Spectral characteristics of MWIR band-pass filters at cryogenic temperature

In order to research the reliability of filters at cryogenic temperature, Ge-based and sapphire-based mid-wave infrared (MWIR) band-pass filters were coated, and the experiments at cryogenic temperature for 5 min,15 min,25 min,35 min and even more than 300 min were done,respectively, the transmittance curves were showed. Furthermore, the impact of cryogenic temperature on the spectral characteristics and fastness after were studied.The transmittance of passband zone reduces with the time increasing at cryogenic temperature, and there is a small shift of 13nm for center wavelength, resulted from the relative variation of the high and low refractivities of deposited coatings which can induce the ratio between center and cut-off wavelengths to change. Compared to the sapphire substrate,Ge substrate is more sensitive to temperature,so the fall range of transmittance shows obvious(about more than 10%)when the bandpass filters transit from room temperature to cryogenic temperature. The fastness of both the Ge-based and sapphire-based MWIR band-pass filters have never been damaged at cryogenic temperature for no more than 300min.