Abstract

We consider a single mode laser with a Raman gain medium, described by a large number of independent 3-level atoms in a lambda - configuration. In the limit of a good cavity, one can adiabatically eliminate the atoms and obtain a Fokker-Planck equation for the generalized P-representation of the laser field. Sufficiently above threshold we can solve this Fokker-Planck equation analytically and obtain explicit expressions for the stationary field distribution, the Mandel Q parameter and the phase diffusion rate. We find an optimum Mandel Q parameter of Q~-1/2 over a large range of parameters. This behaviour is demonstrated in Fig. 1, where we plot Q as a function of the pump field strength R and the intracavity intensity. Of course this is connected with a strong suppression of the low frequency quantum fluctuations in the laser output. A possible physical origin of this intensity noise reduction can be found, if one looks at the atomic level scheme in a so called dressed state picture. In this picture the two lasing levels are split into a Autler- Townes doublets and the upper lasing level is shifted out of resonance with the pump light. The magnitude of this shift, which determins the efficiency of the pumplight absorption, strongly depends on the intracavity laser intensity. Hence we have a dynamic feedback mechanism between laser intensity and pumping, which tends to reduce the intracavity laser intensity fluctuations.

© 1992 IQEC