What Is Optical Coating

- Oct 11, 2018-

Optical coating-IKS PVD


 Optical coating is a process of coating on the optical parts surface of layer or multilayer metal (or medium) thin film. The purpose of coating optical parts is to reduce or increase light reflection, beam splitting, color separation, filter and polarization. The commonly used coating methods include vacuum coating (a kind of physical coating) and chemical coating




The coating is to use physical or chemical method in material surface plating on a transparent layer of electrolyte membrane, or coated with a layer of metal film, the purpose is to change material surface reflection and transmission characteristics. Within the scope of the visible and infrared bands, most of the reflectivity of the metal can reach 78% ~ 98%, but not higher than 98%. Both for CO2 laser, the use of copper, molybdenum, silicon and germanium, etc to make reflectors, germanium and gallium arsenide, zinc selenide and transmission optical element as the output window material, or for YAG laser adopt ordinary optical glass as a mirror, output mirror and transmission optical element material, can't meet the requirements of more than 99% of the total reflection mirror. Different applications require different transmittance of the output mirror, so the optical coating method must be used. For CO2 laser in the infrared wave band, the commonly used coating material with yttrium fluoride, fluoride, praseodymium, germanium, etc; For the near infrared band or visible band of YAG laser lamp, common coating materials include zinc sulfide, magnesium fluoride, titanium dioxide, zirconia, etc. In addition to high reflectance and translucency films, special films can be plated to reflect one wavelength and transmit to another wavelength, such as the spectroscopic film in laser frequency doubling technology.


Basic principle of optical coating


Optical interference is widely used in thin film optics. The common method of optical thin film technology is to apply thin film on glass substrate by means of vacuum sputtering, which is used to control the reflectance and transmittance of the base plate to the incident beam to meet different needs. In order to eliminate the reflection loss on the optical part surface and improve the imaging quality, a layer or multi-layer transparent dielectric film is coated. With the development of laser technology, there are different requirements for the reflectivity and transmittance of the film layer, which promotes the development of multi-layer high reflection film and broadband permeability film. For various applications, we use high reflection film to produce polarizing reflective film, color spectrophotometer, cold film and interference filter etc. Optical parts after surface coating, on the membrane layers of multiple reflection and transmission of light, the formation of multiple beam interference and control film refractive index and thickness of different intensity distribution can be obtained, this is the basic principle of interference in the coating.


Coating process


Optical thin films are realized in high vacuum coating cavities. Conventional coating process requires higher substrate temperature (usually at about 300 ); More advanced techniques, such as IAD, can be performed at room temperature. IAD process not only produces films with better physical properties than conventional coating processes, but can also be applied to plastic substrates. Vacuum the main system is composed of two cryogenic pumps. The control modules of electron beam evaporation, IAD deposition, light control, heater control, vacuum control and automatic process control are all on the front panel of the coater.

The two electron gun sources are located on both sides of the substrate, surrounded by a circular hood and covered by the baffle. The ion source is in the middle, and the light control window is in front of the ion source. At the top of the vacuum chamber, the vacuum chamber has a planetary system with six circular fixtures. The fixture is used to place the coated optical element. The use of planetary systems is the preferred method to ensure the uniform distribution of evaporated material in the fixture area. The clamp rotates on a common axis and rotates on its own axis. The optical control and crystal control are in the middle of the planetary drive mechanism. The large opening on the back leads to the attached high vacuum pump. The base heating system consists of four quartz lamps, two on each side of the vacuum chamber.

The traditional method of thin film deposition has always been thermal evaporation or using resistance heating evaporation source or electron beam evaporation source. The properties of the films are mainly determined by the energy of the deposited atoms, and the energy of the atoms in traditional evaporation is only about 0.1ev. IAD deposition results in direct deposition of ionized steam and increases the activation energy for the growing film, usually at the order of 50eV. Ion sources improve the properties of conventional electron beam evaporation by pointing the beam from the ion gun to the substrate surface and the growing film. Thin film optical properties, such as refractive index, absorption and laser damage threshold, mainly dependent on the microstructure of membrane. The microstructure of the films may be affected by the residual air pressure and substrate temperature. If the evaporation deposited atoms have a low migration rate on the base surface, the film will contain micropores. As the film is exposed to moist air, these pores are gradually filled with moisture.

The filling density is defined as the ratio of the volume of the solid portion of the film to the total volume of the film (including voids and micropores). For optical thin films, the filling density is usually 0.75 ~ 1.0, most of which are 0.85 ~ 0.95 and rarely reach 1.0. The filling density less than l makes the refractive index of the evaporated material lower than that of its block. In the process of deposition, the thickness of each layer by optical or quartz crystal monitor. Each of these technologies has advantages and disadvantages, which are not discussed here. The common point is that when the materials are vaporized, they are used in a vacuum. Therefore, the refractive index is the refractive index of the vaporized materials in a vacuum, rather than the refractive index of materials exposed to moist air. The moisture absorbed by the film replaces micropores and interstices, resulting in increased refractive index of the film. As the physical thickness of the film remains unchanged, this increase in refractive index is accompanied by the corresponding increase in optical thickness, which in turn causes the film's spectral characteristics to drift towards the direction of long waves. In order to reduce the spectral drift caused by the volume and quantity of micropores in the membrane layer, high-energy ions were used to transfer their momentum to the atoms of the evaporating material, thus greatly increasing the migration rate of the atoms of the material during the condensation at the base surface.


The refractive index of the coating

According to the basic theory of electromagnetism, transmission and reflection of different media are mentioned. If n1 perpendicular incident by the media to n2 reflectivity = [(n2 - n1)/(n1 + n2) ^ 2 = 4 n1n2 penetration rate/(n1 + n2) ^ 2

Examples: if the refractive index of air is 1.0, the refractive index of a coating (for example: 1.5), nc glass refractive index n (for example: 1.8) (1) by air directly into the glass transmittance = 4 x 1.0 x 1.8 2 / (1 + 1.8) = 91.84% (2) by air into coating and then into the glass transmittance = [4 x 1.0 x 1.5 / (1 + 1.5) 2] x [4 * 1.5 * 1.8 (1.5 + 1.8) / 2] = 95.2%

Visible coated glass will increase light transmittance. In addition to this formula, we can calculate the light penetrates the both sides of the lens, found that even a piece of the beautiful lens refractive index (1.8), the penetrability of about 85%. With a coating (refractive index of 1.5), the transmittance can reach 91%. The importance of optical coating can be seen.


Coating thickness

We already know that the transmittance is related to the refractive index of the coating, but we don't know about its thickness. However, if we can work on the thickness of the coating, we will find the difference between the reflected light A and the reflected light B. If nc x 2 d = (N + 1/2) lambda where N = 0,1,2,3,4,5... Lambda for light wavelengths in the air can cause the reflected light of specific wavelengths have destructive effect, so the color of the reflected light to change. For example, if the thickness of the coating caused by cancellation of the green light, reflected light will appear red. Many telescopes on the market that look like red lenses are made using this principle. Even so, the transmitted light not slant red phenomenon. In many complex optical system, reflection suppression is a very important work. Therefore, different coating thickness is used to eliminate the reflected light of different frequency between a set of lenses. So the more advanced the optical system, the more colors will be found.


Optical coating materials

Common optical coating material has the following kinds:

 1, magnesium fluoride

Material characteristics: colorless square crystal system powder, high purity, with its preparation of optical coating can improve the transmittance, no point of collapse.

2, silica

Material characteristics: colorless, transparent crystal, high melting point, high hardness, good chemical stability. With high purity, high quality Si02 coating was prepared with it, with good evaporation state and no bursting point. According to the use requirements are divided into ultraviolet, infrared and visible light.

3, zirconium oxide

 Material characteristics White heavy and amorphous, high refractive index and high temperature resistance, chemical stability, high purity, with its preparation of zirconia coating with high quality, not the point of collapse.

 ZY-1913 Decorative Vacuum Coating Machine