Cristian Prodaniuc

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.

\lambda

Direct detection systems with advanced modulation schemes are of great importance in metropolitan networks, because of their low cost and low power requirements. In particular, PAM-4 has attracted considerable attention, but has significant transmission distance limitations in the C-band. To extend its reach, we used a dual drive Mach-Zehnder modulator to generate a chromatic dispersion (CD) pre-compensated signal with an extra (j-1) multiplication to align the optical carrier and the modulated optical signal; by doing so, we achieved successful 128 Gbit/s transmission over an 80 km SSMF link, the longest reported reach of single lane 100 Gbit/s PAM-4 signals over DCF-free links. Synchronized bandwidth pre-compensation was also used, to reduce the influence of bandwidth-limitations.

Data-Center Interconnects

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.

Transmission of single lane 128 Gbit/s PAM-4 signals over an 80 km SSMF link, enabled by DDMZM aided dispersion pre-compensation

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 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.

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

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

We experimentally demonstrate 56-Gbps optical interconnection over 1 km SSMF by using 10G 1550nm VCSEL. PAM4 and Poly-PAM4 MLSE algorithms are developed to optimize the system performance and overcome the chirp-chromatic-dispersion induced power degradation after 1 km fiber transmission.

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

We analyzed the performance of fractionally spaced maximum likelihood sequence estimation (MLSE) equalizers in optical systems using optical and electrical components with cut-off frequencies much less than the baud rate. It was demonstrated that signals suffering from optical and electrical impairments can be efficiently equalized in cheap direct-detection optical receivers using MLSE equalizers working only with one sample per symbol that significantly reduces the receiver complexity.

56-Gbps optical interconnection over 1 Km SSMF with 10G 1550nm VCSEL enabled by flexible PAM4 MLSE

We discuss the transmission over short-reach networks employing VCSELs for rack-to-rack optical interconnects in datacenters. The penalties imposed are compensated by DSP at the receiver. We analyze the DSP complexity and we infer on its impact on the BER performance of the system.

Fractionally spaced MLSE in bandlimited optical systems using intensity modulation and direct detection

Unlike ultralong coherent optical systems that seriously suffer from fiber nonlinearities, short-reach noncoherent systems such as data center interconnections, which utilize small, cheap, and low-bandwidth components, are sensitive to nonlinearities that are mainly produced by devices responsible for electrical signal amplification, modulation, and demodulation. One of the most promising schemes for these applications is the four-level pulse amplitude modulation format combined with intensity modulation and direct detection; however, it can be significantly degraded by linear and nonlinear intersymbol interference. Linear and nonlinear signal degradation can efficiently be handled by different types of equalizers. In many cases, the straightforward linear equalizer cannot lower the error rate at the acceptable level. Therefore, much stronger equalizers based on nonlinear models such as the Volterra series are proposed. Volterra filter that can also be orthogonalized by the Wiener model is well described in the existing literature, and, in this paper, we investigate the most critical points related to high-speed Volterra filter design and implementation. Several experiments are carried out in order to indicate filter requirements/complexity, acquisition, and stability. We also provide a simple guidance for filter complexity reduction and useful hints for channel acquisition.