Design of reflective filter based on metal-dielectric thin films for radiation wavelength of carbon dioxide

The radiation wavelength of the carbon dioxide is 4.3μm. It is a kind of background noise that affect the image contrast in infrared imaging system seriously. Mirror is an important optical element in infrared optical system. This paper intends to design a 4.3μm wavelength filter base on the mirror. So, it is called reflective filter. It can replace the original mirror in the infrared optical system in order to filter out the radiation wavelength of the carbon dioxide and enhance the image quality. The reflective filter consists of metal and dielectric films. The absorption of the Al film which is the underlayer has been induced by the outer side dielectric multilayer films at 4.3μm. And it will keep high reflectivity in other wavelength at the same time. The design result as follow was obtained after optimization design. The average reflectivity is about 98% in the range of 3.7-4.2μm and 4.4-4.8μm (reflection bands) and the reflectivity is less than 10% at 4.3μm (absorption band) when the incident angle is 45°. The reflection phase of the metal-dielectric films was analyzed. The electric field distribution of reflection bands and absorption band was shown respectively. At last, in order to filter out the peak as much as possible, the design method and result of absorption band widening of the reflective filter was shown. Compare the all dielectric transmission minus filter, the layer number of the reflective filter is fewer and the total thickness of the coating is lower. Therefore, the manufacturing process became easier. The reliability became higher. More important, a better parameter of filter was obtained.