A method includes placing an electronic die and a photonic die over a carrier, with a back surface of the electronic die and a front surface of the photonic die facing the carrier. The method further includes encapsulating the electronic die and the photonic die in an encapsulant, planarizing the encapsulant until an electrical connector of the electronic die and a conductive feature of the photonic die are revealed, and forming redistribution lines over the encapsulant. The redistribution lines electrically connect the electronic die to the photonic die. An optical coupler is attached to the photonic die. An optical fiber attached to the optical coupler is configured to optically couple to the photonic die.