Narrowband Tunable Optical Filter Using Fiber Bragg Gratings

Research at Langley Research Center has developed a special fiber- Bragg grating optical filter for use in aircraft or spaceborne differential absorption lidar (DIAL) systems for measuring water vapor in the atmosphere of the Earth. The filter is an optical fiber containing two Bragg gratings that afford high reflectance in 10-pm-wide wavelength bands at wavelengths of 946.0 and 949.5 nm. The optical fiber would be glued to a piezoelectric ceramic, to which a voltage could be applied to stretch the gratings and thereby adjust their peak-reflection wavelengths to correspond to atmospheric water vapor lines of interest. The concept of multiple Bragg gratings in a single optical fiber tuning such gratings by stretching the fiber is not new. The novelty of this research lies partly in the application of these concepts to make tunable ultra-narrowband filters for the specific water vapor wavelengths in question. Another element of novelty in the proposal lies in the design of the DIAL instrument in which the filters would be used: The design calls for a unique optical receiver that would couple a lidar signal from a telescope to a filter of the type proposed, then using an optical circulator the light would be detected.