Ground Based Test Results for Broad Band LIDAR

Our team of personnel from Northrop Grumman Aerospace Systems (NGAS) and Goddard Space Flight Center (GSFC) is developing two new lidar systems for column CO2 measurement based on an innovative new lidar technique using a Fabry-Perot based detector and a broadband laser source which replaces the narrow band laser commonly used. Our lidar is capable of mitigating inaccuracy associated with atmospherically induced variations in CO2 absorption line shape and strength and reduces the number of individual different wavelength lasers required from three or more to only one.It also reduces the requirement for source wavelength stability, instead putting this responsibility on the Fabry-Perot based receiver. There is a great need for measurements of atmospheric carbon dioxide concentration with high spatial and temporal resolution for global and regional studies of the carbon cycle. Such measurements will better resolve the linkage between global warming and anthropogenic CO2 emissions. In the Decadal Survey of Earth Science the National Research Council recommended that NASA develop, build, and fly a laser based system for precision measurement of total carbon dioxide column (the ASCENDS mission). The mission demands measurements of CO2 to a precision of 1 ppm out of the total ~400 ppm column in order to locate sources and sinks. Achieving this 400:1 precision is made more difficult due to the strong dependence on changes in atmospheric pressure and temperature of atmospheric carbon dioxide absorption line position, shape, and strength. Most lidar systems currently under development for remote sensing of atmospheric CO2 require multiple lasers operating at different, very narrow bandwidth wavelengths in order to resolve these effects. Our approach requires only a single laser and the wavelength stability requirements are much less stringent than those for the multiple laser approaches. Since 2007 GSFC has been developing a lidar using their broadband detection scheme and a 1.57 μm superluminescent light emitting diode (SLED) amplified by an optical parametric amplifier (OPA). In 2008 NGAS, leveraging expertise in thulium (Tm) fiber laser systems and recognizing the merit of the broadband approach, suggested a partnership with GSFC to develop a broadband lidar operating at 2.05 μm. Such a system takes advantage of the broad Tm-fiber gain spectrum and the inherent mechanical robustness, compact size, simple power scalability, efficiency and high beam quality offered by fiber lasers. In early 2010 NGAS completed development of a laboratory level, highly efficient, Tm-fiber laser that produces a specially formatted pulsed broadband output around 2.05 μm, a spectral region where CO2 has strong atmospheric absorption features. NGAS has loaned this tunable 2.05 μm laser to GSFC which had concurrently developed a 2.05 μm lidar sensor/receiver. In May 2010 the two systems were tested together to provide proof of concept of 2.05 µm broadband detection of CO2. This presentation will present results of ground based testing of the 1.57 μm and the 2.05 μm systems and discuss their potential application as space borne sensors for the ASCENDS mission.