  A Waveguide Port Boundary Condition (WPBC) based on the restriction of the
approximation space is presented in the context of Finite Element Analysis. As
well as reducing the computational domain in the same manner as the traditional
WPBC, the proposed scheme further reduces the degrees of freedom at the
waveguide ports, simplifies the implementation and seamlessly provides
post-processing results such as the reflected power in each waveguide mode. The
boundary condition is thoroughly derived, and numerical examples are used as a
support for the discussion on topics such as the needed number of modes to be
employed at a waveguide port. Finally, a nanograting-based plasmonic sensor is
analysed to illustrate further possibilities of the scheme.
