倒锥透镜型光纤端的出射光场性质

倒锥透镜型光纤端系经过特殊技术加工而成。本文用二阶统计相关函数,采用单色相干光源,计算了倒锥透镜型光纤端的光场分布及其统计相关性质,并与平端的情况作了对比。在实验上,用He-Ne激光作为单色相干光源,对理论进行了验证。     

Polarity Correlator for Conduction Velocity Measurement

Techniques for the estimation of skeletal muscle fiber conduction velocity are of considerable interest. These techniques use, in general, some form of cross correlation or zero-crossing analysis. Cross correlation is a straightforward method of conduction velocity estimation, however, it is difficult to realize low-cost real-time processors. Polarity correlation offers a method which preserves the advantages of cross correlation and satisfies the speed-cost constraint. This paper describes the algorithm for implementation of a polarity cofrelator instrument using a microprocessor. The instrument is tested with deterministic and stochastic signals and used to estimate the conduction velocity of biceps brachii fibers of ten normal subjects. The performance of the instrument is compared to the zero-crossing technique described by Lynn.     

图像处理在光纤光斑微位移传感中的应用

基于多模光纤的多模干涉原理以及图像处理算法,对多模光纤输出端面光斑图像相位谱的灰度共生矩阵特征值进行提取,提出一种新型光斑图样处理算法,并且将其应用于微位移传感。详细分析了以阶跃型多模光纤为基础的单模-多模光纤结构在两光纤径向产生微位移时,位移量与端面光斑图像相位谱特征值之间的关系。对多种结构参数光斑图样的分析表明,两光纤径向微位移与相关性及一致性两特征值成近似线性关系,可以用光斑图像处理的方法进行微位移测量,从而实现传感。实验结果表明与目前常用的计算光斑图像归一化强度内积处理算法相比,多模光纤芯径较小时,线性度、动态范围比较接近;芯径较大时,线性度提高的同时动态范围扩大约一倍。因此所提出的算法稳定性更好,适用范围更广。     

Dispersion Measurement in Optical Fibers Using Supercontinuum Pulses

The chromatic dispersion of an optical fiber is measured using a time-of-flight technique, based on temporally and spectrally resolving a dispersed broadband pulse, on which a spectral fringe pattern has been imposed using an etalon. The technique employs broadband supercontinuum radiation, generated by launching picosecond pulses from a fiber laser into a photonic- crystal fiber. It allows the dispersion of highly dispersive optical fibers and components to be measured with a high spectral resolution over a wide wavelength region. The technique is demonstrated by measuring the dispersion of a dispersion-compensating module over its entire 400-nm transmission band with a subnanometer spectral resolution.     

柚子型光子晶体光纤布拉格光栅理论及实验研究

利用有限元法对一种柚子型光子晶体光纤中的传输模式进行了模拟,得到了各传输模式的有效折射率和模场分布。结合耦合模理论和相关函数方法,对柚子型光子晶体光纤布拉格光栅反射谱进行了理论分析,解释了柚子型光纤光栅出现多个谐振峰的原因;数值分析了光纤纤芯直径和空气孔尺寸对光栅传输谱的影响。结果表明谐振峰波长随纤芯直径的增大向长波方向漂移,而随空气孔增大向短波方向移动,并且不同谐振模式的变化幅度不同;利用相位模板法写制了光子晶体光纤光栅,实验结果与理论分析能够很好地吻合。     

Realization of true-time-delay using cascaded long-period fiber gratings: theory and applications to the optical pulse multiplication and temporal encoder/decoder

This paper proposes and demonstrates a technique for repetition-rate multiplication of an optical pulse train having an arbitrary period or pattern and the optical temporal encoding/decoding for the optical-code-division multiple-access (O-CDMA) system. The technique exploits the difference in the propagation speeds between the core and copropagating cladding modes of a fiber to obtain true-time-delay between the modes traveling in the core mode and the cladding mode, which can be used to achieve pulse multiplication. For the coupling to the cladding mode, long-period fiber gratings (LPGs) were used. A series of cascaded LPGs imprinted in a fiber with a specific separation has been employed to obtain a specific rate of pulse multiplication with a single input pulse. Second, by controlling the separations among the gratings, the temporal encoder/decoder for O-CDMA could be implemented. The principle and the applications of the proposed device are investigated in detail. The effect of the birefringence of fiber and fiber gratings on the system performance in the time and spectral domains is presented. The sensitivity of the cladding modes in a conventional fiber to the perturbations at the cladding has been overcome by replacing the conventional fiber with inner-cladding fiber. The properties and the benefits of using the inner-cladding mode are investigated.     

Core-Surround Shaping of Hollow-Core Photonic Crystal Fiber Via HF Etching

We report on a technique to pattern the shape of the core surround of a hollow-core photonic crystal fiber. Different shapes are fabricated and a relationship between surface mode location and core surround thickness is experimentally observed.     

Recording of volume hologram using a beam pattern from taperedoptical fiber

A photorefractive volume hologram was recorded and probed using light diffracted from a tapered optical fiber as a reference beam. A single-mode fiber (SMF) was chemically etched and tapered to give a complicated beam pattern, and it is shown that the tapered optical fiber can be utilized to increase the storage density of the volume hologram. Spatial selectivity of the volume hologram with this method was increased by two times compared to the normal SMF referencing, which is due to the fact that the complicated beam pattern has little correlation with its shifted version     

Frequency correlation function of modal noise in multimode opticalfibers

After transmission of highly coherent light through a multimode optical fiber, it is possible to observe a speckle pattern, namely, a modal noise, at its exit face caused by the interference of many different propagation modes. Changes of the source frequency can also cause modal noise variations in time at the exit face. The statistics of the modal noise appearing at the exit face of a fiber can thus be used to study the properties of the fiber and source. Measurements have been made of the frequency correlation function of modal noise in multimode optical fiber not only at the exit face of the fiber but also at a nearby point. A geometrical approximation is used to derive the frequency correlation function at the detecting position for various detecting positions. Experimental results are given and compared with the theory     

分布式光纤光栅传感系统信号解调技术

光纤光栅是近年来光纤传感领域的研究热点,波长编码信号解调是实现光纤光栅分布式传感网络的关键.本文分析了分布式光纤光栅传感系统信号解调的一般原理,总结了系统信号解调过程中存在的主要技术难点,分类评述了几种典型的解调技术的工作机理,并重点分析了其特点和性能,同时提出了一种宽带光源/副载波解调技术.该技术具有实时性好,成本低等优点.     

全分布式光纤应力传感器的研究新进展

全面综述了全分布式光纤应力传感器的发展概况.对光后向散射技术、光学非线性效应、模式耦合技术和光干涉技术的主要研究方案进行了评述,重点介绍和分析了实现高空间分辨率和动态应力测量的新技术.最后展望并讨论了全分布式应力传感器的发展趋势和应用前景.     

光纤Bragg光栅温度补偿方法的研究

应变和温度变化都会引起光纤Bragg光栅（FBG）反射波长的漂移,即所谓的交叉敏感问题,它是制约FBG传感检测技术实用化的“瓶颈”.从应变、温度交叉敏感的物理机制出发,阐述了光纤光栅温度补偿的基本原理,介绍了几种国内外常用的光纤光栅无源温度补偿的方法,并详细分析了每种方法的优缺点.     

光纤光栅傅里叶变换光谱分析的研究

阐述了光纤光栅用作分光元件时 ,通过测量光纤光栅附近区干涉条纹 ,经快速傅里叶变换达到测量光谱的原理。讨论了波长测量范围和光谱分辨率。给出了在设计光纤光栅和确定探测器参数时应该考虑的几个问题。该光谱分析方法具有光谱分辨率高、结构简单、造价低等优点。     

激光侦察告警技术的现状及发展趋势

目前,激光侦察告警技术发展迅速,技术日趋成熟。文中首先介绍激光告警中的光纤前端、高精度方位分辨、多波长告警、多元相关探测和多目标处理等先进技术,并分析了激光告警设备的作战效果,最后综述了激光告警设备的新发展。     

Measurement of Index Modulation along Fiber Bragg Gratings by Side Scattering and Local Heating Techniques

A method for measurement of index modulation profile along fiber Bragg gratings is described which uses a hot finger that selectively heats some fringes. Results obtained from this local heating technique and side scattering technique are reviewed and compared.     

光纤低相干干涉测量法的光谱域处理技术

使用低相干、宽带光源可以让光纤传感器系统获得高分辨率和大动态范围.光谱域处理技术在具有这种特点的光源下应运而生.该技术的特点是除了使用传感干涉仪以外,还必须用第二个处理干涉仪来提取所需的条纹图样信息.使用交叉的梳状谱也是其特点之一.使用这种技术的光纤传感器系统可以实现多种物理量的测量,而且其结构简单、成本低廉.     

Optical Generation of Binary Phase-Coded Direct-Sequence UWB Signals Using a Multichannel Chirped Fiber Bragg Grating

A novel technique to generate binary phase-coded, direct-sequence ultra-wideband (DS-UWB) signals in the optical domain is proposed and demonstrated. In the proposed system, the wavelengths from a laser array are modulated by a Gaussian pulse, which is sent to a multichannel optical frequency discriminator, to generate a UWB monocycle or doublet pulse sequence with a predetermined phase-code pattern. By tuning the wavelengths of the laser array, or by tuning the states of polarization of the wavelengths, the generated pulse shape and code pattern can be changed. The key device in the system is the multichannel dispersive chirped fiber Bragg grating (FBG), which functions, in combination with a dispersive fiber, as a multichannel frequency discriminator with a step-increased group-delay response, to ensure the generated UWB sequence to have uniform time spacing among the chips and to compensate for the fiber-induced chromatic dispersion. The proposed scheme is experimentally demonstrated. A multichannel chirped FBG is designed and fabricated. Binary phase-coded DS-UWB signals with different code patterns are experimentally generated.      

Azimuthal far-field analysis for the measurement of the effectivecutoff wavelength in single-mode fibers-effects of curvature, length,and index profile

The spectral analysis of the optical power transmitted through a bow tie slit rotating in the far-field pattern of a single-mode fiber provides an easy and accurate mean of determining the effective cutoff wavelength of the fiber. The principle of this technique is described. Theoretical considerations show that the ratio between the optical powers carried by the LP₁₁ mode and the fundamental mode, respectively, is directly related to the amplitude of two particular lines in the spectrum. The degree of coherence of the analyzed field is shown to be related to the amplitude of a third line. Owing to the sensitivity of the method, the detectable amount of the total power which is contained in the LP₁₁ mode is as small as 0.5%. The effects of length and curvature on the effective cutoff wavelength are measured over wide ranges for different kinds of single-mode fibers. The behavior of a quadruple-clad flattened dispersion fiber is shown to be somewhat different from that of a typical depressed cladding fiber     

光纤光栅原理与应用(Ⅱ)──光纤光栅制作方法

简要分析了光子诱导所致的光纤光栅形成机理，介绍了光纤光栅的几种制作方法及相关技术，如紫外激光干涉条纹图下的横向侧面曝光法，光学衍射相位掩膜法，点-点写入法及光纤光栅的在线制作方法。     

Refractive-index-profile determination of single-mode optical fibers by a propagation-mode near-field scanning technique

A new technique is presented for determining the refractive-index profile of a single-mode optical fiber. The profile is calculated from the measurement of the near-field mode pattern by a new equation derived from the scalar wave equation, A result of computer simulation shows that the method is useful for practical use in measurement systems.