Optics are employed in virtually every area of military operations, from vision systems and target designators used by troops on the ground, through guidance systems utilized in both manned and unmanned aircraft, to reconnaissance and surveillance packages carried by satellites in Earth orbit. These optics are often subjected to large variations in ambient temperature and humidity, as well as contact with abrasive or corrosive materials (such as sand or salt spray). Thin film coatings, which are almost universally required on military optics, must be able to physically withstand these stressors, as well as deliver their design performance in an environment where “failure is not an option.” But, increasingly, achieving these ends must also be balanced with cost.
An antireflective or anti-reflection (AR) coating is a type of optical coating applied to the surface of lenses and other optical elements to reduce reflection. Due to Fresnel reflection, as light passes from air through an uncoated glass substrate approximately 4% of the light will be reflected at each interface. This results in a total transmission of only 92% of the incident light, which can be extremely detrimental in many applications. Excess reflected light reduces throughput and can lead to laser-induced damage in laser applications. Anti-reflection (AR) coatings are applied to optical surfaces to increase the throughput of a system and reduce hazards caused by reflections that travel backwards through the system and create ghost images.
In typical imaging systems, this improves efficiency since less light is lost due to reflection. AR coatings are especially important for systems containing multiple transmitting optical elements. In complex systems such as telescopes and microscopes, the reduction in reflections also improves the contrast of the image by eliminating stray light. This is especially important in planetary astronomy. In other applications, the primary benefit is the elimination of the reflection itself, such as a coating on eyeglass lenses that makes the eyes of the wearer more visible to others, or a coating to reduce the glint from a covert viewer’s binoculars or telescopic sight. Back reflections also destabilize laser systems by allowing unwanted light to enter the laser cavity. Many low-light systems incorporate AR coated optics to allow for efficient use of light.
Many coatings consist of transparent thin film structures with alternating layers of contrasting refractive index. Layer thicknesses are chosen to produce destructive interference in the beams reflected from the interfaces, and constructive interference in the corresponding transmitted beams. This makes the structure’s performance change with wavelength and incident angle, so that color effects often appear at oblique angles. A wavelength range must be specified when designing or ordering such coatings, but good performance can often be achieved for a relatively wide range of frequencies: usually a choice of IR, visible, or UV is offered.
Military Coating Requirements
Military applications often require high performance coatings that can withstand large environmental shifts, high laser power and exposure to contaminants. Virtually all optical components used in military applications, such as target designation, rangefinding and IR countermeasures, employ thin film coatings to somehow modify their transmission and reflection characteristics.
An optical thin film consists of one or more layers of coating material, with individual layer thicknesses typically ranging from a few nanometers to several microns. Achieving target performance requires tight control of deposition to produce the desired sequence, uniformity, material thicknesses and indices of refraction of these layers.
Higher coating density, or more precisely, lower porosity, also prevents water molecules from entering the film when it is exposed to high humidity. Moisture absorption changes layer refractive index, which shifts the coating performance curve to longer wavelengths. This so called “wet/dry shift” is generally not a problem in broadband coatings, such as antireflection (AR) and most high reflection coatings, but can have a serious impact on coatings intended for narrowband performance or those with a sharp band edge. Examples of these are bandpass filters, edge (short or long wave pass) filters and notch filters (which reflect a single laser wavelength and transmit everything else). These are all coatings widely employed in military systems, including target designators, multispectral imaging sensors, and countermeasures.
For most military applications, there are a few key parameters which are most critical to proper coating performance. The first of these is coating hardness. Hard coatings resist damage due to repeated cleaning or abrasion from particulates like sand. Here, there is a progression from evaporation, which produces the least dense and softest films, through to sputtering and LPCVD which both produce highly densified, hard coatings.
In the most general terms, coatings become more expensive to fabricate as the number of layers increases and/or the index and thickness tolerances on those layers become tighter. Obviously, the goal of the coating specifier is to ensure that the component meets its performance targets, but it’s important to make sure that specifications are framed in a way that doesn’t needlessly drive up cost.
Antireflection (AR) coating
Performance in an antireflection (AR) coating is typically specified by either the maximum allowable reflectance at a single wavelength or the average allowable reflectance over a particular wavelength range. For AR coatings intended for single wavelength, single angle of incidence use, very high performance can be obtained; less than 0.1% reflectance per surface at visible wavelengths on glass substrates is not at all uncommon. It becomes increasingly difficult to maintain high performance in an AR coating as either spectral bandwidth or angular range is increased.
Broadband anti-reflection (BBAR) coatings are designed to improve transmission over a much wider waveband. They are commonly used with broad-spectrum light sources and lasers with multiple-harmonic generation. BBAR coatings typically do not achieve very reflectivity values, but are more versatile because of their wider transmission band. In addition to being applied to transmissive optical components including lenses and windows, AR coatings are also used on laser crystals and nonlinear crystals to minimize reflections, as Fresnel reflections occur where air and the crystal meet.
The particular functionality required for many military applications often necessitates overcoming some of the most difficult performance challenges already identified. For example, rangefinders/target designators typically utilize multispectral operation, functioning simultaneously in the visible, at 1064 nm, at the “eyesafe” wavelength of 1.54 µm, as well as in the mid-IR (3 – 5 µm). These coatings are also frequently specified to function over large angular ranges, and to exhibit a high degree of polarization insensitivity
Producing multi-wavelength AR coatings that operate in both the visible/near infrared and the mid-infrared or thermal infrared can also be challenging because of the limited number of materials that simultaneously transmit in these regions. Specifically, many materials that
transmit in the visible don’t work above about 5 µm, which makes it more difficult, and therefore costly, to produce coatings that work in both these spectral ranges.
The drive to minimize system size and weight, especially in man portable and airborne systems, may motivate the optical designer to scale down component diameters. However, shrinking the diameter of a given power laser beam causes an increase in its power density. Therefore, laser damage threshold often becomes a concern. The need to incorporate numerous layers in order to achieve advanced functionality can also result in relatively thick films which may exhibit high mechanical stress. That can be a problem when the optical system designer specifies components with a relatively high aspect (diameter to thickness) ratio to minimize weight. High coating stress can actually warp these thin components out of their original shape, thus increasing the wavefront distortion of the component and overall system.
Finally, military systems may experience wide swings in temperature and humidity, and are sometimes exposed to salt spray, smoke or other airborne contaminants. Some coating types absorb water, which, together with changes in temperature, can shift coating performance.
Therefore, coating performance stability and mechanical durability (and the ability to be repeatedly cleaned) can be significant considerations.
US army develops moth inspired camouflage for military equipment
The US army has filed a patent for a new method of stamping an anti-reflective coating onto optical components. The recent tech development is inspired by masters of camouflage already found in the animal kingdom: moths. More specifically, moths’ anti-reflective eyespots, which allow the nocturnal creatures to evade predators by preventing light from bouncing off them. Moths are nocturnal and have evolved eyes with a periodic, surface-graded structure so that light bends instead of reflecting.
“The surface of the eye of a moth is covered by bumps that are each roughly 200 nm high and whose centers are spaced approximately 300 nm apart. Since the bumps are smaller than the wavelength of visible light, visible light sees the surface as having a continuous refractive index gradient between the air and the medium, which decreases reflection by effectively removing the air-lens interface. Thus, the bumps serve as an anti-reflective coating on the eye of the moth,” according to the Army’s patent application.
Vincent Schnee from the U.S. Army’s Night Vision and Electronic Sensors Directorate is the only inventor named in the patent filing made public on August 26, 2021. Instead of laser etching individual optical elements, which takes days and days, Schnee is thinking like a press operator at a newspaper. This anti-reflective coating would allow soldiers to remain completely hidden during nighttime operations without any stray light reflections giving away their position.
The document explains that the first step is to cover a silicon cylinder in the moth-eye pattern using two-photon lithography, a micro-3D printing technique using a tightly focused laser beam. The etched cylinder is then used as a rolling stamp, pressing the moth-eye pattern on the optical lens using epoxy with germanium nanoparticles as its ink.
“This allows perfect matching of indices of refraction between the optical element and the anti-reflective coating,” according to Schnee’s patent application. “Such a perfect match is not easily achieved with conventional layered dielectric anti-reflective coatings.”
Brian Metzger, a senior technology manager at TechLink, said there are many applications for the technology.
“In addition to improving the optical components used by soldiers, this tech could be used to increase the efficiency of solar panels, cut glare on flat-panel TVs, outdoor signage, or to increase the visual aesthetic of plexiglass around hockey arenas,” Metzger said.