A wavelength-selective filter device (10; 20; 30; 40) for integration into a photonic integrated circuit, the filter device comprising: - a first optical element patterned in or on a substrate (11, 12), the first optical element being configured for receiving radiation incident thereon and for directing at least partially said received radiation into a direction defined by a first angle (α), - a second optical element patterned in or on a substrate (11, 12, 16), the second optical element being a diffractive element and extending longitudinally and arranged distantly to the first optical element, the second optical element being configured for receiving said directed radiation under an angle of incidence and adapted for diffracting said directed radiation under a second angle (β), said second angle depending on wavelength, wherein the first optical element is directing said received radiation into a propagation medium, said directed radiation propagating through the propagation medium until being received at the second optical element, the propagation medium being formed from a material being different from any material of said substrate of said first and said second optical element, wherein the second angle is such that, for a single reference wavelength, the diffracted radiation is directed into said propagation medium for advancing in said propagation medium towards a predetermined position on the first optical element or on a further optical element for filtering radiation having a wavelength substantially matching the reference wavelength from radiation having a substantially different wavelength, whereby the reference wavelength is uniquely determined for the entire device, and characterized in that the second optical element is adapted for back-reflecting into said propagation medium said directed radiation for advancing in said propagation medium along the same elevation angular direction (θ) but with opposite propagation sense as compared to the radiation incident on the second optical element, said elevation angular direction (θ) being defined with respect to a flat surface of the substrate of the second optical element. - a first optical element patterned in or on a substrate (11, 12), the first optical element being configured for receiving radiation incident thereon and for directing at least partially said received radiation into a direction defined by a first angle (α), - a second optical element patterned in or on a substrate (11, 12, 16), the second optical element being a diffractive element and extending longitudinally and arranged distantly to the first optical element, the second optical element being configured for receiving said directed radiation under an angle of incidence and adapted for diffracting said directed radiation under a second angle (β), said second angle depending on wavelength, wherein the first optical element is directing said received radiation into a propagation medium, said directed radiation propagating through the propagation medium until being received at the second optical element, the propagation medium being formed from a material being different from any material of said substrate of said first and said second optical element, wherein the second angle is such that, for a single reference wavelength, the diffracted radiation is directed into said propagation medium for advancing in said propagation medium towards a predetermined position on the first optical element or on a further optical element for filtering radiation having a wavelength substantially matching the reference wavelength from radiation having a substantially different wavelength, whereby the reference wavelength is uniquely determined for the entire device, and characterized in that the second optical element is adapted for back-reflecting into said propagation medium said directed radiation for advancing in said propagation medium along the same elevation angular direction (θ) but with opposite propagation sense as compared to the radiation incident on the second optical element, said elevation angular direction (θ) being defined with respect to a flat surface of the substrate of the second optical element.