# MZM Optimization of PAM-4 Transmission in Data Center Interconnect
## 4 Lab Measurements
### Experimental Results  
In Figure 4a, three BER curves vs. OSNR are presented, each one associated with different MZM bias points. The optimization was performed by choosing the optimal modulator bias which obtains the lowest BER at OSNR of 38 dB. The '+' (blue), '*' (green) and 'o' (red) curves show the performance of quadrature point of 5 V bias (PAM-4 levels are equally spaced), and uneven PAM bias points of 4.5 V and 4.08 V, respectively. The performance improvement of the '*' (green) and the 'o' (red) curves is associated with the uneven PAM transmission and the increase of OMAouter, by using MZM bias points below quadrature. The OMAouter is defined as the difference between the power of level '3' and the power of level '0'. Since the booster EDFA output (Paverage) is configured to a fixed output optical average power of +6 dBm, the reduction of MZM bias reduces the average power at the EDFA input which increases the EDFA gain and in turn, increases the OMAouter at the EDFA output. The OMAouters of the curves marked with '+' (blue), '*' (green) and 'o' (red) shown in Figure 4a are 650 \(\upmu\)W, 780 \(\upmu\)W and 840 \(\upmu\)W, respectively. Consequently, the OSNR improvement of the '*' (green) curve compared to the '+' (blue) curve (quadrature bias point) due to higher OMAouter is roughly 0.8 dB (10\(\cdot\)log (780/650)). On the other hand, Figure 4a reveals that the overall OSNR sensitivity improvement of the '*' (green) curve compared to the '+' (blue) curve is 3 dB (at BER of 1e-3). The additional OSNR sensitivity improvement of 2.2 dB can be explained by the uneven PAM modulation in the case of the '*' (green) curve conditions, contributing better immunity to the multiplicative signal-spontaneous (signal dependent) noise at the higher PAM levels, leading to additional OSNR performance improvements. Similarly, the signal improvement of the 'o' (red) curve vs. the '+' (blue) curve due to higher OMAouter is roughly 1.1 dB (10\(\cdot\)log (840/650)), while the overall OSNR sensitivity improvement of the red curves compared to the '+' (blue) curve is 5.3 dB. Again, this can be explained by the uneven PAM transmission which leads to additional 4.2 dB of OSNR sensitivity improvement. Also observed in Figure 4a is the fact that in the case of bias of 4.08 V, the BER reduces slowly at large OSNR values (larger than 41 dB). This is due to the fact that under large OSNR conditions, while operating the MZM at low bias point, the dominant BER limiting factor becomes the nonlinearly of the transmitted signal. On the other hand, under lower OSNR conditions, the multiplicative ASE noise dominates, and the proposed uneven PAM scheme outperforms the \"classical\" MZM operation at its quadrature point. Figure 4b presents two BER curves vs. OSNR, where each one is related to a different MZM bias point and a different MZM input voltage swing, such that both curves introduce the same OMAouter of 650 \(\upmu\)W. It is shown that the bias point of 4.08 V and 800 mVpp voltage swing (green curve '*') outperforms the quadrature bias point of 5 V and 1 Vpp swing (blue curve '+') by 4 dB. In this experiment, the improvement is exclusively related to the uneven power levels spacing introduced by the lower bias point, as both curves were generated under identical OMAouter conditions. This value of 4 dB is in a very good agreement with the predicted improvement of the same bias point introduced by the uneven PAM-4 case discussed in Figure 4a. It should also be noted that the signal-dependent ASE noise reduces as the OSNR increases, essentially decreasing the improvement of the uneven PAM transmission. The uneven PAM-4 net gain (OSNR improvement) at BER of 1e-3 versus various bias points is presented in Figure 5a. A clear monotonic performance improvement versus uneven bias point is observed, manifesting the advantage of the proposed optical pre-distortion scheme. Figure 5b presents two BER curves vs. OSNR, where the '+' (blue), and 'o' (red) are related to 5 V bias point (quadrature) and 4.08 V bias point, respectively.