  We present a novel method for detecting nonlinearities, due to quantum
electrodynamics through photon-photon scattering, in Maxwell's equation. The
photon-photon scattering gives rise to self-interaction terms, which are
similar to the nonlinearities due to the polarisation in nonlinear optics.
These self-interaction terms vanish in the limit of parallel propagating waves,
but if instead of parallel propagating waves the modes generated in wavesguides
are used, there will be a non-zero total effect. Based on this idea, we
calculate the nonlinear excitation of new modes and estimate the strength of
this effect. Furthermore, we suggest a principal experimental setup.
