Design and performance analysis of code families for multi-dimensional optical CDMA

A code design algorithm for application in multi-dimensional optical code division multiple access (MD-OCDMA) for asynchronous optical fibre communication is proposed. Two-dimensional (2D) wavelength-time or space-time OCDMA and three-dimensional (3D) space-wavelength-time OCDMA are subsets of MD-OCDMA. Some applications and the performance analysis of the algorithm in 2D multipulse per row codes and 3D multipulse per plane codes are shown. In the applications discussed, this design ensures a maximum cross-correlation of '1' between any two codes. The performance metrics studied are the probability of error due to multiple-access interference for different numbers of active users and optimum temporal length for different values of cardinality. The performance analysis shows that the proposed 2D design offers very low probability of error due to multiple-access interference at lower cardinality when compared with other 2D designs using equivalent code dimension. A comparison of the proposed 3D design with an existing 3D design shows better performance at lower cardinality. The 3D designs show better performance when compared with the 2D designs.     

Construction and performance analysis of 2-D codes for M-ary OFFH?CDMA systems

A new family of two-dimensional (2-D) codes constructed by combining frequency-hop and time-spreading codes is presented. The proposed codes are designed to overcome the limitation imposed on the maximum permissible number of simultaneous users in optical code-division multiple-access (OCDMA) systems. To increase the data transmission rate, the 2-D codes employ a M-ary signalling scheme. The proposed family of codes has the favourable characteristics of a zero autocorrelation constraint and a cross-correlation constraint of one. The performance of the 2-D code is analysed in terms of the bit error rate (BER) and is compared with that of a conventional OCDMA system using optical orthogonal code (OOC). The numerical and simulation results reveal that the proposed system outperforms conventional OCDMA systems both in terms of the BER and the data transmission rate. In addition, the system capacity is sufficient to ensure reliable communication (BERles10^-9) in local area networks     

Photonic encoder and decoder for optical code-division multiplexing with time-to-space converters and angle-multiplexed holograms

A photonic encoder-decoder pair for optical code-division multiplexing (OCDM) that uses time-to-space converters and angle-multiplexed holograms is proposed. The encoder converts the pulse from each input port into a specific code and multiplexes input signals into the output port. The hologram in the decoder generates a correlation waveform between the transmitted code and the recorded code. The performance of the OCDM system with the encoder-decoder pair is estimated. The maximum spectral efficiency for 8-bit length orthogonal codes in the worst case at a bit-error rate of 10(-9) is 0.17 (bits/s)/Hz when the number of channels is 8.     

Design considerations of high performance optical code division multiple access: a new spectral amplitude code based on laser and light emitting diode light source

A new code of optical code division multiple access (OCDMA) based on spectral amplitude coding (SAC) is described and analysed. The coding technique is called random diagonal (RD) code. One of the important properties of this code is that the cross-correlation at data segment is always zero, which means that phase intensity induced noise (PIIN) is reduced. From the construction of RD code sequence, the authors can see that the RD code is constructed using code segment and data segment. Using this code property, RD code is implemented using coherent source (multi-laser) and incoherent source (light emitting diode) for the code segment and data segment, respectively. RD code using two multi-sources (incoherent and coherent) can be considered as an effective way for maximising the capacity while minimising the cost of SAC-OCDMA. It is shown that the system using this new code matrices not only suppresses PIIN, but also allows a larger number of active users compared with other codes.     

All-optical code-division-multiplexing technique supporting multirate data communications and local-area-network interconnections

An efficient all-optical code-division-multiplexing (AOCDM) technique is proposed to support multirate data communications and local-area-network (LAN) interconnections with multiple protocols. To achieve this goal, we use a strict optical orthogonal code (OOC) in multirate AOCDM systems to guarantee that both cross- and autocorrelation constraints are minimum (i.e., 1) for incoherent optical processing. In contrast, the use of a conventional OOC may result in correlation constraints of 2, which in turn can degrade system performance. Moreover, implementation issues on AOCDM systems are discussed. These include the design of low-cost optical transmitters and the trade-off among coherence time, spectral width, and pulse width for AOCDM systems. It is shown that multirate AOCDM systems have a high operation flexibility to support data communications and LAN interconnections of both equal and multiple bit rates. For multirate data transmissions, the proposed system can have a better bandwidth efficiency and a lower bit error rate than a system that uses a conventional OOC.     

OCDMA系统二维2D-OOSC方阵码的参数优化和性能分析

给出了二维λ-t光正交方阵码2D-OOSC的设计方案, 重点讨论了2D-OOSC码字的相关性及码字参数对系统性能的影响。详细分析了2D-OOSC系统的用户容量以及由多用户干扰引起的误码性能。研究表明，在占用相同带宽的条件下，系统的用户容量远大于基于1D-OOC的WDM-CDMA混合系统，特别是在传输距离不太远的局域网中，光纤色散引起的传输损失较小。基于2D-OOSC地址码的 OCDMA系统有着现实的应用价值。     

Internally coded time-hopping coherent ultra-short light pulse code division multiple access scheme with optical amplifier and its performance analysis using additive noise model

The internally coded time-hopping coherent ultra-short light pulse code division multiple access (CULP CDMA) scheme (recently introduced) with an optical amplifier is described and its performance in fibreoptic communication systems is analysed. In accordance with the important role of optical amplifiers in optical communication systems, a preamplifier at the input of the receiver is used in order to compensate the losses because of the spectral encoder, spectral decoder and optical fibre path. The authors evaluate the bit error rate of the system considering the effects of the multiple access interference, noise because of the optical amplifier and thermal noise using saddle point approximation, and compare the results with those of the conventional CULP CDMA system with and without an amplifier. The numerical results indicate a substantial improvement in the performance of the coded system in comparison with the uncoded one. In addition, the negative effect of amplifier noise in the proposed scheme is much less than that of the conventional CULP CDMA system.     

一种新的二维光正交码的设计与性能分析

针对二维光码分多址(OCDMA)系统地址码,本文在单重合序列(OCS)的基础上,构造了一种新的二维光正交码EQC/OCS.该地址码是以扩展二次同余码(EQC)为时间扩频伪随机序列,以OCS或截短OCS为波长跳频伪随机序列进行构造的.与EQC/PC相比,EQC/OCS的波长数可以是任意整数,不局限于素数.EQC/OCS不仅构造灵活,而且充分利用了系统的有效波长资源.当二维OCDMA系统的有效波长数不是素数时,在相同码长和码重的情况下,EQC/OCS增加了二维OCDMA系统的码字容量,从而增加了接入用户数.另一方面,与EQC/PC相比,EQC/OCS的码字的自相关和互相关限保持不变.     

Cylindrical-type nanometer-resolution laser diffractive optical encoder

A new, to our knowledge, design for a cylindrical-type diffractive optical encoder is proposed. The wave-front aberrations induced by the power of the rotation disk in this encoder can be canceled out completely. Wave-front-aberration cancellation and desensitization to the grating misalignment are achieved by means of positioning the virtual point source, which was induced by the cylindrical grating with respect to two sets of modified telescopes with a magnification ratio of one: 1x telescopes. For evaluating the performance envelope of this newly designed optical system a code v-based optical-design software program was adopted to simulate the performance of the optical system. From tolerance-analysis results it was found that this newly developed cylindrical encoder system has the capability to compensate for most aberrations and, in addition, possesses a high tolerance for optical-component misalignment. For verifying the performance of the developed system the cylindrical diffractive encoder system was cross-referenced with a Hewlett-Packard Model HP-5529 laser interferometer positioning signal. The experimental results confirm the merits of this newly developed cylindrical encoder.     

Investigation of an optical multiple PPM link over a highly dispersive optical channel

A performance analysis of (X/Y) multiple pulse position modulation (PPM) systems, in which X denotes the number of data slots and Y the number of pulses, operating over a plastic optical fibre channel has been described. The effects of receiver noise and channel dispersion are accounted for and the manner in which the erasure, wrong-slot and false-alarm errors affect the system performance is examined. The receiver/decoder uses slope detection and a maximum likelihood sequence detector. As the analysis of any (x/y) multiple PPM system is extremely time-consuming, a novel automated solution was designed to predict the equivalent pulse code modulation (PCM) error rates of specific sequences and to simplify the task. A measure of coding quality that accounts for efficiency of coding and bandwidth expansion has also been proposed. Using this measure, original results show that a (12/6) system is the most efficient for a wide range of bandwidths.     

光谱分析仪光学系统的优化设计方法

提出一种光谱分析仪光学系统的优化设计方法。该方法以光线光学为依据,在光源和光瞳上以高密度取样,将追迹实际取样光线得到的点列图作为评价依据, 根据光谱分析系统的评价指标,将整个系统以单色器入口为分界点分为两个子系统,分别对其进行优化设计,研制了对结果的后处理模块,并将其集成到光学设计软件中去。给出了一个原子吸收分光光度计光学系统实例,使用波段为 190~860nm。设计结果和样品实验表明,该系统达到 0.3nm 的光谱分辨力要求。     

High-speed free-space interconnect based on optical ring topology: experimental demonstration

The design and implementation of a high-speed optical-ring-topology-based free-space optical interconnect is described. This interconnect system operates at 500 MHz and consists of 16 laser transmitters, a four-channel free-space interconnect, and a high-speed receiver. A nearest-neighbor interconnect is demonstrated. At the data rate of 500 MHz the total system throughput is 8 Gbits/s. The system can easily be operated at much higher data rates since the rate is limited only by the electronic circuitry. A discussion is given about device issues such as optical switching devices, and practical system-design issues such as integration and interface with current electronic systems are considered. This interconnect is promising to the implementation of ultrafast massively parallel single-instruction multiple-data machines.     

Tolerancing of board-level-free-space optical interconnects

For optical interconnects to become a mature technology they must be amenable to electronic packaging technology. Two main obstacles to including free-space optical interconnects are alignment and heat-dissipation issues. Here we study the issues of alignment tolerancing that are due to assembly and manufacturing variations (passive-element tolerancing) over long board-level distances (>10 cm) for free-space optical interconnects. We also combine these variations with active optoelectronic device variations (active-element tolerancing). We demonstrate a computer-aided analysis procedure that permits one to determine both active- and passive-element tolerances needed to achieve some system-level specification, such as yield or cost. The procedure that we employ relies on developing a detailed design of the system to be studied in a standard optical design program, such as code v. Using information from this model, we can determine the integrated power falling on the detector, which we term optical throughput, by performing Gaussian propagation or general Fresnel propagation (if significant vignetting occurs). This optical throughput can be used to determine system-level performance criteria, such as bit-error rate. With this computer-aided analysis technique, a sensitivity analysis of all the variations under study is made on a system with realistic board-level interconnect distances to find each perturbation's relative effects (with other perturbations set to 0) on the power falling on the detector. This information is used to set initial tolerances for subsequent tolerancing analysis and design runs. A tolerancing analysis by Monte Carlo techniques is applied to determine if the yield or cost (yield is denned as the percentage of systems that have acceptable system performance) is acceptable. With a technique called parametric sampling, a subsequent tolerancing design run can be applied to optimize this yield or cost with little increase in computation. We study a design example and show that most of the tolerances can be achieved with current technology.     

Unipolar codes for optical spectral-amplitude code-division multiple-access systems based on combinatorial designs

A novel class of optical signature codes based on combinatorial designs is proposed for optical spectral-amplitude code-division multiple-access (CDMA) systems. It is applicable to both synchronous and asynchronous incoherent optical CDMA and is compatible with both frequency-encoded and time-spreading schemes. Simplicity of construction, larger code cardinality and larger flexibility in cross-correlation (CC) control make the proposed code family an interesting candidate for future optical CDMA applications that require a large number of simultaneous users. It has been shown that the system performance can be significantly improved by using the proposed codes with ideal in-phase CC in preference to Hadamard codes.     

恒星干涉仪延迟线系统地面随机振动响应分析

研究地面随机微振动对恒星干涉仪关键部件-光学延迟线系统性能的影响。利用美国蓝氏(Lansmont)集团生产的SAVER 3X型三轴振动仪实测地面随机振动信号,以输出的G加速度功率谱密度为激励,运用Workbench有限元分析软件计算该结构系统的随机振动响应;根据抛物镜面节点数据拟合出抛物镜面的Zernike多项式系数、抛物镜刚体平移量和抛物镜面PV和RMS值;将Zernike多项式系数导入光学系统中分析光学延迟线系统光学性能;最后利用ELCOWAT 3000电子自准直仪测量抛物镜刚体平移量,分析实际测量结果和计算理论结果偏差。分析计算结果与实验数据最大偏差7.6%,计算抛物镜面形RMS值为9.6 nm,PV值为46.1 nm,波前差为0.043λ。目前的光学延迟线的地面振动满足光学延迟线系统的稳定性要求。使用动态光学性能标准分析判断微振动对系统光学性能的影响程度,为恒星干涉仪其他子系统优化设计和隔振补偿措施提供参考。     

Learning and recall algorithm for optical associative memory using a bistable spatial light modulator

A learning and recall algorithm for optical associative memory based on the conventional correlationlearning method with three effective improvements (sparse-encoding method, constant-total-activity method, and binary memory) is proposed from a viewpoint of practical implementation. It is shown that the algorithm is suitable for implementation with a bistable spatial light modulator such as a ferroelectric liquid-crystal spatial light modulator, which has high resolution and a fast response time. The results of theoretical analysis and simulations indicate that the algorithm permits an associative-memory system with a large memory capacity to be realized. An example of an optical system for executing this algorithm is proposed. To determine the performance specifications that are required for the various optical components within the system, we simulate and evaluate the effect of noise (which is caused by nonideal components) on system performance. These results show that the system is robust in the presence of predicted noise levels.     

Color encoding for polychromatic single-channel optical pattern recognition

The common multichannel system for recognizing colored images is replaced by a color-encoded single-channel system. Amethod inspired by the Munsell color system is used for encoding the different colors as phase and amplitude functions. It is shown that for many practical cases the phase information part of the color code is sufficient for obtaining good results. An implementation based on a liquid-crystal television panel that works in a phase-modulation mode is suggested. Computer simulations that demonstrate the capabilities of the suggested method are given as well as a comparison with previously published multichannel performance.     

Wavelength-encoding/temporal-spreading optical code division multiple-access system with in-fiber chirped moiré gratings

We propose an optical code division multiple-access (OCDMA) system that uses in-fiber chirped moiré gratings (CMG's) for encoding and decoding of broadband pulses. In reflection the wavelength-selective and dispersive nature of CMG's can be used to implement wavelength-encoding/temporal-spreading OCDMA. We give examples of codes designed around the constraints imposed by the encoding devices and present numerical simulations that demonstrate the proposed concept.     

System performances of optical space code-division multiple-access-based fiber-optic two-dimensional parallel data link

Optical space code-division multiple access is a scheme to multiplex and link data between two-dimensional processors such as smart pixels and spatial light modulators or arrays of optical sources like vertical-cavity surface-emitting lasers. We examine the multiplexing characteristics of optical space code-division multiple access by using optical orthogonal signature patterns. The probability density function of interference noise in interfering optical orthogonal signature patterns is calculated. The bit-error rate is derived from the result and plotted as a function of receiver threshold, code length, code weight, and number of users. Furthermore, we propose a prethresholding method to suppress the interference noise, and we experimentally verify that the method works effectively in improving system performance.     

光空间调制技术的研究进展

光空间调制(OSM)作为一种新型的光多输入多输出(OMIMO)技术,利用空间域激光器索引号额外携带信息,有效地提高了系统的传输速率和能量效率;同时,由于每符号周期仅激活一个激光器传递信息,较好地解决了传统OMIMO系统中的信道干扰和同步等问题。本文首先介绍了现有的几种光空间调制技术,概括和总结其在国内外的研究现状。此外,从传输速率、频谱效率、误码率(BER)和计算复杂度等四个方面对现有的OSM、光空移键控(OSSK)、增强型光空间调制(EOSM)和差分光空间调制(DOSM)等方案进行了比较分析。最后,指出了OSM中亟需解决的关键性问题及其未来的发展方向。