Zhixue He

Coherent optical DFT-Spread OFDM transmission using orthogonal band multiplexing

Coherent optical OFDM (CO-OFDM) combined with orthogonal band multiplexing provides a scalable and flexible solution for achieving ultra high-speed rate. Among many CO-OFDM implementations, digital Fourier transform spread (DFT-S) CO-OFDM is proposed to mitigate fiber nonlinearity in long-haul transmission. In this paper, we first illustrate the principle of DFT-S OFDM. We then experimentally evaluate the performance of coherent optical DFT-S OFDM in a band-multiplexed transmission system. Compared with conventional clipping methods, DFT-S OFDM can reduce the OFDM peak-to-average power ratio (PAPR) value without suffering from the interference of the neighboring bands. With the benefit of much reduced PAPR, we successfully demonstrate 1.45 Tb/s DFT-S OFDM over 480 km SSMF transmission.

432-Gb/s Direct-Detection Optical OFDM Superchannel Transmission Over 3040-km SSMF

We propose a novel multiple pilot-carriers assisted direct-detection optical orthogonal frequency division multiplexing (DDO-OFDM) superchannel transmission scheme. With the proposed scheme, a 432-Gb/s transmission over 3040-km standard single mode fiber (SSMF) is successfully demonstrated. The nonlinear effect of correlated/uncorrelated sub-band loading is also investigated.

Investigation of Coherent Optical Multiband DFT-S OFDM in Long Haul Transmission

Multiband discrete Fourier transform spread (DFT-S) is a promising technology to reduce peak-to-average power ratio and mitigate the fiber nonlinearity. We investigate DFT-S coherent optical orthogonal frequency division multiplexing (CO-OFDM) in terms of computational complexity and nonlinearity. With the nonlinearity improvement, 450-Gb/s two-band DFT-S CO-OFDM outperforms conventional CO-OFDM, and is successfully transmitted over a 9280-km fiber link.

Experimental Demonstration of CO-OFDM Optical Network with Heterogeneous ROADM Nodes and Variable Channel Bit-Rates

We experimentally demonstrate a CO-OFDM optical network employing two types of ROADMs that take advantage of coherent-detection capability for wavelength selection. The operations of optical channel switching, bandwidth elasticity, and wavelength tunability are verified.

Spectrally efficient localized carrier distribution scheme for multiple-user DFT-S OFDM RoF- PON wireless access systems

We propose a modified localized carrier distribution scheme based on multi-tone generation to generate 60 GHz mm-wave for different wireless users and it improves the carrier utilization efficiency by 33.3%. The principle of multiple-user discrete Fourier transform spread optical orthogonal frequency-division multiplexing (DFT-S OFDM) Radio-over-fiber (RoF) system is presented. This multiple-user system is applicable to passive optical network (PON). Then we demonstrate a 8 x 4.65 Gb/s multiple-user DFT-S OFDM RoF-PON wireless access system over 40 km fiber link and 60 GHz wireless link using two localized carrier distribution scheme with different spectral efficiency. Compared to conventional OFDM, 2.3 dB reduction of receiver power using DFT-S OFDM modulation scheme and the calculated BER performance for 8 wireless users clearly demonstrates the feasibility of this spectrally efficient multiple-user RoF-PON scheme.

Impacts of backscattering noises on upstream signals in full-duplex bidirectional PONs

The backscattering noises introduced by Rayleigh and stimulated Brillouin scattering have been experimentally studied by means of their spectrum broadening, the scattering power variation and their impacts on upstream signals with different transmission fiber lengths and incident powers in a single-fiber bidirectional passive optical network (PON) communication system. The results show that both spontaneous scattering and simulated scattering can take place. The power and spectrum of backscattering noises are determined by the downstream launch power, laser linewidth and transmission fiber length. With the transmission length increasing, the power of backscattering noises gets higher, the spectrum of the backscattering noise broadens and the simulated threshold power decreases. The backscattering noise can beat with uplink light to modulate envelop of upstream signal resulting in degradation of BER greatly. Under the condition of one single channel for the second next generation PON (NG-PON2), the fiber length is 40km and downstream launch power is up to 11dBm. At this time, the backscattering noises are easy to be stimulated and the scattering power rises up from -20dBm to 10dBm, which can overwhelm the US signal. The spectrum of the optical beat interference noise also rises up with fiber length, which causes the uplinks BER degradation. The experimental results are significant for mitigation of backscattering noises under the condition of bidirectional PONs.

Phase Noise Canceled Polarization-Insensitive All-Optical Wavelength Conversion of 557-Gb/s PDM-OFDM Signal Using Coherent Dual-Pump

We study the effect of phase noise canceled polarization-insensitive all-optical wavelength conversion (AOWC) of orthogonal frequency-division multiplexing (OFDM) signals, which is based on four-wave-mixing in high-nonlinear optical fiber using dual-pump. An AOWC experiment of polarization division multiplexing OFDM (PDM-OFDM) 8/16/32-QAM signals is investigated. A DFB laser with linewidth of 10 MHz is employed as the coherent dual-pump seed. The measured results show that the received signal after wavelength conversion by utilizing coherent DFB dual-pump has the same BER performance as back-to-back. Furthermore, AOWC of 557-Gb/s superchannel discrete Fourier transform-spread PDM-OFDM with eight-QAM (DFTS PDM-OFDM 8-QAM) signal is proposed and experimentally demonstrated based on the studied coherent dual-pump scheme. Negligible OSNR penalty (<;0.8 dB) is observed at 7% FEC limit (BER = 3.8 × 10-3) after wavelength conversion. Our demonstration shows that the phase noise transferred from the pumps can be effectively eliminated. It also shows that AOWC enables practical implementation on the polarization multiplexing dynamic optical networks at the nodes where wavelengths conflict.

Independent component analysis based channel equalization for 6 × 6 MIMO-OFDM transmission over few-mode fiber

We propose and experimentally demonstrate two independent component analysis (ICA) based channel equalizers (CEs) for 6 × 6 MIMO-OFDM transmission over few-mode fiber. Compared with the conventional channel equalizer based on training symbols (TSs-CE), the proposed two ICA-based channel equalizers (ICA-CE-I and ICA-CE-II) can achieve comparable performances, while requiring much less training symbols. Consequently, the overheads for channel equalization can be substantially reduced from 13.7% to 0.4% and 2.6%, respectively. Meanwhile, we also experimentally investigate the convergence speed of the proposed ICA-based CEs.

Colorless Long-Reach Duplex WDM-PON With Rayleigh Backscattering Noise Mitigation Using Orthogonal Codes

We propose and experimentally demonstrate a novel colorless full-duplex passive optical network (PON) access architecture. Utilizing orthogonal codes and correlation receiving methods, the novel PON can mitigate the optical beat interference (OBI) noise induced by Rayleigh backscattering (RB). A pair of electrical orthogonal codes is generated and modulated at the same wavelength in the optical line terminal (OLT). Then, modulated optical signals are transmitted from the OLT to the optical network units (ONUs). One of the codes is for downstream signal coding, and the other is used as the upstream seed. In the ONU, the upstream signal is remodulated without erasing the downstream signal. Neither extra centralized continuous wave (CW) light sources nor gain-saturated reflective semiconductor optical amplifier is required. By using these orthogonal codes, the spectral overlap between upstream signal and downstream signal in the full-duplex system is reduced, which can mitigate the OBI noise significantly. The performance of transmission and power margin is investigated through the experiments with different transmission distances. The remodulated upstream signals are recovered by correlation algorithm in the OLT. By correlation algorithm, we can get coding gain, which is important in long-reach transmissions. Because of coding gain and the OBI noise mitigation, a link power margin of 4-10 dB can be achieved for 5 Gb/s downstream and 1.25 Gb/s upstream, when the transmission distance is from 20 to 70 km.

Demonstration of a spectrum efficient 8 × 1.25 Gb∕s electrical code-division multiplexing passive optical network based on wavelet packet transform coding

Abstract. An electrical code-division multiplexing passive optical network (ECDM-PON) method is proposed in which a spectrum-efficient orthogonal coding based on wavelet packet is used. Comparing to the normal Walsh or Gold code which had been used widely in ECDM-PON systems, the proposed coding can reduce bandwidth by about 40% at 8×1.25  Gb/s, which can reduce the optical devices’ bandwidth of the whole system. The principle of the coding is shown. Through experiments, a 1.25  Gb/s per channel by 8-channels downstream transmission of a 10  Gb/s ECDM-PON demonstrates the efficiency of our proposed system. Only 6-GHz bandwidth is enough for transmitting the 8×1.25  Gb/s signal using our proposed method which, achieves a spectrum efficiency of about 1.67  bit/s/Hz. Error free transmission is achieved using 8-chip electrical codes over 20-km standard single mode fiber (SSMF).