Anti-reflection Coating, or Anti-reflective Coating, often abbreviated as AR Coating is a kind of optical coating designed to lower the reflectance of optical elements and hence increase the optical transmission. The function of anti-reflection coatings is to enhance the throughput and reduce backward reflections/ghost images due to Fresnel reflection at the interface of the optical substrates. AR coatings are one of the most common types of optical coatings, serving indispensable roles on a wide range of transmissive optical components like optical lenses, optical windows, laser lenses, and laser optics, where high transmission to a specific wavelength or wavelength ranges and low loss are required, or to prevent the harmful effect of reflected beams. AR coatings are also applied on the optical surfaces of laser crystals and nonlinear crystals. With AR coatings, the loss inefficiencies due to reflection when light propagates in optical components can be decreased.
Note that AR coatings are dependent on the spectral range and AOI. Therefore AR coatings are specified as offering high transmission at certain wavelengths/wavebands, and angle of incidence.
AR coatings are often adhered at the front or the rear side of optical components where the light incident on and exits. AR coatings significantly raise the overall transmission efficiencies of optics, for instance, a glass optical component loses 4% transmission of light when light enters the glass due to Fresnel reflection, and loses a further 4% when light exits. When a system contains multiple such uncoated glass elements, the total loss accumulated will be massive. AR coatings are also useful for laser cavities to prevent damage from back reflections.
Optical anti-reflection coatings in the market are almost all dielectric AR coatings, designed and manufactured using thin films of dielectric materials such as oxides. The basic design principle of dielectric AR coating is to utilize the discretely layered structure of materials with different refractive indices that result in the desired interference effect. The thicknesses are carefully calculated to produce destructive interference in the beams reflected from the interfaces, and constructive interference in the corresponding transmitted beams.
Anti-reflective AR coatings could consist of either one layer or multiple layers of thin films.
Single-layer dielectric AR coatings have one layer of dielectric thin film. A typical example of single-layer ar coating is single-layer MgF2 coating. Single layer MgF2 coatings exhibit low optical loss, high laser damage resistance, and a wide transmission range from 0.12μm to 8μm. The MgF2 is very compatible with substrates made of glass, sapphire, and ZnS. However, the limitation of MgF2 single layer coating is that its performance tends to change as the substrate glass changes.
Multiple layer dielectric AR coatings are constructed by complicated layering of thin films so that reduced spectral or angular dependence of the AR coating can be obtained. Multi-layer AR coatings are designed using intricate numerical methods and thin-film computer software to optimize optical transmission across a wider range of wavelengths or incident angles. When engineering optical coatings, a common challenge is the balance between maximizing residual reflectance (the light that is reflected instead of transmitted) versus maximizing the bandwidth (the range of wavelengths that the coating can be effective over). Laser line V coatings offer the highest transmission over a small wavelength range around a center wavelength, giving the reflectance curve a characteristic "V" shape, in contrast with broadband ar (BBAR) coatings that offer wider wavelength range coverage but with inferior reflectivities.
Hangzhou Shalom EO specializes in providing custom optical coatings tailored to suit your demand, this page highlights our AR coating services. We provide various types of Anti-reflection coatings, including:
1. Broadband AR (BBAR) coatings are designed for increasing transmission rates and efficiencies over a wide wavelength range. BBAR coatings are more versatile than V coatings as BBAR coatings have wider transmission spectral ranges, although BBAR coatings in general do not provide minimized reflectance as V coatings. BBAR coatings are applied to broadband laser sources or lasers with multiple-harmonic generation.
2. Laser line V coatings with minimized reflections and optimized transmission centered around a central wavelength. The laser line coatings are excellent for laser windows, lenses, and laser crystals where single wavelength lasers are applied, as well as narrow full width-half max (FWHM) light sources. V coatings can be composed of one, two, or multiple layers of thin films. Most V coatings contain two layers, the simplest case would be a single-layer V coating, but adding layers helps to correct the transmission bandwidth of the V coating, and compensates when coating material with a suitable refractive index can’t be found. Multiple thin films can also increase the range of AOI.
3. Dual Wavelength or Multiple Wavelength AR coatings. Dual wavelength AR coating that offers high transparencies to two wavelengths or triple, multiple wavelength AR coatings that provide utmost transmission simultaneously for multiple wavelengths are available.
4. Single-layer MgF2 coatings as a simple and low-cost alternative to muti-layer dielectric ar coatings. The single layer MgF2 ar coatings exhibit hard and durable qualities and are suitable for commercial optical components. Optics with MgF2 coatings work well for UV applications.
Coatings for different wavelength ranges are available, including UV anti-reflection coatings, visible anti-reflection coatings, and infrared anti-reflection coatings. Our ion-assisted deposition (IAD) e-beam coating technique enhances a number of critical factors for optical coatings, including densities, hardness, and adhesion with greater control of surface texture and microstructures. Our AR coating services enable the seamless adhesion onto miscellaneous substrate materials like optical glass and crystals, and the anti-reflective coatings can be applied to optical lenses, windows, prisms, laser windows, lenses, laser crystals, and NLO crystals of diverse shapes like flat, wedge, plano-convex/concave, double-convex/concave, meniscus, aspheres, etc.
Application Notes:
How to calculate the thickness and refractive index of AR coatings:
Basically, the design principle of AR coating is using destructive interference to cancel the Fresnel reflection. When light hits the surface of an AR coating, part of it is reflected from the top of the thin film, and part of it penetrates the film and reflects from the film-substrate interface. For destructive interference to occur, the relative phase difference between these two reflected beams must be half a wavelength, λ/2.
To achieve the path difference of λ/2, the optical thickness of the coating (which is the physical thickness multiplied by the refractive index of the film) must be an odd integer multiple of λ/4 (i.e., (2m+1)λ/4, where m is an integer), where λ is the design wavelength or a wavelength chosen to minimize reflection. When selecting an AR coating, one of the most important considerations is the spectral range. If you use the coating outside this design wavelength range, the performance can be compromised.
The refractive index of the optical coating material (nc) can be calculated, as an approximation value, using the formula below:
nc=(n1*n2)1/2
Where n1 is the refractive index of the incident medium and n2 is the refractive index of the substrate medium.
