In-service Monitoring of a 100 Gbaud-class Coherent Transmitter Using a 5 GHz Photodetector

Device imperfections are the main limitation in high baud rate coherent transmitters. Because these imperfections vary with temperature and age, an in-service monitoring system is necessary. With the monitored information, such imperfections can be compensated for using classic technologies. Considering cost and power consumption, a monitor using a low-bandwidth observation channel (OC) composed of photodetector (PD) and monitor circuit is preferred. However, implementing such a monitor is challenging. Even when only linear imperfections are considered, they affect the amplitude and phase of the modulated optical signal, while the PD detects only the amplitude. Additionally, the flexibility to design test signals is lost during the in-service stage. Using an algorithm developed for system identification of a Wiener–Hammerstein model when the memoryless nonlinear function is known, it is possible to monitor the linear imperfections of a coherent transmitter with a low bandwidth OC. In this study, we present experimental verification of such a low-bandwidth OC monitor. The linear characteristics and imperfections of a 96 Gbaud coherent transmitter are monitored using a 5 GHz PD. The normalized mean square error is −23 dB. With imperfection compensation based on the monitored information, the system Q factor is only 0.5 dB lower than the ideal compensation. One disadvantage of this low-cost and low-power consumption monitor is its slow convergence speed, which is attributed to the low OC bandwidth. However, a convergence time of 0.5 ms is achieved when using a 5 GHz PD, which is still acceptable for monitoring purposes because the imperfections change with temperature, and temperature stability occurs on the millisecond scale. This drawback is analyzed through theoretical derivation and simulation, and is attributed to the gradient vanishing effect caused by the limited OC bandwidth.