1. An ultra-Narrowband Magneto-Optical Filter for Daylight Observations of the Mesospheric Potassium Layer Jonathan S. Friedman Arecibo Observatory Seminar 9 August 2007 Jonathan S. Friedman Arecibo Observatory Seminar 9 August 2007

2. Arecibo Observatory, 9 August 2007

3. The solar spectrum near the K D-lines is heavily populated with O 2 rotational absorption lines. In fact, the D 2 line in potassium overlaps one such line. Potassium has coronal absorption bands, i.e. Fraunhofer lines. The D 1 Fraunhofer absorption is shown at right.

4. Spectral Filtering Arecibo Observatory, 9 August 2007 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.

5. Advantages of the Faraday Anomalous Dispersion Optical Filter Arecibo Observatory, 9 August 2007 Adjustable-width, ultra-narrowband optical filtering. Flat-topped filter No wavelength drift Low cost ($20K) vs. multiple etalons (>$50K)

6. What is a Faraday Filter? (AKA Magneto-Optical Filter or 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

8. Physical Principles (continued) Modeling the Faraday Filter Arecibo Observatory, 9 August 2007

9. The AO FADOF and its spectral effect Arecibo Observatory, 9 August 2007

10. A FADOF can also be used for Rayleigh Wind Lidar Arecibo Observatory, 9 August 2007

11. The Arecibo FADOF Design Arecibo Observatory, 9 August 2007

12. Performance Tests (for 1 channel) Arecibo Observatory, 9 August 2007 Performance Tests (for 2 channels)

13. K FADOF performance at Tenerife (28°N, 16.5° W) Arecibo Observatory, 9 August 2007

14. 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 line-center throughput. 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 line-center throughput. Arecibo Observatory, 9 August 2007

15. Extra Slides

16. K Energy Level Diagram

17. Filter curve for 2-pass Version Normalized transmissions of emissions due to the three laser wavelengths: 0.98440.98440.9845