Key technologies for terabit/second WDM systems with high spectralefficiency of over 1 bit/s/Hz

To fully utilize a limited gain bandwidth of about 35 nm (4.4 THz) in an erbium-doped fiber amplifier, an increase in signal spectral efficiency is required. In this paper, we investigate the key technologies to achieve terabit/second wavelength-division multiplexing (WDM) systems with over 1 bit/s/Hz spectral efficiency. Optical duobinary signals, which have narrower optical spectra than conventional intensity modulation signals, were applied to such dense WDM systems. The measured minimum channel spacing for 20-Gbit/s optical duobinary signals was 32 GHz and a spectral efficiency of over 0.6 bit/s/Hz was reached. By using polarization interleave multiplexing, spectral efficiency was expected to reach 1.2 bit/s/Hz in an ideal case with no polarization dependencies along the transmission lines. In such ultradense WDM systems with narrower channel spacing, stabilizing the wavelengths of laser diodes is an important issue for achieving stable operation over long periods. To do this, we developed a simple and flexible wavelength stabilization system which uses a multiwavelength meter. The wavelengths for 116 channels with 35-GHz spacing were stabilized within ±150 MHz. The stabilization system is applicable to ultradense WDM signals with a spectral efficiency of over 1 bit/s/Hz by employing wavelength interleave multiplexing and an optical selector switch. On the basis of these investigations, we demonstrated a 2.6-Tbit/s (20 Gbit/s×132 channels) WDM transmission by using optical duobinary signals. In addition, 1.28-Tbit/s (20 Gbit/s×64 channels) WDM transmission with a high spectral efficiency of 1 bit/s/Hz was achieved by using polarization interleave multiplexing     

Dispersion-managed high-capacity ultra-long-haul transmission

The recent progress of dispersion-managed transmission technologies for ultra-long-haul wavelength-division-multiplexing (WDM) systems is reviewed. First, we discuss the dispersion-management technologies for 10 Gb/s-based WDM systems. Next, dispersion-managed technologies for higher bit rate applications are reviewed, including dispersion-managed soliton transmission. Finally, we compare the effectiveness of different modulation formats for achieving ultradense WDM transmission for future applications.     

Duty-ratio control of nonlinear phase noise in dispersion-managed WDM transmissions using RZ-DPSK modulation at 10 Gb/s

The authors compare analytical and numerical estimates, showing that the nonlinear phase noise of short optical pulses associated with the coupling between amplified spontaneous emission noise and fiber nonlinearity may be controlled by adjusting the duty cycle of the return-to-zero (RZ) signal modulation format. The impact of this effect in the optimization of the performance of 10-Gb/s dispersion-managed wavelength division multiplexed (WDM) systems using RZ-differential phase-shift keying (DPSK) modulation is discussed. By extensive numerical simulations, it is shown that the transmission quality of ultradense WDM systems using the RZ-DPSK modulation format may be significantly enhanced by optimizing the duty cycle of the RZ pulses.     

Characteristics of optical duobinary signals in terabit/s capacity,high-spectral efficiency WDM systems

Optical duobinary signals have been applied to dense wavelength division multiplexing (WDM) systems with high-spectral efficiency to fully utilize a limited gain bandwidth of about 35 nm (4.4 THz) for an erbium-doped fiber amplifier (EDFA). These signals are one type of partial response signals and have narrower bandwidth than conventional intensity modulation (IM) signals. Thus, the use of these signals should make possible to attain ultradense WDM systems. In this paper, characteristics of optical duobinary and IM signals in ultradense WDM systems are compared through experimental evaluations at 20 Gb/s. High-spectral efficiency of 0.6 b/s/Hz was reached in this demonstration     

Investigation of system upgradability over installed fiber-optic cable using 40-Gb/s WDM signals toward multiterabit optical networks

An investigation of system upgradability of installed fiber-optic cable was conducted using 40-Gb/s wavelength-division-multiplexing (WDM) signals toward multiterabit optical networks. A field trial of 63-channel 40-Gb/s dispersion-managed soliton WDM signal transmission was successfully demonstrated over 320-km (4 /spl times/ 80-km) installed nonzero-dispersion-shifted fibers. The average Q factor of 15.4 dB was obtained, and very stable long-term bit-error-ratio performance was confirmed without polarization-mode dispersion compensation. This system upgradability investigation in the field environment provided the confidence to introduce 40-Gb/s technologies and effectively to construct multiterabit optical networks following the demand increase in the future.     

Study of 16 Tbit/s WDM transmission system derived from the CO-OFDM with PDM 16-QAM

In this paper,a wavelength division multiplexing (WDM) transmission system derived from the coherent optical orthogonal frequency division multiplexing (CO-OFDM) with polarization division multiplexing (PDM) and 16-order quadrature amplitude modulation (QAM) is studied. A simulation of 80-channel WDM transmission system with data rate of 200 Gbit/s is built, and the transmission performance of the system is analyzed. The simulation results show that the system Q value of the WDM channels at 16 Tbit/s with a spectral efficiency of 7.14 bit/s/Hz is potentially over 10.0 dB for a long haul transmission up to 1800 km in a standard single-mode fiber.     

Extending transmission distance of high-density WDM systems usingpost transmitter fiber Raman amplifiers

The authors investigate the behavior of multichannel signals in a high-density wavelength division multiplexing (WDM) system with the consideration of stimulated Raman scattering. A parameter to evaluate the depth of Raman crosstalk is provided, and the transmission distance limitation imposed by Raman crosstalk is analyzed. To extend the system transmission distance, a post-transmitter fiber Raman amplifier is used. The allocation of signal channels on the pump gain profile is discussed. It is found that there exists an optimum pump power which results in a maximum amplifier gain. The example shows that the transmission distance limitation imposed by Raman crosstalk can be extended by 20 km for a 30-channel system with 0.25-dB/km fiber loss     

Performance of a WDM network based on stimulated Brillouinscattering

The use of stimulated Brillouin scattering as a tunable narrowband amplifier to demodulate, amplify, and select channels in a densely packed WDM (wave-division multiplexed) network is discussed. A 128-channel system configuration is tested with channel separation as small as 1.5 GHz and error rates below 10^-10. Direct FSK (frequency-shift-keyed) modulation of AlGaAs lasers and direct detection receivers are used. The experimental margin and noise analysis indicate that a 1000-channel system can be constructed with only these same components     

基于相干光正交频分复用的WDM传输系统及其性能分析

相干光正交频分复用由于其良好的传输性能成为近年来光传输领域的研究热点,波分复用技术可以在光纤中通过增加并行波长的数量来提高系统的容量,将CO-OFDM和WDM技术结合,可以构造出高速率、大容量、低成本的光传输网络。文章首先对基于CO-OFDM的WDM传输系统的理论模型和基本原理进行了研究,然后对基于CO-OFDM的100Gb/s×32-信道WDM传输系统进行了仿真分析。并研究了该系统的传输性能。结果表明：在没有任何光纤的色散及非线性补偿的情况下,当信号速率为3.2 Tb/s时,系统的Q因子高于16.0 dB,在标准单模光纤中的传输距离可达1500km。     

Fluoride-based erbium-doped fiber amplifier with inherently flat gain spectrum

We successfully developed a fluoride-based Er/sup 3+/-doped fiber amplifier (F-EDFA). An average signal gain of 26 dB was achieved for 8 channel wavelength division multiplexed (WDM) signals in the 1532-1560 nm wavelength region with a gain excursion of less than 1.5 dB at an input signal power of -20 dBm per channel. Furthermore, we studied the amplification characteristics of the F-EDFA for WDM signals. The following experimental results were obtained. (1) For an 8-channel WDM signal in the 1532 to 1560 nm wavelength region, the gain excursion between channels can be suppressed to within 1.5 dB. However, the wavelength region allowing a gain excursion of 1.5 dB, is between 1536-1560 nm for the silica-based Er/sup 3+/-doped fiber amplifier. (2) F-EDFAs have a flat gain region between 1534-1542 nm. The gain excursion of this region is less than 0.2 dB for WDM signals.     

Performance of nonlinear interchannel crosstalk pre-compensation at zero-dispersion wavelength using carrier phase-locked WDM

The increase in WDM transmission distance at zero-dispersion wavelength made possible by pre-compensation to offset nonlinear interchannel crosstalk is studied. A simulation shows that pre-compensation allows higher fibre launched power and increases the transmission distance by 3.3 times for a 50 GHz-spaced 8-channel WDM system. Pre-compensation is shown to be limited by the undeterministic nonlinear distortion induced by the accumulated amplifier spontaneous emission noise from repeater amplifiers. Also, optical power tolerance of the pre-distorted waveform generated at the transmitter is studied.     

面向电子战射频光传输的宽带射频信号杂散产生机理及优化方法

针对电子战系统的宽带射频信号波分复用传输,通过理论分析给出了微波光子链路中各关键参量对杂散信号的影响规律,提出了非均匀信道间隔分配的除杂优化方法。该方法通过微调各通道的波长,使四波混频产生的杂散频率超过电子战接收机带宽,从而降低杂散对系统的影响。该方案实现简单,且无需改变现有电子战系统架构,具有较好的工程实现性。实验结果表明:与等波长间隔相比,5通道非均匀波分复用信号传输1 km时,在接收机带宽内四波混频引起的杂散得到了显著优化,动态范围提升了26 dB以上。     

High-performance operation of monolithically-integrated multipurpose reconfigurable optical add-drop multiplexer

WDM operation of a novel monolithically-integrated 16-channel reconfigurable add-drop multiplexer (ROADM), demonstrating high crosstalk suppression and cascadability to eight nodes for 10 Gbit/s NRZ signals, is reported. The device provides separate wavelength add/drop ports and a common wavelength-multiplexed add/drop port for high system functionality.     

Gain-flattened Er/sup 3+/-doped fiber amplifier for a WDM signal in the 1.57-1.60-/spl mu/m wavelength region

A gain-flattened Er/sup 3+/-doped silica-based fiber amplifier (EDFA) has been constructed for a 1.58-/spl mu/m band WDM signal. This EDFA exhibits uniform amplification characteristics with a gain excursion of 0.9 dB for a four-channel WDM signal in the 1.57-1.60 /spl mu/m wavelength region. The average signal gain and the noise figure for the WDM signal are 29.5 dB and less than 6.3 dB, respectively. The use of this EDFA in parallel with a 1.55-/spl mu/m band EDFA will expand the WDM transmission wavelength region.     

A general and rigorous WDM receiver model targeting 10-40-Gb/schannel bit rates

We present a general and rigorously formulated dynamic receiver model aiming at 10-40-Gb/s wavelength division multiplexing (WDM) system design applications. A demultiplexing (DEMUX) characteristic with periodic transfer function has been treated in detail and it has been indicated how the model should be adjusted to take into consideration a general type of noise spectral density (NSD). The bit error ratio (BER) is evaluated accounting for the influence of non-Gaussian detected amplified spontaneous emission (ASE) noise, noise correlation between stochastic noise samples in the receiver, the gain and effective noise figure variation with wavelength of optical amplifiers, channel crosstalk, and intersymbol interference (ISI) effects caused by nonideal signal modulation, fiber dispersion, fiber nonlinearities, optical MUX, and DEMUX filtering and the impulse response of the electrical low-pass filter in the receiver. Also, the influence of shot and thermal noise is taken into account. Numerical results for the BER are presented considering a realistic 16-channel 10-Gb/s WDM system operating in the C-band using normal transmission fibers and including cascaded erbium-doped fiber amplifiers (EDFAs) with dispersion compensating fibers     

Performance Analysis of CSRZ-OOK With Pairwise or Pulse-to-Pulse Alternate Polarization

The performance of 40-Gb/s carrier-suppressed return-to-zero on-off keying (CSRZ-OOK) combined with either a pairwise or a pulse-to-pulse alternate polarization (APol) is investigated numerically. It is shown that these modulation formats offer a superior suppression of intrachannel four-wave mixing compared to conventional APol-formats. They are also tolerant to narrowband optical filtering and thus suitable for ultradense wavelength-division multiplexing (WDM) systems with a spectral efficiency of 0.8 bit/s/Hz.     

Experimental Demonstration and Numerical Simulation of 107-Gb/s High Spectral Efficiency Coherent Optical OFDM

Orthogonal frequency-division multiplexing (OFDM) is a multicarrier modulation format in which the data are transmitted with a set of orthogonal subcarriers. Recently, this modulation format has been actively explored in the field of optical communications to take advantages of its high spectral efficiency and resilience to chromatic and polarization dispersion. However, to realize the optical OFDM at 100 Gb/s and beyond requires extremely high electronic bandwidth for the electronic signal processing elements. In this paper, we investigate orthogonal-band-multiplexed OFDM (OBM-OFDM) as a suitable modulation and multiplexing scheme for achieving bandwidth scalable and spectral efficient long-haul transmission systems. The OBM-OFDM signal can be implemented in either RF domain, or optical domain, or a combination of both domains. Using the scheme of OBM-OFDM, we show the successful transmission of 107 Gb/s data rate over 1000-km standard single-mode fiber (SSMF) without optical dispersion compensation and without Raman amplification. The demonstrated OBM-OFDM system is realized in optical domain which employs 2 $times$ 2 MIMO-OFDM signal processing and achieves high optical spectral efficiency of 3.3 bit/s/Hz using 4-QAM encoding. Additionally, we perform numerical simulation of 107-Gb/s CO-OFDM transmission for both single-channel and wavelength-division-multiplexed (WDM) systems. We find that the $Q$ -factor of OBM-OFDM measured using uniform filling of OFDM subbands is in fact more conservative, in particular, is 1.2 dB and 0.4 dB lower than using random filling for single-channel and WDM systems, respectively.      

全光纤波分复用传输系统实验研究

本文报道了一种新颖的波分复用系统－全光纤集成型系统：光发射机由全光纤激光器构成，光传输过程中各节点的上／下载由光纤光栅ADM承担，传输中继则由掺饵光纤放大器完成，在这一全光纤系统上，首次成功地实现了1．2Gbit/s的非归零码、1．2Gbit/s的归零码和2．5GHz、5GHz的模拟信号经100km的传输和下载。     

宽带混合光纤放大器研究

在讨论了宽带混合光纤放大器研究方法的基础上 ,设计了一个增益平坦带宽为 11.4THz的宽带混合光纤放大器 ,并对其增益和噪声特性进行了详细研究 ,最后将其应用于一个 2 0信道的WDM系统 ,实现了 6 4 0km无误码传输     

10 Gbit/s four-channel wavelength- and polarisation-divisionmultiplexing transmission over 340 km with 0.5 nm channel spacing

The authors propose a novel WDM transmission technique combined with polarisation-division multiplexing (PDM) which doubles the frequency utilisation efficiency. This technique is used to transmit a 10 Gbit/s four-channel WDM signal with 0.5 nm channel spacing over 340 km and the signal is demultiplexed using an optical filter having 0.5 nm FWHM and a polarisation beamsplitter