Optical 90° hybrids for demodulating optical signals with QPSK modulation format

We discuss several kinds of waveguide-based 90° hybrids in terms of operating bandwidth and potential to monolithic integration with balanced photodiodes. Compared with conventional optical 90° hybrids based on a 4 × 4 multimode interference coupler, the optical 90° hybrid we have proposed exhibits much broader operating bandwidth by ~36 nm. In addition, we shows that by using a tapered MMI structure, the device length of the proposed 90° hybrid can be made much shorter by a factor of ~3/5, still covering the C-band spectral range.