## 5 Experimental Results and Discussion  
### Transmission Performance with Single-Arm Driven Mode  
Figure 6a,b displays the received signal spectra with differential driven mode and single-arm driven mode after 20 km SSMF transmission, respectively. Note that the signal spectra in Figure 6 were normalized to keep the scale consistent with theoretical curves in Figure 2 as a validation. The normalization operation can also provide a fair comparison among different bias voltages. As shown in Figure 6a, the signal spectrum at BTB (dashed black curve) was plotted as reference. One can observe that, after transmission, the signal spectra fell around 14 GHz, resulting in severe ISI. Almost the same spectra were obtained for all the bias voltage, varying from 4.0 V to 5.2 V, which fits well with Equation (4). On the other hand, the spectra were quite different with single-arm driven mode. Note that low-arm single driven mode with bias around \(3\pi/4\) is utilized in Figure 6b. Similar to Figure 2d, larger bias voltage leads to a larger bandwidth at the cost of suppressing the frequency components around zero. By using single-arm driven mode, the 3-dB  
Figure 5: (**a**) Experimental setup. ECL: external cavity laser; AWG: arbitrary waveform generator; EA: electrical amplifier; SSMF: standard single-mode fiber; EDFA: erbium-doped fiber amplifier; VOA: variable optical attenuator; PD: photodiode; DSO: digital storage oscilloscope; Tx: transmitter; Rc: receiver. (**b**) transmitter- and (**c**) receiver-side DSP stacks. RRC: root raised cosine; FFE: feed-forward equalizer; VNE: Volterra nonlinear equalizer. bandwidth achieved larger than 16 GHz, which was closer to the Nyquist bandwidth of the 34 Gbaud signal. We then quantitatively investigated the BER performance of 34 Gbaud PAM-6 and PAM-8 signals after transmission as a function of bias voltage, as in Figure 7a,b, respectively. For differential driven mode, the BERs of PAM-6/8 deteriorated to larger than 0.1, mainly due to the frequency null inside signal bandwidth. Even though VNE was applied, the BERs could hardly be brought down to below 0.1. It can be explained that the structures of both FFE and VNE were only capable of addressing frequency poles, rather than zeros. For single-arm driven mode, the BERs of PAM-6 and PAM-8 were successfully reduced to 2.78 \(\times\) 10\({}^{-2}\) and 6.08 \(\times\) 10\({}^{-2}\) with FFE only, at the optimal bias voltage of 5.7 V. Further improvement can be obtained by applying VNE for SSBI mitigation. In this case, the BERs of PAM-6 and PAM-8 were 1.41 \(\times\) 10\({}^{-3}\) and 1.37 \(\times\) 10\({}^{-2}\), which were lower than the 7% and 20% HD-FEC thresholds, respectively. The optimal bias voltage with VNE was smaller than with FFE, because a higher carrier component was needed to suppress the influence of SSBI for FFE. Figure 6: Received signal spectra versus bias voltage with (**a**) differential driven mode and (**b**) single-arm driven mode after 20 km SSMF transmission. Figure 7: Measured BER versus bias voltage for (**a**) PAM-6 and (**b**) PAM-8 signals after 20 km SSMF transmission with differential and single-arm driven mode. Measured ROP sensitivities of (**c**) PAM-6 and (**d**) PAM-8 signals after 20 km SSMF transmission and (**e**) PAM-6 and (**f**) PAM-8 signals at BTB, respectively. Figure 7c,d depicts the ROP sensitivities of PAM-6 and PAM-8 signals after 20 km SSMF transmission, respectively. The optimized bias voltages of 4.3 V and 5.4 V were chosen for differential and single-arm driven mode, respectively. At 7% and 20% HD-FEC thresholds, the ROP sensitivities were \(-3.7\) dBm and 0.2 dBm for PAM-6 and PAM-8 signals. We also measured the ROP penalty of utilizing the single-arm driven mode at the BTB scenario, as in Figure 7e,f. Since bias voltage can be easily adjusted in practical implementation, the curves were tested under optimal voltage of 4.3 V, which was approximately the quadrature point.