## 3 Experiment, Results and Discussion
To increase the spectral efficiency of each DWDM wavelength channel, we apply bit-loadings to different OFDM subcarriers. Figure 9a-c show the signal-to-noise ratio (SNR) and bit-loadings for each subcarrier in different channels at the 1550.12 nm (ITU-34) wavelength channel. We can see that there is a notch near the 4 th subcarrier, as shown in Figure 9a-c, and this is introduced by our electrical amplifiers. The highest bit-loading of 5 can be achieved, which corresponds to 32-quadrature amplitude modulation (QAM). The SNR and bit-loadings drops at the high frequency region are due to the bandwidth limitation of our RTO, and not to the SiPh 3 \(\times\) 3 switch. Figure 9d,e illustrate the typical constellation diagrams of 4-QAM, 8-QAM, 16-QAM and 32-QAM of the ITU-34 wavelength channel. According to our experimental results, the average data rates of CH1, CH2 and CH3 are 58.71 Gbit/s, 46.64 Gbit/s and 53.65 Gbit/s, respectively. The total capacity of this 3 \(\times\) 3 mode switch can achieve 7.623 Tbit/s (i.e., 53 Gbit/s \(\times\) 3 modes \(\times\) 48 wavelengths). Finally, we experimentally characterize the MDM optical power divider. We evaluate the BER performances of all the 48 DWDM wavelength channels, and the BER performances after power divisions of TE\({}_{0}\), TE\({}_{1}\), and TE\({}_{2}\) modes are shown in Figure 10a-c, respectively. Similarly, we select 1530.33 nm (ITU-16), 1539.77 nm (ITU-22), 1550.12 nm (ITU-34), 1559.79 nm (ITU-47) and 1564.68 nm (ITU-59) for display. We measure that every wavelength channel can satisfy the HD-FEC threshold. We also utilize the bit-loadings for different OFDM subcarriers to improve the spectral efficiency. Figure 11a-c show the SNR and bit-loadings for each subcarrier in different channels at the 1550.12 nm (ITU-34) wavelength channel. According to our experimental results, the average data rates of TE\({}_{0}\), TE\({}_{1}\) and TE\({}_{2}\) mode division are 59.62 Gbit/s, 58.79 Gbit/s and 52.69 Gbit/s, respec  
Figure 8: Measured BER curves of some selected wavelength channels of the 3 \(\times\) 3 modes switch (a) CH1, (b) CH2 and (c) CH3. Figure 7: Measured relative crosstalk of the MDM power divider for power divisions of (**a**) TE\({}_{0}\), (b) TE\({}_{1}\), (c) TE\({}_{2}\) modes. tively. The total capacity of this 3-dB MDM optical power divider can achieve 8.211 Tbit/s (i.e., 57.02 Gbit/s \(\times\) 3 modes \(\times\) 48 wavelengths). To summarize our experiment, we can observe that the 3 \(\times\) 3 modes switch can achieve a total capacity of 7.623 Tbit/ss, while the 3-mode 3 dB power divider can achieve a total capacity of 8.211 Tbit/s. Hence, the whole system will be limited by the switch, and the expected total capacity of the whole system is 7.623 Tbit/s. Figure 11: Experimental SNR and bit-loadings for each subcarrier at the power divided (**a**) TE\({}_{0}\), (**b**) TE\({}_{1}\) and (**c**) TE\({}_{2}\) modes at the 1550.12 nm (ITU-34) wavelength channel. Typical constellations of (**d**) 4-QAM, (**e**) 8-QAM, (**f**) 16-QAM, (**g**) 32-QAM. Figure 9: Experimental SNR and bit-loadings for each subcarrier in (**a**) CH\(1\), (**b**) CH\(2\) and (**c**) CH\(3\) at the 1550.12 nm (ITU-34) wavelength channel. Typical constellations of (**d**) 4-QAM, (**e**) 8-QAM, (**f**) 16-QAM, (**g**) 32-QAM.