Performance of two-stage discrete fiber Raman amplifiers with and without all-optical gain clamping

Using numerical simulations, we analyze the properties of two-stage discrete fiber Raman amplifiers without and with all-optical gain clamping. In both cases a two-stage amplifier can be designed to have the same gain characteristics as a single-stage amplifier with improved noise performance by use of either the same total length of gain fiber (but with increased pump power) or total pump power (but with increased total length of gain fiber).     

The gain and noise figure of Yb-Er-codoped fiber amplifiers based on the temperature-dependent model

A novel temperature-dependent model for Yb³⁺-Er³⁺-codoped fiber amplifier (EYDFA) based on the energy transfer from Yb³⁺ to Er³⁺ is established. Using appropriate fiber and energy transfer parameters, the coupled rate equations is numerically solved at 25 and 40 °C. The pumping powers are 100 and 200 mW at a pump wavelength of 1060 nm. The signal gain and noise characteristics of a 0.3 m erbium/ytterbium co-doped fiber (EYDF) in a single-pass configuration are investigated by using 1, 10 and 100 μW signals at 1535 nm. A maximum signal gain of 40.5 dB and a corresponding noise figure of 3.65 dB at the temperature of 25 °C are achieved.     

不同掺杂浓度光纤拉曼增益特性的研究

研究了受激拉曼散射增益系数和增益谱与泵浦功率、光纤掺杂浓度的关系,得出增益系数和峰值频移量由光纤特性决定,而与泵浦功率无关的结论。在对二氧化硅分子建模计算后,进一步利用LM算法对光纤的拉曼增益谱进行高斯分解。分解图谱表明掺杂物破坏了二氧化硅的分子结构,并在拉曼增益范围内产生了新的峰值,从理论上证明了光纤掺杂物对拉曼增益系数和增益谱的影响。     

Efficient numerical method of freeform lens design for arbitrary irradiance shaping

A computational method to design a lens with a flat entrance surface and a freeform exit surface that can transform a collimated, generally non-uniform input beam into a beam with a desired irradiance distribution of arbitrary shape is presented. The methodology is based on non-linear elliptic partial differential equations, known as Monge-Ampère PDEs. This paper describes an original numerical algorithm to solve this problem by applying the Gauss-Seidel method with simplified boundary conditions. A joint MATLAB-ZEMAX environment is used to implement and verify the method. To prove the efficiency of the proposed approach, an exemplary study where the designed lens is faced with the challenging illumination task is shown. An analysis of solution stability, iteration-to-iteration ray mapping evolution (attached in video format), depth of focus and non-zero étendue efficiency is performed.     

Analytical formula for the transient response of erbium-doped fiber amplifiers

In wavelength-division-multiplexing systems with optical amplifiers, channel drop or add can cause power changes in the existing channels. We present an analytical formula for the transient power of the existing channels as a function of time when one or more input channel is dropped or added on an erbium-doped fiber amplifier (EDFA). Although this formula was originally derived from perturbation theory of single-stage EDFA's, it is a good approximation of large-power excursion cases as well as two-stage EDFA's.     

FRA中增益平坦与抽运光数目的数值模拟与优化

采用神经网络的方法,利用其对于输入、输出参量已知,但解析关系未知的函数具有良好逼近能力的这一特点,构造了双层(2-5-1 型)反向传播(BP)神经网络。利用数值优化技术的 LM 算法训练该网络,获得了宽带拉曼光纤放大器(FRA)中增益带宽、增益平坦度与抽运光数目配置的非线性依赖关系。模拟结果表明:在增益平坦度确定的条件下,不同增益带宽处,抽运光数目必存在下限值。     

地铁车致隧道与土体振动的高效时-频混合预测方法

提出了一种地铁列车引起的隧道与土体振动的高效时-频混合预测方法。该方法基于两步法开展。通过建立车辆-轨道-隧道-土体2D多体动力学/有限元模型,在时域中获取轨-隧相互作用力;将转化后的作用力施加在2.5D有限元-最佳匹配层模型上,在频域-波数域中求解隧道-土体系统的动力响应。同时,为提高频域-波数域计算效率,提出了波数范围随激励频率变化的高效波数采样方案。数值算例中,依次通过与时域3D有限元方法和传统固定波数域采样方案对比,对所提方法的准确性和高效性进行了验证。结果表明,采用时-频混合方法计算的观测点竖向振动速度时程曲线在波形和幅值上与时域3D有限元方法计算结果吻合较好。就振动速度级而言,时-频混合方法的计算结果略小于时域方法,但两种方法计算结果的相对误差小于3%。同时,所提出的高效波数采样方案能较好地预测不同频率荷载下的有效波数范围。除2 Hz以下低频振动外,高效波数采样方案与传统采样方案计算的观测点竖向振动速度1/3倍频曲线几乎吻合。就计算效率而言,高效波数采样方案计算时间为固定波数域采样方案计算时间的3/5。     

A numerical approach for predicting the failure locus of fiber reinforced composites under combined transverse compression and axial tension

A computational model based on the finite element method is presented for the estimation of strength of a fiber-reinforced lamina subjected to a combination of the transverse compression and axial tension. A complex damage mechanism including fiber breakage, fiber/matrix debonding and matrix plastic deformation is reproduced in the proposed model by using appropriate constitutive equations. The numerical simulation of mechanical response of the unidirectional lamina under biaxial loading is used to obtained the failure locus. Subsequently, the model is verified against an analytical solution and experimental data. It was found that the numerical calculations agree better with experimental results than analytical predictions.     

Measurement of gain tilt and its contribution to composite second-order distortion in the analog-digital transmission systems with Er/Yb fiber amplifiers

Fiber-optic-based analog or digital community antenna television systems experience composite second-order (CSO) distortion caused by the interaction between the gain tilt of the doped fiber amplifier and the laser chirp due to modulation. The gain tilt for an analog-digital transmission system with a high-power erbium/ytterbium-codoped fiber amplifier has been experimentally measured and its contribution to the CSO distortion of the system is evaluated. The results are in good agreement when compared with the direct measurement of the CSO distortion of the transmission system with and without the amplifier. The dependence of the gain tilt on the modulation frequency and wavelength of the input light is also investigated.     

New distortion measurement method for booster amplifiers

The usual method of distortion measurement of an amplifier in a feedback loop involves breaking the loop, which may change the dc and (nonlinear) interstage loading conditions. A new method is presented and proven with theory and experiment, permitting distortion measurement of a final or booster stage within an amplifier. The new method does not require breaking of the feedback loop. Measurements are carried out with linear and nonlinear loudspeaker load     

High-power 980-nm pump laser modules for erbium-doped fiber amplifiers

High-power fiber-coupled pump modules with an emission wavelength of 980 nm and output powers in excess of 200 mW, as measured at the fiber end, have been fabricated. Laser-to-fiber coupling efficiencies in excess of 77% are observed to be in excellent agreement with a mathematical model. Also, the output power stability of these modules has been monitored during severe temperature cycling, and the relative power variation was less than 4% for all the tested modules.     

Optical fiber probe for biomedical Raman spectroscopy

In vitro experiments have demonstrated the ability of Raman spectroscopy to diagnose a wide variety of diseases. Recent in vivo investigations performed with optical fiber probes were promising but generally limited to easily accessible organs, often requiring relatively long collection times. We have implemented an optical design strategy to utilize system throughput fully by characterizing the Raman distribution from tissue. This scheme optimizes collection efficiency, minimizes noise, and has resulted in small-diameter, highly efficient Raman probes that are capable of collecting high-quality data in 1 s. Performance has been tested through simulations and experiments with tissue models and several in vitro tissue types, demonstrating that this new design can advance Raman spectroscopy as a clinically practical technique.     

Efficient techniques for predicting viscoelastic behavior of sublaminates

Even for linear elastic behavior, stress analysis of thick laminated composites can be very computation intensive if every lamina is modeled discretely. In such cases, modeling of individual lamina is impractical and the homogenization method for sublaminates becomes essential. In the current work, 3D homogenization formulas for an elastic sublaminate, which were derived by the authors in previous work, were utilized to determine the 3D effective properties for a viscoelastic sublaminate. The properties were determined by three methods that exploited the 3D elastic homogenization formulas: (i) quasi-elastic method, (ii) correspondence principle, and (iii) direct time integration of the incremental viscoelastic equations. The finite element method with discrete modeling of the plies was used to obtain reference solutions. The effective viscoelastic properties obtained using the three methods based on the elastic homogenization formulas were in very good agreement with the reference solution. Among these methods, the quasi-elastic method was found to be both accurate and the simplest method in determining the effective properties. The methods were also used to predict the stress response of a sublaminate to different strain histories. The direct time integration method using the 3D elastic homogenization formulas performs accurately and efficiently for this problem.     

用打靶法求解双向泵浦的拉曼放大器传输方程

提出了用打靶法计算双向泵浦的拉曼放大器传输方程的算法。该算法不仅可以求解前向泵浦、背向泵浦的拉曼放大器传输方程,也可以计算双向泵浦的拉曼放大器传输方程。该算法利用了先猜测,再修正的逐渐逼近的方法来求解传输方程。利用该算法,对用三波长拉曼光纤激光器泵浦的拉曼放大器进行了优化设计,在波长分别为 1427nm,1445nm 和 1466nm,功率分别为500mW,120mW,400mW 时,获得了 1dB 带宽约为 40nm 的结果,并用试验进行了验证。     

受激拉曼增益激光雷达测量CO2气体

激光雷达探测大气中的CO2气体目前还处在探索阶段,在差分吸收原理基础上,利用气体的受激拉曼散射增益效应,设计出测量CO2气体的非线性拉曼激光雷达系统,并模拟实验得到了增益开关两种状态下的光子计数信号,从而反演出CO2气体的浓度分布结果,在垂直方向上,CO2气体浓度随距离变化不大,合肥地区的浓度含量大约在400ppm左右。     

Fast analysis method for polarization-dependent performance of a concave diffraction grating with total-internal-reflection facets

A fast simulation method for a waveguide-based concave grating with total-internal-reflection (TIR) facets is presented using the Kirchhoff-Huygens principle. Unlike the conventional scalar method, modifications are made to take into account the influence of the Goos-H?nchen (GH) shift. The simple method is in good agreement with a numerical method based on rigorous coupled-wave analysis for a wide range of practical device parameters and can provide an insightful physical explanation for the numerical results. It is shown that the GH shift is a main contributing factor for the loss and the polarization-dependent loss of an etched diffraction grating demultiplexer with TIR facets.     

Micro-optical fiber probe for use in an intravascular Raman endoscope

We believe that we have developed the narrowest optical-fiber Raman probe ever reported, 600 microm in total diameter, that can be inserted into coronary arteries. The selection of suitable optical fibers, filters, and a processing method is discussed. Custom-made filters attached to the front end of a probe eliminate the background Raman signals of the optical fiber itself. The experimental evaluation of various optical fibers is carried out for the selection of suitable fibers. Measurement of the Raman spectra of an atherosclerotic lesion of a rabbit artery in vitro demonstrates the excellent performance of the micro-Raman probe.     

Modeling and optimization of add-drop dynamics in gain-clamped fiber amplifiers

A new dynamic model for gain-clamped fiber amplifiers is presented. The model can simulate the transient behavior of a gain-clamped erbium-doped fiber amplifier as a function of its controlling parameters. Results show the detailed dependence of the amplifier gain dynamics on launched pump power and reveal the pump-power penalty incurred in implementing gain control with minimal transients to power disturbances. The effect on the transients of varying the laser cavity feedback coupling is also presented. Model accuracy is verified by the excellent quantitative agreement with dynamic measurements over a range of operating conditions. Results show how an increase in pump power can effectively suppress both the dynamic and the steady-state signal power transients in wavelength-division multiplexing channel add-drop operations.