Fig. 1. Spectrum and constellation diagram of 400G with 25 Gsymbol/s. (a) dual-16QAM and (b) four-QPSK.

Fig. 2. Setup of simulation. SSMF: standard single-mode fiber, OSA: optical spectrum analyzer.

Fig. 3. Signal OSNR versus input power of the 100G single channel system and the three 400G single-channel systems, considering both ASE and NLI noises in uncompensated links with 5 fiber spans.

Fig. 4. Signal OSNRs of the 100G system and the three 400G systems with different noises considered. (a) ASE noise only, (b) NLI noise only, and (c) both ASE and NLI noises. The total bit rates are 1.2 Tb/s for all four systems.

Fig. 5. Signal OSNR of 400G systems with the ASE and NLI noises versus (a) the input channel power and (b) the average signal power in the fiber. The total bit rates are 2.4 Tb/s for all three systems.

Fig. 6. Signal OSNRs versus the fiber span number of the 100G system and the three 400G systems with both ASE and NLI noises in uncompensated links. The input power of each system is optimized to get the maximum OSNR [as shown in Fig. 4(c)]. The total bit rates are 1.2 Tb/s for all four systems.

Fig. 7. Signal OSNRs of the 100G system and the three 400G systems versus the subcarrier power. The total bit rates are 1.2 Tb/s for all four systems.