Numerical simulation and observed rotational relaxation in CW and pulsed HBr-filled hollow-core fiber lasers

Gas-filled hollow-core fiber (HCF) lasers have emerged as a promising technology for generating mid-infrared lasers. A four-energy level system laser model is presented to predict the performance of optically pumped HBr-filled HCF lasers under continuous wave (CW) and pulsed excitations. The steady state condition is considered in CW pumping and the characteristics of simulated population density and power distribution along HCF are investigated. The finite-difference time-domain method is employed in pulsed pumping and the simulated evolutions of pump pulse and laser pulse at different positions along the HCF are studied. In addition, the phenomena of rotational relaxation in HBr-filled HCF lasers are investigated experimentally for the first time, to the best of our knowledge, showing that using the absorption lines away from the strongest absorption lines and tuning the pump wavelength deviating from the center of the absorption line makes the rotational relaxation occur easily. The demonstration is conductive to reveal the underlying mechanism of such gas-filled HCF lasers.