First-order PMD monitoring for NRZ data using RF clock regeneration techniques

We propose and demonstrate two novel techniques for 10 Gb/s polarization-mode-dispersion (PMD) monitoring for NRZ signals that use a regenerated RF clock tone as a monitoring signal. Our techniques regenerate the RF clock tone that is usually absent after square-law detection in the electrical NRZ data spectrum (in the absence of dispersion). Our first technique uses a dispersive element in the monitoring tap-line to put the beat terms between the optical clock sidebands and the carrier in phase and thus regenerates the RF clock tone after detection. Our second technique involves the use of an optical filter that is centered at the bit rate frequency on either the upper or lower sideband of the optical spectrum, removing one of the sidebands and thus preventing the beating that normally cancels the RF clock tone. We show (theoretically, via simulation, and experimentally) the effect that PMD has on these regenerated RF clock tones. We also demonstrate PMD compensation at 10 Gb/s using these techniques for monitoring and show a 6-dB improvement in the 1% power penalty tail. Our techniques are simple, do not require modification at the transmitter, and can be applied to WDM systems via the use of a multichannel dispersive element or a tunable filter swept across all channels.     

Degree-of-polarization-based PMD monitoring for subcarrier-multiplexed signals via equalized carrier/sideband filtering

We demonstrate a method for differential-group-delay (DGD) and polarization-mode-dispersion (PMD) monitoring using the degree-of-polarization (DOP) in subcarrier-multiplexed (SCM) systems. Traditional SCM signal show very little DOP sensitivity to DGD/PMD due to the low modulation depth used for generating SCM signals. We use a narrow-band optical filter to equalize the power of the carrier and one of the sidebands by offsetting the filter from the carrier, enabling PMD and DGD monitoring and more than tripling the DOP sensitivity to DGD/PMD. Our technique is simple, uses only a single optical filter, and can be applied to both single- and double-sideband (SSB and DSB) SCM signals as well as single and multisubcarrier systems. Additionally, we show that our monitoring technique is robust to the chromatic dispersion-induced radio-frequency (RF) power fading effect seen in DSB SCM signals. Using this technique to enhance the DOP sensitivity to DGD/PMD and generate a feedback signal to a PMD compensator (PMDC), we obtain an 11-dB improvement in the 5% RF power tail.     

A novel chromatic dispersion monitoring technique for 16/64-QAM system based on asynchronous amplitude histogram

A novel chromatic dispersion (CD) monitoring technique based on asynchronous amplitude histogram (AAH) for higher order modulation formats is proposed in this paper. Without demodulating the signal, in the monitoring scheme, the received signal is sampled asynchronously, and thus clock information and high-speed sampling units are unnecessary, resulting in low cost and high reliability. Simulations of CD monitoring technique for non-return-to-zero/return-to-zero (NRZ/RZ) 16- and 64-quadrature amplitude modulation (QAM) systems with different optical signal-to-noise ratios (OSNRs) and duty cycles are investigated, and the tolerance of the scheme is also discussed. Simulation results show that the presented CD monitoring technique with high sensitivity can be applied to monitor the residual CD of a transmission link in the next-generation optical networks.     

Improved all optical chromatic dispersion monitoring method for WDM systems

In this paper we propose an improved all optical chromatic dispersion (CD) monitoring method applicable to wavelength division multiplexing (WDM) systems with intensity or phase modulation format. This method can operate over a much larger bandwidth and CD monitoring range and is insensitive to optical signal-to-noise ratio (OSNR) and polarization mode dispersion (PMD). These improvements are achieved by utilizing a wavelength tunable laser as the probe to maintain the appropriate phase-matching condition and a loop-back controlled tunable CD compensator to discriminate CD distortions from PMD and OSNR influences. Numerical simulations show that by this method the operational bandwidth and CD monitoring range can be expanded to cover C + L bands and ±1600 ps/nm, respectively, for different modulation formats and the sensitivity is higher than the other methods proposed before.     

偏振副载波调制在光纤色散与PMD监测技术中的应用

利用偏振副载波调制(SCM)的方法,协同监测光纤通信系统的色散(CD)和偏振模色散(PMD).通过发送端叠加2个不同频率的副载波、接收端监测2副载波对应的功率和功率比值的方法协同监测光纤通信系统的CD和PMD.接收端副载波功率随CD和PMD的增加而增加,其比值随CD的增加而增加、随PMD的增加而减小,通过监测2副载波的功率和其比值,可以实现CD与PMD的协同监测.     

PMD and chirp effects suppression in RF tone-based chromatic dispersion monitoring

We investigate the effects of polarization-mode dispersion (PMD) and chirp on radio-frequency tone-based chromatic dispersion (CD) monitoring method, and show that the PMD and chirp induce significant CD monitoring errors. We propose a CD monitoring technique to suppress these effects. Experiment results show that the CD monitoring errors induced by PMD and chirp fluctuation are greatly suppressed.     

PMD-induced transmission penalties in polarization-multiplexed transmission

In this paper, we investigate for the first time chromatic dispersion and nonlinearity tolerances in the presence of polarization-mode dispersion (PMD) for polarization-multiplexed (POLMUX) 2 /spl times/ 10-Gb/s nonreturn-to-zero (NRZ) transmission. In polarization-multiplexing, the interaction between fiber nonlinearity and PMD can lower the nonlinear tolerance beyond the tolerances evident when considering both transmission penalties separately; the combined penalties are significantly worse than in the case for non-POLMUX transmission. In this paper, we show, through simulations comparing POLMUX with non-POMUX transmission in the presence of nonlinearity, a reduction of about a factor of three in PMD tolerance. In addition, we show that the dispersion tolerance of POLMUX transmission is severely limited in the presence of PMD. For example, a 40-ps differential group delay (DGD) with worst case coupling of the polarization channels into the fiber lowers the dispersion tolerance, resulting in a 1-dB eye-opening penalty (EOP), from 1200 to 450 ps/nm. We conclude that the interaction between PMD, chromatic dispersion, and nonlinearity leads to the worst signal impairments in POLMUX transmission and increases the effort of using polarization-multiplexing as a modulation format.     

A novel chromatic dispersion monitoring method for 400 Gbit/s 256 QAM fiber-optic system based on asynchronous amplitude sampling

A novel chromatic dispersion (CD) monitoring technique based on asynchronous amplitude sampling (AAS) is proposed for a higher modulation format and higher rate system. The dispersion and other impairment factors can be separated with the definition of monitoring parameter M. A 400 Gbit/s 256 quadrature amplitude modulation (QAM) system is built using Optisystem13.0 beta software. Simulations of CD monitoring technique for different bandwidths of sampling Gaussian filter, optical signal to noise ratios (OSNRs) and duty cycles are investigated, and the tolerance is also discussed. Simulation results show that the method can be less affected by noise, and a higher accuracy of 600 ps/(nm.km) can be achieved. The technique supports a wide range of data traffic and enhances operation flexibility of optical networks.     

Tracking and separation of time-varying principal states of polarization in optical fiber link for first-order PMD compensation and its filter-dependent performance

We propose a simple method for tracking and separating time-varying principal states of polarization (PSP) occurring in a fiber-optic transmission link with polarization mode dispersion (PMD) for use in PMD compensation. The proposed method uses as a feedback monitoring signal the bandpass-filtered RF power at bit-rate frequency for NRZ signal format. We demonstrated the operating principle of the method through theoretical simulation and experiment using an automatically adaptive PMD compensator employing a single polarization beam splitter (PBS). The effects of electrical filtering on the PSP tracking performance were also investigated by using three types of filters, i.e., a low-pass filter (LPF), a band-pass filter (BPF), and a high-pass filter (HPF). The simulation results showed that only a BPF centered at bit-rate frequency was found to allow for tracing and separation of two PSPs via PBS. The proposed method, when applied to a conventional PMD compensator that alternately controls a polarization controller and a delay line, enables separation of PSP control from differential-group-delay (DGD) control, thus allowing fast tracking of rapidly changing PSP in a PMD-impaired optical fiber link and reducing compensation time.     

Optical Performance Monitoring of Phase-Modulated Signals Using Asynchronous Amplitude Histogram Analysis

As optical networks continue to grow towards high capacity and high flexibility, new transmission technologies are being introduced. In order to maintain the quality of signal and control over network in the transparent domains, optical performance monitoring (OPM) systems are becoming a necessity. Phase modulation formats emerge as the solution of choice in transparent domains because of their sensitivity, spectral efficiency, and resilience to optical impairments. In this paper, we demonstrate a flexible OPM method for phase-modulated signals using asynchronous amplitude histogram analysis. We show numerically and experimentally the monitoring of optical signal-to-noise ratio (OSNR), chromatic dispersion (CD), and polarization-mode dispersion (PMD) for differential phase-shift keying (DPSK) and differential quadrature phase-shift keying (DQPSK) signals. The OSNR can be measured within range of 20-35 dB and accumulated chromatic dispersion between 600 and 600 ps/nm. The asynchronous amplitude histogram monitoring method is proved to be a precise and versatile monitoring tool for high-capacity optical networks.     

100-Gb/s DQPSK Transmission Experiment Without OTDM for 100G Ethernet Transport

In order to realize a future 100-Gb Ethernet (100 GbE) transport, 100-Gb/s transmission without 100-GHz-class electronics and optical time-division-multiplexing technique was demonstrated. By using a differential quadrature phase-shift-keying (DQPSK) modulation format and commercially available electronics, 2- and 50-km transmissions of 100-Gb/s signal were successfully achieved over a standard single mode fiber. The receiver sensitivity, chromatic dispersion, and differential group delay tolerances of 100-Gb/s DQPSK signal were also evaluated. Through these evaluations, the possibility of DQPSK modulation for future 100-GbE transport is verified     

不同调制码型的偏振模色散补偿研究

在40Gb/s的光纤通信系统中,用三段模拟器模拟光纤传输中的一阶与二阶偏振模色散(PMD),采用单偏振态的偏振度(DOP)作为多级PMD补偿器的反馈信号,对归零(RZ)码、非归零(NRZ)码、载波抑制归零(CSRZ)码和啁啾归零(CRZ)码分别进行四个自由度的一级、六个自由度的二级和十个自由度的三级PMD补偿器的补偿.仿真得到了四种码型的DOP与误码率(BER)的关系,同时补偿后的BER、DOP和PMD的变化关系表明,对RZ和NRZ码采用二级补偿器的效果比一级和三级补偿器要好得多,对CSRZ码和CRZ码采用单偏振态的DOP作为反馈信号进行PMD补偿的效果并不理想.     

Theoretical and experimental analysis of the dependence of a signal's degree of polarization on the optical data spectrum

We show theoretically and experimentally the relationship between a signal's degree of polarization (DOP), all-order polarization mode dispersion (PMD), and the optical spectrum (and hence the data modulation format and pulse width), and that these effects must be taken into account when using the DOP for differential group delay (DGD) monitoring. We explain the theory behind how all-order PMD affects a signal's DOP, and observe the pulse-width dependence for 10-, 20-, and 40-Gb/s return-to-zero (RZ) systems as the duty cycle changes. We then analyze and show (via simulation and experimentation) the effects of different data modulation formats (RZ, carrier-suppressed RZ, alternate-chirped RZ, and differential phase-shift keying) on the DOP in a DGD monitor. We conclude that the measurable DGD range and DOP sensitivity in DOP-based DGD monitors are dependent on a signal's pulse width and the data modulation format. We also show the theory behind the effects of first- and second-order PMD on the maximum and minimum DOP.     

高非线性光纤中并行交叉相位调制的偏振膜色散监测方法

利用信号光和插入的连续泵浦光之间产生的交叉相位调制（XPM）效应,提出了一种基于并联的XPM效应来监测光相位调制信号的一阶偏振模色散（PMD）的新技术。泵浦光的光谱会随着信号光中PMD和色散（CD）的变化而发生变化,所以导致泵浦光的光功率发生变化,在并联的一个支路中抑制PMD的影响,利用并联的两路同一波段泵浦光功率的差值来进行监测。仿真结果显示,新的技术可以实现对40 Gb/s非归零差分四相移相键控（NRZ-DQPSK）光信号从0~20 ps的监测。在20 ps的监测范围内,新技术的动态范围大于3 dB,可以用来进行准确的监测。对信号速率、色散、泵浦光功率和滤波器带宽对新技术的影响做了详细的研究。     

FSK Modulation Scheme for High-Speed Optical Transmission

In this paper,we describe the generation,detection,and performance of frequency-shift keying (FSK) for high-speed optical transmission and label switching.A non-return-to-zero (NRZ) FSK signal is generated by using two continuous-wave (CW) lasers,one Mach-Zehnder modulator (MZM),and one Mach-Zehnder delay interferometer (MZDI).An RZ-FSK signal is generated by cascading a dual-arm MZM,which is driven by a sinusoidal voltage at half the bit rate.Demodulation can be achieved on 1 bit rate through one MZDI or an array waveguide grating (AWG) demultiplexer with balanced detection.We perform numerical simulation on two types of frequency modulation schemes using MZM or PM,and we determine the effect of frequency tone spacing (FTS) on the generated FSK signal.In the proposed scheme,a novel frequency modulation format has transmission advantages compared with traditional modulation formats such as RZ and differential phase-shift keying (DPSK),under varying dispersion management.The performance of an RZ-FSK signal in a 4 × 40 Gb/s WDM transmission system is discussed.We experiment on transparent wavelength conversion based on four-wave mixing (FWM) in a semiconductor optical amplifier (SOA) and in a highly nonlinear dispersion shifted fiber (HNDSF) for a 40 Gb/s RZ-FSK signal.The feasibility of all-optical signal processing of a high-speed RZ-FSK signal is confirmed.We also determine the receiver power penalty for the RZ-FSK signal after a 100 km standard single-mode fiber (SMF) transmission link with matching dispersion compensating fiber (DCF),under the post-compensation management scheme.Because the frequency modulation format is orthogonal to intensity modulation and vector modulation (polarization shift keying),it can be used in the context of the combined modulation format to decrease the data rate or enhance the symbol rate.It can also be used in orthogonal label-switching as the modulation format for the payload or the label.As an example,we propose a simple orthogonal optical label switching technique based on 40 Gb/s FSK payload and 2.5 Gb/s intensity modulated (IM) label.     

Electronic compensation of PMD and chromatic dispersion with an ICin 10 Gbit/s transmission system

A system experiment is reported in which polarisation mode dispersion (PMD) and chromatic dispersion (CD) compensation was achieved using an SiGe IC linear equaliser, with a transmission rate of 100 bit/s over 95 km of standard single-mode fibre (DL=1600 ps/nm). Full equalisation was obtained, with a differential group delay (DGD) of <65 ps and a penalty reduction to 3.4 dB for DGD=100 ps     

基于射频信号2阶零功率的链路色散测量技术

为了适应高速率大色散光纤信道对链路色散的精确补偿要求,研究并提出了一种基于射频信号2阶零功率点的信道色散测量方案。采用信号两边带的位相差来测量光纤链路中的色散值,通过在发射端加载射频信号,可以得到接收端射频信号功率大小随链路色散值的周期性变化关系,从仿真光纤链路色散随射频信号频率的变化曲线获得接收端射频信号2阶零功率点位置。结果表明,基于射频信号2阶零功率的信号测量方案色散测量误差可控制在±10ps/nm范围内。相较于1阶零功率点,基于射频信号2阶零功率点信道色散测量方案可满足高速率、长距离大色散光纤信道对色散值的精确测量需求。     

D8PSK调制格式研究及其传输性能分析

研究了基于相位调制原理的新型调制格式差分八相相移键控(D8PSK),给出了串联和并联两种信号生成方案,对包括非归零码(NRZ)、33%占空比归零码(33%RZ)、50%占空比归零码(50%RZ)、载波抑制归零码(CSRZ)4种不同码型的生成方式进行了说明,对信号的接收过程和预编码公式进行了理论推导,并对系统进行了仿真。仿真系统中信号总速率达到100 Gbit/s,信号生成采用串联方式,信号接收采用4个马赫-曾德尔时延干涉仪(MZI)平衡检测接收机,色散补偿方式采用对称补偿,通过眼图可以看到其接收效果良好。最后对不同码型的D8PSK与DQPSK系统在抵抗群速度色散效应、非线性效应及偏振模色散效应3种主要影响系统传输性能的因素方面进行了分析比较。当眼图张开度代价为1dB时,CSRZ-D8PSK信号的群速度色散容限、非线性容限及偏振模色散容限分别为190ps/nm、4.4 dBm及24 ps,结果表明其具有较好的传输性能。     

Impact of chromatic and polarization-mode dispersions on DPSK systems using interferometric demodulation and direct detection

We study the impact of chromatic dispersion (CD) and first-order polarization-mode dispersion (PMD) on systems using binary differential phase-shift keying (2-DPSK) or quaternary DPSK (4-DPSK) with nonreturn-to-zero (NRZ) or return-to-zero (RZ) formats. These signals are received using optical preamplification, interferometric demodulation, and direct detection. We consider the linear propagation regime and compute optical power penalties at fixed bit-error ratio (BER). In order to evaluate the BER precisely taking account amplifier noise, arbitrary pulse shapes, arbitrary optical and electrical filtering, CD, and PMD, we introduce a novel model for DPSK systems and compute the BER using a method recently proposed by Forestieri for on-off keying (OOK) systems. We show that when properly applied, the method yields highly accurate results for DPSK systems. We have found that when either the NRZ or RZ format is used, 2-DPSK exhibits lower power penalties than OOK in the presence of CD and first-order PMD. RZ-2-DPSK, as compared with NRZ-2-DPSK, incurs smaller penalties due to PMD, but offers no advantage in terms of CD. 4-DPSK, as it has twice the symbol duration of OOK or 2-DPSK for a given bit rate, incurs much lower CD and PMD power penalties than either of these techniques. RZ-4-DPSK is especially promising, as it offers CD and PMD penalties significantly smaller than all other techniques, including NRZ-4-DPSK.     

Multicast-enabled high-speed VCSEL technology for flexible data center networks

We experimentally demonstrate the multiple signal modulation on a single class 10 G vertical cavity surface emitting laser (VCSEL) carrier at 1 310 nm for next generation multicast-enabled data center networks. A 10 Gbit/s data signal is directly modulated onto a single mode VCSEL carrier. To maximize carrier spectral efficiency, a 2 GHz reference frequency (RF) clock tone is simultaneously modulated on the VCSEL phase attribute. The inherent VCSEL orthogonal polarization bistability with changing bias current is further exploited in transmission of a polarization based pulse per second (PPS) timing clock signal. Therefore, we simultaneously transmit a 10 Gbit/s directly modulated data, 2 GHz phase modulated RF and a polarization-based PPS clock signals using a single mode 10 GHz bandwidth VCSEL carrier. It is the first time that a single class 10 G VCSEL carrier is reported to transmit a directly modulated data, phase modulated RF clock and polarization based PPS timing signal simultaneously in a single wavelength. A of G.652 single mode fibre (SMF) transmission over 3.21 km is experimentally attained. A receiver sensitivity of ?15.60 dBm is experimentally obtained for the directly modulated 10 Gbit/s data signal. A 3.21-km-long SMF transmission introduces a penalty of 0.23 dB to the data signal. The contribution of a 2 GHz phase modulated RF and a polarization-based PPS clock signal to this penalty is found to be 0.03 dB. An RF single-side band (SSB) phase noise values of ?82.36 dBc/Hz and ?77.97 dBc/Hz are attained without and with simultaneous directly modulated data and polarization-based PPS clock signals respectively for a 3.21-km-long SMF transmission. This work provides an alternative efficient and cost effective technique for simultaneous high-speed multiple information transmission to different network nodes within a data center network through shared network infrastructure.