Fabio Bottoni

All DFB-Based Coherent UDWDM PON With 6.25 GHz Spacing and a

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.

{>}{\rm 40}~{\rm dB}

We report the first demonstration of a 40 Gb/s orthogonal frequency division multiplexing (OFDM) optical system exploiting a common 1 GHz bandwidth R-SOA and direct detection. To this aim, we combined the use of two techniques, i.e., the adaptive OFDM signaling and the optical equalization. Adaptive OFDM (with optimized bit- and power-loading) allowed us to increase the spectral efficiency and the resilience to chromatic dispersion. Optical equalization was obtained by the interplay between the chirped signal produced by the reflective semiconductor optical amplifiers (R-SOA) and a detuned WDM demultiplexer. This allowed us to increase the useful channel bandwidth to > 10 GHz. By this combination and using state of the art A/D and D/A converters, a 40 Gb/s signal is successfully transmitted over a 26 km SMF optical ring.

Power Budget

We experimentally demonstrate a cost-effective coherent 10 Gb/s system for passive optical networks, exploiting off-the-shelf DFB lasers and a phase-diversity receiver based on a simple 3 × 3 fiber coupler. Since the system uses a simple amplitude-shift keying format, no complex electronic processing is required and there is no need of frequency/phase stabilization of the local oscillator, whose frequency can change by more than ±1 GHz with no significant performance variation. The system has a 40 dB loss budget and is, therefore, compatible with the high losses of practical optical distribution networks, where power splitting is used to distribute the signal to a high number of users. Error-free 10-Gb/s transmission at the FEC limit is obtained after transmission over up to 66 km of G.652 single mode fiber. Polarization-independent operation is also demonstrated with a simple modification of the detection scheme, without duplicating components, and with a small variation of the sensitivity. The limited complexity indicates the potential for a cost-effective implementation, which makes it compatible with the strictly cost-aware access networks environment, even for high-end services.

40 Gb/s Single R-SOA Transmission by Optical Equalization and Adaptive OFDM

We report enhanced 10 Gb/s operation of directly modulated bandwidth-limited reflective semiconductor optical amplifiers. By using a single suitable arrayed waveguide grating we achieve simultaneously WDM demultiplexing and optical equalization. Compared to previous approaches, the proposed system results significantly more tolerant to seeding wavelength drifts. This removes the need for wavelength lockers, additional electronic equalization or complex digital signal processing. Uniform C-band operations are obtained experimentally with < 2 dB power penalty within a wavelength drift of 10 GHz (which doubles the ITU-T standard recommendations).

Coherent Systems for Low-Cost 10 Gb/s Optical Access Networks

A 1.25 Gb/s ASK PON system with -51dBm sensitivity (at BER=2-103) is enabled by a polarization-independent coherent receiver that needs no DSP (nor ADC). The system just uses common DFBs and commercial electronic devices and has 52 dB dynamic range.

Enhanced 10 Gb/s operations of directly modulated reflective semiconductor optical amplifiers without electronic equalization

We report the first experimental demonstration of 25Gb/s operation of 1GHz bandwidth-limited Reflective Semiconductor Optical Amplifier based on DMT modulation and optical equalization. This combination is highly resilient to chromatic dispersion.

Coherent PON system with high-sensitivity polarization-independent receiver and no ADC/DSP

A 1.25-Gb/s ASK passive optical network (PON) system with a -51 dBm pre-FEC sensitivity (at BER = 2 × 10-3) is enabled by a real-time polarization-independent coherent receiver (PI-RX) that needs no DSP (nor ADC). The receiver, which only makes use of common DFBs and commercially available electronic ICs, is targeted for use on a 6.25-GHz UD-WDM grid and has a 52-dB dynamic range. Measurements of the robustness of the PI-RX against backreflections and crosstalk from coexistent adjacent channels show that this preliminary implementation is suitable for UD-WDM-PON systems with frequency spacing down to 5 GHz.

25 Gb/s operation of 1-GHz bandwidth R-SOA by using DMT and optical equalization

This paper presents the first experimental achievements of the COCONUT project which aims at demonstrating ultra-dense Wavelength Division Multiplexing Passive Optical Networks (udWDM-PON) by means of low-cost coherent solutions. COCONUT should exploit low-cost optical components and simple electronics so that the cost of typical line terminals would be affordable to the end-users. Among different technical approaches considered within the project; we focus here on the solutions based on simple OOK-NRZ format. We developed an original improvement of the conventional phase-diversity coherent detection schemes providing for polarization independent operations without doubling the receiver structure. As a result we demonstrated that passive networks of high-power budget (> 45 dB i.e. up to 60 km reach and more than 64-way power splitting) are feasible over a 6.25 GHz grid by using a limited number of photodiodes a simplified analog processing and free-running DFBs.

Polarization-Independent Coherent Real-Time Analog Receiver for PON Access Systems

We demonstrate a bidirectional coherent PON based on a direct-modulated upstream signal reused as LO. The proposed system achieves -46/-48 dBm downstream/upstream sensitivity at BER=10-3.

Low cost coherent receivers for UD-WDM NRZ systems in access networks

An UDWDM-PON with simple real-time ASK and DPSK transceivers at 6.25 GHZ channel spacing is validated in a field-trial with deployed fiber with power budget above 40 dB. This work presents final results of the Coconut European Project.