# Advanced Modulation Formats for 400 Gbps Optical Networks and AI-Based Format Recognition
## 2 Materials and Methods
### Result Analysis
When the single span is 120 km and the launch power is 15 dBm, the 4 \(\times\) 100 Gbps DQPSK signal achieves optimal transmission performance. At this point, the Q-factors for the four channels are 9.8 dB, 10.8 dB, 11.5 dB, and 9.9 dB, respectively. The performance difference between these channels and the four wavebands of Apol-CRZ-FSK is also minimal. However, when the launch power increases to 20 dBm, the Q-factors for the four signals drop to 5.7 dB, 6.2 dB, 6.4 dB, and 6.2 dB, respectively. These values are  
Figure 3: The relation among SMF length, Q-factor, and launch power for the four wavelength channels of 4 \(\times\) 100 Gbps Apol-CRZ-FSK signal transmission: (**a**) first channel; (**b**) second channel; (**c**) third channel; (**d**) last channel. significantly lower than the Q-factors of the four signals of the 4 \(\times\) 100 Gbps Apol-CRZ-FSK under the same conditions, with differences of 8.8 dB, 6.9 dB, 9.4 dB, and 8.2 dB, respectively. At a launch power of 19 dBm, the maximum transmission distance for a single span is 145 km, which is 23 km less than that of the 4 \(\times\) 100 Gbps Apol-CRZ-FSK signals. Based on these data, it is evident that when the launch power is low and the impact of nonlinear effects is minimal, Apol-CRZ-FSK does not exhibit a significant performance advantage. However, as the optical power gradually increases and the transmission distance extends, nonlinear effects become the primary factor affecting system performance. In such cases, the Apol-CRZ-FSK signal demonstrates superior nonlinearity resistance, outperforming the other two signals in terms of both signal Q-factor and transmission distance. Based on the research of single-span transmission performance mentioned above, Figure 6 analyzes the transmission performance differences during multi-span transmission. In this scheme, each span is first passed through an EDFA amplifier to amplify the signal power before entering a single-mode fiber for transmission. Subsequently, dispersion compensation fibers of corresponding lengths are used for post-dispersion compensation. Due to the influence of factors such as four-wave mixing, cross-phase modulation, and dispersion in the 4 \(\times\) 100 Gbps system, there are certain differences in the transmission performance of the four channels. The transmission distance of the channel with the worst performance should be selected as the transmission distance of the system. As can be seen from the figure, the maximum transmission distance of the 4 \(\times\) 100 Gbps Apol-CRZ-FSK signal is 1740 km, at which point the Q-factors of the four channels are 6.1 dB, 7.0 dB, 6.3 dB, and 6.7 dB, respectively. The maximum transmission distance of the 4 \(\times\) 100 Gbps DQPSK signal is 1260 km, which is 480 km shorter than the Apol-CRZ-FSK signal. The maximum  
Figure 4: The relation among SMF length, Q-factor and launch power for the four wavelength channels of 4 \(\times\) 100 Gbps CRZ-FSK signal transmission: (**a**) first channel; (**b**) second channel; (**c**) third channel; (**d**) last channel. transmission distance of the CRZ-FSK signal is 760 km, which is 980 km shorter than the Apol-CRZ-FSK signal. Figure 5: The relation among SMF length, Q-factor and launch power for the four wavelength channels of 4 \(\times\) 100 Gbps DQPSK signal transmission: (**a**) first channel; (**b**) second channel; (**c**) third channel; (**d**) last channel. Figure 6: Performance analysis and comparison of three signals in different distances. Figure 7 displays the eye diagrams of the first channel (Figure 7a), second channel (Figure 7b), third channel (Figure 7c), and last channel (Figure 7d) for each of the three signals when the launch power is 6 dBm and the signals are transmitted for 1500 km. At this point, the Apol-CRZ-FSK signal exhibits superior transmission performance.