It is described a wavelength selective grating-based filter (100), comprising a planar waveguide (4, 4a, 4b) including a core (2) surrounded by a cladding (1), the cladding (1) including a lower cladding (5), the- core (2) being placed above the lower cladding (5), a lateral cladding (7) adjacent to a first and a second opposite lateral sides (13,14) of the core (2), and an upper cladding (6), said upper cladding ( 6) being positioned above said core (2) and lateral cladding (7). The waveguide (4) also comprises a grating structure including a first and a second plurality (8, 9) of grating trenches (11) formed in the lateral cladding (7) in proximity of the first and second opposite lateral sides (13,14) of the core (2), respectively. The first and second (8,9). plurality of grating trenches (11) are covered by the upper cladding (6).
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