Optical bistability by an atomic vapor in a focusing Fabry-Perot cavity

We report experimental observations of dispersive- and absorptive-optical bistability due to saturation of the $D_1$ line of atomic sodium in a tight-focused Fabry-Perot cavity. Under the conditions of large optical power broadening and substantial collisional broadening by argon buffer gas used for part of the data, the sodium very nearly behaves as a two-level homogeneously broadened absorber. It thus yields a test of the two-level mean-field optical-bistability model. That model, including homogeneous broadening, fails to account quantitatively for the power levels and power dependencies of the cavity transmission. In particular, this model predicts too rapid an onset of atomic saturation with cavity power. In contrast, predictions of a two-level Gaussian-field optical-bistability model for the dependence of transmitted power as a function of laser power and frequency are confirmed by the experimental data. However, there is a remaining discrepancy in the absolute value of the cavity power.