Wavelength division multiplexing (WDM) optical networks include numerous wavelengths and nodes, dynamic reconfiguration of transmission paths, and dynamic scheduling of services/resources. Thus, a low-cost and highly reliable online monitoring technology is urgently required to ensure the safe and stable operation of WDM networks. It is the cornerstone technology to ensure the safe and efficient operation of the optical network and effectively reduce operation and maintenance costs. Currently, optical network monitoring obtains the spectral information of each channel using devices such as tunable optical filters. From this information, the channel performance such as the optical modulation format and optical signal-to-noise ratio (OSNR) can be obtained. Another scheme uses a high-speed service signal coherence receiver with complex digital signal processing (DSP) to monitor the channel performance; however, this method can only realize the end-to-end optical performance parameter monitoring of a single wavelength channel. The timeliness of monitoring cannot be guaranteed. Exciting schemes generally have problems such as high cost, poor monitoring reliability, inability to monitor the wavelength channel status at intermediate nodes in real time, and complex system structures. They cannot meet the requirements of low-cost and high-reliability optical performance monitoring for the new generation of large-scale complex optical networks. To address these problems, a low-cost and high-efficiency optical performance monitoring scheme based on a quadratic pulse amplitude modulation (PAM4) optical label is proposed for WDM optical networks.

This paper proposed an optical performance monitoring scheme based on the PAM4 optical label that uses a PAM4 modulation format digital label to carry more monitoring information and improve the timeliness of optical network monitoring. Using the DSP unit at the service transmitter, time-domain digital labels with specific frequency pilot tones were loaded flexibly into the corresponding wavelength channels without the need for additional digital-to-analog converters (DACs) or custom optical modulators. Approximately 1% optical power was separated using an optical coupler at the monitoring node. Subsequently, a low-bandwidth photodetector (PD) and a low-speed analog-to-digital converter (ADC) were used to receive optical labels from all channels. Using a specially designed optical label demodulation and processing algorithm, the optical power and OSNR of all the wavelength channels could be obtained accurately. Simultaneously, the digital label monitoring information loaded onto the corresponding wavelength channel could be recovered. Thus, the low-cost and highly reliable monitoring of the wavelength channel performance was achieved.

A simulation platform of a 16 GBaud polarization multiplexing (PM) QPSK/16QAM WDM optical transmission system for a C-band eight-channel 25-span transmission was built to verify the feasibility and accuracy of the proposed WDM optical network monitoring scheme based on the PAM4 optical label. The simulation results show that the method of calculating the channel optical power [Fig. 5(b)] using the peak of the spectrum at the pilot tone significantly reduces the power monitoring error caused by the complex high-order harmonics in the PAM4 digital label compared with the method of spectral integral using the OOK/DPSK label. Moreover, the correction of the label mean value significantly reduces the monitoring error caused by the uneven label distribution [Fig. 5(c)]. After the 25-span long-distance transmission, the channel optical power monitoring error does not exceed 0.65 dB [Fig. 5(d)], the OSNR estimation error does not exceed 0.6 dB (Fig. 6), and the performance is slightly better than that based on low-order modulation format optical label such as DPSK (Fig. 7). The experimental results show that the power monitoring errors of the QPSK and 16QAM systems are less than 0.3 dB when different label modulation depths and different mean values of digital label signals are used in a 80 km optical transmission.

To meet the requirements of multi-channel, multi-parameter, low-cost, and highly reliable online monitoring for WDM optical networks, a new monitoring scheme based on the PAM4 digital optical label is proposed. We innovatively proposes a complete set of mechanisms for loading, detecting, and processing the PAM4 optical label, as well as an error correction method for optical power monitoring based on the PAM4 optical label sequence characteristics. It can accurately and efficiently monitor the optical power and OSNR of all wavelength channels in real time. Based on the established WDM multi-channel and multi-span transmission simulation platform, the monitoring performance after a 25-span WDM optical transmission was simulated and analyzed. The results show that the performance of the proposed PAM4 label-based optical power monitoring is significantly improved using the designed power monitoring error correction method. The maximum optical power error does not exceed 0.65 dB, and the OSNR estimation error does not exceed 0.6 dB. The performance of the proposed PAM4 label-based monitoring scheme is slightly better than those of the DPSK and OOK low-order optical label-based monitoring schemes. In addition, an offline experimental platform was constructed using a PD with a bandwidth of 200 MHz and an ADC with a sampling rate of 400 MSa/s. The experimental results show that the PAM4 label can be accurately recovered using the proposed scheme, and the optical power monitoring error is less than 0.3 dB. These results demonstrate that the proposed scheme is cost-effective, easy to deploy on a large scale, reliable, and efficient for WDM optical network monitoring.