  We discuss one possible method for suppression of the nonlinear effect of a
parametric oscillatory instability in a Fabry-Perot cavity of the next
generation of gravitational wave detectors by determination of optimal radii of
curvature of interferometers' mirrors. Varying the radii of curvature of the
mirrors we can change the frequencies of the optical modes of the Fabry-Perot
resonators and distributions of optical fields in optical modes, which, in
turn, can change the number of unstable modes, leading to parametric
oscillatory instability. The determination of the optimal values of the radii
of curvature of mirrors to reduce the number of unstable modes by thermal
lensing is fulfilled.
