Cascaded tilted fiber Bragg grating for enhanced refractive index sensing

We proposed and experimentally demonstrated a cascaded tilted fiber Bragg grating(TFBG)for enhanced refractive index sensing.The TFBG is UV-inscribed in series in ordinary single-mode fiber(SMF)and reduced-diameter SMF with the same tilt angle,and then excites two sets of superposed spectral combs of cladding modes.The cascaded TFBG with total length of 18 mm has a much wider wavelength range over 100 nm and narrower wavelength separation than that of a TFBG only in the SMF,enabling an enlarged range and a higher accuracy of refractive index measurement.The fabricated TFBG with the merits of enhanced sensing capability and temperature self-calibration presents great potentials in the biochemical sensing applications.

Pure Bending Characteristic of Tilted Fiber Bragg Grating

a novel structure of the pure macro- bending sensor based on the tilted fiber Bragg grating （TFBG） is proposed. The TFBG located in the half circle with the different diameters is bent at a constant angle with respect to the tilted grating planes. With the variations of the curvature, the core-mode resonance is unchanged and the transmission power of cladding modes detected by the photodiodes varies linearly with curvature, while the ghost mode changes by the form of two-order polynomial. So we can use the transmission power of ghost mode or other cladding modes to detect bending curvature as shape sensor. From a practical point of view, the sensor proposed here is simple, low cost and easy to implement. Moreover, it is possible to make a temperature-insensitive shape sensor due to the same temperature characteristic between the core mode and the cladding modes.

UV-Inscribed Optical Fiber Gratings in Mid-IR Range and Their Laser Applications

We have UV-inscribed fiber Bragg gratings （FBGs）, long-period gratings （LPGs） and tilted fiber gratings （TFGs） into mid-IR 2μm range using three common optical fiber grating fabrication techniques （two-beam holographic, phase mask and point-by-point）. The fabricated FBGs have been evaluated for thermal and strain response. It has been revealed that the FBG devices with responses in mid-IR range are much more sensitive to temperature than that in near-IR range. To explore the unique cladding mode coupling function, we have investigated the thermal and refractive index sensitivities of LPGs and identified that the coupled cladding modes in mid-IR range are also much more sensitive to temperature and surrounding medium refractive index change. The 45° tilted fiber gratings （45°-TFGs） as polarizing devices in mid-IR have been investigated for their polarization extinction characteristics. As efficient reflection filters and in-cavity polarizers, the mid-IR FBGs and 45°-TFGs have been employed in fiber laser cavity to realize multi-wavelength 2μm Tm-doped CW and mode locked fiber lasers, respectively.

基于TFBG边缘滤波的FBG温度传感器

为了提高系统的温度灵敏度,设计了一种基于倾斜光纤光栅(TFBG)边缘滤波的光纤布喇格光栅(FBG)温度传感器。该系统在常规的TFBG边缘滤波解调系统上引入一个光纤环形镜,将从TFBG透射的光信号反射回来,使其能再次通过TFBG,以增加TFBG透射谱的深度,并对传感光栅进行封装。实验结果表明:封装前和封装后的传感光栅在所测量的温度区间内中心波长的漂移量分别为0.55 nm和7 nm,封装后的漂移量扩大了十多倍;同时,由于TFBG边缘滤波解调传递函数的斜率得到提高,传感器波长解调后的温度灵敏度也得到了改善,是未级联光纤环形镜传感器的1.6~2.4倍。

光纤Bragg光栅传声器设计

在详细分析光纤Bragg光栅(FBG)振动传感原理的基础上,设计了基于悬臂梁结构的光纤光栅传声器。系统采用结构简单、解调速度快的线性斜边滤波解调方法,选用1 531 nm/1 551 nm的CWDM作为滤波器件,其边带斜率为13 dB/nm.分别进行了单频信号(300 Hz和4 kHz)测试实验及声音信号测试实验,表明该传声器具有良好的频率检测性能和较高的信噪比,可以无失真地传递人的声音。而且,该系统可以实现光源复用,降低系统成本。

倾斜Bragg光纤光栅反射谱特性理论和实验研究

光纤B ragg光栅在通信和传感领域有着广泛的应用。根据倾斜光纤B ragg光栅的结构特点,利用耦合模原理,运用传输矩阵法,模拟分析了倾斜角度对光纤B ragg光栅反射特性的影响,并用实验对模拟结果进行了验证。实验和模拟结果表明:随着倾斜角的增大,中心反射波长向长波方向移动,反射带宽变窄,反射率减小。这个结论对倾斜B ragg光纤光栅的生产和应用有一定的指导作用。

Tilted Fiber Bragg Gratings:Principle and Sensing Applications

In this paper,the mode coupling mechanism of tilted fiber Bragg gratings(TFBGs)is briefly introduced at first.And a general review on the fabrication,theoretical and experimental research development of TFBGs is presented from a worldwide perspective,followed by an introduction of our current research work on TFBGs at the Institute of Modern Optics,Nankai University(IMONK),including TFBG sensors for single-parameter measurements,temperature cross sensitivity of TFBG sensors,and TFBG-based interrogation technique.Finally,we would make a summary of the related key techniques and a remark on prospects of the research and applications of TFBGs.

Fabrication of kW-level chirped and tilted fiber Bragg gratings and filtering of stimulated Raman scattering in high-power CW oscillators

Suppression of stimulated Raman scattering(SRS)by means of chirped and tilted fiber Bragg gratings(CTFBGs)has become a key topic.However,research on high-power systems is still lacking due to two problems.Firstly,after the inscription,there are a large number of hydroxyl compounds and hydrogen molecules in CTFBGs that cause significant heating due to their strong infrared absorption.Secondly,CTFBGs can couple Stokes light from the core to the cladding and the coating,which causes serious heating in the coating of the CTFBG.Aimed at overcoming these bottlenecks,a process that combines constant-low-temperature and variable-high-temperature annealing is used to reduce the thermal slope of the CTFBG.Also,a segmented-corrosion cladding power stripping technology is used on the CTFBG to remove the Stokes light which is coupled to the cladding,which solves the problem of overheating in the coating of the CTFBG.Thereby,a CTFBG with both a kilowatt-level power-carrying load and the ability to suppress SRS in a fiber laser has been developed.Further,we establish a kW-level CW oscillator to test the CTFBG.Experimental results demonstrate that the power-carrying load of the CTFBG is close to 1 kW,the thermal slope is lower than 0.015 ℃/W,and the SRS suppression ratio is nearly 23 dB.

Fiber laser twist sensor with hybrid few-mode tilted Bragg grating and few-mode long period grating

A twist sensor with hybrid few-mode tilted fiber Bragg grating(FM-TFBG) and few-mode long period grating(FM-LPG) in fiber laser cavity is demonstrated. The FM-LPG is utilized to excite LP11 core mode. The FM-TFBG is used for sensing. The transverse modes at 1 553.9 nm and 1 550.5 nm are LP01 and LP21 core modes, respectively, which are coupled from forward-propagating LP11 core mode. These two excitation wavelengths have opposite variation tendencies, which participate in sensing. The twist sensitivity of 0.16 dB/° from-40° to 40° is achieved. The proposed sensor has potentially used for structure monitoring in many areas.

Experimental research on cholesterol solution concentration sensing based on tilted fiber Bragg grating

The cholesterol solution concentration sensing characteristics based on tilted fiber Bragg grating(TFBG) are investigated by means of theoretical analysis and experiments. We prepare two groups of cholesterol solutions with the same concentration range and different refractive index ranges. The sensitivity of the two groups of solutions was 11.83 pm·m L/mg and 124.79 pm·m L/mg, respectively. The results show that the sensitivity of cholesterol solution can be improved by adjusting the refractive index range. This conclusion is valuable for measuring the concentration of fat-soluble solution.

All-fiber ellipsometer for nanoscale dielectric coatings

Multiple mode resonance shifts in tilted fiber Bragg gratings(TFBGs)are used to simultaneously measure the thickness and the refractive index of TiO_(2) thin films formed by Atomic Layer Deposition(ALD)on optical fibers.This is achieved by comparing the experimental wavelength shifts of 8 TFBG resonances during the deposition process with simulated shifts from a range of thicknesses(T)and values of the real part of the refractive index(n).The minimization of an error function computed for each(n,T)pair then provides a solution for the thickness and refractive index of the deposited film and,a posteriori,to verify the deposition rate throughout the process from the time evolution of the wavelength shift data.Validations of the results were carried out with a conventional ellipsometer on flat witness samples deposited simultaneously with the fiber and with scanning electron measurements on cut pieces of the fiber itself.The final values obtained by the TFBG(n=2.25,final thickness of 185 nm)were both within 4%of the validation measurements.This approach provides a method to measure the formation of nanoscale dielectric coatings on fibers in situ for applications that require precise thicknesses and refractive indices,such as the optical fiber sensor field.Furthermore,the TFBG can also be used as a process monitor for deposition on other substrates for deposition methods that produce uniform coatings on dissimilar shaped substrates,such as ALD.

一种面向结构沉降的改进型FBG传感器

为了提高光纤布喇格光栅(FBG)型沉降传感器的灵敏度,降低在沉降过程中由于倾斜与温漂所造成的测量偏差,设计了一种面向结构沉降的改进型FBG传感器。该传感器将传统悬臂结构改为直拉结构,减少因沉降倾斜导致的测量误差;通过精确计算2组FBG的波长偏移量的比值,实现对沉降量的准确测量。测试结果表明:该传感器的应变响应量为5.2×10^(-5)ε/N,在0~50 mm内波长偏移量与沉降量成线性关系,其偏差均值优于0.69 mm,灵敏度均值为107.45 pm/mm;测试结果波动均低于1.0%,明显优于传统FBG传感器结构,验证了其具有更好的抗干扰能力。

温度对线路覆冰监测光纤光栅倾角传感器性能的影响

针对现有架空输电线路覆冰在线监测系统存在的易受电磁干扰、使用寿命短等缺点,开发出了用于输电线路覆冰监测的光纤布喇格光栅(fiber Bragg grating,FBG)拉力倾角传感器;研究了温度对该传感器的影响。利用温控箱试验传感器零点温度影响的结果表明,使用温度内温度变化引起的拉力测量零点变化<±0.1%时,倾角测量零点变化<±0.67%;气候室中试验研究覆冰气候环境下开发的拉力倾角传感器的拉力测量误差<0.17%。户外试验表明研发的光纤布喇格光栅拉力倾角传感器在长期低温环境下(-15°C～+3°C)运行稳定,传感器测量未出现蠕变衰减现象。

基于倾斜光纤光栅的传感解调技术

该文设计了一种基于倾斜光纤光栅(tilted fiber Bragg grating,TFBG)的不受温度影响的光纤布拉格光栅(fiber Bragg grating,FBG)应变解调系统.将倾斜光纤光栅用作边沿滤波器,当周围环境温度变化时,利用倾斜光纤光栅纤芯模与包层模的温度特性与普通光纤布拉格光栅相同这一特点,无需另加温度补偿,就可以实现FBG应变传感的动态解调,消除温度噪声对应变信号的影响.由实验可知,当温度在25,℃到39,℃范围变化时,解调系统的性能基本不发生改变,解调范围达到6 nm.

倾斜光纤光栅谱特性

基于倾斜光纤光栅耦合模理论,采用数值分析法研究了前后向导模耦合时倾斜角度和调制深度对倾斜光纤光栅反射谱的影响,以及导模和辐射模耦合时针对不同的入射光偏振态在倾斜角度、调制深度、光栅长度等不同时倾斜光纤光栅透射谱(或反射谱)的特性.研究结果表明,在前后向导模耦合时,不同偏振态入射光的光谱几乎相同;而对于辐射模耦合,由于反射谱的包络受消光系数的调制,而不同偏振态入射光的消光系数不同,并且倾斜角度越大不同偏振态的入射光的消光系数差别越大,故在研究辐射模耦合时需要将偏振态予以考虑.

基于数据融合的倾斜光纤光栅应变测量

倾斜光纤光栅的透射谱中除了纤芯模之外还有一系列包层模。倾斜光纤光栅各个模式的应变传感特性是不同的,因此,可将每一个模式作为一只应变传感器测量应变。提出了利用倾斜光纤光栅的多个模式作为应变测量的多只传感器,在此数据测量基础上,采用数据融合方法提高数据测量的精度,从而得到应变的准确信息。这种方法计算简便,编程容易,在实际测量系统中具有很好的应用价值。

可分离温度影响的FBG应变测量方法

光纤布拉格光栅在通信和传感领域具有广泛的应用。利用光纤布拉格光栅中心波长的偏移,可以测量温度和应变等多种物理量,但必须解决光栅对温度和应变的交叉敏感问题。该文简要分析了光纤光栅作为传感器的基本原理及其优点,设计了利用参考光栅法分离温度影响以及利用掺铒光纤的自发发射放大特性分离温度影响的2种应变测量方法。最后,介绍了一种利用倾斜光纤光栅的主模和边模对布拉格光栅中心波长的偏移进行解调的方法,该方法成功地分离了温度对应变测量的影响。

数据融合在光纤光栅传感网络中的应用

网络化是光纤光栅传感的一个发展方向.在光纤光栅传感网络中,不但产生的大量数据需要实时处理,还需要对数据进行筛选、总结,从而得到被测量更加准确的信息.倾斜光纤光栅是一种特殊的光纤光栅,其透射谱中除了纤芯模之外还有一系列包层模,因此可将每一个模式作为一个传感器.以光纤光栅传感网络中的温度测量为研究对象,首次提出利用倾斜光纤光栅的多个模式作为温度测量的多个传感器,采用数据融合的方法得到更加准确的温度信息.这种方法计算简便,编程容易,在实际测量系统中具有很好的应用价值.

基于倾斜布拉格光纤光栅的高灵敏度折射率传感器

制备了不同倾斜角度的倾斜布拉格光纤光栅,用于测量周围环境的折射率.当环境折射率增加时,高阶包层模谐振峰会逐渐消失,即将逐渐消失的高阶包层模谐振峰的波长会随环境折射率的增大而线性增大.测得的折射率灵敏度在500 nm/RIU以上.通过选择适当的倾斜角度,制得的光纤传感器可以满足不同范围的折射率测量.

倾角对闪耀光纤光栅辐射模耦合波长的影响

不同角度下,闪耀光栅发生辐射模耦合的波长有所不同。文章利用相位匹配条件定性分析了辐射模耦合波长随倾斜角度的变化;使用耦合模方程对闪耀光栅的透射谱特性进行数值仿真,仿真结果与相位匹配条件分析得到的结果一致:若设计Bragg波长保持不变,当光栅倾角变大时,发生辐射模耦合的波长变短。通过比较0～45°范围内s和p偏振情况下耦合波长的差异,得出辐射模耦合的偏振依赖特性,此特性可用于传感检测。