Characteristics of a Laser System for Atmospheric Absorption and Air Pollution Experiments

Abstract

The design and performance of a double-beam laser transmitter and receiver system is described. The transmitter consists of two cw helium-neon lasers, one working at 0.6328 ?m (used as a guide beam), and the other at 3.392 ?m, and two beam expanders which result in transmitted beam divergences of 0.1 and 0.4 mrad, respectively. As a receiver, an 8-inch Newton telescope coupled to a triglycine sulphate pyroelectric detector has been used. Using the transmitter in the laboratory, the value of the absorption coefficient of methane in dry air at 760 Torr for 3.392-?m laser radiation was measured to be k = 1.09 ± 0.01 Torr?1 m?1. The value of absorption coefficient is 20% higher for air containing 7% by volume of water vapor. In outdoor measurements a homogeneous concentration of methane of 1 ppmv decreases the transmitted signal by 10% for an effective path length of 130 m. The system was used for detection of point leaks of methane. For a beam transmitted 1 m above the ground level a simulated leak of 100 cm3 of methane (released in 1 min) caused an 8% decrease of transmitted signal. The range of the transmitter-receiver system is 0.5 km for the full beam (1/e2 width) collected by the receiver or 5 km with one additional beam expander. The system appears to be suitable for communication experiments, long-path or folded-path spectroscopy, and atmospheric pollution measurements.