Options, feasibility, and availability of 2x4 90° hybrids for coherent optical systems

Coherent optical transmission systems offer enhanced possibilities concerning multilevel modulation and the electronic compensation of transmission impairments. One of the key components in a homodyne in-phase/quadrature (IQ) receiver is a 2 × 4 90° hybrid. However, this component is only commercially available in a form that requires an additional active control loop. This paper presents the different possible alternatives for the realization of the 2 × 4 90° hybrids and discusses their availability and properties, as well as the practical implementation considerations.