FEATURE OPTICAL POLISHING
‘This is the big bottleneck in automating polishing: process unpredictability’
g
and heat it to 100 million degrees Celsius to achieve ignition – that is, nuclear fusion. Albrecht added: ‘To achieve the high fluences required at NIF, for each substrate, you’re allowed to have essentially one 10µm defect on the entire surface, and you’re controlling the figure to about 60nm peak.’ Equally impressive has been Zygo’s
somewhat unheralded work enabling gravitational wave astronomy. For the now famous Ligo and Virgo facilities, as well as Kagra, an upcoming gravitational wave observatory in Japan, extremely precise optical components generating minimal scatter over kilometres have been the key enabling technology. ‘The optics inside of those interferometers are quite insane,’ said Albrecht. ‘The tolerance is less than 1 Angstrom RMS, which sounds okay, but we’re measuring across several different spatial scales.’ What this means is that the optical component is smooth from the level of less than a micron up to full aperture, the result being no scatter at all.
Automated future? With Zygo controlling so many variables in the polishing process to allow them to quickly and repeatedly produce super- polished surfaces, the next step seems obvious – full automation. But Albrecht is somewhat pessimistic that this will happen any time soon. ‘I think that it could be automated, but it’s a major challenge because there are so many variables involved, and it’s very rare that you would have the same scenario reoccur.’ Professor David Walker, University of Huddersfield, UK, shares Albrecht’s view. ‘We are nowhere near that yet,’ he said. ‘But my long-term dream is that we can do for optical manufacturing what Tesla is doing for cars – build a machine with appropriate
Optical element being readied for installation at Ligo
metrology and sensor technologies, so that you can put a rough piece of glass in one end and get a finished piece out the other with no manual intervention.’ Walker is at the forefront of making
this dream a reality. At Zeeko, a small but market-leading polishing and metrology provider based in Coalville, UK, he has been involved in the development of a series of CNC precision polishing machines capable of achieving finishes as good as 2 to 3 Angstrom, and form accuracies better than λ/20 P-V. ‘The core Zeeko technology in these machines is an inflated bonnet process where the body can adapt to different curvatures on a surface,’ Walker explained. ‘It effectively takes the established craft process and automates it.’ But Zeeko’s machines, and those of competitors such as Logitech based in Glasgow, UK, are not infallible. ‘With a Zeeko machine and a fair wind, you’ll get about 80 per cent convergence, which means you tried to remove an error and 20 per cent of it is left,’ said Walker. In other cases – whether using a Zeeko machine or employing any other polishing process – Walker said that sometimes things just go wrong, like removing too much material or introducing unexpected local defects. ‘Why? Nobody has real insight into the fundamentals of why this happens.’ Moreover, human decision-making is still
High energy optic from MPO’s TT range 22 Electro Optics May 2022
required at various touch points during the Zeeko process: in defining process-chains for specific materials, geometries and tolerances; in interpreting metrology results and making appropriate decisions; and in deciding when to end the polishing process.
Walker is trying to get to the nub of
these problems. Like Zygo, he has quizzed master opticians, but for a longer-term aim. In a study conducted alongside a team of researchers from the University of Huddersfield and University of Nottingham, a psychologist extracted knowledge from three skilled operators working on a challenging glass optic task. The gleaned information was then used to construct flow charts showing the sequence of process steps performed. This and further studies are intended to build understanding of the master optician’s decision-making processes in a way that would be digestible for future autonomous intelligent systems capable of manufacturing high-quality optical components unsupervised. His most ambitious project – funded by
the UK’s Engineering and Physical Sciences Research Council – aims to illuminate the most fundamental mystery in polishing and finishing: exactly how does it work? ‘This is the big bottleneck in automating polishing – process unpredictability due to the basic physics and chemistry not being understood,’ he said. By studying the micro- and macroscopic details of how abrasive slurry traverses the boundary between the tool and component’s surface during finishing, Walker and a host of collaborators hope to shed light on the problem. ‘The idea is to develop a predictive model that will relate things you can measure in real time like forces, temperatures, vibration, to predict what the instantaneous removal rate is,’ he explained. ‘If you can do this, you might have a method of improving process convergence.’ EO
@electrooptics |
www.electrooptics.com
Manx Precision Optics
Caltech/MIT/LIGO Laboratory
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44
