Photodiode with improved power absorption

A photodiode for detecting an optical signal, comprising:

• a substrate that extends mainly in a plane,
• first and second contact terminals,
• first and second waveguides that have, over their entire length, different effective propagation indices, each of these waveguides including a core surrounded by a cladding, the optical signal propagating essentially inside the core, the cores of the first and second waveguides each extending mainly parallel to the plane of the substrate and being coupled optically to one another, the core of the second waveguide being made from germanium or from SiGe so as to transform the optical signal into electrical charges,
• first and second oppositely doped regions that electrically connect the core of the second waveguide, respectively, to the first and second contact terminals, such that the magnitude of the current between the first and second contact terminals varies depending on the power of the optical signal absorbed by the core of the second waveguide,
• wherein:
• the core of the first waveguide terminates in a tapered termination that extends parallel to the plane of the substrate,
• the cores of the first and second waveguides are separated by a low-index layer to form a stacked structure, with the core of the first waveguide being above or below the core of the second waveguide,
• the photodiode includes a matching strip that extends opposite the tapered termination on one side and opposite the core of the second waveguide on the opposite side, the matching strip being coupled optically to the core of the second waveguide by an evanescent coupling and including a first zone inside which its effective propagation index is equal to the effective propagation index of a second zone of the tapered termination situated opposite the first zone, these first and second zones thus optically coupling the tapered termination to the matching strip through a modal coupling, and
• the low-index layer extends parallel to the plane of the substrate between the matching strip and the tapered termination, the low-index layer being made from a material whose refractive index is less than 0.8nmin, where nmin is the value of the smallest refractive index chosen from the group consisting of the refractive indices of the materials used to form the cores of the first and second waveguides, respectively, the refractive indices being measured, in the same conditions, at the wavelength λ of the optical signal.