Multi-measurement scheme for a fiber-optic sensor based on a single long-period grating

In this paper, we present and discuss a scheme to perform simultaneous measurement of multi-perturbations using a single Long-Period Grating (LPG). It was shown that different spectral shifts in the resonant bands of the transmitted spectrum of an LPG sensor can be used for measuring concurrent perturbations. The functioning mechanism of this multi-parameter sensing scheme is discussed and used to perform simultaneous sensing of temperature, mechanical strain and refractive index variations.     

Mechanism and characteristics of asymmetrically phase-shifted corrugated long-period fiber grating fabricated by burning fiber coating and etching cladding technology

In this letter, a new phase-shifted corrugated long-period fiber grating (PS-CLPFG) manufacture method is proposed based on burning the fiber coating and etching cladding method. We analyze the effect of phase amplitude, locations, and numbers on transmission spectrum. A proof-of-concept experiment is performed, and a good agreement of theoretical analysis and experiment results is obtained. In addition, a relative easier method to control band-pass and band-rejection characteristics is reached. Moreover, the obvious sidelobes in the passband of multiple-π PS-CLPFG can be effectively suppressed.     

Tunable microwave generation based on a dual-wavelength single-longitudinal-mode fiber laser using a phase-shifted grating on a triangular cantilever

Frequency tunable microwave signal generation, based on a dual-wavelength single-longitudinal-mode (SLM) erbium-doped fiber (EDF) laser, incorporating a phase-shifted fiber Bragg grating (PS-FBG) with two π-phase shifts, is demonstrated. In the proposed configuration, the PS-FBG with two ultranarrow transmission bands is embedded in a triangular cantilever to serve as a wavelength spacing tunable filter with a fixed center wavelength by applying various strains on the cantilever. A section of unpumped EDF is employed as a saturable absorber to ensure SLM operation in each of the two lasing lines. By beating the two wavelengths at a photodiode, a tunable microwave signal ranging from 8.835 to 24.360 GHz is successfully achieved.     

Dependence of fringe spacing on the grating separation in a long-period fiber grating pair

The spectral spacing of the interference fringes formed by a pair of long-period fiber gratings was investigated. The variation of the fringe spacing was measured while the separation between the gratings was changed from 22 to 500 mm. When the grating separation was much longer than the length of the individual grating, the inverse of the fringe spacing became linearly proportional to the grating separation and to the differential effective group index of the fiber. In the third stop band of the grating pair, made along a dispersion-shifted fiber centered at 1.55 mum, the differential effective group index was calculated to be ~6.4 x 10(-3), which is approximately twice the differential effective index of the fiber. The discrepancy between the two indices was observed to decrease with the band order, a phenomenon that is explained by the first-order dispersion of the fiber. The measured interference fringes were not regularly spaced in the frequency domain, but regular spacing is required in wavelength-division multiplexing communication systems. Analysis of the second-order dispersion of the fiber and the grating-induced nonlinear phase shift within grating regions as the factors that induce chirping on the fringe spacing is presented.     

基于相移光纤光栅传感器的碳纤维增强树脂复合材料基体裂纹超声探伤

复合材料内部的微小裂纹常会引起后续严重的破坏,因此需要对其进行检测。然而超声探伤复合材料基体裂纹非常困难。本文搭建了一个具有高灵敏度、大带宽的相移光纤光栅超声传感系统,利用此系统探测了在正交铺层碳纤维增强树脂复合材料板中传播的Lamb波。对Lamb波进行数据处理发现,随着三点弯曲实验产生的基体裂纹个数增加,Lamb波的幅值和频谱峰值线性减少。通过和传统压电传感器比较表明,相移光纤光栅传感器测得的Lamb波信号随复合材料基体裂纹数的增加其幅值具有更高的下降速率,表明相移光纤光栅传感器更适合于复合材料基体裂纹的超声探伤。研究表明,新开发的传感系统能够探测到中心频率为300 kHz的微弱超声信号,并能够对碳纤维增强树脂复合材料板中微小基体裂纹个数进行精确评估。      

Optical coupling method utilizing a lensed fiber integrated with a long-period fiber grating

A novel optical coupling scheme that uses a lensed fiber integrated with a long-period fiber grating (LPFG) is proposed. Two experiments are performed to demonstrate the validity of such a scheme in single-mode-fiber-single-mode-fiber (SMF-to-SMF) and laser-diode-single-mode-fiber (LD-to-SMF) coupling setups. The measured results show that for an appropriate lens radius the addition of a LPFG will lead to a higher coupling efficiency over a longer range of working distance than without the LPFG. Coupling efficiencies of ~78% and 35% are achieved for corresponding working distances of ~250 and 110 mum, 1-dB longitudinal tolerances of ~40 and 26 mum, and 1-dB transverse tolerances of ~7.6 and 2.6 mum for SMF-to-SMF and LD-to-SMF, respectively.     

基于双折射效应的光纤光栅称重方法研究

与通常的光纤光栅传感器原理不同,本文基于光纤在外界径向载荷作用下产生的双折射效应为原理,建立了一套简单的称重实验装置,并对其检测原理进行了详细的分析。实验表明,这种光纤光栅称重传感器适合大载荷情况的重量检测问题,特别适合于公路计重收费系统中对车辆载重的监测领域,其称重灵敏度可达5kg。     

Thermal tuning of mechanically induced long-period fiber grating

We have developed a wideband tunable optical filter that uses a long-period fiber grating (LPFG) in which both resonance wavelength and its signal attenuation can be adjusted. We create the grating mechanically by pressing a spring coil to an optical fiber. We achieve continuous fine tuning of wavelength and attenuation by varying the temperature of the LPFG. The adjustable ranges of the LPFG are more than 200 nm in resonance wavelength and more than 10 dB in signal attenuation.     

Optical fiber long-period grating humidity sensor with poly(ethylene oxide)/cobalt chloride coating

A long-period fiber grating (LPFG) humidity sensor is reported utilizing poly(ethylene oxide)/cobalt chloride (PEO/CoCl2) as a hybrid hygrosensitive cladding coating. A thin overlay of the material is deposited on the LPFG and with exposure to different ambient humidity levels, its spectral properties are modified. The material parameters associated with the sensing mechanism may include those of refractive index, absorption, and morphological alterations of the overlaid material. Relative humidity variations in the range from 50% to 95% have been detected with a resolution better than 0.2%. The response time constant of the fiber sensor is of the order of a few hundred milliseconds.     

基于FBG轴向拉力的液位测量传感器

根据光纤布喇格光栅(fiber Bragg grating,FBG)的传感原理,提出了一种基于波纹管结构的FBG液位传感器,利用补偿FBG(FBG1)与液位FBG(FBG2)具有相同的温度灵敏度进行温度补偿,将FBG2固定在波纹管内,液压产生的张力可以使波纹管的自由端产生位移,从而使固定在里面的FBG2产生轴向应变进而影响布喇格中心波长的改变.通过布喇格中心波长的改变来监测液位的变化,在40~340mm的液位变化范围内进行了实验研究.结果表明,光纤布喇格光栅中心反射波长漂移对液位呈现良好的线性关系,线性拟合度大于0.998,灵敏度为15.7pm/cm,理论测量范围可达到3m.说明通过改变波纹管的波纹数目和其他参数,可实现对传感器测量范围和灵敏度的调整,以满足各种应用场合.     

Temperature sensor based on the power reflected by a Bragg grating in a tapered fiber

We present a temperature sensor based on two chirped gratings made in optical fibers tapered by fusion. One of the gratings has a metallic shielding and acts as sensor element, whereas the second grating provides a reference signal. The sensor is interrogated by measuring the power reflected by the two gratings, and the system has an accuracy of 0.05 degrees C over a linear operation range of more than 10 degrees C that can be adjusted in the fabrication process.     

Optical low-coherence reflectometry based on long-period grating Mach-Zehnder interferometers

The optical low-coherent interferometric technology for long-period grating (LPG) Mach-Zehnder interferometers is described. By including the coupling and recoupling behaviors of a LPG pair, a numerical model is developed to analyze the output reflectogram of the system. The effects of the grating interval, grating length, grating strength, and light source on the output reflectogram have been comprehensively discussed, which reveals that the low-coherence reflectometry offers the capability of interrogating the multiplexed sensors based on LPG pairs. A comparison of the calculated and experimental results is presented, and an excellent agreement between the simulation and the measurement is shown.     

Temperature-insensitive fiber Bragg grating tilt sensor

A temperature-insensitive optical fiber tilt sensor is presented. The sensor scheme uses a prestrained fiber Bragg grating to sense the strain, which depends on the tilt angle. To compensate for the temperature effect, materials that have different linear thermal expansion behaviors are used for implementation of the sensor body. The differentiation in the linear thermal expansion would then cause a counter effect to the original temperature effect. Experimental tests show an accuracy of +/-0.167 degrees in tilt angle measurement. A temperature stability better than +/-0.33 degrees over the temperature range from 27 degrees C to 75 degrees C is demonstrated. The resolution 0.0067 degrees in tilt angle measurement is achieved by using our preliminary sensor with a dimension of 1 6 x 5 x 5 cm(3).     

A new ultra long-period fiber grating for measuring torsional characteristics

In this paper, we proposed a novel kind of periodic helical ultra-long period fiber grating (U-LPFG) for measuring the torsion rate and distinguishing the torsion direction. The sensor was fabricated by spirally deforming a common fiber by local heating for 4 periods. This method of fabrication was not only easy to implement, but also further increased the sensitivity of the grating to the torsion because of the spiral structure. This paper introduced the torsional characteristics of this sensor in detail and drew some relevant conclusions. Experimental results showed that the twist sensitivity of 0.085?nm/(rad/m). Meanwhile, the temperature sensing characteristic of the proposed U-LPFG was also tested as 68?pm/°C within the range of 30?°C ～210?°C. Therefore, this proposed sensor could achieve good results in simultaneously measuring the torsion rate and distinguishing the torsion direction, and it may have great potential in some practical applications.     

Characteristics of a long-period fiber grating with reduced cladding for refractive index sensing

The sensitivity to surrounding refractive index (SRI) of a long-period fiber grating (LPFG) can be effectively improved by decreasing the cladding radius. When the cladding is reduced, a three-layer model is necessary to evaluate the effective refractive index (ERI) of the core mode. A variation of SRI can induce a greater resonant wavelength shift when the core mode is coupled to a higher-order cladding mode. However, as the cladding is reduced further, the highest-order cladding mode would be cut off, i.e. the number of cladding modes that a given fiber structure can support would be less; thus, the higher-order cladding modes that can be used for higher sensitivity are limited. Hence, the implementation of high sensitivity for SRI sensing with cladding-reduced LPFGs is dependent on the proper combination of cladding radius and cladding mode order. Based on the vector coupled-mode theory, the transmission spectrum and sensitivity are numerically analyzed with respect to the cladding radius, which shows that the SRI sensitivity of the HE₁₂ mode with cladding radius a ₂?=?20?µm is 32 times as high as that with a ₂?=?62.5?µm and the SRI resolution is available to the order of 10^?7.     

Simultaneous temperature and strain measurements performed by a step-changed arc-induced long-period fiber grating

A compact sensor based on step-changed arc-induced long-period fiber gratings was implemented to discriminate between temperature and strain. The proposed sensor consists of a single long-period grating with two sections written consecutively in the SMF-28 fiber using the electric arc discharge technique. The two sections have the same period but different fabrication parameters. The operation of the sensor relies on the existence of a difference between the values of temperature and strain sensitivity of two neighboring resonances observed in the spectrum of the step-changed grating. The temperature and strain resolutions obtained for the sensor are 0.2 degrees C and 35 micro epsilon, respectively.     

光纤Bragg光栅应变传感器在霍普金森压杆上的冲击试验研究

利用复合材料封装光纤Bragg光栅(FBG)设计制作了用于测量高速碰撞或爆炸与冲击作用下混凝土内部动态应变的FBG应变传感器。在霍普金森压杆上对FBG在封装前的裸光栅和封装后的FBG传感器分别进行了高速冲击试验,试验表明：设计的FBG传感器的瞬态响应上升时间小于20μs,具有良好的动态响应特性,可以用于工程中混凝土结构内高速冲击下的动态应变测试     

基片式高灵敏度光纤光栅温度传感器

光纤光栅是一种新型光学无源器件,现已普遍运用于传感测量方面,但由于裸光纤光栅的灵敏度比较低,需要对其进行增敏封装处理。该文提出一种基片式封装结构,运用铝和殷钢两种不同热膨胀系数的材料,进行过渡配合,当铝受热产生变形,在铝和殷钢之间形成挤压力,这种挤压力减弱横向延伸,增强纵向两端延伸,使得光纤光栅应变增大。通过水浴加热实验对比分析裸光纤光栅、封装的光纤光栅和改进封装后的光纤光栅的温度特性,实验结果表明改进的基片式封装结构温度传感器的线性相关系数为0.996,其灵敏度为33.21 pm/℃,是裸光纤光栅的3.224倍,比单一铝材料封装灵敏度增大6.41 pm/℃,可广泛运用于多种场合的温度测量,具有实际的应用价值。     

Evanescent wave long-period fiber bragg grating as an immobilized antibody biosensor

An immunosensor using a long-period grating (LPG) was used for sensitive detection of antibody-antigen reactions. Goat anti-human IgG (antibody) was immobilized on the surface of the LPG, and detection of specific antibody-antigen binding was investigated. This sensor operates using total internal reflection where an evanescent field interacts with bound antibody immobilized over the grating region. The reaction between antibody and antigen altered the LPG transmission spectrum and was monitored in real time as a change in refractive index, thereby eliminating the need for labeling antigen molecules. Human IgG binding was observed to be concentration dependent over a range of 2-100 microg mL-1, and equilibrium bound antigen levels could be attained in approximately 5 min using an initial rate determination. Binding specificity was confirmed using human interleukin-2 and bovine serum albumin as controls, and nonspecific adsorption of proteins did not significantly interfere with detection of binding. Antigen detection in a heterogeneous protein mixture and in crude cell lysate from Escherichia coli was also confirmed. Moreover, regeneration of the LPG surface via diethylamine treatment resulted in approximately 80% removal of bound antigen. Subsequently, fibers reexposed to antigen retained greater than 85% of the initial signal after five consecutive regeneration cycles.     

In-fiber reflection mode interferometer based on a long-period grating for external refractive-index measurement

We present two novel schemes for refractometry based on a long-period fiber grating- (LPG-) based Michelson interferometer. These schemes are designed to overcome the measurement dependence of previously demonstrated LPG-based refractometry on the immersion depth. The first utilizes an unshielded LPG and the second, a shielded one. Both schemes were tested over a certain refractive-index range, and the measurement of glucose concentration in water was experimentally demonstrated. In addition, the temperature sensitivity of the two schemes is discussed.