Stimulated Emission from Radiation Trapping Atoms: The Copper Vapor Laser
Stimulated emission in the atomic copper vapor laser takes place in a rather unique way in that the upper levels of the laser transitions (2P states) are electric dipole–coupled to the ground state (2S), the lower laser levels (2D) are metastable, and yet the gain is one of the highest among all the visible lasers . It is generally believed that the radiation trapping in the 2P–2S resonant transitions greatly enhance the transition probabilities for the 2P–2D transitions, thereby making the lasing action possible for the Cu atoms. There have been theoretical studies to analyze the pumping mechanism and to understand the spectral composition of the copper vapor laser in connection with the efforts to develop high-power copper vapor lasers [2,3]. In these studies, radiation trapping has been treated by Holstein’s theory which is based on a transport equation and the Beer’s law absorption coefficient . An earlier work by Leonard  analyzed the copper vapor laser by means of a three level laser model without considering the radiation trapping. As far as we know, however, there has been no analysis of the stimulated emission in the atomic copper vapor from the perspective of Rabi oscillations between the 2P — 2S states. It is the purpose of this paper to present a semiclassical calculation of the gain of the stimulated emission in the atomic copper vapor and discuss some new features that arise in the Rabi oscillation picture . For instance the gain becomes proportional to the entire ground state atomic density, modulated by some factor that depends on the intensity of the trapped fields, in contrast to the usual laser systems where the gain is proportional to the population inversion density . Also, there appears to be a “conservation law” which relates the gains of the two laser transitions (yellow and green).
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