Ultra-dense WDM-PON 6.25 GHz spaced 8×1 Gb/s based on a simplified coherent-detection scheme.

We demonstrate experimentally a novel type of coherent low cost Gigabit-to-the-User Ultra-Dense-Wavelength Division Multiplexing (UD-WDM) PON, featuring 6.25 GHz channel spacing and long reach. Polarization-independent coherent detection is achieved by exploiting a novel scheme which requires only a 3 × 3 coupler, three photodiodes, basic analogue processing and a common DFB as local oscillator (LO). This avoids the conventional polarization diversity approach. The DFB LO is free running, i.e. not locked in frequency, and is tuned to detect any of the eight channels by simply changing its temperature in a range of 2° C. We achieve 70 km long-reach transmission plus 30 dB attenuation, for a total of > 45 dB optical distribution network loss. This indicates that this solution could be effectively exploited to overlay existing PON infrastructures by UD-WDM.