Gold, M.P.; Hartog, A.H.: 'Ultra-long-range OTDR in single-mode fibres at 1.3 μm', Electronics Letters, 1983, 19, (13), p. 463-464, DOI: 10.1049/el:19830315 IET Digital Library, https://digital-library.theiet.org/;jsessionid=1th4rmaw623cg.x-iet-live-01content/journals/10.1049/el_19830315