## 3 Experimental Validation
### Data Plane Transmission Performance Evaluation  
The transmission performance is further evaluated in this section. Our evaluation follows a stepwise approach, starting with 50-gigabyte-per-second traffic transmission for all the validated combinations of ToRs. This is followed by a demonstration of the end-to-end 50 Gb/s packet switching experiment. Firstly, an experiment was set up with a one-cluster 4 \(\times\) 4-rack DCN prototype, which was implemented by the SWS and a 4 \(\times\) 4 AWGR with 200-gigahertz channel spacing, as shown in Figure 6. Four lasers and four SOAs were employed to implement the SWS prototype. The central wavelength of each laser output was set to match the wavelength routing map of the AWGR. For each transmission evaluation, the SOA gates of the SWS were turned on/off in order to set the central wavelength of the transmitters to assess each of the transmission paths between the source and destination ToRs of the DCN. Since the optical modulator and AWGR were polarization-dependent, polarization controllers were used to align the polarization state of the laser beam with the Mach-Zehnder modulator (MZM) and AWGR separately. The 50 Gb/s PRBS-31 NRZ-OOK signals were then coupled into a triplet lens collimator (Thorlabs TC18 FC-1550) to reach the AWGR through an optical wireless path of 2 m. Since the AWGR forwarded the light signal between the input and output ports with a fixed routing map, the optical signal from the transmission ToR was switched to reach each destination ToR by switching the SOA gates of the SWS. At the destination ToR, the transmission performance was evaluated by a BER tester to measure the bit error rate of each transmission link. The back-to-back (BB) measurement was carried out by connecting the output of the MZM directly to the BER tester. Figure 7 shows the BER curves measured at 50 Gb/s for all the 12 links. The results confirm a nearly identical system performance error-free operation with a power penalty of less than 2 dB at BER < 1 \(\times\) 10\({}^{-9}\) with respect to BtB transmission. This 2 dB penalty was mainly due to the decrease in the SNR values in addition to spectral filtering by the AWGR. In the second experiment setup, shown in Figure 8, an optical packet-switched proof-of-concept testbed of a 4 \(\times\) 4-rack OW-DCN operating at 50 Gb/s was implemented. The central wavelength of ToR1 and ToR2's optical packets was fast-switched by the FPGA-controlled SWS for transmitting the 50 Gb/s packetized signal to ToR3 or ToR4. In order to achieve fast and accurate packet switching at 50 Gb/s, an FPGA that controlled the SWSs of ToR1 and ToR2 was time-synchronized with the 50-gigabyte-per-second pattern generator, which drove the 50-gigabyte-per-second MZM. Four SOAs were employed to demonstrate the full dynamic fast packet-switch transmission between ToR1 and ToR2, to reach ToR3 or ToR4. Specifically, optical packets at 50 Gb/s OOK with a 7 ns (350 bits) payload time and 1 ns (50 bits) guard time were generated, as shown in Figure 9a. Figure 9b shows one of the FPGA's control signals for SOA. The time duration of this control signal was chosen in accordance with the packet duration of 8 ns. When the optical packets had to be transmitted to the ToR3 or ToR4, a trigger signal from the pattern generator was sent to the FPGA for triggering the generation of the control signals simultaneously. This guaranteed an accurate time synchronization so that the wavelength switching occurred right in the middle of the gap between two consecutive packets. Moreover, guard time of 1 ns was also well designed to match the fast switching time of the SOAs. Figure 6: (a). Experimental setup of one cluster with 4 \(\times\) 4-rack OW-DCN. BERT: bit error-rate tester; (b) the actual setup. Figure 7: BER curves versus received power at 50 Gb/s for the links between ToRs.