We experimentally demonstrate an innovative Ultra Dense Wavelength Division Multiplexing (UDWDM) Passive Optical Networks (PON) that implements the full λ-to-the-user concept in a filterless distribution network. Key element of the proposed system is a novel class of coherent transceivers, purposely developed with a non-conventional technical approach. Indeed, they are designed and realized to avoid D/A-A/D converter stages and Digital Signal Processing (DSP) in favor of simple analog processing so that they match system, cost and power consumption requirements of the access networks without sacrificing the overall performance. These coherent transceivers target different use case scenarios (residential, business, fixed, wireless) still keeping perfect compatibility and coexistence with legacy infrastructures installed to support gray, Time Division Multiplexed (TDM) PON systems. Moreover, the availability of coherent transceivers of different cost/performance ratios allows for deployments of different quality service grades. In this paper, we report the successful field trial of the proposed systems in a testbed where 14 UDWDM channels (and one legacy E-PON system) are transmitted simultaneously in a dark-fiber network deployed in the city of Pisa (Italy), delivering real-time and/or test traffic. The trial demonstrated filterless operations (each remote node selects individually its own UDWDM channel on a fine 6.25 GHz grid), real-time GbE transmissions (by using either fully analog or light digital signal processing), multirate transmission (1.25 and 10 Gb/s/), high Optical Distribution Network loss (18 ÷ 40 dB) as well as a bidirectional channel monitoring system.

a b s t r a c tWe report a new scheme to support, cost efficiently, ultra-dense wavelength division multiplexing (UDWDM) for optical access networks. As validating experiment, we apply phase modulation of a reflective semiconductor optical amplifier (RSOA) at the ONU with a single DFB, and simplified coherent receiver at OLT for upstream. We extend the limited 3-dB modulation bandwidth of available uncooled To-can packaged RSOA ( $400 MHz) and operate it at 3.125 Gb/s with the optimal performance for phase modulation using small and large signal measurement characteristics. The optimal condition is selected at input power of 0 dB m, with 70 mA bias condition. The sensitivities at 3.125 Gb/s (at BER¼ 10 À 3 ) for heterodyne and intrady Q2 ne detection reach À 34.3 dB m and À 38.8 dB m, respectively.

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