光纤中受激拉曼散射连续谱研究

我们在实验中研究了自相位调制效应和受激四光子混频效应对受激拉曼散射光谱的影响。在较强的泵浦光和适当的入射条件下,得到九阶受激拉曼散射的级联 Stokes 连续平滑谱带,带宽3500cm~(-1),其强度可与530nm 泵浦光强相比拟。     

Performance evaluation of SCM-WDM communication in the presence of SRS induced crosstalk for different types of fiber

In this paper, the SRS-induced crosstalk has been evaluated in a SCM-WDM communication link at different modulation frequencies for various type of fibers. Results show that SRS-induced crosstalk dominates at low frequency. As the dispersion and effective area of fiber (A_eff) decreases, initially the crosstalk remains high and then it decreases with increase in modulation frequency. The present work shows that out of five different types of fibers, Standard Single Mode Fiber (SMF) has minimum crosstalk (−53 to −64 dB) and True Wave fiber (TW) has maximum crosstalk (−47 to −48 dB).     

Effect of four-wave mixing in WDM optical fibre systems

Wavelength division multiplexing (WDM) can both significantly enhance transmission capacity and provide more flexibility in optical network design. Through the use of erbium-doped fibre amplifiers (EDFAs), it is possible to build long-distance transparent optical transmission links without electrical regenerators. In such systems, fibre nonlinearities are likely to impose a transmission limit due to increased total interaction length. There are a number of optical nonlinear effects in optical fibres, such as stimulated Raman scattering (SRS), stimulated Brillouin scattering (SBS), carrier-induced phase modulation and four-wave mixing (FWM). Out of these SRS and FWM are the dominant effects. In this paper, an algorithm has been suggested to study the effect of FWM in the total system noise considering the combined effect of SRS and FWM in the presence of amplified spontaneous noise (ASE). It has been found from the study that to maximize the signal-to-noise ratio of the transmitted signal in a WDM system FWM noise needs to be reduced as this is the dominant noise factor.     

Effect of pulse walk off and data formats on stimulated Raman scattering in fiber optic communication systems

A novel numerical method has been presented here which is capable of taking into account pulse walk off effect and different data formats such as non return to zero (NRZ), return to zero (RZ) and optical Manchester while evaluating the effect of stimulated Raman scattering (SRS) in dense wavelength division multiplexed (DWDM)-based fiber optic communication systems. Power tilt because of SRS effect in the DWDM systems has been calculated at different data rates and data formats.     

Suppression of first order stimulated Raman scattering in erbium-doped fiber laser based LIDAR transmitters through induced bending loss

A model is presented for suppression of first-order stimulated Raman scattering (SRS) in pulsed Er-doped fiber amplifiers with wavelength-selective induced bending loss. The model estimates the effectiveness of SRS suppression through fiber coiling in a pulsed amplifier, considering both the single- and dual-clad fiber cases. The basic approach is to fit analytical expressions to the evolution of a quasi-CW signal in the amplifier. Then, assuming that SRS does not deplete the signal, exact analytical expressions for the propagation of the Stokes wave are determined, from which the SRS threshold is estimated. Finally, example calculations are presented, and the model is used to determine a fiber geometry for the suppression of SRS through fiber coiling.     

Performance comparison of wavelet packet transform based and conventional coherent optical OFDM transmission system

In this work, the performance of WPT-COOFDM system is investigated and compared to that of FFT-COOFDM system over a fiber link. Simulation results show that the longer length of wavelet filters achieve a better performance, whereas the complexity is higher. For different wavelet mother functions employed in WPT-COOFDM systems, the chromatic dispersion robust of John64E wavelet outperforms that of other wavelets, and which could be a viable alternative for coherent optical OFDM to be considered in short distance transmissions. The simulation results also show that most of the developed wavelet mother functions mainly for image processing are not suitable for COOFDM transmission for its sensitivity to chromatic dispersion.     

基于混合光纤放大器的波分复用光纤传输系统研究

在研究了入纤功率、光器件的插入损耗以及非归零码中1码出现的概率对系统误码率影响的基础上,提出了一种基于混合光纤放大器的波分复用(WDM)光纤传输系统设计方案,并对该系统的噪声特性进行了详细地研究。提出了一种抑制前向噪声的有效方法,实现了20路WDM信号320km无误码传输。试验结果对基于混合光纤放大器的波分复用光纤传输系统的设计具有一定的指导意义。     

Investigation of stimulated rotational Raman scattering of the high-power broadband laser with applied angular dispersion

Smoothing by spectral dispersion （SSD） leads to considerable improvement on laser-irradiation uniformity in far field for fusion lasers. Phase modulation in time and spectral angular dispersion （SAD） across the beam introduced by SSD will affect the stimulated rotational Raman scattering （SRRS） gain in the near field. This paper focuses on the influence of SAD on SRRS gain under different laser conditions. Results show that the SAD will aggravate the generation of SRRS when the laser initial additional phase is constant. On the contrary, the SAD can reduce the SRRS gain if appropriate SSD parameters are adopted when the laser initial additional phase is variable. SSD has a certain application prospect in SRRS suppression.     

Numerical analysis of stimulated Brillouin scattering in high-power double-clad fiber lasers

A theoretical analysis of stimulated Brillouin scattering (SBS) in linear cavity Yb³⁺-doped double-clad fiber lasers is presented by solving the steady-state rate equations with the SBS. The effects of cavity length, fiber core diameter, input mirror reflectivity at Stokes wavelength, Yb³⁺ concentration and laser linewidth on the SBS are discussed. Numerical results show that the SBS threshold power can be improved by shortening the cavity length, using large mode area fiber, reducing the input mirror reflectivity at Stokes wavelength, lowering the Yb³⁺ concentration and broadening the laser linewidth, and the influence of the laser linewidth on the SBS threshold power is more noticeable than other system parameters.     

Impact of 2OD and 3OD on SRS- and XPM-induced crosstalk in SCM-WDM optical transmission link

In this paper, the impact of second-order dispersion (2OD), third-order dispersion (3OD) and modulation frequency over stimulated Raman scattering (SRS)- and cross-phase modulation (XPM)-induced crosstalk in sub-carrier-multiplexed (SCM) wavelength division multiplexed (WDM) transmission link has been analyzed. It has been observed that there is significant effect of 2OD, 3OD and modulation frequency on the SRS- and XPM-induced crosstalk in a SCM-WDM transmission link. Here the results for SRS- and XPM-induced crosstalk have been reported with independent and combined higher-order dispersion. It has been observed that XPM-induced crosstalk lies between [−52.8 to −45.3] and [−94.7 to −78.6] dB in the presence of 2OD and 3OD respectively for modulation frequencies varied from 500 MHz to 2.0 GHz, while it is in the range of [−94.4 to −84] and [−128.5 to −117] dB when both SRS and XPM are taken into consideration.     

Performance comparison of wavelength shift keying WDM system and conventional on-off WDM system in presence of four-wave mixing

Performance of wavelength shift keying (WSK) wavelength division multiplexed (WDM) system is analyzed and compared to that of conventional on-off keying (OOK) WDM system in respect of four-wave mixing (FWM) effect. WSK technique employs symmetric wavelength assignment and balanced detection to cancel FWM interference to first order. WSK–WDM outperforms conventional OOK–WDM at all signal levels and permits higher allowable input power, lower power penalty and higher transmission distance for a given bit-error rate of 10⁻⁹. WSK–WDM is also found to be noise resistant.     

Effect of a fluorescing impurity on the characteristics of stimulated Raman scattering from acetone

Asymmetry in the intensities between backward and forward stimulated Raman scattered radiations (BSRS and FSRS respectively) in acetone is investigated in the presence of a fluorescing impurity (rhodamine 6G). In the case of pure acetone above a threshold pump power, BSRS becomes more intense than FSRS. On the other hand intensity of BSRS decreases with concentration of the fluorophore in solution, while that of FSRS is found to increase. It is observed that absorption saturation also affects the asymmetry between FSRS and BSRS.     

Transmission of 40 Gb/s × 8 WDM channels with 100 GHz spacing using short-period dispersion-managed fiber

We report on the 320 Gb/s (40 Gb/s × 8 channels) WDM transmission experiment using short-period dispersion-managed fiber. After 320-km transmission, the average Q-factor was measured to be better than 20.4 dB. In addition, we compared the performances of various types of fibers in 40-Gb/s based systems. The results show that the SPDMF was robust to both the intra-channel and inter-channel nonlinearities due to its small average dispersion and large local dispersion.     

Numerical analysis of multi-wavelength cascaded Raman fiber lasers based on genetic algorithm

A novel numerical method is presented for the calculations of the coupled propagation equations in multi-wavelength Raman fiber lasers. By taking the advantages of genetic algorithm and shooting method, only a few of the best individuals at each generation are chosen to implement several shootings in order to accelerate theirs converging. The output characteristics of an all-fiber three-wavelength Raman fiber laser have been analyzed based on the proposed algorithm. Results show that the total output power linearly depends on the pump power with a slope efficiency of ∼51%. For the three output Stokes, the slope efficiencies of the longer wavelengths are larger than that of the shorter ones because the optical energy at the Stokes with shorter wavelengths allows for a transfer of optical power to the longer Stokes via stimulated Raman scattering. We also find that the total output power degrades by less than 10% by adjusting the output-coupler reflectivity and is insensitive to the variation of the Raman fiber length over a large range.     

Numerical analysis of stimulated inelastic scatterings in ytterbium-doped double-clad fiber amplifier with multi-ns-duration and multi-hundred-kW peak-power output

We investigate the stimulated Brillouin scattering (SBS) and the stimulated Raman scattering (SRS) in an ytterbium-doped double-clad fiber amplifier that outputs optical pulses with multi-ns-duration and multi-hundred-kW peak-power. The ytterbium-doped double-clad fiber amplifier is simulated by a model which is composed of a set of propagation-rate equations. The simulated results show that SBS and SRS will deform the output signal pulse in both the time domain and the spectral domain, and degrade the performance of the ytterbium-doped double-clad fiber amplifier seriously. It is shown in our simulation that the troublesome SBS can be effectively suppressed by broadening the signal linewidth to a critical value of 0.07 nm in our calculation, and the effect of SRS can be suppressed using a large-mode-area fiber with proper length. The model and the simulated results are very useful for designing an ultra-high-power ytterbium-doped double-clad fiber amplifier.     

Continuous-wave pumped, all-fiber optical parametric oscillator assisted by stimulated Raman scattering

A continuous-wave pumped, all-fiber optical parametric oscillator (OPO) around 1523 nm based on the mixing interaction of parametric process and stimulated Raman scattering (SRS) effects are investigated in this paper. We study and give the detailed analysis of the generation and output characteristics of all-fiber OPO with the influence of the SRS effects. Experimental results show that there exists the saturation output effect for this kind of OPO due to the coexistence of first order SRS effect, second order SRS and the optical parametric oscillation. The maximal output power of OPO is about 100 mW as the pump power reaches 1.4 W. Furthermore, a depolarized L-band super-continuum light source from 1570 nm to 1640 nm based on the combined interaction of parametric process and SRS effects can also be obtained.     

Pulse compression effect based on stimulated Brillouin scattering light storage in an optical fiber

The pulse compression effect in SBS light storage is numerically investigated. We demonstrate theoretically that the compressed width of retrieved data-pulses is not only related with temporal profile of data-pulse, but also spectrum and temple profile of control-pulse. The data-pulses with steep rising edge can be compressed after they are retrieved. The optimum compression effect takes place when a 2-ns-long exponential data-pulse interacts with a 1.5-ns-long chirped Gaussian control-pulse, where the data-pulse width is compressed to 1.73-ns-long and the readout efficiency is the biggest. A 2-ns-long rectangular data-pulse is also compressed to 1.73-ns-long when it encounters to a 1.5-ns-long rectangular control-pulse, but the readout efficiency isn’t maximum value. The results are significant for increasing storage capacity and variable bits all-optical buffering.     

Zero-broadening of the retrieved data-pulse based on stimulated Brillouin scattering light storage in an optical fiber

It was numerically demonstrated that a 2-ns-long Gaussian shaped and a 2-ns-long rectangular shaped data-pulse are fully retrieved as original via stimulated Brillouin scattering light storage process in an optical fiber. Zero-broadening without distortion data-pulse light storage is realized. In our study, for a given data-pulse, there are two conditions for it is fully retrieved: (1) the spectrum of the control-pulse is engineered so that the spectral width of the pulses is changed, making possible to match at best the data-pulse spectrum. (2) control-pulse temporal distribution profile must be suitable for the data-pulse. We also proved that the readout efficiency of completely retrieved data-pulse is the biggest. Because the Gaussian shaped data-pulse relative optical spectral bandwidth is the narrowest, it is easier to be retrieved.     

Improved analysis for SRS- and XPM-induced crosstalk in SCM-WDM transmission link in the presence of HOD

In this paper, the improved analysis for SRS- and XPM-induced crosstalk has been reported. The modified expression for XPM-induced crosstalk has been obtained and SRS- and XPM-induced crosstalks have been reported at varied walkoff parameter, modulation frequency, input optical power and transmission distance. It has been observed that there is exponent decrease in SRS-induced crosstalk with the increase in modulation frequency from 0 to 2.0 GHz. It varies with the increase in length and lie in the range of (−114 to −122.4) dB and (−115.5 to −124.4) dB at 20 and 100 km, respectively. Moreover, it increases exponentially with the increase in input optical power and lies in the range of (−121.6 to −130.6) dB at 10 mW and grows exponentially up to the range of (−114 to −122.8) dB at 60 mW optical powers at walkoff parameter of (13.6, 27.2, 54.4 and 81.6) ps/km. It has been observed that the XPM-induced crosstalk increases exponentially with the increase in transmission distance and modulation frequency for 2OD and 3OD. Furthermore, it has been found that the total SRS- and XPM-induced crosstalk rises exponentially with fluctuations with the increase in modulation frequency and transmission length in the presence of combined effect of 2OD and 3OD at varied walkoff parameters.     

The transmission performance degradation of the optical millimeter-wave signals by fiber chromatic dispersion

The influence of the fiber chromatic dispersion on double sideband (DSB), optical carrier suppression (OCS), and single sideband (SSB) optical mm-wave signals is investigated based on the Taylor expansion of the propagation constant and is verified by simulation. According to our theoretical results, the fading effect suppresses the signal power of the DSB optical mm-wave periodically in a cosine-like pattern, and it can be described by the zero-order Taylor expansion of the propagation constant. For the optical mm-wave with the signal modulated on two or more tones, the bit pulses of the mm-wave signal are distorted by the dispersion-inducing bit walk-off effect between tones, which is expressed by the first-order Taylor expansion of the propagation constant. Moreover, as the signal rate and the transmission distance are increased further, higher-order Taylor expansion of the propagation constant still degrades the optical mm-wave signal even if both the fading effect and the bit walk-off effect are eliminated completely. The distortion of the signal pulses of SSB optical mm-wave is derived based on the second-order Taylor expansion of the propagation constant. This degradation is verified by the simulation with the eye diagram evolution of the SSB optical mm-wave signal.