Abstract: An active optical device comprises a packaged optical chip, an optical fiber coupled to the chip to receive an optical output from it, and a photodetector located within the package for receiving radiation light responsive to the amplitude of that optical output. The radiation light is produced by destructive interference in a pair of waveguides that merge together at a chosen small angle and sufficiently smoothly for a substantial part of the light (ideally nearly all of it) to propagate through the substrate of the chip close to the output waveguide and preferably in only two directions (corresponding to a first-order diffraction). The optical fiber is coupled to the chip by means of a fiber support (such as a fiber block or a ferrule) made of transparent material, and the photodetector is positioned to receive the radiation light at least partly through the fiber support and at least partly by reflection from the interior of the package.