Fig. 1. Principle of Ahs based on signal spectra

Fig. 2. Process of spectrum preprocessing. (a) Original spectrum; (b) spectrum after downsampling; (c) spectrum after filtering

Fig. 3. Obtained Ahs under different modulation formats based on principle described in Fig. 1. (a) 64QAM-16QAM-4QAM; (b) 64QAM-64QAM-16QAM; (c) 4QAM-4QAM-64QAM; (d) 4QAM-4QAM-4QAM

Fig. 4. Structural diagram of MT-DNN

Fig. 5. Principle of parameter-based TL

Fig. 6. Schematic diagram of three-channel WDM coherent optical communication system

Fig. 7. MFI accuracy and MAE of OSNR monitoring varying with σ₁

Fig. 8. Monitoring results of MT-DNN with different optimizers. (a) MFI accuracy and MAE of OSNR monitoring; (b) loss of MFI and OSNR monitoring

Fig. 9. Results of MFI and OSNR monitoring under different training set sizes

Fig. 10. Confusion matrix of MFI accuracy

Fig. 11. MAE of OSNR monitoring for ten different modulation formats and OSNR monitoring error for all signals in testing phase. (a) MAE of OSNR monitoring; (b) OSNR monitoring error

Fig. 12. MFI accuracies and MAEs of three-channel OSNR parallel monitoring of TL-MT-DNN and w/o TL-MT-DNN varying with epoch in training phase.(a) MFI accuracy; (b) MAE of three-channel OSNR parallel monitoring

Fig. 13. Confusion matrix of MFI accuracy and OSNR monitoring errors of three channels for trained TL-MT-DNN based on Eq. (6) in testing phase. (a) Confusion matrix of MFI accuracy; (b) OSNR monitoring error of channel 1; (c) OSNR monitoring error of channel 2; (d) OSNR monitoring error of channel 3