OPTICAL POLISHING FEATURE


‘The optics inside those interferometers [Ligo, Virgo and Kagra gravitational wave detectors] are quite insane’


lithography, we’re making optics where the figure and roughness have picometre- type requirements,’ he explained. In these circumstances, a mote of dust or even a person’s body temperature are enough to scramble the process. This is why finishing at Zygo is conducted in ultra clean rooms with minimal particle counts in the atmosphere, and with much of the work being conducted remotely from outside the room itself. But the team is not only controlling the


Bespoke instrumentation for measuring process forces being installed on a Zeeko CNC polishing machine


‘It’s an extremely powerful laser,’ said MPO co-founder and managing director, Dr Helmut Kessler. ‘We are one of the few manufacturers worldwide who can meet those threshold levels.’ A large part of why is the company’s understanding of LIDT. ‘The old theory for damage threshold was basically that it was thermally induced from impurities,’ explained Kessler. ‘We thought: “Well, if that were true, why do coatings with no discernible impurities work for small optics but cause large optics to blow up?”’ The team found that thermal damage is more likely driven by the coating structure and how the coating can dissipate heat.


Illuminating a dark art Even now, being able to produce fine optics in the way MPO and its competitors do is considered a dark art. Stories circulate of master opticians with eccentric methods, such as one who adds a particular brand of vinegar and another who flicks cigar ash into their polishing slurry. Though MPO doesn’t deploy such unconventional tricks, the company is like all others in the industry in holding its polishing secrets close. ‘It’s the environment – the humidity,


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the temperature, what pitch you use, what the stock removal rate is, what grit you use,’ said Kessler. ‘They’re the sort of parameters that you obviously don’t disclose.’ Although MPO tries to control as


many of these parameters as possible, like many others in the industry, Kessler regards the polishing process as non- deterministic. ‘You probably don’t know all the parameters because there are so many small contributors,’ he said. ‘For example, the optic might be almost finished, about to come off the polishing line, and for whatever reason it picks up a scratch and it throws you back three days.’ However, this idea that there are just some aspects of polishing that will always remain uncontrollable is starting to be challenged. Founded in 1970, Zygo is a global manufacturer of high-precision optical components and advanced optical metrology systems. Michael Albrecht, Zygo product line director of optics, has seen a lot more determinism entering the polishing and finishing process in his time at the company. ‘In some of the more challenging applications, like extreme ultraviolet


room conditions and atmosphere more precisely, they are also gently teasing out details of the dark art of polishing from the company’s master opticians. ‘Zygo has relied on the expertise of master opticians for years – and we still do,’ Albrecht said. ‘But more recently, we’ve been focusing on pulling back the curtain on these coveted skills and finding ways to automate them to increase production throughput of high precision optics.’ Another crucial element helping control


the process is Zygo’s in-house development of metrology tools. ‘The better we make the optics, the better the metrology is. The better the metrology is, the better we can make the optics, and so they keep feeding each other,’ Albrecht explained. ‘As we’re able to better control things like mid spatial frequency and high spatial frequency and roughness in the optics that go inside of the interferometers, then the interferometers become more powerful.’ With collaboration and funding from


LLNL’s National Ignition Facility (NIF), Zygo has a fleet of large polishing machines that are designed to make metre-class laser- grade optics. This means they can produce up to 80 extremely precise 1m diagonal substrates per month – a feat that has allowed Zygo to deliver roughly 8,000 optics to NIF up to now. Among these optics are more than 3,000 amplifier slabs, which amplify a pulse of photons into NIF’s 192 laser beams to focus them on a target smaller than a pencil eraser. The purpose is to compress the target to 100 times the density of lead


May 2022 Electro Optics 21


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Credit Zeeko


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