All DFB-Based Coherent UDWDM PON With 6.25 GHz Spacing and a ${>}{\rm 40}~{\rm dB}$ Power Budget

We demonstrate an ultra dense WDM passive optical network based on a simplified low-cost coherent receiver and free-running DFB lasers. The proposed 1.25 Gb/s system provides a high power budget , which can be exploited to support both large splitting ratio (1 × 128) and long reach (exceeding 60 km). The homodyne receiver is significantly simplified since it does not make use of any DSP and does not require phase/frequency locking; therefore, it allows for the use of a common free-running DFB as local oscillator rather than expensive low phase noise laser. Here, the proposed system is evaluated experimentally by using 5 × 1.25 Gb/s (gross data rate) channels on a 6.25 GHz grid. In the coherent receiver, a single free-running DFB laser was able to select each individual channel by thermal tuning and showed uniform performance. A -48 dBm average sensitivity at FEC level over a 66 km reach leaves enough margin for additional 30 dB splitting losses in the optical distribution network.