Photoacoustic trace detection of methane using compact solid-state lasers

Methane was excited in the near-infrared (NIR) region using a pulsed grazing-incidence periodically poled lithium niobate optical parametric oscillator (OPO) and modulated diode laser radiation at 1650 nm as light sources and detected photoacoustically employing single tube and double tube acoustic resonators coupled with acoustic filters. The output power of the OPO was similar to 60 mW at the acoustic resonance frequency of 2.12 kHz, and therefore much higher than that of the diode laser radiation with 2 mW. By combining the more powerful OPO with the more sensitive differential cell with two resonator tubes (sensitivity: alpha W-min = 1.2 x 10(-9) cm(-1) W), an ultimate sensitivity of 20-30 parts per billion (ppbV) is estimated for methane detection with the signal beam of the OPO in the near-infrared and sub-ppbV sensitivity in the mid-infrared region with the idler beam.