Design of broadband LP01↔LP02 mode converter based on special dual-core fiber for dispersion compensation

A novel broadband LP01?LP02 mode converter for dispersion compensation based on special dual-core fiber is theoretically investigated by using the coupled-mode theory. The simulated mode converter has ～22 nm bandwidth with a conversion efficiency of over 80%. Furthermore, this noncomplete conversion only introduces the insertion loss rather than multipath interference resulting from the residual LP01 mode. Finally, one optimal scheme for broadening the bandwidth of high-efficiency conversion has been proposed by longitudinally tapering the dual-core fiber. The simulation results show that the conversion bandwidth can be improved to ～31 nm by tapering with a scaling range of only 2%.     

Thermal drawing of high-density macroscopic arrays of well-ordered sub-5-nm-diameter nanowires

We investigate the lower limit of nanowire diameters stably produced by the process of thermal fiber drawing and fiber tapering. A centimeter-scale macroscopic cylindrical preform containing the nanowire material in the core encased in a polymer scaffold cladding is thermally drawn in the viscous state to a fiber. By cascading several iterations of the process, continuous reduction of the diameter of an amorphous semiconducting chalcogenide glass is demonstrated. Starting from a 10-mm-diameter rod we thermally draw hundreds of meters of continuous sub-5-nm-diameter nanowires. Using this approach, we produce macroscopic lengths of high-density, well-ordered, globally oriented nanowire arrays.     

Compound parabolic tapered fiber for fiber coupling with a highly divergent source

A novel fiber tapering shape, which is based on compound parabolic geometry, is proposed to increase the acceptance angle of a compound parabolic concentrator. The proposed design is described by use of ray optics on a step-index multimode fiber.     

Bitapered fiber coupling characteristics between single-mode single-core fiber and single-mode multicore fiber

By splicing and tapering at the fusion point of one-core single-mode fiber and three- or four-core single-mode fiber, an effective bitapered fiber coupling technique is implemented. Based on the beam propagation method, the bitapered coupling characteristics between the one-core fiber and the multicore single-mode fiber are simulated and analyzed. The theoretical prediction is confirmed by the experimental results, and the difference between the simulation and the experimental results is also discussed.     

Miniature tapered photonic crystal fiber interferometer with enhanced sensitivity by acid microdroplets etching

We fabricate a miniature tapered photonic crystal fiber (PCF) interferometer with enhanced sensitivity by acid microdroplets etching. This method is very simple and cost effective, avoiding elongating the PCF, moving and refixing the device during etching, and measuring. The refractive index sensing properties with different PCF diameters are investigated both theoretically and experimentally. The tapering velocity can be controlled by the microdroplet size and position. The sensitivity greatly increases (five times, 750?nm/RIU) and the size decreases after slightly tapering the PCF. The device keeps low temperature dependence before and after tapering. More uniformly and thinly tapered PCFs can be realized with higher sensitivity (～100 times) by optimizing the etching process.     

In-line optical fiber sensors based on cladded multimode tapered fibers

The use of uniform-waist cladded multimode tapered optical fibers is demonstrated for evanescent wave spectroscopy and sensors. The tapering is a simple, low-loss process and consists of stretching the fiber while it is being heated with an oscillating flame torch. As examples, a refractive-index sensor and a hydrogen sensor are demonstrated by use of a conventional graded-index multimode optical fiber. Also, absorbance spectra are measured while the tapers are immersed in an absorbing liquid. It is found experimentally that the uniform waist is the part of the taper that contributes most to the sensor sensitivity. The taper waist diameter may also be used to adjust the sensor dynamic range.     

碳纳米管对纳米炭复合纤维结构与性能的影响研究

采用不同方法将多壁碳纳米管(MWNTs)混合于中间相沥青中,利用氮压式单孔纺丝机进行纺丝,经预氧化、碳化后制得了纳米炭复合纤维,对比研究了混合方式对纳米炭复合纤维的结构和性能的影响.研究结果表明,MWNTs的加入最终改变了纤维的微观结构,尤其经浓酸处理后显著提高了纳米炭复合纤维的力学性能.     

Long-period grating fabricated by periodically tapering standard single-mode fiber

We fabricated an asymmetric long-period grating (LPG) by periodically tapering a section of standard single-mode fiber using a resistive filament heating. The LPG exhibits large peak transmission attenuation of -30.31 dB with only 22 periods in a 1.0 cm long optical fiber and possesses unique characteristics for sensing applications. The bending and strain sensitivities are 1.74 nm m and 1.11 pm/mu epsilon, respectively. The polarization dependent loss is large, up to 11.65 dB, which is caused by an asymmetric index profile in the cross section of the tapered LPG.     

Adaptively controlled supercontinuum pulse from a microstructure fiber for two-photon excited fluorescence microscopy

Selective fluorescence excitation of specific molecular species is demonstrated by using coherent control of two-photon excitation with supercontinuum pulses generated with a microstructure fiber. Pulse shaping prior to pulse propagation through the fiber is controlled by a self-learning optimization loop so that the highest fluorescence signal contrast between two fluorescent proteins is obtainable. The self-learning optimization loop successfully controls both the optical nonlinarity of the microstructure fiber and the two-photon excitation of the fluorescent proteins.     

Cylindrically etched carbon-fiber microelectrodes for low-noise amperometric recording of cellular secretion

The most important sources of noise with disk-shaped carbon-fiber microelectrodes (CFMEs) are the exposed cut disk face of the fiber itself and the seal region between the carbon fiber and the applied insulating layer. To reduce noise and to fabricate simple, reproducible low-noise CFMEs, we sealed commercially available carbon fibers in pulled glass pipets and then we performed cylindrical etching of the fiber extending beyond the glass sheath, followed by insulation with anodic electrophoretic deposition of paint. The resulting CFMEs had electroactive carbon disks with radii as small as ～0.5 μm. The noise of such electrodes was minimized by virtue of a design that ensures a good seal between the carbon fiber and its insulation and a reduced diameter of the exposed carbon. In contrast to CFMEs made of conically etched carbon fibers, cylindrically etched CFMEs offer the significant advantage that they can be easily reused: The cylindrically etched region extends over several hundreds of micrometers and, therefore, can be cut back repeatedly to expose a fresh carbon surface of uniform diameter. The low noise and small size of these electrodes make them ideal for the high-sensitivity measurements demanded in studies of single-vesicle transmitter release from secretory cells. Furthermore, the small cross-sectional diameter of the tips allows them to be used in restricted spaces, such as inside the tapering micrometer-diameter tips of melted and pulled glass microcapillaries (e.g., patch pipets).     

Multimode interference tapered fiber refractive index sensors

Real-time monitoring of the fabrication process of tapering down a multimode-interference-based fiber structure is presented. The device is composed of a pure silica multimode fiber (MMF) with an initial 125?μm diameter spliced between two single-mode fibers. The process allows a thin MMF with adjustable parameters to obtain a high signal transmittance, arising from constructive interference among the guided modes at the output end of the MMF. Tapered structures with waist diameters as low as 55?μm were easily fabricated without the limitation of fragile splices or difficulty in controlling lateral fiber alignments. The sensing device is shown to be sensitive to the external environment, and a maximum sensitivity of 2946 nm/refractive index unit in the refractive index range of 1.42-1.43 was attained.     

Effect of physicochemical structure of natural fiber on transverse thermal conductivity of unidirectional abaca/bamboo fiber composites

To study the effect of the microstructure of natural fiber on the transverse thermal conductivity of unidirectional composite, abaca and bamboo fibers were unidirectionally aligned to fabricate epoxy composites by a resin transfer molding (RTM) technique. The transverse thermal conductivity of these two types of composites was measured in a steady-state platform. X-ray diffractometer and scanning electron microscopy were applied to analyze the microstructure and morphology of both fibers and composites. The results indicated that the transverse thermal conductivity showed two types of tendencies with fiber content increasing: increasing for bamboo fiber composites, and decreasing for abaca fiber composites. The microstructure and theoretical analysis suggest that the lumen structure plays a great role rather than crystal structures and chemical compounds on the transverse thermal conductivity of unidirectional composites, which is useful for further development and design of natural fiber reinforced composites with better thermal insulation property for people’s daily life.     

碳纳米管纤维/环氧树脂复合材料的界面剪切强度及微观结构

研究了碳纳米管纤维的微观结构和拉伸性能,并进一步分析了其与环氧树脂形成界面剪切强度及微观结构。采用单丝断裂试验测试了碳纳米管纤维/环氧树脂复合材料体系的界面剪切强度,结合单丝断裂过程中的偏光显微镜照片、复合材料的拉曼谱图和断口扫描电镜照片,研究了碳纳米管纤维/环氧树脂复合材料界面的微观结构。结果表明: 碳纳米管纤维/环氧树脂复合材料的界面剪切强度约为14 MPa;在碳纳米管纤维和环氧树脂形成界面的过程中,环氧树脂可以浸渍纤维,形成具有一定厚度的复合相,这种浸渍过程和界面相的形成都有利于碳纳米管纤维与基体之间的连接。     

Optical-fiber sensor using tailored porous sol-gel fiber core

A new concept in optical-fiber chemical sensors, the active fiber core optical sensor (AFCOS), is presented. In this sensor, the fiber core acts as a transducer. The sensitivity of an AFCOS sensor is compared with that of an active coating [evanescent wave (EW)] based optical-fiber sensor. Requirements for a fiber core to act as a chemical sensor are discussed. Novel techniques for making a porous sol-gel silica fiber, doping chemical reagents into the fiber, and constructing a chemical sensor using the porous fiber as a transducer have been developed. The microstructure of the fabricated sol-gel silica fiber and the effect of the fiber's microstructure on the capability of the porous sol-gel silica fiber for guiding light are discussed. A humidity sensor employing a CoCl/sub 2/-doped porous sol-gel fiber as a transducer has been constructed as an example. The test results for the humidity sensor verified a theoretical analysis indicating that an optical-fiber chemical sensor using an active fiber core as a transducer has a much higher sensitivity than that of an EW-based sensor.     

卤化银多晶红外光纤显微结构与相关性能的研究

借助ＴＥＭ、ＸＲＤ、硬度微观测量等方法,研究了卤化银多晶红外光纤的显微结构与相关性能的关系．发现光纤是以ＡｇＣｌ_ｘＢｒ_１－ｘ。多晶体为主骨架,纳米晶粒Ａｇ镶嵌其中．由纳米晶粒带来的损耗光纤的散射损耗增加很小,即ＡＮλ^－４通过控制光纤的制备工艺条件,可以改变光纤的晶粒大小和光纤硬度．     

Thermo-mechanical fatigue modeling of advanced metal matrix composites in the presence of microstructural details

An analytical model developed for predicting the inelastic response of metal matrix composites subjected to axisymmetric loading is employed to investigate the behavior of SiC---Ti composites under thermo-mechanical fatigue loading. The model is based on the concentric cylinder assemblage consisting of arbitrary numbers of elastic or inelastic sublayers with isotropic, transversely isotropic, or orthotropic, temperature-dependent properties. In the present work, the inelastic response of the titanium matrix is modeled by the Bodner-Partom unified viscoplastic theory. These features of the model allow the investigation of microstructural effects (such as the layered morphology of the SCS-6 fiber, including the weak carbon coating, and matrix microstructure) and rate-dependent response of the matrix on the fatigue behavior.

In this paper, we employ the predictions of the multiple concentric cylinder model to study the effects of the layered morphology of the SCS-6 SiC fiber and two-phase microstructure of the Ti-15-3 matrix on the response of a SiC---Ti composite under thermo-mechanical fatigue loading. It is shown that ignoring the microstructure can lead to significant errors in the predictions of the internal stress and inelastic strain distributions.     

高温下短纤维增强金属基复合材料界面的微观结构和热残余应力状态研究

利用透射电镜观察了δ-Al₂O₃短纤维增强Al-5.5Mg合金复合材料界面在不同环境温度下的微观结构特征。 同时, 基于该类复合材料的单纤维模型, 利用弹塑性有限元分析方法, 研究了在不同温度下界面热残余应力的大小和分布情况, 并讨论了热残余应力对界面行为的影响。 最后, 讨论了界面的微观结构和热残余应力特征对复合材料整体性能的影响。 研究表明, 不同环境温度下, 界面具有不同的微观结构和热残余应力特征, 这些特征的变化将引起复合材料整体性能的明显变化。     

Study of a fiber optic humidity sensor based on agarose gel

An optical fiber humidity sensor was fabricated using a hydrophilic gel (agarose) deposited on the tapered plastic optical fiber (POF). The sensing element, agarose, can absorb and exude moisture from/to the ambience, thereby altering its refractive index and changing its ability to modulate the intensity of light that propagates through the fiber. Thus, the operating principle of the sensor is based on the intensity modulation technique, which utilizes a tapered POF probe coated with agarose that is sensitive to humidity. The POF, which was fabricated using an etching method, has a waist diameter of 0.45 mm and tapering length of 10 mm. As the relative humidity varies from 50% to 80%, the output voltage of the sensor with agarose gel of 0.5% weight content decreases linearly from 2.24 mV to 1.55 mV. The agarose-based sensor produces a sensitivity of 0.0228 mV/%, with a slope linearity of more than 98.36%. The tapered fiber with agarose gel of 1% weight content produces a sensitivity of 0.0103 mV/% with a slope linearity of more than 94.95% and a limit of detection of 2.635%, while the tapered fiber with agarose gel of 1.5% weight content produces a sensitivity of 0.0079 mV/% with a slope linearity of more than 98.53% and a limit of detection of 6.853%. The fiber with agarose gel of 0.5% weight content shows higher sensitivity compared to that of 1% and 1.5% due to the effect of pore size, which changes with concentration. The results demonstrate that agarose-based optical fiber sensors are both sensitive and efficient for economical and flexible measurements of humidity.     

高强度南极磷虾蛋白/海藻复合纤维的制备与表征

以戊二醛为增强改性剂,利用湿法纺丝制备含有预埋增强剂的南极磷虾蛋白/海藻（AKP/SA）初生纤维,通过初生纤维纺丝线在线热交联技术实现热交联反应从而制得高强度南极磷虾蛋白/海藻（HAKP/SA）复合纤维。利用旋转黏度计确定了增强交联反应的热交联温度,研究了交联度对复合纤维微观结构以及吸湿性能的影响,测试了增强后复合纤维力学性能和结晶性能的变化。结果表明:80℃时戊二醛能有效发生热交联反应;AKP/SA复合纤维具有较好的吸湿性能,吸湿率为7.3%,随着交联度的提高,复合纤维的吸湿性能降低,吸湿平衡时间缩短;HAKP/SA复合纤维仍以非晶形态存在,表面存在不均匀的沟槽结构;力学性能测试表明增强后的纤维断裂强度提高了13%。