Intracavity photoacoustic sensing of water vapor with a continuously tunable external-cavity quantum-cascade laser operating near 5.5 μm

A gas sensor integrating a pulsed external-cavity quantum-cascade laser and a photoacoustic (PA) cell placed inside the laser cavity is described. The laser design allows a continuously tunable and mode-hop-free operation with an estimated instrument-limited linewidth of 0.063 cm(-1) in Littrow or Littman-Metcalf cavity configurations. Due to higher intracavity power levels, the PA signal for the cell located within the laser cavity is 80-320 times stronger than the signal for the same cell placed outside the cavity. The sensor shows a good capability to identify individual absorption lines of isotopic H-2 O-16, H-2 O-8, and H-2 O-17 species within a narrow spectral interval from 1814 to 1821 cm(-1) at natural isotope abundances. (C) 2016 Optical Society of America