Unmanned aircraft systems (UAS) and their applications are experiencing rapid growth, especially in the area of remote sensing. Advances in propulsion; airframe materials; communications, command and control (C3) systems; cameras and detectors; and image processing are combining to continuously improve UAS capabilities. Most optical systems used in unmanned aircraft systems function over the visible (VIS) and near-infrared (NIR) wavelength ranges. Increasing in importance is multispectral or hyperspectral imaging which can combine information from VIS, NIR, mid-wave infrared (MWIR), and long-wave infrared (LWIR) wavelength regions to provide enhanced detection, such as chemical, vehicular, or terrain identification capabilities not available by the use of imaging alone. This article will cover some of the basics of optical thin film coatings as applied to electro-optical systems commonly used in unmanned systems (Figure 1).
Substrate performance dominates the overall performance of a coated optic. The useful transmittance range of a window material is the most important characteristic, followed by mechanical properties. Various optical glasses are the most durable and versatile of substrates. They have excellent compatibility with coating processes and high hardness. Colored or dyed glasses can be used in conjunction with coatings, for example, to block visible light and only permit transmittance in the NIR. One of the most important substrates is synthetic sapphire, Al2O3. Sapphire is extremely hard and durable and has a useful transmittance range extending from 0.3 microns out to nearly 6 microns. The increasing use of sapphire as a substrate for blue LEDs has driven prices down and capabilities up, making it an even more desirable material for optical coatings.
Other substrates used in the MWIR include germanium (Ge), silicon (Si), and zinc sulfide (ZnS). Recent advancements in “multispectral” ZnS, involving hot isostatic pressing, have resulted in “water clear” substrates that can be used from 0.4 microns all the way to 20 microns. However, ZnS is mechanically inferior to the other materials because its relative softness renders it vulnerable to damage by rain and solid particle erosion. It is a useful substrate material where the combination of subsonic airspeed and the need for multispectral thermal imaging occur together.
Polymer substrates are getting more attention because of the potential for low weight and high strength-to-weight ratio. There is a wide variety of polymer resins available, though many of them are poorly suited to vacuum deposition processes because of plasticizers, mold release agents, and mold flow agents added to the resin formulation. Care must be taken by the coating designer to identify “metallizing grade” resins. Many high-performance plastics such as polycarbonate (PC), polyetheretherketone (PEEK), and polyimide (PI) are compatible with thin-film deposition processes. Polymer substrates can pose additional challenges because of their high thermal expansion coefficient compared to the dielectric materials used in coatings.
Coating Composition and Properties
Specialized optical interference coatings provide enhanced transmittance or reflectance in desired “in-band” spectral regions. This usually occurs at the expense of transmittance or reflectance in “out-of-band” regions where no performance requirement exists.
Most durable thin film coatings today are made using sputter deposition, a plasma-based physical vapor deposition technique. Sputtering results in metal oxide coatings that have excellent adhesion and durability. Thin-film interference coatings are made using alternating layers of low- and high-refractive-index materials. Silicon dioxide (SiO2) is the most common and most practical low-index material used in sputtered coatings, although higher-index aluminum oxide (Al2O3) can be used in some applications. Various high-index materials are used in sputter deposition including zirconium dioxide (ZrO2), titanium dioxide (TiO2), niobium pentoxide (Nb2O5), and tantalum pentoxide (Ta2O5).
Another physical vapor deposition (PVD) technique is evaporation, which can be done using electron-beam or resistance heating. Evaporated material structures are often softer and have less adhesion than sputtered coatings; however, the use of evaporation gives the coating producer access to materials that would be difficult or uneconomical to produce by sputtering. Ultimately, the system designer and coating manufacturer have to converge on a set of requirements that includes optical and durability performance, and this drives material selection.