tEchnology SputtEr coAtingS Improve performance
Stephen Mounsey investigates the dense world of sputter-deposited optical coatings
uring the communications boom of the early 1990s, new sputter coating techniques allowed telecoms developers to produce filters able
to reflect all wavelengths outside a very narrow transmission band. These so-called dense wavelength division multiplexing (DWDM) filters now allow many signals to be transmitted down a single fibre optic cable before being easily de-multiplexed, each at a slightly different wavelength. These sputter coating techniques have found more and more applications in coating high-precision optical components, where the technique allows optical performance that would otherwise be impossible. The durability of sputter coatings means that they can be used with lasers at high power densities, or in the most hostile of ambient environments. In order to understand what sputter coating can offer customers looking for high-end optical components, it is necessary to consider the alternatives. Prior to the development of sputtering, optical coatings
with sputtering D
were produced through evaporative physical vapour deposition processes.
Although evaporative coatings still account for most optical coatings produced, they have many shortcomings, as Mark Damery, VP of sales at REO, a spokesman for coatings specialist REO (Research Electro Optics, Boulder, Colorado), describes: ‘The majority of thin films fabricated for optical applications are produced through traditional evaporative means; this means that you take [the coating] material and heat it up, either by passing an electric current through it or by hitting it with an electron beam [or “e-beam”], until it gets hot enough to boil and vaporise. This is all done in a vacuum chamber, so that the resulting vapour fills the chamber before re- condensing on every surface,’ he says, noting that the parts to be coated are placed within that chamber. ‘You may have several different coating materials in the chamber that can be rotated into the path of the electron beam one after the other, so that you can produce a coating consisting of several different layered materials.’
A sputter chamber for producing optical coatings by the high-energy ion beam sputtering technique. Image courtesy of DSI
The evaporative process, Damery explains,
is not very energetic; as the coating material re- condenses on the surface of the optic, it does not encounter the substrate with a high energy, and the resulting coating is relatively porous. This porosity, he explains, is responsible for many of the limitations of optics coated by e-beam or other evaporative processes, because the voids in the coatings absorb moisture from the ambient environment. Moisture subsequently changes both the shape and refractive index of the coating, preventing it from functioning properly. In the case of laser optics, moisture in a porous coating can also lower the optic’s damage threshold. In harsh environments such as salt spray or steam, traditional evaporative coatings are even more limited in usefulness. ‘The bottom line on traditional evaporative coatings is that they have issues in terms of stability under changes of temperature or humidity,’ says Damery.
24 ElEctro opticS l MAY 2011
www.electrooptics.com
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