Active controlled fiber optical 90 degrees hybrid for coherent communications

A description is given of a lossless, active controlled fiber-optic 90 degrees -hybrid coupler consisting of four fiber couplers for use in a phase-diversity receiver. The four output signals are proportional to E/sub 1/+or-E/sub 2/ and E/sub 1/+or-E/sub 2/*exp(j90 degrees ), where E/sub 1/ and E/sub 2/ are the complex amplitudes of the two input signals. The hybrid comprising four 3-dB monomode fiber couplers and two phase shifters was designed for the 1.5- mu m wavelength range. The principle of operation is described and experimental results are presented. The maximum phase error is smaller than 0.5.>