‘Perfect’ PAM4 Serial Digital-Optical Conversion

We develop a ‘perfect’ design for PAM4 Segmented Mach-Zehnder Modulators (SEMZM), featuring a fully linear (uniform) optical DAC (oDAC) staircase (i.e., having equi-spaced PAM4 levels) and no intrinsic modulation loss (the qualifier ‘perfect’ pertains to ideal conditions, barring optical excess losses and other impairments). This is achieved by leveraging our recently introduced maximin optimization strategy, enabling to locate the hitherto unknown optimal operating point in the serial oDAC parameters space, gaining ~7 dB relative to conventional PAM4 designs (typically based on a pair of segment electrodes in 2:1 lengths ratio and using 3 dB modulation backoff in order to compensate for degradation in oDAC linearity). The same photonic design may be reused to generate DAC-free coherent PAM4 (bipolar symmetric field-domain equi-spaced max-full-scale constellations), for efficient I/Q tributaries of 16QAM.