Multispectral Imager Utilizing A Variable Grating Mode LC Device Combined with a Fabry-Perot Etalon

We consider multispectral imaging not just as a problem of providing a reconfigurable filter (variable Fabry-Perot etalon, acousto-optic tunable filter, light-induced holographic variable grating) but rather as a problem of overall system design. We find that the only variable dispersion element (VDE) capable of avoiding aliasing of the spectral images at the imaging (CCD) plane is a combination of a variable grating with a fixed Fabry-Perot etalon. We propose to build the VDE, using a variable grating mode (VGM) effect occurring in some liquid crystal (LC) mixtures. Under a dc voltage applied to such a mixture, a phase grating appears in the direction perpendicular to the quiescent state alignment of the LC molecules. Moreover, by increasing the dc bias we can linearly change the induced grating frequency over a wide range. Since the induced refractive index modulation is rather significant (up to 0.2) we expect to achieve theoretically limited diffraction efficiency. We propose a concept of a multispectral imager (MI) that combines the VGM device with an inexpensive, low-finesse, fixed Fabry-Perot (FP) etalon, and will select a narrow spectral band for each consecutive image. Thus, no image degradation is expected while rapid switching of spectral bands occurs. Phase I of this project will experimentally demonstrate the feasibility of the proposed VGM-FP combination for bandpass selection in the MI.