Silicon Photonic Ring-Assisted MZI for 50 Gb/s DAC-Less and DSP-Free PAM-4 Transmission

We present the design and characterization of a silicon photonic ring-assisted Mach-Zehnder interferometer for four-level pulse amplitude modulation (PAM-4) short-reach transmission without the use of digital-to-analog converter (DAC) or digital signal processing (DSP). The PAM-4 optical signals are generated by driving two microring modulators with independent 2-level radio frequency signals. The device is operated at 25 Gbaud and the PAM-4 optical signals are transmitted over 2 km of standard single mode fiber. The bit error rate is estimated to be lower than the hard-decision forward error correction threshold of 3.8 × 10 -3 , showing a successful DAC-less and DSP-free PAM-4 transmission at 50 Gb/s.