A method includes forming a stack of semiconductor die. The stack includes a first semiconductor die, a second semiconductor die and a third semiconductor die. The first semiconductor die is stacked above the second semiconductor die and the third semiconductor die is stacked above the first semiconductor die. A first optical transmitter and a first optical receiver are provided in the first semiconductor die, a second optical transmitter is provided in the second semiconductor die, and a second optical receiver is provided in the third semiconductor die. A first optical signal is transmitted from the first optical transmitter in the first semiconductor die to the second optical receiver in the third semiconductor die. A second optical signal is transmitted from the second optical transmitter in the second semiconductor die to the first optical receiver in the first semiconductor die.