  We design an efficient coupler to transmit light from a strip waveguide into
the flatband slow mode of a photonic crystal waveguide with ring-shaped holes.
The coupler is a section of a photonic crystal waveguide with a higher group
velocity, obtained by different ring dimensions. We demonstrate coupling
efficiency in excess of 95% over the 8 nm wavelength range where the photonic
crystal waveguide exhibits a quasi constant group velocity vg = c/37. An
analysis based on the small Fabry-P\'erot resonances in the simulated
transmission spectra is introduced and used for studying the effect of the
coupler length and for evaluating the coupling efficiency in different parts of
the coupler. The mode conversion efficiency within the coupler is more than
99.7% over the wavelength range of interest. The parasitic reflectance in the
coupler, which depends on the propagation constant mismatch between the slow
mode and the coupler mode, is lower than 0.6% within this wavelength range.
