Sensitive Detection of Methane Using Tunable Multimode Diode Laser Absorption Spectroscopy at 1.675 μm

A multimode laser absorption spectroscopy system was built using a tunable multimode diode laser around 1.675 μm. To improve the sensitivity of the system, a discrete lens-type multipass cell with a path length of 100 m was employed as an absorption cell for the measurement. Methane concentrations were obtained from the relationship between the normalized signal peak heights of the measured and reference signals. The accuracy and sensitivity of the system are estimated to be 3.3% and 101 ppbv, respectively. The system was used to measure the concentration of methane in the air.