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In the 400 Gbit/s transmission system based on C + L band spectrum and QPSK modulation format, the short-wavelength signal power will be shifted to the long-wavelength signal due to the presence of the stimulated Raman scattering (SRS) effect, which will seriously affect the performance of the transmission system as the transmission span accumulates. The solution is to set the gain and gain slopes of the C-band amplifier and L-band amplifier appropriately at each optical amplifier site, and adjust the signal power of each channel through the WSS at the transmitting end and the WSS at the DGE site, so that the flatness of the channel power at the receiving end can be controlled in a reasonable range, thus guaranteeing the transmission performance of the system. However, the system fault will destroy the originally set channel power, which will seriously affect the transmission performance of the system. In this paper, filling channel device combined with output power locking of amplifiers used in a 400 Gbit/s system based on C + L band and QPSK modulation format to provide the protection for the system is proposed and demonstrated for the first time, which gives different solutions for sudden fault at different locations of the system, and provides a reference for the channel power management of multi-band optical transmission systems in the future.

A multifunctional optical signal processing scheme for 10-Gbaud quadrature phase shift keying (QPSK) and binary phase shift keying (BPSK) signals based on four-wave mixing (FWM) in highly nonlinear fiber (HNLF) is proposed. Wavelength-division-multiplexing (WDM) multicast, wavelength conversion, modulation format conversion, and hybrid modulation format exclusive-OR (HMF-XOR) logic gates are realized simultaneously by simulation. One-to-three WDM multicast of 20-Gbps QPSK signals are achieved paying the optical signal-to-noise ratio (OSNR) penalties <0.8  dB at the bit error rate (BER) of 10 -3 . The converted BPSK signals with high performance are obtained; they are generated from wavelength conversion of 10-Gbps BPSK and format conversion from 20-Gpbs QPSK. In the case of HFM-XOR logic gates, we discuss the extension application in all-optical encryption. Therefore, both theoretical analysis and simulation results are conducted to analyze the feasibility of the dual-channel all-optical encryption for QPSK and BPSK signals. The OSNR penalties of decrypted QPSK and BPSK signals at the BER of 10 -3 are 1 dB and 0.7 dB, respectively. The concentrated constellations and clear eye diagrams indicate the high performance of decrypted signals.