We demonstrate a coherent 10 Gb/s PON system solution exploiting a direct modulated laser (DML) transmitter to obtain long reach (>100 km). The system exploits a simplified coherent receiver. This receiver, which is similar to the scheme that we recently proposed, had to be adapted to obtain polarization-independent operation with a chirped signal from a DML and is still based on simple optical and electrical components. Thanks to the combined use of coherent-RX and DML, the system achieves a link budget of 43 dB for 105 km transmission distance with no need of dispersion compensation. This makes it a cost-effective, simple, and robust solution for satisfying the increasing capacity request of access networks.

COCONUT project demonstrated an innovative DWDM Passive Optical Network implementing the full λ-to-the-user concept in a filterless distribution fiber network. We make use of an innovative class of coherent transceivers that exploit the advantages of coherent detection in terms of sensitivity and channel selectivity in a cost efficient manner. The different transceivers developed target different use case scenarios, showing perfect compatibility with legacy infrastructures installed. In this paper, we report the successful trial of the proposed system in a testbed where 4 different coherent transceivers (plus one legacy E-PON system) ran simultaneously, operating over 14 DWDM channels, in a dark-fiber network in the city of Pisa, delivering real-time and/or test traffic. The testbed demonstrated filterless operations, multi-rate transmission (from 1.25 to 10 Gb/s/λ), high ODN loss (18 - 40 dB) as well as a bi-directional channel monitoring system.

A coherent TX-RX prototype is presented including a real-time polarization-independent coherent receiver (PI-RX) with analog processing (no DSP). The prototype, realised by means of common DFBs and commercially available RF electronic ICs, is targeted for use on a 6.25 GHz Ultra Dense Wavelength Division Multiplexed (UD-WDM) grid and features up to 50 dB of loss budget. Robustness of the PI-RX against crosstalk from coexistent adjacent channels is demonstrated. Using two such prototypes, bidirectional transmission in a 8x1.25G UD-WDM system configuration with 6.25 GHz frequency spacing and 45 dB ODN loss is shown with channel selection by means of thermal tuning of the local oscillator.

Simplified coherent detection of directly-modulated DFB transmitters allows for 55 dB ODN power budget in downstream of a 40 Gb/s DML-based WDM-PON with 110 km reach. The solution can enhance NGPON2, addressing long reach and high power budget links (>50 dB).

An UDWDM-PON with simple real-time ASK and DPSK transceivers is validated in a fieldtrial with deployed fibre achieving -47 dBm sensitivity at BER=2.10-3 with 1.25 Gbit/s effective user bitrate. Real data traffic transmission and EPON coexistence are also demonstrated.

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.

We introduce a novel algorithm for real-time ONU activation (λ - assignment) and reconfiguration (λ - tuning) in WDM coherent PONs. The algorithm prevents service interruption of lighted channels and it is experimentally demonstrated by using real-time fully-analog coherent transceivers.

Advantages offered by coherent detection systems in access networks are not limited to the increase of power budget due to the superior sensitivity. Channel selectivity is another intriguing feature offered by coherent systems that can be exploited to enable advanced system and network functionalities for access systems. Among them, we focus here on the implementation of filterless optical networks and network reconfiguration capabilities which might be required to satisfy dynamic load balancing requests and new terminal activations. We show that in the access domain these functionalities do not require DSP-aided coherent receivers, but can be easily realized by means of simpler, fully analogue real-time coherent terminals matching the access network low-cost paradigm. This paper discusses these concepts and how they can be experimentally implemented by using a novel wavelength allocation algorithm and real-time analogue coherent transceivers based on DFB lasers, whose wavelengths are tuned by a simple temperature control.

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.

Processing of high-frequency, wide-band signals is conveniently performed in the optical domain due to availability of optical filters which intrinsically have a wide-band response. On the other end, there are several application scenarios where very sharp pass-band filtering is required. In these cases, realizing the processing in the optical domain might not be technically simple, although filters of extremely high Q-factor have been demonstrated. This paper presents the feasibility of signal processing based on very sharp electrical filtering (< 1 GHz around 193 THz) assisted by optical down-conversion, i.e., by means of optical coherent detection. In particular, we show experimentally two applications where the filtering capability is exploited to perform signal processing: the demodulation of phase-modulated optical signals and the implementation of a chirp-managed transmission link. Other possible applications of this technique will be also discussed at the presentation.

Using fully-analogue and commercially available electronics, we realize a real-time coherent ONU receiver for Ultra-Dense WDM-PON based on intensity-modulated signals. The receiver also requires common photonic components and has -47dBm sensitivity at FEC level.

This paper presents the implementations and test results of the novel polarization-independent receiver for ASK format, realized in the framework of COCONUT FP7 Project. COCONUT exploits low-cost optical components and simple electronics, so that the cost of typical line terminals would be affordable to the end-users. Among the 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 polarization-independent coherent detection is feasible within the constraint of limited cost typical of passive access networks. The developed receivers can allow for high-power budget (> 45 dB and ultra-dense WDM operation (6.25 GHz grid).

A 1.25 Gb/s ASK PON system with -51dBm sensitivity (at BER=2-10 3 ) 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.

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.

This paper presents the most significant results achieved in the last two years by our group in coherent ultra-dense-WDM PON for lambda-to-the-user access. Here we mostly focus on the architectures based on simple OOK-NRZ modulation format. We demonstrated the real-time implementation of our coherent receiver and we proved its compatibility with direct modulated laser (DML). Receiver sensitivities as low as -48 dBm at BER=1.10-3 for 1.25 Gb/s has been observed, showing the feasibility of a coherent ONU receiver based on devices commonly available and with limited cost. The system support quite high ODN losses and allows the lambda-to-the-user approach.

We demonstrate 10-Gb/s transmission over more than 130 km of G.652 fiber using a simple directly modulated distributed feedback (DFB) laser and a new “chirp-managed” approach based on a low-complexity coherent receiver. The “chirp-managed” effect, which has been obtained in the past by optical spectral reshaping, is conveniently achieved here by means of simple electrical filtering of the received signal at the intermediate frequency. Due to coherent detection, the receiver sensitivity was -38 dBm at bit error rate (BER) 1/4 1.8 χ 10-3. No colored filters and no dispersion compensation (optical or digital signal processing (DSP)-based) were used at the receiver side. We show that the coherent system presented here, based on off-the-shelf components, is robust against variations of DFB chirp, local oscillator detuning, and electrical filter bandwidth over ranges wide enough to guarantee a simple and cost-effective implementation for 10-Gb/s long-reach passive optical network applications.

We experimentally demostrate a 6.25 GHz grid WDM-PON based on commercially available directly-modulated DFB transmitters driven with a 650mV(pp) and simplified coherent receivers. The system can support 35dB distribution network losses and allows the lambda-to-the-user approach.

We demonstrate a coherent OFDM-PON based on a simplified coherent receiver, which exploits a DFB local oscillator without phase/frequency locking. We achieve ultra-narrow channel spacing (Δν = 1.6 GHz) and high-power budget (43 dB/35 dB) for 1.25/10 Gb/s.

131 km transmission (typical LR-PON distance) at 10Gb/s over G.652 fiber is demonstrated exploiting a direct modulated (DM) DFB laser, coherent receiver and electrical filtering obtaining an innovative chirp managed approach. No dispersion compensation (optical or DSP) is exploited.

The operation of an extended power budget long reach access network is experimentally demonstrated using commercially available small form pluggable directly modulated transmitters and a simplified coherent receiver free from digital signal processing and phase-locked loops. An ad-hoc choice of photodiode bandwidth allows no penalty compared with external modulation based transmitters.