The present invention concerns an optical sensor located on a preferred azimuthal portion of the side of an optical fiber. An optical fiber having two opposite ends, a core and a cladding and at least one sensing area is disclosed. Each of the sensing areas is located between the two opposite ends and each has a longitudinal, a radial and an azimuthal portion of the fiber that has been removed and replaced by a sensing material. The sensing collected light is thus representative of a parameter to be measured, such as temperature. Placing the sensing material in a lateral side portion of the fiber increases the sensitivity of the sensor, particularly when it comes to contact temperature measurement in vivo.