Title: Development of a Pulsed 2-micron Laser Transmitter for CO2 Sensing from Space
Author: Upendra Singh
Organization: NASA Langley Research Center
Co-Authors: Jirong Yu, Yingxin Bai, Mulugeta Petros and Robert T. Menzies

Carbon dioxide (CO2) has been recognized as one of the most important greenhouse gases. It is essential for the study of global warming to accurately measure the CO2 concentration in the atmosphere and continuously record its variation. Studies of the carbon cycle are limited by the tools available to precisely measure CO2 concentrations by remote sensing. Active sensing, using the Integrated Path Differential Absorption (IPDA) approach, permits measurements day and night, at all latitudes and seasons.

Most active sensors under development use a continuous-wave (CW) laser with the goal of making column integrated measurements. This approach is limited in that the topographical scattering target must be at a known distance (requiring an ancillary laser altimeter) and cannot distinguish the desired signals reflected from topography from those reflected from clouds or dense aerosol layers. There exists a critical need to develop energetic pulsed transmitter for use at 2μm to make accurate column-averaged measurements of CO2 from space, as well column and profile measurement of CO2 from ground and airborne platform.

The development of a high pulse energy 2-μm laser transmitter for high-precision CO2 measurements from space leverages years of NASA investment in solid-state laser technology. Under NASA Laser Risk Reduction Program, funded by Earth Science Technology Office, researchers at NASA Langley Research Center developed an injection seeded, high repetition rate, Q-switched Ho:YLF laser transmitter for CO2 Differential Absorption Lidar/IPDA (profile/column) measurements from ground and airborne platforms. This master-slave laser system has high optical-to-optical efficiency and seeding success rate. NASA LaRC’s 2-micron pulsed laser transmitter possesses advantages over current passive and CW active sensors. First, the pulsed format provides a built-in means for determining range to the scattering target and effectively filtering out the scattering from thin clouds and aerosols, thus eliminating a source of measurement bias. Second, by concentrating the laser energy into a pulse, sufficient backscatter signal strength can be obtained from aerosol scattering rather than relying on a hard target at a known distance. Third, the absorption line at the 2.05 µm band is ideally suited for the CO2 concentration measurement. In particular, the weighting function of 2 µm is optimum for measurement in the lower troposphere where the sources and sinks of CO2 are located.

The planned laser transmitter development will lead to a Tm:Fiber pumped Ho:YLF laser transmitter capable of delivering 65 mJ at 50 Hz at on-line wavelength and 50 mJ at 50 Hz at off-line wavelength. The planned laser technology development and performance capabilities are a major step forward in the laser transmitter requirements called out in recent comprehensive system studies, e.g., the European Space Agency (ESA) exploration mission studies, A-SCOPE, for future CO2 column density measurements from space. The planned laser technology development is relevant to NASA’s earth science priorities, such as NASA ASCENDS mission for space-based CO2 column density measurements. This presentation will provide an overview of the current status of laser transmitter development and describe future technology development to meet the transmitter requirement for a space-based column averaged measurement of CO2 concentration.