  We propose to excite a large number of coherent magnons with high momentum in
optical cavities. This is achieved by two counterpropagating optical modes that
are detuned by the frequency of a selected magnon, similar to stimulated Raman
scattering. In sub-mm size yttrium iron garnet spheres, a mW laser input power
generates 10^6-10^8 coherent magnons. The large magnon population enhances
Brillouin light scattering, a probe suitable to access their quantum
properties.
