Improvement of the fiber raman soliton laser for femtosecond optical pulse generation

Femtosecond optical pulses generated from a synchronously pumped fiber Raman soliton laser (FRASL) have been shown to have large excess noise and high background light (i.e., the pedestal) levels. In this paper, to improve the FRASL, the operation characteristics of the FRASL are investigated both theoretically and experimentally. It is shown that real femtosecond soliton oscillation in the FRASL can be obtained only when the soliton self-frequency shift (SSFS) effect in the fibers is suppressed and proper choices of both the Stokes oscillation wavelength and the pump power level are required for the SSFS suppression in the FRASL. By using a tunable all-fiber Raman ring laser, optical pulses as short as 400 fs with a low white AM noise level of -120 dBc / Hz have been generated from the compact FRASL with SSFS suppression. Based on the theoretical analyses, we propose to use an intracavity saturable absorber to prevent the generation of high-level Stokes background light in the FRASL, and the feasibility of this method is shown by numerical simulations.     

Broadly wavelength- and pulse width-tunable high-repetition rate light pulses from soliton self-frequency shifting photonic crystal fiber integrated with a frequency doubling crystal

Soliton self-frequency shift (SSFS) in a photonic crystal fiber (PCF) pumped by a long-cavity mode-locked Cr:forsterite laser is integrated with second harmonic generation (SHG) in a nonlinear crystal to generate ultrashort light pulses tunable within the range of wavelengths from 680 to 1800?nm at a repetition rate of 20?MHz. The pulse width of the second harmonic output is tuned from 70 to 600?fs by varying the thickness of the nonlinear crystal, beam-focusing geometry, and the wavelength of the soliton PCF output. Wavelength-tunable pulses generated through a combination of SSFS and SHG are ideally suited for coherent Raman microspectroscopy at high repetition rates, as verified by experiments on synthetic diamond and polystyrene films.     

拉曼增益对高双折射光纤中暗孤子俘获的影响

从高双折射光纤中含有拉曼增益的耦合非线性薛定谔方程出发, 利用拉格朗日方法, 推导出了暗孤子俘获的阈值, 并利用快速分步傅里叶变换, 模拟了孤子的两个正交偏振分量的演化, 对比了两种方法得到的阈值, 探究了暗孤子俘获受拉曼增益的影响. 研究发现解析解所得阈值比数值解偏小, 且群速度失配越小时, 二者符合得越好; 并且拉曼增益减小了暗孤子的俘获阈值, 当平行拉曼增益增大时, 俘获阈值减小加快.     

Adiabatic, closed-form approach to the highly efficient analysis of a fiber Raman amplifier problem

We propose a novel framework for the solution of a general fiber Raman amplifier problem by use of a closed integral form of a Raman equation. Treating the given problem as an adiabatic system and taking the Raman process as the perturbation parameter, we can seek the solution along the iteration axis rather than the fiber propagation axis, permitting an orders-of-magnitude increase for the product of convergence speed and spatial resolution in the numerical assessment.     

Evolution of the nth Stokes-wave due to stimulated Raman scattering in single-mode fibers

The evolution of the nth analytical solutions of commonly used Raman equations, analyzed by numerical simulation and experimentally, is reported. In the experiment, a 1 km undoped single-mode fiber was pumped with an ytterbium doped fiber laser system (FL) in continuous wave regime at 1064 nm in a free running configuration. We showed that it is possible to obtain up to the nth power thresholds and the maximum power for each Stokes wave by using compact analytical solutions as a first approximation in a simple, quick process.     

Cancellation of Raman self-frequency shift for compression of optical pulses

We study to which extent a fiber soliton can be manipulated by a specially chosen continuous pump wave. A group velocity matched pump scatters at the soliton, which is compressed due to the energy/momentum transfer. As the pump scattering is very sensitive to the velocity matching condition, soliton compression is quickly destroyed by the soliton self-frequency shift (SSFS). This is especially true for ultrashort pulses: SSFS inevitably impairs the degree of compression. We demonstrate numerically that soliton enhancement can be restored to some extent and the compressed soliton can be stabilized, provided that SSFS is canceled by a second pump wave. Still the available compression degree is considerably smaller than that in the Raman-free nonlinear fibers.     

Optimizational 6-bit all-optical quantization with soliton self-frequency shift and pre-chirp spectral compression techniques based on photonic crystal fiber

In this paper, we optimize a proposed all-optical quantization scheme based on soliton self-frequency shift(SSFS)and pre-chirp spectral compression techniques. A 10m-long high-nonlinear photonic crystal fiber(PCF) is used as an SSFS medium relevant to the power of the sampled optical pulses. Furthermore, a 10m-long dispersion flattened hybrid cladding hexagonal-octagonal PCF(6/8-PCF) is utilized as a spectral compression medium to further enhance the resolution. Simulation results show that 6-bit quantization resolution is still obtained when a 100m-long dispersion-increasing fiber(DIF)is replaced by a 6/8-PCF in spectral compression module.     

低双折射光纤中拉曼增益对光偏振态的影响

利用包含拉曼增益的耦合非线性薛定谔方程, 导出了斯托克斯参量表示的含拉曼增益的杜芬方程, 通过椭圆积分求出了杜芬方程的解析解, 重点分析了光波在低双折射光纤中传播时, 拉曼增益对其偏振态演化的影响结果表明当输入功率与运动常量满足一定关系时, 拉曼增益改变了光波传输时其偏振态演化周期.     

拉曼增益系数可分离光纤中的受激拉曼散射

给出了拉曼增益系数分离光纤的定义,在考虑了N个单色光之间的两两受激拉曼散射,得到了这种光纤中单向传输的N个不同的单色光之间的受激拉曼散射（SRS）稳态耦合波方程的解析解。它适用于各单色光功率任意大小、光频率分布不均匀时的一般情况。这一结果可应用于光纤通信系统、拉曼光纤放大器和拉曼光纤激光器的设计与分析。最后把解析解与数值解进行了比较,两者取得了很好的一致。     

Adiabatic Soliton Compresion in an Exponentially Dispersion-Decreasing Fiber Amplifier

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Numerical simulation of propagation of a femtosecond optical soliton in a single-mode fiber with allowance for the imaginary part of Raman response

We consider the effect of Raman inertial response of a medium on the stability of a first-order femtosecond soliton. Numerical solution to the high-order nonlinear Schrödinger equation, with the complex Raman term, describing propagation of a femtosecond optical soliton in a single-mode fiber, is obtained. It is shown that a soliton solution of the high-order nonlinear Schrödinger equation exists under certain conditions imposed on the equation coefficients. These conditions lead to limitations on the wavelength, fiber type, and the highest energy. Results of numerical solutions are in agreement with available experimental data.     

Supercontinuum generation in the irregular point of hollow-core photonic crystal fiber

Broadband supercontinuums (SC) are generated by soliton self-frequency shift (SSFS) of hollow-core photonic crystal fiber (HC-PCF) in our laboratory. With the pump works at 810 nm when the pump power increase from 400 to 600 mW, the Raman Soliton shifts from 2089 to 2215 nm, the bandwidth of SC increases from 2213 to 2320 nm. The ultra-violet part of SC is below 180 nm, and the mid-infrared part of SC exceeds 2500 nm. Moreover, the influence of pump power on SC is also analyzed.     

拉曼增益对孤子传输特性的影响

利用考虑拉曼增益效应的非线性薛定谔方程, 在忽略光纤损耗的情况下, 采用基于MATLAB的分步傅里叶数值算法, 得出线性算符和非线性算符具体的表达式, 分步作用于光孤子脉冲传输方程, 仿真模拟了光孤子在光纤中传输时的演变. 与不考虑拉曼增益的光孤子在光纤中传输相对比, 探析了拉曼增益对孤子传输特性的影响.拉曼增益会破坏孤子的传输周期, 导致孤子在光纤中传输时快速衰减, 并且影响程度和输入孤子的脉冲峰值功率大小有关, 拉曼增益对基态孤子和高阶孤子的影响也不相同. 关键词： 拉曼增益 孤子 对称分步傅里叶法 非线性薛定谔方程     

快速数值差分递推改进算法在超宽连续谱模拟中的应用研究

对快速数值差分递推公式进行了改进,使之能够求解带有脉冲自陡峭项和脉冲内喇曼散射项的非线性薛定谔方程.通过与传统孤子解析结果及分步傅里叶方法数值结果的对比分析表明,快速数值差分递推改进算法是一种快速而准确的数值计算方法,它不仅能够同步考虑光学媒质中的群速色散作用和非线性克尔作用,而且将所有高阶非线性项对光脉冲传输的影响也...     

在同种光纤中实现RFA增益平坦化的方法

提出了一种新的光纤拉曼放大器设计方法:在同一种光纤内运用不同波长的泵浦光源达到前增益后补偿的目的,最终实现在放大器输出端信号光增益谱平坦化。通过对拉曼增益谱前后沿做线性化处理,化简稳态SRS耦合波方程的解析解,最终在输出端得到了一个固定的功率输出值。模拟结果表明:所设计的放大器具有增益平坦度好、增益高的优点,且设计方法简单。该方案为增益平坦化的拉曼光纤放大器设计提供了一种新的思路。     

Polarization-dependent efficient Cherenkov radiation at visible wavelengths in hollow-core photonic crystal fiber cladding

Efficient Cherenkov radiation (CR) is experimentally generated by a soliton self-frequency shift (SSFS) in a knot of hollow-core photonic crystal fiber (HC-PCF). When the angle of the half-wave plate is rotated from 0° to 45° , the Raman soliton shifts from 2227 to 2300 nm, the output power of the CR increases 8.15 times, and the maximum output power ratio of the CR at 556 nm to the residual pump is estimated to be 20:1. The width of the output optical spectrum at visible wavelengths broadens from 25 to 45 nm, and the conversion efficiency of the CR can be above 28%. Moreover, the influences of the pump polarization and wavelength on the CR are studied, and the corresponding nonlinear processes are discussed.     

High power linearly polarized Raman fiber laser at 1120 nm

An all-fiber linearly polarized Raman fiber laser at 1120 nm is demonstrated. With a 1 070-nm linearly polarized Yb-doped fiber laser as pump source, an output of up to 7.7 W at 1120 nm is obtained with an optical efficiency of 55%. The polarization extinction ratio of the linearly polarized Raman fiber is higher than 18 dB. A numerical simulation model is developed to determine the Raman coefficient of the gain fiber and to evaluate the laser performance. The spectral isolation between the Raman fiber laser and the pump fiber laser is determined to be necessary for further improvements of performance.     

一种增益平坦的光子晶体拉曼光纤放大器

为解决传统拉曼放大器增益系数低和增益不平坦的问题,采用级联光子晶体光纤的设计方法设计了一种增益平坦的拉曼光纤放大器.采用受激拉曼散射效应的稳态分析理论,分析了光子晶体光纤的拉曼增益谱,建立了拉曼放大器的理论模型.通过解耦合方程,推导了实现增益平坦的约束条件,发现光纤长度和泵浦功率是影响拉曼光纤放大器增益平坦度的两个参数.仿真结果表明,在1 508～1 544 nm的带宽范围内,实现了一个增益高达21 dB,增益平坦度仅为0.14 dB的光子晶体拉曼光纤放大器,可在光纤通信系统应用中发挥重要作用.     

双重瑞利散射导致的多路干涉噪声对光纤喇曼放大器噪声系数的影响

在没有对喇曼增益分布做任何假设的情况下,利用所推导的光纤喇曼放大器噪声系数的解析表达式,研究了不同光纤长度、不同信号输入功率、不同增益、不同瑞利散射下双重瑞利散射导致的多路干涉噪声对喇曼放大器噪声系数的影响.结果表明随着光纤长度,信号输入功率、喇曼增益和瑞利散射的增加,多路干涉噪声对噪声系数的劣化越严重.但通过合理的优化光纤长度、控制喇曼增益可以取得噪声和增益间的最佳平衡.     

Effective simulation method for parametric signal-noise interaction in transmission fibers

We propose a new method for effective numerical simulation of transmission system performance and study of correlated noise evolution along an optical fiber with nonlinear parametric interaction between the amplified spontaneous emission (ASE) and the modulated optical signal. The method is based on an evaluation of the noise covariance matrix by using full nonlinear Schr?dinger equation (NLSE) and an analytical model for the optical receiver. Using extensive brute-force Monte Carlo simulation as a verification tool, we test the accuracy of the method and illustrate the analytical receiver model limitations in the case of moderate as well as substantial growth of non-Gaussian optical noise along the optical fiber transmission link.