The effect of cooperative atomic interactions on laser linewidth is investigated by suitably generalizing the Scully-Lamb theory of the laser. In particular, the single- and two-atom interactions with the field are included. It is shown that, under certain conditions, the cooperative interactions tend to reduce the natural linewidth.
As the laser linewidth arises under the influence of spontaneous emission, it is of considerable interest to find effects that decrease it. In this paper it is considered the effect of cooperative atomic interactions on the linewidth of a single-mode laser. It can be shown by suitably generalizing the Scully-Lamb theory that the cooperative effects tend to decrease the natural linewidth of the laser at high intensities. Considering the interaction of two-two- level atoms with a single-mode quantized field and the effect of cooperative atomic interactions it was obtained an expression for the natural linewidth of the system. Based on the theoretical models we obtain the numerical results for the laser linewidth as a function of the cooperation parameter. We then present a set of graphs by suitably plotting these data and we discuss the results.
The question of how the light output of a laser and its fluctuations depend on the optical-cavity length at a given frequency is investigated theoretically for a laser oscillating in a single Gaussian mode. The treatment is quantum mechanical, and is based on the Scully-Lamb laser model, except that a perturbation expansion is used and possible cooperative atomic effects are included. It is shown, with the help of some reasonable approximations, that the probability distribution of the photon-occupation number can be cast into a form that is similar to the Scully-Lamb formula, but with coefficients that depend on cavity length in a more complicated way. Curves are presented that illustrate the behavi\`or and should lend themselves to experimental test.
The effect of counter-rotating terms in the interaction Hamiltonian on the photon statistics is investigated for a single-mode laser. The treatment is based on the Scully-Lamb model. A perturbation expansion is used to derive an equation of motion for the field-density matrix. The counter-rotating terms are found to lead to both transient and secular effects. With some reasonable approximations the steady-state photon-number distribution is derived and it is shown that the counter-rotating terms tend to raise the threshold of laser action and broaden the photon-number distribution. The relative size of these effects is found to be very small and depends on the square of the ratio of the atomic linewidth and the transition frequency.
Thomas et al. (1981, 1982) proposed the generation of cross correlation between two laser fields interacting with a three-level system as a means of reducing noise and subsequently exploited this property in the observation of very narrow Ramsey fringes. Cross correlation has been discussed theoretically by Dalton and Knight (1982, 1983) and shown to have interesting effects in population trapping. For such effects to be important, the cross correlation coefficient must be as large as possible. The authors discuss the degree of correlation between the two modes of a two-mode laser using the approach of Scully and Lamb (1967, 1968) and show it can be large. They also evaluate the linewidths of the two laser modes. They find that if the laser parameters for the two modes are equal, the two-mode linewidth is one half the value of the linewidth of the corresponding single-mode laser, well above threshold.