We describe all solid state differential absorption lidar (DIAL) based on the mid-infrared (IR) tunable Optical
Parametric Oscillator (OPO). Generation of tunable mid-infrared laser radiation using a two stage tandem OPO was
demonstrated. The first stage was based on the nonlinear KTP crystal and produced up to 45 mJ of 1.57 μm radiation,
while pumped by a commercial Q-switched Nd:YAG laser. The quality of signal beam was improved by the use of
unstable resonator. The AgGaSe<sub>2</sub> crystal was used in the second stage OPO. Idler energies up to 1 mJ were generated at
this stage within tuning range from 6 to 12 μm. The receiver consisted of a 250 mm gold mirror telescope, two channel
detection system and control electronics. We have designed a photoacoustic cell for wavelength calibration of lidar.
Preliminary lidar field test results are presented.
High sensitivity and selectivity of gas/vapour detection are achieved employing registration of laser photoacoustic spectra. The lasers are usually operated in the continuous-wave (CW) single-frequency mode. The tuning range of the single CW laser system is not sufficient to cover spectral bands of variety of gases/vapours of interest. The optical parametric oscillator (OPO) systems are more preferential for multi component laser analyzers allowing the simultaneous measurement of different gases or pollutants. Pumped by the same 7 ns duration pulse of Nd:YAG laser and its harmonics, two OPO systems were tested. One system generates in the 0.7-1.9 μm range and covers overtones of stretching vibrations and combination vibrations of hydrogen atoms in the analyte molecule. Other system generating in the 5-11 μm range covers vibrations of molecular characteristic groups ("fingerprints"). Photoacoustic spectra of nitro compound vapours, e.g. nitromethane, nitroethane, nitropropane, nitrobenzene and nitrotoluene, also spectra of methane and water vapour were measured and compared to simulated spectra derived with the aid of HITRAN data base and to the literature spectral data. Photoacoustic detection thresholds are evaluated from the ratios of measured signal to the registration noise.
We describe a new project (acronym LISATNAS) approved by the Lithuanian Research Council in 2003 devoted to the development of differential absorption lidar (DIAL) and stationary spectrometric systems based on the mid-infrared tunable Optical Parametric Oscillator (OPO), pumped by compact Q-switched lasers. The purpose of the project is to construct a mobile infrared lidar, assembled in the truck for selective pollutant analysis - possessing spatial resolution of a few meters in the distance range extending from hundred of meters to a few kilometers. A reliable cascade mid-IR generation scheme was developed. Pulse energies up to milijoule in mid-IR have been already obtained using nonlinear AgGaSe<sub>2</sub> crystal. Optoacoustic and multipass cells were constructed for stationary spectrometers. Preliminary results with detection of CO<sub>2</sub>, CH<sub>4</sub>, H<sub>2</sub>O and other gases in the ppm concentration range show good sensitivity. Special pollutants were synthesized by chemical group of the project for spectrometric experiments: multiatomic nitrocompounds, such as trinitrotoluen (TNT) or trotyl, DNT (dinitrotoluoen), MNT (mononitrotoluoen) and RDX (heksahydro-1.3.5-triazyn). The mobile DIAL system based on the tunable laser in the 8-12 μm region, 10" goldmirror telescope, MCT cooled detector with control electronics is under construction and should be finished in 2005.
Optical and sorptive properties of cellulose have been investigated by three methods covering the wide 7000 - 80 cm<SUP>-1</SUP> spectral range. Attenuated total reflection spectra showed shift of sorbed water bands to shorter wavelengths under paper moisture increase. Surface electromagnetic waves (SEW) propagation at metal - Ge (film) - paper system were investigated firstly in far infrared region. Four slopes in the SEW intensity dependence versus time under paper drying were observed. Near infrared diffuse transmittance revealed a band of bonded to cellulose water at 1.53 micrometers which shows 0.08 micrometer red shift compared to 1.45 micrometer band of free water. Damping constants of cellulose sheets of various thickness were determined in the 140 - 85 cm<SUP>-1</SUP> range by the surface electromagnetic wave method. Sorptive properties of cellulose are compared to those of other sorbers made of divided silica.