Kaiheng Zou

Orthogonal-band-multiplexed offset-QAM optical superchannel generation and coherent detection.

Nowadays the Internet not only has fast growing data traffic, but also has a fast growing number of on-line devices. This leads to high demand of capacity and flexibility of the future networks. The conventional Orthogonal Frequency Division Multiplexing (OFDM) and Nyquist pulse shaping signals have the advantage of high spectral efficiency when consisting of superchannels in the Wavelength-Division-Multiplexing (WDM) way. However, they face a cost issue when the spectral granularity of the superchannel is decreased to support more users. This paper proposes for the first time the scheme of Orthogonal-band-multiplexed offset-Quadrature Amplitude Modulation (OBM-OQAM) superchannel. OBM-OQAM superchannel provides large capacity and high spectral efficiency. Furthermore, it has the advantage of offering subbands of variable symbol rate without changing the system configuration. We provide a proof-of-principle demonstration of OBM-OQAM superchannel transmission. In our experiment, 400 Gbps 16 Quadrature Amplitude Modulation (QAM) OBM-OQAM superchannel transmission over 400 km Standard Single Mode Fiber (SSMF) is conducted. The experimental results show that the OBM-OQAM signal has low penalty in multi-band aggregation.

1 λ × 1.44 Tb/s free-space IM-DD transmission employing OAM multiplexing and PDM

We report the experimental demonstration of single wavelength terabit free-space intensity modulation direct detection (IM-DD) system employing both orbital angular momentum (OAM) multiplexing and polarization division multiplexing (PDM). In our experiment, 12 OAM modes with two orthogonal polarization states are used to generate 24 channels for transmission. Each channel carries 30 Gbaud Nyquist PAM-4 signal. Therefore an aggregate gross capacity record of 1.44 Tb/s (12 × 2 × 30 × 2 Gb/s) is acheived with a modulation efficiency of 48 bits/symbol. After 0.8m free-space transmission, the bit error rates (BERs) of all the channels are below the 20% hard-decision forward error correction (HD-FEC) threshold of 1.5 × 10(-2). After applying the decision directed recursive least square (DD-RLS) based filter and post filter, the BERs of two polarizations can be reduced from 5.3 × 10(-3) and 7.3 × 10(-3) to 2.2 × 10(-3) and 3.4 × 10(-3), respectively.

High Speed Band-Limited 850-nm VCSEL Link Based on Time-Domain Interference Elimination

Short-distance optical interconnection among servers in data centers has attracted the attentions of a multitude of researchers. The method to make the optical link with high transmission capacity and low cost becomes increasingly crucial. Considering these requirements, we propose a new equalization method for time-domain interference elimination in this letter, employed in a band-limited 850-nm vertical cavity surface-emitting laser (VCSEL) link. The method is named simplified maximum likelihood sequence estimation (MLSE), which reduces about 87.8% computational complexity in digital signal processing compared with the conventional MLSE. Based on the proposed simplified-MLSE and combined with the feed-forward equalizer (FFE), a 60-Gb/s non-return-to-zero (NRZ)-modulated VCSEL link could transmit via 56-m OM4 multimode fiber (MMF), and the bit error rate (BER) is still lower than the 7% hard-decision forward error correction (FEC) threshold. The 3-dB bandwidth of the employed 850-nm VCSEL in this transmission system is only 18 GHz, fulfilling the principle of cost-efficient optical interconnects and showing the enormous potential of our proposal in data centers.

Phase Conjugated and Transparent Wavelength Conversions of Nyquist 16-QAM Signals Employing a Single-Layer Graphene Coated Fiber Device

We fabricate a nonlinear optical device based on a fiber pigtail cross-section coated with a single-layer graphene grown by chemical vapor deposition (CVD) method. Using the fabricated graphene-assisted nonlinear optical device and employing Nyquist 16-ary quadrature amplitude modulation (16-QAM) signal, we experimentally demonstrate phase conjugated wavelength conversion by degenerate four-wave mixing (FWM) and transparent wavelength conversion by non-degenerate FWM in graphene. We study the conversion efficiency as functions of the pump power and pump wavelength and evaluate the bit-error rate (BER) performance. We also compare the time-varying symbol sequence for graphene-assisted phase conjugated and transparent wavelength conversions of Nyquist 16-QAM signal.

Fiber Nonlinearity Mitigation in 32-Gbaud 16QAM Nyquist-WDM Systems

Fiber nonlinearity distortion limits the achievable transmission distance and channel capacity of optical transmission systems. In this paper, we demonstrate an experiment of 1400-km standard single mode fiber transmission of 11 × 32 Gbaud 16-ary quadrature amplitude modulation signals with an emphasis on nonlinearity mitigation. Based on the first-order perturbation theory, we present the nonlinearity mitigation principle of the conjugate data repetition (CDR) scheme in a polarization division multiplexed wavelength division multiplexed scenario. Then, we apply several nonlinearity mitigation methods such as CDR, phase-conjugated twin waves (PCTW), digital back propagation (DBP), and the recursive least-squares (RLS) algorithm based filtering to the experiment, respectively. Optical Kerr effects of self-phase modulation, intra- and interchannel cross-phase modulation are considered. The experimental results show that CDR demonstrates similar performance to PCTW. By introducing the CDR scheme, the average bit error rate is reduced from 1.92 × 10-2 to 1.76 × 10-3 at the cost of halving the data rate. This corresponds to 3.2-dB Q2 factor promotion, including 1.5 dB improvement in nonlinearity mitigation. In contrast, 1.0-dB Q2 factor promotion is achieved by employing RLS-based filtering and DBP together without sacrificing the data rate.

1.2 Tb/s (12×100Gb/s) Nyquist 32-QAM Subcarrier Modulation WDM Transmission with Direct Detection

We demonstrate direct-detection WDM transmission over 25km SSMF of 1.2Tb/s (12×100Gb/s) Nyquist 32-QAM half-cycle subcarrier modulation signals with the single channel 20GBaud and a channel frequency grid of 50GHz.

800 Gb/s (8 × 100 Gb/s) Nyquist Half-Cycle Single-Sideband Modulation Direct-Detection Transmission Over 320 km SSMF at C-band

Direct detection shows great attraction in short- and medium-reach transmission for its low cost and easy integration. Especially, single-sideband (SSB) Nyquist subcarrier modulation is a promising technology due to its high spectral efficiency and strong tolerance to chromatic dispersion. In this paper, we experimentally demonstrate 800 Gb/s (8 × 100 Gb/s) direct-detection wavelength-division multiplexing (WDM) transmission over 320 km standard single-mode fiber (SSMF) with Nyquist 16-ary quadrature amplitude modulation (16-QAM) and half-cycle SSB subcarrier modulation. Each channel carries 25 Gbd 16-QAM SSB signal and the channel spacing is 35 GHz. The nonlinear performance is discussed for both single channel and WDM systems. The bit error rate of the WDM system transmitting over 320 km SSMF is 6.73 × 10–3. Considering 20% hard-decision forward error correction and frame redundancy, a net bit rate of 650.4 Gb/s is achieved with a spectral efficiency of 2.32 b/s/Hz. With this spectral efficient modulation technique, our study achieves C-band high-capacity direct-detection WDM systems with metro distance transmission as long as 320 km SSMF.

Single Carrier 400G Transmission With Single-Ended Heterodyne Detection

We demonstrate single channel 448 Gb/s (56 GBaud) and wavelength division multiplexed (WDM) 1.792-Tb/s transmission of Nyquist polarization division multiplexed 16-ary quadrature amplitude modulation (16-QAM) signal by employing heterodyne detection with single-ended photodiodes. The signal-signal beat interference induced by square-law detection of the single-ended photodiodes is overcome by the Kramers-Kronig scheme. For both single channel 160-km standard single mode fiber (SSMF) transmission and WDM 80-km SSMF transmission, the bit-error rates are below the 7% hard-decision forward error correction threshold of

A 70 Gbps NRZ optical link based on 850 nm band-limited VCSEL for data-center intra-connects

4.5\times 10^{-3}

Optical PAM-4 signal generation using a silicon Mach-Zehnder optical modulator

. To the best of our knowledge, this letter is the first experiment of single carrier 400G optical transport with heterodyne detection. For WDM operation of high symbol rate heterodyne system, a record net spectral efficiency of 5.71 b/s/Hz is achieved.