Nonreciprocal Optical Modulation for Colorless Integrated Optical Transceivers in Passive Optical Networks

Wavelength remodulation schemes reuse the optical carrier received in a downstream signal for the upstream link and are cost-effective in eliminating the need for specific wavelength sources at the optical networking units (ONUs). We describe a study comparing the performance of a wavelength-remodulated wavelength-division-multiplexed (WDM) passive optical network (PON) using amplitude modulation [amplitude-shift keying (ASK)] downstream and phase modulation [differential phase-shift keying (DPSK)] upstream with a system using DPSK downstream and ASK upstream based on nonreciprocal modulators. We measured the conditions needed for both downstream and upstream signals to achieve a high extinction ratio (ER) and error-free operation at 10 Gb/s. Both schemes can give error-free operation. ASK downstream with orthogonally remodulated DPSK upstream (ASK/DPSK) may potentially be more cost-effective because it can use a common decoder in the central office instead of requiring individual DPSK decoders at each ONU. However, DPSK downstream with orthogonally remodulated ASK upstream (DPSK/ASK) is more suitable for long-reach access networks since it has a better power budget for long-reach PON.