Stefano Calabro

Technology Options for 400 Gb/s PM-16QAM Flex-Grid Network Upgrades

In this letter, we report on 400 Gb/s polarization multiplexed 16-state quadrature amplitude modulation (PM-16QAM) transponder variants and flex-grid network upgrade configurations. We address transponder subcarrier granularity, and demonstrate that the performance improvement, from dual-carrier to quad-carrier super-channel configuration, is limited to ~ 1.4 dB (in Q-factor, at power spectral density of 10-1 mW/GHz), at the cost of doubled hardware requirements. In view of that, we establish the performance improvements, for a dual-carrier 400 Gb/s PM-16QAM transceiver, as a function of increasing forward error correction overhead (FEC-OH) and spectral inversion based super-channel fiber nonlinearity compensation (SNLC-SI). We show that increasing the FEC-OH improves the transmission performance, at the cost of significant power consumption requirements, alternatively, employing SNLC-SI, at a lower FEC-OH, is a more power efficient solution. In particular, for homogeneous and heterogeneous launch power based network configurations, SNLC-SI enables ~ 23% and ~ 45% reach improvements at maximum considered FEC-OH (45%). At a fixed distance, it enables ~ 25% and ~ 50% power savings, respectively, compared with FEC-OH employing linear compensation only.

Joint adaptive pre-compensation of transmitter I/Q skew and frequency response for high order modulation formats and high baud rates

We present a digital algorithm for joint pre-compensation of the low-pass frequency response and I/Q skew in transmitters. Experimental results for DP-16QAM to DP-256QAM at 37.41 GBaud are presented.

A Robust Adaptive Pre-Distortion Method for Optical Communication Transmitters

Present and next generation optical communication systems are constantly being developed to operate at higher baud rates and higher modulation formats. It then becomes inevitable to consider the various linear and nonlinear effects due to the imperfect components at the transmitter. State-of-the-art digital-to-analog converters, high bandwidth driver amplifiers, and dual-polarization (DP) Mach-Zehnder modulators are far from being ideal and present distortions in the form of bandwidth limitation, transmitter I/Q skew, and nonlinear effects. In this letter, we propose a new robust pre-distortion method to mitigate the undesirable linear and non-linear distortions of all electrical and optical transmitter components simultaneously. A step-by-step derivation and implementation of the algorithm are discussed and put forward. The performance of the algorithm is experimentally assessed across DP-4QAM, DP-8QAM, DP-16QAM, DP-32QAM, and DP-64QAM up to the signaling rates of 56 GBd. Significant improvements in the required optical signal-to-noise ratio at bit-error rate (BER) of 10-2 are demonstrated.

Record field demonstration of C-band multi-terabit 16QAM, 32QAM and 64QAM over 762km of SSMF

We demonstrate a record 38.4Tb/s 64QAM C-band transmission over 762km of field deployed SSMF fiber, connecting Lyon and Marseille, France, employing hybrid EDFA-Raman amplification and achieve a spectral efficiency of 8b/s/Hz. Furthermore, we demonstrate, for the first time to our knowledge, flexi-rate transmission over the same commercial link, using quad-carrier 1Tb/s 16QAM (24.0 Tb/s), 1Tb/s 32QAM (32.0 Tb/s) and 1.2Tb/s 64QAM (38.4 Tb/s).

Digital Compensation of Bandwidth Limitations for High-Speed DACs and ADCs

In this paper, we present a novel digital preemphasis algorithm to compensate for the electrical bandwidth limitations at the transceiver also by taking into account the quantization noise introduced by the signal digitalization. The proposed method is based on the minimization of the mean square error between the desired input signals and the output signals of the digital-to-analog converter/analog-to-digital converter (DAC/ADC), when assuming the knowledge of the DAC/ADC frequency responses. Though this paper focuses on the DAC/ADC compensation, the introduced method could be applied to electrical bandwidth limitations caused by any other component within the transponder. The performance of the algorithm is assessed in optical back-to-back configuration by comparing it against the case without digital preemphasis and with a previously published method to compensate for DAC bandwidth limitations. Our analysis shows that when utilizing realistic descriptions of the DAC/ADC, the proposed digital preemphasis (at the transmitter) or digital compensation (at the transmitter and receiver) can considerably increase the maximum transmittable symbol rate for the case of advanced modulation formats. For example, the maximum symbol rate can be ideally increased up to ~ 60% for the case of 16QAM when employing the high-speed DAC with a -3 dB electrical bandwidth of ~16 GHz and with six effective number of bits. Furthermore, we evaluate the impact of additional noise sources, based on experimental measurements, envisioning potential for further improvements of the digital preemphasis module. Finally, we experimentally verified our algorithm, for the specific case of polarization-multiplexed 16QAM, showing a considerable match between simulation and lab results.

FEC overhead and fiber nonlinearity mitigation: Performance and power consumption tradeoffs

Increasing the FEC overhead necessitates substantial power-consumption requirements, alternatively, super-channel nonlinear mitigation ensures 30% reduced power-consumption, for a fixed distance at a lower overhead, and up to 35% improved reach, at a fixed FEC overhead.

Constellation Expansion for 100G Transmission

In this letter, we focus on coherently detected, non differentially encoded, polarisation division multiplexed 100G communication systems; through incorporating phase noise in the channel model, we assess the net coding gain achievable by various constellations that can be used in lieu of the conventional quadrature phase-shift keying (QPSK). Whereas the optical community put much effort into the optimization of the forward error correction codes for QPSK, trying to reduce the gap to the Shannon limit, we show that a potential gain of more than 2 dB is still available if different constellations are adopted.

Interchannel nonlinear transmission penalties in polarization-multiplexed 2 x 10 Gbit/s differential phase-shift keying transmission

We discuss the performance of a multichannel 2×10Gbit?s polarization-multiplexed differential phase-shift keying transmission system. Through simulations and transmission experiments we find that, despite the constant power envelope of non-return-to-zero phase-shift keying modulation formats, polarization multiplexing strongly reduces the nonlinear tolerance and transmission performance at a 2×10Gbit?s line rate with multichannel transmission. This results in a 10 dB power penalty when comparing single- and nine-channel transmission. Additionally, multichannel impairments in differential phase-shift keying and on–off keying are compared for 2×10Gbit?s polarization-multiplexed transmission.

Multi-flex field trial over 762km of G.652 SSMF using programmable modulation formats up to 64QAM

We demonstrate next-generation network upgrade scenarios using flexi-format (PM-QPSK→PM-64QAM) and flexi-rate (100G→300G) transmission over field-deployed fiber (762km). The back-to-back penalties are limited to ~2.6dB, whereas after transmission, available margin in excess of ~7.6dB is reported.

Constellation expansion and multi-symbol detection for differentially encoded 100G systems

We propose an approach to differentially encoded 100G transmission that improves both spectral efficiency and OSNR performance over conventional DQPSK-based systems, demonstrating a practical performance beyond the theoretical limit of the conventional approach.