All-fiber 90 degrees optical hybrid for coherent communications

A 90 degrees optical hybrid is a four-port optical device with two inputs and two outputs. The two output signals are proportional to E(1) + E(2) and E(1) + E(2) exp(j90 degrees ), respectively, where E(1) and E(2) are the complex amplitudes of the two input signals. The 90 degrees hybrids are needed in many applications, including homodyne optical receivers in both phase-locked and phase-diversity configurations. In this paper, the principle of operation of an all-fiber 90 degrees hybrid is described, and an experimental unit using connectorized in-line single-mode fiber components is reported.