Dispersion slope equalizer for dispersion shifted fiber using alattice-form programmable optical filter on a planar lightwave circuit

This paper reports an integrated-optic dispersion slope (third-order dispersion) equalizer for dispersion shifted fiber which employs a lattice-form programmable optical filter on a planar lightwave circuit (PLC). This dispersion slope equalizer consists of nine symmetrical interferometers interleaved with eight asymmetrical interferometers. The performance of the equalizer is evaluated numerically. We confirm experimentally that this equalizer is effective in reducing the pulse waveform deterioration caused by the dispersion slope. In addition, the equalizer improves the power penalty of a 200-Gb/s, 100-km, time-division multiplexed optical transmission experiment     

Exact compensation for both chromatic dispersion and Kerr effect ina transmission fiber using optical phase conjugation

We propose a new method to compensate exactly for both chromatic dispersion and self-phase modulation in a transmission fiber, where the light intensity changes due to fiber loss and amplifier gain. This method utilizes optical phase conjugation (OPC). The pulse shape is precompensated before OPC by transmission through a fiber with large dispersion. A computer simulation demonstrates effective compensation for waveform distortion in a 40 Gb/s NRZ intensity-modulated light transmission     

Design theory of long-distance optical transmission systems usingmidway optical phase conjugation

This paper describes the basic system principle, the residual waveform distortion, the optimal design method, the ultimate transmission capacity, and its future improvement of the long-distance optical fiber transmission system using midway optical phase conjugation. We show that the periodic power variation in an optical amplifier chain and the dispersion value fluctuation from span to span along the system cause imperfect compensation for waveform distortion in the midway optical phase conjugation system. The optimal design method for suppressing the residual waveform distortion is proposed and confirmed by using computer simulations     

Integrated dispersion slope equalizer of AWG-based optical CDMA for radio-over-fiber transmissions

Chromatic dispersion from optical fiber is the most important problem that produces temporal skews and destroys the rectangular structure of code patterns in the spectra-amplitude-coding optical code-division multiple-access (SAC-OCDMA) system. Thus, the balance detection does not work perfectly and the system performance will be degraded. In order to improve the distortion, we apply cascaded Mach–Zehnder interferometers (MZIs) to design a dispersion slope equalizer for the SAC-OCDMA system integrated with arrayed-waveguide grating (AWG) router coder. The dispersion slopes of a cascaded MZI compensator could be adjusted by the arm length differences of MZIs and be complementary with the fiber links. In this study, we present a set of length differences for 10 km long single-mode fiber (SMF) and verify the compensation scheme by relative delay time slope and signal-to-interference ratio (SIR). The dispersion slope equalizer with perfect complementary slope successfully compensates the dispersion from SMF and the system performance with dispersion slope equalizer is highly improved.     

Effect of chirping-induced waveform distortion on the performanceof direct detection receivers using traveling-wave semiconductor opticalpreamplifiers

The influence of chirping-induced waveform distortion on the performance of multigigabit-per-second traveling-wave semiconductor optical amplifier (TWSOA)/p-i-n direct detection receivers is evaluated. The results are based on a novel method of evaluating the probability of error in the presence of the signal-spontaneous and spontaneous-spontaneous beat noise components. Laser chirping causes the dependence of the receiver sensitivity on the fiber dispersion coefficient×length product DL to be different for TWSOA/p-i-n and avalanche photodiode (APD) receivers. Compared to the APD receiver, the sensitivity of the TWSOA/p-i-n receiver degrades less quickly. So for cases of practical interest, the TWSOA/p-i-n receiver is more tolerant of chirping-induced waveform distortion     

啁啾光脉冲在高阶色散控制光纤链中的稳定传输

基于一种二阶和三阶色散补偿的光纤级联系统模型 ,用数值法对啁啾皮秒光脉冲作了传输模拟。结果表明 ,完全补偿的高阶色散控制系统消除了三阶色散所引起的脉冲边沿部的振荡 ,减弱了脉冲峰的时间移动 ;另外 ,在确定的配置下 ,给输入脉冲附加一最佳的频率啁啾 ,可使得色散控制孤子稳定传输。脉冲宽度和啁啾以及光强度都围绕在初始值附近波动 ,在每个补偿周期末端 ,基本恢复到初始值 ;最佳啁啾的选取与二阶色散的配置有关 ,与三阶色散的配置无关 ;文中画出了 10 0Gbit/s码率的 64位随机高斯光脉冲序列在完全补偿系统中传输 10 0 0 0km后的眼图。由清晰的眼图可知 ,这种完全补偿系统减弱了脉冲之间的相互作用     

Reconfigurable Dispersion Equalizer Based on Phase-Apodized Fiber Bragg Gratings

We present a novel dispersion equalizer design for the compensation of chromatic dispersion and chromatic dispersion slope in WDM systems. The device is based on a cascade of complex quasi-periodic chirped fiber Bragg gratings. We show that the use of a low chirp results in the distribution of the resonating cavities along the optical fiber length, which allows reconfiguration of the spectral characteristics by the application of a temperature profile. This paper exposes in detail the numerical techniques used in the optimization of the fiber Bragg grating filters taking into account fabrication imperfections. We present a specific design for a 32-channel dispersion equalizer for 10 Gbit/s and a 50-GHz channel spacing. We examine the spectral characterization of a device fabricated using a phase-apodized mask for various settings of the chromatic dispersion profile. We demonstrate a tuning range of $pm {800}~{rm ps}/{rm nm}$ over a bandwidth of 30 GHz. On average, the standard deviation of the phase ripple was below 0.1 rad. Finally, we evaluate the performance of this device by bit error rate measurements.      

Computationally Efficient Nonlinearity Compensation for Coherent Fiber-Optic Systems

Split-step digital backward propagation (DBP) can be combined with coherent detection to compensate for fiber nonlinear impairments.A large number of DBP steps is usually needed for a long-haul fiber system,and this creates a heavy computational load.In a trade-off between complexity and performance,interchannel nonlinearity can be disregarded in order to simplify the DBP algorithm.The number of steps can also be reduced at the expense of performance.In periodic dispersion-managed long-haul transmission systems,optical waveform distortion is dominated by chromatic dispersion.As a result,the nonlinearity of the optical signal repeats in every dispersion period.Because of this periodic behavior,DBP of many fiber spans can be folded into one span.Using this distance-folded DBP method,the required computation for a transoceanic transmission system with full inline dispersion compensation can be reduced by up to two orders of magnitude with negligible penalty.The folded DBP method can be modified to compensate for nonlinearity in fiber links with non-zero residual dispersion per span.     

Transmission performance of chirp-controlled signal by usingsemiconductor optical amplifier

We examine the fiber transmission performance of the optical signal whose chirp is controlled by utilizing phase modulation in semiconductor optical amplifier (SOA) with both simulations and experiments. This chirp control technique converts a positive chirp created by electroabsorption (EA) modulator into negative chirp, which reduces the waveform degradation due to the chromatic dispersion in transmission over standard single-mode fiber (SMF). It also provides an optical gain that is sufficient to compensate the insertion loss of the EA modulator. We investigate how the chirp control is affected by the input power to the SOA and the carrier lifetime of the SOA. As the SOA input power increases, the negative chirp becomes large, while the waveform is largely distorted due to gain saturation. However, the waveform distortion at high SOA input powers can be shaped by using a frequency discriminator. The acceleration of the carrier lifetime also reduces the waveform distortion due to gain saturation. We demonstrate that the chirp control technique is effective even for a high bit rate optical signal up to 10 Gb/s, when the carrier lifetime is expedited by optical pumping     

Compensation for the self-steepening effects in optical fiber communication system using midway optical phase conjugation

Effects of self-steepening (SS) of chirped Gaussian pulses on optical fiber communication system using midway optical phase conjugation (OPC) are analyzed. Dynamic evolution of the ultrashort pulses is simulated numerically. It is found that OPC cannot compensate for pulse waveform distortion due to SS.The initial chirp of pulses and dispersion can counteract SS and improve the compensation performance for the distortion.     

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     

Performance evaluation of prechirped RZ and CS-RZ formats inhigh-speed transmission systems with dispersion management

This paper describes the transmission performance of prechirped return-to-zero (RZ) and prechirped carrier-suppressed return-to-zero (CS-RZ) signals over a periodically dispersion-compensated transmission line. We analyze the transmission characteristics of both formats, taking account the transmitter configuration expected, in which pulse chirping is generated by using both a phase modulator and a linear dispersion compensating device. We also discuss the dependence of the transmission characteristics on phase modulation, pre- and postcompensating dispersion, and receiver optical and electrical filter widths. We show that, in single-channel transmission, phase modulation effectively reduces the intrachannel nonlinear interaction and improves the transmission performance. Next, we discuss the transmission characteristics of chirped RZ and chirped CS-RZ signals in dense wavelength division multiplexed (DWDM) signal transmission. In 100-GHz spaced 40-Gb/s-per-channel systems, it is shown that the phase modulation must be carefully optimized in order to minimize the linear crosstalk and waveform distortion induced by the intra- and interchannel nonlinear interaction in the transmission fiber     

Computer simulation of high-bit-rate optical fiber transmission using single-frequency lasers

Chirp-induced dispersion penalties in high-bit-rate optical fiber transmission are assessed using numerical integration of laser rate equations and a Fourier transform fiber dispersion routine. The roles of the imposed modulation waveform and laser design parameters are evaluated from computer generated eye diagrams and simple analytical observations. Consistent with experiment, we find device dependent optimum laser extinction ratios. In addition, we address the delicate balance between nonlinear chirp-induced dispersion penalties and the speed limitations imposed by linear current filtering on both the laser transmitter and the receiver. These considerations become increasingly important at higher bit rates such as 8 Gbit/s.     

Fiber nonlinearity and dispersion mitigation in 40-Gb/s NRZ WDM transmission using a multichannel optical equalizer

We investigate the mitigation of distortion due to self-phase modulation and dispersion in 40-Gb/s nonreturn-to-zero wavelength-division-multiplexed transmission using a multichannel optical equalizer. The ability of the equalizer to reduce the signal degradation due to fiber nonlinearities was demonstrated. We achieved 21-channel transmission at BER <10/sup -9/ without forward error correction over 750 km of true wave reduced slope fiber with 107-km amplifier span lengths.     

Optical power budget enhancement in next-generation DDO-OFDM-based optical access networks using square root module

A passive optical network based on orthogonal frequency division multiplexing (OFDM-PON) gives improved performance for high speed optical access network due to its greater resistance to fiber dispersion and higher bandwidth efficiency. In optical fiber communication, chromatic dispersion (CD) is a linear distortion in fiber, but it is converted into nonlinear distortion due to square-law characteristic of photo diode detector at the receiver side resulting in degradation of performance. To compensate for this nonlinear distortion, we proposed to use a linearized receiver circuit with square root module (SRM) device which can improve the performance of system in terms of CD tolerance. In this paper, we have analytically analyzed the performance of OFDM-PON system with and without SRM device for direct-detection optical OFDM-PON (DDO-OFDM-PON) system. At BER of \(10^{-3}\), which is the limit of forward error correction, there is 11.1 and 13.5 dB improvement in optical budget with SRM for downstream and upstream direction, respectively, as compared to conventional DDO-OFDM-PON system.     

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     

Cumulative waveform distortion in cascaded optical amplifierrepeaters for multigigabit IM/DD systems

An analysis was conducted of a cumulative pattern-dependent waveform distortion in cascaded semiconductor laser and Er³⁺-doped fiber amplifiers. At 2.5 Gb/s, cumulative waveform distortion limits the number of cascaded amplifiers to about 20 for the semiconductor amplifiers. The Er³⁺-doped fiber amplifier is relatively unaffected-over 100 stages can be cascaded. The Er³⁺ amplifier is seen to be the better choice for long-haul multigigabit systems     

Fiber Bragg grating fabrication for dispersion slope compensation

Group-delay dispersion slope causes signal deterioration in long-distance high-data-rate communication systems. We report the fabrication of high-quality apodized and chirped fiber gratings with dispersion slopes as high as -1800 ps/nm/sup -2/ over a 0.7-mn bandwidth-sufficient to compensate for the dispersion slope of >25000-km length of standard fiber at a wavelength of 1550 nm. We believe this is the first time that fiber Bragg gratings have been specifically designed and fabricated for dispersion slope compensation. The fabrication technique uses a standard unchirped phase mask and two scans of the inscribing UV beam.     

Bit-error performance of a newly designed external-cavity diode laser for uncooled WDM applications

The impact of fiber chromatic dispersion and an isolated mode hop on the bit-error performance of an external-cavity diode laser newly designed for uncooled wavelength-division-multiplexing (WDM) applications was investigated. Bit-error-rate measurements confirmed that the power penalty resulting from optical waveform distortion after a transmission through 80 km of standard single-mode fiber (SMF) was suppressed to about 1 dB. Moreover, it was confirmed that the deterioration in the bit-error performance caused by an isolated mode hop can be disregarded.     

160-gb/s adaptive dispersion equalization using an asynchronous dispersion-induced chirp monitor

This paper describes an adaptive dispersion equalizer (ADE) that uses an asynchronous dispersion-induced chirp monitor and the detailed study of the first demonstration of 160-Gb/s adaptive dispersion equalization. The device successfully equalized the dispersion change over a 40/spl deg/C temperature range (from 5/spl deg/C to 45/spl deg/C) and the dispersion slope of an 80-km dispersion-shifted fiber (DSF). The ADE will enhance the feasibility of 160-Gb/s optical transmission systems.