MRI-R2: Development of Next-generation Imaging Spectrometer Based on a Tunable Liquid Crystal Filter

Trustees Of Boston University, Boston

Investigators

Abstract

This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5). This research develops a solid state, frequency-agile, imaging spectrometer based on an electronically tunable optical filter. The instrument--referred to as the Liquid-Crystal (LC) Hyperspectral Imager (LCHSI)--solves the difficult problem of imaging transient events at multiple wavelengths simultaneously. At the heart of the LCHSI is a Fabry-Perot etalon whose transmission characteristic is controlled via an electric field applied across a liquid-crystal substrate injected into the gap. The substrate is etched into four electrically-isolated regions which may be individually controlled. The resulting quadrant filter is then coupled to beam diverting prisms to produce four narrow-band images on a single detector. Each channel may be individually tuned at millisecond cadence to thousands of narrow-band color combinations. The instrument thus combines the wavelength agility of a dispersion-based hyperspectral imager (HSI) with the imaging ability of a filter-wheel camera. This architecture has several advantages over competing multi-spectral and hyper-spectral systems. The combination of simultaneous multi-channel imaging and electronic tuning makes the LCHSI extremely versatile. The use of a single optical chain and a solid-state filter reduces size, mass, cost, and complexity. Static filters degrade over time resulting in a drift of their center wavelength, rendering them unusable; since an LC filter is tunable, it does not degrade, and need only be calibrated periodically. The LC filter is also extremely robust; the technology is thus a candidate for next-generation space-borne imaging systems. Although the applications of this technology are extremely broad, the initial demonstration of the LCHSI will be in the field of aeronomy, targeting five common emissions in the airglow and auroral spectrum: 427.8, 557.7, 630, 732 and 777.4 nm. The 732 and 777.4 lines can be tuned in a single quadrant, thus providing a test of the wavelength-hopping capability, as well as the on- and off-band sampling mode. Testing will be carried out at the NSF Upper Atmospheric Facilities at Arecibo, Puerto Rico, and Sondrestrom, Greenland.