An incoming DPSK optical signal is first split into two beams with equal intensities, in which one beam is delayed in space by an optical path difference that introduces a time delay corresponding to one bit. The two beams in the two paths are then coherently recombined to interfere each other constructively or destructively.
An incoming DPSK signal is initially divided into two beams of equal strength, where one beam experiences a delay due to an optical path difference that equates to a time interval corresponding to one bit. The two beams are then recombined coherently to produce either constructive or destructive interference.