Fig. 3. Up to 200 Gbit s⁻¹ PAM4 signalling.

a Experimental setup for generating PAM4 signals from our fabricated SPH modulator. AWG arbitrary waveform generator. Inset shows the driving electrical PAM4 signals with a peak-to-peak voltage of 1.3 V. b, c Measured optical eye diagrams of PAM4 signals at data rates of 104 Gbit s⁻¹ (b) and 112 Gbit s⁻¹ (c). d Measured BER curves as functions of the received optical power for 88, 104, and 112 Gbit s⁻¹ PAM4 signals. e, f Measured optical eye diagrams of Nyquist PAM4 signals at data rates of 184 Gbit s⁻¹ (e) and 200 Gbit s⁻¹ (f). g Measured BER curves as functions of the received optical power for 160, 184, and 200 Gbit s⁻¹ Nyquist PAM4 signals. The pre-FEC BER thresholds for hard-decision (HD) and KP4 FECs⁴², that is, 3.8 × 10⁻³ and 2.2 × 10⁻⁴, respectively, are plotted for reference.