by the incoherent summation of the intensities of the reflected fields in the image/detector plane. But now in the case of partially coherent illumination, not only the amplitude but also the phase information is of utmost importance for the realistic modelling of the chromatic confocal signal. In the literature review to the best of our knowledge, we have not come across a modelling approach for chromatic confocal signals that offers the possibility to preserve the phase information of all the modes that propagate through the conventional two-pass chromatic confocal optical system. In this work, first, the number of propagating modes through the optical system is determined. Here the specifications of the light source such as spatial as well as spectral distribution of the light source and the illumination function received at the entrance of the chromatic confocal system play an important role. Secondly, all these modes for the complete wavelength range have to be propagated through the system based on the wave-optical model. Finally, for all wavelengths, the coherent summation is done in the image/detector plane using the amplitude and phase information. The flexibility of this modelling approach is unprecedented and it not only offers a better understanding of chromatic confocal imaging but also opens new horizons in the areas such as artificial intelligence and machine learning where computer-assisted methods are employed that offer the realization of a more accurate, robust and eventually cost-efficient chromatic confocal measurement device.

The vision for the future of high value manufacturing, as embodied in ‘Industrie 4.0’, is one where autonomous ‘smart’ manufacturing processes are able to deliver bespoke products on-demand by deploying ‘right first-time’ fabrication techniques while aiding progression towards targets for waste reduction and carbon neutrality. While the vision is clear enough, the technical challenges associated with the realisation of autonomous manufacturing processes are substantial. Autonomous manufacturing requires measurement systems to be integrated much more closely with the manufacturing process than current sensor technologies allow, in order to generate the process feedback needed to ensure the workpiece being created meets its specification. It is predominantly the size and weight of conventional optical instrumentation that limit the potential for sensor integration either in-situ, on-machine or in-process. Recent advances in nanophotonics and specifically metasurface technology opens up a new route whereby a step-change reduction in size and weight of optical measurement systems can be achieved. Such systems will revolutionise the way optical measurement is deployed in manufacturing systems and represent a keystone technology upon which future manufacturing will evolve. We demonstrate the readiness of such metasurfaces to realise next-generation optical sensors through the example of a confocal microscope based on a single metasurface without the need for the additional optical elements found a conventional optical embodiment e.g. beamsplitters. The sensor concept is based on a metasurface consisting of two interlaced lenses, which focus independent off-axis points to a common point on the optical axis. We show how placing a point source and a point detector at these two off-axis points it is possible to realise an ultra-compact confocal sensor apparatus comprising a single optical element. Through this example we discuss the potential for a new generation of sensor technologies based on simplified optical assemblies which have great potential for both miniaturisation and cost-reduction.

Endoscopy is essential for biomedical imaging and diagnosis as well 3D Metrology and process monitoring in confined spaces. Conventionally 3D endoscopes are limited in the minimally achievable diameter by distal scanning or stereo optics to several millimetres. Recent progress and perspectives towards 3D submillimetre endoscopes using coherent fibre bundles (CFBs) is discussed. While CFBs allow for diameters of only a few 100 µm, they are commonly used for relaying intensity patterns, only. This is due to the complex-valued optical transfer function (OTF), which results in a time and wavelength depended phase scrambling. Furthermore, CFBs offer only a few 10,000 fibre cores limiting the achievable space-bandwidth product. Different approaches based on holography, digital optical phase conjugation, 2P-Polymerisation-based 3D printing and deep learning for enabling lensless 3D single-shot imaging with sub-micron resolution are introduced and compared. Optical OTF compensation can be realized using diffractive optical elements (DOEs). For static distortions, static DOEs printed by 2P-polymerization are sufficient. While dynamic distortion require an in-situ calibration and programmable DOEs such as spatial light modulators. OTF independent compensation can be realized by a low coherence common path interferometry. A second approach is based on circumventing the complex valued OTF by evaluating intensity information only, just like conventional lens based endoscopes. In order to achieve capability, an optical diffuser substitutes the distal lens. The diffuse scattering of light is used to code 3D object information in speckle patterns and relay this information through the CFB. Deconvolution algorithms or artificial neural networks can perform the decoding on the distal side numerically. The novel application areas for these single shot 3D measurements will be highlighted.

Coherent fiber bundles (CFB) are commonly used for endoscopic imaging, e.g. in biomedicine like non-destructive in-vivo diagnostics on brain tissue or inside cochlear. Usually, a CFB with several 10,000 cores is employed, allowing pixelated imaging of only intensity information due to unknown phase deviations for each single core, limiting the applicable optical measurement methods. However, using holographic image formation at the distal fiber facet enables 3D imaging by detecting only intensity information through the fiber. We demonstrate a miniaturized lensless endoscope for vibration measurements through a coherent fiber bundle.