Sensitivity of long-wave infrared intracavity laser absorption vapor detector

A quantum cascade laser at IR wavelengths with an open external cavity presents an opportunity for spectral sensing of molecular compounds that have low vapor pressure. The sensitivity of such a system is potentially very high due to extraordinarily long effective optical paths that can be achieved in an active cavity. We demonstrate here an external cavity mid-IR QCL molecular absorption sensor using a fixed Fabry-Perot etalon as the spectrum analyzer. The system is sensitive to the water vapor present in the laboratory air with an absorption coefficient of just 1 x 10(-7) cm(-1). The system has sufficient sensitivity to detect the TNT vapor at room temperature.