Adams, M.J.; Payne, D.N.; Sladen, F.M.E.: 'Mode transit times in near-parabolic-index optical fibres', Electronics Letters, 1975, 11, (16), p. 389-391, DOI: 10.1049/el:19750295
IET Digital Library, https://digital-library.theiet.org/;jsessionid=3q266jnd8kolu.x-iet-live-01content/journals/10.1049/el_19750295