Abstract
A laser transmitter has been developed and incorporated into a micro-pulse differential absorption lidar (DIAL) for water vapor profiling in the lower troposphere as an important step towards long-term autonomous field operation. The laser transmitter utilizes two distributed Bragg reflector (DBR) diode lasers to injection seed a pulsed tapered semiconductor optical amplifier (TSOA), and is capable of producing up to 10 mJ of pulse energy with a 1 ms pulse duration and a 10 kHz pulse repetition frequency. The on-line wavelength of the laser transmitter can operate anywhere along the water vapor absorption feature centered at 828.187 nm (in vacuum) depending on the prevailing atmospheric conditions, while the off-line wavelength operates at 828.287 nm. This laser transmitter has been incorporated into a DIAL instrument utilizing a 35.6 cm Schmidt-Cassegrain telescope and fiber coupled avalanche photodiode (APD) operating in the photon counting mode. The performance of the DIAL instrument was demonstrated over a ten-day observation period. During this observation period, data from radiosondes were used to retrieve water vapor number density profiles for comparisons with the number density profiles retrieved from the DIAL data.
Highlights
The planetary boundary layer is the bottommost part of the troposphere and typically ranges from the Earth’s surface to several hundred meters, during stable nighttime conditions, to several kilometers, during the day, when solar induced convection causes shear and turbulence in the air parcel near the surface
Raman lidar uses the characteristic frequency shift resulting from the inelastic Raman scattering to identify the molecular species of interest while differential absorption lidar (DIAL) utilizes the elastically scattered signals from two closely spaced laser transmitter wavelengths, the first associated with an absorption line for the molecule of interest, referred to as the on-line wavelength, and the second minimally affected by any molecular absorption, referred to as the off-line wavelength
The difference in the strength of the on-line and off-line return signals results from molecular absorption so that with knowledge of the absorption cross section, the ratio of scattered light collected by the DIAL receiver at the on-line and off-line wavelengths can be used to determine the molecular number density
Summary
The planetary boundary layer is the bottommost part of the troposphere and typically ranges from the Earth’s surface to several hundred meters, during stable nighttime conditions, to several kilometers, during the day, when solar induced convection causes shear and turbulence in the air parcel near the surface. Recognizing the potential of the micro-pulse DIAL technique, Machol et al [15] developed a diode-laser-based DIAL instrument based on distributed feedback (DFB) diode lasers and tapered semiconductor optical amplifiers (TSOAs) and demonstrated initial water vapor retrievals up to 2.5 km during nighttime operation. TSOA [16,17,18,19,20] This instrument has retrieved daytime water vapor profiles up to 4 km and nighttime water vapor profiles up to 6 km, providing the first demonstration of a diode-laser-based micro-pulse. The data products retrieved from the DIAL instrument during this observation period include the normalized relative backscatter and water vapor number density [19] During this observational period, Vaisala RS92 radiosondes were launched providing temperature, pressure, and relative humidity profiles.
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