Comparison between NRZ and RZ signal formats for in-line amplifiertransmission in the zero-dispersion regime

Nonreturn-to-zero (NRZ) and return-to-zero (RZ) signal formats are experimentally and numerically compared for single-channel long-distance transmission in an in-line amplifier system with dispersion management providing average zero dispersion and local nonzero dispersion at an interval equal to the in-line amplifier spacing. Among a 20-ps RZ signal, a 40-ps RZ signal, and an NRZ signal transmitted in 10 Gb/s straight-line experiments, the last signal achieves the longest transmission distance of 6000 km while the others are limited to 4400 km. Numerical simulations explain these results well and show that, along with linear amplified spontaneous emission (ASE) accumulation, signal waveform distortion due to the combined effect of higher order group-velocity dispersion (GVD) and self-phase modulation (SPM) dominates the performance. Nonlinear optical noise enhancement is not obvious because of the fiber dispersion arrangement. Signals with large pulse widths are less affected by the combined effect, while small-width signals yield superior initial signal-to-noise ratio (SNR) as determined by optical noise. A detailed simulation indicates that a pulse width of about 60 ps is optimum for long distance transmission under the fiber dispersion arranged in this paper     

Ultralong lightwave systems with incomplete dispersioncompensations

Ultralong nonreturn-to-zero optical transmission systems with incomplete dispersion compensations are studied. The dispersion of transmission fiber is periodically under- or overcompensated. Postdispersion compensation (PDC) at the receiver is used to compensate for the residual dispersion caused by incomplete compensation and to tailor the signal pulse shape. Formulas estimating the change of pulse width in the absence of amplifier noise during signal transmission and after PDC are given. During signal transmission, pulse width may be compressed or broadened by the combined effect of the dispersion and self-phase modulation (SPM). The change of pulse width nearly increases with the square of the distance during signal transmission. With amplifier noise, system performance evaluated by Q factor is studied. Several types of transmission fibers are considered. The Q factor can be significantly improved by proper PDC. Signal pulse is compressed when PDC is optimized. The characteristics of the maximum Q factor and the residual dispersion are studied, in which PDC is optimized. The results show that to achieve the best system performance, fiber dispersion should be undercompensated for positive dispersion parameter and overcompensated for negative dispersion parameter. The optimal fiber dispersion lies in the range from 4 to 10 ps/km/nm for the considered systems, and the optimal ratio of residual dispersion and fiber dispersion is about 1%     

Full-Field Electronic Dispersion Compensation of a 10 Gbit/s OOK Signal Over 4$,times,$ 124 km Field-Installed Single-Mode Fibre

We experimentally demonstrate the use of full-field electronic dispersion compensation (EDC) to achieve a bit error rate of 5times10^{- 5} at 22.3 dB optical signal-to-noise ratio for single-channel 10 Gbit/s on-off keyed signal after transmission over 496 km field-installed single-mode fibre with an amplifier spacing of 124 km. This performance is achieved by designing the EDC so as to avoid electronic amplification of the noise content of the signal during full-field reconstruction. We also investigate the tolerance of the system to key signal processing parameters, and numerically demonstrate that single-channel 2160 km single mode fibre transmission without in-line optical dispersion compensation can be achieved using this technique with 80 km amplifier spacing and optimized system parameters.     

10Gbit／s级联光纤放大器通信系统的传输距离

本文数值模拟了强度调制光信号在级联光纤放大器常规ＳＭＦ通信系统中的传输,在模拟中主要考虑了自相位调制、群带色散和ＡＳＥ噪声。我们使用负色散补偿光纤去补偿群速色散和自相位调制。结果表明如果色散得到很好的补偿,当放大器的间距减少到５０ｋＭ时,无误码２０５０ｋｍ传输是可能的。     

Experimental evaluation of chromatic dispersion distortion inoptical CPFSK transmission systems

Receiver sensitivity degradations due to fiber chromatic dispersion without chromatic dispersion compensation in multigigabit optical continuous-phase frequency-shift-keying (CPFSK) transmission systems are evaluated experimentally and theoretically. Transmission distance limited by the chromatic dispersion of 15 ps/nm/km is estimated to be 130 km at 5 Gb/s and 33 km at 10 Gb/s     

Theoretical analysis of system limitation for AM-DD/NRZ opticaltransmission systems using in-line phase-sensitive amplifiers

This paper shows the theoretically derived performance of single channel, amplitude modulation/direct detection optical transmission systems using in-line optical phase-sensitive amplifiers (PSA's). The calculations take into account the degradation of the signal-to-noise power ratio (SNR) and intersymbol interference (ISI) due to the distortion of transmitted signal pulses. The SNR is analyzed by considering not only amplifier noise and fiber loss but also noise enhancement by four-wave mixing in the transmission fiber. The ISI is estimated by eye-pattern degradation of the transmitted signal numerically calculated using the nonlinear Schrodinger equation. The regenerative repeater spacing of in-line PSA systems limited by SNR and ISI can be expanded by approximately 3 to 10 times that of in-line EDFA systems, in the case of |D|⩽0.1 ps/mn/km dispersion fiber systems transmitting a 40-Gb/s signal     

A study on the relationship between DWDM transmission performance and chromatic dispersion

In this letter, we numerically study the relationship between 40-Gb/s-based dense wavelength-division-multiplexing (DWDM) transmission performance and chromatic dispersion in two different transmission lines. We show that the optimum chromatic dispersion region for improving the DWDM transmission performance varies with the type of transmission line. We also show that a hybrid transmission line has a greater potential to resist any change in the dispersion slope compensation characteristics than a single fiber transmission line with a dispersion compensation fiber module. Finally, we show that chromatic dispersion of more than 12 ps/nm/km in a hybrid transmission line is optimum for a 40-Gb/s-based DWDM transmission system by taking the design of a dispersion compensating fiber into account.     

2500 km-10 Gbps RZ transmission system based on dispersion compensation CFBGs without electric regenerator

The characteristics of chirped fiber Bragg gratings (CFBGs) are optimized so that the ripple coefficient of the power reflectivity spectrum and group time delay are less than 1 dB and |± 15| ps, group delay is about 2600 ps/nm, polarization module dispersion is very small, PMD<2 ps, -3 dB bandwidth is about 0.35 nm, and insertion loss is about 4-5 dBm. Using dispersion compensation CFBG, a 2500 km-10 Gbps RZ optical signal transmission system on G.652 fiber was successfully demonstrated without an electric regenerator by optimizing dispersion management and loss management. The RZ optical signal was generated through a two-stage modulation method. At 2081 km, the power penalty of transmission is about 3 dB (conditions: RZ signal, BER = 10-12, PRBS = 1023 - 1); At 2560 km, the power penalty is about 5 dB. It is superior to the system using NRZ under the same conditions.     

Dispersion Compensation Configuration Applying a Combination of EDC and Tunable-ODC for Extended $L$-Band WDM Transmission

We propose effective polarization-mode dispersion (PMD) and chromatic dispersion (CD) compensation configuration for practical use in the field, taking into consideration the system cost and the field condition such as the CD changes due to fiber temperature dependence or transmission route change, and rapidly fluctuate state of polarization of the signal lightwave. Our configuration combines electric dispersion compensators (EDCs) with fixed taps in each channel with a multichannel tunable optical dispersion compensator (T-ODC) that can simultaneously compensate all channels' accumulated CD. We demonstrate widely extended $L$-band 43-Gb/s-based wavelength-division-multiplexing transmission over 450 km using our dispersion compensation configuration, and obtain effectiveness that the PMD penalty is suppressed by 2 dB at a differential group delay of 33 ps by using the EDC and that the CD penalty is improved by about 2 dB at the accumulated CD change of $+{/}-$ 30 ps/nm by using T-ODC and EDC.      

Fibre transmission of 10 Gbit/s signals following wavelengthconversion using a travelling-wave semiconductor optical amplifier

The wavelength of an intensity-modulated signal at 10 Gbit/s was translated from 1546 to 1531 nm using cross gain-compression in a semiconductor optical amplifier. The shifted signal was transmitted with less than 1 dB penalty over a 121 km span of dispersion-shifted fibre with two in-line amplifiers. A dispersion penalty of nearly 2 dB is measured for transmission of 6 Gbit/s data over 20 km conventional fibre     

Adjustable dispersion-compensation devices with wavelength tunability based on enhanced thermal chirping of fiber Bragg gratings

We demonstrate a dispersion compensation device based on thermal chirping of a glued fiber Bragg grating. The device can change the group velocity dispersion (GVD) while maintaining a center wavelength or change the center wavelength while preserving a GVD value. The GVD can be tuned from -122.5 to -57 ps/nm with a center wavelength at 1552.9 nm. On the other hand, the center wavelength can be shifted by 2.2 nm with a GVD value around -105 ps/nm.     

Design theory of long-distance WDM dispersion-managed transmissionsystem

This paper describes a novel design theory of long distance wavelength division multiplexed (WDM) dispersion-managed optical transmission systems. Assuming that the transmission distance, bit rate, and number of WDM channels are initially known, we investigate the optimum dispersion allocation and input power per channel to achieve the minimum channel spacing. Based on the design guidelines for single-channel and multichannel systems, we establish the optimal design strategy. Details of the design procedure are demonstrated for 2.5-, 5-, and 10-Gb/s 10000 km WDM systems by using computer simulations. Next, we study the impact of the fiber dispersion slope on the usable wavelength span, and show that the attainable capacity of the representative 5-Gb/s 10000 km WDM system employing the postcompensation scheme can not exceed 100 Gb/s. Finally, we propose several techniques to approach the ultimate capacity of the WDM system and show that up to 1 Tb/s (200×5 Gb/s) 10000 km system can be implemented without utilizing the in-line dispersion slope compensation scheme. We also discuss the 10 Gb/s-10000 km WDM system employing in-line dispersion slope compensation     

基于SOA啁啾管理的连续可调谐色度色散补偿的研究

提出了一种新型的可小范围连续调谐的色度色散(CD)补偿方案.该CD补偿方案包括一个半导体光放大器(SOA)和一段固定长度的色散补偿光纤(DCF).利用SOA的交叉相位调制(XPM)效应,通过调节SOA的偏置电流和控制脉冲光的强度,可以对进入SOA的光信号引入不同大小的附加啁啾量,从而可以利用固定长度的DCF得到补偿后的无啁啾光信号.实验中,实现了10 Gb/s可调谐CD补偿器,在无需替换DCF的情况下,实现了补偿范围为-40 ps/nm到60 ps/nm的连续可调谐CD补偿.     

Negative dispersion-flattened fibre suitable for 10 Gbit/s directly-modulated signal transmission in whole telecommunication band

New dispersion-flattened fibres having negative dispersion of about -8 ps/nm/km over the whole telecommunication band are developed. Transmissions with 10 Gbit/s directly modulated LDs over 51 km at 1275 nm and over 80 km at 1550 nm are realised without dispersion compensation.     

Spectrally efficient optical label insertion/extraction technique using an optical single sideband filter

A novel label insertion technique, using an adaptive optical single sideband (OSSB) filter, is experimentally shown. The OSSB filter is used to suppress one of the sidebands of the 40-Gb/s payload signal, and a 2.5-Gb/s intensity modulated signal is inserted as a label in the suppressed sideband. Lower label-payload crosstalk is observed using the OSSB filter compared to the absence of sideband suppression, allowing a reduction of 5 dB in the label power without additional penalty. The enhanced tolerance to group velocity dispersion (GVD) of the payload is experimentally assessed and a 5-dB penalty is observed for 136 ps/nm of accumulated dispersion. Additionally, simulation results show the efficient use of electrical dispersion compensation to improve the GVD tolerance, allowing the doubling of the dispersion tolerance.     

Unrepeated transmission of 20-Gb/s optical quadrature phase-shift-keying signal over 200-km standard single-mode fiber based on digital processing of homodyne-detected signal for Group-velocity dispersion compensation

We demonstrate unrepeated optical transmission of 20-Gb/s quadrature phase-shift-keying (QPSK) signals over a 200-km-long standard single-mode fiber (SMF) without using any optical dispersion compensator. By employing optical homodyne detection, which can restore the entire information of the complex amplitude of the transmitted signal, group-velocity dispersion (GVD) of the SMF can be compensated electrically by a linear equalizer at the receiver. From off-line bit-error-rate measurements, we find that a simple transversal filter implemented in digital signal processing circuits after homodyne detection can effectively cancel the fiber GVD of up to 4000 ps/nm, enabling successful 20-Gb/s QPSK transmission.     

Waveform degradation due to dispersion effects in an optical CPFSKsystem

Waveform degradation due to polarization and chromatic dispersions in a single-mode fiber is calculated for a coherent CPFSK signal. For a single-mode fiber with polarization dispersion of ⩽1 ps, chromatic dispersion almost dominates the system. However, if a fiber has polarization dispersion of more than a few picoseconds and a chromatic dispersion of less than 0.1 ps/km/nm, which can be attained by using a dispersion-shifted fiber and/or by electric dispersion compensation, polarization dispersion will restrict transmission capacity. For instance, polarization dispersion of 5 ps will restrict a bit rate by ~60 Gb/s when chromatic dispersion is fully reduced using a dispersion-shifted fiber or applying electrical equalization     

Residual Chromatic-Dispersion Monitoring and Dynamic Compensation in 40-Gb/s Systems

A dynamic chromatic-dispersion-compensation system is implemented with a residual chromatic-dispersion monitoring module based on self-frequency shift effects in a semiconductor optical amplifier. Experimental results show that the compensation range of the system is 100 ps/nm and the residual chromatic dispersion of the transmission link is less than 5 ps/nm.     

Postdetection Adjustable Simultaneous Compensation of DGD and GVD in a 40-Gb/s Optical Single-Sideband System

Results on postdetection compensation of group velocity dispersion (GVD) and differential group delay (DGD) in a 40-Gb/s optical single-sideband system are presented. An electrically adjustable transversal filter structure and a microstrip line are used as electrical compensators. Our results show that 408 ps/nm of GVD or 18 ps of DGD, considered separately, are compensated with less than 1.3-dB optical signal-to-noise ratio (OSNR) penalty to back-to-back without compensator, for an extinction ratio (ER) of 6 dB. The simultaneous effect of 12.3 ps of DGD and 374 ps/nm of GVD is compensated with less than 2-dB OSNR penalty, also for ER of 6 dB. Simulations performed support the experimental results.     

System performance of coherent transmission over cascaded in-linefiber amplifiers

The limitations of cascaded in-line amplifier systems using coherent modulation-demodulation schemes are examined by evaluating the product of the data rate and the transmission distance. The linear amplified spontaneous emission (ASE) accumulation is shown to make the maximum value of the data rate-distance product increase proportionally with the ratio of the amplifier output signal power to the noise figure. It is also shown that the Kerr-nonlinearity-induced phase noise limits the product of the data rate and the third power of the distance, the maximum value of which is inversely proportional to both the amplifier output signal power and the noise figure. The fiber dispersion is known to limit the product of the distance and the square of the data rate by causing waveform distortion. By taking these three relations into account, it is concluded that coherent signal transmission has a maximum in-line amplifier system length of 10³-10⁴ km in the gigabit-per-second range. Among these three factors, the nonlinearity-induced phase noise has the greatest impact