Common Functions of Optical Film
Optical Films are used in various applications of optical systems, including laser systems, optical communications, optical displays, optical storage, etc. The main optical Film devices include reflective films, anti-reflective films, polarizing films, interference filters and beamsplitters.
What is Optical Coating?
Optical Coating, is the process of coating one or more Films on the surface of polished optical parts in order to change the beam transmission characteristics of light waves, including the functions of transmission, reflection, absorption, scattering, polarization and phase change of light, or to achieve other specific purposes by means of optical coatings.
How to choose the right optical film?
1, need to clarify its applicable spectral range, such as the need for coating in the ultraviolet band, or in the visible or infrared band, and so on.
2ã€Define the size of the angle of incidence of the film layer to meet the specific use of this optical element in the optical system.
3ã€Define the size of the film layer characteristic value of concern after coating, for example, whether the reflectivity needs to be lower than 0.5%, or the reflectivity should be higher than 96% and so on.
4ã€According to the different scenarios of application, the optical film has different functions, to choose the correct film layer.
Common Functions of Optical Film
A. Reflection (HR Coating)
Reflective films are film systems used to increase the reflectivity of optical surfaces. Optical surfaces coated with reflective films can be used as mirrors. These reflective films are commonly used in laser systems, optical mirrors, and telescope mirrors to achieve high reflectivity over a wide spectrum of wavelengths.
Under different reflectivity index, the corresponding coating film system is different, the common reflective film is roughly two kinds of film system choice, one is metal film, such as aluminum film, silver film, gold film, etc. The thickness of the metal film is usually thinner, the surface shape of the surface is well maintained, and the reflectivity can reach up to the level of 97%. Common metal has a large extinction coefficient. When the light beam from the air incident on the metal surface, into the metal within the light amplitude of the rapid attenuation, so that the light energy into the metal inside the corresponding reduction in the reflected light energy increases. The larger the extinction coefficient, the more rapidly the light amplitude decays, the less light energy enters the metal, and the higher the reflectivity. In the ultraviolet region commonly used metal thin material is aluminum, commonly used in the visible region of aluminum and silver, commonly used in the infrared region of gold, silver and copper, in addition, chromium and platinum are often used as a number of special film film material.
Another is the plating of dielectric film, dielectric film is the use of different dielectric materials superimposed on each other to form a multilayer film, such as magnesium fluoride (MgF2) and tantalum pentoxide (Ta2O5) with the formation of the dielectric reflective film, you can achieve a fairly high (for example, 99.99%) level of reflectivity. However, the thickness of the dielectric film is usually on the thick side, so it is not easy to maintain a highly accurate surface shape. As an example, the following figure gives the reflectivity curves of gold, silver, aluminum and electrolyte films coated on the surface of BK7 material in the visible light band:
In the above figure, the beam is incident at 0 degree, the red line represents the gold film, the green line represents the silver film, the blue line represents the aluminum film, and the black line represents the dielectric film, in which the dielectric film is made up of 10 layers of MgF2 and 11 layers of Ta2O5, the total thickness of MgF2 is 66 nm and the total thickness of Ta2O5 is 99 nm, and the reflectance of the dielectric film that they constitute is the highest.
Aluminized high reflector
B. Anti-Reflection Reduction (AR Coating)
Reduced reflection film is also known as transmittance enhancement film, the role of the reduced reflection film is in a certain spectral range, reduce the reflectivity of the optical surface, thereby increasing the transmittance of the light beam in the components of the indicator, but also to reduce the impact of stray light of the system. Reflection-reducing films are usually made of one or more layers of different electrolyte materials superimposed on each other. Here again, magnesium fluoride and tantalum pentoxide can be used. In the figure below, the surface reflectance curves of the BK7 material in the visible wavelength band are given for the uncoated, coated single-layer dielectric film and the multilayer dielectric film:
In the above figure, the beam is incident at 0 degree, the red line represents the polished surface of BK7 without coating, and the reflectivity is about 4.2%, the green line is the reflectivity curve of a single-layer MgF2 film coated on the substrate, and the thickness of the film layer is about 99nm, which can be seen that the reflectivity has been significantly reduced. The blue line is a multilayer dielectric film with 3 layers of MgF2 and 3 layers of Ta2O5 coated on the substrate, and at this time the surface reflectivity is already below 0.5%. Coating materials are not limited to MgF2 and Ta2O5, in practice, you can choose a lot of coating materials to use, mainly fluoride, oxides and other compounds, commonly used fluoride also includes LaF3, YF3, YbF3, etc., commonly used oxides also include SiO2, TiO2, ZrO2, HfO2, etc., as well as ZnS, ZnSe and other compounds.
Sapphire chip anti-reflection
C.spectroscopy (Beam Splitting)
Through the coating, the light beam incident on the coated surface can be divided into two beams of light, one beam of light transmitted through, the other beam of light is reflected, and the ratio of the light intensity of the two beams of light can be defined in the design of the film system. Commonly, there are 1:1 beamsplitting films, i.e., half of the light intensity is transmitted and half of the light intensity is reflected. In addition, there are 1:2 ratios or other ratios of beamsplitting films in practical applications.
Beam splitter, dielectric film, short wave pass
D. Attenuating
Through the coating of flat products, can make the intensity of the light beam through it weaker, but the disappearance of the beam is not reflected back by the flat sheet, but was absorbed, so as to realize the function of light beam attenuation. We usually wear sunglasses, it can realize the absorption of ultraviolet rays and light intensity attenuation, play the purpose of eye protection. In optical instruments, there is a constant attenuation rate of the attenuation piece, that is, the product of each position can be achieved on the incident beam to the same degree of attenuation, there is also a gradual attenuation rate of the attenuation piece, when the gradual attenuation piece of the product around the center of the rotation of a week, the light intensity of the transmittance of the rate can be from the strong to the weak continuous change.
E. Light filtering (Filtering)
Used for the filtering function of the component is usually called filter, it can make a specific spectral band of light through the light will not be interested in the spectral band of the beam filtered out, filters in the optical instrument is very widely used, in accordance with the classification of spectral characteristics, can be divided into band pass filter, short wavelength pass filter, long wavelength pass filters and other types of filters. In the photography industry, filters are often used as a color filter, so as to obtain a specific artistic effect of the photo.
F. Polarizing Control
As we know, linearly polarized light is divided into P light and S light, the two vibration directions are perpendicular to each other, the propagation is also independent of each other. Through the coating, the polarization state of the incident beam can be controlled, common applications such as polarizer, which can turn natural light into polarized light, thus widely used in 3D movie glasses, LCD display occasions. The polarizing beam splitter prism (PBS), commonly used in optical instruments, can be used to split the incident beam into P light and S light, which are 90 degrees from each other, by coating the beveled surface of a glued right-angle prism, which plays the role of polarization, and the schematic diagram of the PBS is shown below:
G. Phase Change
As we mentioned earlier, optical film is usually a layer or a superposition of layers of media, when the light beam is reflected or refracted between the interface layers of different media, the phase of the different polarization states of light will change, therefore, according to this feature, optical films can be used to achieve the phase regulation of the incident light in the P light and S light, common applications are phase matching, phase delay and so on.
H. Functions of Non-Optical Properties
There is also a class of optical films that do not have optical features, but are needed in practice. For example, when the optical surface needs to be waterproof, it needs to be plated with a hydrophobic film, the film layer can make the water droplets on the lens is not easy to attach, just like the effect of water droplets on the lotus leaf, was efficiently drained away. There are also some occasions, the need for optical surfaces with a conductive function, this time, it is necessary to plating ITO (Indium Tin Oxide) film, this kind of film in the visible wavelength band has a good transmittance, at the same time with excellent conductivity, so as to provide transparent conductive function for flat panel displays, touch screens, solar cells and other electronic products and equipment.
To summarize, optical Film is an indispensable part of various advanced optoelectronic technologies, which not only improves system performance, but also is a necessary means to meet design goals.
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