Xuezhi Hong

Demonstration of optical steganography transmission using temporal phase coded optical signals with spectral notch filtering

A novel approach is proposed and experimentally demonstrated for optical steganography transmission in WDM networks using temporal phase coded optical signals with spectral notch filtering. A temporal phase coded stealth channel is temporally and spectrally overlaid onto a public WDM channel. Direct detection of the public channel is achieved in the presence of the stealth channel. The interference from the public channel is suppressed by spectral notching before the detection of the optical stealth signal. The approach is shown to have good compatibility and robustness to the existing WDM network for optical steganography transmission.

Elimination of Multiple Access Interference in Ultrashort Pulse OCDMA Through Nonlinear Polarization Rotation

A multiple access interference elimination scheme based on nonlinear polarization rotation (NPR) is proposed for ultrashort pulse optical code-division multiple access (OCDMA). This scheme is experimentally investigated in a four-user and 2.5-Gb/s coherent OCDMA system encoded/decoded by 127-chip superstructured fiber Bragg gratings. The measured bit-error-rate results of the NPR-based thresholder are also compared with those of a conventional supercontinuum-based thresholder in both the back-to-back and 20-km transmission cases. Our proposed scheme has the advantage of colorless operation over C-band.

Passive optical interconnects at top of the rack: offering high energy efficiency for datacenters.

This paper introduces a new concept, namely passive optical interconnect at top of the rack in the datacenter networks, and investigates several architectures, which use only passive optical components to interconnect different servers. In such a manner, the proposed schemes are able to offer higher bandwidth and significantly improve energy efficiency compared to their electronic counterpart that is based on commodity switches. The proposed passive optical interconnect schemes are experimentally demonstrated in order to validate the transmission performance. Besides, an assessment in terms of energy consumption and cost has also been carried out, which shows our proposed concept can significantly outperform the conventional commodity switches on energy efficiency while keeping the cost in the similar level.

Linearly interpolated sub-symbol optical phase noise suppression in CO-OFDM system

An optical phase noise suppression algorithm, LI-SCPEC, based on phase linear interpolation and sub-symbol processing is proposed for CO-OFDM system. By increasing the temporal resolution of carrier phase tracking through dividing one symbol into several sub-blocks, i.e., sub-symbols, inter-carrier-interference (ICI) mitigation is achieved in the proposed algorithm. Linear interpolation is employed to obtain a reliable temporal reference for sub-symbol phase estimation. The new algorithm, with only a few number of sub-symbols (N(B) = 4), can provide a considerably larger laser linewidth tolerance than several other ICI mitigation algorithms as demonstrated by Monte-Carlo simulations. Numerical analysis verifies that the best performance is achieved with an optimal and moderate number of sub-symbols. Complexity analysis shows that the required number of complex-valued multiplications is independent of the number of sub-symbols used in the proposed algorithm.

An Integrated Optical Mixer Based on SU8 Polymer for PDM-QPSK Demodulation

This letter presents the design and fabrication of an optical mixer based on SU8 polymer for the demodulation of polarization division multiplexed-quadrature phase shift keying (PDM-QPSK) signals. The mixer consists of two novel polarization beam splitters (PBSs) and two 4 × 4 multimode interference (MMI) couplers. It is the first polymer-based optical mixer incorporated with PDM functionality. The experimental results show good performances of the mixer.

Proposal of Inverse Pulse Position Modulation for Downstream Signal in Remodulation PON With PolSK-Modulated Multicast Overlay

Inverse pulse position modulation (PPM) is proposed for downstream data modulation in an optical network unit-source-free wavelength-division-multiplexed passive optical network, while optical amplitude shift keying (ASK) modulation is used for upstream data remodulation. In such a PON, polarization shift keying (PolSK) modulation overlay can be supported on the downstream signals for multicasting services. Our experimental results show that the inverse PPM optical ASK modulation for downstream data has small channel crosstalk with PolSK multicast data and ASK-remodulated upstream data. Compared with the approaches based on the Manchester code or inverse return-to-zero code, the proposed scheme based on inverse PPM has a better power margin, indicating a longer transmission distance and a higher tolerance to link loss.

Low-complexity optical phase noise suppression in CO-OFDM system using recursive principal components elimination.

A low-complexity optical phase noise suppression approach based on recursive principal components elimination, R-PCE, is proposed and theoretically derived for CO-OFDM systems. Through frequency domain principal components estimation and elimination, signal distortion caused by optical phase noise is mitigated by R-PCE. Since matrix inversion and domain transformation are completely avoided, compared with the case of the orthogonal basis expansion algorithm (L = 3) that offers a similar laser linewidth tolerance, the computational complexities of multiple principal components estimation are drastically reduced in the R-PCE by factors of about 7 and 5 for q = 3 and 4, respectively. The feasibility of optical phase noise suppression with the R-PCE and its decision-aided version (DA-R-PCE) in the QPSK/16QAM CO-OFDM system are demonstrated by Monte-Carlo simulations, which verify that R-PCE with only a few number of principal components q ( = 3) provides a significantly larger laser linewidth tolerance than conventional algorithms, including the common phase error compensation algorithm and linear interpolation algorithm. Numerical results show that the optimal performance of R-PCE and DA-R-PCE can be achieved with a moderate q, which is beneficial for low-complexity hardware implementation.

Demonstration of Low-Cost Uplink Transmission in a Coherent OCDMA PON Using Gain-Switched Fabry–Pérot Lasers With External Injection

We propose and demonstrate a simple and low-cost uplink transmission scheme for coherent optical code-division multiple access (OCDMA) passive optical networks (PONs), using gain-switched Fabry-Perot (GS-FP) lasers with external injection as local light sources at optical network units. Experimental results based on the two-user 1.25-Gb/s OCDMA system confirm the feasibility of this scheme. The system performance is compared with that of the OCDMA system using a conventional mode-locked laser (MLL). The auto-correlation peak to the maximum wing level (P/W) ratio is also given as the central wavelength of the GS-FP laser varies. The present low-cost scheme is highly preferable for OCDMA PON applications.

Passive optical interconnects based on cascading wavelength routing devices for datacenters: A cross-layer perspective

A general passive optical interconnect (POI) architecture based on cascading wavelength routing devices is proposed for datacenters. An analysis shows that with a proper port configuration, the logic topology of the proposed POI is equivalent to a DCell network, which has rich connectivity, leading to a high fault tolerance. Compared with the existing arrayed waveguide grating (AWG)-based architecture, such as H-LION, the proposed architecture has a higher AWG port utilization and lower cabling complexity that result in better scalability. Meanwhile, an impairment-aware scalability analysis is carried out to estimate the impact of in-band crosstalk on the size of the POI. Furthermore, a cross-layer investigation of the proposed POI provides a guideline to select proper physical-layer implementations, including optical interface structure and design of the transceiver, for acceptable network performance. The numerical results reveal that, with a moderate number of shortest paths K and degrees of node D, (a) the bidirectional optical interface is more suitable for large-scale datacenter networks than the unidirectional one, (b) an array of fixed receivers can be replaced by just a small number of tunable receivers with negligible degradation in network performance, and (c) decreasing 50% of the tuning range of the receivers causes only a minor increase of blocking probability, which demonstrates a good balance between the complexity of the transceiver design and network performance.

Low-complexity frequency domain nonlinear compensation for OFDM based high-speed visible light communication systems with light emitting diodes

A novel frequency domain nonlinear compensation method, FD-NC, is proposed for orthogonal frequency division multiplexing (OFDM) based visible light communication (VLC) system. By tackling the memory nonlinear impairments from light emitting diodes (LEDs) in the frequency domain rather than in the time domain, the proposed method has much lower computational complexity than the conventional time domain Volterra nonlinear compensation method (TD-NC). Both theoretical derivation and experimental investigation of the proposed method in OFDM based VLC systems with four types of commercial LEDs are presented. The results of experiments show that the proposed low-complexity FD-NC method with a moderate truncation factor achieves a performance comparable to that of the TD-NC. The application of FD-NC method in the bit-power loading OFDM VLC system is also experimentally demonstrated. Compared with the linear equalization case, at a bit error rate (BER) of 3.8 × 10-3 (a), the transmission distance of a 960 Mbps VLC system can be extended from 0.7 m to 1.8 m by the FD-NC, and (b) the achievable system capacity can be enhanced by 18.7%~36.5% for transmission distance in the range of 0.5 m~2 m with the FD-NC. The complexity analysis shows that the required number of real-valued multiplications (RNRM) of the FD-NC is independent of linear or nonlinear memory length. The reduction of RNRM achieved by the FD-NC over the TD-NC becomes more profound for a larger nonlinear memory length or a smaller truncation factor.