  We present a general approach for coupling a specific mode in a planar
photonic crystal (PC) waveguide to a desired free-space mode. We apply this
approach to a W1 PC waveguide by introducing small index perturbations to
selectively couple a particular transverse mode to an approximately Gaussian,
slowly diverging free space mode. This "perturbative photonic crystal waveguide
coupler" (PPCWC) enables efficient interconversion between selectable
propagating photonic crystal and free space modes with minimal design
perturbations.
