Demonstration of continuous multiple access with homodyne and image-rejection heterodyne coherent receivers using directly modulated laser transmitters

This work presents a comprehensive set of experiments for the multipoint-to-point coherent passive optical network (PON), where the wavelength locking between the optical network unit (ONU) and optical line termination lasers is critical, especially if operating in burst mode. Here, we test the performance of continuous multiple access in a splitter-based PON with both an ultra-dense wavelength division multiplexing and RF-subcarrier multiplexing configuration, with simple distributed feedback (DFB) lasers along non-return to zero and pulse amplitude modulation of four levels. Most interestingly, we test a spectrally efficient heterodyne receiver with image-frequency rejection and polarization independence based on the {{3}} \times {{3}} optical front-end. Two users at the same intermediate frequency are detected simultaneously avoiding image frequency interference while minimizing complexity, with transmissions of 2.5 Gb/s. We provide comparison with an asynchronous homodyne receiver. The achieved results demonstrate the feasibility of continuous multiple access using thermally controlled ONUs with conventional DFBs as an enhanced alternative to commercial time division multiplexing access.