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We present the theoretical analysis and the numerical simulations of a polarization-independent coherent receiver for binary intensity-modulated (IM) signals. The receiver is designed for access networks or data centers and is DSP-free. It is based on a 3 × 3 symmetric coupler with phase-diversity detection. To achieve polarization independence, the receiver conveniently exploits the third input of the 3 × 3 coupler, which was previously left unused. We first introduce the scheme of the receiver and derive the analytical expressions for the final output signal, which is produced by analog processing. We then confirm the approach by numerical simulations. These are used to assess the practical potential and indicate the possible limitations. We finally discuss the role of the key parameters that must be considered in upgrading to higher bit rates and the extension to other modulation formats.

Some of the different aspects of the DSP enabled receivers in access networks will be addressed in this article. A revision of the main topics which may impact DSP design will be discussed through application examples. Modulation formats, KK receivers, and coherent receivers are analyzed at light of the DSP enabled receivers’ impact. Also, relevant are FEC and burst mode which are debated at the light of PON requirements. Simplified analog coherent receivers are presented as a use case to serve as simplification path for non-DSP high performance receivers. On the other end, aiming at discussing current PON limits, an ultra dense wavelength division multiplexing (UDWDM) use case is also presented where several advanced topics like pulse shaping, high density, and simplified DSP operations are considered.

A new pair of optical wireless modems has been realized, which exploit visible light communication to transmit Ethernet signals through water. The modem prototypes were finally tested in sea waters at La Spezia harbor; they successfully transmitted 10 Mbit/s 10Base-T signals over up to 10 m, notwithstanding the high turbidity and the strong sunlight. Final tests included the integration with SUNRISE testbed and the use with a moving robot, remotely operated. Commercial components were used to realize the modems; thus, we expect that the key design concepts can be used as a starting point for practical deployment of this technology.

©2017 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes, creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works. We experimentally demonstrate an innovative ultradense wavelength division multiplexing (UDWDM) passive optical networks (PON) that implements the full ¿-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 nonconventional technical approach. Indeed, they are designed and realized to avoid D/A-A/D converter stages and digital signal processing 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 co-existence with legacy infrastructures installed to support gray, time division multiplexed 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 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.

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.

We describe the operating principle, practical implementation, and experimental evaluation of a synchronous homodyne coherent optical receiver based on an optical phase-locking scheme that combines optical injection locking of the semiconductor laser local oscillator (LO) with low-speed electronic feedback to give both a large locking bandwidth and a wide tracking range. We demonstrate phase-error variance as low as 0.002 rad2 in 10-GHz bandwidth for locking to a continuous-wave (CW) signal for a combined signal and LO linewidth of 1.5 MHz (full-width at half-maximum), and robust phase locking to a 10-Gb/s binary amplitude-shift-keyed (ASK) signal, enabling synchronous back-to-back demodulation of the signal with low bit error ratio (BER

We experimentally investigate the real transparency of four-wave mixing (FWM) in semiconductor optical amplifiers to modulation formats involving intensity, phase, and polarization multiplexing. We exploit two different FWM polarization-independent schemes (that make use of two pumps) to wavelength-convert 40 Gb/s single-polarization and 80 Gb/s polarization-multiplexed signals in case of both nonreturn-to-zero ON -OFF keying (NRZ-OOK) and NRZ differential phase-shift keying modulation formats. We found that, although FWM conversion is transparent to modulation formats employing phase and intensity, polarization-multiplexed signals pose serious limitations to all-optical processing transparency.

We analytically compare the system performance of various types of in-line optical regenerators in long distance amplified links. The evolution of the probability density function (pdf) of the optical signal intensity is evaluated, and bit error ratio (BER) is estimated numerically. Critical issues of common solutions are found