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

Abstract

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.

Accurate Power-Efficient Format-Scalable Multi-Parallel Optical Digital-to-Analogue Conversion

“…In the case of an SEMZM type of oDAC, s labels each of the modulation segments (thus in SEMZM, a gate is a modulation segment), consisting of two partial electrodes, push-pull driven just like the top|bottom electrodes of an MZM in (4). Evidently the segments of an SEMZM are not full 1-bit MZM gates.…”

“…Rather, each segment is a dual-drive Phase Modulator (PM) (essentially a dual-drive MZM without end couplers). The top|bottom PMs are driven per 'adopted push-pull' scheme (4).…”

“…In contrast, in our proposed bias scheme (7) for MZM-based IGs, the drives are push-pull unipolar, and the MZM arms we are not statically biased to differ in phase, but rather the two MZM arms are symmetric (in the absence of applied drive waveforms) in the sense that they have the same static optical lengths. We henceforth refer to our IG phase-bias&drive scheme (4) and (5), as Symmetric Bias (SB), to have it distinguished from conventional Quadrature Bias (QB). In the Appendix A section we compare the two MZM IG schemes, proving their functional equivalence, and considering additional engineering aspects of the electrical drives for IGs and PGs.…”

“…We conclude that a common physical realization, based on a pair of parallel MZMs, with a 50:50 splitter and a variable (tunable) combiner set to either 1 3 : 2 3 or 1 5 : 4 5 respectively, may dually serve both 2MP DD and 2MP COH generation of PAM4 or bipolar PAM4. This is an example of reconfigurability of the PAM4 MPMZM (see more on the optical reprogramming topic in Section 8)…”

A Novel Silicon Forward-Biased PIN Mach–Zehnder Modulator with Two Operating States

“…Although using pre-emphasis signals is an effective method to improve the high frequency of the PIN modulator, it needs an extra digital signal processor (DSP) to generate pre-emphasis signals, which increases the difficulty of driving circuits. In [ 41 ], the E-O bandwidth of the PIN modulator is greatly improved by integrating a passive RC equalizer, which reduced the equivalent capacitance of the circuit. This method does not need to modify the shape of the electronic signal, and no additional power consumption occurs.…”