Principles of Optics: Optical Thin Film Coatings for Reducing Surface Reflection
Do you all know about anti reflective coatings? Let's learn about the relevant knowledge of optical principles today.
Definition: Optical thin film coating used to reduce surface reflection.
Anti reflective coating is a thin dielectric coating applied on an optical surface to reduce the reflectivity of a certain wavelength region of light on the surface. In most cases, the working principle used is that the reflected waves at different optical interfaces undergo decoherence interference and cancel each other out.
Single layer anti reflective coating
The simplest case is under normal incidence, where the designed anti reflective film coating uses a material with a refractive index close to the geometric mean of the refractive indices of two adjacent media, and an optical thickness of one quarter wavelength. At this point, the reflected waves at the two interfaces are of equal magnitude, and after interference and cancellation, they cancel each other out.
This method has two limiting factors:
It is not always possible to find materials with suitable refractive index, especially when the refractive index of the bulk medium is low.
Single layer coatings can only work in very limited bandwidth situations.
multilayered coatings
If suitable single-layer coating materials cannot be found, or if anti reflection properties are required over a large wavelength range (where light in different wavelength ranges needs to have them simultaneously or at different incident angles), more complex designs are needed, usually using numerical calculation methods. When designing multi-layer dielectric coatings, a balance needs to be made between low residual reflectivity and large bandwidth. V-coating only exhibits good properties within a very narrow bandwidth (on the order of 10nm), while broadband coating can have moderate performance over a wide wavelength range.
Figure 1: Reflectance curves of anti reflective coatings optimized numerically on BK7 glass substrates for 1064nm and 532nm light. Two layers of coatings were used, namely TiO2 and SiO2.
In addition to the above properties, it is also necessary to consider the tolerance of the growth process: many complex coating designs can only achieve good performance through very precise machining processes. Therefore, the tolerance of the growth process is an important factor in practical design.
Design method
Simple types of anti reflective coatings with fewer layers are usually designed using analytical methods. In more complex designs, numerical optimization algorithms are required, which are similar to the methods used in dielectric reflectors. The design results are often difficult to understand, as they come from the complex interference effects of reflected light at different interfaces, resulting in anti reflection properties.
Gradient refractive index coating
The material composition of gradient refractive index coatings gradually changes, so there are many possible properties for this type of coating. The simplest case is that the refractive index of two optical materials changes gently within a few wavelength ranges, which can suppress reflection over a wide range of wavelengths and incident angles. However, it is actually difficult to achieve because optical surfaces come into contact with air, and the refractive index of solid materials differs greatly from that of air.
One solution is to use nano optical methods to create sub wavelength pyramid shaped structures or similar structures (moth eye structures), whose refractive index slowly changes to 1 by reducing the amount of solid material slowly. However, other solutions can also be used without using nano optics, especially by coating gradient refractive index layers on multi-layer coatings. This can achieve anti reflection properties over a wide bandwidth without using materials with very low refractive indices over a large angle range.
Strong absorption coating
A special type of anti reflective coating is a coating that contains very thin strongly absorbing materials. Its thickness may only be a few tens of nanometers, which is much smaller than the thickness of typical anti reflective coatings, because the imaginary part of the propagation constant of this medium is very large and can produce significant phase changes.
The incident light is strongly absorbed by this structure rather than transmitted. This anti reflective structure is also known as a photon metamaterial due to its subwavelength structure, but understanding this property can be described by simple interference phenomena.
application
Anti reflective coatings are commonly used on optical components to reduce optical losses and some harmful effects caused by reflected light beams. The proportion of residual reflection at a given wavelength and incident angle is on the order of 0.2%, or can be optimized to be smaller (within a limited bandwidth).
When applied in the field of vision, the suppression of reflections that can be achieved is smaller, as the coating needs to operate over a wide range of wavelengths and incident angles. Anti reflective coatings are commonly used on laser crystals and nonlinear crystals. At this point, the non-uniform thermal expansion from the crystal can lead to additional issues.
In most cases, anti reflective coatings are applied to optical interfaces with an area of several square millimeters. However, this coating can also be applied to the fiber end face, sometimes even on the protective sleeve and assembly connector.
There are many technical difficulties at this time, such as leakage of polymer protective sleeves into vacuum containers, and some fiber optic ports being treated as a whole, but special sputtering methods have also been developed to eliminate these problems. The obtained coating performance can be comparable to that of ordinary surface coatings, at least comparable to simple coating designs with a small number of layers.
damage threshold
In addition to reflective properties, the optical damage threshold of anti reflective coatings also needs to be considered, for example, when used in Q-switched lasers. Anti reflective coatings can have higher or lower damage thresholds than substrates depending on the materials used.
Even with a given coating material, the damage threshold of the coating varies depending on the preparation techniques used. The ion beam sputtering method can achieve a higher damage threshold.
Editor: Sichuan Jinzhongde Science and Technology Research Institute
