Theoretical analysis of multimode fiber amplifiers and lasers containing spatial filters

By the use of the coupling theory of the modes and the multimode rate equations, the performances of the multimode fiber amplifiers or lasers containing a spatial filter have been discussed. After the rate equations applicable to amplifiers are solved, the beam quality factors of the output from a multimode fiber amplifier with and without a single mode filter have been evaluated and compared. By the use of the analytical expression of the threshold pump power applicable to the multimode fiber lasers with the spatial filter and doped with quasi two-energy-level ions, and two example fibers had been selected which may guide 4 and 9 transversal modes, respectively, the threshold pump power for each transversal fiber mode has been evaluated. The results indicate that the single mode filter, does introduce a very strong discrimination for the modes between the high order modes and the fundamental mode. This should considerably favor the operation of the fundamental mode in the lasers with a homogeneously broadened active medium.     

Temperature dependence of macrobending loss in all-fiber bend loss edge filter

A theoretical model for macrobending-induced temperature dependent loss (TDL) for a standard singlemode fiber (SMF28) with dual coating layers is presented, with good agreement demonstrated between theoretical calculations and experimental results. The impact of temperature on two examples of an all-fiber based edge filter is also investigated theoretically and experimentally and using the developed model, it is shown that it is possible to predict the impact of temperature variations on an all-fiber based edge filter.     

Variable temperature system using vortex tube cooling and fiber optic temperature measurement for low temperature magic angle spinning NMR

We describe the construction and operation of a variable temperature (VT) system for a high field fast magic angle spinning (MAS) probe. The probe is used in NMR investigations of biological macromolecules, where stable setting and continuous measurement of the temperature over periods of several days are required in order to prevent sample overheating and degradation. The VT system described is used at and below room temperature. A vortex tube is used to provide cooling in the temperature range of -20 to 20 degrees C, while a liquid nitrogen-cooled heat exchanger is used below -20 degrees C. Using this arrangement, the lowest temperature that is practically achievable is -140 degrees C. Measurement of the air temperature near the spinning rotor is accomplished using a fiber optic thermometer that utilizes the temperature dependence of the absorption edge of GaAs. The absorption edge of GaAs also has a magnetic field dependence that we have measured and corrected for. This dependence was calibrated at several field strengths using the well-known temperature dependence of the (1)H chemical shift difference of the protons in methanol.     

Propagating beam analysis of fiber wavelength filters with a depressed-index section

A wavelength filter consisting of single-mode and few-mode fibers is investigated numerically. A simple finite-difference beam-propagation method, in which a transparent boundary condition can be imposed, is developed for circularly symmetric waveguides. After confirming the validity of the numerical method by the mode-mismatch loss, we calculate the propagating field in the fiber wavelength filter, in which interference between LP₀₁, and LP₀₂ modes occurs. To improve the filtering operation, a depressed-index fiber is employed for the few-mode fiber. The effects of the radius and refractive index of the depressed section on the transmission power are revealed and discussed. Power is suppressed to less than 0.1% at 1.3 μm, while maintaining power transmission of more than 85% at 1.55 μm. It is also found that the filtering operation shifts to higher wavelengths as the input power is increased when we choose a self-focusing nonlinear material in the depressed section.     

Distributed temperature measurement using a Fabry-Perot effect based chirped fiber Bragg grating

A temperature distribution sensing system based on the reflection spectrum of a 70 mm apodized chirped fiber Bragg grating has been demonstrated. Reflection variations created by the Fabry-Perot effect due to the intra-grating wavelength shift are presented. By fitting a parametric transfer matrix model of the grating response to measured spectra, the temperature change, position and width of a localized temperature change are obtained. This system is particularly attractive of its simplicity, high spatial resolution and ability in measuring a non-monotonic distribution. Experiments to measure two localized heating regions along the grating indicate that the technique is promising for measuring more complex temperature profiles.     

带光纤环的分布式光纤拉曼温度系统

 为了抑制分布式光纤拉曼温度系统的温漂和瑞利散射光窜扰反斯托克斯散射光,在传感光纤前端盲区后放置光纤取样环,用瑞利散射光解调反斯托克斯散射光及用光纤取样环的温度计算光纤线上其它点的温度,提高了系统的测温精度和稳定性。采用功率100 mW,波长1.55 μm,脉宽10 ns脉冲激光器和15 dB前置光纤放大器,100M14bitA/D转换卡及DSP作数字平均构建光纤拉曼温度系统的实验,实现了测温误差在±0.03 ℃内。     

基于卡尔曼滤波的分布式光纤Raman测温系统

分布式光纤喇曼测温系统采用了线性逼近的方法进行温度解调,并通过灵敏度分析法获取了相应的线性解调参数。为了提高系统的精度,在信号累加平均去噪的基础上,采用了卡尔曼滤波的方法,通过对前一时刻的估计值与当前测量值的递推,实现了对存在测量噪声的温度信号的最优化估计,完成了对现场采集信号的实时更新和处理。实验验证了该滤波方案的可行性,随着外界环境温度的变化,测温系统能够得到合理的温度曲线,测量误差小于1℃。系统响应性良好,性能稳定,能够适应复杂的环境变化。     

InGaAsP/InP long wavelength high efficiency edge emitting LED for single mode fiber optic communication

This article reports on a 1.3-μm edge emitting LED that has a high coupling efficiency to a single mode fiber and a high fiber output power. ^1-4This E²LED was prepared using VIPS (V-grooved inner stripe on P-type substrate) technique with InP and InGaAsP layers suppressing the lasing action.     

基于长周期光纤光栅的可变体机翼翼表温度测试系统研究

为实现可变体机翼翼表温度的高精度的实时检测,设计了一种基于长周期光纤光栅(LPG)的温度测试系统.该系统采用长周期光纤光栅作为温度传感元件,高双折射光纤环镜作为边缘滤波器件.在边缘滤波理论研究的基础上,提出了一种降噪增敏的算法.实验中将LPG粘贴在可变体机翼翼表材料的表面,当外界温度发生变化时,LPG的谐振波长发生偏移...     

高功率光纤中传输光模式与其波长相关性研究

针对高功率光纤激光模式诊断和光谱诊断的需求,研究了光纤中传输模式的波长相关性,数值计算了光纤中各个模式的模场分布随波长的变化曲线及相应的光束质量,采用双傅里叶变换F~2法实际测量了光纤模式成分与波长的关系曲线.结果表明,光纤中各个模式的模场分布随波长变化,波长越长,模场面积越大;模式的光束质量随波长变化不大,但在截止频率附近明显变差;光纤中各个模式的功率占比与波长有关.     

埋入式光纤光栅应变测量系统的设计

埋入式光纤光栅应变传感器作为大型土木结构健康监测的智能元件具有很好的发展前景。讨论了埋入式光纤光栅应变传感器的传感机理，设计了埋入式光纤光栅应变测量系统。从力学角度阐述了光栅传感探头的结构和制作工艺。采用可调谐F-P滤波器对光栅信号进行解调，解决了应变与温度交叉敏感问题。系统对应变的测量范围为0～1500με。通过理论分析证实了系统方案的可行性。     

CdSe量子点滤光片尺寸、温度依赖的光学特性

为了缩小光谱仪体积使之适用于军事卫星等领域,本文将胶体量子点作为滤光材料,研究了CdSe胶体量子点滤光片的光学特性.本文采用热注入法合成出了高质量的CdSe胶体量子点,经过对苯二胺消光处理制备成CdSe胶体量子点滤光片.利用透射电子显微镜(TEM)进行样品形貌结构的表征及粒径尺寸的测量,并分别在不同温度下进行了紫外.可...     

Structural and wavelength dependence of mode power distribution for a broadband dispersion compensating microstructured fiber

Approximate empirical analysis of mode power distribution (MPD) carried by the fundamental mode is newly investigated based on a broadband dispersion compensating microstructured fiber (MF). The fraction of modal power in the core region, η, is defined with the help of extending the applicability of well-established classical optical fiber theories to MFs. In doing so, the influences of structural parameters and wavelength on MPD characteristics of the fundamental mode are systematically analyzed in detail based on simple physically consistent concepts of conventional fibers. As a result, it is shown that for the optimum MF design in Ref. [11] as a multimode fiber, when the number of guiding modes increases, the mode power ratio of the fundamental mode constantly increases from 69.4%; in addition, we find that as wavelength increases within 1.2-1.6 μm, mode power confinement ability of the fundamental mode is lessened for single-mode propagation, whereas for multimode propagation it becomes enhanced.     

Fresnel-reflection-based fiber sensor for on-line measurement of ambient temperature

A fiber sensor with a simple structure for measuring the environment temperature is presented. The sensor uses only a diode laser as light source, three couplers, two photodetectors and two sensing fiber ends with protective cladding. One of the sensing fiber ends is covered by solidified epoxy resin with a metal cover. The measurement principle is based on relative Fresnel reflective intensity. Various ambient temperatures are measured in the setup. The measured data is fitted to linear equation very well with the value of R² of 0.9982. Applying the relative technique, the errors resulted from fluctuation of light source and other influences of environment are effectively eliminated, and the stability for long time measurement can be improved.     

高温超导滤波器用斯特林制冷机温度控制算法设计

在深空通信领域,利用高温超导滤波技术,可大幅提高接收机的灵敏度、信噪比和抗干扰能力。由于高温超导滤波器工作温度较低,需要斯特林制冷机进行制冷。斯特林制冷机温度的稳定性将直接影响高温超导滤波器的性能,文章对高温超导滤波器用斯特林制冷机温度控制算法进行了分析和设计,并给出了基于高温超导滤波器试验装置的温度控制曲线。     

Adaptive Bit allocation algorithm based on unsaturated bandwidth of multimode fiber for optical OFDM system

An adaptive bit allocation algorithm for Orthogonal Frequency Division Multiplexing (OFDM) multimode fiber communication system is proposed in this paper, aimed at utilizing the infinite bandwidth of multimode fiber in high frequency region. The algorithm breaks through the limitation that bits are formerly allocated only in fixed bandwidth, and is proved to be effective by simulation. Compared with greedy algorithm, new algorithm is easy to implement, and lowers bit error rate (BER) of each subcarrier effectively.     

Simultaneous strain and temperature measurement using a highly birefringence fiber loop mirror and a long-period grating written in a photonic crystal fiber

We present a new design for simultaneous strain and temperature measurement using a high-birefringence fiber loop mirror (HiBi-FLM) concatenated with a temperature-insensitive long-period grating (LPG) written in a photonic crystal fiber (PCF). The FLM acts as a sensor head, while the LPG in PCF serves as a filter to convert wavelength variation to optical power change. By measuring the wavelength variation and the power difference of two near peaks in the spectral response of this configuration, simultaneous strain and temperature measurement is obtained.     

热释电双波长光纤测温仪的精度分析

在介绍一种利用钽酸锂热释电探测器实现的实用化双波长光纤测温仪的基础上,着重讨论了反射辐射、探测器周围环境的热辐射、仪器工作波长的带宽,以及光路中选择性吸收气体的光谱吸收等多种因素对仪器测温精度的影响,并提出了相应的抑制措施。实验表明,采取相应的抗干扰措施后,在系统要求的测温范围400—1360℃内,其测温精度符合设计要求。     

Experimental results of the performance of a laser fiber as a remote sensor of temperature

We present experimental results demonstrating the performance of an erbium-doped silica fiber as a remote temperature sensor. The sensor is based on the fluorescence intensity-ratio change of two spectral bands as a function of temperature in the wavelength interval from 515 to 570 nm. We apply a radiometric analysis to the fluorescence spectrum that we have measured to determine the optimal spectral bands to use in the power ratio of the sensor. The spectral bands used in the power ratio, with best performance, are 525–535 nm/555–565 nm, with a signal–noise ratio of 57 and 56 dB, respectively. The sensor sensitivity is about , and the resolution is approximately .     

Simultaneous measurement of strain and temperature based on a fiber Bragg grating combined with a high-birefringence fiber loop mirror

We present an all-fiber sensor for simultaneous measurement of temperature and strain. The sensing head is formed by introducing a fiber Bragg grating into a high-birefringence fiber loop mirror that acts as a Mach-Zehnder interferometer for temperature and strain discrimination. A sensing resolution of ±1 °C in temperature and ±21 με in strain has been experimentally achieved over a temperature range of 60 °C and strain range of 600 με.