Quantum noise reduction in a laser by cascade coherent excitation

Abstract

We study the statistical properties of a three-level laserwhere cascade coherent excitation as a two-step process isused as a coherent pump and a laser field is generated on theupper transition. It is shown that the laser intensityfluctuations are suppressed up to 50% below the shot noiselimit and that the laser linewidth is reduced below theSchawlow-Townes formula by more than one order of magnitudeof the laser intensity. By transforming to the picture dressedby the coherent field on the upper transition, we identify atransfer of the lower coherent excitation to be responsiblefor the quantum noise reduction. Cascade coherent excitationturns out to be a new way in which one can create a pathwayfor recycling of active electrons and regularization of laserexcitation process. The conditions for the physicalrealization of the Raman scheme (Ritschet al 1992 Europhys. Lett. 19 7) as a limiting case is discussed.