Quaternary optical packets generated by fiber four-wave mixing

Recent work suggests that fiber four-wave mixing may be used to merge two wavelength multiplexed binary channels into a single quaternary signal. In this letter, we investigate the application of this technique to the optical packets generation considering that a 10-Gb/s payload is optically multiplexed to a 2.5-Gb/s label. Computer simulations are used to analyze the propagation of the generated quaternary optical packet in the dominance of amplified spontaneous emission noise. Results suggest that the optical packet may be propagated through metropolitan area networks length scales and that an average power around 0 dBm should be used to minimize the packet bit-error rate.     

OCDMA-coded free-space optical links for wireless optical-mesh networks

We propose a wireless optical-mesh network based on optical code-division multiple-access (OCDMA)-coded free-space optical links. The performance of the proposed network under intermediate and strong turbulence channel conditions is investigated for synchronous and asynchronous OCDMA. We show that synchronous OCDMA using complementary Walsh-Hadamard codes operating at a raw bit rate of 622 Mb/s per user can achieve a bit-error rate of 10/sup -9/. This is achievable in channels with strong turbulence and high temporal correlation, or in channels with intermediate turbulence conditions; reasonable conditions based on channel characteristics inferred from the refractive index spectrum. For asynchronous OCDMA using optical orthogonal codes (OOCs), using a lower bound, we show that asynchronous OCDMA using OOCs cannot achieve acceptable performance for the same channel conditions, due to interference limitations.     

Performance bounds for coded free-space optical communications through atmospheric turbulence channels

Error-control codes can help to mitigate atmospheric turbulence-induced signal fading in free-space optical communication links using intensity modulation/direct detection (IM/DD). Error performance bound analysis can yield simple analytical upper bounds or approximations to the bit-error probability. We first derive an upper bound on the pairwise codeword-error probability for transmission through channels with correlated turbulence-induced fading, which involves complicated multidimensional integration. To simplify the computations, we derive an approximate upper bound under the assumption of weak turbulence. The accuracy of this approximation under weak turbulence is verified by numerical simulation. Its invalidity when applied to strong turbulence is also shown. This simple approximate upper bound to the pairwise codeword-error probability is then applied to derive an upper bound to the bit-error probability for block codes, convolutional codes, and turbo codes for free-space optical communication through weak atmospheric turbulence channels. We also discuss the choice of interleaver length in block codes and turbo codes based on numerical evaluation of our performance bounds.     

混沌空间光通信研究进展

随着空间通信技术的迅猛发展,人们对信息安全的需求愈发迫切。基于半导体激光器的空间激光通信凭借其终端体积小、功耗低、高带宽以及无电磁频谱约束等特点,已广泛应用于空间高速通信领域。激光混沌通信技术作为一种在物理层对空间光通信加密的安全技术手段,逐渐成为了空间光通信的研究热点。结合当前国内外自由空间光通信、混沌激光通信、混沌空间光通信的发展历程,介绍了近年来混沌空间光通信关键技术的研究进展,主要包括自由空间光通信快速跟瞄技术、混沌空间光通信大气湍流抑制技术和激光混沌空间同步技术。最后结合当前混沌空间光通信发展现状与不足,对混沌空间光通信的研究方向和可借鉴的关键技术进行了展望,旨在为该领域的进一步发展提供参考和借鉴。     

High-speed optical FSK modulator for optical packet labeling

We described a novel optical label swapping (OLS) technique for optical packet systems using frequency-shift-keying (FSK) optical labeling. High-speed optical FSK signal can be generated by using an external FSK modulator consisting of four optical phase modulators. The FSK modulator was based on optical single-sideband (SSB) modulation technique, and comprised of traveling-wave electrodes for high-speed frequency switching. We demonstrate 10 Gbps FSK transmission, and simultaneous modulation by FSK and intensity modulation (IM). OLS using double-sideband modulation was also demonstrated, where this technique can be used for a bundled wavelength-domain-multiplexing (WDM) channels without using an array of pumping light sources.     

Free-space digital optical systems

Within the past 15 years there has been significant progress in the development of two-dimensional arrays of optical and optoelectronic devices. This progress has, in turn, led to the construction of several free-space digital optical system demonstrators. The first was an optical master-slave flip-flop using Hughes liquid-crystal light valves as optical logic gates and computer-generated holograms as the gate-to-gate interconnects. This was demonstrated at USC in 1984. Since then there have been numerous demonstrations of free-space digital optical systems including a simple optical computing system (1990) and five switching fabrics designated System₁ (1988), System₂ (1989), System₃ (1990), System₄ (1991) and System₅ (1993). The main focus of this paper will be to describe the five switching fabric demonstrators constructed by AT&T in Naperville, IL. The paper will begin with an overview of the SEED technology which was the device platform used by the demonstrators. This will be followed by a discussion of the architecture, optics, and optomechanics developed for each of the five demonstrators     

Fundamental limits in optical communication

This paper addresses fundamental limits on the ability to communicate over optical fiber and optical free-space channels. The underlying limits on the sensitivity of optical receivers dictated by quantum effects are reviewed. Receiver sensitivity limitations caused by background light, dark current, post detection amplifier noise, and transmitter imperfection are then considered. Finally the limitations of the channels related to input coupling, loss, and delay distortion are reviewed. Throughout, comparisons are made between the optical systems and their microwave counterparts to highlight differences and the origins of those differences.     

Approaches to optical Internet packet switching

Wavelength-division multiplexing is currently being deployed in telecommunications networks in order to satisfy the increased demand for capacity brought about by the explosion in Internet use. The most widely accepted network evolution prediction is via an extension of these initial predominantly point-to-point deployments, with limited system functionalities, into highly interconnected networks supporting circuit-switched paths. While current applications of WDM focus on relatively static usage of individual wavelength channels, optical switching technologies enable fast dynamic allocation of WDM channels. The challenge involves combining the advantages of these relatively coarse-grained WDM techniques with emerging optical switching capabilities to yield a high-throughput optical platform directly underpinning next-generation networks. One alternative longer-term strategy for network evolution employs optical packet switching, providing greater flexibility, functionality, and granularity. This article reviews progress on the definition of optical packet switching and routing networks capable of providing end-to-end optical paths and/or connectionless transport. To date the approaches proposed predominantly use fixed-duration optical packets with lower-bit-rate headers to facilitate processing at the network-node interfaces. Thus, the major advances toward the goal of developing an extensive optical packet-switched layer employing fixed-length packets are summarized, but initial concepts on the support of variable-length IP-like optical packets are also introduced. Particular strategies implementing the crucial optical buffering function at the switching nodes are described, motivated by the network functionalities required within the optical packet layer     

A 2-Gb/s 0.5-/spl mu/m CMOS parallel optical transceiver with fast power-on capability

This paper describes an optical transceiver designed for power-efficient connections within high-speed digital systems, specifically for board- and backplane-level interconnections. A 2-Gb/s, four-channel, dc-coupled differential optical transceiver was fabricated in a 0.5-/spl mu/m complementary metal-oxide-semiconductor (CMOS) silicon-on-sapphire (SoS) process and incorporates fast individual-channel power-down and power-on functions. A dynamic sleep transistor technique is used to turn off transceiver circuits and optical devices during power-down. Differential signaling (using two optical channels per signal) enables self-thresholding and allows the transceiver to quickly return from power-down to normal operation. A free-space optical link system was built to evaluate transceiver performance. Experimental results show power-down and power-on transition times to be within a few nanoseconds. Crosstalk measurements show that these transitions do not significantly impact signal integrity of adjacent active channels.     

Planar-integrated free-space optical fan-out module for MT-connected fiber ribbons

This paper reports on the design, the fabrication, and the testing of a compact planar-integrated free-space optical 1 /spl rarr/ 4 fan-out module for fiber ribbons with multifiber-terminated connectors. It supports 12 parallel optical channels and consists of a cascade of basic cells with 1 /spl rarr/ 2 fan-out. The module was implemented with surface-relief diffractive-phase elements; design and fabrication of the optical system were optimized for low loss by various measures such as the use of dielectric and silver reflective coatings. In experimental tests, a coupling efficiency of -11.4 dB per fan-out channel was obtained.     

On-board baseband switch development using A high speed optical ring

This paper presents work performed at COMSAT Laboratories to develop a prototype on-board baseband switch. The switch design is modular to accommodate different service types, and the architecture features a high-speed optical ring operating at 1 Gb/s to route input (up-link) channels to output (down-link) channels. The switch is inherently a packet switch, but can process either circuit-switched or packet-switched traffic. If the traffic arrives at the satellite in a circuit-switched mode, the input processor packetizes it and passes it on to the switch. The main advantage of the packet approach lies in its simplified control structure. Details of the switch architecture and design, and the status of its implementation, are presented.     

A method for rebroadcasting signals in an optical backplane bussystem

A guided-wave optical backplane bus system intended for use in high-performance board-to-board interconnects is described. Its multiplexed polymeric holograms can implement optical signal broadcast between boards so that all boards share common optical channels. By introducing an active coupler to the doubly multiplexed hologram at the center board, signals received from any board can be rebroadcast to all other boards. We describe the design concepts for a centralized optical backplane and the resulting performance and assembly advantages over previously developed guided-wave and free-space optical backplane bus systems used for broadcasting signals. These advantages include equalized fan-out power, increased interconnect distance, and simpler fabrication     

Error rate performance of coded free-space optical links over strong turbulence channels

Error control coding can be used over free-space optical (FSO) links to mitigate turbulence-induced fading. We present error rate performance bounds for coded FSO communication systems operating over atmospheric turbulence channels, which are modeled as a correlated K distribution under strong turbulence conditions. We derive an upper bound on the pairwise error probability (PEP) and then apply the union-bound technique in conjunction with the derived PEP to obtain upper bounds on the bit error rate. Simulation results are further demonstrated to verify the analytical results.     

Channel capacity and non-uniform signalling for free-space optical intensity channels

This work considers the design of capacityapproaching, non-uniform optical intensity signalling in the presence of average and peak amplitude constraints. Although it is known that the capacity-achieving input distribution is discrete with a finite number of mass points, finding it requires complex non-linear optimization at every SNR. In this work, a simple expression for a capacity-approaching distribution is derived via source entropy maximization. The resulting mutual information using the derived discrete non-uniform input distribution is negligibly far away from the channel capacity. The computation of this distribution is substantially less complex than previous optimization approaches and can be easily computed at different SNRs. A practical algorithm for non-uniform optical intensity signalling is presented using multi-level coding followed by a mapper and multi-stage decoding at the receiver. The proposed signalling is simulated on free-space optical channels and outage capacity is analyzed. A significant gain in both rate and probability of outage is achieved compared to uniform signalling, especially in the case of channels corrupted by fog.     

空间激光通信网络中的全光相位再生技术

围绕空间激光通信网络中高速数据多跳传输应用需求,针对相位调制激光链路经过空间长距离传输后信号质量劣化的问题,研究了基于相位敏感四波混频参量效应实现二进制相位调制高速激光信号的全光相位再生技术。利用Matlab软件数值分析了全光相位再生系统的影响因素,并基于OptiSystem仿真平台搭建了全光相位再生系统。结合高轨-地面站空间激光通信系统链路预算,对速率为10 Gbit/s的DPSK信号光经背靠背、相位噪声劣化以及劣化后全光相位再生处理三种传输场景进了对比分析。模拟仿真结果与数值分析结果均表明,与劣化后未经再生处理的系统相比,全光相位再生处理后的系统误码率平均优化4个数量级,信噪比提升约3 dB,表明该空间激光通信全光相位再生技术可实现相位调制信号的全光相位再生,能够有效提升空间相干激光通信系统的性能,可以应用于空间高速激光通信网络中继节点处的全光数据中继等方向。     

Free-space optical transmission of multimedia satellite datastreams using mid-infrared quantum cascade lasers

Experimental results for an optical free-space high-speed link using direct modulated mid-infrared (λ = 8.1 μm) quantum cascade lasers are presented. A total of 800 digitally encoded multimedia channels were transmitted. The reliability of the system against weather influence (fog) was experimentally compared to that of a near-infrared (λ = 0.85 μm) link     

Error rate performance analysis of coded free-space optical links over gamma-gamma atmospheric turbulence channels

Error control coding can be used over free-space optical (FSO) links to mitigate turbulence-induced fading. In this paper, we derive error performance bounds for coded FSO communication systems operating over atmospheric turbulence channels, considering the recently introduced gamma-gamma turbulence model. We derive a pairwise error probability (PEP) expression and then apply the transfer function technique in conjunction with the derived PEP to obtain upper bounds on the bit error rate. Simulation results are further demonstrated to confirm the analytical results.     

Output power and SNR swings in cascades of EDFAs for circuit- andpacket-switched optical networks

A simple dynamic model of the erbium-doped fiber amplifier (EDFA) that includes self-saturation by amplified spontaneous emission (ASE) is used to analyze the power and signal-to-noise ratio (SNR) transients in wavelength division multiplexed (WDM) optical networks in which signals cross chains of EDFAs from source to destination. The model, which consists of solving sequentially one ordinary differential equation per amplifier, is used to (1) determine power and SNR excursions in the surviving channels along a chain of 35 EDFAs during isolated add-drop events in a 16-channel WDM circuit switching scenario and (2) run Monte Carlo simulations of the first five EDFAs of the same chain fed by burst-mode packet switching traffic on each of the 16 channels. Each packet source is modeled as an ON-OFF asynchronous transfer mode (ATM) source, with ON and OFF times having a heavy-tailed Pareto distribution. The aggregate source model is asymptotically self-similar, and well describes multimedia packet communications. The results are used to examine the influence of average network utilization and source ON-OFF time variance on the probability density function of signal power and SNR at each EDFA output. We demonstrate that self-similar traffic generates sizable power and SNR swings, especially at low network utilization. The simulations also indicate sizable broadening of the power and SNR density functions along the cascade of EDFAs, reaching levels in excess of 9 dBm and 4 dB for the power and SNR swings, respectively, at the 5th EDFA. The effect becomes more pronounced for longer EDFA chains. Such a large broadening may imply serious system impairments in burst-mode WDM packet networks     

Pointing error effects on performance of free-space optical communication systems using SC-QAM signals over atmospheric turbulence channels

In this paper, we theoretically analyze pointing error effects on performance of free-space optical (FSO) communication systems using subcarrier intensity quadrature amplitude modulation (SC-QAM) signals over atmospheric turbulence channels. Unlike previous studies, we take into account both atmospheric turbulence channels and the pointing error effect. In order to model atmospheric turbulence channels, we employ a log-normal distribution for weak-to-moderate turbulent condition and a gamma–gamma distribution for strong turbulent condition. Moreover, we study the pointing error effect by taking into account the influence of beamwidth, aperture size and jitter variance. In addition, we use a combination of these models to analyze the combined effect of atmospheric turbulence and pointing error to FSO/SC-QAM systems. Finally, we derive analytical expressions to evaluate the average symbol error rate (ASER) performance of such systems. Numerical results present the impact of pointing error on the performance of FSO/SC-QAM systems and how we use proper values of aperture size and beamwidth to improve the performance of such systems. In addition, simulation results of FSO/SC-QAM performance over strong atmospheric turbulence and pointing errors show that the closed-form expression can provide a precision for evaluating ASER of such systems.