Efficient approach for modeling collision-induced timing jitter inWDM return-to-zero dispersion-managed systems

A semi-analytical approach that allows us to create an efficient numerical algorithm to accurately evaluate collision-induced timing jitter in wavelength-division-multiplexing (WDM) systems with return-to-zero (RZ) modulation format is developed. The approach agrees well with full numerical simulations. The computational time is reduced by two orders of magnitude compared to full numerical simulations     

Performance Comparison of WDM Systems with Different Duty Cycle

The performances of nonlinear WDM systems with different duty cycle are compared by means of numerical simulation.The numerical results show that the optical pulse with duty cycle of 0.5 is superior to the conventional NRZ modulation schene.The conclusion is different from that of some references.The reason is that inter symbol interference is not included in some references.In fact,inter symbol interference plays an important role in monlinear WDM system.Although the larger the duty cycle is,the stronger the effect of the cross-phase modulation and self-phase modulation on nonlinear WDM is however,the larger the duty cycle is ,the stronger the inter symbol interference is.     

Novel modulation and detection for bandwidth-reduced RZ formats using duobinary-mode splitting in wideband PSK/ASK conversion

This paper proposes a novel duobinary-mode-splitting scheme that uses wideband phase-shift-keying (PSK)/amplitude-shift-keying (ASK) conversion for modulation and detection of bandwidth-reduced return-to-zero (RZ) modulation formats. We have first demonstrated that the proposed scheme greatly simplifies the modulation process of the duobinary carrier-suppressed RZ format (DCS-RZ) based on baseband binary nonreturn-to-zero (NRZ) modulation. We also proposed carrier-suppressed RZ differential-phase-shift-keying format (CS-RZ DPSK) as a novel bandwidth-reduced RZ format by applying the proposed scheme in the detection process. These novel RZ formats are shown to be very useful for dense wavelength-division multiplexed (DWDM) transport systems using high-speed channels, over 40 Gb/s, with spectrum efficiencies higher than 0.4 b/s/Hz. We demonstrate that the proposed modulation and detection scheme greatly simplifies the DWDM transmitter and receiver configuration if the periodicity of the optical PSK/ASK conversion filter equals the WDM channel spacing. The large tolerance of the formats against several fiber nonlinearities and their wide dispersion tolerance characteristics are tested at the channel rate of 43 Gb/s with 100-GHz spacing. The novel CS-RZ DPSK format offers higher nonlinearity tolerance against cross-phase modulation than does the DCS-RZ format.     

Dynamics of the chirped return-to-zero modulation format

We numerically simulated long-distance, high-bit-rate, wavelength division multiplexed (WDM) transmission in dispersion-managed systems. We investigated return-to-zero (RZ) and nonreturn-to-zero (NRZ) modulation formats with amplitude and phase modulation. Consistent with earlier experiments, we find that the chirped return-to-zero (CRZ) modulation format has significant advantages over the NRZ modulation format in WDM systems. We elucidate the physical reasons for these advantages. We then discuss, in detail, the dynamics of the CRZ systems, carefully distinguishing noise effects, single-channel nonlinear effects, and multichannel nonlinear effects. In this way, we provide a physical basis for understanding CRZ systems that should prove useful for future system design. In particular, we find that the pulse evolution is dominated by linear dispersion and that the spread in the eye diagrams is dominated by signal-spontaneous beat noise, just like in linear systems. However, we also find that symmetric dispersion compensation performs better than asymmetric dispersion compensation, due to the effects of nonlinearity. Additionally, we find that interchannel nonlinearities spread the eye diagrams without changing the dynamical behavior. Thus, the system is quasilinear in the sense that its properties resemble those of linear systems, but nonlinearity plays an important role     

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.     

Long-haul WDM transmission using higher order fiber dispersionmanagement

We propose a fiber dispersion management scheme for large-capacity long-haul wavelength division multiplexing (WDM) transmission systems that considers not only second- but also third-order dispersion characteristics using transmission fibers with opposite dispersion signs. It eliminates the waveform distortion of WDM signals that originates from the existence of third-order dispersion, which is a constraint placed on WDM capacity in conventional dispersion management, while reducing the interchannel interaction caused by the interplay of fiber nonlinearity and second-order dispersion. Design concept of the scheme is discussed to show the feasibility of using actual fiber parameters. An experimental investigation on transmission performance regarding the signal pulse format, nonreturn-to-zero (NRZ) and return-to-zero (RZ), and interchannel interaction caused by four-wave mixing (FWM) and cross-phase modulation (XPM) is described for optimizing WDM system performance. It is experimentally shown that RZ pulse transmission is possible without significant spectral broadening over a wide wavelength range in dispersion managed fiber spans. Using these results together with a wideband optical amplifier gain-bandwidth management technique, yields long-distance WDM transmission with the capacity of 25×10 Gb/s over 9288 km     

Alternative modulation formats in N×40 Gb/s WDM standardfiber RZ-transmission systems

A comparison of carrier-suppressed return-to-zero (CSRZ) and single sideband return-to-zero (SSB-RZ) formats is made in an attempt to find the optimum modulation format for high bit rate optical transmission systems. Our results show that CSRZ is superior to return-to-zero (RZ) and SSB-RZ with respect to signal degradation due to Kerr nonlinearities and chromatic dispersion in wavelength division multiplexing (WDM) as well as in single-channel 40-Gb/s systems over standard single-mode fibers (SSMF). It is shown that CSRZ enables a maximum spectral efficiency of approximately 0.7 (b/s)/Hz in a N×40 Gb/s WDM system with equally polarized channels. Furthermore, the CSRZ format in N×40 Gb/s WDM systems shows no further signal degradation compared to single-channel transmission     

利用MZ-DI装置的40Gb/s RZ到NRZ全光码型变换

利用马赫-曾德尔延时干涉(MZ-DI)装置对40Gb/s的归零(RZ)码到非归零(NRZ)码的全光码型转换方案进行研究.利用通信软件模拟了40Gb/s的码型转换过程,实现稳定的不同占空比的RZ码到NRZ码的码型转换.当占空比约为0.3时,转换效果最佳.     

偏振相关损耗对偏振模色散补偿系统性能的影响

数值分析了40Gb／s占空比50％的归零码（RZ）、占空比38％的RZ和载波抑制归零码（CSRZ）3种不同码型系统中偏振相关损耗（PDL）对偏振模色散（PMD）补偿的性能，并和不考虑PDL的情况作了比较。结果表明，由于PDL的存在，导致补偿系统的补偿性能恶化严重，使得损耗概率增加，眼图张开度减小；3种码型中，CSRZ码型系统的缓解性能比其他两种码型要好。     

Wavelength re-generation and re-modulation using optical phase lock loop techniques for 100-Gb/s DQPSK up-stream transmission in DWDM passive optical networks

We demonstrate, by partial experiment and simulation, a re-modulation scheme of the lightwave carrier imbedded in a downstream optical signals under differential quadrature phase shift keying (DQPSK) modulation format for upstream transmission over passive optical networks (PONs) at a bit rate of 100 Gb/s. The recovery of the optical carrier with the precise wavelength is implemented using an injection laser incorporating an optical phase locked loop (OPLL). In the computer simulation, the OPLL is implemented by a Simulink model consisting of interconnected system blocks following exactly the physical phenomena of the hardware structures. This model is then integrated with DQPSK modulation formats for up- and down-transmissions in PONs. Pulse shaping of Non-Return-to-Zero and Return-to-Zero (RZ) of 50% duty cycle and 67% duty cycle are used, respectively. Dispersion tolerance of 25 ps/nm with a bit error rate (BER) of 10⁻⁹ is achieved for both down- and upstream transmissions over fully dispersion compensated 80- km standard SMF and 1.5- km SSMF equivalent dispersion with carrier suppressed RZ pulse shaping (CS-RZ 67%). The contribution to BER by the timing synchronization error at the sampling of the optical network unit for re-modulation and related power penalty of these modulation formats is also investigated. It is noted that using the optical phase locking technique it is possible to remove any cross talks which may be generated from nonlinear effects such as cross-phase modulation, self-phase modulation, and four wave mixing.     

Optimum filter bandwidths for optically preamplified NRZ receivers

We present a comprehensive treatment of optically preamplified direct detection receivers for non-return-to-zero (NRZ) and return-to-zero (RZ) on/off keying modulation, taking into account the influence of different (N)RZ optical pulse shapes, specified at the receiver input, and filter transfer functions; optical Fabry-Perot filters (FPFs) and Bragg gratings as well as electrical fifth-order Bessel and first-order RC low-pass filters are considered. We determine optimum optical and electrical filter bandwidths and analyze the impact of bandwidth deviations on receiver sensitivity. Optimum receiver performance relies on a balance between noise and intersymbol interference (ISI) for NRZ transmission, while for RZ reception detection noise has to be traded against filter-induced signal energy rejection. Both for NRZ and 33% duty cycle RZ, optical filter bandwidths of around twice the data rate are found to be optimum. Receivers using RZ coding are shown to closely approach the quantum limit, and thus to outperform NRZ-based systems by several decibels. We further analyze the impact of important degrading effects on receiver sensitivity and optimum receiver bandwidths, including receiver noise, finite extinction ratio, chirp, and optical carrier frequency (or optical filter center frequency) fluctuations     

All-Optical Conversion From RZ to NRZ Using Gain-Clamped SOA

An all-optical converter from return-to-zero (RZ) pulses to the nonreturn-to-zero (NRZ) format is presented. The converter operates in two stages: the laser generated in a gain-clamped semiconductor optical amplifier (SOA) is modulated with the data signal; afterwards this signal is wavelength-converted by cross-gain modulation in a common SOA. The setup is noninverting and can feature wavelength conversion. Experimental error-free conversion from 5- and 40-ps RZ pulses to NRZ format is presented at 10 Gb/s using a 2¹¹-1 bit sequence     

Return-to-zero modulation with electrically continuously tunable duty cycle using single NRZ modulator

Two optical return-to-zero (RZ) modulation techniques are discussed, both based on a single Mach-Zehnder modulator driven by non-return-to-zero (NRZ) electrical signals. Both methods allow for continuously electrically tunable duty cycles and lead to chirped RZ formats. We demonstrate and discuss two RZ transmitter setups that generate optical RZ signals with electrically continuously tunable duty cycle using a single, NRZ-driven Mach-Zehnder modulator.     

Novel transmitter of dark RZ signal based on external modulator

A novel transmitter to generate a dark RZ signal with tunable duty cycle and extinction ratio is proposed, by modifying the process of precoding, modulating and coding. A dark RZ signal is generated simply by using one dual-arm Mach-Zehnder LiNbO3 modulator. We demonstrate experimentally that this optical dark RZ signal can be directly measured by a conven- tional binary intensity modulation direct detection (IM-DD) receiver. When different values of duty cycles at 2.5 Gbit/s are adjusted, the experimental results show different BER curves and eye diagrams of the optical dark RZ signal.     

NRZ versus RZ in 10-40-Gb/s dispersion-managed WDM transmissionsystems

We compare nonreturn-to-zero (NRZ) with return-to-zero (RZ) modulation format for wavelength-division-multiplexed systems operating at data rates up to 40 Gb/s. We find that in 10-40-Gb/s dispersion-managed systems (single-mode fiber alternating with dispersion compensating fiber), NRZ is more adversely affected by nonlinearities, whereas RZ is more affected by dispersion. In this dispersion map, 10- and 20-Gb/s systems operate better using RZ modulation format because nonlinearity dominates. However, 40-Gb/s systems favor the usage of NRZ because dispersion becomes the key limiting factor at 40 Gb/s     

A novel all-optical label swapping based on RZ-DQPSK/IRZ-ASK combined modulation format

A novel all-optical label swapping based on optical return zero (RZ) differential quadrature phase shifted keying/inverse return zero amplitude shifted keying (RZ-DQPSK/IRZ-ASK) combined modulation format scheme is investigated and analyzed theoretically. Internet protocol (IP) packets can be efficiently labeled and processed using this proposed scheme. Numerical simulation is taken to demonstrate the transmission characteristic of the all-optical label swapping based on RZ-DQPSK/IRZ-ASK modulation format. The transmission performance can be affected by the duty cycle of the IRZ pulse, the IRZ-ASK label extinction ratio, the dispersion compensation ratio, received optical power and the coupling coefficient of the coupler. Results show that the IRZ-ASK label extinction ratio is almost infinite and preferable performance is obtained. The proposed scheme is a practical solution to meet the data rate and cost-efficient of the optical links simultaneously in tomorrow's all-optical label swapping.     

An optical modulation format generation scheme based on spectral filtering and frequency-to-time mapping

An optical modulation format generation scheme based on spectral filtering and frequency-to-time mapping is experimentally demonstrated. Many modulation formats with continuously adjustable duty radio and bit rate can be formed by changing the dispersion of dispersion element and the bandwidth of shaped spectrum in this scheme. In the experiment, non-return-to-zero (NRZ) signal with bit rate of 29.41 Gbit/s and 1/2 duty ratio return-to-zero (RZ) signal with bit rate of 13.51 Gbit/s are obtained. The maximum bit rate of modulation format signal is also analyzed.     

Long-span repeaterless transmission systems with optical amplifiersusing pulse width management

This paper proposes pulse width management in order to extend the repeater spacings of repeaterless transmission systems with optical amplifiers. First, the dependency of receiver sensitivity on duty ratio, receiver response, and fiber dispersion is clarified by numerical analysis. Next, the calculation results of sensitivity as a function of signal format and receiver basedband response are verified experimentally. Moreover, we show that pulse width management which uses return-to-zero (RZ) format with large duty ratio (~0.7) at the transmitter and pulse compression at the receiver increases the repeater gain by ~4.5 dB compared to conventional systems employing nonreturn-to-zero (NRZ) format. Record repeater spacing of 300 km is realized at 10 Gb/s by utilizing pulse width management     

Optimal 40 Gb/s modulation formats for spectrally efficient long-haul DWDM systems

Three modulation formats are compared by numerical simulation of highly dense (75-GHz-spaced for 40 Gb/s channel), long-haul (600-1800 km) wavelength division multiplexed systems with three fiber types. Nonreturn-to-zero (NRZ) format, being the most spectrally compact and the simplest in transmitter and receiver configuration of the three, seems to be capable enough at shorter transmission distances than 1000 km regardless of fiber type. Carrier-suppressed return-to-zero (RZ) format, being the most tolerant to the self-phase modulation effect, showed better performance with fibers having larger chromatic dispersion. However, its transmission distance with low dispersion fibers is severely limited by the four-wave mixing effect. Bit-synchronous intensity modulated differential phase shift keying (IM-DPSK) format seems to be the best choice for a transmission distance beyond 1000 km because of its superior tolerance to optical noise and fiber nonlinear effects regardless of fiber types, despite slightly more complex transmitter and receiver configurations.     

利用SOA和OBPF的40Gb/s RZ到NRZ全光码型变换

全光码型转换技术是实现未来全光网络的关键技术之一,为此提出了一种利用半导体光放大器（SOA）和光带通滤波器（OBPF）的归零（RZ）码到非归零（NRZ）码的信号转换方案。利用通信软件数值模拟了基于40 Gb/s的码型转换,仿真实现了稳定的不同占空比的RZ码到NRZ码的码型转换以及波长转换。