New design of spectral amplitude coding in OCDMA with zero cross-correlation

A zero cross-correlation (ZCC) code is proposed to reduce the impact of system impairment and multiple access interference (MAI) in spectral amplitude coding optical code division multiple access (SAC-OCDMA) system. Bit-error-rate (BER) performance is derived taking into account the effect of some noises. The key to an effective OCDMA system is the choice of efficient address codes with good or almost zero correlation properties for encoding the source. The use of ZCC code can eradicate phase induced intensity noise (PIIN) which will contribute to better BER. Thus, we demonstrate, theoretically, the performance of optical ZCC code. It is shown that optical ZCC code can accommodate more users simultaneously for the typical error rate of optical communication system of 10⁻⁹. The result indicates that the established system not only preserves the capability of suppressing MAI, but also improves bit-error-rate performance as compared to the conventional coders.     

Design and performance analysis of various one dimensional codes using different data formats for OCDMA system

In this paper, the design, implementation and performance analysis of various one dimensional codes in an OCDMA system for different data formats is presented. A number of different codes are used with optical CDMA to improve its error performance. Here, three such codes, optical orthogonal codes (OOC), Walsh Hadamard codes and zero cross-correlation (ZCC) codes have been compared using different data formats, NRZ raised cosine, NRZ rectangular, RZ raised cosine and RZ rectangular. It is found that NRZ raised cosine has the best system performance for all the codes used. After that, the three codes have been compared in terms of the BER, eye diagrams and received optical power using NRZ raised cosine modulation format. It is analyzed that ZCC codes have zero cross-correlation property. The simulation results revealed that ZCC codes can provide a better BER compared to the OOC and Walsh Hadamard codes and it is most suitable to be employed in the OCDMA systems.     

Minimizing correlation effect using zero cross correlation code in spectral amplitude coding optical code division multiple access

The use of minimal multiple access interference (MAI) in code design is investigated. Applying a projection and mapping techniques, a code that has a zero cross correlation (ZCC) between users in optical code division multiple access (OCDMA) is presented in this paper. The system is based on an incoherent light source—LED, spectral amplitude coding (SAC), and direct detection techniques at the receiver. Using power spectral density (PSD) function and Gaussian approximation, we obtain the signal-to-noise ratio (SNR) and the bit-error rate (BER) to measure the code performance. Making a comparison with other existing codes, e.g., Hadamard, MFH and MDW codes, we show that our code performs better at BER 10⁻⁹ in terms of number of simultaneous users. We also demonstrate the comparison between the theoretical and simulation analyses, where the results are close to one another.     

多噪声背景下光码分多址多用户干扰的抑制

针对光码分多址(OCDMA)的多用户干扰问题, 研究采用APD光电检测器的系统中存在多种噪声情况下, 基于最大值判决的光码分多址并行干扰抑制方法的多用户干扰抑制效果。阐述了其工作原理, 即在发送端对比特“0”和比特“1”分别使用一个地址码和它的移位变形进行映射, 以避免发送比特“0”时无光脉冲通过, 实现并行干扰抑制; 接收端判决电路采用最大值判决, 以避免门限判决所引起的误判, 进一步减小多用户干扰。系统采用光正交码作为地址码, 分析推导了系统的误码率表达式。根据所得的误码率表达式, 进行了数值仿真并与其他多用户干扰抑制方法进行比较。仿真结果表明, 本方法对多用户干扰的抑制作用与系统用户数和码重有关; 与引入双硬限幅器方法、多比特传输方法的多用户干扰抑制作用相比, 抑制效果要优于多比特传输的方法; 当接收功率较低时, 该方法优于引入双硬限幅器的方法, 因此可以有效的抑制多用户干扰。     

基于最大值判决及光硬限幅器的光码分多址并行干扰抑制系统

针对光码分多址（OCDMA）的多用户干扰问题,提出一种引入光硬限幅器、基于最大值判决的光码分多址并行干扰抑制系统.该系统在发送端对比特“0”和比特“1”分别使用一个地址码和它的移位变形进行映射,以避免发送比特“0”时无光脉冲通过的问题,实现并行干扰抑制;在接收端光电检测器前引入一个光硬限幅器,判决电路采用最大值判决,以避免门限判决所引起的误判,进一步减小多用户干扰.阐述了其工作原理,系统采用光正交码作为地址码,PIN光电检测器,分析推导了系统的误码率表达式,进而对新型系统的多用户干扰抑制作用进行误码性能仿真.仿真结果表明,当信号光子到达率为25时,新型系统比常规系统的误码性能改善10个数量级以上,比常规并行干扰抑制系统改善2个数量级以上.     

Variable cross-correlation code construction for spectral amplitude coding optical CDMA networks

We present, for the first time, several aspects of incoherent optical code-division multiple access (OCDMA) codes, focusing on the flexible variable cross-correlation code allocation and its potential for future optical networks. We briefly present a new version of the Random Diagonal (RD) codes for Spectral-Amplitude Coding (SAC) OCDMA approaches. We then concentrate on the properties specific to such schemes allowing for its increased scalability and flexibility. The main coding properties are reviewed. The RD codes provide simple matrix constructions compared to the other SAC-OCDMA codes such as Hadamard, MQC and MFH codes. This code possesses such a numerous advantages, including the efficient and easy code construction, simple encoder/decoder design, existence for every natural number n, and variable in-phase cross-correlation and easy to implement using Fiber Bragg Gratings (FBGs). Finally, a new detection scheme called “NAND” detection is developed for the variable cross-correlation RD code.     

基于APD线列的单光子探测计数研究

单光子探测系统可以对单个光子进行探测;探测系统含有探测部分、淬灭电路部分和计数部分;探测部分主要由工作在盖革模式下的雪崩光电二极管组成;在盖革模式下的雪崩光电二极管发生雪崩后不能自然停止,淬灭部分主要为了主动抑制雪崩电流,快速降低雪崩电压,以达到提高探测效率,降低错误计数的目的;APD线列产生多个光子脉冲信号,计数部分的主要功能是对多路光子脉冲信号进行计数、显示并且可以控制每路APD的比较电压,保证每路APD淬灭电路的正常工作。     

Performance evaluation of Fi-Wi network based on SCM–optical code division multiple access architecture

This paper investigates the performance of a spectral amplitude coding (SAC)–OCDMA system design utilizing subcarrier multiplexing techniques for use in the popular fiber-wireless (Fi-Wi) technology. Zero cross correlation code is employed because of its ability to eliminate phase induced intensity noise (PIIN) and the simplicity of using a direct detection technique at the receiving end. The performances are evaluated theoretically to derive the signal-to-noise ratio and results are presented in terms of bit error rates (BER). Results are analyzed to investigate the effect of code weight and variable data rates on the system performance. To validate the results, simulative analysis is also done using Optisys version 6.0. We demonstrated that the ZCC code performed better compared to other code for a BER floor of 10⁻⁹ with larger cardinality of subscribers. Results obviously indicate that system performance can be improved with larger code weight. In addition, the choice of lower bit rate is observed to perform better and more suitable for application in this hybrid Fi-Wi network.     

Noise performance of an InP/InGaAs superlattice avalanche photodiode

The noise performance of an InP/InGaAs Superlattice Avalanche Photodiode (SL-APD) has been studied theoretically to examine its suitability as a detector in optical receiver units. The results indicate that the device which has a very large gain-bandwidth product (450 GHz) provides a high receiver sensitivity compared to conventional APD. The device also exhibits a very low value of excess noise factor which results in a high value of signal-to-noise ratio. The SL-APD is expected to find useful applications as a high sensitivity photodetector in long-haul fibre optic communication systems. For a digital system, the device exhibits a very low value of bit-error rate (BER) even at moderate gain.     

Receivers for optical communications: A comparison of avalanche photodiodes with PIN-FET hybrids

The effects of photodiode bulk leakage current and amplifier noise on receiver sensitivity are analysed using a model described previously [4]. The sensitivity of a receiver using a PIN photodiode can be greatly improved by employing a high-performance microwave FET in the input stage, to the point where its remaining technical disadvantage in comparison with a silicon APD receiver at 800–900 nm may be offset by economic and operational attractions. In systems operating at the optimum transmission wavelength beyond 1.1 m, the PIN-FET hybrid receiver may offer better technical performance than an APD receiver.     

毫秒脉冲激光致硅光电二极管电学损伤的有限元分析及实验研究

为了研究毫秒脉冲激光致硅基PIN光电二极管电学损伤,基于热传导及弹塑性力学理论,在光电二极管内部材料各向同性并且P-I-N三层结构之间满足温度连续和热流平衡条件下,建立毫秒脉冲激光辐照硅基PIN光电二极管二维轴对称模型,采用有限元方法模拟分析了1064 nm Nd:YAG毫秒量级脉冲激光辐照硅基PIN光电二极管的温度场与应力场分布,并实验测量了硅基PIN光电二极管实验前后的电学参数.结果表明,激光辐照硅基PIN光电二极管时,温升使材料表面熔融、烧蚀,并且在空间上存在温度梯度变化,即激光辐照产生的热与应力使光敏面及硅晶格晶键损伤,最终造成光电探测器的探测性能下降.研究结果可为毫秒脉冲激光辐照硅基PIN光电二极管电学损伤机理奠定基础.     

Long wavelength photodetectors

The role of long wavelength systems (1.0μm<λ< 10.0μm) in fiber optics communications is evaluated. For high-bit rate optical telecommunications at 1.3 μm or 1.5 μm, GaInAs p-i-n detectors have emerged as the preferred choice because of their low noise, excellent sensitivity, and high temperature stability. Ge and HgCdTe photodiodes offer nearly equivalent performance and a somewhat more advanced production technology. Beyond 2 μm, HgCdTe would seem to be the clear choice for photodetector applications. Avalanche photodiodes in long wavelength optical fiber links may find uses in situations involving high bit rate transmission using low cost components. Although avalanche gain will always improve the system sensitivity, the LED/APD combination is only half as sensitive as a laser/p-i-n system, as well as being bandwidth limited. Nontelecommunications applications involving data base and fiber guidance systems are discussed, and a prospective look is taken at the uses, such as power transmission, of ultra low-loss fibers in the 2 to 10 μm region of the optical spectrum.     

A divided spectrum balanced detection technique for intensity noise reduction in SAC OCDMA systems

In this paper we propose a simple divided spectrum balanced detection (DSBD) for spectral amplitude coding (SAC) optical code division multiple access (OCDMA) systems. SAC OCDMA systems are limited by phase induced intensity noise (PIIN), which is a signal dependent source of noise. Our proposed technique reduces the PIIN by dividing the spectrum of the signal into two or more, and detecting each spectrum by a different photodiode. The DSBD scheme reduces the detected optical power at photodetection, thus resulting in a higher mitigation of the PIIN. Theoretical results show that DSBD demonstrate noticeable improvement over traditional balanced detection technique, for example an up to 33% increase in the number of active users can be achieved, and at least 1 × 10⁻³ b/s Hz increase in the spectral efficiency is obtained. However, the SDBD is more complex and append more constrains on system components.     

Performance Enhancement for Optical Fast Frequency-hopping CDMA Systems by BCH Channel Coding

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An efficient MAI cancellation technique in Optical CDMA

Optical code division multiple access (OCDMA) is a promising operational system for future backhaul optical networks especially for switching and optical signal multiplexing. However, OCDMA operation is restricted by multiple access interference (MAI) due to the asynchronous nature of transmission. In this study, we discuss an efficient MAI technique for OCDMA systems that depends not only on using hybrid frequency shift keying (FSK) modulation but also on employing the double padded modified prime code (DPMPC) as a signature sequence. MAI cancellation is performed by subtracting a reference signal from the received signal of the desired user. The simulation results show that the performance of the FSK-OCDMA technique is superior to the performance of the PPM-OCDMA technique. Furthermore, the obtained results illustrate that the FSK-OCDMA technique is able to accommodate a large number of simultaneous active users with low error-rate.     

Edge-view photodetector for optical interconnects

A new semiconductor photodetector design is reported. The PIN active area of the photodetector is fabricated on a sloping sidewall of a mesa structure. The photodetector is referred to as an edge-view photodetector (EVPD) and simplifies the integration of optical circuits by direct end coupling, eliminating the need for 45 degrees mirrors. The EVPD geometry can reduce the cost of optical interconnects by simplifying the fabrication process and making possible automatic alignment between photodetectors and waveguides or optical fibers. The EVPD fabrication process and initial measurement results are presented. The main processing steps include deep anisotropic chemical etching, material growth, and lithography on a 3-D surface.     

Tunable 2D time-wavelength optical CDMA encoder for differentiated service provisioning

We propose and demonstrate a reconfigurable tunable encoder for two-dimensional time-wavelength optical code division multiple access (OCDMA). This encoder is capable of creating 2D codes with variable code weight, enabling differentiated service provisioning. The demonstrated novel scheme uses ultra-fast optical delay lines and all-optical sampling while operating at 115 Gchip/s. Our proposed architecture provides an all-optical method of varying the code weight on a bit-per-bit basis, while maintaining the core architecture of a tunable OCDMA encoder.     

Si-PIN photodiode readout for a scintillating optical fiber dosimeter

For more than a decade plastic optical fiber based dosimeters have been developed for medical applications. PMT's have been widely used as the readout photodetector but for a number of applications, Si-PIN photodiodes are a good alternative. They are robust, present good stability over time and have enough sensitivity so that an electrometer can be used as a measuring device. This work describes the tests made with the S9195 Si-PIN photodiode from Hamamatsu used as the readout photodetector for a scintillating optical fiber dosimeter using a BCF-60 polystyrene optical fiber.     

New technique for construction of a zero cross correlation code

This paper proposes a new technique for constructing zero cross correlation codes (ZCC). In the existing techniques, especially for weight w more than one, the weight is always fixed to the maximum number of users (i.e. the code size). To overcome this difficulty, an efficient design is presented. The code design procedure can be easily used to construct variable length and non-constant weight w. A comparison has been established between existing ZCC optical codes and the novel ZCC classes.     

A new code construction with zero cross correlation based on BIBD

This paper presents a newly constructed zero cross correlation code (ZCC) which is based on BIBD (balanced incomplete block design) code. The ZCC (C, w) code is a family of binary sequences of length C and constant Hamming-weight w. Such codes find applications in spectral amplitude-coding optical code division multiple access (SAC-OCDMA). The constructing ZCC codes have a size of C ? N ÿ w + 1, where N is the number of users and C is any prime number. The proposed construction method is not complicated compared to the existing ones.