Simultaneous measurement of refractive index, temperature and strain based on core diameter mismatch and polarization-maintaining FBG

A kind of fiber-optic sensor for simultaneous measurement of refractive index of surrounding medium, temperature and strain is described. Based on core diameter mismatch, a multimode-single mode-multimode (MSM) structure is presented. It is demonstrated that the three parameters can be measured respectively by the interference of the core mode and cladding modes excited in the single mode fiber (SMF). Then combined with a polarization-maintaining fiber Bragg grating (PMFBG) which has different sensing properties from MSM structure, three parameters are measured simultaneously. The experimental results show that PMFBG is insensitive to the refractive index and the refractive index sensitivity of the MSM structure is 96.04 nm·RIU^?1; the temperature sensitivities of the characteristic wavelength for MSM structure and the center wavelengths of fast and slow axes for PMFBG are 0.0911 nm · °C^?1, 0.00976 nm · °C^?1 and 0.0105 nm · °C^?1, respectively; the strain sensitivities of those are ?0.013 nm · μ?^?1, 0.012 nm · μ?^?1 and 0.012 nm · μ?^?1, respectively.     

Fourfold power efficiency improvement in organic light-emitting devices using an embedded nanocomposite scattering layer

It has been demonstrated that internal extraction structures (IES) can be introduced in OLEDs to decrease the ratio of the waveguide mode and simultaneously increase the ratios of the substrate and radiation modes. In this study, titanium oxide (TiO₂) nanoparticles (NPs) combined with transparent photoresist (TPR) were utilized to form an embedded nanocomposite scattering layer between the indium-tin-oxide (ITO) and glass substrate, leading to a significant boost in the out-coupling efficiency of the OLEDs. Inside the nanocomposite scattering layer, NPs of different sizes served distinct functions. The 250 nm-TiO₂ particles were used to induce scattering and diminish the light reflection back to the ITO layer. On the other hand, the refractive index of the TPR can be increased by increasing the concentration of the 25 nm-TiO₂, which reduced the difference in the refractive index between the ITO and TPR and thus multiplied the amount of light entering into the scattering layer. By employing nanocomposite substrate with mixed dual-sized NPs, we obtained power efficiencies of the blue phosphorescent OLEDs that were about 4.3 times higher than that of the control device at the high luminance of 5 × 10³ cd/m².     

Polymer-based scattering layers for internal light extraction from organic light emitting diodes

Efficient light extraction for organic light emitting diodes (OLED) using scalable processes and low-cost materials are important prerequisites for the future commercialization of OLED lighting devices. The light-extraction technology exhibited in this paper uses polymer-based high-refractive index scattering layers processed from solution. The scatter matrix formulation incorporates two types of nanoparticles for refractive index tuning and scattering, respectively. Planarization by the same material in order to reduce surface defects was critical for achieving highly increased device yield. Highly efficient and defect-free large-area (1.8 cm²) white OLED devices were fabricated on top of the scattering layer in a bottom emitter configuration. Light extraction enhancement leads to an overall efficiency gain of up to 81% for luminances of 5000 cd m⁻².     

A digital signal-processing analysis technique for the infrared reflectivity characterization of ion implanted silicon

A new method for the characterization of ion-implanted silicon is proposed. It is based on analysis of the Fourier spectrum of bilinearly transformed infrared reflectance versus wavenumber data of ion-implanted samples. This non-destructive technique has been applied to previously published infrared reflectance data of 〈111〉 and 〈100〉 oriented Si samples which had been implanted with 2.7 MeV phosphorus and 380 keV silicon ions, respectively, and annealed at 500° C for various lengths of time. The refractive index and thickness of the amorphous layer of the as-implanted samples can be measured directly by means of this technique. The position of boundaries between the amorphous, recrystallized and substrate zones, as well as the position of the carrier concentration peak can be determined for the various annealing times. Depending on the annealing time, the recrystallized layer in 〈111〉 silicon has a refractive index which is between 2% and 4% higher than the substrate refractive index, while the difference in refractive index between the amorphous and recrystallized layers is in the order of 5%. In contrast to these results, the presence of the substrate/recrystallized material interface could not be detected in partially recrystallized 〈100〉 silicon by this method, implying that the refractive index step at the substrate/recrystallized material interface is less than 1%. The step in refractive index at the crystalline/amorphous interface in 〈100〉 silicon implanted with a dose of 0.5 x 10¹⁶ cm⁻² silicon ions, was measured to be 12%, and it is reduced to 8% after partial regrowth has occurred. These results confirm the data obtained by a model-based least-squares analysis.     

用于测量折射率的光纤迈克尔逊干涉型传感器

提出了一种基于单模光纤-四芯光纤-薄芯光纤(SMF-FCF-TCF)迈克尔逊干涉结构的折射率传感器。采用直接熔接的方式将各光纤进行熔接,由于各光纤之间纤芯的直径不匹配,因此在光纤的熔接处会发生光的激发和耦合。薄芯光纤端面涂覆有一层银面反射膜并用紫外固化胶进行保护来增强光在端面的反射率。四芯光纤作为传感结构中的耦合器,激发了更多的光进入薄芯光纤的包层中,提升了传感器的灵敏度。对传感器的折射率和温度传感特性分别进行了实验探究,实验结果表明,在折射率1.3333~1.3794范围内的灵敏度为137.317 nm/RIU,线性度为0.999,并且温度对传感器的影响较小。该传感结构熔接方式简单,在折射率测量领域具有一定的应用前景。     

Refractive indices variation with temperature and humidity of optical adhesive

Refractive index and birefringence of optical adhesives is studied under different environmental exposures, such as temperature, humidity, and annealing. Refractive index of the adhesive decreases at a higher rate for the temperature treatment compared to the humidity exposure. A sudden decrease in the rate of refractive index (thermo-optic coefficient, dn/dT), from −2×10⁻⁴°C⁻¹ to −4×10⁻⁴°C⁻¹ was observed near the glass transition temperature. The adhesive’s refractive index (n) was studied during the moisture desorption cycle. Refractive index of the adhesive was found to increase at a constant rate with moisture. The change in refractive index of the adhesive upon moisture absorption was found to be irreversible. Birefringence (T_E — T_M) decreases with higher annealing temperature treatment of the adhesive, because the rate of increase in refractive indices for T_E and T_M modes is not similar. The refractive index in T_M mode increases at a higher rate compared to that in the T_E mode. This suggests that the material has become more isotropic due to annealing. The environmental changes in optical properties of the adhesive are discussed in light of Lorenz-Lorentz equations.     

Nanostructure Core Fiber With Enhanced Performances: Design, Fabrication and Devices

We report a new type of silica-based all-solid fiber with a 2-D nanostructure core. The nanostructure core fiber (NCF) is formed by a 2-D array of high-index rods of sub-wavelength dimensions. We theoretically study the birefringence property of such fibers over a large wavelength range. Large-mode-area (LMA) structure with a typical high birefringence in the order of $10^{-4}$ can be easily realized. The attenuation of the fabricated NCF is as low as 3.5 dB/km at 1550 nm. Higher macro- and micro-bending losses compared with those of the single-mode fiber (SMF) due to the reduced index difference have been observed experimentally, which suggests that the NCF is potentially useful for curvature and strain sensing applications. A fiber Bragg grating (FBG) inscribed in such a novel fiber is side-polished to make use of its evanescent field for refractive index sensing. The refractive index sensitivity obtained is one order of magnitude higher than that of the side-polished FBG in SMF, while the temperature and strain performances are comparable with those of the SMF-based FBG.      

微型准直透镜的研究与设计

自聚焦透镜与多模光纤的折射率分布都遵从平方律分布,基于这个事实,进行了以多模光纤生产工艺制造微型准直透镜的研究和设计。结果表明,该微型准直透镜用作单模光纤准直器,具有良好的准直性能,该器件较之自聚焦透镜,尺寸更小且制造工艺简单。     

采用多模光纤透镜的光纤麦克风理论与实验研究

对现有强度调制型光纤麦克风做了分析和讨论,采用单模光纤(SMF)熔接多模光纤(MMF)的方法设计了一种新的反射式光纤麦克风。适当长度的折射率分布为抛物线形的多模光纤具有准直透镜的作用,直接熔接在单模光纤上,可以使光纤麦克风进一步微型化、成本低廉化。用单频正弦波信号做声源测试该光纤麦克风,输出正弦波电压信号与声源频率一致,振幅随声源振幅线性变化。结果表明,该结构的光纤麦克风在原理和实验上可以用于语音通信中。     

Design and analysis of a refractive index sensor based on dual-core large-mode-area fiber

We present a novel co-axial dual core large-mode-area (LMA) fiber design for refractive index sensing. In a dual-core fiber there is resonant coupling between the two cores, which is strongly affected by the refractive index (RI) of the outermost region. The transmittance of the fiber, therefore, varies sharply with the refractive index of surrounding medium. This characteristic of the proposed structure has been utilized to design a RI sensor. We have analyzed the structure by using the transfer matrix method. Our numerical results show that the proposed sensor is highly sensitive with the resolution of 2.0 × 10⁻⁶ around n_ex = 1.44376. Effect of design parameters on sensitivity of the proposed sensor has also been investigated.     

小角前向散射激光测粒仪中折射率对测量结果的影响

在经典Mie散射理论基础上,利用POWELL反演方法研究了小角前向散射激光测粒仪中输入折射率对测量结果的影响。对于输入折射率与待测颗粒实际折射率没有偏差的情况,数据处理导致的测量误差在0.002%之内;而对于两者存在偏差的情况,当输入折射率在(2.0,4.0)范围内时数据处理误差较小,输入折射率在(1.1,2.0)范围内时数据处理误差显然较大。从而得到结论:在测量过程中应尽可能输入与颗粒实际折射率一致的数值;当无法确知颗粒实际折射率时,比较可靠的办法是适当选择一个大于2.0的数值作为输入折射率进行数据处理     

A Promising Radiation Thermal Protection Coating Based on Lamellar Porous Ca-Cr co-Doped Y3NbO7 Ceramic

Dissipation of heat efficiently from a hot object via radiation while minimizing the inward heat conduction is the key requirement of radiation thermal protection. In this study, a Ca-Cr co-doped Y₃NbO₇ coating with lamellar porous structure is fabricated, which shows an ultra-low thermal conductivity (<0.7 W m⁻¹ K⁻¹) and near-unity emissivity (>0.9) across a broad wavelength range of ≈1–24 µm. This record high emissivity to thermal conductivity ratio (≈1.3) is experimentally and theoretically revealed from a multi-scale perspective. The diffusoin-mediated thermal conduction feature of niobates combined with lamellar porous structure of the coating reduces its thermal conductivity to an impressive 0.5 W m⁻¹ K⁻¹ at 25 °C, surpassing the theoretical amorphous limitation of 0.72 W m⁻¹ K⁻¹. Experiments and FDTD calculation results demonstrate that the intrinsic emissivity dips at shallow extinction wavelengths (1 and 8 µm) and strong phonon-polariton resonances wavelengths (>13 µm) can be effectively compensated by the multiple scattering/absorption and gradual modulation of conical shape/effective refractive index induced by surface micro-protrusion structures, respectively. Furthermore, the coating exhibits robust mechanical and thermal stability with a high bonding strength (18.3 MPa) and thermal expansion coefficient (≈11 × 10⁻⁶ K⁻¹ at 1200 °C) comparable to YSZ, showing great potential in the radiation thermal protection field.     

对PPV衍生物非线性折射率的测量

利用532nm的连续激光对烷氧基非对称聚对苯撑乙烯衍生物的氯仿溶液进行诱导衍射实验,通过观察衍射环的个数可以估算出材料的克尔常数及非线性折射率。实验发现,当入射激光功率达到阈值0.77mW而小于3.36mW时,通过在接收屏处观察光克尔效应对激光横截面上的空间自相位调制而产生的衍射环个数,可以估算出样品溶液的克尔常数γ的大小近似为5.55×10-10m2/W及非线性折射率n2的大小为9.21×10-4esu。当入射激光功率继续增大时,由于热效应对激光横截面上的空间自相位调制产生的衍射环逐步淹没光克尔效应引起的衍射环,接收屏上的衍射图案将发生显著的变化。     

Synthesis and nonlinear optical behavior of Ag nanoparticles in PMMA

In this work we have synthesized silver nanoparticles in Poly (methyl methacrylate) (PMMA). This was achieved by polymerizing the mixture of monomer and corresponding metal compound, followed by post-heating treatment. The linear absorption coefficient of the samples was measured using a spectrophotometer, where an absorption peak at 420 nm was observed. This peak grows up and shifts as a function of the concentration of the radical initiator. The linear refractive index was measured using the Fresnel equations and agrees with previous reported results. The nonlinear properties were obtained using the single lens Z-scan method, where the nonlinear absorption coefficient (Δα) was found between 5.5975514 and 17.9483493 cm⁻¹. The nonlinear refractive index coefficient (Δη) was found to be negative and its value oscillates between 12.9099 E-06 and 22.4276 E-06. Finally, the third-order coefficient (χ⁽³⁾) was calculated in the range of 233-787 E-9 esu.     

基于多模干涉和长周期光纤光栅的温度及折射率同时测量

基于多模干涉理论和长周期光纤光栅(LPFG)的传感特性,提出了一种单模-多模-单模(SMS)结构与LPFG级联的光纤传感器,实现了温度和折射率的同时测量。实验结果表明,SMS结构的干涉谱和LPFG对温度和折射率具有不同响应灵敏度,其温度灵敏度分别为0.017nm/℃和0.060nm/℃;SMS结构对折射率不敏感,而LPFG的折射率灵敏度为-35.60nm/RIU(RIU为折射率单位)。因此利用敏感矩阵,实现对温度和折射率的同时测量,得到温度和折射率的最大测量误差分别为±0.59℃和±0.0013。该结构灵敏度高、结构简单,且不易受电磁等干扰。实验结果具有良好的线性度,在生物化学领域应用前景良好。     

Frozen-in viscoelasticity for novel beam expanders and high-power connectors

Frozen-in viscoelasticity has a significant, yet previously overlooked, impact on the refractive index profile of optical fibers and can serve as the basis for novel fiber devices. Viscoelastic strains can be frozen into an optical fiber during draw, or by cooling down a drawn fiber from high temperature while under tension. The resulting refractive index perturbation can exceed 0.003, exhibits minimal birefringence, and is concentrated in the highest viscosity region of the fiber. We have developed a quantitative theory for the refractive index perturbation, which is in excellent agreement with measured data. We have harnessed frozen-in viscoelasticity to create novel high-performance all-fiber beam expanders achieving mode field diameters in excess of 40 /spl mu/m. These beam expanders are suitable for low-cost high-volume manufacturing and can serve as the basis for high-power single-mode fiber (SMF) connectors, since their lower power density improves reliability during high-power operation. Because of the significant reduction in free-space diffraction associated with the enlarged mode field diameter, these beam expanders may be suitable for MEMS applications, or for coupling between optical fiber terminations and optical fiber sources or detectors.     

Ordered 3D Thin‐Shell Nanolattice Materials with Near‐Unity Refractive Indices

The refractive indices of naturally occurring materials are limited, and there exists an index gap between indices of air and available solid materials. With many photonics and electronics applications, there has been considerable effort in creating artificial materials with optical and dielectric properties similar to air while simultaneously being mechanically stable to bear load. Here, a class of ordered nanolattice materials consisting of periodic thin‐shell structures with near‐unity refractive index and high stiffness is demonstrated. Using a combination of 3D nanolithography and atomic layer deposition, these ordered nanostructured materials have reduced optical scattering and improved mechanical stability compared to existing randomly porous materials. Using ZnO and Al₂O₃ as the building materials, refractive indices from 1.3 down to 1.025 are achieved. The experimental data can be accurately described by Maxwell Garnett effective media theory, which can provide a guide for index design. The demonstrated low‐index, low‐scattering, and high‐stiffness materials can serve as high‐quality optical films in multilayer photonic structures, waveguides, resonators, and ultra‐low‐k dielectrics.     

OMNIDIRECTIONAL HIGH REFLECTOR FOR INFRARED WAVELENGTH

We have purposed the design of omnidirectional high reflectors with wide bandwidth (nearly 6300A⁰) for the infrared wavelength. Using translational matrix method for numerical calculation, it is found that a complete photonic band gap high reflector covering the wavelength region 11500A⁰ to 17800A⁰ is obtained for the quarter wave stack air / (n_L n_H)⁷ / air by using the tellurium (n=4.60) and TiO₂ (n=2.35) as material of high reflective index and SiO₂(n=1.45) low refractive index. Due to high refractive index of contrast of the structure i.e. SiO₂/tellurium, a large complete photonic band gap, omnidirectional high reflector, has been observed for the infrared wavelengths. This type of omnidirectional high reflector can be used in telecommunication. Besides this, we have studied the effective refractive indices (phase and group) of the proposed structures. It is observed that the effective refractive indices (phase and group) of the high omnidirectional reflector band are largely changed near the band edges due to the high refractive index of contrast, and the reflections from the internal interfaces of the structure.     

掺铁铌酸钾晶体中的光感生各向异性散射及其光学限制效应

光学限制器是光学系统的保护装置,其保护特性 是由光限制材料与光强或功率有关 的光学非线性效应所产生的,其中光折变晶体中的光感生散射便为该效应之一,多年来受到 世界研究者的广泛研究和关注。对于这类光感生散射,有人提出它是由晶体的表面和体散射 产生的,研究使用的晶体为 LiNbO₃:Fe、LiTaO₃、BaTiO₃,并使用固定的晶体样品和 聚焦透 镜间距。为了进一步探索该散射的产生机理,本文研究了掺铁铌酸钾晶体在激光照射下产生 的光感生各向异性散射特性,利用Z扫描技术测量了该晶体中的 光感生折射率变化,并对晶 体中光散射形成的光学限制特性进行了研究。分析表明该各向异性散射主要是由掺铁铌酸钾 晶体中光感生的柱面透镜效应所导致的。利用光折变晶体中光感生光散射效应的光学限制器 ,一方面可以通过改变晶体尺寸、透镜焦距、小孔光阑大小等来调整其性质,另一方面因其 利用的是低功率连续波激光进行工作,所以能够承受更大的功率和更长的寿命。     

Built‐In Haze Glass‐Fabric Reinforced Siloxane Hybrid Film for Efficient Organic Light‐Emitting Diodes (OLEDs)

Substrates with high transmittance and high haze are desired for increasing the light outcoupling efficiency of organic light‐emitting diodes (OLEDs). However, most of the polymer films used as substrate have high transmittance and low haze. Herein, a facile route to fabricate a built‐in haze glass‐fabric reinforced siloxane hybrid (GFRH) film having high total transmittance (≈89%) and high haze (≈89%) is reported using the scattering effect induced by refractive index contrast between the glass fabric and the siloxane hybrid (hybrimer). The hybrimer exhibiting large refractive index contrast with the glass fabric is synthesized by removing the phenyl substituents. Besides its optical properties, the hazy GFRH films exhibit smooth surface (R_sq = 0.2 nm), low thermal expansion (13 ppm °C⁻¹), high chemical stability, and dimensional stability. Owing to the outstanding properties of the GFRH film, OLED is successfully fabricated onto the film exhibiting 74% external quantum efficiency enhancement. The hazy GFRH's unique optical properties, excellent thermal stability, outstanding dimensional stability, and the ability to perform as a transparent electrode enable them as a wide ranging substrate for the flexible optoelectronic devices.