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Acta Physica Sinica (Overseas Edition)

Linewidth reduction in an inversionless laser with pump phase fluctuations

Xiangming Hu, PENG JIN-SHENG

Published Feb 1, 1999• 0 citations
Publisher

Abstract

The effect of the pump phase diffusion on the linewidth of a laser without inversion is studied. It is shown that the linewidth at equal intensity may be reduced below that for a coherent driving, enen if the net effect of the pump bandwidth leads to an increase in the linewidth. The physical mechanism is analyzed in the dressed-state picture. Furthermore, the effects of the pump phase noise on the linewidths of an inversionleess laser and a Raman laser are compared with each other.

DOI
10.1088/1004-423x/8/2/008

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The effect of the pump phase diffusion on the linewidth of a laser without inversion is studied. It is shown that the linewidth at equal intensity may be reduced below that for a coherent driving, enen if the net effect of the pump bandwidth leads to an increase in the linewidth. The physical mechanism is analyzed in the dressed-state picture. Furthermore, the effects of the pump phase noise on the linewidths of an inversionleess laser and a Raman laser are compared with each other.
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.
In this paper, a quantum theory of an inversionless laser is presented. The gain medium is considered to consist of three-level atoms in a closed \ensuremath{\Lambda} configuration, with a lower-level microwave coupling. By deriving an equation of motion for the reduced density matrix for the two modes of the field, gain and diffusion are studied. In particular, two mechanisms through which inversionless amplification takes place in the context of this model, namely, the suppression of absorption and mode coupling, are discussed. Stable conditions for inversionless amplification and the attendant phase diffusion are discussed and a comparison is made with conventional, inversion-based lasers. It is shown that lasing without inversion does not necessarily result in a reduced linewidth compared to a conventional laser.
The authors have extended the theory of stimulated Raman scattering to include the effects of laser bandwidths, in both the transient and steady-state regimes. The case of two interacting laser beams, a pump laser and a probe (Stokes) laser, is treated. Using the phase-diffusion model for laser bandwidth, the authors demonstrate that in the absence of dispersion, the forward Raman gain is essentially independent of the laser bandwidths in the high-gain limit, while in the low-gain limit the gain coefficient is inversely proportional to the sum of the bandwidths. It is further shown that when the pump-laser bandwidth is much larger than the linewidth of the Raman medium, the stimulated Stokes output assumes the same spectrum as the pump laser in the high-gain limit. A possible interpretation of these results is discussed assuming a "phase-locking" of the Stokes phase to the fluctuations in the pump-laser phase, due to the nonlinear interaction of the two beams through the Raman medium.