Abstract

We study the nonlinear theory of a laser with an active medium of three-level atoms in the lambda configuration, which are externally driven by a classical electromagnetic field. We study the main features of the laser from a semiclassical and a fully quantum mechanical point of view, assuming general conditions such as off-resonant laser operation and initial atomic preparation. In the case of initial atomic preparation the analysis includes the consideration of phase-locking. We focus our attention on the photon statistics of the laser field, investigating those regions in which the laser field is sub- Poissonian. From previous works^1-5 it is known that a sub-Poissonian laser field can be achieved in externally driven laser systems, for both closed^1,2,4,5 and open³ atomic systems. In those papers reduced intensity fluctuations with a relative variance down to 1/2 and squeezing of up to 50% were found. A more recent work⁵ has found an improvement of this limit, however, by making use of a driving field that itself is sub-Poissonian. In our work we restrict ourselves to classical driving fields and rather explore the full range of atomic parameters, in particular the effects of initial atomic preparation of atoms into a coherent superposition and the detuning of the additionally applied external driving field. With large detunings we can favorably influence the phase-locking condition, favor pure Raman transitions, and reduce spontaneous emission noise. In this physical picture of an open Scully-Lamb laser we found that the laser can be operated in its full nonlinear regime with less population in the upper levels than in the ground state, and at the time we found a strong reduction of photon number fluctuations. For maximal initial coherence between the two lower levels of lambda atoms, the relative variance of the photon number could be even below 1/2, or the Mandel parameter could be below –1/2.

© 1994 IEEE