A Compact Silicon Coherent Receiver Without Waveguide Crossing

A monolithically integrated silicon coherent receiver based on a novel scheme with a crossing-free 900 hybrid optical coupler and two balanced germanium photodetectors is reported. The integrated receiver is compact (footprint is 0.8 × 1.0 mm
²
), and it is demonstrated by working with single-polarization 56-Gb/s quadrature phase-shift keying (QPSK) and 80-Gb/s 16-quadrature amplitude modulation (16-QAM) signals. In particular, QPSK transmission that is back-to-back at 28 Gbaud shows a bit-error rate (BER) below 10
^-4
for an optical signal-to-noise ratio (OSNR) lower than 17 dB, whereas 16QAM at 20 Gbaud shows a BER not better than 3*10
^-2
because of the nonideal behavior of the device. Reasons for the performance limitations are discussed.