Excess noise acquired by a laser beam after propagating through an atomic-potassium vapor

We have found that an intense shot-noise-limited laser beam tuned near the 4 ${}_{}{}^2{}_{}{}^{}$${\mathit{S}}_{1/2}$⇆4 ${}_{}{}^2{}_{}{}^{}$${\mathit{P}}_{3/2}$ potassium resonance transition acquires excess noise after passing through an atomic-potassium vapor cell. The noise is maximum for laser detunings of approximately ±1 GHz and falls to nearly the shot-noise limit for detunings greater than ±3 GHz. We describe the production of this noise in terms of a forward four-wave mixing process involving the laser field and its side modes, which are initially in the vacuum state. We present a fully quantum-mechanical theory of forward four-wave mixing in a system of two-level atoms and use it to predict the noise properties of the transmitted laser beam. The predictions of this theory are in good agreement with the experimental data.