Reduction of phase-difference deviation in 90.DEG. optical hybrid over wide wavelength range

We describe a technique for reducing the wavelength-dependent phase-difference deviation between in-phase and quadrature components in an 90° optical hybrid consisting of four 3-dB couplers and a phase shifter. We fabricated the 90° hybrid with our proposed technique, which uses silica-based planar lightwave circuit technology. The fabricated 90° hybrid exhibits a low phase-difference deviation from 90° of less than 0.7° over the C- and L-bands, and will be useful in a digital coherent receiver for transmission systems operating at 100Gbit/s or more.