Coherent-state optical amplifier: a proposal

A new kind of phase-insensitive optical amplifier is proposed whose output is in the coherent state (ideal laser light) if the input is in the coherent state. In-phase and quadrature modulations are preserved in absolute values. Both modulations can be tapped off. The amplifier employs conventional optical amplifiers, electrical feedback, and all-pass filters. Remarkably, these properties hold when either linear or nonlinear optical amplifiers are employed.