1 Modification of the refractive index response of long period gratings using thin film overlays Imran M. Ishaq*, Antonio Quintela ***, Stephen W. James*, Geoffrey J. Ashwell**, José M. Lopez-Higuera*** and Ralph P. Tatam* *Optical Sensors Group and ** Nanomaterials Group , Centre for Photonics and Optical Engineering, School of Engineering, Cranfield University, Bedford MK43 0AL, UK *** Grupo de Ingenieria Fotonica-Universidad de Cantabria, E.T.S.I.I. y Telecommunicacion, Santander, Spain Abstract The response of a fibre optic long period grating (LPG) to the surrounding refractive index is shown to be modified by the deposition of a thin film of thickness of order 100nm onto the cladding. The LPG becomes sensitive to surrounding refractive indices greater than that of silica, and exhibits an enhanced sensitivity to external refractive indices lower than that of silica. Sensors and Actuators B: Chemical, Volume 107, Issue 2, 29 June 2005, Pages 738-741.