Fig. 1. Diagram of WDM optical network monitoring scheme based on PAM4 digital optical label (FFT: fast Fourier transform; OSNR: optical signal-to-noise ratio)

Fig. 2. Different types of optical network monitoring schemes. (a) Optical filtering based monitoring scheme; (b) coherent detection based monitoring scheme

Fig. 3. Label power and noise power corresponding to different service signal parameters. (a) Different modulation formats; (b) different transmitting rates of 16QAM services

Fig. 4. Influence of modulation depth on label and service. (a) When the service signal is modulated by QPSK, the label power and noise power vary with modulation depth; (b) OSNR limit corresponding to different bit error rate thresholds varies with different modulation depths

Fig. 5. Power monitoring results. (a) Influence of filtering at transmitter; (b) comparison between peak and integral monitoring; (c) comparison of monitoring error before and after power correction; (d) power monitoring error of QPSK/16QAM system

Fig. 6. OSNR estimation results. (a) QPSK system; (b) 16QAM system

Fig. 7. Scheme comparison. (a) Results of power monitoring; (b) results of OSNR estimation

Fig. 8. Schematic diagram of PM-QPSK/16QAM monitoring system offline experiment platform based on PAM4 optical label

Fig. 9. Waveform of PAM4 optical label after low bandwidth PD reception, low-pass filtering envelope and amplitude determination. (a) QPSK system; (b) 16QAM system

Fig. 10. Experimental monitoring results. (a) Power monitoring error corresponding to different modulation depths when service signal is QPSK/16QAM modulated; (b) power monitoring error corresponding to different label sequence mean values when service signal is QPSK/16QAM modulated