A device for optical signal processing includes a first layer, a second layer and a waveguiding layer. A lens is disposed within the first layer and adjacent to a surface of the first layer. The second layer is underneath the first layer and adjacent to another surface of the first layer. The waveguiding layer is located underneath the second layer and configured to waveguide a light beam transmitted in the waveguiding layer. A grating coupler is disposed over the waveguiding layer. The lens is configured to receive, from one of the grating coupler or a light-guiding element, the light beam, and focus the light beam towards another one of the light-guiding element or the grating coupler.