Abstract
Differential Absorption Lidar (DIAL) is a powerful laser-based technique for trace gas profiling of the atmosphere. However, this technique is still under active development requiring precise and accurate wavelength stabilization, as well as accurate spectroscopic parameters of the specific resonance line and the effective absorption cross-section of the system. In this paper we describe a novel master laser system that extends our previous work for robust stabilization to virtually any number of multiple side-line laser wavelengths for the future probing to greater altitudes. In this paper, we also highlight the significance of laser spectral purity on DIAL accuracy, and illustrate a simple re-arrangement of a system for measuring effective absorption cross-section. We present a calibration technique where the laser light is guided to an absorption cell with 33 m path length, and a quantitative number density measurement is then used to obtain the effective absorption cross-section. The same absorption cell is then used for on-line laser stabilization, while microwave beat-frequencies are used to stabilize any number of off-line lasers. We present preliminary results using ∼300 nJ, 1 μs pulses at 3 kHz, with the seed laser operating as a nanojoule transmitter at 822.922 nm, and a receiver consisting of a photomultiplier tube (PMT) coupled to a 356 mm mirror.
Highlights
Differential Absorption Lidar (DIAL) is a powerful technique for both range resolved as well as integrated column measurement of an absorbing atmospheric trace gas [1]
We developed a specific calibration technique based on in-situ measurements, and demonstrated its application to the DIAL system described in this paper
This paper presents an extension to our previously published novel two wavelength DIAL system, with one of the best DIAL wavelength fractional stability figures reported to date [13], for stabilizing three or more laser wavelengths with minimal additional components
Summary
Differential Absorption Lidar (DIAL) is a powerful technique for both range resolved as well as integrated column measurement of an absorbing atmospheric trace gas [1]. The effective absorption cross-section of a specific resonance line needs to be known for quantitative number density na(r) measurements, and HITRAN is not necessarily a suitable reference for this data. For this reason, we developed a specific calibration technique based on in-situ measurements, and demonstrated its application to the DIAL system described in this paper. A single strong absorption line can be utilized, with multiple side-line wavelengths tuned slightly off-resonance for the desired optical depth, to provide the adjustable range and sensitivity [9] Atmospheric pressure affects both the position, as well as the width of all molecular resonances. The vacuum wavenumber ν, intensity ST and half-widths γa and γs come from the respective database releases, while the peak Voigt cross-section σp is calculated from the respective parameters using the Whiting-Olivero [15] [16] method, with Doppler, air- and self-broadening at STP and a volume mixing ratio of 0.9%
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