Reflective semiconductor optical amplifier has been widely employed in wavelength-division-multiplexed passive optical network (WDM-PON) for implementing colorless optical network unit. WDM-PONs using a single-feeder fiber is economically beneficial, but vulnerable to beat noise resulted from distributed and discrete reflections. In this paper, we present a novel technique exploiting simple digital signal processing to mitigate the impairments in single-feeder WDM-PON by means of introducing correlations between signal levels in a coding process. The correlation properties of the coded sequence permits overall spectrum shaping to suppress the low-frequency components. We apply dicode code, the simplest correlative level code that enables direct current balance, in the uplink of a WDM-PON with downstream continuous-wave seed light or in the downlink of a WDM-PON with remodulated upstream signal. In the first case, the system's reflection tolerance is substantially enhanced via the dicode-coded modulation in the uplink. Moreover, the system reach is extended by 15 and 25 km for data rate of 2.5 and 1.25 Gb/s, respectively. In the second case, a 60 km full-duplex WDM-PON with 10 Gb/s dicode-coded downlink and 2.5 Gb/s remodulated uplink is demonstrated with high robustness against the remodulation and reflection noise.
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