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 is composed of two reflection stacks and separated by even number of compartments. Each of these structures is called a cavity, and some filters may contain up to eight cavities. There are many different variants of bandpass filters, but in this discussion, we will only consider the all-dielectric and metal dielectric types.
The all-dielectric type consists of two high-reflective mirrors separated by a dielectric isolation layer. These mirrors are composed of alternating high-index and low-index materials, and the stacked reflectivity sometimes exceeds 99.99%. The central wavelength and bandwidth of the filter can be changed by changing the thickness of the isolation layer and/or the reflection layer. This type of filter shows high transmission rate in the passband, but the range of out-of-band blocking is limited. In order to make up for this deficiency, an additional blocking component is added, which can be fully dielectric or metal dielectric depending on the required blocking range. This additional blocking component will eliminate any unnecessary out-of-band radiation, but will also reduce the overall throughput of the filter.
The metal dielectric type is similar to the full dielectric type, but the difference is that it uses the metal interlayer to replace the dielectric layer. Although this type of filter has excellent out-of-band blocking and high-pass transmission capability, it lacks the steep cutoff slope of typical two-cavity and three-cavity filters. Metal dielectric type is mainly used for bandpass filter in ultraviolet radiation. However, when far infrared ray needs to be rejected, a form of inductive transmission type can be used as an additional blocking component.
