热可调液晶填充微结构聚合物光纤设计及特性分析

通过在微结构聚合物光纤(mPOF)的包层空气孔中填充液晶材料获得了高度热可调的带隙型mPOF. 带隙随温度的增加发生显著的蓝移, 带隙上边界的温度灵敏度可达-5.5 nm/℃. 采用全矢量有限元方法对模场特性以及基模有效模场面积的分析结果表明, 该填充液晶的mPOF在带隙的中心波长附近具有大的有效模场面积, 和相同结构未填充液晶的mPOF连接时具有较高的功率耦合效率. 研究结论为mPOF在温度传感领域的应用及各种可调光纤器件的制备提供了理论参考.     

基于高折射率液体填充的花瓣形微结构光纤可调滤模特性

提出一种新型的可调滤模光纤结构,利用纤芯模式与微结构包层形成的超模群之间的耦合实现选择性滤模,采用花瓣形包层结构使包层中传输的模式更容易产生高的泄漏损耗;提出以液体填充包层介质柱,使包层形成的超模群有效折射率区间可以通过环境温度来调节,从而达到可调选择性滤模目的.利用液体柱的LP11模所形成的超模群,有效增大了其工作带宽和温度调谐范围.数值模拟结果表明,采用长度仅为71.4 mm的滤模光纤,可以使特定的抑制模式损耗达到20 dB以上,而其他模式损耗均在1 dB以下.提出的光纤可以在少模光纤传输系统中作为滤模器使用,以降低模式转换器、复用器/解复用器以及光开关和光路由等的模式串扰.     

Theoretical Research on Sensitivity for Resonance Wavelength of Long Period Fiber Grating

In a long period fiber grating(LPFG), the forward-propagating guided mode in the core is coupled to a number of co-propagating lossy cladding modes at resonance wavelengths. Sensitivity of resonance wavelength against temperature, axial strain, and external refractive index is modeled in this paper. Schemes for temperature compensation and enhanced wavelength tuning of LPFG are proposed and analyzed. Experiments show accordant results with simulations. Temperature coefficient of the LPFG compensated by means of axial strain is below 1.8 nm/100℃. For the LPFG with polymer recoating layer, the enhanced tuning range is more than 70 nm.     

Theoretical Research on Sensitivity for Resonance Wavelength of Long Period Fiber Grating

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Coupling to circularly asymmetric modes via long-period gratings made in a standard straight fiber

We demonstrate long-period fiber gratings made in a straight standard optical fiber via a CO₂ laser side irradiation that couples light between modes of different azimuthal symmetry. We show that such coupling can be even stronger than that to the azimuthally symmetric modes, provided the photoinduced asymmetry in the refractive index change across the fiber cross-section is high enough. This suggests that some previously-observed phenomena with CO₂-written LPGs not observed with other types of LPGs may be attributed to LPG resonances that couple light into the cladding modes with higher azimuthal symmetry. We show it on an example of bend-induced LPG resonance position tuning with the resonance depth maintained constant.     

Measurement of modal dispersion in optical fiber by means of acousto-optic coupling

We show that frequency-wavelength tuning characteristics of acousto-optic coupling can be used for measuring the difference of effective index, group index, and chromatic dispersion between core and cladding modes in single-mode fibers. Chromatic dispersion measurements of a 30-cm-long conventional single-mode fiber, a nonzero dispersion-shifted fiber, and a dispersion-compensating fiber with this new method are presented for the wavelength range 1500-1600 nm. Qualitative agreement with independently measured data is obtained.     

Analysis of a widely tunable long-period grating by use of an ultrathin cladding layer and higher-order cladding mode coupling

A widely tunable long-period grating in single-mode fiber is analyzed by use of an ultrathin cladding layer and higher-order cladding mode coupling. The numerical simulation shows that a 225-nm tuning range in the newly designed ultrathin long-period grating (cladding thickness, 35 microm) with third-order cladding mode coupling can be obtained. The analyzed tuning range is seven times wider than those of the other known long-period gratings. We believe that the proposed highly sensitive long-period grating will be widely used as a gain-flattening filter for ultrawideband optical amplifiers and fast tunable filters in dynamic optical communication systems.     

Highly tunable birefringent microstructured optical fiber

We demonstrate a method for introducing and dynamically tuning birefringence in a microstructured optical fiber. Waveguide asymmetry in the fiber is obtained by selective filling of air holes with polymer, and tunability is achieved by temperature tuning of the polymer's index. The fiber is tapered such that the mode field expands into the cladding and efficiently overlaps the polymer that has been infused into the air holes, ensuring enhanced tunability and low splice loss. Experimental results are compared with numerical simulations made with the beam propagation method and confirm birefringence tuning that corresponds to a phase change of 6pi for a 1-cm length of fiber.     

Polymer Optical Fiber Photosensitivities and Highly Tunable Fiber Gratings

In this paper, recent progress in the investigation of photosensitivities of polymer optical fibers and the development of polymer optical fiber grating is reported. Photosensitivities in various polymer fibers, including doped and undoped, multimode and single-mode polymer fibers, have been experimentally characterized and evaluated for fiber grating application. In particular, the wavelength dependence of material absorption and photosensitivity has been found essential to the fabrication of polymer optical fiber grating. Based on the results of photosensitivity research, polymer optical fiber gratings have been successfully fabricated. More importantly, polymer optical fiber grating has been demonstrated highly tunable, with a tuning range more than 70 nm. The unique feature of high tunability in polymer optical fiber grating has great potential in various applications including optical fiber WDM systems and fiber sensor systems. Several important issues that remain to be investigated will also be discussed.     

Polymer Optical Fiber Photosensitivities and Highly Tunable Fiber Gratings

In this paper, recent progress in the investigation of photosensitivities of polymer optical fibers and the development of polymer optical fiber grating is reported. Photosensitivities in various polymer fibers, including doped and undoped, multimode and single-mode polymer fibers, have been experimentally characterized and evaluated for fiber grating application. In particular, the wavelength dependence of material absorption and photosensitivity has been found essential to the fabrication of polymer optical fiber grating. Based on the results of photosensitivity research, polymer optical fiber gratings have been successfully fabricated. More importantly, polymer optical fiber grating has been demonstrated highly tunable, with a tuning range more than 70 nm. The unique feature of high tunability in polymer optical fiber grating has great potential in various applications including optical fiber WDM systems and fiber sensor systems. Several important issues that remain to be investigated will also be discussed.     

长周期光纤光栅中的包层模谐振

本文给出了光纤中包层模的电场形式,指出包层膜的电场形式与弱导光纤中导模的电场形式不同,对光纤中基模与包层模电场沿光纤横截面分布的重叠情况的分析表明,长周期光纤光栅除了能将HE₁₁导模耦合到HE_1p包层模外,也可以将其耦合到EH_1p包层模。基于耦合模理论的计算表明,耦合到EH_1p和EH_1,p-1包层模的耦合波长相近,对于典型的单模光纤,当p>4时耦合到这两个模式的耦合系数是可比的。     

Broadband micro-Michelson interferometer with multi-optical-path beating using a sphered-end hollow fiber

We demonstrate a high-sensitivity broadband (1250-1650 nm) fiber micro-Michelson interferometer using a single-mode fiber end-spliced with a sphered-end hollow-core fiber. The hollow core is slightly smaller than the solid core of a single-mode fiber, so the fractional power of the core mode is converted into cladding modes. The excited cladding modes propagate at distinct optical paths along the hollow-core fiber and have individual foci outside the spherical lens. The reflected core mode, generated at the solid core-air interface, and the reflected cladding modes, generated at external material, interfere with each other to produce beating in the interference signals.     

单模手征光纤研究

对圆芯和内外包层都为手征介质构成的双包层手征光纤（W型）进行了解求解,导出了模式场解及模式特征方程,然后用数值计算的方法研究了手征参数对模式的色散曲线和截止频率的影响,结果发现在双包层光纤的纤芯中引入手征或在单包层光纤的包层中引入手征,都可以实现单模工作。     

Tunable optical add-drop multiplexer based on long-period fiber gratings for coarse wavelength division multiplexing systems

We propose and experimentally demonstrate a voltage-controllable add-drop multiplexer with a tunable coupler based on long-period fiber gratings (LPFGs). The tunable coupler is based on cladding mode coupling between two parallel LPFGs with divided coil heaters, which controls the resonant wavelength positions of the two LPFGs. The wavelength tunability of the tunable coupler is measured to be -0.45 nm/degrees C in the temperature range from 20 to 120 degrees C. The large tuning range of optical signals (approximately 50.54 nm) from 1502.32 to 1552.86 nm, which covers both the short (S) and the conventional (C) bands, is achieved. The channel isolation is as high as approximately 40 dB. It has several advantages, including a broad tuning range of wavelength in both the S- and C-bands, high channel isolation, no backreflection, and so on.     

微结构芯大模场平顶光纤及其传输特性分析

通过在纤芯引入非均匀分布的高折射率介质柱, 获得了大模场面积、平顶型模场分布的新型微结构光纤. 运用数值仿真, 分析了光纤在不同的结构参数下的模场面积、基模模场分布等特性. 研究结果表明: 通过合理设计光纤的包层和纤芯参数, 可以实现有效的平顶模场分布. 在工作波长为1064nm时, 平顶光纤的基模模场面积可以达到1655μm². 关键词： 大模场面积 平顶光纤 微结构芯 数值仿真     

Design of Topas microstructured fiber with ultra-flattened chromatic dispersion and high birefringence

A microstructured polymer optical fiber (mPOF) with both ultra-flattened near-zero chromatic dispersion and high birefringence based on Topas cyclic olefin copolymer is designed. Three rings of uniform elliptical air holes are arranged in triangular lattice in the cladding and an extra small defected hole is introduced in the fiber core. Guided modes, dispersion, birefringence and mode confinement properties of the designed mPOF are investigated by using the full-vector finite element method. Dispersion values between ± 0.5 ps/km/nm over the wavelength 1.1-1.7 μm and high birefringence of the order of 10⁻³ are obtained for the optimized fiber structure. Low confinement losses and small effective mode area are obtained at the same time. The relatively simple architecture of the proposed Topas mPOF can be fabricated by our extrusion-stretching techniques.     

基于包层模的光纤布拉格光栅折射率传感特性

提出了基于光纤布拉格光栅(FBG)包层模式的折射率传感方案。实验中,利用不同浓度的丙三醇水溶液作为外界折射率传感溶液,采用氢氟酸溶液化学腐蚀的方法来减小光纤包层的直径以增大包层模式对外界折射率的敏感度,研究了腐蚀后光纤布拉格光栅包层模式的耦合波长对外部折射率的变化关系。实验结果表明在1.3300~1.4584的折射率范围内,包层模式耦合波长随外界折射率增大而增大,在接近光纤包层折射率处具有很高的折射率灵敏度,最大达到了172 nm/riu(refractive index unit)。而且,包层模谐振的光谱半峰全宽(约0.07 nm)仅为布拉格纤芯模谐振光谱半峰全宽的1/4,能够获得更好的传感精度。     

New plastic optical fiber using polycarbonate core and fluorescence-doped fiber for high temperature use

This article describes the development of a plastic optical fiber composed of a polycarbonate core with a glass transition temperature of 150°C, and a cladding of newly developed poly-4-methyl penten-1, which softens at 173°C. This cladding is suitable for use at temperatures up to 130°C. The minimum optical attenuation is 0.8 dB/m at 765 nm in the near-infrared region. The cause of the attenuation of the PC-core fiber was analyzed and the intrinsic loss limit was estimated to be 0.4 dB/m at 765 nm. The fiber has excellent characteristics, including thermal stability up to 125°C, high flexibility, high strength, and self-extinguishing properties.

The polycarbonate core fiber, doped organic fluorescing materials, has also been developed for automotive uses such as light guide and illuminator. Light can be transmitted through this fiber with incident optical beam perpendicular to the fiber as well as the beam parallel to the fiber.     

New plastic optical fiber using polycarbonate core and fluorescence-doped fiber for high temperature use

This article describes the development of a plastic optical fiber composed of a polycarbonate core with a glass transition temperature of 150°C, and a cladding of newly developed poly-4-methyl penten-1, which softens at 173°C. This cladding is suitable for use at temperatures up to 130°C. The minimum optical attenuation is 0.8 dB/m at 765 nm in the near-infrared region. The cause of the attenuation of the PC-core fiber was analyzed and the intrinsic loss limit was estimated to be 0.4 dB/m at 765 nm. The fiber has excellent characteristics, including thermal stability up to 125°C, high flexibility, high strength, and self-extinguishing properties.

The polycarbonate core fiber, doped organic fluorescing materials, has also been developed for automotive uses such as light guide and illuminator. Light can be transmitted through this fiber with incident optical beam perpendicular to the fiber as well as the beam parallel to the fiber.     

Investigation on characteristics of W-type fiber with an inner cladding made of negative refractive index materials

A doubly clad optical fiber with an inner cladding made of negative refractive index material was proposed and the modes distribution, dispersion and time delay were investigated by fully vectorial numerical method. The results indicate that HE₁₂ mode can be operated as a single mode, the cut-off frequency of modes shifts to low normalized frequency with increasing thickness of inner cladding made of negative index materials, large negative dispersion and time delay can be obtained and tailored by the inner cladding. These results provide theoretical basis for designing novel fibers or optical devices.