# Low-Cost 400 Gbps DR4 Silicon Photonics Transmitter for Short-Reach Datacenter Application
## 4 DR4 Transmitter Test  
To analyze the general performance of the silicon photonics engine, we integrated it into an evaluation board, as shown in Figure 4. The engine design followed DR4 standards with a size of 6 mm \(\times\) 4 mm. DC controls were added to the engine through top and bottom wire bonding, and AC signals were coupled to the engine through wire bonding on the right. The fibers were aligned to the edge couplers in the engine, and the TIA chip was bonded to the photodiode array in the engine. The AC inputs and TIA outputs went to GPPO connectors at the end of the evaluation board. The fiber array was inserted into the engine and fixed into the V-grooves, forming parallel optical lanes. The other end of the fiber array terminated in an MPO-12 connector. Note that the scale of the assembly, regardless of the temporary evaluation board, is compliant with the size constraints of the QSFP-DD MSA [30]. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Ref.**} & **EO Bandwidth** & **Vpi\(\times\) L** & **Loss** & **Data Rate** & \\  & **(GHz)** & **(V-cm)** & **(dB)** & **(Gb/s)** & **Type** \\ \hline
[17] & 26 & 1.4 & 8 & 64 (QPSK) & MZI \\
[18] & 20 & 0.8 & 10 & 256 (DP-16-QAM) & MZI \\
[28] & 50 & 0.52 & 4.2 & 112 (PAM-4) & MR \\
[29] & 35 & 1.8 & 5 & 72 (NR2) & SISCAP 1  \\ Our design & 43 & 2.5 & 5 & 106 (PAM-4) & MZI \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Key parameter comparison among different modulator types. Figure 3: Test and simulation S-parameter results of MZM under differential drive: (**a**) reflection; (**b**) transmission. The large signal experiment setup is shown in Figure 5. Here, we only tested its performance as a transmitter. A bit error rate tester (BERT), which can work up to 64.8 GHz clock speed, generates the PAM-4 signal and was pre-amplified by a BERT amplifier (Amp.) to 0.57 V\({}_{\text{pp}}\). Then, the signal was amplified to 1.8 V\({}_{\text{pp}}\) (3.6 V\({}_{\text{ppd}}\)) by a high-speed, linear driver and sent to the Balun, where the differential pair was formed. The electrical connection between Balun and device under test (DUT) was extremely short, in order to minimize skew between the differential pair. The differential signal was fed into the silicon photonics engine, lane by lane, through the evaluation board. After optical signal modulation, it was sent to a Digital Communication Analyzer (DCA) for data recovery. The System Impulse Response Correction (SIRC) was enabled to improve the response of the reference filter inside the DCA and to de-embed its bandwidth limitation. A 5-tap FFE was used when running the Transmitter and Dispersion Eye Closure Quaternary (TDECQ) algorithm. In Figure 6, clear open eyes are observed for all lanes in the silicon photonics engine at a data rate of 53.125 Gbaud/s. To be specific, lane 1 has an Extinction Ratio (ER) of 5.4 dB and a TDECQ of 3.04 dB; lane 2 has an ER of 5.3 dB and a TDECQ of 3.19 dB; lane 3 has an ER of 5.3 dB and a TDECQ of 2.98 dB; lane 4 has an ER of 5.0 dB and a TDECQ of 2.87 dB. Thus, the PAM-4 eyes, in principle, would not approach the bit error rate limit on the receiver side. Note, that the high-speed response of the transmitter drops because of the relatively long and lossy RF routing between the driver and the modulator, including the GPPO cable and connector, metal trace on the evaluation board, and wire bonds. In a compact QSFP-DD package, where all the components shown in Figure 1 are attached on a high-speed substrate, we expect still better eye performance and lower TDECQ < 2 dB. Figure 4: Microscope view of the assembled silicon photonics engine. Figure 5: Experimental setup for transmitter PAM-4 signal test. BERT: bit error rate tester; Amp. : amplifier; DUT: device under test; DCA: Digital Communication Analyzer. Finally, as a 5 dB ER was achieved with a 1.8 V\({}_{\rm pp}\) driving voltage and the optical loss is within 5 dB, the MZM was functioning without consuming many resources electrically and optically.