The present disclosure relates to optical systems and methods for their manufacture. An example optical system includes a first substrate having a mounting surface and a spacer structure having at least one cavity. The spacer structure is coupled to the mounting surface of the first substrate. The optical system also includes a light-emitter device that is coupled to the spacer structure and a detector device coupled to the first substrate such that the at least one detector device is disposed within the at least one cavity of the spacer structure. The optical system also includes a second substrate that mounts a lens and a waveguide and is coupled to the spacer structure. The optical system also includes a shim coupled between the second surface of the spacer structure and a mounting surface of the second substrate.