Types of optical coatings for optical filters
Optical coating refers to the process of coating one or more layers of metal or dielectric thin films on the surface of optical components. The purpose of coating the surface of optical components is to reduce or increase the requirements for light separation, reflection, beam separation, filtering, cutoff, polarization, etc. Vacuum coating and chemical coating are commonly used.
The optical coating thin film design system includes optical thin films such as anti reflective films, reflective films, bandpass filters, cutoff filters, and depolarization films.
The residual reflection of anti reflection film and anti reflection film in narrowband and broadband is very low, with residual reflection in narrowband less than 0.1% and average residual reflection in broadband less than 0.3%. In various optical systems, it is used to reduce the reflectivity of optical surfaces with different refractive indices, and is commonly used in monochromatic or white light sources (such as laser wavelengths). The commonly used three or four layers can cover the visible light region, making it difficult to design and produce anti reflection films in a wide wavelength range.
Reflective films are selected to improve the reflectivity of optical devices based on their bandwidth, reflectivity, hardness, and cost. Metal mirrors are commonly used for broadband reflection, but do not require high reflectivity. All dielectric reflectors are commonly used in laser technology because the reflectivity of laser technology is very high.
Bandpass filters include combination bandpass filters, all dielectric bandpass filters, and induced transmission bandpass filters, further divided into single cavity, dual cavity, and multi cavity. The main indicators are center wavelength, half bandwidth, transmittance, wave coefficient, cutoff depth, and cutoff bandwidth. Metal bandpass filters have few layers, simple preparation, low cut-off bandwidth, but low peak transmittance (such as 30%). In situations where requirements are not high, cut-off errors are common. All dielectric bandpass filters have high peak transmission and a large cut-off depth, but a narrow cut-off bandwidth. The bandwidth is controlled by colored glass and cut-off filters, and metal dielectric bandpass filters are within their limits. Inductive transmission bandpass filters have high cutoff and transmittance, and have certain application value.
There are two types of cut-off filters: long wave pass and short wave pass. The main indicators include cut-off degree, cut-off depth, cut-off bandwidth, steepness, wavelength, passband transmittance, and ripple. It is mainly used to separate two bands, so it is also known as dichroic mirror, color matching, ultraviolet isolation, infrared isolation, etc.
There are two types of polarizing films: planar polarizing film and cubic prism polarizing film. The former is based on the difference in effective refractive index of oblique incident light, so the bandwidth is different and the polarization band is narrow, while the latter is based on all interfaces to satisfy the Brewster angle condition, with a large polarization bandwidth and an extinction ratio of up to 2000 or higher. The polarizing film does not require bonding and is used under the condition of substrate Brewster angle. It is commonly used in high-power laser systems. Due to its large polarization bandwidth and high polarization degree, cubic prism polarizing film requires internal adhesion, coating, and high stability within the film layer. Therefore, it has been widely used in fields such as liquid crystal display technology.
The depolarization film includes two types: metal and all dielectric. The so-called depolarization refers to the transmission or reflection of p and s components being the same in a certain wavelength band when the incident angle is tilted.