Coherent time- and wavelength-division multiplexed point-to-multipoint optical access network using low-cost DFB lasers enabled by a frequency comb

Coherent reception, along with time- and wavelength-division multiplexing (TWDM), is a promising concept to simultaneously support multiple services in future high-speed point-to-multipoint passive optical networks (PONs).
The current TWDM-PON standard, known as Next-Generation PON 2 (NG-PON2) cannot easily be migrated to a coherent system, because NG-PON2 is based on intensity modulation and direct detection (IM/DD). In addition, it employs tunable-lasers, tunable optical filters and cyclic arrayed-waveguide gratings. Here, we investigate a novel coherent TWDM-PON architecture based on a frequency comb source in the optical line terminal (OLT), and thermally-tuned distributed-feedback (DFB) lasers in the optical network units (ONUs). For downstream operation, we broadcast multiple copies of two 25GBd dual-polarization quadrature phase shift keying (DP-QPSK) signals, resulting in a total PON downstream capacity of 200Gbit/s. The copies of the downstream signals are spanning ±2nm (±250GHz). In the ONUs, we align the wavelengths of the DFB lasers, which act as local oscillators (LOs), to one of the downstream signals by a thermal heater or by changing the direct current. In upstream, the already aligned DFB lasers act as transmit lasers and the frequency comb as LO. We demonstrate TWDM upstream by emulating two ONUs with 25GBd DP-QPSK, resulting in a total PON upstream capacity of 200Gbit/s.