An electrically controlled liquid-crystal (LC) interference filter by light orientation for infrared multispectral imaging

This paper presents an electrically controlled liquid-crystal (LC) interference filter in the infrared multispectral imaging system. Based on the theory of multi-beam interference, the proposed structure utilizes a pair of parallel flat plates with a reflectivity of ~95% to form a Fabry-Perot (FP) cavity, realizing constructive interferences and destructive interferences in specific wavelengths for incident infrared broad-spectra. The filtering performance of the proposed structure can be controlled by incorporating electrode layers based on the electro-optical properties of LC. We simulate the filtering characteristics of the proposed structure according to the elastic continuum theory of LC. Then the zinc selenide (ZnSe)-based LC filter with a preset cavity depth of 50 μm was fabricated with light orientation instead of the conventional friction orientation method. An infrared spectra test system was built to measure the performance of the fabricated interference filter. The experimental results successfully demonstrate the filtering characteristics covering three important infrared window bands in accordance with the simulated results by applying different root mean square (RMS) voltages. Also, the fabricated filter brings a sharper transmission peak waveform in a higher frequency infrared band. The proposed filter has a low cost and weight and is easy to install, laying the foundation for developing full-wave infrared multispectral imaging systems.