Excess noise imparted on a laser beam after it propagates through atomic-potassium vapor

Abstract

We have observed that a single laser beam that has propagated through an atomic potassium vapor can acquire excess noise. In particular, we find that even if the incident laser beam is shot noise limited, the transmitted laser beam contains noise in excess of its shot noise limit. We interpret this observation in terms of the amplification of vacuum sidemodes of the incident laser beam through forward four-wave mixing processes. These amplified sidebands then beat with the incident laser beam at the photodetector to create excess noise. The excess noise is maximum when the laser frequency is detuned from the potassium resonance by approximately one linewidth on either side of the resonance. We are presently developing a quantitative theoretical model to describe the process that produces this excess noise. We believe that this excess noise can degrade the noise properties of any nonlinear optical interaction that utilizes atomic vapors.