The authors report on the downlink performance of a 10 Gb/s long-reach and ultra-dense wavelength-division multiplexed passive optical network, based on a multicarrier transmitter realized by using an externally injected gain-switched distributed-feedback laser diode. Each of the comb channels, spaced by 10 GHz, is modulated with a 3 Gbaud dual polarization quadrature phase shift keying signal that included a 20% overhead for forward error correction. Frequency selectivity and enhanced receiver sensitivity is achieved by employing a digital coherent receiver to receive the signal. Experimental results achieved in a back-to-back and 100 km transmission scenarios show an excellent worst case receiver sensitivity of -44 dBm. . 2013 Optical Society of America.