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The Basics of Bandpass Filters

Bandpass filters are used to transmit the required wavelength range and cut off the unwanted wavelength range. Their design is essentially a thin film formed by vacuum deposition technology, which consists of two reflective stacks separated by even-numbered spacer layers. Each of these structures is called a cavity, and some filters may contain up to eight cavities.


All-dielectric bandpass filters


The all-dielectric type bandpass filter consists of two high-reflection mirrors, which are separated by a dielectric isolation layer. These mirrors are composed of alternating high refractive index and low refractive index materials, and the reflectivity of the stack sometimes exceeds 99.99%. By changing the thickness of the isolation layer or the reflective layer, the center wavelength and bandwidth of the bandpass filter can be changed. This type of filter shows a high transmission rate in the passband, but the range of out-of-band blocking is limited. To make up for this shortcoming, an additional barrier component is added, which can be fully dielectric or metal dielectric depending on the required barrier range. This additional blocking component will eliminate any unnecessary out-of-band radiation, but it will also reduce the overall throughput of the bandpass filter.


Metal-dielectric bandpass filters


The metal dielectric type is similar to the full dielectric type, except that it uses a metal spacer layer instead of the dielectric layer. Although this type of bandpass filter has excellent out-of-band blocking and high-passband transmission capabilities, it lacks the steep cut-off slopes of typical two-cavity and three-cavity bandpass filters. The metal dielectric type is mainly used for bandpass filters in ultraviolet light. However, when it is necessary to reject far infrared rays, a form of inductive transmission type can be used as an additional blocking component.

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