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 in Littrow or Littman-Metcalf cavity configurations.