The present invention relates to optical fibre sensors and, more particularly to a few­ mode optical fibre sensor configured to measure ambient temperature and refractive index (RI) simultaneously. In particular, the present invention is a sensor for measuring ambient refractive index (RI) and temperature simultaneously, comprising: the few-mode optical fibre having a Bragg grating configured to reflect more than one Bragg wavelengths (M0, A1) given by each light propagating mode (LP01, LP11) when incident light strikes the grating, wherein the end of the few-mode fibre, including the inscribed region, is etched to the extent where the reflected wavelengths (M0, A1) given by each mode (LP01, LP11) are able to discriminate RI changes from temperature changes, in which the wavelength shift ( M0, A1) of each mode is in linear relationship to the temperature and refractive index change so that the actual value of the ambient RI and temperature can be calculated based on a matrix algorithm which takes into consideration cross sensitivity factors (K).