Influence of molecular absorption on propagation of CO2 laser radiation in terrestrial atmosphere (review)

The following topics are reviewed from the point of view of the molecular absorption in the atmosphere: 1) propagation of the P20 line of the 00⁰1–10⁰0 transition (λ≈10.6 μ) emitted by a ¹²C¹⁶O₂ laser; 2) propagation of various lines of the 00⁰1–10⁰0 and 00⁰1–02⁰0 transitions emitted by ¹²C¹⁶O₂ lasers; 3) propagation of radiation emitted by ¹³C¹⁶O₂ and ¹²C¹⁸O₂ lasers (00⁰1–10⁰0 and 00⁰1–02⁰0 transitions). It is stressed that three main factors should be considered in reviewing the atmospheric absorption: The continuous absorption of the water vapor, resonance absorption by CO₂ molecules, and absorption due to the accidental coincidence of a laser emission line with a telluric absorption line of some gas component of the terrestrial atmosphere. The relative role of these factors depends on the actual laser and on the conditions in the atmosphere, such as temperature, humidity, and pressure. Preference should be given to those lasers for which the absorption due to the last two factors is small compared with the continuous absorption by the water vapor. The latter is constant to within (± 20%) throughout the wavelength range 9–11 μ, where the laser radiation under consideration is located. It follows that the ¹³C¹⁶O₂ and ¹²C¹⁸O₂ lasers should be preferred because the concentration of the corresponding isotopes in the atmosphere is approximately 100 times lower than those of the isotopes in ¹²C¹⁶O₂.