Development and experimental evaluation of a dual BWOPO source for differential absorption measurements

Sammanfattning

This thesis investigates the development and experimental evaluation of a differentialabsorption source based on two backward-wave optical parametric oscillators (BWOPOs).The source was designed to generate pulse-to-pulse alternating online and offline idlerwavelengths near the CO2 absorption feature at 2051 nm, with the aim of assessing itssuitability for future differential absorption LIDAR applications.The two BWOPO outputs were recombined to produce collinear alternating idlerpulses, which were transmitted through a gas cell containing controlled CO2/N2 mix-tures. The measured differential optical depths were then compared with simulationscalculated from tabulated absorption cross-section values under the experimental pres-sure, temperature, and path-length conditions.The measured differential optical depth increased with CO2 mixing ratio, demon-strating that the source was sensitive to the targeted CO2 absorption feature. However,the measured values were significantly lower than predicted. The discrepancy couldnot be explained by finite BWOPO spectral broadening or gas absorption saturationalone. Instead, the results suggest that uncertainties in the effective CO2 columndensity, wavelength alignment, relative online/offline pulse-amplitude stability, andpossible detector nonlinearity limited the quantitative accuracy of the measurements.Overall, this work demonstrates the feasibility of using a dual-BWOPO source fordifferential absorption measurements, while identifying the main experimental limita-tions that must be addressed for future quantitative DIAL applications.