# Real-Time Spatial-Division Multiplexing Transmission with Commercial 400 Gb/s Transponders
## 2 Real-Time SDM Transmission over Single-Mode WC-MCFs  
The cross-section of the single-mode MCF and the number of the cores are shown in Figure 1a. The MCF consists of seven single-mode, weakly coupled hexagonally arranged cores,among which core 1 is the center core. The core and cladding diameters are 7.9 and 42 \(\upmu\)m, respectively. The coating diameter is 245 \(\upmu\)m which is the same as the standard SMF. In order to suppress the inter-core crosstalk, the core-to-core pitch is set to 41.5 \(\upmu\)m. Besides, a low-index trench is employed outside each core, which could also decrease the bending loss. The measured refractive index of the MCF is shown in Figure 1b. The MCF has an average attenuation of about 0.21 dB/km and a dispersion coefficient of 20.9 ps/nm/km at 1550 nm. The photo of the fan-in (or fan-out) device is shown in Figure 1c. The cladding of seven standard SMFs is reduced to 41.5 \(\upmu\)m by chemical etching process, which is the same as the pitch of the MCF. The seven thin-cladding fibers are gathered into the closest packed structure in a glass capillary and the end face is mechanically polished. The fiber bundle is actively aligned with the MCF through a six-dimensional adjustment stand and bonded with adhesive. We construct two MCF links which are composed of single-roll 60 km MCF or 20-roll 3 km MCFs with a pair of FIFO devices. The 20-roll 3 km MCFs are spliced one by one to simulate the deployment of fiber cables in real telecommunication networks. The program of the fusion splicer (Fujikura 100 P+) is finely optimized to minimize the splice loss. The two MCF links are characterized and the results are shown in Tables 1 and 2. The diagonal elements of the matrices are link losses, while the nondiagonal elements are inter-core crosstalk. As can be seen, the link losses of core 1 (center core) in the two links are \(-14.0\) dB and \(-16.2\) dB, respectively. The average link losses of outer cores in the two links are \(-15.25\) dB and \(-20.6\) dB, respectively. For the 20-roll MCFs, the average splice losses of center core and outer cores are 0.116 dB and 0.281 dB per splice point, respectively. It can be seen that each of the six outer cores has an extra splice loss. This is because the outer core requires not only concentric alignment, but also the angular alignment. The maximum core-to-core crosstalk of the single-roll MCF and the 20-roll MCF are \(-48.48\) dB and \(-48.2\) dB, respectively. No obvious crosstalk degradation is observed after splicing. The low link loss and inter-core crosstalk of the 20-roll MCFs shows its great potential to be used in real telecommunication transmission systems. The experimental setup of the real-time SDM transmission over two MCF links is illustrated in Figure 2a. The center core (core 1) and two outer cores (core 2 and 4) are chosen to test the transmission performance, since the inter-core crosstalk is minor and the outer cores have similar characteristics. The optical transceivers used in this experiment are 75 GBaud commercial 400 G dual-polarization (DP) quadrature amplitude modulation (QAM) transponders, which are installed on Digital China.2 platform. Nine transponders are divided into three sets for three cores under test. Each set consists of three transponders which are assigned the central frequencies of 191.6 THz, 193.4 THz and 196.0 THz across the C band. At the transmitter, three wavelength-selective switches (WSSs) are used as wavelength MUXes. The optical spectrum at the output of a WSS after WDM is shown in Figure 2a. Then, three single-mode erbium-doped fiber amplifiers (EDFAs) are followed to adjust the input power to balance the loss of the three cores in the MCF link. The MCF  
Figure 1: (**a**) Cross-section of the weakly coupled 7-core fiber. (**b**) Refractive index profile of the MCF. (**c**) Photo of a fan-in (or fan-out) device. link consists of a fan-in device, 60 km or 20 \(\times\) 3 km MCFs and a fan-out device.