Kun Qiu

Tunable optical frequency comb enabled scalable and cost-effective multiuser orthogonal frequency-division multiple access passive optical network with source-free optical network units

We propose and experimentally demonstrate a multiuser orthogonal frequency-division multiple access passive optical network (OFDMA-PON) with source-free optical network units (ONUs), enabled by tunable optical frequency comb generation technology. By cascading a phase modulator (PM) and an intensity modulator and dynamically controlling the peak-to-peak voltage of a PM driven signal, a tunable optical frequency comb source can be generated. It is utilized to assist the configuration of a multiple source-free ONUs enhanced OFDMA-PON where simultaneous and interference-free multiuser upstream transmission over a single wavelength can be efficiently supported. The proposed multiuser OFDMA-PON is scalable and cost effective, and its feasibility is successfully verified by experiment.

Polarization demultiplexing in stokes space for coherent optical PDM-OFDM

We propose a polarization demultiplexing method in Stokes space for coherent optical PDM-OFDM without inserting training symbols. The experimental demonstration of fiber transmissions shows good performance, comparable with that of using training symbols.

Scalable and Reconfigurable All-Optical VPN for OFDM-Based Metro-Access Integrated Network

In this paper, we propose an orthogonal frequency division multiplexing (OFDM)-based metro-access integrated network (MAIN), which enables scalable all-optical virtual private network (VPN) communication for the first time. Two types of VPN communication, inter-VPN and intra-VPN, are introduced in the proposed MAIN. By placing the VPN light sources in central office (CO) and cascading a wavelength de-multiplexer (DeMUX) with an M* M optical on-off in CO and each central node (CN), the centralized dynamic management of the VPN sub-bands and communications can be achieved in CO. Meanwhile, by employing the proposed VPN sub-bands allocation scheme and recycling process, the scalability and the reconfigurability of all-optical VPN are significantly improved. Theoretical analysis of VPN scalability has been conducted to obtain the minimal and maximal numbers of the multi-VPNs communication that the MAIN can support simultaneously. Besides, the performance degradation of OFDM modulation introduced by the broadened line-width of laser has been studied and discussed. The intra-VPN and inter-VPN communications at 10-Gbit/s with 16 quadrature amplitude modulation in the proposed OFDM MAIN system are successfully simulated and the results show the feasibility of the proposed scheme.

Proposal for all-optical generation of multiple-frequency millimeter-wave signals for RoF system with multiple base stations using FWM in SOA.

An approach for the multiple-frequency millimeter-wave (mm-wave) signals generation is proposed and demonstrated, specifically, which can be applied to a radio-over-fiber (RoF) system with multiple base stations (BSs). In this scheme, optical double sideband (ODSB) modulation is achieved using a Mach-Zehnder modulator (MZM) to generate the two-sideband signals. New frequencies of the optical signals are obtained by using four-wave mixing (FWM) in a semiconductor optical amplifier (SOA). At the BSs, two different frequencies are achieved using a comb optical filter (COF), and which then input a photodiode (PD) to generate the mm-wave signals with the frequencies of 20, 40 or 60 GHz for different BSs, by mixing of these frequencies components. Experimental results verify that the proposed multiple-frequency mm-wave signals generation scheme for a RoF system with multiple base stations can work properly.

Joint PAPR Reduction and Physical Layer Security Enhancement in OFDMA-PON

For joint peak-to-average power ratio (PAPR) reduction and physical layer security enhancement, we propose a chaos IQ-encryption-based optimal frame transmission technique in an orthogonal frequency-division multiple access-based passive optical network (OFDMA-PON). The chaos IQ-encryption technique is utilized to enhance the physical layer security. In encrypting, the in-phase (I) and quadrature-phase (Q) parts of the quadrature amplitude modulation (QAM) symbols are coded with two phase sequences separately, which are generated using a 2-D logistic map. The encrypted OFDM symbols comprise an OFDM frame, and the frame with the minimum PAPR is transmitted to the optical network unit side. Thus, the transmitted OFDM signal is of a joint low PAPR and high physical layer security. In the demonstration, an 11.32-Gb/s encrypted 16 QAM OFDM signal has been experimentally transmitted over 25-km standard single mode fiber in an intensity-modulation/direct-detection OFDMA-PON.

All-optical virtual private network and ONUs communication in optical OFDM-based PON system.

We propose and demonstrate a novel scheme, which enables all-optical virtual private network (VPN) and all-optical optical network units (ONUs) inter-communications in optical orthogonal frequency-division multiplexing-based passive optical network (OFDM-PON) system using the subcarrier bands allocation for the first time (to our knowledge). We consider the intra-VPN and inter-VPN communications which correspond to two different cases: VPN communication among ONUs in one group and in different groups. The proposed scheme can provide the enhanced security and a more flexible configuration for VPN users compared to the VPN in WDM-PON or TDM-PON systems. The all-optical VPN and inter-ONU communications at 10-Gbit/s with 16 quadrature amplitude modulation (16 QAM) for the proposed optical OFDM-PON system are demonstrated. These results verify that the proposed scheme is feasible.

Chaos Coding-Based QAM IQ-Encryption for Improved Security in OFDMA-PON

In this letter, we propose a quadrature amplitude modulation (QAM) encryption method based on chaos coding for security improvement in orthogonal frequency-division multiple access-based passive optical network (OFDMA-PON) systems. In the encrypted QAM symbols, the real (\(I\) ) and imaginary (\(Q\) ) parts are separately coded with key sequences, which are generated by a modified Logistic mapping. Iteration parameters of the mapping formula are set as the security keys, which are determined in each optical network unit (ONU) and then sent to optical line terminal (OLT). The key sequences for different ONUs are generated with their corresponding security keys. According to the subcarrier allocation information, downstream signal is encrypted in the OLT and then decrypted in each ONU with distinct key sequences. Encryption of 10-Gb/s 16QAM OFDM and 12.5-Gb/s 32QAM OFDM signals has been successfully implemented over 30-km standard single mode fiber in the security improved OFDMA-PON.

Secure passive optical network based on chaos synchronization

A physical-enhanced secure passive optical network (PON) based on chaos synchronization is proposed and numerically demonstrated. In this scheme, the chaotic output of an external-cavity semiconductor laser is used as the transmission carrier in both downstream and upstream directions, the chaos modulation technology is used to encrypt the downstream data, and the multiplexed subcarrier-modulation technology is adopted for the upstream transmission. Simulation results demonstrate that both the downstream data and the upstream data encrypted into the chaotic carriers can be successfully decrypted; moreover, the security of downstream can be enhanced by properly increasing the bit rate, and the upstream security can be maintained at a high level. The proposed PON affords secure all-optical access at the physical layer.

Experimental demonstration of novel source-free ONUs in bidirectional RF up-converted optical OFDM-PON utilizing polarization multiplexing

We propose and experimentally demonstrate a novel cost-effective optical orthogonal frequency-division multiplexing-based passive optical network (OFDM-PON) system, wherein all optical network units (ONUs) are source-free not only in the optical domain but also in the electric domain, by utilizing polarization multiplexing (PolMUX) in the downlink transmission. Two pure optical bands with a frequency interval of 10 GHz and downlink up-converted 10 GHz OFDM signal are carried in two orthogonal states of polarization (SOPs), respectively. 10 GHz radio frequency (RF) source can be generated by a heterodyne of two pure optical bands after polarization beam splitting in each ONU, therefore it can be used to down-convert the downlink OFDM signal and up-convert the uplink OFDM signal. In the whole bidirectional up-converted OFDM-PON system, only one single RF source is employed in the optical line terminal (OLT). Experimental results successfully verify the feasibility of our proposed cost-effective optical OFDM-PON system.

Digital coherent superposition of optical OFDM subcarrier pairs with Hermitian symmetry for phase noise mitigation

Digital coherent superposition (DCS) provides an approach to combat fiber nonlinearities by trading off the spectrum efficiency. In analogy, we extend the concept of DCS to the optical OFDM subcarrier pairs with Hermitian symmetry to combat the linear and nonlinear phase noise. At the transmitter, we simply use a real-valued OFDM signal to drive a Mach-Zehnder (MZ) intensity modulator biased at the null point and the so-generated OFDM signal is Hermitian in the frequency domain. At receiver, after the conventional OFDM signal processing, we conduct DCS of the optical OFDM subcarrier pairs, which requires only conjugation and summation. We show that the inter-carrier-interference (ICI) due to phase noise can be reduced because of the Hermitain symmetry. In a simulation, this method improves the tolerance to the laser phase noise. In a nonlinear WDM transmission experiment, this method also achieves better performance under the influence of cross phase modulation (XPM).