Geodesic deviation in a nonlinear gravitational wave spacetime
  The tidal effects generated by a nonlinear gravitational wave are investigated in double-null v - u coordinates, as an exact solution of Einstein's field equations. The components $\xi^{v}$ and $\xi^{u}$ of the separation vector behave as in flat space but the transversal components $\xi^{x}$ and $\xi^{y}$ depend nonlinearly on $v$ through the Bessel and Neumann functions, far from the null surface $v = 0$. We show that the same results are obtained by means of the tetrad formalism. 
Hristu Culetu
gr-qc hep-th