CD-tolerant 2-channel OTDM system using alternating-phase pulses and SiP transmitter for data-center networks

In-phase and quadrature (IQ)-interleaved 2-channel optical time-division multiplexing (OTDM) is a promising approach to overcoming transmitter bandwidth limitations in high-speed intensity-modulation and direct-detection systems. However, we show analytically that IQ-interleaved OTDM signals generated with conventional constant-phase pulses suffer from a severe chromatic dispersion (CD)-induced power imbalance between two multiplexed channels, which limits the 200-Gb/s pulse-amplitude modulation 4-level (PAM4) transmission reach to less than 1 km in the C-band. To address this fundamental limitation, we propose to use sinusoidally modulated alternating-phase pulses (with a 50-GHz repetition rate) generated by a null-biased Mach-Zehnder modulator driven by a 25-GHz electrical clock. We demonstrate analytically and through simulation that these pulses are inherently immune to CD-induced power imbalance regardless of the sign of the relative phase between the two return-to-zero (RZ) PAM4 tributaries during IQ combining (ϕ_RZ2 - ϕ_RZ1 = ±90°), and further demonstrate through simulation that the +90° configuration is preferable to the -90° counterpart, since it extends the transmission reach to 6 km. To fully exploit this advantage, we further propose a two-stage equalization scheme in which a half-symbol-spaced symbol-timing recovery block suppresses aliasing before an adaptive channel equalizer that compensates residual inter-symbol interference. The proposed system is experimentally implemented using a silicon photonics transmitter chip. Transmission of 200-Gb/s C-band PAM4 signals is experimentally demonstrated over 1.9 km, validating the feasibility of C-band 2-km data-center networks, while simulation results further show transmission reach of up to 6 km, suggesting that sufficient margin can be secured for future 400-Gb/s/λ operation.