Comparison of the performance of optically amplified transmissionsystems

In this paper, we show a comparison among the performance of single- and multichannel optical systems considering NRZ and soliton signals. The results have been obtained by means of numerical simulations, taking into consideration the chromatic dispersion, the Kerr nonlinearity, the fiber loss, and the ASE noise of the optical amplifiers. The performance of IM-DD and coherent asynchronous ASK systems have been evaluated in terms of the Q factor. We have considered the propagation in links encompassing conventional step-index fibers, DS fibers and in links with two different dispersion management techniques in which the chromatic dispersion is varied along the propagation distance both in randomly and in deterministically way. The effects of the in-line filtering process are mainly investigated in soliton propagation     

Simultaneous compensation of laser chirp, Kerr effect, anddispersion in 10-Gb/s long-haul transmission systems

The effects of chirp and fiber nonlinearity in a directly modulated 10-Gb/s intensity-modulated direct-detection (IM-DD) fiber transmission system are investigated by simulation, and a simple and flexible technique for compensating these effects is discussed. Self-phase-modulation (SPM) in optical fiber can be equalized by an anomalous dispersion fiber, whereas pulse broadening caused by laser transient chirp can be compensated by normal dispersion. Using these characteristics, laser transient chirp, SPM, and fiber dispersion can be simultaneously compensated by equalizing fibers inserted within certain intervals. Optimum compensation is always realizable for such fixed equalizing fibers, since the magnitude of SPM can be controlled by changing the optical power in the fiber. Simulation suggests that this technique enables 10-Gb/s, 100-km fiber transmission by direct modulation     

高速密集波分复用系统中的二级调制格式研究

论述了强度调制直接检测(IM-DD)高速密集波分复用(DWDM)系统的二级调制原理，并从马赫-曾德尔调制器的传递特性出发推导了四种调制格式[全频率调制归零码(FFMRZ)、半频率调制归零码(HFMRZ)、单边带调制归零码(SSBRZ)、载波抑制归零码(CSRZ)]。根据信号光眼图及归一化频谱描述了各种码型的时频特征。在此基础上分别对各种码型的色散容限、非线性容限进行了数值仿真，通过比较眼图张开度损伤(EOP)发现对于单波长系统载波抑制归零码传输性能最优。最后利用全面的密集波分复用系统模型计算了各种码型的Q因子，发现载波抑制归零码的性能仍然保持最优，这种优势在考虑偏振模色散(PMD)的情况下更为突出。因此载波抑制归零码是强度调制直接检测系统较好的选择。     

Performance evaluation of the different types offiber-chromatic-dispersion equalization for IM-DD ultralong-distanceoptical communication systems with Er-doped fiber amplifiers

We have evaluated the transmission performance of different fiber-chromatic-dispersion-equalization methodologies for ultralong distance IM-DD optical communication systems that use Er-doped fiber amplifier repeaters. The experiment used a 1000 km fiber loop consisting of 30 dispersion-shifted fiber spans and 31 Er-doped fiber amplifiers. We changed the insertion point of the normal single-mode fiber for equalization to change the shape of the accumulated chromatic dispersion. Comparison of the longest transmission distance and the width of the 9000 km transmissible window are discussed for several types of dispersion equalization. The results indicate that the best type of the dispersion equalization for ultralong distance IM-DD optical communication systems is to install dispersion-shifted fibers with short sections of normal single-mode fibers to compensate the accumulated dispersion     

Analysis of laser phase noise to intensity noise conversion bychromatic dispersion in intensity modulation and direct detectionoptical-fiber transmission

Laser phase noise conversion to intensity noise due to fiber chromatic dispersion is analyzed by deriving the noise power spectral density. Theory predicts that the phase-modulation-amplitude-modulation conversion noise is a principal limiting factor of the gigabit-per-second nonregenerative transmission using an external modulator when the linewidth of the laser transmitter is above several tens of megahertz and the total chromatic dispersion of fibers exceeds several thousand picoseconds per nanometer. This fact is confirmed by the 2.4-Gb/s transmission experiments using multiple inline Er-doped fiber amplifiers. The system penalty due to this noise in the intensity modulation and direct detection (IM-DD) optical transmission using an external modulator is evaluated     

Phase Shift Technique for the Measurement of Chromatic Dispersion in Optical Fibers Using LED's

A sinusoidal technique is reported, which allows simple and accurate measurements of chromatic dispersion in optical fibers. It is based on the phase shift which a sinusoidally modulated light beam undergoes while traveling along a fiber when its wavelength is changed. The choice of a multiple LED's source permits the continuous spectral covering from 750 to 1600 nm; easily available instrumentation and devices are needed for the measurement setup. The technique is reported in detail by showing results obtained in multimode fibers; statistical evaluation of its accuracy and a comparison with conventional methods are carried out. An accuracy of a few picosecond in relative delay and of /spl I.chemc/1 ps/nm /spl dot/ km in chromatic dispersion are demonstrated, that compare very favorably with the existing techniques.     

色散管理传输系统中克尔效应对偏振模色散的补偿研究

光纤的随机双折射效应可导致脉冲无规展宽即偏振模色散(PMD)。在零路径色散管理孤子传输系统中，二阶色散和三阶色散效应均被完全补偿，克尔效应成为一种有害因素会使脉冲变窄，但是当光纤的随机双折射被考虑时，克尔效应正好与PMD相互抵消，使光脉冲准稳定传输，不同的光纤偏振模色散参数分别对应不同的最佳系统功率。此外，如果考虑不同偏振方向的损耗差异，则即使在最佳匹配条件下，微小的偏振损耗差异也可产生很大的脉宽波动。因此，偏振相关损耗是影响脉冲传输质量的相当重要的因素，不论在理论计算还是在工程设计中都应当认真考虑。     

Prechirp technique as a linear dispersion compensation forultrahigh-speed long-span intensity modulation directed detectionoptical communication systems

Prechirp technique, as a linear dispersion compensation for intensity modulation direct detection (IM-DD) optical transmission systems, has been investigated theoretically and experimentally. This technique is based on a predistortion technique in an optical transmitter. Implementation to the ordinary IM-DD optical transmitter, which uses an external intensity modulator, is easily realized merely by adding a small injection current modulation to a semiconductor laser diode, allowable optical transmission fiber dispersion will be more than doubled with this technique, modified prechirp technique, which utilizes a time division superimposing prechirped bit streams, has also been investigated to achieve greater dispersion compensation capability     

应用于接入网段的低成本OFDM射频信号光传输系统的研究

应用于接入网段的OFDM射频信号光传输系统宜采用直接调制降低成本和技术复杂度,但射频信号直接调制方式对激光器性能和光纤色散提出更大的挑战。研究了OFDM射频信号直接调制的光传输系统(OFDM直调RoF系统)的传输特性,定量分析了直接调制激光器性能、光纤色散、载波频率及OFDM子载波数等关键参数对OFDM直调RoF系统性能的影响,提出了对OFDM直调RoF系统设计及关键参数设置的建议,研究结果对低成本和低技术复杂度的RoF接入网设计有参考价值和指导意义。     

三种载波抑制归零码光调制格式性能

首先分析了基于载波抑制归零码的光载波抑制归零-开关键控(CSRZ-OOK)、载波抑制归零-差分相移键控(CSRZ-DPSK)和载波抑制归零-差分正交相移键控(CSRZ-DQPSK) 三种光调制格式的时域和频域特性,并数字仿真了其在40 Gbit/s单信道光纤系统中的传输性能.以Q值、眼图和误码率(BER)作为性能评价指标,研究了这三种调制格式对色散(CD)、偏振模色散(PMD)和非线性效应的抑制作用.结果表明:CSRZ-DQPSK相对于CSRZ-OOK、CSRZ-DPSK对色散、偏振模色散和非线性效应有非常好的容忍能力.     

All-Optical 10 and 40 Gbit/s RZ-to-NRZ Format and Wavelength Conversion Using Semiconductor Optical Amplifiers

We demonstrate that the cross-gain compression (XGC) in a semiconductor optical amplifier can produce effective return to zero (RZ)-to-nonreturn to zero (NRZ) format conversion. This technique is experimentally investigated at 10 and 40 Gbit/s. At 10 Gbit/s, the format adaptation allows for a very high pulsewidth increase, i.e., from 10 to 100 ps. The output 10 Gbit/s NRZ signal is transmitted on metro-like links with no chromatic dispersion compensation.      

Impact of dispersion, PMD, and PDL on the performance of spectrum-sliced incoherent light sources using gain-saturated semiconductor optical amplifiers

The effects of chromatic dispersion (CD), polarization-mode dispersion (PMD), and polarization-dependent loss (PDL) on the intensity noise suppression of spectrum-sliced incoherent light sources achieved by using gain-saturated (GS) semiconductor optical amplifiers (SOAs) are investigated. Passing the spectrum-sliced incoherent light through SOAs, the excess intensity noise (EIN) originating from beating of spontaneous emission against itself can be greatly reduced. However, since the noise suppression is achieved by an elaborate balancing between numerous frequency/polarization components of light, thus, forming a high correlation between them, it is vulnerable to frequency/polarization-dependent optical phenomena. Through Q-factor and bit error rate (BER) measurements, this paper shows that CD, PMD, or PDL deteriorates the SOA-based noise suppression technique by breaking the correlation. Spectral analysis is also performed to investigate the frequency dependency of these effects. It is shown that CD and PMD negate the noise suppression giving rise to intensity noise from high frequencies, whereas there is no frequency dependence for PDL effects. Therefore, CD-, PMD-, or PDL-induced penalties for incoherent light sources using the SOA-based noise suppression technique are considerably greater than those produced by pulse broadening or distortion alone.     

Novel OTDR Technique for Measuring Relative-Index Differences of Fiber Links

A novel technique is proposed for measuring the relative-index difference of the fiber links using an optical time-domain reflectometer (OTDR). The material dispersion of the fiber links can be estimated by using this method. Moreover, the chromatic dispersion distribution of a fiber link is successfully estimated by combining this technique with the conventional OTDR technique     

Phase-shift technique for the measurement of chromatic dispersion in single-mode optical fibres using LEDs

The phase-shift technique has been successfully extended to chromatic dispersion measurements in single-mode optical fibres. Using only two sinusoidally modulated LEDs operating at 1.33 and 1.47 ?m, accurate results have been achieved in the spectral region of interest, in good agreement with those obtained with the Nd-Yag laser and Raman fibre technique.     

Performance analysis of spectrally encoded hybrid WDM-OCDMA network employing optical orthogonal modulation format against eavesdropper

In this paper, the spectrally encoded hybrid WDM-OCDMA network employing 60 Gbps non-return to zero/differential quadrature phase shift keying (NRZ/DQPSK) orthogonally modulated data signal operating over 100 km SMF+DCF has been proposed. In proposed hybrid WDM-OCDMA network, the orthogonal modulation formats are used to enhance per channel capacity and the spectral amplitude optical coding for enhancing the confidentiality of data from unauthorized user or eavesdropper. The impact of input power, transmission distance and energy per bit to noise power spectral density ratio (E_b/N₀) on the performance of WDM-OCDMA network in terms of output optical power, timing diagram, BER, Q-factor and probability of error free code detection has been investigated. It can also be determined that orthogonal modulation formats are promising option to increase per channel capacity as compared to conventional modulation format and these are less vulnerable to chromatic dispersion (CD) and polarization mode dispersion (PMD). Meanwhile, the performance of proposed hybrid network is compared with existing OCDMA network which show the feasibility of proposed WDM-OCDMA network in future generation optical networking.     

Chromatic Dispersion Monitoring of CSRZ Signal for Optimum Compensation Using Extracted Clock‐Frequency Component

This paper presents a chromatic dispersion monitoring technique using a clock‐frequency component for carrier‐suppressed return‐to‐zero (CSRZ) signal. The clock‐frequency component is extracted by a clock‐extraction (CE) process. To discover which CE methods are most efficient for dispersion monitoring, we evaluate the monitoring performance of each extracted clock signal. We also evaluate the monitoring ability to detect the optimum amount of dispersion compensation when optical nonlinearity exists, since it is more important in nonlinear transmission systems. We demonstrate efficient CE methods of CSRZ signal to monitor chromatic dispersion for optimum compensation in high‐speed optical communication systems.     

Transmission of 160-Gb/s Modified Manchester modulation over WDM system

The paper examines the performance of Modified Manchester (MM) modulation scheme over wavelength division multiplexing (WDM) in high-speed optical communication links. The MM as a new modulation technique has a narrow spectral width compared to conventional Manchester coding, which allows its implementation in WDM systems beneficial. In this study, the performance characteristics of MM and conventional Manchester modulation formats are assessed in WDM system at 10 Gb/s bitrate for each channel, for the least allowable channel spacing as well as tolerance to chromatic dispersion (CD). It is revealed from the results of simulation that MM performs meaningfully well in comparison with conventional Manchester in terms of tolerance against narrow optical filtering, spectral efficiency, which is improved by 32% and CD tolerance, which is improved by +100 ps/nm. Sixteen wavelength channels (16 × 10 Gb/s) are modulated to provide 160 Gb/s data capacity, which was transmitted successfully over 224 km standard single mode fibre (SSMF) using MM while the conventional Manchester only covered about 157 km.     

Small Signal Transfer Function of Periodically Dispersion-Compensated Optical Link for Intensity Modulated Signals

We present analytical expressions for the transfer function of periodically dispersion compensated optical links within intensity-modulated optical transmission systems. The accuracy of the proposed expressions is confirmed through the simulation based on the split-step Fourier method. With the transfer function, we thoroughly investigate the effects of precompensation and nonzero residual chromatic dispersion (CD) on the performance of periodically dispersion compensated optical links. We also introduce a fast and simple technique to determine the optimum link condition, such as the optimum amount of precompensation and residual CD, and show that there is an optimum monitoring frequency in the CD monitoring technique based on the sub-carrier tone.      

光双二进制技术在城域光网络中的应用

对一种新型光部分响应数字传输技术即光双二进制技术的特点与发展趋势进行了深入研究和介绍。在详细阐述光双二进制系统的实现原理及主要实现方式,并介绍该技术的主要性能优越性以及近来研究成果的基础上,认为由于其与非归零等传统调制方式相比所具有的色散容忍度大、频谱窄、频带利用率高等诸多优点,使其在城域光纤网中的广泛应用正在日益成为可能。