Low-loss silica-based 90 deg optical hybrid in c band based on 4 x 4 multimode interference coupler for coherent receiver

A silica-based 90 deg optical hybrid based on a 4 x 4 multimode interference coupler with a refractive index contrast of 0.36% is designed and fabricated for coherent receiver. The measured results of this device show that the common mode rejection ratios are better than 23 dB and excess loss including a coupling loss of -0.3 dB is less than -0.9 dB in C band (1530 to 1565 nm). The phase error is less than 4 deg. The designed low-cost 90 deg optical hybrid is promising for further improving the sensitivity of quadrature phase shift keying coherent receivers.