By Oleh J. Sniezko, Ray S. Thomas
The authors presented analysis and processing results of the vast data collected during characterization of the broadband optical links used for expanding broadband bandwidth to 1 GHz and for supporting full digital load.
Three different types of load for optical links were analyzed:
- A combination of the CW load with test QAM channels,
- A combination of the NTSC modulated analog channel load with test QAM channels, and
- Full digital load of QAM channels.
The results indicate that the dominant types of distortion to digital 256-QAM signals are nonlinear products generated in optical links. These distortions affect the digital signals before laser clipping occurs. Alignment of the optical laser transmitters is critical to realize full capacity of the optical links up to 1 GHz. Similarly, the parameters of the QAM signal FEC, and especially the interleaver settings, are important to achieve this goal. The paper presents the conclusions and lists some pointers to apply in laser transmitter alignment.