Fotini Karinou

Directly PAM-4 Modulated 1530-nm VCSEL Enabling 56 Gb/s/

We demonstrate a 56 Gb/s four-level pulse-amplitude modulation (PAM-4) transmission using direct detection and a long-wavelength 18-GHz bandwidth vertical-cavity surface-emitting laser as directly modulated light source for short-reach inter- and intra-connects in datacenters and short-reach networks. Error-free transmission over 2 km at 7% hard-decision forward-error correction threshold is achieved by applying powerful equalization schemes at the receiver side. Three equalization schemes, i.e., a maximum likelihood estimation (MLSE), a feed-forward equalizer (FFE), and a combination of the FFE and the MLSE are thoroughly investigated, and the performance comparison between them is carried out.

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We experimentally demonstrate Tx DSP-free 112-Gbit/s PAM-4 transmission over 20 km SMF using commercially available 1298 nm 25-GHz TOSA and ROSA devices. Superior performance is achieved by using timing recovery supported DSP at the receiver.

Data-Center Interconnects

We experimentally demonstrate the possibility of transmitting 112  Gb/s PAM-4 signals over 30-km standard single-mode fibers with commercial 20-GHz components. The impact on system performance of three different equalization schemes-feed-forward equalizer (FFE), Volterra filter (VF), and maximum likelihood sequence estimator (MLSE)-is investigated and compared. We prove that the |x| and x|x| components of the VF can efficiently reduce the chirp-induced signal skew distortion, and that an MLSE with only four states can be used to remove the error floor. In contrast, the performance of FFE is worse than that of VF with a 4-state MLSE, even when a 64-state MLSE is used.

Performance and DSP complexity evaluation of a 112-Gbit/s PAM-4 transceiver employing a 25-GHz TOSA and ROSA

We demonstrate the transmission of 28 Gb/s NRZ-OOK over DCF-free links up to 10 km using a monolithic VCSEL, direct detection and an MLSE-based receiver for low-cost, short-reach interconnects. DSP complexity is also investigated in terms of performance optimization.

Cost-effective single-lane 112 Gb/s solution for mobile fronthaul and access applications

We demonstrate the generation and transmission of a 28-Gb/s intensity modulated optical signal over single mode fiber (SMF) links up to 1, 2, 5, and 10 km employing a 18-GHz 3-dB bandwidth monolithic vertical-cavity surface-emitting laser (VCSEL) based on buried tunnel junction technology with regrown n-doped InP material. Inexpensive technologies such as nonreturn-to-zero on-off keying modulation format, direct detection as well as digital signal processing-based receivers make the transmission feasible for chromatic dispersion (CD) uncompensated SMF links up to 10 km at the 7% hard-decision (HD) forward error correction (FEC) limit. Three different equalizers for the receiver side are investigated in this paper in terms of performance optimization, i.e., the maximum likelihood sequence estimation equalizer (MLSE), the feed-forward equalizer (FFE) and a hybrid scheme that comprises of an FFE and an MLSE equalizer (FFE/MLSE) in cascaded form. Our experimental results indicate that MLSE scheme outperforms the other two counterparts in all transmission scenarios. Performance optimization in terms of MLSE complexity is also presented. Less complex implementations like FFE equalizer can provide reliable transmission below the 7% HD-FEC limit for links up to 5 km. The FFE/MLSE scheme provides similar performance as MLSE for links up to 5 km, while its performance deteriorates for 10 and 15 km links as signal distortion due to CD becomes too severe to recover the signal. A timing recovery unit is employed before the equalizers in order to compensate for phase offsets and improve the signal quality. Our proposed scheme proves a promising candidate to enable VCSEL-based short reach optical interconnects in data centers and metro-access area.

28 Gb/s NRZ-OOK using 1530-nm VCSEL, direct detection and MLSE receiver for optical interconnects

We investigate two trellis-coded modulation (TCM) schemes for use in next-generation low-cost 400G optical networks. Their performance is evaluated against that of four-level pulse amplitude modulation (PAM-4). We experimentally demonstrate that TCM outperforms PAM-4 at bit rates between 56 and 80 Gbps and thus is a valid candidate for 8λ × 50G implementations of future 400G wave division multiplexing (WDM) networks. Due to the severe bandwidth limitations of the experimental setup, the feed-forward equalizer and the maximum likelihood sequence estimation equalizer are employed in order to achieve bit error rates below the KP4 threshold of 3 × 10-4. We also show that, at 56 Gbps and 1300 nm wavelength, TCM enables optical links of up to 40 km, whereas PAM-4 can only provide transmissions of up to 30 km. We observe that TCM performance degrades faster than that of PAM-4 when we increase the bit rate. For rates higher than 80 Gbps, TCM is not able to improve upon PAM-4 anymore. Thus, only PAM-4 is fit for 4λ × 100G WDM networks, unless higher bandwidth components are used.

1.55-μm Long-Wavelength VCSEL-Based Optical Interconnects for Short-Reach Networks

The performance of two equalization techniques is experimentally compared using a 1530-nm VCSEL, 56Gb/s/A PAM-4, and direct detection over 2 km links for datacenter interconnect applications. Results on the performance evaluation in terms of DSP implementation and complexity are presented.

Performance Comparison Between 4D Trellis Coded Modulation and PAM-4 for Low-Cost 400 Gbps WDM Optical Networks

In this paper, we present an accurate blind method for chromatic dispersion estimation in coherent optical receivers. The method is independent on modulation formats and signal spectra. Additionally, the estimation method is proven to be robust against sampling phase, polarization mode dispersion, carrier phase and frequency offset, and clock frequency offset. Excellent performance is demonstrated even in extremely bandlimited systems, so-called faster than Nyquist systems.

Experimental performance evaluation of equalization techniques for 56 Gb/s PAM-4 VCSEL-based optical interconnects

In contrast to the more prominent discrete-variable quantum key distribution (DV-QKD), which requires specialized hardware like single-photon detectors, the continuous-variable version (CV-QKD) promises low-cost and high-performance implementations by leveraging mature telecommunication technology. In this paper we demonstrate a simplified CV-QKD architecture based on analog frontends and digitizers for mobile communication systems and standard optical components. The high-fidelity, software-defined receiver and transmitter allow us to shift complexity from the analog to the digital domain, thus simplifying the system, improving robustness and paving the way for a low-cost implementation.

Efficient and low-complexity chromatic dispersion estimation in coherent optical systems

Low complexity, single-stage frequency-domain equalization supported by blind CD estimation and training-aided 2×2 MIMO channel estimation is proposed and experimentally demonstrated in a 32 GBaud Nyquist PDM-QPSK transmission system.