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We have proposed and experimentally demonstrated a 6 × 40-Gb/s long reach dense wavelength division multiplexed passive optical network downlink system operating over 80 km of standard single mode fiber. The six optical channels with 20 GHz spacing are generated by an externally injected gain switched optical comb source. Each comb line is imposed with a 6-GBd polarization division multiplexed quadrature amplitude modulation, which included a 20% overhead for forward error correction. A worst case channel sensitivity of -35.6 dBm (54 photons/bit) at a bit error rate of 1.5×10 ^-2 was achieved, yielding a system loss budget of 40 dB.