Bit-error rate of M-QAM signal and its analysis model for compositedistortions in AM/QAM hybrid transmission

In cable television (CATV) systems that deliver digital video services by employing a hybrid fiber and coaxial network, it is important to consider digital channel deterioration due to composite distortions, as well as the amplitude-modulation vestigial-sideband channel. Furthermore, it is essential to clearly estimate a digital channel's quality in designing CATV transmission systems. However, there have been few studies on statistical models and an analysis method to estimate the bit-error rate (BER) of M-ary quadrature amplitude-modulation signals for composite distortions. This paper shows statistical properties of composite distortions and a new BER analysis method to handle them. In the evaluations, the amplitude distributions of composite distortions are given by Weibull distributions with skewness levels of 2.3 for composite second-order distortion and 1.5 for composite triple-beat distortion. We propose a new BER analysis method that uses the Weibull distributions for the composite distortion statistical features and demonstrate the effectiveness of the new BER analysis method in estimating BER by comparing its measurement results to calculation results     

Second-order distortion improvements or degradations brought byerbium-doped fiber amplifiers in analog links using directly modulatedlasers

Within lightwave analog amplitude-modulated (AM) CATV systems using directly modulated lasers, erbium-doped fiber amplifiers (EDFA's) act upon the signal distortion because of the interaction between the laser chirp and the EDFA wavelength-dependent gain. This interaction is theoretically investigated in order to predict the EDFA-induced distortion. The relevant gain tilt characteristic for analog applications and the way to measure it are described. Expected and measured distortions at the EDFA output are in excellent agreement. Fiber amplifiers are found to decrease the signal distortion level when the gain tilt is negative, i.e., for wavelength above the gain maximum     

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     

Fundamental second-order nonlinear distortions in analog AM CATVtransport systems based on single frequency semiconductor lasers

The author analyzes the fundamental second-order nonlinearities and their composite distortions in lightwave analog AM CATV transport systems based on single frequency semiconductor lasers. The nonlinear interaction between the photons and the electrons in the laser, the nonlinear dynamics of laser spatial hole burning, and the nonlinear interaction of fiber chromatic dispersion and laser frequency chirping are strongly dependent on the CATV carrier frequencies. The author found that the overall distortions increase for higher carrier frequencies and longer fiber spans. The distortion levels, however, are within the critical requirements for current and future CATV transport systems     

Comparison between erbium-doped fluoride and silica fiberamplifiers in an AM-CATV transmission system

Interest behavior of the flattened dynamic gain of an erbium-doped fluorozirconate fiber amplifier is demonstrated in a lightwave CATV system including a 50-channel subcarrier multiplexer. Using two analog distributed-feedback (DFB) laser diodes, one with a relatively high chirp and the other with a very low chirp, a comparison between amplifiers made of erbium-doped silica and fluoride fiber demonstrates the significant influence of the laser FM response when the spectral dynamic gain of the amplifier is not constant     

Fundamental limits for linearity of CATV lasers

We have considered nonlinear distortions in a subcarrier multiplexed optical transmission system arising from carrier heating effects in the source laser. We included two sources of carrier heating: free-carrier absorption of the coherent radiation in the cavity, and the power flux into the active layer, associated with the input current. Resulting from the very existence of modulated optical and electrical signals, these effects cannot be avoided in any laser structure. Simple analytic expressions have been derived for the intermodulation distortion in multichannel single-mode lasers. For an 80-channel 60-540 MHz system with 3% optical modulation depth per channel, the composite second-order (CSO) distortion brought about by the hot-carrier effects is near the tolerance limit for CATV systems     

Nonlinear distortions and compensations of DFB laser diode inAM-VSB lightwave CATV applications

The nonlinear distortions of a directly modulated laser diode applied to AM-VSB lightwave CATV systems are studied from the viewpoint of rate equations. A simple method for calculating composite second-order distortion (CSO) is proposed, which is called the quasidynamic approach. The contribution to nonlinearity from fiber dispersion and laser chirping is also discussed. The dependences on the variations of parameters of rate equations are investigated to determine the dominant factor for CSO degradation. This approach explains quite well the experimental results and suggests a method for constructing a predistortion circuit with at least 10 dB improvement     

Analysis of the second- and third-order intermodulation distortionin DFB lasers including dynamic spatial hole burning effect

Analysis of second and third order intermodulation distortion characteristics of semiconductor DFB lasers is, for the first time, performed, including dynamic longitudinal spatial hole burning (LSHB) effect. We have shown theoretically that the third order intermodulation distortion can be lower than the calculated curve without LSHB effect, which is confirmed with experiments     

Analysis of AC gain tilt in erbium-doped fiber amplifiers

We analyze theoretically and experimentally the dependence of AC gain tilt variations on gain variations in homogeneously broadened erbium-doped fiber amplifiers. Theoretically, we find that this dependence can be evaluated from the fiber's absorption spectrum. This is experimentally verified for gain variations induced by a pump-power reduction. For AM CATV, AC gain tilt variations limit how much the gain can vary before composite second order distortions become too large. From this point of view, for an erbium-doped alumino-germanosilicate fiber, an operating wavelength between 1542-1555 nm is more suitable than other ones in the long wavelength range of the EDFA, normally considered for AM CATV.     

AM—CATV传输系统中掺铒氟化物和石英光纤放大器的比较

本文介绍了掺铒氟化物光纤的功能,并对光几何参数非常接近的EDSFA和EDFFA在AM-VSB CATV系统内进行了对比,在对比过程中采用了两只不同波长（1542nm和1561nm）,啾声特性亦不同的模拟DFB信号激励器。     

Self-suppression effect of longitudinal spatial hole burning inabsorptive-grating gain-coupled DFB lasers

Longitudinal spatial hole burning (LSHB) induces degradation of longitudinal-mode stability in distributed-feedback (DFB) lasers. Measurement of frequency modulation characteristics has revealed that, in absorptive-grating gain-coupled DFB lasers, the LSHB diminishes as power increases. This anomalous behavior has been qualitatively explained by a theoretical analysis that took into account the saturable nature of the absorption of the gain-coupled grating. This LSHB suppression effect is advantageous for high-power single-longitudinal-mode operation of DFB lasers     

初始啁啾和光纤损耗对皮秒脉冲孤子效应压缩的影响

运用分步傅立叶方法数值模拟了皮秒脉冲在单模光纤中的传输 ,计算和分析了初始啁啾和光纤损耗对皮秒脉冲孤子效应压缩的影响。结果表明 ,两者对具有一定峰值功率的脉冲孤子效应压缩的影响具有互补性 ,即两者对脉冲压缩的作用相反。进一步研究表明 ,对在损耗光纤中传输的皮秒脉冲 ,如果给其加适当大小的初始啁啾将得到很好的压缩效果     

光子晶体光纤在有线电视传输系统中的应用

文章介绍了一种利用光子晶体光纤作为宽带色散补偿装置的直接幅度调制残留边带(AM-VSB)有线电视传输系统,测试结果表明,与未加补偿的外调制方式相比,本系统具有更高的载噪比(CNR),更优的复合二阶失真(CSO)和复合三阶差拍性能(CTB).将光子晶体光纤用作色散补偿,不但可以应用到CATV系统中,还可以广泛应用于其它宽带高速长距离光纤通信领域.     

AM-CATV光纤传输系统中EDFA非线性失真的补偿

分析计算了掺铒光纤放大器（ＥＤＦＡ）在ＡＭ－ＣＡＴＶ光纤传输系统中引入的非线性失真，在总结了两种补偿方法的同时，提出了一种新的补偿方法——光补偿，计算机模拟和实验结果表明，它对ＥＤＦＡ的非线性失真有一定的改善。最后对这三种补偿方法进行了简单的比较     

Lightwave subcarrier CATV transmission systems

The design and performance of multichannel amplitude-modulated (AM) vestigial-sideband (VSB) lightwave cable television (CATV) systems are described. Requirements on linearity and noise are derived, and factors limiting the performance of the laser transmitters and receivers are discussed. For the high-performance lasers the carrier-to-noise ratio and composite second- and third-order distortions are acceptable for video trunk systems. Impairments because of fiber reflections and dispersion and mode partition fluctuations in the laser are discussed. Feedforward, feedback, and predistortion are discussed; difficulties with each prevent immediate application. The use of LiNbO₃ external modulators and high-power solid-state lasers is considered. The third-order distortion and insertion loss of the modulator more than counteract the high available laser power (100 mW), making this alternative unattractive unless a third-order linearizer can be implemented     

Improved direct-modulation characteristics of a semiconductor laserby FM/IM conversion through an interferometer

The behavior of the light signal converted by means of a Mach-Zehnder interferometer is analyzed under the assumption of a three-tone modulation of the emitting laser source. Illustrations are given of the signal at the interferometer output, showing the frequency response, the second- and third-order intermodulation distortions (IMDs), with a direct comparison between intensity modulation (IM) and optical frequency modulation (FM). The FM characteristics are plotted for the different cases where the spurious IM component and the term of chirp formula involving the gain compression factor are taken into account or not. Successful results are obtained in agreement with previous studies. In addition, this work shows that the FM-to-IM signal would exhibit lower distortion levels than the conventional direct IM, placing less stringent requirements on the laser linearity     

Clipping distortion in lightwave CATV systems: models, simulations,and measurements

Theoretical, numerical, and experimental investigations of clipping distortions in CATV systems are reported. The applicability of A.A.M. Saleh's (1989) calculation of the carrier-to-nonlinear-distortion ratio (CNLD) is extended by incorporating a more precise spectral analysis and further analytic results. An effective transfer function model which spectrally resolves the clipping distortion at intermodulation products (IMPs) of all orders and frequencies, and features closed-form analytic calculation of the second- and third-order distortions (CSO and CTB) from basic principles, as well as the CNLD, is introduced. It is found that Saleh's model slightly overestimates the simulation results for the CNLD, while the new model is in essential agreement with the simulation for all three distortion measures. Experimental measurements of the CSO, CTB, and CNLD over a 50-dB range in distortion exhibited excellent agreement with the simulations and the new model over the entire range. The unified nature of the model allows standard CATV, CSO, and CTB measurements to be theoretically connected to the CNLD and clipping boundary     

Upgrade of coaxial CATV networks for upstream high speed digitalcommunication

There is a growing interest to extend existing coax CATV networks to enable interactive multimedia services. In this paper we describe necessary modifications and additions of CATV network components to provide broadband upstream channels in the range of 6 to 40 MHz. To investigate linear and nonlinear distortions, reflexions, noise and ingress, suitable digital modulation schemes and adaptive equalization, the CATV network-components are modelled and a computer simulation program using PTOLEMY is established. It is shown that all 3370 subscribers of an existing CATV sub-network can be provided with up to 25 kbit/s collision free on the physical network layer. Using appropriate teletraffic and media access control protocols, user bit rates of up to 2 Mbit/s are feasible in the upstream direction     

光发射机中MZ型波导调制器产生CTB的分析

AM－CATV外调制光发射机中的MZ型波导调制器，其转移特性呈正弦曲线。当多路信号同时调制时，将产生组合二次失真（CSO）和组合三次差拍（CTB）。本文对CTB进行了理论分析，并对我国450、550MHz二种CATV系统调制时的CTB进行了计算。这对非线性补偿电路的设计具有一定的指导价值。     

掺铒光纤放大器中孤子脉冲获得线性啁啾的研究

为了研究孤子脉冲经掺铒光纤放大器演变后的啁啾特性,采用了对称分步傅立叶的数值方法,对孤子脉冲在不同增益条件下的演变和线性啁啾特性进行了数值仿真。数值计算的结果表明：当满足增益系数μ远大于1时,基孤子脉冲在放大器中传输能够获得很好的线性啁啾,且发现脉冲的啁啾特性受选取的放大器长度参量的影响;啁啾越明显的脉冲,线性啁啾所占的区域越窄。进一步研究还发现初始啁啾系数和孤子阶数对脉冲获得的线性啁啾均有很大影响,初始正啁啾系数和高孤子阶数均能增强输出脉冲的线性啁啾特性。因此,利用孤子脉冲在掺铒光纤放大器中传输的非线性效应能够获得良好的线性啁啾,这对实现高质量的脉冲压缩是很有帮助的。