Laser Long-Path Absorption Lidar Technique for Measuring Methane Using Gas Correlation Method

Abstract

We studied a new laser long-path absorption lidar technique using a gas correlation method for measuring methane in the atmosphere. The method employs a broad-band multimode infrared pulsed laser and a cell containing methane as the correlation filter. The correlation filter is used to analyze the spectra of the transmitted and received pulses. We obtained a simple equation for deriving the density of the target molecule from an analogy of the differential absorption method. The measurement method was evaluated by a simulation study to measure methane in the 3 µm region using a solid-state optical parametric oscillator. The result shows that atmospheric methane can be measured by a system using the reflection from a hard target, and that the methane density derived by a simple equation has good linearity in the practical density range in methane measurements. It also indicates that the measurement is not sensitive to fluctuations in the multimode laser spectrum. The measurement system is compact and useful for various field measurements of atmospheric methane.