  In this paper, we compute the Morse index for a free boundary minimal
submanifold from data of two simpler problems. The first one is the
corresponding problem with fixed boundary condition; and the second is
associated with the Dirichlet-to-Neumann map for Jacobi fields. As an
application, we show that the Morse index of a free boundary minimal annulus is
equal to 4 if and only if it is the critical catenoid.
