## 5 High Capacity 2 \(\times\) 2 OPS Node with Contention Resolution and Fiber Transmission
### Transmission Experimental Setup  
Figure 14 shows the experimental setup of a transmitter for DWDM PDM-16-QAM optical packets. Compared with the setup introduced in Figure 7, the improved setup of Figure 14 introduces two enhancements. First, arbitrary waveform generators (AWGs) were used to generate the driving signal for the modulators as opposed to two-bit digital to analog converters (DACs) made of discrete components. Secondly, the AWGs were used to carve the signal in order to generate the optical packets and the OOK gate based on an MZM was removed from the setup. The AWGs used to drive the modulators had an electrical bandwidth of 16 GHz and a resolution of eight bits. The four-level I and Q electrical tributaries input to the modulator were equalized with a Nyquist 128-symbol square root raised cosine (SRRC) filter with a roll-off factor of 0.25, followed by a 1025-tap pre-emphasis filter to compensate for the DACs' non-ideal frequency response. Notably, additional MZM modulators were not used in this work to carve the continuous wave (CW) signal into optical packets owing to the ability of the AWGs to set both I and Q tributaries to the null point of the DPMZMs. The data rate of the optical packets was increased from 160 Gb/s to 200 Gb/s and the decorrelation between the X and Y polarization was six symbols, in order to yield a total capacity of 12.8 Tbit/s. The back-to-back error vector magnitude of the generated PDM-16 QAM signal was about 10% without any receiver equalization (with the exception of the receiver's matched SRRC filter). The format of the optical packet train and packet labels after the transmitter was similar to the one shown on the inset of Figure 7. After the transmitter, the optical signal was split into two to simulate two transmitters, named TX1 and TX2. After the transmitter, the set of eight packets from TX1 and TX2 were propagated through different 50 km SMFs plus DCF spans and were input to both input ports of the 2 \(\times\) 2 OPS node, described in Section 4. In this experiment, the OPS node was configured to forward optical packets with label A of TX1 and TX2 to the same output port in order to emulate packet contention. To resolve this packet contention, FDLs were used as an optical buffer. The buffer manager calculated the delay time required for each packet to avoid collisions and controlled a set of PLZT switching matrices to send each packet to appropriate FDLs having 0, 100 and 200 ns, respectively. On the other hand, optical packets with  
Figure 13: BER of the 64 wavelength channels after buffering in the OPS node for each of the eight packets with label A from TX1 and TX2, shown Figure 12b. label B (only from TX2) were sent to the alternative output port without the buffering process (drop process). This whole processing is illustrated in Figure 15, which shows the waveforms processed in the data plane of the OPS node. In the label switching process, packets with label A and label B were separated and four packets with label B of TX2 were output (marked in Figure 15 as DROP). Optical packets with label A were output after the buffering process without any packet loss (labelled FORWARD on Figure 15). The optical packets were received by a polarization diverse coherent receiver followed by a digitizer as described in Section 3.1. Asynchronous optical packet detection was not considered in this experiment and known timing from the transmitter was used to digitally chop the optical  
Figure 14: General experimental description/setup of the packet buffering and transmission experiment. (**a**) Packet transmitter; and (**b**) general diagram of the OPS transmission experimental setup. Figure 15: Waveforms of the packets processed in the 2 \(\times\) 2 OPS node. DROP and FORWARD, along with back-to-back, were the waveforms at the input of the coherent receiver (ECOC 2014). packets for further DSP processing, simulating a null PER.