Optical sensor suitable for pressure measurements comprising a sensor head, said sensor head including: - a diaphragm having a first surface in contact with a fluid of which the pressure has to be measured and a second surface opposite to the first surface, - a body with a tubular cavity, - means (92, 10) arranged inside the tubular cavity, associated with the second surface of the diaphragm and movable longitudinally inside said tubular cavity in response to the deformation of the diaphragm, said means including a reflective surface (92, 102), - an end part of waveguide means arranged inside the tubular cavity and having the end surface faced to and not in contact with the reflective surface (92, 102) of said means, said waveguide means being connected to a light source and a receiver respectively to send to said means the light beam deriving from the light source and to collect at the receiver the light beam reflected from the reflective surface of said means, the receiver is adapted to detect the the intensity of the collected light beam being dependent on the distance between the end surface of the end part of the waveguide means and the reflective surface, characterized in that: the end surface of the end part of the waveguide means is tilted with respect to a plane orthogonal to the optic axis of a first angle (α) having a value such that the incident angle (θ a) of the light beam deriving from the light source is less than the critical angle (θ c-air) between the waveguide means and the air and greater than the critical angle (θ c) of the waveguide means, and the reflective surface (102, 92) of said means is tilted with respect said plane orthogonal to the optic axis of a second angle (β) equal to the escaping angle of the light beam from said end surface of the end part of the waveguide means. - a diaphragm having a first surface in contact with a fluid of which the pressure has to be measured and a second surface opposite to the first surface, - a body with a tubular cavity, - means (92, 10) arranged inside the tubular cavity, associated with the second surface of the diaphragm and movable longitudinally inside said tubular cavity in response to the deformation of the diaphragm, said means including a reflective surface (92, 102), - an end part of waveguide means arranged inside the tubular cavity and having the end surface faced to and not in contact with the reflective surface (92, 102) of said means, said waveguide means being connected to a light source and a receiver respectively to send to said means the light beam deriving from the light source and to collect at the receiver the light beam reflected from the reflective surface of said means, the receiver is adapted to detect the the intensity of the collected light beam being dependent on the distance between the end surface of the end part of the waveguide means and the reflective surface, characterized in that: the end surface of the end part of the waveguide means is tilted with respect to a plane orthogonal to the optic axis of a first angle (α) having a value such that the incident angle (θ a) of the light beam deriving from the light source is less than the critical angle (θ c-air) between the waveguide means and the air and greater than the critical angle (θ c) of the waveguide means, and the reflective surface (102, 92) of said means is tilted with respect said plane orthogonal to the optic axis of a second angle (β) equal to the escaping angle of the light beam from said end surface of the end part of the waveguide means. the end surface of the end part of the waveguide means is tilted with respect to a plane orthogonal to the optic axis of a first angle (α) having a value such that the incident angle (θ a) of the light beam deriving from the light source is less than the critical angle (θ c-air) between the waveguide means and the air and greater than the critical angle (θ c) of the waveguide means, and the reflective surface (102, 92) of said means is tilted with respect said plane orthogonal to the optic axis of a second angle (β) equal to the escaping angle of the light beam from said end surface of the end part of the waveguide means.