Comment on "Measurement of the phase refractive index of a photonic crystal fiber mode".

In a recent Letter [Opt. Lett.43, 2571 (2018)OPLEDP0146-959210.1364/OL.43.002571], Vengelis et al. have claimed to implement a new experimental technique for measuring the phase refractive index of a photonic crystal fiber mode. Unfortunately, while the experimental effort is appreciated, one of the key assumptions was inaccurate. Taking this error into account reveals that the measured value was the fiber's group index, rather than its phase refractive index.