Group Index of Refraction

Hi, I have been comparing OTDRs by JDSU and Exfo. In the JDSU OTDRs there is a parameter defined in the datasheets known as "Group Index of Refraction" or "Group Refractive Index" or "Group Index Range". I have tried searching it online and in other OTDR manufacturer datasheets but as yet haven't found any success. Can anyone please enlighten me as to what this term means and what is its effect on OTDRs dynamic range and other functionality?