What is Optical Coating?
Optical coating refers to the process of coating one (or multiple layers) of metal (or dielectric) films on the surface of optical parts. The purpose of coating the surface of optical parts is to reduce or increase light reflection, beam splitting, color separation, filtering, polarization and other requirements. Commonly used coating methods include vacuum coating (a kind of physical coating) and chemical coating.
product description
Coating is to coat a transparent electrolyte film or a metal film on the surface of the material by physical or chemical methods, the purpose is to change the reflection and transmission characteristics of the material surface.
In the visible and infrared wavelength range, the reflectivity of most metals can reach 78% to 98%, but not higher than 98%. Whether for CO2 lasers, copper, molybdenum, silicon, germanium, etc. are used to make mirrors, germanium, gallium arsenide, zinc selenide are used as output window and transmission optical element materials, or ordinary optical glass is used for YAG lasers as mirrors , output mirror and transmission optical element materials, can not meet the requirements of more than 99% of total reflection mirror. The output mirror has different transmittance requirements in different applications, so optical coating methods must be used.
For the mid-infrared band of CO2 laser lamps, the commonly used coating materials are yttrium fluoride, praseodymium fluoride, germanium, etc.; for the near-infrared or visible light band of YAG laser lamps, the commonly used coating materials are zinc sulfide, magnesium fluoride, titanium dioxide, oxide Zirconium, etc. In addition to high-reflection film and anti-reflection film, it can also be coated with special films that increase reflection at a certain wavelength and increase transmission at another wavelength, such as beam splitting films in laser frequency doubling technology.
Fundamentals of Optical Coatings
Interference of light is widely used in thin film optics. The common method of optical thin film technology is to coat a thin film on a glass substrate by means of vacuum sputtering, which is generally used to control the reflectivity and transmittance of the substrate to the incident beam to meet different needs. In order to eliminate the reflection loss on the surface of optical parts and improve the imaging quality, one or more layers of transparent dielectric films are coated, which are called anti-reflection films or anti-reflection films. With the development of laser technology, there are different requirements for the reflectivity and transmittance of the coating layer, which promotes the development of multi-layer high-reflection coatings and broadband anti-reflection coatings. For various application needs, high-reflection films are used to manufacture polarized reflective films, color beam splitting films, luminescent films and interference filters.
After the surface of the optical parts is coated, the light is reflected and transmitted multiple times on the film layer to form multi-beam interference, and the refractive index and thickness of the film layer can be controlled to obtain different intensity distributions. This is the basic principle of interference coating.
Coating process
Optical thin films are realized in a high-vacuum coating chamber. Conventional coating processes require elevated substrate temperatures (typically around 300°C); more advanced techniques, such as ion-assisted deposition (IAD), can be performed at room temperature. The IAD process not only produces films with better physical properties than conventional coating processes, but can also be applied to substrates made of plastic. The main vacuum pumping system consists of two cryopumps. The control modules for e-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 either side of the base plate, surrounded by an annular cover and covered by baffles. The ion source is in the middle, and the light control window is in front of the ion source. The vacuum chamber contains a planetary system with 6 circular grippers. Fixtures are used to place coated optics. Using a planetary system is the preferred method to ensure that the evaporated material is evenly distributed in the gripper area. The gripper rotates about a common axis while simultaneously rotating about its own axis. The light control and crystal control are located in the middle of the planetary drive mechanism, and the drive shaft shields the crystal control. A large opening on the back leads to an additional high vacuum pump. The substrate heating system consists of 4 quartz lamps, two on each side of the vacuum chamber.
The traditional method of thin film deposition has been thermal evaporation, either with resistively heated evaporation sources or with electron beam evaporation sources. The film properties are mainly determined by the energy of the deposited atoms, which are only about 0.1 eV in conventional evaporation. IAD deposition results in direct deposition of ionized vapor and adds activation energy to the growing film, typically on the order of 50 eV. The ion source improves the film properties of conventional electron beam evaporation by directing the beam from the ion gun to the substrate surface and the growing film.
The optical properties of thin films, such as refractive index, absorption and laser damage threshold, mainly depend on the microstructure of the film layer. Film material, residual gas pressure, and substrate temperature can all affect the microstructure of the film. If the evaporatively deposited atoms have low mobility on the substrate surface, the film will contain micropores. When the film is exposed to moist air, these pores are gradually filled with water vapor.
Packing 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 pores. For optical films, the packing density is usually 0.75 to 1.0, mostly 0.85 to 0.95, and rarely reaches 1.0. Packing densities less than 1 cause the refractive index of the evaporated material to be lower than that of its bulk.
During deposition, the thickness of each layer is monitored by an optical or quartz crystal. Both techniques have their own advantages and disadvantages, which are not discussed here. What they have in common is that they are all used in a vacuum when the material is evaporated, so the index of refraction is the index of refraction of the evaporated material in vacuum, not the index of refraction of the material exposed to moist air. The moisture absorbed by the film replaces the pores and voids, causing the refractive index of the film to increase. Since the physical thickness of the film remains constant, this increase in refractive index is accompanied by a corresponding increase in optical thickness, which in turn causes a shift in the spectral properties of the film toward longer wavelengths. To reduce this spectral shift caused by the volume and number of micropores within the film, energetic ions are employed to transfer their momentum to the evaporating material atoms, thereby greatly increasing the mobility of the material atoms during condensation at the substrate surface .
Refractive index of coating
According to the basic theory of electromagnetism, the transmission and reflection of different media are mentioned.
If it is vertically incident from medium n1 to n2
Reflectivity=[ (n2－n1) / (n1+n2) ]^2
Penetration rate=4n1n2 / (n1+n2)^2
Example explanation:
If the refractive index of air is 1.0, the refractive index of the coating is nc (for example: 1.5), the refractive index of glass is n (for example: 1.8) (1) The air directly enters the glass
Penetration rate = 4×1.0×1.8 / ( 1+1.8 )2=91.84%
(2) The penetration rate of the glass entering the coating from the air = [ 4×1.0×1.5 / ( 1+1.5 )2] × [ 4×1.5×1.8 / ( 1.5+1.8 )2]=95.2%
Optical Coater
It can be seen that coated glass will increase the transmittance. In addition, from this formula, we can calculate that light penetrates both sides of the lens and find that even a perfect lens (refractive index 1.8) has a transmittance of about 85%. If a layer of coating (refractive index 1.5) is added, the transmittance can reach 91%. The importance of visible optical coatings.
coating thickness
We already know that transmittance is related to the refractive index of the coating, but not its thickness. However, if we can work on the thickness of the coating, we will find that the reflected light A and the reflected light B differ by an optical path difference of nc×2D. if
nc×2D=(N+ 1/2)λ where N= 0,1,2,3,4,5…..λ is the wavelength of light in air
This will cause the reflected light of that particular wavelength to have a destructive effect, so the color of the reflected light will change.
Optical Coater
For example, if the thickness of the coating causes the cancellation of green light, the reflected light will appear red. Many telescopes on the market that look like red lenses are made using this principle. Nevertheless, the transmitted light has no reddishness.
In many complex optical systems, the suppression of reflected light is a very important task. Therefore, between a group of lenses, different coating thicknesses will be used to eliminate reflected light of different frequencies. Therefore, the more advanced the optical system, the more colors of reflected light will be found.
Optical Coating Materials
Common optical coating materials are as follows:
1. Magnesium fluoride
Material characteristics: colorless tetragonal crystal powder, high purity, using it to prepare optical coating can improve transmittance without collapse point.
2. Silica
Material characteristics: colorless transparent crystal, high melting point, high hardness, good chemical stability. The purity is high, and it is used to prepare high-quality SiO2 coating, and the evaporation state is good, and there is no collapse point. According to the requirements of use, it is divided into ultraviolet, infrared and visible light.
3. Zirconia
Material characteristics White heavy crystalline state, with high refractive index and high temperature resistance, stable chemical properties, high purity, use it to prepare high-quality zirconia coatings without collapse points.
