Abstract: An optical device includes a waveguide, the waveguide having a core surrounded by a cladding, the cladding including a lower cladding, the core being placed above the lower cladding, a lateral cladding adjacent to a first and a second opposite lateral sides of the core, and an over-cladding, the over-cladding being positioned above the core and lateral cladding. The core and lateral cladding define a guiding layer. A grating structure is formed in the guiding layer, which includes a plurality of empty trenches. The over-cladding includes a cap layer, the cap layer having a first refractive index and being in contact with the grating structure. The first refractive index of the cap layer is substantially identical to the refractive index of the lateral cladding in contact with the cap layer. Additionally, the cap layer is located above the trenches and forms bridges connecting each couple of adjacent trenches of the plurality, so that voids are formed in the trenches.