An effective carrier phase estimation scheme in faster than Nyquist WDM transmission system

We propose a carrier phase estimation method using unequaled digital pilot tone in a faster than Nyquist WDM transmission system, in which the digital pilot tone is employed to estimate laser phase noise induced by the lasers located at the transmitter and the receiver ends. The method can effectively overcome the scale of the inter-symbol interference introduced by FTN signaling, with the good features of high phase noise tolerance and simple hardware complexity.     

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.     

Low-Cost Multiparameter Optical Performance Monitoring Based on Polarization Modulation

A novel technique to conduct optical performance monitoring of WDM networks is proposed, implemented, and evaluated. The method is based on channel-individual polarization modulation of the wavelength channels at the transmitter, which enables low-cost monitor nodes consisting only of one polarizer followed by low-speed electronics. Despite the few optical components, the channel Stokes vectors can be measured and subsequently channel power, wavelength, optical signal-to-noise ratio, differential group delay are obtained. The Stokes vector precision is measured to be 2$^{circ}$ (rms), channel power accuracy $pm 0.1 $ dB with a 20-dB dynamic range, and the wavelength accuracy is $pm {30} $ pm. The OPM system was implemented in the field to monitor five WDM channels in an 820-km-long installed transmission link.      

Tunable Parametric All-Fiber Short-Wavelength IR Transmitter

In this paper, the performance of an all-fiber short-wave IR (SWIR) transmitter with wideband tunability and high peak power is reported. Transmitter construction relied on parametric process in highly nonlinear fiber to convert a seed laser at 1260 nm to 2155 nm SWIR channel with record 39 dB efficiency and translation over 900 nm spectral range. We demonstrated 61 W of peak converted power at 2 $mu$m and 26 W at 2.15 $mu$ m. Efficient conversion was made possible by engineered fourth-order dispersion of the highly nonlinear fiber and construction of a low-noise pump source in a 1550 nm band.      

A Low-Noise Broadband Light Source for a WDM-PON Based on Mutually Injected Fabry–PÉrot Laser Diodes With RF Modulation

We demonstrated improved performance of a low-noise broadband light source (BLS) based on mutually injected Fabry–PÉrot laser diodes for a large capacity and high bit-rate wavelength-division-multiplexed passive optical network (WDM-PON). The 3-dB linewidth and relative intensity noise of each mode of the low-noise BLS are improved by radio-frequency modulation, where the frequency was detuned from the fundamental noise peak of the low-noise BLS. Then, a 622-Mb/s WDM-PON was demonstrated. Furthermore, a 1-Gb/s WDM-PON can be realized with Manchester coding at a channel spacing of 100 GHz. Thus, the proposed low-noise BLS can be employed to realize a highly scalable WDM-PON.      

Demonstration of timing skew compensation for bit-parallel WDM datatransmission with picosecond precision

We demonstrate transmission of seven wavelength-division-multiplexed (WDM) bit-parallel channels with a total of 15-nm spectral span over a 2.5-km standard single-mode fiber/dispersion-compensating fiber link with less than 3-ps timing skew. The synchronized WDM channels are generated by spectrally slicing pulses from a single femtosecond fiber laser using a femtosecond pulse shaper. The small residual timing skew arises from the residual dispersion slope of the link. We measure a dispersion slope of D'=0.017 ps/km/mn², which is roughly four times less than for an equivalent length of dispersion-shifted fiber. Our work shows that the dispersion-compensating fiber technique could significantly reduce the timing skew for WDM bit-parallel transmission over a several-kilometer fiber link     

Transmission capacity of optical path overhead transfer schemeusing pilot tone for optical path network

Photonic networks based on the optical path concept and wavelength division multiplexing (WDM) technology require unique operation, administration, and maintenance (OAM) functions. In order to realize the required OAM functions, the optical path network must support an effective management information transfer method. The method that superimposes a pilot tone on the optical signal appears very interesting for optical path overhead transfer. The pilot tone transmission capacity is determined by the carrier to noise ratio which depends on the power spectral density of the optical signal. The pilot tone transmission capacity of an optical path network employing WDM technology is elucidated; 4.5 kb/s transmission can be realized when the pilot tone modulation index is set at 3%     

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.      

Experimental Investigation of Delay-Tap Sampling Technique for Online Monitoring of RZ-DQPSK Signals

We experimentally demonstrate optical performance monitoring of 20-Gb/s return-to-zero differential quadrature phase-shift keying signals using an asynchronous delay-tap sampling technique. This method allows online monitoring of accumulated chromatic dispersion in the range from $-$600 to $+$600 ps/nm. We also show evaluation of optical signal-to-noise ratio at a level of 6.7 dB and monitor operation at input power of $-$ 16 dBm.      

Impact of Optical Transmission on Multiband OFDM Ultra-Wideband Wireless System With Fiber Distribution

Multiband (MB) orthogonal frequency-division multiplexing (OFDM) ultra-wideband (UWB) wireless, which provides high data rate access, is required to be distributed by using optical fiber. The performance of MB-OFDM UWB over fiber transmission system is investigated considering optical modulation and demodulation impact. Theoretical analysis of the effect of fiber dispersion, optical transmitter, and optical receiver response on system performance is carried out considering amplitude and phase distortion. Experiments are conducted and verified by our theoretical analysis and good agreement is obtained. It is found that RF modulation index of $sim {hbox {4}}%$ is optimum for optical transmitter with Mach–Zehnder modulator, and optical receiver with Chebyshev-II response is the best for MB-OFDM UWB over fiber. Compared to back-to-back UWB over fiber, optical transmission is mainly limited by laser phase noise converted relative intensity noise and phase distortion induced by fiber dispersion when optimum modulation index is used. Higher modulation index is limited by amplitude and phase distortion to OFDM signal induced by optical transmitter and receiver response nonlinearities and fiber dispersion and the spectral mask. It is also found that highly received optical power is required for transmission of MB-OFDM UWB signal over fiber.      

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     

Transoceanic WDM System with More Than 100 Gb/s Capacity

This paper reviews several key technologies to realize a transoceanic wavelength-division-multiplexing (WDM) system with more than 100 Gb/s capacity. The key technologies include a novel gain equalization scheme, a broadband erbium-doped fiber amplifier, a chromatic dispersion compensation technique at a transmitter, and a RZ modulation format. Employing these new technologies, we successfully demonstrated a 32 channel 5.3 Gb/s (total capacity of 170 Gb/s) WDM signal transmission over 9879 km.     

1.6 Tbit/s(40×40 Gbit/s)光通信传输系统

在国家自然科学基金网(NSFCNet)上已实现由400 km×10 Gbit/s传输链路直接升级的一路400 km×40 Gbit/s光传输实验的基础上,采用自行研制的40×40 Gbit/s载波抑制归零(CS-RZ)码多波长光发送源,进行了160 km的1.6 Tbit/s(40×40 Gbit/s)波分复用(WDM)光传输实验。实验结果表明,对于常规中短距离10 Gbit/s传输链路可以直接升级至40 Gbit/s。但是由于40 Gbit/s传输系统的色散容限小于60 ps/nm,而且传输光纤与色散补偿模块的色散斜率不匹配,要实现40通道40 Gbit/s的传输,必须对40个信道分别进行精细的色散补偿。这也说明,对于宽带的40 Gbit/s多波长系统,有必要优化设计或更新传输链路。     

A multiwavelength source having precise channel spacing for WDMsystems

We report a wavelength-division-multiplexed (WDM) source in which a combination of a periodically driven Mach-Zehnder modulator and a nonlinear fiber are used to generate an optical spectrum that consists of several wavelength components precisely spaced by 64 GHz (>0.5 nm). The 13 strongest spectral components are within a 10-dB optical power range and their performance as channel sources in a WDM transmitter have been evaluated. We also propose and demonstrate a technique to generate an optical spectrum in which the separation between the major components is four times the modulator drive frequency     

Transparent ultra-long-haul DWDM networks with "broadcast-and-select" OADM/OXC architecture

We describe an experimental realization of ultra-long-haul (ULH) networks with dynamically reconfigurable transparent optical add-drop multiplexers (OADMs) and optical cross-connects (OXCs). A simple new approach to dispersion management in ULH dense-wavelength-division-multiplexing (DWDM) transparent optical networks is proposed and implemented, which enables excellent transmission performance while avoiding dispersion compensation on a connection-by-connection basis. We demonstrate "broadcast-and-select" node architectures that take full advantage of this method. Our implementation of signal leveling ensures minimum variations of path-averaged power among the wavelength-division-multiplexing (WDM) channels between the dynamic gain-equalizing nodes and results in uniform nonlinear and spontaneous-emission penalties across the WDM spectrum. We achieve 80/spl times/10.7-Gb/s DWDM networking over 4160 km (52 spans/spl times/80 km each) of all-Raman-amplified symmetric dispersion-managed fiber and 13 concatenated OADMs or 320/spl times/320 wavelength-port OXCs with 320-km node spacing. The WDM channels use 50-GHz grid in C band and the simple nonreturn-to-zero (NRZ) modulation format. The measured Q values exhibit more than a 1.8-dB margin over the forward-error correction threshold for 10/sup -15/ bit-error-rate operation. We compare these results with point-to-point transmission of 80/spl times/10-Gb/s NRZ WDM signals over 4160 km without OADM/OXC and provide detailed characterization of penalties due to optical signal-to-noise-ratio degradation, filter concatenation, and crosstalk.     

密集波分复用系统中波长控制技术研究

本文利用半导体激光器的热阻模型,研究分析了密集波分复用系统中光源的波长控制技术。利用本文的理论指导设计的特定波长的光源已经应用于武汉邮电科学研究院研制的济南—青岛８×２．５Ｇｂｉｔ／ｓ密集波分复用实验工程。     

Performance of Forward-Error Correction Code in 10-Gb/s RSOA-Based WDM PON

We investigate the performance of the forward-error correction (FEC) code for the 10-Gb/s wavelength-division- multiplexed passive optical network (WDM PON) implemented by using reflective semiconductor optical amplifiers (RSOAs) with extremely limited modulation bandwidth and the electronic equalizers to compensate for the degradations resulting from the use of such RSOAs. We show that the error occurrences in this network strongly depend on the bit pattern and the burst errors are likely to occur. Thus, it is important to use the FEC code capable of correcting the burst errors such as Reed–Solomon (RS) code. In addition, since a significant penalty can be induced by the increased line rate resulting from the use of the FEC code, it is necessary to find the optimum redundancy required to minimize the bit-error rate. We also evaluate the tolerance to the chromatic dispersion of the proposed 10-Gb/s WDM PON implemented by using the RS code with the optimum redundancy.      

Performance Comparison of Duobinary Formats for 40-Gb/s and Mixed 10/40-Gb/s Long-Haul WDM Transmission on SSMF and LEAF Fibers

Duobinary formats are today considered as being one of the most promising cost-effective solutions for the deployment of 40-Gb/s technology on existing 10-Gb/s WDM long-haul transmission infrastructures. Various methods for generating duobinary formats have been developed in the past few years but to our knowledge their respective performances for 40-Gb/s WDM transmission have never been really compared. In this paper, we made an extensive numerical evaluation of the robustness of these different types of duobinary transmitter to accumulation of ASE noise, chromatic dispersion, PMD but also to single-channel and WDM 40-Gb/s transmission impairments on standard single-mode fiber. A numerical evaluation of the ability of duobinary format for mixed 10/40-Gb/s WDM long-haul transmission with 50-GHz channel spacing is also led, on both standard single-mode and LEAF fibers, and compared to DQPSK format. In order to clearly identify the limiting transmission effects on each of these two fiber types, the assessment of the performance of a 50-GHz spaced WDM 40-Gb/s long-haul transmission using either duobinary or DQPSK channels only is implemented at last.      

WDM系统中色散的实时监测

简单地推导了相位调制下的单音频率在传输光纤中其功率与光纤色散、光纤距离之间的关系，并介绍了在DWDM系统中可以利用相位调制法进行实时监测色散。此方法对于40GHz系统的色散实时监测提供了很好的参考方法。仿真结果表明，在未考虑光纤非线性效应的情况下有很高的精确性，当考虑非线性效应时，其传输距离会由于传输速率的影响而受到限制。     

Quasicontinuous tunable fiber-ring laser applied as localoscillator in an absolute calibrated spectrometer for WDM systems

A special tunable fiber-ring laser is introduced. With this laser as local oscillator, a calibrated optical spectrometer was realized, which allows the determination of the wavelength of all transmitters of a wavelet division multiplexing (WDM) system in one sweep. The system is based on a fiber ring laser including a high finesse Fabry-Perot resonator with a high free spectral range and a 1-km-long dispersion shifted fiber. To achieve a high wavelength determination accuracy two auxiliary signals are generated: one with a Krypton filled absorption tube and another with a simple, silica based, periodical filter. With that, the wavelength of every transmitter of a WDM signal can be determined with an absolute frequency accuracy of better than ±0.8 GHz in a range of 1527-1562 nm in one sweep. The relative frequency accuracy is better than ±0.3 GHz