Andrea Chiuchiarelli

Homodyne Coherent Optical Receiver Using an Optical Injection Phase-Lock Loop

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 <; 10- 10) and improved performance compared to direct detection at low optical SNR (OSNR). By locking to a low-power CW pilot carrier in the polarization orthogonal to the data signal, demodulation of binary phase-shift-keyed (BPSK) data has been achieved, with the required OSNR at BER = 10-3 reduced by 3 dB compared to demodulation of ASK data under the same conditions. The OSNR penalty after transmission of the ASK and BPSK signals over 40 km of standard single-mode fiber was 1-2 dB at BER = 10-3, indicating that the chromatic dispersion sensitivity of the coherent receiver is similar to that for direct detection, and verifying that the scheme for locking to the orthogonal pilot is applicable to transmission systems, provided that optical polarization tracking is employed. In addition, we demonstrate frequency-selective operation of the coherent receiver, demultiplexing and demodulating one of a pair of equal power, 10 Gb/s, ASK channels separated by 17.5 GHz, with OSNR penalty at BER = 10- 3 of 1.5 dB compared to single-channel operation.

Enhancing Resilience to Rayleigh Crosstalk by Means of Line Coding and Electrical Filtering

We demonstrate a technique that ameliorates the performance of bidirectional systems impaired by in-band Rayleigh crosstalk, which is common in passive optical access networks based on centralized remote wavelength feeding. The technique is based on the combined use of a suitable line coding and ad hoc postdetection electrical filtering. Therefore, it can be seamless implemented in optical access systems and it is easily scalable with the data rate. We experimentally assess the technique by using a 8B10B coded 1.25-Gb/s signal. We report an increased tolerance to the signal-to-crosstalk ratio by about 5 dB.

Effective homodyne optical phase locking to PSK signal by means of 8b10b line coding

We demonstrate a novel technique that allows effective homodyne optical phase locking to a phase shift keying (PSK) signal with a residual carrier. We exploit 8b10b coding of the signal in order to reduce its low frequency spectral content, suppressing the data-to-phase crosstalk effect. In a transmission experiment on a 10 Gb/s binary PSK signal (8 Gb/s before coding), we achieved transmission over 215 km of dispersion-compensated, installed single-mode fibre, with no penalty compared to back-to-back at a bit error ratio of 10(-3). The proposed solution is applicable to other modulation formats, including multi-level amplitude and/or phase modulation formats.

Uncooled and Polarization Independent Operation of Self-Seeded Fabry–Pérot Lasers for WDM-PONs

We present for the first time an experimental characterization of uncooled and polarization-insensitive operations of directly modulated self-seeded Fabry-Pérot laser diodes (FP-LDs) for wavelength division multiplexed passive optical networks (WDM-PONs). Polarization-insensitive operations are achieved through the combination of nonreciprocal optical elements placed both at the FP-LD and the WDM distribution node, which is placed within the PON. Compared to self-seeded reflective semiconductor optical amplifiers, the FP-LDs show narrower emission linewidth, resulting in improved resilience to chromatic dispersion. The 1.25-Gb/s WDM system operation is assessed on 32 channels in the C-band over a 100-GHz grid and in a wide range of ambient temperature (0°C-60°C). The results shown in this letter promote the self-seeded uncooled FP-LDs to a potential candidate for WDM-PON deployments.

Low Complexity Advanced DBP Algorithms for Ultra-Long-Haul 400 G Transmission Systems

We experimentally assess the performance versus complexity tradeoff of advanced split-step and Volterra-based digital backpropagation (DBP) techniques, applied to aWDM (75-GHz flexigrid) ultra-long-haul (ULH) transmission system composed of five dual-carrier PM-16 quadratic-amplitude modulation 400 G superchannels. Using the recently proposed weighted Volterra series nonlinear equalizer (W-VSNE), we demonstrate a maximum reach improvement of 600 km, obtained at the expense of only six DBP steps for an entire >5000-km optical link (1000 km/step), representing a ~90% reduction on the total number of DBP steps relatively to the widely used split-step Fourier method (SSFM) implementation. The W-VSNE technique is also shown to be more accurate than the correspondent weighted SSFM (W-SSFM) algorithm when applied in medium (10-50 DBP steps) and low complexity (<;5 DBP steps) regimes. The achieved reduction of computational effort (up to 88%), together with the associated latency savings (up to 90%), demonstrates the feasibility of low complexity and efficient DBP for high-speed ULH transmission systems.

Variable Delay With Directly-Modulated R-SOA and Optical Filters for Adaptive Antenna Radio-Fiber Access

We present an all-optical adaptive-antenna radio over fiber transport system that uses proven, commercially-available components to effectively deliver standard-compliant optical signaling to adaptive multiantenna arrays for current and emerging radio technology implementations. The system is based on a directly-modulated reflective semiconductor amplifier (R-SOA) and exploits the interplay between transmission-line dispersion and tunable optical filtering to achieve flexible true time delay, with 2pi beam steering at the different antennas. The system was characterized, then successfully tested with two types of signals defined in IEEE 802.16 (WiMAX) standard for wireless networks: a 90 Mbps single-carrier signal (64-QAM at 2.4 GHz) and a 78 Mbps multitone orthogonal frequency-division multiple access (OFDMA) signal. The power budget of this configuration supports a 4-element antenna array.

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

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).

Multicarrier Digital Backpropagation for 400G Optical Superchannels

Optical superchannels and digital nonlinear mitigation are key technology options to be considered for the deployment of next-generation optical 400G transmission systems. In this paper, we experimentally assess the performance and complexity of multicarrier digital backpropagation (DBP) for dual- and triple-carrier 400G superchannels based on polarization-multiplexed (PM) 16QAM and 64QAM modulation. As an alternative to the widely used nonlinear compensation based on total-field DBP, we demonstrate that a coupled-equations DBP (CE-DBP) approach can be more computationally efficient and also more robust to a nonideal equalization of the receiver front-end in scenarios with limited sampling rate and electrical bandwidth. Employing a triple-carrier PM-16QAM superchannel placed in a 75-GHz slot of a WDM system, we demonstrate an ultralong-haul signal reach of 6600 km using CE-DBP, corresponding to 32% increase relatively to chromatic dispersion equalization (CDE). Targeting an highly spectral-efficient solution for 400G transmission in metro optical networks, these results are extended to a triple-carrier PM-64QAM superchannel placed in a 50-GHz slot, yielding a maximum signal reach of 1750 km through the use of CE-DBP in single-superchannel propagation, corresponding to approximately 60% increase over CDE.

Polarization independent self-seeding of Fabry-Peròt laser diodes for WDM-PONs

We obtain stable 1.25 Gb/s self-seeded operation of directly-modulated Fabry-Peròt laser diodes without polarization control, at several wavelengths. Benefits against self-seeded Reflective-SOAs are also discussed.

Adaptive antenna system for OFDMA WiMAX radio-over-fiber links using a directly modulated R-SOA and optical filtering

We implement a beam steering system based on a directly-modulated unseeded R-SOA, allowing the distribution of 2.4 GHz 64 QAM OFDMA signals with 2048-subcarriers satisfying IEEE 802.16 e specifications.