弱光反馈对半导体激光器AM和FM噪声谱的影响

一、引言 相干光通信系统具有灵敏度高,选择性好及可采用波分复用等优点。为了实现相干光通信系统,一个首要的问题便是抑制作为发射机振荡器与接收机本地振荡器的LD的噪声。光反馈是抑制LD噪声的有效手段,近年来人们对此进行了大量研究。然而,这些研究一般都局限于LD的线宽。由于LD的场功率谱通常不是洛伦兹型的,因此,必须研究LD的FM和AM噪声功率谱才有实际意义。     

自差型电反馈半导体激光器的调制特性

本文研究了自差型电反馈半导体激光器(LD)的调制特性,指出:电反馈可以抑制 LD 的动态频率漂移(DFS),并讨论了反馈参数对 DFS 的影响。     

基于Turbo码的无线相干光通信系统性能研究

为了提高无线光通信系统的性能,提出并分析了基于Turbo码的无线相干光通信系统。对无线光通信系统中的衰减和噪声进行了分析,并通过Matlab仿真得出了特性曲线。借助系统的误码率,通过对比基于Turbo码的BPSK调制的无线相干光通信系统与BPSK调制的无线非相干光通信系统和未编码无线光通信系统性能,得到无线相干光通信系统借助Turbo编码技术,可以提高系统的抗干扰能力,节省发射光功率,提高了系统性能。     

Gamma-gamma大气湍流下自由空间光通信的性能

自由空间光通信在各种应用中得到了广泛关注,但大气湍流效应导致系统链路性能恶化.假设大气湍流信道是无记忆平稳遍历并且为加性高斯白噪声(AWGN),自由空间光通信系统采用开关键控(OOK)强度调制直接检测(ID/DD),研究了gamma-gamma大气湍流信道下自由空间光通信的性能.利用MeijerG函数推导出了自由空间光...     

二维大气无线光码分多址通信系统

在无线光通信系统中采用光码分多址(OCDMA)技术,可以充分利用其丰富带宽,提高系统性能。考虑背景光噪声、探测器噪声、多用户干扰和大气闪烁等影响因素,给出了二维无线光码分多址(2D-WOCDMA)系统模型,在该模型中采用了脉冲位置调制(PPM)和光纤布拉格光栅(FBG)编解码器。通过数值分析,详细讨论了该无线光通信系统的性能。结果表明,大气闪烁是影响二维无线光码分多址通信系统性能的重要因素,当其对数方差较大时,系统难以实现通信,需要采用诸如多用户检测、信道编码及阵列接收等技术提高系统的性能;二维无线光码分多址通信系统适合采用1550 nm波段;该通信系统还受背景光、雪崩光电二极管(APD)增益和调制消光比等因素的影响。     

高性能LD准直光源的设计与测试

单纵模二极激管光器(LD)在光盘数据存贮、光信息处理、计量测试、自由空间光通信、激光打印机和准直装置等领域的应用急剧增加。虽然其具有小体积、低功耗以及能直接调制的特点,但其光学与电气特性有别于气体激光器。光束具有较大的发散性以及     

激光二极管动态调制分析模型及其应用

本文给出一个新的适于电路分析的激光二极管(LD)动态调制分析模型.讨论了增益压缩因子及调制电流幅度对LD开关时间及弛张振荡的影响.发现对于给定的ε,有一最佳调制电流幅度,使得LD开关时间最小,弛张振荡幅度很小.对一含有缓冲放大级的光电集成光发射机的开关特性进行了计算机辅助分析,讨论了调制信号幅度及LD,驱动电路的开关特性对OEIC开关性能的影响.     

可编程调制的智能化激光二极管驱动电源设计

设计了一种能对输出电流实现多种波形调制的半导体激光二极管(LD)驱动电源。该驱动电源嵌入单片机,控制灵活,结构简单、性能价格比高。可用于LD和LED的输出光功率直接调制,也适用于LD输出光频率调制,具有通用性。     

CATV光网络设备原理及应用：第六讲 光发射机的射频激励功能电路

(上接第14期) 光发射机的任务是把从前端送来的高频电视信号变成光信号,使其能在光导纤维中传输.光发射机按照调制方式划分为调频、调幅和数字调制等多种.目前用得最多的是多路调幅光发射机,在调幅光发射机中按照强度调制的方式不同又分为直接调制光发射机和外调制光发射机.直接调制光发射机是利用高频电视信号(将信号电流叠加到激光器的偏置电流上)来控制半导体激光器的偏流,进而控制激光器的输出光强.外调制光发射机是在激光器输出激光之后,让其通过一个外调制器,使激光的强度随多路调幅信号电压而改变.无论哪种形式的光发射机都具有射频激励单元电路,此功能电路的好坏直接决定着光发射机指标的优劣.鉴于目前市场上直接调制光发射机占大多数,此处RF驱动电路的介绍偏重于直接调制光发射机.     

光通信系统中信道模型与FEC码型构造的分析

对光通信系统中光发射机产生的噪声、光纤中的噪声、放大器的放大的自发辐射(ASE)噪声和光接收机噪声等噪声进行了分析与探讨.用中心极限定理将其中一些噪声综合化归成高斯噪声,再利用随机过程知识将光通信系统信道中的噪声简化为高斯噪声,从而建立了光通信系统的信道模型,即加性高斯白噪声信道模型,基于此信道模型对光通信系统中误码率进行了理论分析.并基于这些分析以及对光通信系统中前向纠错(FEC)码型的分析,提出了光通信系统中FEC码型的主要构造方法.     

Linearity and CNR Improvement Technologies against Optical Reflection in Fiber-Oriented Wireless Access Systems

It is important to consider optical reflectionconditions when designing fiber-oriented wirelessaccess systems around subcarrier optical transmissionsince the noise characteristic can be significantlydegraded by optical reflection, especially if narrowspectral width devices are used. The superluminescentdiode (SLD) offers stable noise and 3rd-orderintermodulation distortion (IM3) characteristics thatfollow the ideal cubic law, even under multipleoptical reflection. The SLD-predistorter combinationis proposed as a way to overcome the SLD's narrowdynamic range. It is experimentally found that thepredistorter reduces IM3 by 8 dB. The receiversensitivity of a practical system that uses theSLD-predistorter combination is discussed from theviewpoint of overall design. A superimposedsubcarrier modulation (SSM) technique is also proposedin order to reduce the reflection noise. Weexperimentally confirm that the SSM technique reducesthe noise degradation and provides stable opticaltransmission. An SSM design methodology is presentedthat takes both noise and linearity into account. TheSLD-predistorter and the SSM scheme make it possibleto realize stable transmission performance and widerdynamic range in the presence of optical reflection.     

Turbo-coded optical PPM communication systems

A number of parallel concatenated convolutionally coded (PCCC) photon communication systems is introduced and investigated. It is assumed that the optical channel is an intensity modulated (IM) channel and that the received optical signal is detected using a direct-detection (DD) scheme. Two modes of operation are considered. In one scenario, it is assumed that the receiver is limited by shot noise (i.e., negligible receiver thermal noise, or Poisson channel). In the other case, we consider a nonnegligible receiver thermal noise where an avalanche photodetector (APD) is employed to detect the received optical signal. It is also considered that the modulation scheme is the binary pulse-position modulation (PPM). With the aid of the best available upper bounds, the performance of the rate 1/n PCCC encoded optical PPM systems is assessed in terms of the upper bound on the system bit error rate (BER) for the shot-noise-limited IM/DD systems with nonnegligible background noise and for the thermal-noise-limited systems with APD detectors when a uniform interleaver is used. Numerical results for the rate 1/3 PCCC encoded PPM channels are presented. The numerical results demonstrate the enormous potential of this novel coding scheme in enhancing the performance of the aforementioned optical channels by a sizeable margin across the board     

Effect of third-order intermodulation on radio-over-fiber systems by a dual-electrode Mach-Zehnder modulator with ODSB and OSSB signals

Third-order intermodulation (IM3) is a very important issue as a degradation factor of system performance in the range of high input signal power. In this paper, the effect of IM3 from a dual-electrode Mach-Zehnder modulator (DEMZM) and a photodetector (PD) is analyzed for optical single-sideband (OSSB) and optical double-sideband (ODSB) signals incorporating fiber dispersion. In addition, the optimum input signal power and the signal-to-noise-and-distortion ratio (SNDR) for the two cases are also investigated to optimize the performance of the entire system. In the case of OSSB signals, the fundamental components are robust against fiber dispersion, whereas their IM3 components are still sensitive to fiber dispersion. Subsequently, the SNDR for OSSB signals fluctuated to within 6 dB in the relatively high input power range due to fiber dispersion. In the case of ODSB signals, both powers of the fundamental and IM3 components are attenuated. However, the power attenuation of IM3 due to fiber dispersion is significantly faster than that of the fundamental. Thus, the SNDR for ODSB signals is improved as fiber dispersion increases until the power of IM3 is greater than that of the additive noise level.     

Signal extraction with frequency arrangement (SEFA) andsuperimposed subcarrier modulation (SSM) schemes in fiber-orientedwireless access systems

This paper describes technologies for the improvement of the noise and nonlinearity characteristics of wireless access systems based on subcarrier optical transmission. First, the superimposed subcarrier modulation (SSM) technique is proposed to reduce the reflection noise induced by fiber connectors. We experimentally confirmed that the proposed method reduces this noise, and we found the optimum frequency and power of the superimposed subcarrier, SSM design methodology is also presented, taking both noise and nonlinearity into account. Second, signal extraction with frequency arrangement (SEFA) is proposed for increasing the optical modulation index, because the third-order intermodulation distortion (IM3) produced by the optical devices can be ignored. Simulation results show that SEFA greatly improves the transmission performance in terms of both the ratio of desired-to-undesired signal and the receiver sensitivity under Rayleigh fading. Combining the SSM and SEFA is also considered, for use with FP-LDs or DFB-LDs. These technologies will enable cheaper FP-LD implementation in fiber-oriented wireless access systems     

External optical feedback effects on intensity noise ofvertical-cavity surface-emitting lasers

The authors have experimentally evaluated the effect of external optical feedback on the relative intensity noise (RIN) of a vertical-cavity surface-emitting laser (VCSEL) diode. In the absence of optical feedback, the smallest RIN is found to be -135 dB/Hz. For optical feedback levels approaching -25 dB, the RIN is degraded by about 20 dB. The authors have measured feedback-induced power penalties in a 500-Mb/s intensity-modulation/direct-detection (IM/DD) link and determined that the penalty exceeds 1 dB if the optical feedback ratio is larger than -30 dB     

Lightpath routing and spectrum allocation over elastic optical networks in content provisioning with cloud migration

In this paper, an improved receiver based on diversity combining is proposed to improve the bit error rate (BER) performance of layered asymmetrically clipped optical fast orthogonal frequency division multiplexing (ACO-FOFDM) for intensity-modulated and direct-detected (IM/DD) optical transmission systems. Layered ACO-FOFDM can compensate the weakness of traditional ACO-FOFDM in low spectral efficiency, the utilization of discrete cosine transform in FOFDM system instead of fast Fourier transform in OFDM system can reduce the computational complexity without any influence on BER performance. The BER performances of layered ACO-FOFDM system with improved receiver based on diversity combining and DC-offset FOFDM (DCO-FOFDM) system with optimal DC-bias are compared at the same spectral efficiency. Simulation results show that under different optical bit energy to noise power ratios, layered ACO-FOFDM system with improved receiver has 2.86, 5.26 and 5.72 dB BER performance advantages at forward error correction limit over DCO-FOFDM system when the spectral efficiencies are 1, 2 and 3 bits/s/Hz, respectively. Layered ACO-FOFDM system with improved receiver based on diversity combining is suitable for application in the adaptive IM/DD systems with zero DC-bias.     

Direct detection of optical digital transmission based onpolarization shift keying modulation

Receiver structures for the direct detection of binary and multilevel digital optical modulation schemes employing the modulation of the state of polarization of light, or polarization shift keying (POLSK), are proposed and accurately analyzed, in the presence of optical amplifier ASE noise and electrical receiver noise. A comprehensive set of results shows that the performance of the binary system is approximately 3 dB better than IM/DD (on peak optical power), and stays so for a wide range of optical filter bandwidths. As a somewhat unexpected result, the multilevel schemes show a lower sensitivity to the use of a wide optical filter than the binary one. As a consequence, transmitting 3 b/symbol on a cubic constellation on the Poincare sphere brings about virtually no penalty, whereas transmitting 2 b/symbol has a better performance than binary transmission, for even relatively low values of the optical filter bandwidth. A clear explanation of this phenomenon has been found. These results suggest that narrow-bandwidth and therefore low-dispersion, multilevel, POLSK transmission could be performed with no penalty. Together with the low excitation of nonlinear effects in the fiber that polarization modulation seems to ensure, these results make POLSK schemes look promising for very-long-haul transmission     

A compensation method for dispersion-induced third-orderintermodulation distortion using an etalon

A method to compensate dispersion-induced third-order intermodulation distortion (IM3_CD) in subcarrier multiplexing (SCM) optical transmission systems is proposed. It is confirmed theoretically and experimentally that IM3_CD can be compensated by interference-induced third-order intermodulation distortion (IM3_IN). Simple analytic expressions of IM3_CD and IM3_IN are derived from the same expression of optical electric field which includes exact phase term taking into account the phase of laser chirp. From the theoretical analyses, we clarified the conditions of IM3_CD-compensation. IM3_IN can be easily generated by an etalon which is a kind of interferometer and a optical glass plate. More than 30 dB of IM3_CD -compensation was achieved, and IM3_CD-compensation was achieved regardless of the transmission distance by adjusting tilt angle of the etalon. Using this method, high quality signal transmission can be realized even though the wavelength of laser diode (LD) is not coincident with the fiber zero dispersion wavelength     

Effects of Intensity Modulation of Light Source on Brillouin Optical Correlation Domain Analysis

The effects of the intensity modulation (IM) of light source on fiber-distributed sensors based on Brillouin optical correlation domain analysis (BOCDA) are analyzed by numerical simulation, and the results are experimentally confirmed. We show that the shape of the Brillouin gain spectrum in the BOCDA system has a particular dependence on the optical spectrum of light source and that it can be controlled and tailored by proper modification of the optical spectrum using an additional IM. In the experiments, we applied several IM waveforms based on the simulation results for confirmation. Additionally, a distributed strain measurement along a 1-km optical fiber with a 30-cm spatial resolution is demonstrated by applying a proper IM scheme, which is the longest range reported using the BOCDA system     

Rigorous assessment of small-signal analysis for linear anddispersive optical communication systems operating near zero-dispersionwavelength

This paper presents a rigorous small-signal theory for linear single-mode fibers taking into account the first- and second-order fiber dispersion. From this theory, exact small signal intensity modulation (IM) and frequency modulation (FM) fiber transfer functions are presented. Exact expressions of the intensity and frequency noise spectra at fiber output due to laser noise taking into account all Langevin noise terms are also derived. Accurate numerical simulations of the IM and FM fiber transfer functions, and intensity and frequency noise spectra after linear transmission along single-mode fiber are compared with theoretical predictions and very good agreement is achieved. In addition, the theoretical predictions are compared with other author's results and the discrepancies are thoroughly explained. A new expression for the transmission distance which can lead to further significant reduction of intensity noise spectrum in systems using single-mode lasers with reduced linewidth enhancement factor is presented. The theoretical and simulation results show that the magnitude of the small-signal IM and FM fiber transfer functions, and the intensity and frequency noise spectra after linear single-mode fiber transmission are not affected by second-order fiber dispersion. The theory indicates that second-order fiber dispersion solely introduces delay distortion in the IM and PM fiber transfer functions. So, with linear transmission second-order dispersion effects on the intensity and frequency noise have proved to be irrelevant even for very long broad-band fiber systems