Recently observed nonlinear-propagation and optical-limiting effects of nanosecond and picosecond laser pulses through a fiber are analyzed with a model that accounts for various molecular photonic-absorption processes including linear, two-photon, intermediate, and excited-state absorptions. Explicit expressions for the laser-induced molecular-level density changes, the thermal–density effects following photoabsorption, and their effects on the laser propagation transmission are obtained for conditions corresponding to the experimental situations. These theoretical considerations are found to correlate very well with experimental results for the transmission of picosecond and nanosecond laser pulses through the nonlinear fiber. Our analyses show that in the picosecond regime, nonlinear photonic absorptions are efficient optical-limiting processes, whereas in the nanosecond regime, thermal–density effects are the dominant contributor. We also identify a particular nonlinear core liquid that gives very low optical-limiting thresholds and clamped transmission for picosecond as well as nanosecond laser pulses.
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