Compact, Automated Differential Absorption Lidar for Tropospheric Profiling of Water Vapor

Knowledge of the vertical distribution of water vapor in the lower troposphere is centrally important for understanding climate change, high impact weather systems, and accurate weather forecasting. Currently, meteorological balloon-borne radiosondes are used to measure water vapor profiles but these only provide information at the time of the launch (twice daily), and so only provide limited spatial and temporal coverage. New instruments for monitoring water vapor profiles in real time are needed to provide a better understanding of the thermodynamic state of the atmosphere and to improve mesoscale weather forecasting. These new observational instruments need to operate autonomously in a cost effective manner to provide high value.

Researchers at Montana State University (MSU), in collaboration with Physical Sciences Inc., are working to develop a diode laser-based DIfferential Absorption Lidar (DIAL) for water vapor profiling in the lower troposphere. This DIAL instrument utilizes two fiber-coupled, distributed Bragg grating diode lasers (DBR's) operating near 820 nm. The on-line laser's frequency is locked to the center (or side-line) frequency of a water vapor absorption line and the off-line laser's frequency is chosen to be near the first but removed from any absorption. These DBR's are used to injection seed a pulsed, tapered semiconductor optical amplifier (TSOA) driven by a pulsed current supply. One laser wavelength is selected by a MEMS optical switch to be passed to the TSOA. Data is collected for several seconds before changing to the other frequency. The laser transmitter can produce between 5 and 15 μJ in a 1 μs pulse at a 10 kHz pulse repetition rate. Backscattered light is collected by a 35.6 cm diameter Schmidt-Cassegrain telescope and is split between near and far field detection channels. Each channel uses a filtered avalanche photodiode (APD) in conjunction with a multichannel scalar (MSC) to monitor the return signal. The far field channel uses a stabilized etalon, along with a narrow bandpass dielectric filter to reduce clear sky background radiance. The near field channel provides detection for ranges from ~300 to 1 km, while the far filed channel covers the 1 to 5 km range.

This presentation will review ongoing work to improve system performance to meet the needs of the research community. Details on the two detection channels, including the etalon, as well as improvements to the transmitter optical train, will be presented. The design process for enhanced mechanical stability and robust packaging for long term deployment will be reviewed. Field demonstration plans for the water vapor DIAL will be discussed including deployment of the next generation instrument at the Howard University Beltsville Center for Climate System Observation (BCCSO) for inter-comparison with the Howard University Raman lidar.