Optical film is one or more layers of film coated on the surface of optical parts to change the surface characteristics of optical parts. It can be a metal film, a dielectric film or a combination of these two types of films. Optical films are an indispensable part of various advanced optoelectronic technologies. They can not only improve system performance, but are also a necessary means to meet design goals. The application fields of optical films involve all aspects of optical systems, including laser systems, optical communications, Light display, light storage, etc. The main optical thin film devices include reflective films, anti-reflection films, polarizing films, interference filters, beam splitters, etc. They have been widely used in the national economy and national defense construction, and have gained increasing attention from scientific and technological workers.
1. Definition of optical film
The application of optical films began in the 1930s. Optical films have been widely used in the fields of optics and optoelectronics to manufacture various optical instruments. Preparation conditions require high and precise conditions.
The definition of optical film is: it involves the thin and uniform dielectric film layer attached to the surface of the optical device during the propagation path of light, and its characteristics such as reflection, transmission (refraction) and polarization when passing through the layered dielectric film layer. Achieve the special forms of light we want, such as total transmission of light within a certain or multiple wavelength ranges, total reflection of light, or polarization separation.
2. Thin film interference principle
1. Wave nature of light
In the 1860s, the American physicist Maxwell developed the electromagnetic theory and pointed out that light is an electromagnetic wave, which brought the wave theory to a quite perfect stage.
It can be seen from the wave-particle duality of light that light is the same as radio waves, X-rays,? Rays are all electromagnetic waves, but their frequencies are different. The relationship between the wavelength λ, frequency u and propagation velocity V of electromagnetic waves is:
V=λu
Since electromagnetic waves of various frequencies propagate at the same speed in vacuum, electromagnetic waves with different frequencies have different wavelengths. High frequency wavelengths are short, and low frequency wavelengths are long. For the sake of comparison, it can be divided into radio waves, infrared rays, visible light, ultraviolet rays, X-rays and? The size of the wavelength (or frequency) of rays, etc., arrange them into a spectrum. This spectrum is called the electromagnetic spectrum.
2. Thin film interference
The film can be a transparent solid, a liquid, or a thin layer of gas sandwiched between two pieces of glass. The incident light is reflected by the upper surface of the film to obtain the first beam of light. The refracted light is reflected by the lower surface of the film and refracted by the upper surface to obtain the second beam of light. These two beams of light are separated by the same incident vibration on the same side of the film. , is coherent light and belongs to fractional amplitude interference. If the light source is an extended light source (area light source), interference can only be observed in a specific overlapping area of two coherent beams, so it is localized interference. For a planar film with two surfaces parallel to each other, the interference fringes are localized at infinity, usually observed in its image focal plane with the help of a converging lens; for a wedge-shaped film, the interference fringes are localized near the film.
3. Classification of optical film characteristics
The main optical thin film devices include reflective films, anti-reflective films, polarizing films, interference filters, beam splitters, etc. They are widely used in the national economy and national defense construction, and have received increasing attention from scientific and technical workers. For example, the use of anti-reflection coatings can reduce the luminous flux loss of complex optical lenses by ten times; the use of mirrors with high reflective film ratios can double the output power of lasers; the use of optical films can improve the efficiency and efficiency of silicon cells. stability.