Application of Intracavity Laser Spectroscopy for Pollution Detection

Summary

Intracavity laser spectroscopy offers a very high enhancement of the sensitivity of differential absorption measurements. It is based on an important feature of multimode lasers: the high sensitivity of the emission spectrum to the narrow line absorption inside the resonator. Intracavity absorption spectra are similar to conventional absorption spectra, where the length of the absorption cell 1 = c•t, where c is the velocity of light and t is the duration of the laser pulse. Practically, the equivalent optical absorption length of 1000 km is realized in experiments with lasers of 3 ms pulse duration. This enables the detection of spectral lines less than 10⁻⁹ cm⁻¹. Most of the measurements up to now were carried out with dye lasers, colour centre lasers, and glass lasers in the visible and near infrared range. A large number of new atmospheric absorption lines were detected with these lasers. Most of them are H₂O and O₂ absorption lines, but some are due to pollutants. The presence of as little as 10 ppb of NO₂ in the atmosphere is detected in the range of 600 nm. High concentration sensitivity to pollutants can be obtained with the lasers operating in the medium IR range, the spectral range of strong molecular resonant absorption. The most suitable lasers for this range are diode lasers. A very sensitive portable intracavity gas detector can be built with diode lasers for pollution measurements in the field.