An optical device for a biological or chemical sensor includes: an absorption cavity configured to receive a biological or chemical medium; an injection waveguide to inject an analysis light beam into the absorption cavity; and an extraction waveguide to extract a measurement light beam, corresponding to the analysis light beam after transit through the absorption cavity. The absorption cavity has a shape of a right cylinder with an elliptical base. One end of the injection waveguide is placed at a first focus of the ellipse and one and of the extraction waveguide is placed at a second focus of the ellipse. A core of the injection waveguide and a core of the extraction waveguide each have a tip-shaped end, with a constant height in a plane parallel to the generatrix of the right cylinder and a tip-shaped cross section in planes parallel to the base of the right cylinder.