A Ring-Based 25 Gb/s DAC-Less PAM-4 Modulator

A novel PAM-4 12.5 Gbaud/s inter-coupling modulator based on a Mach-Zehnder interferometer assisted ring resonator is demonstrated. The 25 Gb/s PAM-4 optical signal is generated by driving the modulator with two independent electrical signals with an amplitude of 3 V
_pp
at 2 V reverse bias. Reverse bias operation mode of the modulator enables achieving an electro-optic (EO) bandwidth of approximately 14 GHz without preemphasis of the electrical driving signals or equalization techniques. This configuration successfully simplifies the operation of the modulator in generating PAM-4 signals without the need for a digital-to-analog converter (DAC). Experimental results show proper generation of a PRBS data stream. The proposed modulator has a compact footprint of 0.48 mm
²
with a bandwidth density of 52.08 Gb/s/mm
²
.