Spot-size converter for optical mode conversion and coupling between two waveguides

A spot-size converter for coupling light between a first waveguide and a second waveguide respectively supporting a first and a second propagation modes, the spot-size converter extending along a longitudinal waveguiding axis and comprising a transition region that includes:

• a first part of waveguiding structure having a first end and a second end opposed to the first end,
• wherein the first end thereof is coupled to the first waveguide to receive light therefrom or transmit light thereto in the first propagation mode;
• and
• a second part of waveguiding structure having a first end and a second end opposed to the first end,
• wherein the first end thereof is coupled to the second waveguide to transmit light thereto or receive light therefrom in the second propagation mode,
• the second part of waveguiding structure having a plurality of high-index elements arranged in multiple vertically spaced rows and horizontally spaced columns, extending along the longitudinal waveguiding axis at least partially over the first part of waveguiding structure so as to define a low-index region wherein the mode of the first waveguide progressively transforms into the mode of the second waveguide, thereby enabling light propagation via a mode of a combined system of the first and second parts of waveguiding structure.