Birch, R.D.: 'Fabrication and characterisation of circularly birefringent helical fibres', Electronics Letters, 1987, 23, (1), p. 50-52, DOI: 10.1049/el:19870037
IET Digital Library, https://digital-library.theiet.org/;jsessionid=84io8bj9mgdjt.x-iet-live-01content/journals/10.1049/el_19870037