长周期光纤光栅折射率传感器的结构优化

为了提高长周期光纤光栅对环境介质折射率的传感灵敏度,提出一种长周期光纤光栅的周期和包层半径的结构优化.基于长周期光纤光栅的耦合模理论,分析了长周期光纤光栅的周期和包层半径的大小分别与环境介质折射率传感灵敏度的关系,讨论了长周期光纤光栅的周期和包层半径对折射率传感的影响以及控制光栅周期与包层半径对折射率传感的重要性.为使优化的长周期光纤光栅具有实用性,谐振波长设计在1.55 μm的常规波长范围,经过多次摸拟实验,提出最佳优化参量为：Λ=380 μm,rcl=17 μm,对环境介质折射率从1.26~1.38不同值的实验测试,折射率传感灵敏度达到0.000 12,长周期光纤光栅的结构优化获得理想的预期效果.     

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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Simultaneous measurement of refractive index and temperature using dual-period grapefruit microstructured fiber grating

By cascading the long period fiber grating (LPFG) and fiber Bragg grating (FBG) in grapefruit microstructured fiber, a novel dual-period fiber grating sensor is proposed. The refractive index and temperature are measured simultaneously by using the different sensitivity of FBG and LPFG. The relationship between dual-period fiber grating transmission spectrum and refractive index, resonant wavelengths and temperature are analyzed theoretically, respectively. The simulation results show that the accuracy of the sensor in measuring refractive index and temperature is estimated to be 2319.6 nm/RIU in a range from 1.33 to 1.36 and 0.017 nm/°C from 0 °C to 100 °C, respectively. Thus, the sensor has high refractive index sensitivity, and can provide the theoretical foundation for the optical fiber biosensor.     

Widely tunable long-period fiber grating with nm-thick higher refractive index film overlay

By introducing a four-layer step-index waveguide modeling, the characteristics of long-period fiber grating (LPFG) with an nm-thick film overlay, which has higher refractive index than that of fiber cladding are investigated in detail. The influence of both the overlay thickness and refractive index on the tuning ability of LPFG is analyzed. The numerical results demonstrate that spectral response of LPFG is divided into three distinct regions as the overlay deposition increases and the shift of resonant wavelength is drastic in some special thickness range. In conjunction with higher-order cladding mode coupling and fiber cladding etching, the sensitivity of LPFG to the overlay refractive index is enhanced significantly and over 120 nm resonant wavelength tunable range is achieved.     

Design of a coated thinly clad chalcogenide long-period fiber grating refractive index sensor based on dual-peak resonance near the phase matching turning point

A novel method for designing chalcogenide long-period fiber grating (LPFG) sensors based on the dual-peak resonance effect of the LPFG near the phase matching turning point (PMTP) is presented. Refractive index sensing in a high-refractive-index chalcogenide fiber is achieved with a coated thinly clad film. The dual-peak resonant characteristics near the PMTP and the refractive index sensing properties of the LPFG are analyzed first by the phase-matching condition of the LPFG. The effects of film parameters and cladding radius on the sensitivity of refractive index sensing are further discussed. The sensor is optimized by selecting the appropriate film parameters and cladding radius. Simulation results show that the ambient refractive index sensitivity of a dual-peak coated thinly clad chalcogenide LPFG at the PMTP can be 2400 nm/RIU, which is significantly higher than that of non-optimized gratings. It has great application potential in the field of chemical sensing and biosensors.     

长周期光纤光栅折射率传感器的结构优化

为了提高长周期光纤光栅对环境介质折射率的传感灵敏度,提出一种长周期光纤光栅的周期和包层半径的结构优化.基于长周期光纤光栅的耦合模理论,分析了长周期光纤光栅的周期和包层半径的大小分别与环境介质折射率传感灵敏度的关系,讨论了长周期光纤光栅的周期和包层半径对折射率传感的影响以及控制光栅周期与包层半径对折射率传感的重要性.为使...     

基于模式转换及双峰谐振效应的薄包层镀膜长周期光纤光栅特性研究

基于耦合模理论,首先研究了镀膜长周期光纤光栅(LPFG)高阶包层模的模式转换,划分了高阶包层模的非模式转换区及模式转换区。分析了高阶包层模有效折射率随薄膜厚度增加的响应特性,包层模谐振波长在模式转换区的偏移量要大于非模式转换区。在此基础上,研究了不同包层半径下高阶包层模谐振波长随光栅周期的变化情况,结果表明,相同包层半径下模式转换区内双峰间距的偏移量大于非模式转换区;无论在模式转换区还是非模式转换区,包层半径的减小将增加双峰间距的偏移量。最后分析了不同包层半径下的高阶包层模双峰透射谱在模式转换区及非模式转换区内的折射率响应,进而提出了薄包层镀膜LPFG的优化设计方案,当选定敏感膜层厚度及折射率处于镀膜LPFG的模式转换区内,光栅周期靠近相位匹配转折点时,将得到灵敏度高于传统LPFG双峰传感器的镀膜LPFG折射率型双峰传感器;而减小包层半径,将进一步提高传感器的分辨本领。     

长周期光纤光栅的折射率敏感特性

利用光波导的耦合模理论分析了长周期光纤光栅(LPFG)的折射率传感特性,给出了LPFG 的谐振波长相对于环境折射率变化时的漂移量解析表达式.对 LPFG 的折射率传感特性进行了数值模拟.结果表明:在光栅周期不变的情况下,当包层折射率小于且接近外界环境折射率时,波长的漂移量增大,且对应的模次越高、包层半径越小、包层折射率越小,波长漂移量越大,即 LPFG 对应于外界折射率传感灵敏度得到显著提高;当外界环境折射率大于包层折射率时,光栅的谐振波长将近似不变.     

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.     

啁啾长周期光纤光栅的超宽带滤波特性及其切趾优化

基于光纤光栅的模式耦合理论,采用传输矩阵法对啁啾长周期光纤光栅(LPFG)的超宽带滤波特性进行了分析。研究表明当啁啾LPFG的纤芯模同时与同向传输的多个阶次的包层模发生耦合,且与多个不同阶次包层模对应的谐振峰交叠时,其传输谱带宽可扩展到500nm以上,可用做超宽带带阻滤波器。传输谱带宽随模式序数、啁啾系数、光栅长度、周期和折射率调制量的增加单调递增,但随光纤包层半径的增大单调递减。采用高斯折射率切趾技术抑制了传输谱旁瓣,为设计超宽带滤波器提供了新的方法。     

对弯曲不敏感的长周期光纤光栅传感器

发现了用高频CO₂激光写入的长周期光纤光栅的谐振波长对弯曲灵敏度在圆周的不同方向上呈现周期性,且在两个特定的对称方向上对弯曲不敏感(即弯曲曲率达到1.1m^-1,谐振波长仅漂移-0.018nm).据此提出新型对弯曲不敏感的传感器和可调灵敏度弯曲传感器的设计方案,可望从根本上解决长周期光纤光栅在测量中存在的温度、应变或折射率与弯曲之间的交叉敏感问题.     

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。     

Enhancement of sensitivity in long-period fiber gratings with deposition of low-refractive-index materials

It was proved [Opt. Lett. 30, 720 (2005)] that the deposition of an overlay of higher refractive index than that of the cladding on a long-period fiber grating (LPFG) causes large shifts in the attenuation bands induced by the grating. The result is an enhancement of the sensitivity of the LPFG to variations in the ambient and overlay refractive indices or the overlay thickness. The limitation of the previous design to materials with higher refractive indices than that of the cladding of the LPFG is overcome with a five-layer model. To this purpose, a first overlay of higher refractive index than that of the cladding of the LPFG will enhance the sensitivity of the device to variations in the refractive index of a second overlay of lower refractive index than that of the cladding of the LPFG. Moreover, it is proved that, if the second overlay is thick enough, its behavior resembles that of an infinite layer.     

长周期光纤光栅检测混合油的折射率

为了检测柴油煤油混合油的折射率特性,利用耦合模理论分析长周期光纤光栅折射率传感特性,建立长周期光纤光栅中心波长、透射谱峰值损耗与混合油品中柴油含量的关系.结果表明:在混合油中,随着柴油含量的增加,长周期光纤光栅波长发生蓝移,当柴油含量达到使混合油折射率等于包层折射率时,中心波长漂移最大,柴油含量每改变1%,中心波长平均漂移0.622 7nm;柴油含量继续增加,混合油折射率大于包层折射率时,光栅中心波长基本保持不变,但是峰值损耗逐渐增大,带宽减小,之后柴油含量每增加1%,峰值损耗线性增加0.154 6dB.该方法在检测混合燃料各组分含量具有潜在的应用前景.     

长周期光纤光栅模式与耦合的研究

长周期光纤光栅是在光纤纤芯中沿轴向形成折射率周期性调制的带阻型光纤器件。与光纤布拉格光栅不同，它表现为前向传播的纤芯基模与同向传播的各阶次包层模式在特定波长的耦合。研究了纤芯基模、包层模式（HE1t/EH1t）及其有效折射率随波长的变化，研究了纤芯基模与包层模式（HE1t/EH1t）的耦合系数。研究表明，纤芯基模与一阶低次包层模式的有效折射率随波长增大而减小，纤芯基模与HE1t产生的耦合系数远大于与EH1t的耦合系数，并且包层模式次数较低时，耦合系数随次数的升高而增加。这与以前的研究结果有所不同。     

Optimization of photonic bandgap fiber long period grating refractive-index sensors

Long period gratings in low-index contrast solid-core photonic bandgap fibers are a promising platform for fiber-based fluid refractive index sensing with very low detection limits. We provide a comprehensive investigation of the possibilities for refractive index sensing using that principle in a commercial photonic crystal fiber filled with a fluid: using an acoustic grating, we map out the cladding bands, and use this data to optimize a long period grating’s sensitivity. We then implement the optimized long period grating, again using an acoustic grating, and directly measure its sensitivity to refractive index. We demonstrate a sensitivity of 17,900 nm/RIU (6.94 nm/°C) which corresponds to a smallest detectable index change of the fluid of 8.4 × 10⁻⁶.     

Bend-insensitive long-period fiber grating sensors

A novel bend-insensitive long-period fiber grating (LPFG) sensor written using focused CO₂ laser pulses is demonstrated, for the first time, to our knowledge. It is found that the central wavelength shift of such a LPFG is only −0.018 nm even for a curvature of 1.1 m⁻¹ at the most bend-insensitive position of the LPFG. Experimental results show that the bend sensitivity of the central wavelength of the LPFG has a periodic distribution along its circular directions. Such a bend-insensitive sensor could be used to solve the problem of cross-sensitivity between bend and other measurands, such as temperature, strain or refractive index, which is an unsolved problem for LPFG sensors in practice. In addition, the bend sensitivity of the LPFG can be adjusted by selecting its circular positions.     

金属包层长周期光纤光栅的理论和实验研究

金属包层长周期光纤光栅可用于对谐振波长的调谐。通过实验和理论研究了长周期光纤光栅在沉积金属包层前后谐振波长的变化规律。分析了金属包层光纤的特点,给出了金属包层光纤包层模的本征方程,并给出了这一复本征方程的求解方法。长周期光纤光栅在沉积金属包层后,对低阶包层模,谐振波长会向长波方向偏移,模次增加,偏移会增大;对高阶包层模,谐振波长向短波方向偏移。不同金属包层,谐振波长的偏移量也有一定差别。所给出的理论分析方法,可用于预测谐振波长的偏移方向和大小,对设计和制做这类长周期光纤光栅提供理论参考。