The Theoretical Study Of Lasing Without Inversion In An Open Four-level System

The study of lasing without inversion (LWI) is an important and active research front field in international lasing physics nowadays. The study of LWI has not only an important theoretical value, but also wide application prospects. As a new mechanism of producing laser, LWI have received considerable attention for their potential to obtain laser light in spectral domains, e.g. the x-ray or even y-ray range, where conventional methods based on population inversion are not available or are difficult to implement.We presented at first the theoretical model of the open four-level LWI system. In this paper, we mainly studied the effect of system's parameters varying on the gain, dispersion and population difference, and the effect of the driving field phase fluctuation and the effect of propagation on the open LWI system. This paper consists of four chapters.In Chapter 1, we explained the significance for studying LWI and the basic principle of producing LWI, introduced simply the current research state and the main contents for the important study subject at present.In Chapter 2, the model of an open four-level and its nonlinear analytical solution in the steady state were presented on the basis of the closed four-level system. Using numerical results from the nonlinear analytical solution, we analyzed the effect of system's parameters varying on the gain, dispersion and population difference, and discussed the difference between the nonlinear and linear solution.In Chapter 3, we studied the effect of the driving field phase-fluctuation the open four-level non-inversion lasing system. The phase-fluctuation of driving field results in thefinite line-width of the driving field. We obtained the analytical solution in the steady state in the phase fluctuation of driving field. Using the numerical calculation results of the analytical solution, we discussed the effects of the finite line-width of driving field, the detuning of the driving and probe fields, the Rabi frequency of the driving field, the ratio of the atomic injection rates and the atomic exit rate on the gain and dispersion of the lasing without population inversion.In Chapter 4, under regarding the Doppler effects due to atomic motion and regardless of the Doppler effects, we studied the propagation effects on lasing without inversion in the open four-level system, respectively. When ignoring the Doppler effects, we discussed the effects of the propagation length on the probe gain, the Rabi frequency of the driving and probe fields, and discussed the effect of the driving field Rabi frequency at the entrance of the active medium on the probe gain for a propagation length of 5.2 cm. When considering the Doppler effects and the probe and driving fields and the atomic motion co-propagating, we analyzed the probe gain, the Rabi frequency of the driving and probe fields and the populations as the functions of the propagation length and studied respectively the probe gain for a propagation length of 5.2 cm as the function of the driving field Rabi frequency at the entrance of the active medium and as the function of the Doppler broadening. We contrasted the numerical simulation results in the four transmission cases among the probe field, the driving field and the atomic motion.