Tunable Optical Delay Using Four-Wave Mixing in a 35-cm Highly Nonlinear Bismuth-Oxide Fiber and Group Velocity Dispersion

In this paper, an optically controlled tunable delay scheme has been proposed using four-wave mixing (FWM) wavelength conversion in a 35-cm highly nonlinear bismuth-oxide fiber (Bi-NLF) together with group velocity dispersion (GVD) in a chirped fiber Bragg grating (CFBG). The Bi-NLF offers a very large nonlinearity and gives rise to significant FWM over a short fiber segment. With the use of a CFBG, a delay range over 185 ps has been experimentally demonstrated. To investigate the performance of the tunable delay, we have applied the scheme for variable delays of 10-Gb/s amplitude-shift keying (ASK) and differential phase-shift keying (DPSK) data signals. The bit error rate (BER) measurements show a power penalty of less than 3.5 dB for both amplitude- and phase-modulated data formats. To further increase the delay time, the CFBG has been replaced by a dispersion compensated fiber (DCF) that provides a wider bandwidth of operation. A variable delay up to 840 ps has been obtained using dual-pump FWM that offers a conversion bandwidth of about 40 nm. The large conversion range helps to minimize GVD-induced pulse distortion as a shorter DCF can be used for a given delay. The Bi-NLF provides an enhanced stimulated Brillouin scattering (SBS) threshold, a reduced latency, and an increased compactness of the approach for practical applications.     

Self-Pumping Wavelength Conversion for DPSK Signals and DQPSK Generation Through Four-Wave Mixing in Highly Nonlinear Optical Fiber

A novel scheme is proposed to achieve self-pumping wavelength conversion for two differential phase-shift keying (DPSK) signals at different wavelengths through four-wave mixing (FWM) effect in a highly nonlinear optical fiber. By changing the phase modulation depths to pi/2 for both of the DPSK signals, the two signals can be multiplexed to generate a differential quadrature phase-shift keying signal. The simulations and experimental results demonstrate the feasibility of phase manipulations for phase-shift keying signals through the FWM process     

色散管理 WDM 系统中简并 FWM 噪声的研究

在入射脉冲为啁啾高斯脉冲条件下,考虑到各信道内比特序列初始相位和比特模式的随机性,建立了啁啾高斯脉冲色散管理波分复用系统中的简并四波混频噪声的半解析理论计算模型。并详细推导了啁啾高斯脉冲在色散管理光纤链路中的脉冲振幅表达式。色散图由两段群速度色散绝对值大小相等、符号相反且长度可变的光纤构成。通过对此模型仿真计算得到：脉冲特征宽度对 FWM 噪声标准差的影响比较明显,FWM 噪声标准差值随脉冲特征宽度的增加而增加;当脉冲特征宽度较小时,啁啾参数值对 FWM 效应产生较大的影响,优化啁啾参数值可进一步降低 FWM 噪声标准差值;优化的色散管理方案可以使得噪声标准差有较小的值;对局部群速度色散系数绝对值进行优化,可得到较小噪声标准差值。     

Four-Wave Mixing Between Coherent Signal and Incoherent Pump Light in Nonlinear Fiber

We report an experimental and theoretical investigation of the four-wave mixing (FWM) process between a coherent signal light and an incoherent pump light in a highly nonlinear fiber. In the experiment, two main phenomena are observed: 1) the peak power of the spectrum of the coherent signal light decreases along the fiber rather than be amplified; 2) the spectrum of the coherent signal is broadened. A theory of the FWM process between incoherent pump and coherent signal is proposed by considering the incoherence of the pump which has amplitude and phase fluctuations. The theoretical results agree with the experimental results.     

Four-Wave Mixing Crosstalk Measurement in a Highly Doped L-Band Erbium-Doped Fiber Amplifier by Using Half of the Signal Channels

We propose a four-wave mixing (FWM) crosstalk measurement method for a signal with 50 GHz spaced wavelength channels in a highly doped L-band erbium-doped fiber amplifier (EDFA). The method uses the crosstalk measured by using a signal with 100 GHz spaced wavelength channels. This approach is based on analyses of the channel spacing dependence of FWM signal generation and the effect of the gain evolution of each signal channel along the EDF during FWM signal generation at a specific frequency. It is shown that reducing the channel spacing from 100 to 50 GHz has little impact on the efficiency of FWM generation if the gain evolution is the dominant term in FWM generation. It is also shown that the contributing ratio of the gain evolution of each channel is almost the same for the two signals. These results enable us to develop a measurement method for estimating the FWM crosstalk in an L-band EDFA applied to a wavelength-division multiplexing system with a 50 GHz spaced signal by using a 100 GHz spaced signal and a compensation factor. The validity of the measurement method is successfully demonstrated experimentally.      

Performance Enhancement of Multiwavelength Conversion of RZ-DPSK Based on Four-Wave Mixing in Semiconductor Optical Amplifier

In this letter, we propose two methods to enhance the performance of multichannel wavelength conversion of return-to-zero differential phase-shift-keying signal in a semiconductor optical amplifier (SOA) based on four-wave-mixing effect. The first one is optimizing the relative time delay of the multichannel signal to reduce the crosstalk-induced penalty in SOA. The second one is using the synchronous clock pumping to raise the power of converted signal. The experiment on wavelength conversion of two channels at 10-Gb/s signal showed the power penalty was improved more than 1 dB in the first method and about 1.5 dB in the second method.     

Highly Functional All-Optical Control Using Ultrafast Nonlinear Effects in Optical Fibers

Using the optical fibers and ultrashort pulses, we can demonstrate highly functional optical control. We have succeeded in generation of low-noise, ultraflat, high-quality supercontinuum (SC). Recently, octave spanning high-quality SC is also generated. The carrier envelope offset frequency measurement and ultrahigh resolution optical coherence tomography are demonstrated using SC. We can also generate widely wavelength-tunable ultrashort soliton pulses using soliton self-frequency shift (SSFS). So far, several new techniques have been proposed and demonstrated using SSFS. The author has also discovered novel two-pulse trapping phenomena. The ultrafast all-optical switching is demonstrated using the pulse trapping techniques, and principles and characteristics are described.      

密集波分复用系统中四波混频影响的研究

掺铒光纤放大器（EDFA）的引入带来了许多非线性效应,其中四波混频（FWM）现象对密集波分复用（DWDM）系统造成了较大的不利影响,必须采取相应的措施予以克服。文中首先分析了FWM产生的机理和影响混频效率的主要因素,然后研究了FWM对DWDM系统性能的影响,最后提出了几种克服FWM对DWDM系统性能影响的方法。     

基于SOA双泵浦FWM全光波长变换的研究

本文报道了半导体光放大器(SOA)在偏振方向相互垂直的双泵浦光作用下,四波混频(FWM)效应实现的全光波长变换实验,其变换效率和信噪比在55nm的调谐范围内较高并且基本稳定.     

Dual-Wavelength Mode-Locked Erbium-Doped Fiber Ring Laser Using Highly Nonlinear Fiber

We demonstrate a stable dual-wavelength actively mode-locked erbium-doped fiber laser operating at 20 GHz. A highly nonlinear fiber is used to compensate the frequency chirp caused by regular single-mode fiber in the optical cavity. Dual-wavelength pulse trains with pulsewidths $sim$1.4 ps are obtained.      

All-Optical Chromatic Dispersion Monitoring for Phase-Modulated Signals Utilizing Cross-Phase Modulation in a Highly Nonlinear Fiber

We propose and experimentally demonstrate an all-optical chromatic dispersion (CD) monitoring technique for phase-modulated signals utilizing the cross-phase-modulation effect between the input signal and the inserted continuous-wave probe. The probe's optical spectrum changes with the accumulated CD on the input signal, indicating that the optical power variations can be measured for monitoring. The experimental results show that this technique can monitor up to 120 ps/nm of CD for a 40-Gb/s return-to-zero differential phase-shift keying (RZ-DPSK) transmission system, with the maximum measured optical power increment of 16.5 dB. The applicability of this monitoring technique to higher bit-rate phase-modulated signals, such as 80-Gb/s RZ differential quadrature phase-shift keying and 80-Gb/s polarization-multiplexed RZ-DPSK, is also investigated via simulation.      

基于半导体光放大器的四波混频效应对正交频分复用光信号进行全光波长变换

实验研究了基于半导体光放大器(SOA)的四波混频(FWM)效应的单抽运光正交频分复用(OFDM)信号的波长变换系统.信号光源和抽运光源分别由两个不同输出波长的可调分布反馈式激光器(DFB-LD)产生.信号光源经2.5 Gb/s OFDM的电信号直接调制后再和抽运光源耦合,经光放大器后在SOA实现波长变换.实验结果显示,耦合信号经SOA四波混频效应后,产生新波长的信号光将携带OFDM信号,且转换效率与信号光和抽运光的功率、波长以及两者的偏振夹角有关.同时也测量了转换的OFDM信号的功率-误码曲线和接收星座图.     

Two-Stage SPM-Based All-Optical 2R Regeneration by Bidirectional Use of a Highly Nonlinear Fiber

Two-stage all-optical 2R (reamplification and reshaping) signal regeneration based on spectrum broadening due to self-phase modulation in a nonlinear fiber and subsequent off-centered filtering is demonstrated by the use of only one fiber spool in which the signal is transmitted twice in opposite directions. The two-stage configuration allows wavelength shift-free operation of the regenerator. Recirculating-loop signal transmission and regeneration experiment shows that the bidirectional 2R regenerator extends transmission distance by a factor more than two for an unequally-spaced 40 Gb/s short-pulse train with minimum pulse separation of 12.5 ps. Numerical simulation for assessing the influence of Rayleigh backscattering in the bidirectional configuration is performed, which shows that although some influence of Rayleigh backscattering can appear at higher operation speeds such as 80 Gb/s, strong noise reduction is still achievable by the bidirectional two-stage regeneration.      

光纤中的四波混频及其受色散的影响

讨论了四波混频的效率及色散对四波混频的影响，并推导了四波混频光的效率公式。另外，在光纤线路中由于每段光纤的零色散波长略有不同，四波混频的发生与有均匀零色散波长光纤中的不同，本文对这种情况下的四波混频也进行了分析研究。     

Pulse Pedestal Suppression Using Four-Wave Mixing in an SOA

Experimental results are presented demonstrating how four-wave mixing in a semiconductor optical amplifier can be used to remove pulse pedestals introduced due to nonlinearities which occur upon pulse propagation in an optical system. Such pedestals would degrade the performance of an optical time-division-multiplexed system due to coherent interaction between channels. An improvement of the temporal pulse suppression ratio to greater than 30 dB is achieved regardless of the level of the pulse pedestal on the input signal. This improvement takes place simultaneously with wavelength conversion and compression of the optical pulse.     

Four-Wave Mixing of Pulsed Signal in Dispersion-Shifted Fiber With Pump Depletion

The output signal and idler pulses generated by dispersion-shifted-fiber-based four-wave mixing (FWM) with pulsed signal input are analyzed. A set of analytical expressions for the amplitude and phase of the output pulses is derived. Analytical analysis shows that the generated idler pulse is far away from the phase conjugation of the input signal, and that the output signal is not an amplified replica of the input signal. These phenomena are due to both the phase distortion caused by self- and cross-phase modulation and the temporal envelope broadening resulting from pump depletion. Besides, in the frequency domain, the spectral inversion between signal/idler will be destroyed. The implications of the phase and shape distortions in all-optical signal processing by FWM are discussed. The induced phase distortions of idler pulses in dual pumps case are found to be much smaller than those of the single-pump case.     

光纤无线通信系统中用高非线性光纤实现全光频率上转换

研究了一种基于高非线件光纤(HNLF)中交叉相位调制效应的全光频率上转换射频耦合到光纤无线通信(ROF)系统.数值计算结果表明,由于交叉相化调制引起的调制不稳定性.波长1.54μm,重复频率为40 GHz的抽运光可使波长为1.56μm,载有速率为2.5 Gbit/s的非归零码作为下行链路数据的弱信号光光波分裂,产生与载波距离为40 GHz且与载波相干的两个一阶调制边带,抽运光脉宽、抽运光功率和光纤长度对载波与边带功率差有较大影响.仿真实验结果证实了以上原理,速率为2.5 Gbit/s的数据信号在高非线性光纤中被上转换到40 GHz毫米波上.信号光功牢为0 dBm时,得到的优化光纤长度为600 m,抽运光功率为17 dBm.     

Polarization-Independent Optical Demultiplexing Using XPM-Induced Wavelength Shifting in Highly Nonlinear Fiber

Polarization-independent all-optical demultiplexing of 160-Gb/s optical time-division-multiplexed data based on cross-phase-modulation-induced wavelength shifting in highly nonlinear fiber (HNLF) has been experimentally demonstrated with a maximum power penalty of 0.8 dB due to signal polarization variations. The method uses a simple and easy-to-implement single-path polarization-diversity scheme by placing a short piece of polarization-maintaining fiber before the HNLF.     

Extending the Gain Bandwidth of Combined Raman-Parametric Fiber Amplifiers Using Highly Nonlinear Fiber

The authors demonstrate extended flat gain bandwidth by combining Raman and parametric processes. The increase in bandwidth is accomplished by extending the Raman gain region to the longer wavelength side of the Raman gain by using parametric gain. Two different configurations of the amplifier are investigated. The associated gain and bit error performance is characterized. A gain of about 15 dB, with gain flatness of 5 dB is measured.     

波分复用系统的四波混频特性研究

本文对波分复用（WDM）光纤通信系统中的四波混频（FWM）特性进行了研究，得到了WDM可用于由多段不同长度，不同色散值的光纤组成的多级放大WDM系统FWM效应的计算方法，进而分析了单级、多级放大和色散补偿这3种特例的FWM特性，研究结果表明，标准单模光纤与高负色散值色散移位光纤的组合可以有效地抑制FWM混频效应。