Light amplification without stimulated emission: Beyond the standard quantum limit to the laser linewidth

The standard quantum limit to the linewidth of a laser for which the gain medium can be adiabatically eliminated is l05k/2n̄ . Here k is the intensity damping rate and n̄ the mean photon number. This contains equal contributions from the loss and gain processes, so that simple arguments which attribute the linewidth wholly to phase noise from spontaneous gain are wrong. I show that an unstimulated gain process actually introduces no phase noise, so that the ultimate quantum limit to the linewidth comes from the loss alone and is equal to lult5k/4n̄ . I investigate a number of physical gain mechanisms which attempt to achieve gain without phase noise: a linear atom-field coupling with a finite interaction time, a nonlinear atom-field coupling, and adiabatic photon transfer using a counterintuitive pulse sequence. The first at best reaches the standard limit l0, the second reaches 3 4 l0, and the third reaches the ultimate limit of lult5 1 2 l0. @S1050-2947~99!03711-7#