# Advanced Modulation Formats for 400 Gbps Optical Networks and AI-Based Format Recognition
## 2 Materials and Methods
### Result Analysis  
In the 4 \(\times\) 100 Gbps Apol-CRZ-FSK communication system, as the launch power varies from \(-5\) dBm to 30 dBm, the OSNR of the signal increases with the increase in launch power, resulting in a gradual increase in the Q-factor of each channel signal. Figure 3 shows that the Q-factors of the four channels are 9.0 dB, 10.0 dB, 8.5 dB, and 10.0 dB, respectively, when the transmission distance is 120 km, and the launch power is 15 dBm. When the launch power is 20 dBm, the signal achieves optimal transmission performance, with Q-factors of 14.5 dB, 13.1 dB, 15.8 dB, and 14.4 dB for the four channels, respectively. As the launch power continues to increase, the impact of nonlinear effects such as four-wave mixing and cross-phase modulation caused by small-wavelength spacing and high-power density becomes increasingly severe, leading to continued degradation of the signal. When the launch power is 26 dBm, the maximum transmission distance of the 4 \(\times\) 100 Gbps Apol-CRZ-FSK signals over a single span is 168 km. To better assess the performance advantages and disadvantages, a comparative analysis was conducted among the 4 \(\times\) 100 Gbps Apol-CRZ-FSK, CRZ-FSK and DQPSK systems under identical conditions. The single-channel 100 Gbps CRZ-FSK setup is depicted in Figure 1b, where signals from a four-channel 100 Gbps CRZ-FSK transmitter, employing a wavelength spacing of 100 GHz, are multiplexed and transmitted through a single fiber. The system configurations for the line side and receiver are identical to those used in the 4 \(\times\) 100 Gbps Apol-CRZ-FSK, as illustrated in Figures 1e and 1f, respectively. Figure 2b presents the optical spectrum of the 4 \(\times\) 100 Gbps CRZ-FSK signals at the transmission side. Examining the transmission performance of the 4 \(\times\) 100 Gbps CRZ-FSK communication system in Figure 4, it is observed that at a transmission distance of 120 km and launch power of 15 dBm, the Q-factors of the four channels are 9.7 dB, 10.3 dB, 9.7 dB, and 9.5 dB, respectively. At this point, the difference in signal transmission performance between the CRZ-FSK and Apol-CRZ-FSK signals is minimal, approximately 1 dB. However, when the launch power is increased to 20 dBm, the Q-factors of the four channels of the 4 \(\times\) 100 Gbps CRZ-FSK signals drop to 11.6 dB, 12.3 dB, 11.3 dB, and 11.9 dB, which are lower than those of the 4 \(\times\) 100 Gbps Apol-CRZ-FSK signals by 2.9 dB, 0.8 dB, 4.5 dB, and 2.5 dB, respectively. Furthermore, at a launch power of 22 dBm, the maximum transmission  
Figure 2: Spectral diagram of a 4 \(\times\) 100 Gbps signals: (a) Apol-CRZ-FSK; (b) CRZ-FSK; (c) DQPSK. distance over a single span is 152 km, a decrease of 16 km compared to the 4 \(\times\) 100 Gbps Apol-CRZ-FSK signals. For the 4 \(\times\) 100 Gbps DQPSK system, a phase modulator and an MZM are used to generate each 100 Gbps DQPSK signal at the transmitting side. The modulation depth of the first phase modulator is \(\pi\)/2, and the modulation depth of the second MZM is \(\pi\). The DQPSK modulation format has only half the symbol rate under the condition of a fixed transmission bit rate, which compresses the spectrum and allows for higher spectral efficiency in DWDM systems. It also improves resistance to dispersion. The signal from the four-channel 100 Gbps DQPSK transmitter with a wavelength spacing of 100 GHz is coupled through a wavelength multiplexer, and the resulting signal spectrum is depicted in Figure 2c. After long-distance transmission through the transmission line shown in Figure 1e, the 4 \(\times\) 100 Gbps DQPSK signal is separated into four 100 Gbps signals by a demultiplexer at the receiving side. Each signal passes through an optical bandpass filter, is delayed by one symbol interval, and then multiplied by itself. After low-pass filtering and sampling decision, the original digital information can be directly recovered. Figure 5 illustrates the transmission performance of a 4 \(\times\) 100 Gbps DQPSK signal.