Bragg gratings written in Sn-Er-Ge-codoped silica fiber: investigation of photosensitivity, thermal stability, and sensing potential

Bragg gratings were fabricated in an Sn-Er-Ge-codoped silica fiber with a phase mask and ultraviolet radiation from a 248-nm KrF excimer laser. The photosensitivity of the fiber was examined by studying the initial growth rate of the gratings written into it. The thermal stability of the gratings was investigated and modeled in terms of both the refractive-index modulation and the effective refractive index of the fiber core. It was shown that the temperature-induced irreversible shift in the Bragg wavelength could not be predicted by the isothermal decay of the refractive-index modulation. Finally, the potential of the gratings written into the fiber is discussed in terms of their use in high-temperature-sensing applications.     

Analysis on the saturation of refractive index modulation in fiber Bragg gratings (FBGs) written by partially coherent UV beams

We present an analysis on the saturation of refractive index modulation of fiber Bragg gratings written in nonhydrogenated Ge-B co-doped single-mode photosensitive optical fiber by partially coherent pulsed UV beams. The UV beams of different spatial coherence properties were generated by second harmonic conversion of high repetition rate, high average power copper vapor laser (CVL) oscillators with different optical resonators. It is observed that for UV beams of higher spatial coherence, the fiber Bragg grating reflectivity growth was faster and saturation of refractive index modulation was higher. The experimental results are explained with the help of a physical model based on exponential decay of defect centers per unit volume on UV absorption in the fiber core. The subsequent increase in the refractive index was attributed to the structural modification and densification of the fiber core.     

基于狭缝整形的低损耗飞秒光纤光栅制备

为了解决飞秒激光逐点法制备的光纤布拉格光栅（Fiber Bragg Gratings, FBG）损耗较高的问题,利用逐面法完成飞秒光纤光栅制备的实验研究。运用高斯光束传播的基本理论,通过狭缝整形技术对聚焦的飞秒激光能量分布进行等高宽整形,突破在光纤横截面内诱导产生圆形折射率调制的难点,最终制备得到低损耗的飞秒光纤光栅。开展不同狭缝宽度制备FBG的光谱特性对比实验,结果表明：利用光斑直径为5.0 mm的飞秒激光光束刻写FBG时,采用宽度为1.7 mm的狭缝制备得到的FBG插入损耗降低至0.15 dB,短波损耗降低至0.5 dB,验证了基于狭缝整形的低损耗飞秒光纤光栅制备方法的有效性。针对狭缝法制备的FBG反射率分散问题,提出控制折射率匹配液填充量以及调整飞秒激光能量的方法,并优化光束聚焦流程,成功降低FBG反射率的分散度。本研究对推动飞秒光纤光栅在大容量、高链路损耗等环境中的实际应用具有重要意义。     

Surface-relief fiber Bragg gratings for sensing applications

We present a new type of fiber Bragg grating (FBG) in which we etch the grating into the flat surface of a D-shaped optical fiber. Instead of being written in the core of the fiber, as are standard FBGs, these surface-relief FBGs are placed in the cladding above the core. These gratings are a viable alternative to standard FBGs for sensing applications. We describe the fabrication process for etching Bragg gratings into the surface of D-fibers and demonstrate their performance as temperature sensors.     

Polarization analysis of surface-relief D-fiber Bragg gratings

Surface-relief fiber Bragg gratings exhibit substantially more polarization dependence than standard fiber Bragg gratings. Using D-fiber with different core orientations, surface-relief gratings are analyzed and fabricated to determine the polarization dependence. We show that the largest Bragg reflection occurs for the polarization state with a dominant TE field component parallel to the flat surface of the fiber. The polarization dependence is adjusted by changing the index of refraction of the surrounding media and by fabricating the surface relief grating using rotated core D-fiber.     

Characteristics of short-period blazed fiber Bragg gratings for use as macro-bending sensors

The characteristics of short-period blazed fiber Bragg gratings for use as macro-bending sensors are discussed. This sensor is able to detect macro bending with the transmitted power variation of the first side mode in the blazed fiber Bragg grating. Since an incident ray experiences different variations of tilt angles with respect to bending direction, the blazed fiber Bragg grating has different coupling efficiencies of the first side mode, which can be reduced considerably in the case of twisted blazed fiber Bragg gratings.     

Analysis of fiber bragg gratings by a side-diffraction interference technique

A new nondestructive, noncontact, and sensitive technique for fiber Bragg grating geometry and index-fault location measurements is presented. Two plane-wave probe laser beams are incident upon the grating from the side at angles that satisfy the Bragg-reflection condition. An interference pattern is formed behind the fiber between the first-order diffracted beam (from one probe beam) and the zero-order transmitted beam (from the second probe beam). The axial grating index modulation and the grating period are functions of the fringe visibility and the fringe period, respectively. The method is sensitive and is applicable even in the case of relatively weak gratings. Unchirped and chirped Bragg gratings have been studied with the proposed technique. We demonstrate accuracies of 1 x 10(-4) for measurement of the index modulation and 0.01 nm for measurement of the period. As well as for the analysis of most already-fabricated gratings, this technique is useful for in situ analysis of a long fiber Bragg grating as such a grating is translated along its axis during the fabrication process.     

Analysis of thermal decay and prediction of operational lifetime for a type I boron-germanium codoped fiber Bragg grating

The thermal decay of a type I fiber Bragg grating written at 248 nm in boron-germanium codoped silica fiber was examined in terms of its reflectivity and Bragg wavelength change. In addition to the decay in reflectivity, which was observed, a shift in Bragg wavelength over the temperature range considered was seen. A mechanism for the decay in the reflectivity was developed and modeled according to a power law, and the results were compared with those from the aging curve approach. The wavelength shift was simulated by modification of the power law, which was also found to fit well to the experimental data. Temperature-induced reversible and irreversible change in the grating characteristic were observed and considered to be a means to predict the working lifetime of the grating at comparatively low temperatures. Accelerated aging was also reviewed and compared in terms of reflectivity and Bragg wavelength shift. It was shown that the temperature-induced irreversible shift in the Bragg wavelengths could not be predicted by use of the isothermal decay of the refractive-index modulation. The results were discussed within the framework of the current theoretical approaches for predicting the stability of gratings of this type.     

Pressure Vessel Wall Thinning Detection Using Multiple Pairs of Fiber Bragg Gratings For Unbalanced Strain Measurements

This paper proposes a method for using a pair of identical fiber Bragg gratings to measure the unbalanced strain factor in an isothermal-mechanical system. Cascading two identical fiber Bragg gratings, the unbalanced strain factor caused by the disturbance can be interrogated from the peak difference of the two reflecting Bragg wavelengths. If the mechanical system is in an isothermal environment, the thermal variations in the Bragg grating detection system can be automatically compensated. To verify the feasibility of the unbalanced strain detection using a pair of fiber Bragg gratings, the wall-thinning monitor in a pressure vessel system was investigated. The proposed technique is easy to install, immune to electro-magnetic and thermal interference, and highly sensitive in-time to remote monitoring for fiber optic smart structures.A finite element model (FEM) is used to simulate the wall thinning in a pressure vessel system. The minimum detectable unbalanced strain and cross-talk between two reflecting Bragg wavelengths were investigated.ROC Revised for Journal of Nondestructive Evaluation, 2000     

Comparison of isochronal and isothermal decays of bragg gratings written through continuous-wave exposure of an unloaded germanosilicate fiber

We compare the results of thermally induced isochronal and isothermal decays of fiber Bragg gratings written through cw exposure of an unloaded germanosilicate fiber. We show that isochronal step decays can be used to predict isothermal decays, provided that some corrections are carried out to take into account a reversible change in grating reflectivity induced by the increase of the fiber temperature. The isochronal accelerated-aging method enables one to sample most of the initial distribution of trapped site energies in a fairly short time. Taking advantage of this property of the method, we show that the initial distribution for a weak grating is similar to those for stronger gratings. The consequences of this observation are discussed within the framework of the various reaction pathway model.     

Post-hydrogen-loaded draw tower fiber Bragg gratings and their thermal regeneration

The idea of Bragg gratings generated during the drawing process of a fiber dates back almost 20 years. The technical improvement of the draw tower grating (DTG) process today results in highly reliable and cost-effective Bragg gratings for versatile application in the optical fiber sensor market. Because of the single-pulse exposure of the fiber, the gratings behave typically like type I gratings with respect to their temperature stability. This means that such gratings only work up to temperatures of about 300?°C. To increase temperature stability, we combined DTG arrays with hydrogen postloading and a thermal regeneration process that enables their use in high-temperature environments. The regenerated draw tower gratings are demonstrated to be suitable for temperatures of more than 800?°C.     

Apodized fiber Bragg gratings manufactured with the phase plate process

We present a manufacturing method based on the dynamic use of phase plates to photowrite Bragg gratings. This process allows for control of the local value of the index modulation envelope in the grating. The application to apodized fiber Bragg gratings is discussed.     

Inverse scattering algorithm for reconstructing lossy fiber Bragg gratings

We demonstrate an inverse scattering algorithm for reconstructing the structure of lossy fiber Bragg gratings. The algorithm enables us to extract the profiles of the refractive index and the loss coefficient along the grating from the grating transmission spectrum and from the reflection spectra, measured from both sides of the grating. Such an algorithm can be used to develop novel distributed evanescent-wave fiber Bragg sensors that measure the change in both the refractive index and the attenuation coefficient of the medium surrounding the grating. The algorithm can also be used to analyze and to design fiber Bragg gratings written in fiber amplifiers. A novel method to overcome instability problems in extracting the parameters of the lossy grating is introduced. The new method also makes it possible to reduce the spectral resolution needed to accurately extract the grating parameters.     

Intensity-referenced and temperature-independent curvature-sensing concept based on chirped fiber Bragg gratings

An intensity-referenced temperature-independent curvature-measurement technique that uses a smart composite that comprises two chirped fiber Bragg gratings is demonstrated. The two gratings are embedded on opposite sides of the composite laminate and act simultaneously as curvature sensors and as wavelength discriminators, enabling a temperature-independent intensity-based scheme to measure radius of curvature. Also, the system's performance is independent of arbitrary power losses that are induced in the lead fibers to the sensing head. It is demonstrated that the measurement range depends on the relative positions of the chirped fiber Bragg gratings and on their spectral bandwidths. By using two chirped fiber Bragg gratings with bandwidths W1 = 2.8 nm and W2 = 3.7 nm and with central wavelengths at lambda 01 = 1560.3 nm and lambda 02 = 1563.7 nm, we obtained a resolution of 1.6 mm/square root of Hz for the measurement of the radius of curvature (approximately R = 350 mm) over the measurement range 190 mm < R < infinity.     

Thermal decay of fiber Bragg gratings of positive and negative index changes formed at 193 nm in a boron-codoped germanosilicate fiber

A complex grating decay process is observed at elevated temperatures as predicted by a recently proposed three-energy-level model. We have also measured thermal stability of fiber gratings of both positive and negative index changes in a boron-codoped germanosilicate fiber in order to characterize the energy levels of the system and to predict grating lifetimes. The negative index gratings are found to be able to operate at 300 degrees C for more than 25 years without significant degradation.     

Fabrication and characterization of a diffraction-grating transducer in thin polybutadiene rubber film for sensing dynamical strain

Surface relief gratings are holographically fabricated in thin polybutadiene rubber films produced by both spin coating and dip coating on glass and metal substrates. These thin-film gratings are characterized for their application as efficient transducers for detecting dynamic strain in solids. The performance of these rubber-grating transducers is compared to surface-mounted fiber Bragg gratings for a range of frequencies between 50 Hz and 30 kHz. Dynamic-strain sensitivity around 1 nepsilon/radicalHz is recorded for thin rubber-film grating transducers.     

Fabrication of advanced fiber Bragg gratings by use of sequential writing with a continuous-wave ultraviolet laser source

We present a novel scheme based on sequential writing for fabrication of advanced fiber Bragg gratings. As opposed to earlier sequential methods this technique uses a cw UV laser source and allows for very precise control and repetitivity of the formation of the gratings. Furthermore it is possible to use high average irradiances without destroying the fiber, resulting in considerable reduction in fabrication time for complex gratings. The method has been applied to several test gratings, which proved its versatility and quality.     

光纤Bragg光栅的特性及应用研究

介绍了光纤光栅研制及其应用研究的情况。研究包括：光纤光栅的基本特性,光纤光栅外腔半导体单频和锁模激光器、光纤光栅稳频光纤环形激光器和主动锁模光纤激光器、基于光纤光栅的波长转换器和基于啾啁光纤光栅的可调谐激光器等。     

Distributed measurement of static strain in an optical fiber with multiple bragg gratings at nominally equal wavelengths

The Fourier transform relationship between the reflected light froma Bragg grating and the complex spatial modulation of the Bragg grating is used to produce a distributed strain sensing system. A tunable external cavity diode laser along with a reference reflector in anoptical fiber are used to produce a measurement of the phase and amplitude of the reflected light from the modulated Bragg grating as a function of wavelength. The system is demonstrated with 22 Bragg gratings in a single fiber on a cantilever beam and compared with foil strain gauge readings.     

分布式OFBG-GFRP传感筋在粘结性能试验中的应用

提出了一种新的利用分布式OFBG-GFRP智能传感筋研究GFRP筋与混凝土之间粘结特性的方法.在OFBG-GFRP的拉挤过程中,多个串连的光栅随同母材,即GFRP筋一同拉挤成型.早期的性能试验表明OFBG-GFRP筋不仅可以作为优良的增强材料使用,还具有理想的和稳定的传感特性.对3个带有OFBG-GFRP筋的混凝土粘结试件进行了直接拔出试验,在试验中自动记录了3个试件中的14个光栅的波长变化数据,这些数据连同记录的拔出力水平一起用来研究GFRP筋与混凝土之间的粘结性能.分析结果表明,GFRP筋与混凝土之间的粘结应变沿埋置深度基本呈现倒三角分布,且随着拔出力的增大,粘结应变分布也发生较为明显的变化,记录的光栅数据也很好地印证了试验过程中发生的试验现象.试验表明,所提方法可以在不影响FRP筋与混凝土之间粘结性能的情况下展开研究.