Fig. 5: Generation and measurement of quadrature amplitude modulation (QAM) signals with symbol rates of up to 320 GBd using OAWG and OAWM techniques, and comparison to competing approaches.

a Optical spectra of 16QAM signals with symbol rates ranging from 80 GBd to 320 GBd generated using spectrally sliced OAWG and measured using a high-resolution optical spectrum analyzer (AP2060, Apex Technologies, Marcoussis, France, resolution bandwidth 100 MHz). For better comparison, all spectra are normalized to the out-of-band amplified spontaneous emission (ASE) noise level. We indicate the frequencies \(f_{1}, \;f_{2}, \;f_{3}, f_{4}\) of the Tx comb, for which a constant free spectral range (FSR) is maintained throughout the experiment. b Constellation signal-to-noise ratio (CSNR) as a function of the symbol rate for 16QAM (blue dots and solid blue line) and 32QAM signals (red dots and red dashed line)—measured in an optical back-to-back (ob2b) configuration and after transmission over 87 km of single-mode fiber (blue and red cross, see inset). The results are compared to other high-symbol-rate optical signaling experiments that rely on single digital-to-analog converters (DACs)^9,46,47, or photonic-electronic^13,14,30 (circular markers) or purely electronic^{36,37,38,39,40,41,42,43,44,45,48,49,50,51,52} (square markers) multiplexing techniques. References ^40,43,51 demonstrate pulse-amplitude modulation (PAM) signaling, whereas the other publications show QAM signals. References ^41,42,43 and^48,49,50,51 use the commercially available signal generators Keysight M8199A and M8199B, respectively, which rely two time-interleaved DAC channels. Insets: Exemplary constellation diagrams for 16QAM and 32QAM 320 GBd signals and measured bit-error ratio (BER) obtained for the optical back-to-back configuration