Remote System for Detection of Low-Levels of Methane Based on Photonic Crystal Fibres and Wavelength Modulation Spectroscopy. - Joel Pedro Carvalho; Hartmut Lehmann; Hartmut Bartelt; Filipe Magalhães; Rodrigo Amezcua Correa; José Luís Santos; Jan Van Roosbroeck; Francisco M. Araújo; Luís Alberto Ferreira; J. C. Knight (J. Sensors, 2009)