A Compact Silicon Coherent Receiver Without Waveguide Crossing

Abstract

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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), 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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> 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 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> because of the nonideal behavior of the device. Reasons for the performance limitations are discussed.