Statistical-noise properties of an optical amplifier utilizing two-beam coupling in atomic-potassium vapor

We have measured the gain and statistical-noise properties of a weak probe beam amplified through two-beam coupling in atomic-potassium vapor for both the Rabi and stimulated Rayleigh gain features. The probe-beam gain was observed to be as large as 125 for the Rabi feature, whereas the gain was typically less than 2 for the Rayleigh feature. The rms noise at the Rabi gain feature was approximately three times greater than the ideal amplifier quantum-noise limit. For the Rayleigh gain feature, the rms noise was 12 to 15 times greater than the ideal amplifier quantum-noise limit. We present a fully quantum-mechanical theory of two-beam coupling in a system of two-level atoms which includes the effects of atomic motion and pump-beam absorption. The predictions of this theory are in good agreement with the experimental data.