This paper describes a measurement technique for the characterization of 3 × 3 couplers for use in passive homodyne systems based on phase diversity detection, such as interferometric sensors1 and coherent homodyne communications.2 Ideally, 3×3 fiber couplers (6-port hybrid devices) show a ± 120° phase difference between the outputs and, therefore, contain a linear combination of inphase IP and quadrature Q components of the phase. Phase detection can then be accomplished by deriving 90° phase shifted outputs by appropriately combining (sum/difference) these three output signals. To extract stable outputs representing Q and IP information, the phase offsets and power splitting ratios of the couplers must be stable. If this is not satisfied, phase errors in the phase detection process occur, which, particularly in the case of interferometric sensors, leads to distortion in the output.
© 1989 Optical Society of AmericaPDF Article