Study and preparation of mid-wave infrared 3μm～5μm broad band pass filter

The plating process of the mid-wave infrared broad-band pass filter is difficult because of multi-layers, and the total thickness of film layers is very big ( the thickness is up to about 10 m). Through analysis of the design features of several infrared band pass filters, and combining with the actual plating technology, the 3 m～5 m broad-band pass filter with germanium substrate was designed by using the method of combining long-wavepass, shortwave-pass and irregular film design technology. The design reduces the total thickness of film ( about 8.65 m) greatly as well as the deposition time, and the firmness of the film is improved. The film obtains excellent spectral properties by changing the evaporation rate of film materials, correcting the scan mode of the electron gun when vaporizing the ZnS material, and adjusting the loading method of the evaporation material in the crucible. The average transmittance is more than 96% in the passband and less than 1% in cut-off region.