Laser and photodetector coupled by planar waveguides

An optical apparatus comprises: a waveguide substrate; three planar optical waveguides formed on the substrate, each comprising a transmission core and cladding; a laser positioned to launch its optical output to propagate along the first waveguide; a photodetector positioned to receive an optical signal propagating along the second waveguide; and a lateral splitter core formed on the substrate for (i) transferring a first fraction of laser optical output propagating along the first waveguide to the second waveguide, and (ii) transferring a second fraction of the laser optical output propagating along the first waveguide to the third waveguide.