New stimulated brillouin laser with an induced cavity

A paradoxical decrease in the generation threshold of high Stokes and anti-Stokes components of stimulated Brillouin scattering in water has been detected for the first time to our knowledge upon the displacement of the caustic of a focused laser beam from the bulk of water to air. The mechanism of generation of components caused by the four-wave mixing on counterpropagating beams when the water surface serves as the mirror of an induced cavity of a stimulated Brillouin laser is discussed. An upper bound $R\ll0.02$ has been obtained for the reflection coefficient $R$ of distributed feedback at stimulated Brillouin scattering in water.