FEATURE OPTICAL POLISHING
Smooth operators: the dark art of optical polishing
Benjamin Skuse asks why, after 400 years of polishing optics, the process hasn’t been automated
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you’re trying to shape, and in polishing, it’s softer,’ said Peter Thelin, a former master optician at Lawrence Livermore National Laboratory (LLNL) who now runs Thelin Optical Consulting. Polishing and finishing (also called
ravitational wave detectors Ligo and Virgo have opened a new window into the cosmos, detecting the faint ripples of
distant colliding black holes and neutron stars. The Large Hadron Collider has already given us the Higgs boson and promises even more insights into the fundamental constituents of the universe as it begins its next run after another upgrade. And the new James Webb Space Telescope is on the verge of probing deeper in time and space than humanity has ever achieved before to spy on the very first galaxies that formed in the early universe. These extraordinary big science instruments rely on a host of modern technologies, but they would be useless without the extreme precision optics that advances in polishing and finishing have provided over recent decades. Producing a finished optical component
capable of controlling light in a desired manner follows the same process, regardless of the material or shape required. From a block or other basic shape of material, an initial blank is hewn that is roughly the desired dimensions. This blank then goes through several stages of grinding, at each step using softer tool materials and generally smaller grit size. Grinding brings the surface to within 1 to 5µm of the finished shape, but leaves a frosted appearance. This is why polishing is key. ‘I differentiate polishing from grinding by saying that, in grinding, the lap [working tool] is harder than what
20 Electro Optics May 2022
corrective polishing) finalises the optical component’s surface form and, most importantly, removes surface and subsurface damage caused by the various stages of machining, leaving behind a smooth, shiny finish. During his 44 years in the industry, Thelin has seen dramatic changes in polishing, yet the fundamental process remains the same. ‘The techniques are very similar, but the more high finesse you want, the more you have to control a bunch of variables so that they’re all very, very consistent,’ he said. ‘But all of these little variables become harder to control and it becomes harder and harder to predict.’ Manx Precision Optics (MPO) is a
leader in providing such super-smooth optical finishes. Based on the Isle of
Man, the small family-owned company manufactures optical components for some of the world’s most powerful laser systems. Using a traditional pitch polishing process, MPO automates some of the early polishing stages, but final finishing is done by experienced craftspeople operating continuous polishers. The end result is a smooth surface, generally 3 to 5 Angstrom RMS roughness, though 1 to 2 Angstrom is also possible. Working with ELI Beamlines in the Czech Republic, in 2019 MPO applied their process to manufacture optical elements with new coatings for the BIVOJ laser, a pulsed nanosecond diode-pumped, solid-state laser developed at the Science and Technology Facility Council’s Central Laser Facility in Didcot, UK. These new optical elements allowed the BIVOJ team to extend the limits of the laser’s existing laser-induced damage threshold (LIDT), and thereby beat its own world record for kilowatt laser sources in its class by almost 40 per cent.
Manx Precision Optics’ polishing lab @electrooptics |
www.electrooptics.com
Manx Precision Optics
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