During the last 20 years much attention has been given to the study of propagation of short intense laser pulses for which the peak power exceeds the critical power of self-focusing, Pcr. For a laser power P < Pcr, a dynamic equilibrium between the Kerr self-focusing, diffraction and defocusing caused by laser-ionized plasma result in the production of a high intensity laser filament in air within which a variety of nonlinear optical phenomena are observed. However, research in the 0.8-1 m range so far has shown a fundamental limitation of guided energy to a few mJ transported within an ~100 m single channel.
A long-wavelength, 0~10 m CO2 laser is a promising candidate for nonlinear guiding because expected high Pcr values according to the modeling should allow for the increase of energy (and therefore power) in a self-guided beam from mJ (GW) to few Joules (TW). During the last decade a significant progress has been achieved in amplification of picosecond pulses to terawatt and recently to <10 TW power level at UCLA and ATF BNL. Such powerful 10 m lasers open possibility for nonlinear propagation studies in an atmospheric window with high transmission. As a natural first step in a our program on picosecond CO2 laser filamentation, we have made first measurements of Kerr coefficients of air and air constituents around 10 m. We also undertook direct measurements of n2 of air by analyzing nonlinear self-focusing in air using a ~3 ps, 600 GW pulses of the BNL CO2 laser.
Several original schemes for intracavity frequency doubling of CO2 laser radiation (TEA and cw systems) in AgGaSe2 nonlinear crystals have been proposed and studied experimentally. Computer modeling was used for optimization of the optical schemes and cavity parameters. The enhancement by several fold in conversion efficiency was achieved for intracavity SHG in comparison with traditional schemes. A 60% peak power efficiency and 15% energy conversion efficiency has been obtained with a TEA CO2 laser. More than 100 mW at 5 micrometers was generated in the crystal with L equals 17 mm, which is a record output for SHG of a cw CO2 laser. This output is more than 10 times higher than 5 micrometers power measured with standard focusing the CO2 laser beam.
A comparative analysis of the efficiency of generation of the second harmonic of a TEA CO-2) laser in Te, ZnGeP2, GaSe, AgGaSe2, and Tl3AsSe3 single crystals as well as of the absorption properties of these crystals and the damage threshold of their surface has been performed on the basis of experimental data. High-optical-quality AgGaSe2 crystals holding much promise for the second harmonic generation have been grown. Phisico-chemical features of the process of growing and annealing of these crystals were investigated. It was revealed that the efficiency of the conversion of the CO2 laser radiation in ZnGeP2 and AgGaSe2 crystals can be significantly increased at the expense of shortening of laser pulses, which can be effected by a simple optical device -- a plasma gate.
The method of frequency conversion based on the use of nonlinear crystal as an output coupler of laser cavity has been proposed and experimentally tested. Frequency doubling characteristics of such a nonlinear output coupler have been studied. It is shown that the second harmonic generation efficiency of a TE carbon-dioxide laser output in this case is several times higher than for traditional focusing methods at the comparable energy parameters of the fundamental frequency radiation.
The present-day ecological situation calls for lidar systems based on CO2 lasers. However, the spectral tuning range of a conventional CO2 laser is limited by a discrete set of oscillation lines on the regular bands 0001 - 1000, 0001 - 0200. Therefore, the possibility of using such lasers for atmospheric monitoring is limited and depends on accidental coincidence of radiation frequencies and absorption resonances of chemical compounds polluting the atmosphere. Besides, the presence of carbon dioxide in the environment leads to significant losses due to the absorption at its transmission in the atmosphere for considerable distances. We and other investigators obtained powerful radiation pulses at a sufficiently high efficiency in a TEA CO2 laser on nontraditional bands: the sequence (0002 - 1001, 0002 - 0201) and the hot (0111-1110) bands, which more than doubles the radiation spectrum of conventional CO2 lasers thus widening their potentialities for atmospheric monitoring. It is also important that the radiation of nontraditional bands practically is not absorbed by atmospheric carbon dioxide. In this paper, we present a UV-preionization TEA CO2 laser tunable over the lines of regular and nontraditional bands specially designed for atmospheric monitoring. Developing it, we took into account the fact that atmospheric monitoring places certain demands on lasers: high stability of output performances, a comparatively narrow oscillation line, low beam divergence, possibility of working in pulse-periodic regime. Besides, the laser has high energy performances and the widest possible spectral range of tuning. Special consideration was given to the real problem of tunable CO2 lasers -- automatization of the process of tuning over oscillation lines. On the basis of this laser a lidar system was created which has passed an in- the-field test to satisfaction. With its use small concentrations (at a tolerable-concentration level) of a number of gaseous pollutants was determined.
In the paper, a cw CO2 (CO) laser with automatic tuning over oscillation lines and stabilized output parameters is described. The transfer of oscillation from carbon dioxide to carbon monoxide only takes place when the series-produced sealed-off active element GL-501 is replaced by the constructively identical element GL-509. The laser oscillates in the 9.1 - 11.3 micrometers range (100 vibrational-rotational lines of the 0001-1000(0200) band and more than 20 lines of the 0111-1110 band's P-branch of the CO2 molecule when GL-501 is used) and in the 5.2 - 6.4 micrometers range (more than 100 vibrational-rotational lines of the CO molecule when GL-509 is used). The output power in the single-mode regime is more than 10 W at oscillation on CO2 (on weak lines -- 1 W at least) and more than 1 W at oscillation on CO (on weak lines -- 0.05 W at least). The CO2 (CO) laser with the above parameters operated satisfactorily for a long time as a component of a trace gas analyzer and it was used also for local control of gaseous atmospheric pollutants by the method of PA-detection. For the purpose of widening spectral possibilities of the CO2 (CO) laser for working gas analyzer problems the oscillation of the CO2 laser radiation second harmonic in the AgGaSe2 crystal of a high optical quality has been obtained. The convertor efficiency factor about 0.15% with the laser output power approximately 5 W directed onto crystal has been realized.