An Optical 2×4 90° Hybrid Based on a Birefringent Crystal for a Coherent Receiver in a Free-Space Optical Communication System

The design and fabrication of an optical 2×4 90° hybrid based on birefringent crystals for a coherent receiver in a free−space optical communication system are presented. For the quadrature receiver, two pairs of 180° phase−shift outputs are obtained and one pair has a phase difference of 90° with respect to the other. The 90° hybrid comprises two pairs of stacked birefringent plates, a phase retardation plate and an analyser birefringent plate. The testing results measured by the heterodyne method verify that the 2×4 optical 90° hybrid can work correctly and effectively. The phase compensation and further optimization schemes are also proposed.