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We experimentally demonstrate efficient optical coupling into a slow light photonic crystal waveguide (PCW) that is independent of the group velocity of the guided mode. With a group index taper to match the group velocity between a strip waveguide and a PCW, the optical coupling efficiency is nearly constant throughout the spectrum of the defect-mode, including the slow light region near the band edge. Compared to strip-PCW butt-coupling without a group index taper, our measurement results show a 20 dB enhancement in coupling efficiency with 5 dB less Fabry–Perot fluctuations. The measurements show excellent agreement with two-dimensional finite-difference time domain simulations.