Femtosecond XFROG in the middle infrared: A novel approach to standoff detection

The capability to produce femtosecond laser pulses with wavelengths in the atmospheric absorption window requires a new understanding of pulse propagation effects. In this work, we characterize the changes in temporal propagation of middle infrared femtosecond laser pulses by cross-correlation frequency resolved optical gating (XFROG). The temporally distorted infrared pulses are cross-correlated with 800 nm pulses by a four-wave mixing process in air. For the first time, we investigate these propagation effects through gas molecules that are not present in the atmosphere. Each molecule is shown to have a unique effect on the temporal propagation of the pulse that is wavelength dependent. We verify our experimental data with simulations based on a KramersKronig transformation of spectral data from the HITRAN database. The propagation effects are similar to optical free induction decay. Multiple vibrational and rovibrational absorption lines are excited by the middle infrared pulse and constructive interference occurs at various delay times relative to the initial pulse. The constructive interference impresses a unique fingerprint onto the pulse because the spectral lines of each molecule are unique. The fingerprint behaves as a nonlinear function related to the molecular concentration. To account for this, a regression model is developed to predict the concentration of unknown gas species. The middle infrared beam is the only laser beam sensitive to the analytes. Thus, standoff detection is a possibility since the XFROG can be performed locally.