Development And Application Of Innovative Folded-path Optical Cells For High-sensitivity Trace Gas Sensing

Over the last few decades,monitoring of trace gas composition of the atmosphere has come to loom large in scientific and public concerns.Such knowledge is crucial to deepen our understanding of the sources/concentrations of key atmospheric species,of their roles in atmospheric chemical processes and of their impacts on local air quality,public health and global climate change.Laser absorption spectroscopy is an efficient analytical tool for optical metrology of trace gases.Multi-pass cells(MPCs)or optical cavities are usually used for long path absorption spectroscopy to enhance measurement sensitivity.Commercial optical multi-pass cells are bulky,expensive,and unsuitable for field monitoring applications requiring costeffective,compact,and lightweight optical sensors,in particular for network and unmanned aerial vehicle(UAV)observations.To further enhance the measurement sensitivity,dielectric mirror-based cavities are used,which allows one to reach dozens of kilometers of optical path length.However,coating on the cavity mirror,used to achieve high-reflectivity,is wavelength-dependent which limits the bandwidth of the mirror’s high reflectivity to 5-15%around the designed central wavelength.As a result,cavities formed by dielectric mirrors are only suitable for measurements in a relatively narrow spectral range.The present Ph D thesis reports on the developments of innovative folded-path dense spot patterned multi-pass cells and a prism-based optical cavity,dedicated to high-sensitivity trace gas sensing.The main works achieved during the Ph D work are as follows :(1)Development of a novel calculation model(patented n°CN109407310A)for the design of folded-path optical absorption cells with various dense spot patterns in a small size.One-circle spot-patterned multi-pass cell(MPC),seven-circle spot-patterned MPC,and single-line spot-patterned MPC were designed and applied for use in optical sensors of CO,CH4,and H2 O vapor.(2)Development of a broadband optical cavity based on prisms used as cavity reflectors.Relying on total internal reflection combined with light incidence at Brewster’s angle to realize low losses and quasi-wavelength independent high reflectivity over a broad spectral range.This new approach offers a high-finesse optical cavity operating in a wide spectral region from the near-UV to the near infrared.Its optical characteristics and spectroscopic performance are evaluated using absorption spectra of NO2,NO3 radicals,and H2 O vapor over a broad spectral region in the visible and in the near infrared,respectively.