Sound amplification by stimulated emission of radiation zero on the average. Nevertheless, if the active medium is located in a resonator, then a standing mode can be excited in it. Particles then bunch under the action of the acoustic radiation forces. In this case, the oscillations of the bubbles are self-synchronized and the useful mode amplifies. The similarity of this with the Free-electron laser is useful to understand the theoretical concepts of the scheme. In a FEL, electrons move through magnetic periodic systems producing electromagnetic radiation. The radiation of the electrons is initially incoherent but then on account of the interaction with the useful electromagnetic wave they
Sound amplification by stimulated emission of radiation introduced around 1995 The pumping is created by mechanical oscillations of a cylindrical resonator and the phase bunching of bubbles is realized by acoustic radiation forces. A notable fact is that gas bubbles can only oscillate under an external action, but not spontaneously. According to other proposed schemes, the electrostriction oscillations of the dispersed particle volumes in the cylindrical resonator are realized by an alternating electromagnetic field. However, a SASER scheme with an alternating electric field as the pump has a limitation. A very large amplitude of electric field (up to tens of kV/cm) is required to realize the amplification.