Nanoscale Coatings Improve Fused Silica Surfaces

Laser systems are used in a wide range of applications such as energy generation, medical devices and materials processing, as well as in distance sensors and precision instruments for both civilian and military uses.

However, there are technical limitations to the performance of substrates, whether they are diamond, sapphire or fused silica, in that they need to be coated with a nanoscale thin film to provide anti-reflection properties. The problem is that these coatings are not as durable environmentally as the substrate itself.

A solution to this problem is to replace these thin films with textures that are directly etched on to the substrate surfaces. These textures can be configured to provide properties enabling high reflection, anti-reflection and polarisation or wavelength filtering.

These textures provide wide acceptance angles of the impacting laser beams, as wide bandwidths give the substrate a high efficiency. It also provides other surface effects such as water resistance or hydrophobicity and a resistance to chemical adsorption from compounds around the laser environment.

Designers have long been concerned with the transmission losses caused by thin film coatings on optical substrates in research as well as commercial laser systems. Some of these coatings, such as sol gel, tend to be very delicate but are used because of cost considerations. Their advantage has been that they are resistant to laser damage. The more efficient anti-reflection coatings can cost three to five times more.

Water adsorption has also been a serious problem in mid-range infrared laser systems. The etching of the textures, usually termed “moth eye” in the academic literature, has solved this problem to a large extent.

Chemical damage to substrates caused by carbon contamination and hydrocarbon adsorption can also limit a laser’s efficiency. This can be solved by etching a durable anti-reflection coating on to sapphire, a substrate that is resistant to damage from alkali vapours.

These anti-reflective coatings can be applied to the end facets fused silica fibre lasers. This can avoid damage to the end facets and lessens retro reflections that bounce back through the fibres.

Different textures can be made to suit different substrates. A zinc selenide compound that has been metal ion doped can be fabricated on to sapphire as well as fused silica and vapour cell windows. This can also be etched on to diamond windows that are used in carbon-dioxide powered lasers for extreme ultraviolet lithography systems.

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