Laser Remote Sensing for Environmental Monitoring

Sammanfattning: This thesis studies the techniques of laser remote sensing and their applications in environmental monitoring, as documented in several published papers.

The environment where the human being live is degrading with an accelerating speed. Quantitative monitoring characterizes the quality of the environment and offers possibilities to solve environmental problems. Laser remote sensing actively probes physical quantities with advantages of, e.g., large spatial coverage and little environmental influence, and hence becomes a very suitable tool for environmental analysis.

Light detection and ranging (LIDAR) measures the backscattered light from remote targets using a short-pulsed laser. Three varieties of LIDAR techniques, based on a vehicle-carried laboratory, have been developed in the work within the thesis. Elastic LIDAR measures the elastic backscattering from the atmospheric aerosols, and has been used to comparatively study their vertical distribution above a Swedish rural area and a Chinese Magacity. Differential absorption LIDAR (DIAL) is able to analyze the distribution of trace pollutants in the atmosphere. Vertical measurements of two serious pollutants, atomic mercury and nitrogen monoxide, have been performed in the same Chinese city. Further, DIAL is also proved to be valid for remote gas analysis in multiple-scattering media. Fluorescence LIDAR can recognize the molecules contained in a remote target, through analyzing the laser-induced fluorescence fingerprint. Such a technique is employed in this thesis mainly for ecological studies, particularly for insect and bird monitoring. Promising performances have been revealed through feasibility tests and field experiments.

Tunable diode laser absorption spectroscopy (TDLAS) attracts research attention for its compactness, cost-effectiveness, and high sensitivity. The signal-to-noise ratio of a TDLAS can be conventionally improved by applying modulation; while in the thesis, a different idea is proposed and demonstrated by operating the absorption spectroscopy on a zero light background. Proof-of-principle experiments are performed and considerations regarding real-world applications are discussed. GAs in Scattering Media Absorption Spectroscopy (GASMAS) employs diode lasers for gas analysis as well, and wavelength modulation is frequently applied for sensitivity enhancement. Such an advanced TDLAS tool is applied in the thesis for food package monitoring.

Finally, fiber-optic remote sensing realizes the environmental monitoring by employing fiber-optic devices, e.g., fiber Bragg gratings (FBGs). Applications of these sensors are usually limited to temperature and stain monitoring. By using a methane catalyst which transfers the ambient methane into heat, FBG sensors become sensitive to methane. The sensitivity can be improved if the sensing FBG is interrogated by another reference grating having a matched spectrum. The multiplexing of such twin-grating-structure sensors is realized, and thus a distributed sensor network is possible to construct.