Quantum cascade laser-based trace detection of gases in the deep-infrared region using phase fluctuation optical heterodyne spectroscopy

We present a spectroscopic technique based upon optical phase-fluctuation spectroscopy for very high levels of sensitivity and specificity with application for detecting the presence of concealed explosives by detection in the vapor phase. The approach enables recent advances in deep-infrared QCL spectroscopic sources to be utilised without the need for cooled detectors and gives multi-pass Herriott-type cell performance from a highly compact form factor. The system has been evaluated in the mid-infrared using a continuous-wave optical parametric oscillator as a spectroscopic excitation source, and Ethane as a sample molecule for detection. With this setup we have demonstrated the specificity of the device by being able to resolve characteristic spectral lines of the molecule of interest against other contaminants in the sample with similar spectral response, and a noise-equivalent sensitivity of 15ppb. Sensitivity is currently limited by ambient mechanical noise and routes to minimize this are considered.