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Intensity modulation and direct detection (IM/DD) with pulse amplitude modulation (PAM) is a cost-effective option for intra- and inter-data center (DC) applications. Due to current demand on DC data traffic, transmission rates beyond 200 Gbit/s/ $\lambda $ are desirable. However, employing commercially available lower bandwidth components introduces severe inter-symbol interference (ISI). In addition, to achieving high-speed data rates, digital signal processing (DSP) schemes like Tomlinson-Harashima precoding (THP) and decision feedback equalizer (DFE) can increase tolerance against ISI. In this letter, we show an experimental comparison of PAM-4, PAM-6 and PAM-8 modulation formats with and without THP on the transmitter-side and Volterra nonlinear equalizer (VNLE) on the receiver-side. We provide also results with and without duobinary (DB) encoding on the transmitter side and DFE on the receiver side. The experiment is conducted using an O-band electro-absorption modulated laser over transmission distances up to 20 km.

We have experimentally shown that probabilistically shaped PAM-8 improves receiver sensitivity by more than 1 dB compared with regular PAM-8 for data rates beyond 200 Gbit/s.

We investigate the performance of non-regular PAM8 constellations and introduce a novel non-uniformly quantized equalizer to improve nonlinearity compensation in 70 GBd PAM8 back-to-back and 1 km SSMF transmission without additional complexity.

Interoperability requirements are discussed for 200Gb/s PAM4 and verified using high-bandwidth electro-absorption modulated lasers (EML) and a concatenated forward error correction (FEC) scheme based on hard-decision zipper code, which can be applied to 800Gb/s-FR4 transmission.

We have shown transmission of 270 Gb/s with PAM-8 at each lane of CWDM4 in the O-band up to 2 km. The BER performance in each lane are very similar at 7% HD-FEC limit.

We demonstrate 402 Gb/s IM/DD transmission using a single DAC directly driving an O-band EML with PAM-8. Considering 15.31%-overhead SD-FEC, this yields to a record net bit rate of 348.62 Gbit/s over 2 km SSMF.

We demonstrate the feasibility of an aggregate data-rate of 1.6Tb/s using 200Gb/s per lane. Eight O-band wavelengths carrying 112GBd PAM4 are employed to demonstrate transmission over 2km of SSMF at the HD-FEC threshold of $1.75\times 10^{-3}$ .

We experimentally demonstrate the transmission of a net rate of 250 Gb/s/lambda using PAM-6 and PAM-8 IMDD over 1-km single-mode fiber using commercially available components. A power sensitivity of -6dBm (-5dBm) is achievable for PAM-6 (PAM-8).

IEEE journal of selected topics in quantum electronics 27(3), 8100212 (2020). doi:10.1109/JSTQE.2020.3041742, Published by IEEE, New York, NY

Recent developments of 5G PHY-layer technology have evidenced the advantages taken by Filter-Bank Multicarrier (FBMC) waveforms against state-of-the-art Orthogonal Frequency Division Multiplexing (OFDM) in terms of bandwidth efficiency and spectrum sharing capabilities. In this paper, we propose the use of FBMC for drone-to-earth multicast transmission in environmental monitoring applications. In particular, we consider FBMC with Offset Quadrature Amplitude Modulation (OQAM) and Hermite polynomials-based prototype filtering in order to efficiently manage the tradeoff between performance and spectral efficiency. The weighted frequency spreading methodology has been employed in order to implement the FBMC system as a simple and computationally-affordable extension of OFDM. The FBMC-based solution has been simulated and tested in a drone based multicast configuration and compared to the standard OFDM solution in terms of bandwidth efficiency, Peak-to-Average Power Ratio (PAPR) and link performance in presence of multipath propagation. As result, FBMC-OQAM exhibits improved throughput efficiency and dramatically decreased Out-of-Band (OOB) emissions, while PAPR and BER performance achieved by the two multicarrier solutions look overall comparable. The spectrum slicing - naturally provided by FBMC and not allowed by OFDM - and long CP avoidance seem key, positive, features, in the framework of future adoption of FBMC in drone-based multicast applications, characterized by augmented flexibility, cognitivity and throughput efficiency.

Subcarrier multiplexing hybrid QAM combined with coherent superposition is proposed to achieve flexibility in spectral efficiency (SE) and reach. Experimental evaluation over SSMF shows scalable SE from 4.9 to 1.2 bit/s/Hz with maximum reach from 2000 to 15500 km, respectively.

The requirements for 600 Gb/s single wavelength transmission in short links, namely data center interconnections, are assessed. It is shown that 600 Gb/s transmission is possible using available components and digital pre-emphasis.

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.

We demonstrate the potential of single-carrier serial line-rate 400Gb/s PM-64QAM transmission over short reach optical links. Performance below pre-FEC BER threshold is achieved over 328km for 96×516Gb/s PM-64QAM, employing EDFA-Raman amplification and LA-PSCF.

A long-haul transmission of 1.28Tb/s quad-subcarrier PM-16QAM based superchannel (5.0bit/s/Hz) employing EDFA only/ EDFA-Raman amplification schemes is reported. Maximum recorded reach is 3,000km/ 2,050km and 3,350km/2,250km without and with DBP for LAPSCF/ SSMF, respectively.

We experimentally compared terabit transmission employing 16QAM Nyquist WDM and QPSK Time-Frequency Packing. The two modulation schemes have been transmitted over the same link configuration. The latter showed slightly better performance in terms of spectral efficiency and reach, at the expense of a more expensive and complex hardware.

Currently available electrical components put a limit on the maximum achievable symbol rate per channel and order of modulation for coherent optical transmission systems. Hence, future optical transmission systems are expected to utilize superchannel structure for the long-haul transmission of data rates as high as 400 Gb/s and 1Tb/s. In this paper we discuss the trade-offs at transmitter and receiver to achieve long-haul reach for 1 Tb/s superchannel.

We optimize the FEC-OH of dual-carrier PM-16QAM 400Gb/s super-channel, considering the tradeoff between FEC net coding gain and performance penalties due to limited DAC/ADC bandwidth. At optimum FEC-OH (~29%), WDM transmission of 400Gb/s super-channel over 2150km LAPSCF is experimentally demonstrated.

Elastic optical networks represent one of the most promising candidates for the imminent upgrade of current fixed-grid based optical systems. This novel architecture based on high efficient spectrum allocation (i.e. higher capacity), is necessary to cope with foreseen future exponential increase of Internet traffic. This work discusses the influence of hardware components on the transmission performance of recently proposed bandwidth variable transponders employing digital signal processing algorithms. Both are key elements for the realization of future elastic optical networks, and only their successful interplay with coherent detection can enable the transmission of different modulation formats at variable symbol and data-rates over flexible links. We evaluate the performance of such transponders when different modulation schemes are generated employing ideal and realistic values for some key hardware components. Finally, we briefly present a couple of examples of mitigation techniques: namely digital pre-distortion and digital back-propagation.

Flex-grid optical networks have evolved as a near-future deployment option to facilitate dynamic and bandwidth intense traffic demands. These networks enable capacity gains by operating on a flexible spectrum, allocating minimum required bandwidth, for a given channel configuration. It is thus important to understand the nonlinear dynamics of various high bit-rate super-channel configurations, and whether such channels should propagate homogenously (uniform channel configuration) or heterogeneously (non-uniform channel configuration), when upgrading the current static network structure to a flex-grid network. In this paper, we report on the spectrum allocation strategies based on the impact of inter-channel fiber nonlinearities, for PM-16QAM channels (240Gb/s, 480Gb/s and 1.2Tb/s) –termed as super-channels, propagating both homogenously, and heterogeneously with 120Gb/s PM-QPSK, 43Gb/s PM-QPSK, and 43Gb/s DPSK traffic. In particular, we show that for high dispersion fibers, both homogenous and heterogeneous spectrum allocation enable similar performance, i.e. the nonlinear impact of hybrid traffic is found to be minimal (less than 0.5dB relative penalties). We further report that in low dispersion fibers, the impact of spectrum allocation is more pronounced, and heterogeneous traffic employing 120Gb/s PM-QPSK neighbors enables the best performance, ~0.5dB better than homogenous transmission. However, the absolute nonlinear impact of co-propagating traffic is more significant, compared to high dispersion fibers, with maximum performance penalties up to 1.5dB.

Transmission of a 4-D optimized format with the same spectral efficiency as PM-QPSK was experimentally investigated. Decision directed equalizer adaptation and phase estimation enabled detection of this format and PM-QPSK and coded POLQAM as well.