Imaging open-path Fourier transform infrared spectrometer for 3D cloud profiling

Abstract

An imaging open-path Fourier transform infrared (I-OP-FTIR) spectrometer is being developed for real-time threedimensional cloud profiling. The system employs a single modulator and a novel optical configuration which projects an array of angularly dispersed IR beams, each of which exhibits comparable throughput to a single channel OP-FTIR, to an array of respective retroreflector arrays remotely located at the opposite side of the test grid. The return light from each retroreflector array is imaged onto respective detectors that record the spatially-resolved interferograms which are subsequently transformed and analyzed for molecular content via advanced multicomponent algorithms. The result is a capability to sensitively, quantitatively, and simultaneously measure the molecular absorbance and associated concentration-pathlength of an open release plume over a spatial region. Use of two or more I-OP-FTIR sensors around the perimeter of the release allows for tomographic reconstruction of the concentration map of each molecular species contained in the plume.This approach realizes the high sensitivity of an OP-FTIR spectrometer without adding the expense and logistical difficulties associated with installing a large number of spectrometer units required for the cloud profiling application. In addition, the active spectral measurement supports detection in zero temperature contrast conditions where the plume is the same temperature as the background. A further reduction in cost and weight is achieved through the use of lowcost plastic press molded retroreflector arrays to return the spatially dispersed open path beams.

Spectroscopic Theory for Chemical Imaging

“…Differential absorption lidar (DIAL) shifts the laser frequency onto and off an analyte absorption peak to derive the analyte absorption. An openpath FTIR array spectrometer has been reported that performs 3D profiling of atmospheric plumes in real time [74].…”

Modular reconfigurable matched spectral filter spectrometer

“…In cases where multiple gases are addressed, there is typically a size and weight penalty because it often requires the integration of multiple lasers or resonant cavities and their associated optics. In scenarios with high intrinsic thermal contrast, such as monitoring exhaust plumes in industrial settings, a passive system can be used instead, which provides the ability to monitor a much broader range of gases with a reduced sensitivity 3,4,5 . Since the target chemicals and the measurement scenarios of interest to NASA may vary over time, OPTRA is currently developing a modular spectrometer for the detection of GHGs that can be reconfigured into an active in situ or passive remote sensor to provide the desired level of sensitivity and flexibility, thereby accommodating a wide range of research interests.…”