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Chinese Physics Letters

Effects of Pump Phase Noise on the Linewidth of a Λ-Laser Without Inversion: Linewidth Reduction

Xiangming Hu, Jin-sheng Peng

Published Aug 1, 1998• 3 citations
Publisher

Abstract

The effects of the pump phase fluctuation on a Λ-laser with and without inversion are investigated. It is shown that the pump phase fluctuation can reduce the linewidth of the laser without inversion. It is also shown that, when the system exhibits a transition from lasing without inversion to lasing with inversion, the laser linewidth can still be reduced.

DOI
10.1088/0256-307x/15/8/010

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Physical Review A•Dec 1, 1995•8 citations
The effects of the pump phase fluctuation on a Λ-laser with and without inversion are investigated. It is shown that the pump phase fluctuation can reduce the linewidth of the laser without inversion. It is also shown that, when the system exhibits a transition from lasing without inversion to lasing with inversion, the laser linewidth can still be reduced.
The influence of driving field phase fluctuation on lasing without population inversion in a three-level cascade system was studied. In the rotating wave and slowly varying envelope approximations, the density-matrix motion equations for the three-level cascade system, considered the influence of driving-field phase diffusion is solved, and the exact linear analytical solutions of the three-level cascade system with the driving field having the phase fluctuation in the steady state are obtained. The dependence of LWI gain, dispersion and the populations on the probe field detuning and the strengths of coherent pumping, respectively, are examined and simulated numerically. It was shown from the results of numerical calculation about the steady-state solutions of three-level cascade system that the gain of lasing without population inversion will be decreased due to driving-field phase-fluctuation The effect ran he compensated by increased driving-field intensities. However, the effect that LWI gain is decreased due to driving- field phase-fluctuation can not be always compensated by increased driving field Rabi frequency. Lasing without population inversion is still obtained even if the linewidth due to driving-field phase-fluctuation is large enough. The presence of the linewidth prevents the cascade system from obtaining a high refractive index along with zero absorption. The cascade system can still exhibit a larger refractive index and zero absorption at the lesser linewidth. The linewidth tends to destroy lasing without population inversion and refractive index enhancement. There is no population inversion for the lasing transition and for the driving transition under the given condition. And the condition without population inversion has nothing to do with the variety of linewidth in the steady-state analytical solutions. That is to say, there is not possibility for the cascade system that a change from lasing with population inversion to lasing without population inversion can occur with the linewidth increasing or with the Rabi frequency of driving-field increasing. The conclusion is very different from that obtained in other inversionless lasing system.
The steady-state behaviour for a Λ-type, three-level atomic system has been analysed by taking into account the effect of the phase fluctuation. It is shown that, for a monochromatic driving field, even if the probe field is in resonance with the corresponding atomic transition, a large refraction index can still be generated; for a finite linewidth of the driving field, inversionless lasing and index enhancement tends to disappear. Furthermore, the linewidth prevents the transparency of the medium.