High-Speed Optical Digital-to-Analog Converter Operation of Compact Two-Segment All-Silicon Mach–Zehnder Modulator

An optical digital-to-analog converter (DAC) is a promising solution for the efficient generation of multilevel modulation signals without a high-speed electrical DAC and a large swing linear driver. We report the design and characterization of an all-Si segmented Mach-Zehnder modulator for an optical DAC transmitter. The modulator comprises a forward-biased positive intrinsic negative (PIN) phase shifter integrated with a passive resistance and capacitance (RC) equalizer (PIN-RC) to optimize a tradeoff between modulation efficiency and analog bandwidth. We successfully confirmed an eye diagram of 90-Gbaud NRZ signal owing to a wide 3-dB bandwidth of 43.9 GHz. Additionally, a 70-Gbaud four-level pulse amplitude modulation (PAM4) waveform can be obtained via the optical DAC operation using a lumped segmented modulator because of its broad electro-optic bandwidth and uniform characteristics. A two-segmented modulator has a compact footprint of 300 μm × 600 μm. Error-free performances based on the patterns of the PRBS11 of NRZ signals were obtained at 50-70 Gbaud. The bit-error-rates of 60-Gbaud PAM4 and 70-Gbaud PAM4 were less than the 7% hard and 25.5% soft decision forward error correction limits, respectively. The optical DAC transmitter using an all-Si segmented PIN-RC modulator is suitable for next-generation high-speed multilevel transmitters.