OPTICAL COATINGS


power from a continuous wave (CW) laser onto the target, causing catastrophic heating, and a number of countries are investing heavily in the technology. In the USA, for instance, the Department of Defense (DoD) has several development programmes under way. As the DoD looks to increase the power of these weapons, it requested at least $669m in the fiscal year 2023 for unclassified research, testing and evaluation, and another $345m for unclassified procurement, the Congressional Research Service reported. According to a report from Global Industry Analysts Inc, the global market for directed energy weapons will grow from $15bn in 2020 to $51bn by 2026. High-power CW directed-energy weapons comprise arrays of fibre or semiconductor- diode lasers that are combined into a single, high-quality beam using spectral beam combining, through which many laser beams, each with a slightly different wavelength, are superimposed onto a


“Current optical coatings on the mirrors are a limiting factor for their [gravitational wave detectors’] sensitivity”


single beam using a device with angular dispersion, such as a prism or grating. The power in the resulting beam is the sum of the powers of the individual beams. The optics used in these systems require highly reflecting and anti-reflective coatings (typically applied by ion-beam sputtering, see below) that must be highly failure-resistant when in use. This has spurred coating facilities to fine-tune their processes to eliminate defects, predict potential failures and validate the quality of their coatings. Optical coatings could help to solve some of our biggest problems. The National Ignition Facility (NIF) is a laser-based inertial confinement fusion research device, located at Lawrence Livermore National Laboratory and working towards the goal of a controlled nuclear fusion reaction that could be exploited as a clean, sustainable form of energy. NIF triggers fusion reactions by aiming nearly 200 high-powered laser beams at the inside of a 1-cm-long hollow metal cylinder. The intense X-rays generated in the process converge on a 2-mm-diameter spherical capsule placed in the middle of the cylinder that contains deuterium and tritium. As the outer portion of the capsule is blasted off, the deuterium and tritium are forced inwards and, for a moment, experience enormous pressures and temperatures –


www.electrooptics.com


high enough that the nuclei fuse, yielding heat, helium nuclei and neutrons. There is one large, deformable mirror for each of NIF’s beams, each using an array of actuators to bend its surface to compensate for wavefront errors in the NIF laser beams. While working at the University of Rochester, Oliver and his colleagues coated these mirrors. Advances in optical coatings could even


play a significant role in enhancing our understanding of the universe. They are a key component in gravitational wave- detectors, called Michelson interferometers. The LIGO observatory in the USA first detected gravitational waves in 2015, opening up an entirely new field of astronomy, through which vibrations in spacetime are measured. Since 2015, gravitational wave detectors have been used to make some spectacular discoveries, including signals from more than 100 pairs of colliding black holes. Gibson and his colleagues at the


University West of Scotland are working as part of a UK-based consortium to develop technologies, including optical coatings for use in two future international gravitational wave detector development projects. These projects – Cosmic Explorer in the USA and the Einstein Telescope in Europe – are in the early stages of design work. They are expected to be fully constructed and operational by the end of the next decade. Gravitational wave detectors work


by bouncing lasers between mirrors suspended at each end of long pipes, often arranged in an L shape. As gravitational waves – the faint ripples in spacetime caused by enormous astronomical events such as the collision of black holes – pass through the detectors, they cause miniscule variations in the distance between the mirrors, as measured by the lasers.


Bigger and heavier mirrors The next generation of detectors will be more ambitious in their design, with lasers bounced between mirrors suspended free of external vibration and placed up to 40km apart, instead of 4km as they are in current detectors. The mirrors will also be bigger and heavier, as they will be double the diameter, at around 60cm. “You’re measuring deviations of 10-23


of a


metre. These are extremely high-precision interferometers and the current optical coatings on the mirrors are a limiting factor for their sensitivity,” says Gibson. “The physical movement of the atoms in the optical coatings gives rise to noise.” The increasing need for optical coatings in a wide variety of industries is driving an evolution in the processes used for their production, according to Gibson. He explains: “Your mobile phone, for example,


FEATURE


Professor Des Gibson and his colleagues at the University West of Scotland are working to develop optical coatings for use in two future international gravitational wave detector development projects


is absolutely jam-packed full of optical coating technology; the cameras, and the transparent conductive oxides that are used for the keypads, which need to be transparent and electrically conducting as well, for instance. There is a whole raft of different types of optical coatings now being used in extremely high-volume consumer electronics, which is driving the need for better techniques.” Ion-assisted electron-beam (IAD e-beam)


evaporative deposition is one of the standard methods used to create coatings for applications where flexibility and low cost are key. Using the technique, an electron gun bombards and vapourises source materials in a vacuum chamber. The resulting vapour condenses onto optical surfaces and forms uniform, low- stress layers of a specified thickness. IAD e-beam coatings feature low losses in the ultraviolet (UV) spectrum and high laser- induced damage thresholds (LIDTs) in the near infrared (NIR) spectrum. It can accommodate a wide range of coating materials, is economical and can be used to deposit materials on large substrates. The process, however, is not precise enough for some applications. This is leading to an increase in the use of sputtering to produce optical coatings, according to Gibson: “Sputtering gives you more durable coatings, it’s easier to control, and the quality of the coatings is much better. So, the market is moving more to sputtering technology.” Ion beam sputtering (IBS) is a repeatable


technology that creates coatings of very high optical quality and stability. During IBS, a high-energy beam of ions bombards


February 2024 Electro Optics 23


Elaine Livingstone


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  |  Page 45  |  Page 46  |  Page 47  |  Page 48