微纳尺度光纤布拉格光栅折射率传感的理论研究

亚波长直径微纳光纤强倏逝场传输的光学特性,使其对周围介质折射率的变化具有极高的灵敏度.本文提出一种基于微纳尺度光纤布拉格光栅(MNFBG)的折射率传感器,结合微纳光纤倏逝场传输和光纤布拉格光栅(FBG)强波长选择的特性来实现高精度折射率传感,对其制备可行性进行了讨论.论文中对MNFBG折射率传感机理进行了深入的理论分析,并使用OptiGrating软件进行了数值模拟,模拟数据显示MNFBG折射率测量的灵敏度随着光纤半径的减小而增加,其中光纤半径为400 nm的MNFBG灵敏度可达到993 nm/RIU,相比于包层蚀刻的FBG灵敏度增加了170倍,说明MNFBG对发展微型化、高灵敏度折射率传感器具有良好的应用前景. 关键词： 微纳光纤 光纤布拉格光栅 折射率传感     

微纳光纤及其锁模激光应用

微纳光纤是一种直径接近或小于传输光波长的纤维波导,由于纤芯和包层折射率差较大,具有强光场约束、强倏逝场、低损耗、反常波导色散、表面均匀性好和机械性能高等特性。近年来,以纳米材料作为饱和吸收体的被动锁模激光器成为超短脉冲激光技术方向的研究热点。得益于微纳光纤的强光场约束能力及大比例倏逝场,纳米材料与微纳光纤的复合结构能显著增强光与物质的相互作用,进而降低该复合结构的饱和吸收阈值,为超短脉冲产生和非线性动力学等研究提供一个新颖而灵活的平台。同时,微纳光纤因具有反常波导色散、光谱滤波、饱和吸收和偏振敏感等特性,在激光器的色散调控、偏振锁模等方面获得应用。介绍了微纳光纤的制备和特性以及在锁模激光方面的典型应用和相关技术的最新进展,并就未来的发展方向进行了展望。     

基于倏逝场耦合的石墨烯波导光传输相位特性仿真与实验研究

石墨烯材料应用到各种光波导器件中正成为新一代光子器件的重要发展方向之一,目前基于石墨烯的光纤和集成光子器件研究越来越受到国内外的重视. 本文建立了一种由微纳光纤耦合光倏逝场,并在石墨烯薄膜中传输的模型. 通过有限元分析法,研究了光在这种石墨烯波导中传输光场的强度分布和相位特性,并通过实验进行了验证. 结果表明,沿着微纳光纤-石墨烯光波导传播的倏逝场的强度分布和相位均受石墨烯材料作用,石墨烯材料能有效聚集和导行波导中传输的高阶模,在单位传输长度上具有更密集的等相位面. 本文提出了一种利用微纳光纤耦合光倏逝场研究石墨烯相位响应特性的新方法,对基于石墨烯波导的新型调制器、滤波器、激光器和传感器等光子器件的设计和应用具有一定的参考意义. 关键词： 石墨烯平面光波导 倏逝波 光场强度 相位     

偏振保持光纤的模式双折射

偏振保持光纤是单模光纤的一种特殊类型,其模式双折射与温度、芯和皮折射率差及椭圆度、热膨胀系数、光纤直径、弹性模量、泊松比、频率、波长、拉丝张力等诸多因素有关,对与偏振保持光纤模式双折射相关的上述因素进行分析,指出光纤模式双折射越高,光纤的偏振态保持就越好,为更好地设计新型保偏光纤或合理选择保偏光纤类型奠定了基础.     

高折射率椭圆芯布拉格光纤的偏振特性

应用带完全匹配层边界条件的全矢量有限元方法,分析了高折射率椭圆芯布拉格光纤的偏振特性,详细讨论了光纤结构参量对模式双折射度以及群速度走离的影响,结果发现:高折射率椭圆芯布拉格光纤的模式双折射度可达10-2量级,比传统保偏光纤至少高出一个量级,并且表现出不同于传统保偏光纤的群速度走离特性;在保持较高双折射度的同时,通过合理的结构设计,可在一定的波长处灵活地获得较大的群速度走离或零走离特性,具有重要的潜在应用价值;最后,简要分析了低折射率区域的折射率变化对偏振特性的影响;研究结果有助于设计高性能的微结构保偏光纤。     

熔锥型保偏光纤耦合器分光比的偏振依赖研究

实验研究了熔锥型保偏光纤耦合器分光比随入射单色线偏振光偏振方向的旋转所呈现的周期性,验证了保偏光纤耦合器分光比的偏振依赖特性.借鉴已成熟的单模光纤耦合器的倏逝场耦合模理论,对这一偏振依赖分光比的周期性规律提出了一种理论解释.在实验结果的规律性基础上,经验拟合出了分光比与单色线偏振光输入偏振角的函数关系.     

椭圆形高折射率芯Bragg光纤的偏振和色散特性

应用全矢量有限元方法,分析了椭圆形高折射率芯Bragg光纤的偏振和色散特性,详细讨论了光纤结构参量对模式双折射的影响,结果发现：椭圆形高折射率芯Bragg光纤的模式双折射在10－3量级,比传统保偏光纤高出一个量级,且随波长的增加而单调递增;模式双折射随包层低折射率材料填充比的增加而减小,但与包层周期的变化几乎无关.此外,提高椭圆率或材料间的折射率比可显著增强模式双折射.研究结果有助于设计高性能的一维微结构保偏光纤.     

高双折射80PbO·20Ga_2O_3中心填充光子晶体光纤特性（英文）

提出一种高折射率材料80PbO·20Ga2O3(PG玻璃)中心填充椭圆芯光子晶体光纤.基于有限元法对光子晶体光纤的双折射特性和模场面积进行数值模拟,并研究了椭圆芯尺寸、椭圆率和孔间距等光纤几何参量对双折射特性的影响.数值研究表明:在1 550nm波长处,双折射高达1.256×10-1,x偏振和y偏振模场面积分别为0.43μm2和0.68μm2;在910nm~1931nm的宽波段范围内,双折射始终保持10-1量级.该光纤可以作为保偏光纤应用于偏振控制、相干通信和光纤传感系统.     

一种高双折射光子晶体光纤模场和偏振特性的研究

采用平面波展开法分析一种填入了聚甲基丙稀酸甲酯并引入大空气孔的高双折射光子晶体光纤的模场和偏振特性,并研究其结构参数变化对偏振特性的影响。研究表明这种高双折射光纤的基模模场具有较强的线偏振特性,模式双折射比普通光子晶体保偏光纤有较大提高。研究结果为光子晶体保偏光纤的开发制作提供了理论基础。     

一种新型高双折射光子晶体光纤

提出了一种新的高双折射光子晶体光纤结构.应用全矢量频域有限差分方法所做的数值分析表明：该结构光纤基模的两个正交偏振态不再简并，其模式呈现很强的线偏振特性,并且模式双折射与结构参数设置有密切关系.通过选择合适的结构参数，可以使之达到10^-2量级，比传统的D型和熊猫型保偏光纤高出2个数量级.合理设计光纤包层的几何结构，可以取得理想的色散效果.这种结构的光子晶体光纤可用于制作具有适当色散特性或偏振特性的保偏光纤及相关光纤器件. 关键词： 光子晶体光纤 模式双折射 偏振特性 频域有限差分法     

Optical microfiber passive components

Optical microfiber waveguides with diameters close to the wavelength of light possess an intriguing combination of properties, such a tight modal confinement, tailorable dispersion, and high nonlinearity, which have been utilized in many passive applications. Here, the key fabrication techniques and optical properties of microfibers are introduced, followed by a discussion of the various passive microfiber devices and sensors. Applications exploiting their strong confinement are reviewed, including harmonic generation, supercontinuum sources, gratings, tips for optical trapping and intracellular sensing and subwavelength light sources, as well as devices based on large evanescent fields such as couplers, interferometers, optical manipulators, sensors, and resonators. Furthermore, the properties and practical intricacies of manufacturing various microfiber resonators are evaluated, with a focus on their applications in sensing ranging from temperature monitoring to current, pressure, refractive index and chemicals detection.     

Wavelength and sensitivity tunable long period gratings fabricated in fluid-cladding microfibers

We report the fabrication of long period gratings in fluid-cladding microfibers by directly focusing a femtosecond laser beam on the microfibers surface to induce periodical modification a long one side of the microfibers.A long period grating is fabricated in a water-cladding microfiber with a diameter of~5μm,which demonstrates a resonant attenuation of 28.53 dB at wavelength of 1588.1 nm with 10 pitches.When water cladding is changed to be refractive index oil of n=1.33 and alcohol solution with concentration of 5%,the resonance wavelength shifts to 1575.1 nm with resonant attenuation of 24.91 dB and 1594.1 nm with resonant attenuation of 35.9 dB,respectively.The long period grating demonstrates different temperature sensitivities of-0.524 nm/℃,-0.767 nm/℃and-1.316 nm/℃for water,alcohol solution and refractive index oil cladding microfibers,respectively,which means the alterable liquid cladding allows the availability of tunable wavelength and sensitivity.The fluid-cladding protects the microfibers from external disturbance and contamination and allows more flexibility in controlling the transmission property and sensing characteristics of long period gratings,which can be used as fiber devices and sensors for chemical,biological,and environmental applications.     

二氧化硅微光纤与一维纳米材料集成器件的原理与应用

微光纤器件由于其优异的光波导特性、较高的机械强度、操作灵活性和易于集成的特性，已经受到越来越多的关注，并且实现了在传感、激光、生物化学等领域的广泛应用。然而，二氧化硅微光纤器件尺寸相对较大、材料折射率较低，使其在微纳尺度的气、液态环境中的应用具有极大的挑战性。一维纳米材料的出现弥补了这一不足，由于其具有更小的尺寸和更高的折射率，基于单纳米线集成的微光纤器件在一些特殊环境中可以实现更高灵敏度和更高空间分辨率，拥有广阔的应用前景。本文从器件制备、耦合原理以及具体应用等方面对纳米线集成的微光纤器件进行了系统的介绍，并简要介绍了国内外以及南京大学近年来在该领域的研究进展。     

Compact microfiber Bragg gratings with high-index contrast

We fabricate fiber Bragg grating (FBG) in microfibers (MFs) using focused ion beam milling technique. By periodically etching 100 nm-depth grooves on the surface of silica MFs with diameters less than 2 μm, evident grating features with transmission dip up to 15 dB are obtained. Because of the high-index contrast of the gratings structure, the length of the microfiber Bragg grating (MFBG) can be reduced to 500 μm level. Using a 518 μm-length 1.8 μm-diameter MFBG, we also demonstrate sensitivity up to 660 nm per refractive index unit (RIU) for refractive index (RI) sensing. The highly compact MFBGs demonstrated here may serve as low-dimensional building blocks for miniaturized photonic components and devices.     

Refractive index sensor using microfiber-based Mach-Zehnder interferometer

A simple and robust refractive index (RI) sensor based on a Mach-Zehnder interferometer has been demonstrated. A section of optical microfiber drawn from silica fiber is employed as the sensing arm. Because of the evanescent field, a slight change of the ambient RI will lead to the variation of the microfiber propagation constant, which will further change the optical length. In order to compensate the variation of the optical length difference, a tunable optical delay line (ODL) is inserted into the other arm. By measuring the delay of the ODL, the ambient RI can be simply demodulated. A high RI sensitivity of about 7159 μm/refractive index unit is achieved at microfiber diameter of 2.0 μm.     

Optical thermometry probe via fiber containing β-NaLuF4:Yb3+/Er3+/Tm3+

In this article, β-NaLuF₄:Yb³⁺/Tm³⁺/Er³⁺ were synthesized by thermal decomposition technique. Yb³⁺/Tm³⁺/Er³⁺ has been firstly manifested to fabricate the microfibers after co-doping with polymer solution. The guiding performance of a microfiber was observed under 980 nm excitation source to prove that it could be used as an active and passive waveguides. The characteristics of optical thermal sensing of Tm³⁺ and Er³⁺ ions in a single microfiber were demonstrated. The FIR method was utilized to evaluate the sensitivity of a single microfiber and maximum sensitivity of activator ions in a microfiber was achieved as 0.00124 and 0.00311 K^-1 with temperature ranging from 303 to 363 K, respectively. Our results suggest that a single microfiber has good sensing stability and has potential applications in the field of thermometry.     

Probing optical microfiber nonuniformities at nanoscale

We demonstrate a novel, simple, and comprehensive method for probing optical microfiber surface and bulk distortions with subnanometer accuracy. The method employs a regular optical fiber as a probe that slides along a microfiber transmitting the fundamental mode. The fraction of radiation power absorbed in the probe depends on the local distribution of the mode propagating in the microfiber. From the measured variation of the absorbed power, we determine the variation of the effective microfiber radius, which takes into account both the microfiber radius and refractive index variations. Furthermore, we verify the cylindrical symmetry of the microfiber nonuniformities by probing the microfiber from different sides. These results explain observed transmission losses in silica microfibers and open broad opportunities for microfiber investigation.     

微米光纤耦合型环境折射率传感器

本文展示一种由两根微米光纤构成的耦合型环境折射率传感器。微米光纤表面存在倏逝波,芯外介质折射率微小的改变将引起倏逝场分布变化,从而导致能量在耦合器两臂中的重新分配。本文通过测量耦合器两端出射光功率的比值来对葡萄糖溶液的浓度进行传感,实验结果表明该微米光纤耦合型传感器的浓度传感灵敏度可达1.58 mol·L~(-1)。本文用耦合模理论详细分析了两根相邻微米光纤的耦合效率与芯外介质折射率之间的关系,发现减小耦合区的光纤的直径,增大入射光波长或增大耦合长度,都可以进一步提高这种传感器的灵敏度。     

Bragg gratings in rectangular microfiber for temperature independent refractive index sensing

We present fiber Bragg grating fabricated in rectangular microfiber for temperature-independent refractive index (RI) measurement. The grating has two Bragg peaks due to the high geometrical birefringence of the rectangular microfiber. The two peaks present different RI responses because the modes along the orthogonal polarizations have different energy fractions in terms of evanescent field outside the silica microfiber and hence the light/liquid interaction strength are different. In contrast, they exhibit identical temperature sensitivities because most mode energy is confined in the microfiber and the thermal-optic effect of silica dominantly determines the temperature response. As a result, temperature-independent RI sensing can be realized by monitoring the wavelength separation between the two peaks.     

High-performance refractive index sensor based on guided-mode resonance in all-dielectric nano-silt array

We propose a high-quality refractive index sensing by utilizing the characteristic guided-mode resonance (GMR) excited in a simple one-dimensional all-dielectric nano-slit array. We demonstrate a figure of merit up to 12000, which is higher than that achieved in most plasmonic refractive index sensors. We show that high-quality sensing performance can be sustained over a broadband range in near infrared region with relatively large variations in both grating depth and angle of incidence. Such a GMR-based all-dielectric sensing device with the ease of fabrication is expected to hold great promise for realizing broadband refractive index sensors with high performance and compactness.