R. Isaac

High Spectral Efficiency 400 Gb/s Transmission Using PDM Time-Domain Hybrid 32–64 QAM and Training-Assisted Carrier Recovery

We report the successful transmission of ten 494.85 Gbit/s DWDM signals on the standard 50 GHz ITU-T grid over 32 × 100 km of ultra-large-area (ULA) fiber. A net spectral efficiency (SE) of 8.25 b/s/Hz was achieved, after excluding the 20% soft-decision forward-error-correction (FEC) overhead. Such a result was accomplished by the use of a recently proposed polarization-division-multiplexed (PDM) time-domain hybrid 32-64 quadrature-amplitude-modulation (QAM) format, along with improved carrier frequency and phase recovery algorithms. It is shown that time-domain hybrid QAM provides a new degree of design freedom to optimize the transmission performance by fine tuning the SE of the modulation format for a specific channel bandwidth and FEC redundancy requirement. In terms of carrier recovery, we demonstrate that 1) hardware efficient estimation and tracking of the frequency offset between the signal and local-oscillator (LO) can be achieved by using a new feedback-based method, and 2) a training-assisted two-stage phase estimation algorithm effectively mitigates cyclic phase slipping problems. This new phase recovery algorithm not only improves the receiver sensitivity by eliminating the need for differential coding and decoding, but also enables an additional equalization stage following the phase recovery. We have shown that the introduction of this additional equalization stage (with larger number of taps) helps reduce the implementation penalty. This paper also presents the first experimental study of the impact of inphase (I) and quadrature (Q) correlation for a high-order QAM. It is shown that an adaptive equalizer could exploit the correlation between I and Q signal components to artificially boost the performance by up to 0.7 dB for a PDM time-domain hybrid 32-64 QAM signal when the equalizer length is significantly longer than I/Q de-correlation delay.

PDM-Nyquist-32QAM for 450-Gb/s Per-Channel WDM Transmission on the 50 GHz ITU-T Grid

We discuss the generation and transmission of 450 Gb/s wavelength-division multiplexed (WDM) channels over the standard 50 GHz ITU-T grid optical network at a net spectral efficiency of 8.4 b/s/Hz. This result is accomplished by the use of Nyquist-shaped, polarization-division-multiplexed (PDM) 32-quadrature amplitude modulation (QAM) and both pre- and post-transmission digital equalization. To overcome the limitation of available digital-to-analog converter bandwidth, a novel method is introduced for the generation of the five-subcarriers of the 450 Gb/s signal. Nearly ideal Nyquist pulse-shaping (roll-off factor = 0.01) enables guard bands of only 200 MHz between subcarriers. To mitigate the narrow optical filtering effects from the 50 GHz-grid reconfigurable optical add-drop multiplexer (ROADM), a broadband optical pulse-shaping method has been proposed and demonstrated. By combined use of electrical and optical shaping techniques, transmission of 5 × 450 Gb/s PDM-Nyquist 32 QAM on the 50 GHz grid over 800 km and one 50 GHz-grid ROADM has been successfully demonstrated.

Spatial Superchannel Routing in a Two-Span ROADM System for Space Division Multiplexing

We report a two-span, 67-km space-division-multiplexed (SDM) wavelength-division-multiplexed (WDM) system incorporating the first reconfigurable optical add-drop multiplexer (ROADM) supporting spatial superchannels and the first cladding-pumped multicore erbium-doped fiber amplifier directly spliced to multicore transmission fiber. The ROADM subsystem utilizes two conventional 1 × 20 wavelength selective switches (WSS) each configured to implement a 7 × (1 × 2) WSS. ROADM performance tests indicate that the subchannel insertion losses, attenuation accuracies, and passband widths are well matched to each other and show no significant penalty, compared to the conventional operating mode for the WSS. For 6 × 40 × 128-Gb/s SDM-WDM polarization-multiplexed quadrature phase-shift-keyed (PM-QPSK) transmission on 50 GHz spacing, optical signal-to-noise ratio penalties are less than 1.6 dB in Add, Drop, and Express paths. In addition, we demonstrate the feasibility of utilizing joint signal processing of subchannels in this two-span, ROADM system.

Joint Digital Signal Processing Receivers for Spatial Superchannels

We discuss the advantages of spatial superchannels for future terabit networks based on space-division multiplexing (SDM), and demonstrate reception of spatial superchannels by a coherent receiver utilizing joint digital signal processing (DSP). In a spatial superchannel, the SDM modes at a given wavelength are routed together, allowing a simplified design of both transponders and optical routing equipment. For example, common-mode impairments can be exploited to streamline the receivers DSP. Our laboratory measurements reveal that the phase fluctuations between the cores of a multicore fiber are strongly correlated, and therefore constitute such a common-mode impairment. We implement master-slave phase recovery of two simultaneous 112-Gbps subchannels in a seven-core fiber, demonstrating reduced processing complexity with no increase in the bit-error ratio. Furthermore, we investigate the feasibility of applying this technique to subchannels carried on separate single-mode fibers, a potential transition strategy to evolve todays fiber networks toward future networks using multicore fibers.

8 450-Gb/s, 50-GHz-spaced, PDM-32QAM transmission over 400km and one 50GHz-grid ROADM

Employing Nyquist-pulse-shaped PDM-32QAM modulation and both pre- and post-transmission digital equalization, we demonstrate 50GHz-spaced, 8 450Gbits/s DWDM transmission over 4 100km of ultra-large-area fiber and one 50GHz-grid WSS-based ROADM at 8.37b/s/Hz spectral efficiency.

A robust real-time 100G transceiver With soft-decision forward error correction [Invited]

We demonstrate a 120 Gb/s coherent polarization-multiplexed quadrature-phase-shift-keyed transceiver with soft-decision forward-error-correction (SD-FEC) coding based on Turbo Product Code. This industry-first transceiver module utilizes a 40 nm complementary metal-oxide semiconductor (CMOS) application-specific integrated circuit with integrated analog-to-digital conversion, digital signal processing and SD-FEC, and is packaged according to a multi-source agreement from the Optical Internetworking Forum. Through several long-haul and ultra-long-haul system experiments (over 1000 km to 3760 km), we validate the robustness of the transceiver and demonstrate its high tolerance to various system impairments, including fiber nonlinearity, chromatic dispersion up to 60,000 ps/nm, polarization mode dispersion, polarization-dependent loss, polarization transients and multiple-path interference.

1200km Transmission of 50GHz spaced, 5×504-Gb/s PDM-32-64 hybrid QAM using electrical and optical spectral shaping

Employing time-domain hybrid QAM and electrical and optical shaping, we successfully transmitted five 504Gbits/s PDM-32/64QAM DWDM signals at 8.4b/s/Hz net spectral efficiency (SE) over a record 12×100km reach (for WDM SE>4b/s/Hz) and one 50GHz-grid ROADM.

4000km transmission of 50GHz spaced, 10×494.85-Gb/s hybrid 32–64QAM using cascaded equalization and training-assisted phase recovery

Employing time-domain hybrid QAM, training-assisted phase recovery and cascaded equalization, we successfully transmitted ten 494.85Gbit/s PDM-32/64QAM DWDM signals at 8.25b/s/Hz net spectral efficiency (SE) over 40×100km, achieving a record terrestrial SE⋅distance product of 33,000-km b/s/Hz.

Demonstration of joint DSP receivers for spatial superchannels

We report lab measurements of joint digital signal processing of simultaneous 112Gbps links in a 7-core fiber. Strongly-correlated phase fluctuations between the cores permit reduced processing complexity with no increase in the bit-error ratio.

Erratum [regarding "High Spectral Efficiency 400 Gb/s Transmission Using PDM Time-Domain Hybrid 32-64 QAM and Training-Assisted Carrier Recovery"]

We report the successful transmission of ten 494.85 Gbit/s DWDM signals on the standard 50 GHz ITU-T grid over 32 × 100 km of ultra-large-area (ULA) fiber. A net spectral efficiency (SE) of 8.25 b/s/Hz was achieved, after excluding the 20% soft-decision forward-error-correction (FEC) overhead. Such a result was accomplished by the use of a recently proposed polarization-division-multiplexed (PDM) time-domain hybrid 32-64 quadrature-amplitude-modulation (QAM) format, along with improved carrier frequency and phase recovery algorithms. It is shown that time-domain hybrid QAM provides a new degree of design freedom to optimize the transmission performance by fine tuning the SE of the modulation format for a specific channel bandwidth and FEC redundancy requirement. In terms of carrier recovery, we demonstrate that 1) hardware efficient estimation and tracking of the frequency offset between the signal and local-oscillator (LO) can be achieved by using a new feedback-based method, and 2) a training-assisted two-stage phase estimation algorithm effectively mitigates cyclic phase slipping problems. This new phase recovery algorithm not only improves the receiver sensitivity by eliminating the need for differential coding and decoding, but also enables an additional equalization stage following the phase recovery. We have shown that the introduction of this additional equalization stage (with larger number of taps) helps reduce the implementation penalty. This paper also presents the first experimental study of the impact of inphase (I) and quadrature (Q) correlation for a high-order QAM. It is shown that an adaptive equalizer could exploit the correlation between I and Q signal components to artificially boost the performance by up to 0.7 dB for a PDM time-domain hybrid 32-64 QAM signal when the equalizer length is significantly longer than I/Q de-correlation delay.