An optical device includes a light-transmitting medium on a base. The light-transmitting medium at least partially defines a free propagation region through which light signals travel. A reflective grating is positioned such that light signals can travel through the free propagation region and be received by the optical grating. The optical grating is configured to reflect the received light signal back into the free propagation region. The optical grating reflects the light signals such that light signals associated with different wavelengths separate as the light signals travel through the free propagation region. The portion of the light-transmitting medium that defines the free propagation region has a facet through with the light signals are transmitted. The grating includes a buffer layer between the facet and a reflecting layer that is configured to reflect the light signals received by the grating.