A 90° optical hybrid is a four-port optical device with two inputs and two outputs. The two output signals are proportional to E1 + E2 and E1 + E2 exp (j90°), where E1 and E2 are the complex amplitudes of the two input signals. The 90° hybrids are needed in many applications, including homodyne optical receivers in both phase-locked1 and phase-diversity2 configurations. Optical hybrids were previously implemented with discrete optical components.3 An optical hybrid implemented entirely in the single-mode fiber domain may be more attractive for single-mode fiber communication systems, since it is fully compatible with other components, and the high insertion loss and alignment problems associated with conventional bulk-optic components can be avoided. The principle ot operation of an all-fiber 90° hybrid is described, and an experimental unit using connectorlzed in-line single-mode fiber components is reported.
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