Direct optical characterisation of DFB lasers for CPFSK system applications

The authors present a new optical characterisation method for single-frequency lasers that are to be applied in Gbit/s CPFSK systems. They use optical delay-demodulation to allow easy matching of the demodulator delay over a wide range of values. It is shown that for a particular device, when used in a 2.4 Gbit/s CPFSK system, the modulation index must be larger than 1.0 to obtain a bit error rate below 10/sup -11/. With a modulation index of 1.5, bit error rates below 5*10/sup -13/ have been measured.<>     

Optimum system parameters for multigigabit CPFSK optical heterodynedetection systems

To obtain a multigigabit continuous-phase frequency-shift-keying (CPFSK) system with a high receiver sensitivity, we theoretically and experimentally investigated the optimum modulation index parameter and IF center frequency, considering the modulation and demodulation baseband widths and the IF bandwidth. In a 6-Gb/s CPFSK experiment, we achieved a receiver sensitivity of -41.6 dBm, or 89 photons/bit. To use the 4- to 13-GHz IF band efficiently, we set the IF center frequency f _IF to 8.7 GHz, or 1.45 times the bit rate. To compensate for the insufficient 5-GHz FM-modulation bandwidth in the transmitter, we set the modulation index parameter β to 1.38, that is, a modulation index m of 0.8     

Waveform distortion evaluation for optical CPFSK heterodyne transmission

Waveform distortion of a continuous phase FSK signal due to fiber chromatic dispersion is measured. The transmission spacing is estimated to be 100 km at 5 Gbit/s with 1.55-μm wavelength. Also the 2-Gbit/s optical CPFSK heterodyne detection transmission experiment is reported. Since there is no chirping degradation, it is possible to transmit the signal through a 200-km single-mode fiber.     

Optimum encoder for trellis-coded CPFSK with ring convolutionalcodes

The authors apply trellis-coding techniques to continuous phase frequency shift keying (CPFSK). Ring convolutional encoders for trellis-coded CPFSK are found by optimising the normalised minimum Euclidean distance for a given total number of states in the overall MLSE receiver. Code searching results show that several optimum encoders give performances which are better than previous schemes     

Phase-noise-insensitive coherent optical heterodyne CPFSK receiver with limiter-discriminator detector

A coherent optical heterodyne CPFSK receiver with a limiter-discriminator (LD) detector, which cannot be degraded by quantum phase noise of lasers when the frequency deviation ratio h is equal to 1, is proposed. The receiver can give the same bit error rate as the ideal digital DPSK system. In addition, 1/2 exp (-SNR) can be considered as the upper bound of the minimum error rate of the CPFSK-LD detector.<>     

An optical FSK heterodyne dual filter detection system for takingadvantage of DFB LD applications

An optimum system configuration for an optical frequency-shift keying (FSK) heterodyne dual-filter detection system with distributed feedback laser diodes (DFB LDs) is investigated, taking into consideration LD phase noise influence. Experimental and theoretical examination show that an IF filter bandwidth greater than 10 times the beat spectral linewidth is necessary to avoid LD phase noise influence. A 301-km long-span transmission experiment has been successfully carried out with an optimum configuration for 34 Mb/s. High receiver sensitivity, -61.8 dBm with more than 10 dB improvement over the direct detection system, has also been achieved. Experimental results at higher bit rates of 140, 200, and 280 Mb/s indicate that a modulation index greater than two is desirable to avoid cross talk between mark and space signals. With sufficient frequency deviations, high receiver sensitivities of -54.7 dBm (140 Mb/s) and -52.5 dBm (200 Mb/s) have been achieved. These represent 9.6- and 9-dB sensitivity improvement, respectively, over direct detection systems. A guide for FSK dual-filter detection system design is derived from the experimental and theoretical results. Potential application regions for a dual-filter detection system with DFB LDs are determined     

FSK heterodyne system experiments at 1.5 ?m using a DFB laser transmitter

140 and 70 Mbit/s FSK heterodyne system experiments are reported using single-filter detection with transmitter laser linewidths of 30 and 60 MHz. The system requirements that must be satisfied to avoid large performance penalties are considered.     

A cold-start 1.7 Gb/s FSK heterodyne detection system usingsingle-electrode DFB lasers with no equalization

A cold-start, 1.7-Gb/s frequency-shift keying (FSK) heterodyne single-filter detection system using ordinary single-electrode distributed feedback (DFB) lasers is discussed. These lasers were frequency-locked to an atomic krypton line at 1.5224 μm independently. Thus, this system can maintain a constant IF without an IF locking circuit and consequently does not require any IF acquisition circuitry for cold-start operation. The measured receiver sensitivity of this system was -41 db (1 mW) at an error rate of 10^-9     

Cassegrain光学天线系统的优化设计

由于空间光通信中采用的激光束的光场分布呈高斯分布,反射镜对光线的遮挡将严重影响天线的传输效率,从而导致通信质量的下降,针对Cassegrain光学天线,对光学天线的系统结构以及次镜遮拦对天线增益的影响进行了分析,并根据高斯光束经过光学系统的变换与传输特性,分析了采用Cassegrain天线时,遮拦比以及入射光束束腰对光束透过率的影响,提出了一种提高光学天线传输效率的新方案,改进后的天线系统,使得传输效率有了显著提高。     

基于DSP的DFB激光器温度控制系统设计

为了提高红外分布反馈式激光器的工作稳定性,减少其受工作温度波动的影响,采用PID控制技术,设计并研制了一种基于DSP的DFB激光器智能温度控制系统,并利用该系统对DFB激光器进行温度测试仿真实验。实验表明,该系统的温度控制精度为±0.1℃,上升时间为19s,达到稳定的时间为70s,超调量为11.3%,温度控制范围为10℃~60℃,优越于基于单片机的激光器温度控制系统,具有一定的应用价值。     

不确定时延网络控制系统的稳定性设计

针对网络环境下的控制系统所具有的网络时延和不确定性特点,考虑最大允许网络诱导时延下界不为零及系统带有不确定性的情况,采用不需事先指定参数的更具一般性的状态反馈控制器,建立状态反馈的闭环网络控制系统模型。通过引入积分不等式方法,对Lyapunov-Krsasovskii泛函中的二次型积分项直接进行界定,避免了对系统进行模型变换和对交叉项的界定所带来的保守性。在此基础上,推导出使网络控制系统鲁棒稳定的条件。最后通过对模型进行Matlab仿真,得到状态响应曲线。系统状态很快趋于稳定,表明该结论的有效性和可行性。     

Optimum frequency deviation for noncoherent CPFSK receivers using narrow-bandwidth optical filters

For CPFSK lightwave systems with noncoherent detection using a Fabry-Perot filter, the optimum frequency deviation is determined experimentally when the EFM response is nonuniform, as for a single-electrode DFB laser, and the filter bandwidth is narrow. Owing to the passband response of the optical filter, the optimum frequency deviation is considerably larger than that for a heterodyne receiver.<>     

光纤延时自外差法测量DFB型LD温度调谐的瞬态特性

以马赫-曾德干涉仪(MZI)原理为基础搭建了臂长差为3km的光纤延时自外差(FOSHI)测量系统,测量了1 582nm分布反馈型(DFB)二极管激光器(LDs)在温度调谐下输出的瞬态特性,包括LDs输出波长及线宽。结果表明,DFB LDs输出光波长随调谐温度呈线性关系,通过对光电流功率谱密度函数的洛伦兹线型拟合得到LDs的线宽为3.24MHz,与3MHz理论值相近。     

Optimum range for DFB laser chirp for fiber-optic AM videotransmission

Simple expressions for the impact of the distributed feedback (DFB) laser chirp on the system noise and distortion specifications of AM CATV fiber links are derived. These· expressions are found to agree well with measured results. There is a narrow range of chirp for which the degradation of both noise and distortion is low. 1310-nm lasers are found to have chirp within the acceptable range. 1550-nm lasers are found to have chirp that is unacceptably large for most CATV applications     

Optimum CPFSK systems with postdetection filtering and better than matched filter performance

A rigorous theoretical model for CPFSK receivers with post-detection filtering is presented. The sensitivity is within 0.3 dB of the best result (i.e. 36 received photons for a 10/sup -9/ error probability) for modulation index between 1.5 and 5. The sensitivity is better than the matched filter sensitivity in some cases. The phase noise tolerance is significantly improved.<>     

光纤激光器透镜耦合系统的优化设计

准直一聚焦双透镜系统是光纤激光器中常用的一种泵浦耦合技术,但是对于大功率LD泵浦源尾纤输出的多模类高斯光束,难以解析表达其中高阶模式的透镜变换规律,根据此设计透镜耦合系统过程也很复杂。本文用几何光学的方法保证数值孔径的匹配,以类高斯光束光斑尺寸作为基模高斯光束束腰,用基模高斯光束传播规律来简化大功率光纤激光器透镜耦合系统的优化设计.最后给出了一个利用MAT-LAB优化工具函数设计双透镜聚焦耦合系统的实例。     

Tuned front-end design for heterodyne optical receivers

A theoretical analysis of the noise performance of optical receivers with front-end tuning, suitable for wideband coherent systems, is presented. An algorithm for choosing the values of the tuning components in the front end so as to minimize the thermal noise output power has been developed. This theory is applied to the well-known simple parallel and serial tuning configurations and also to three more advanced designs. It is shown that any tuning is better than none in the wide-bandwidth designs considered and that the more advanced designs yield up to 12-dB reduction in thermal noise power. Two of the designs can be implemented with discrete components and should yield shot-noise-limited detection with 50-100 μW of local oscillator power in receivers with 5-GHz bandwidth. The practical problem of equalizing the front-end response is considered, and it is shown that good performance can be expected using realizable components     

空间外差光谱仪成像光学系统设计

为满足高光谱傅里叶变换光谱仪对高光谱分辨率、小畸变、像面光谱辐照度均匀、高信噪比以及仪器轻量化小型化的要求,设计了空间外差光谱仪成像光学系统。基于傅里叶变换的空间外差光谱仪空间干涉的特点和对成像系统缩放比、畸变等要求,依据干涉图调制度分析了最恶劣面形改变条件下对干涉仪元件面形误差的要求,并采用前后镜组匹配实现了双远心成像系统的设计。设计结果表明:该光学结构可避免调焦产生的成像系统缩放比改变,有效视场内畸变量约0.1%,传递函数在38.5 lp/mm处物面所有点全视场范围逸0.60。依据仪器视场角对滤光片安装位置和精度提出要求,并对成像系统进行杂散光和照度均匀性分析提出有效抑制杂散光的措施和方法。系统设计满足了空间外差光谱仪对成像的要求,实现了照度均匀、低畸变、离焦情况下缩放比保持不变等。     

A tractable above-threshold model, for the design of DFB andphase-shifted DFB lasers

A model, based on the transfer matrix method, and extended to above-threshold conditions in order to be used in the analysis and design of distributed feedback (DFB) structures, is presented. It provides the oscillation parameters as lasing frequency and net material gain, the linewidth estimation and output power, the photon density, carrier density, and effective refractive index profiles. This model can be used with or without facet reflectivities and with a number of symmetric or asymmetric phase shifts. It takes into account nonuniform envelope material gain and nonuniform index saturation. It is easily adaptable to any structure modifications     

Space-time code design for CPFSK modulation over frequency-nonselective fading channels

We derive a novel space-time code (STC) design criterion for continuous-phase frequency-shift keying (CPFSK) over frequency-nonselective fading channels. Our derivation is based on a specific matrix that is related to the input symbols of the CPFSK modulators. With this code-design criterion, we propose a simple interleaved space-time encoding scheme for CPFSK modulation over frequency-nonselective correlated fading channels to exploit potential temporal and spatial diversity advantages. Such an encoding scheme consists of a ring convolutional encoder and a spatial encoder, between which a convolutional interleaver is placed. A decoding algorithm that generates symbol metrics for the Viterbi decoder of convolutional codes from the spatial modulation trellis is examined. Simulation results confirm that the advantages of the combination of the interleaved convolutional encoding (for temporal diversity) and the spatial encoding (for spatial diversity) are promising for various system parameters.