Structures for a photonics chip, testing methods for a photonics chip, and methods of forming a structure for a photonics chip. A photonics chip includes a first waveguide, a second waveguide, an optical tap coupling the first waveguide to the second waveguide, and a photodetector coupled to the second waveguide. A laser is attached to the photonics chip. The laser is configured to generate laser light directed by the first waveguide to the optical tap.