An ultra-Narrowband Magneto-Optical Filter for Daylight Observations of the Mesospheric Potassium Layer Jonathan Jonathan S. S. Friedman Friedman Arecibo Arecibo Observatory Observatory Seminar
Seminar 9 9 August August 2007 2007 Arecibo Observatory, 9 August 2007 The The solar solar spectrum spectrum near
near the the K K D-lines D-lines is is heavily heavily populated populated with with O O22 rotational
rotational absorption absorption lines. lines. In In fact, fact, the the D D22 line line in in potassium potassium overlaps
overlaps one one such such line. line. Potassium Potassium has has coronal coronal absorption absorption bands,
bands, i.e. i.e. Fraunhofer Fraunhofer lines. lines. The The D D11 Fraunhofer Fraunhofer absorption absorption is is shown
shown at at right. right. Arecibo Observatory, 9 August 2007 Spectral Filtering There are a variety of methods of spectral filtering. Color filters, spectrometers/monochrometers, interference filters and etalons are the most common. These suffer from low line-center throughput, broad bandwidth, poor spectral discrimination,
and/or high cost. Etalons are the only technology that can work in daylight. Etalons suffer from high cost + stability problems, especially when multiple etalons are required. Arecibo Observatory, 9 August 2007 Advantages Advantages of of the the Faraday Faraday Anomalous
Anomalous Dispersion Dispersion Optical Optical Filter Filter Adjustable-width, ultra-narrowband optical filtering. Flat-topped filter No wavelength drift Low cost ($20K) vs. multiple etalons (>$50K) Arecibo Observatory, 9 August 2007
What What is is a a Faraday Faraday Filter? Filter? (AKA (AKA MagnetoMagnetoOptical Optical Filter Filter or or FADOF) FADOF) The Faraday filter uses an atomic vapor cell
in an axial magnetic field to rotate the polarization of light that is resonant with an electronic transition of the atomic species that makes up the vapor. The cell and B-field are sandwiched between crossed polarizers, so light that is not resonant, and passes unimpeded through the cell does not have its polarization rotated and is extinguished. Arecibo Observatory, 9 August 2007 Arecibo Observatory, 9 August 2007
Physical Physical Principles Principles (continued) (continued) Modeling Modeling the the Faraday Faraday Filter Filter QuickTime and a
Video decompressor are needed to see this picture. Arecibo Observatory, 9 August 2007 The AO FADOF and its spectral effect Arecibo Observatory, 9 August 2007 A FADOF can also be used for Rayleigh Wind Lidar
Arecibo Observatory, 9 August 2007 The Arecibo FADOF Design Arecibo Observatory, 9 August 2007 Performance Tests Tests (for Performance (for 1 2 channel) channels)
Arecibo Observatory, 9 August 2007 K FADOF performance at Tenerife (28N, 16.5 W) Arecibo Observatory, 9 August 2007 Summary Understanding of the MLT region requires round-the-clock observations of state variables. The solar background is obviously a huge problem for optical observations.
There are not many useful technologies, and most are costly and unwieldy. The FADOF resolves many issues, providing a useful, economical, efficient and stable means of reducing the solar background with high linecenter throughput. Arecibo Observatory, 9 August 2007 Extra Slides K Energy Level Diagram Filter curve for 2-pass Version
Normalized transmissions of emissions due to the three laser wavelengths: 0.9844 0.9844 0.9845
