Application of optical time-domain reflectometry for the interrogation of fiber Bragg sensors

A reflectometric approach is proposed for the polling and multiplexing of sensitive elements on fiber Bragg gratings (FBGs). The method is based on the power measurement of the radiation reflected by the FBG using a conventional fiber-optic time-domain reflectometer. The multiplexing of sensors is based on the time separation of signals. Requirements on the Bragg diffraction gratings that provide the linear dependence of the received signal on the FBG mechanical stress and temperature are determined. In the measurements of the FBG relative elongation and temperature, the threshold sensitivities are 0.8 × 10^?4 (80 μstrain) and 5°C, respectively. Due to its simplicity and efficiency, the reflectometric method of FBG polling and multiplexing can be used to solve various measurement problems, in particular, the safety monitoring of the stressed-strained elements in building structures.     

All-fiber actively Q-switched fiber laser tuned by a pair of temperature controlled fiber Bragg gratings

We report an all-fiber actively Q-switched erbium-doped fiber laser, where the linear laser cavity mirrors are composed of two fiber Bragg gratings (FBGs). The laser oscillation wavelength could be tuned by this pair of temperature controlled FBGs. The Q-switching is achieved by an all-fiber phase modulation device. Using this system, we could obtain stable Q-switched laser pulses output, which could be optimized by tuning the reflection wavelengths of the two FBGs to be adjacent to each other. Instead of being modulated by the FBG filter in high-speed oscillation, this fiber laser system is operating in the Q-switched regime using an all-fiber phase modulator, producing a more stable laser output spectrum.     

A novel dual-wavelength fiber Bragg grating and its application in fiber ring laser

A novel method for fabricating dual-wavelength fiber Bragg gratings (FBGs) by using one phase mask is developed. The method is based on a double-exposure technique. Our technique lends itself to writing gratings with controllable reflectivity and separation of two Bragg wavelengths. A grating with two equal transmission peaks of 20.25 dB is obtained by this method and the separation of the two Bragg wavelengths is about 0.8 nm. With the grating, we demonstrate a dual-wavelength erbium-doped fiber ring laser whose interval of the two peaks is 0.8 nm. The laser's peak powers can get 3.1 mW above and have a good stability.     

Quasi-distributed temperature sensor combining Fibre Bragg Gratings and temporal reflectometry technique interrogation

Quasi-distributed sensors based on Fibre Bragg Gratings (FBG) usually deal with the concatenation of FBGs of different Bragg wavelengths analyzed through wavelength-sensitive devices. In these sensors, a given wavelengths range is dedicated to one particular FBG. The number of sensing points is directly limited by the source and detector spectral ranges, and by the wavelength spacing between two gratings. This spacing is linked to the maximum possible excursion of the physical parameter to be measured-before superimposition of the reflection spectra of the FBGs.In this paper, an original interrogating device is presented, that allows a very large number of concatenated gratings to be addressed. In this scheme, identical FBGs (same Bragg wavelength and same low reflectivity) are interrogated by the Optical Time Domain Reflectometry technique, for which a commercial device has been extended to a wavelength-tunable system, within an automated experimental set-up. Detection and localization of an important amount of sensing points along a unique optical fibre is demonstrated. Repeatability measurements did exhibit the very good accuracy of the presented sensor.     

用超磁致伸缩调谐光纤光栅的光分/插复用器

提出并研究了一种新型多信道切换的全光分/插复用器,它主要由光纤Bragg光栅(FBG)和一对光环行器组成.采用了一种高效的超磁致伸缩材料(GMM)使FBG产生有效的Bragg波长偏移.用控制电流来调控FBG的应变和Bragg波长偏移.用4只相同的FBG与波长叠加技术相结合,可建立能提供15种不同下路信道方式的OADM.     

基于保偏光纤取样光栅的宽频带滤波方法

提出一种新型刻录在保偏光纤上的取样布喇格光纤光栅.利用耦合模方程分析了此种光纤光栅的光谱特性,并利用切趾函数对光栅的输出谱进行了优化处理,最终可以实现高品质的宽频带多波长滤波.频带宽度为普通取样布喇格光纤光栅的2倍,经过切趾处理后,此种光纤光栅的边模抑制比也有显著提高.     

Sensing properties of fiber Bragg gratings in small-core Ge-doped photonic crystal fibers

We report about fiber Bragg gratings (FBGs) inscribed in two different types of small-core Ge-doped photonic crystal fibers with a UV laser. Sensing properties of the FBGs were systematically investigated by means of demonstrating the responses of Bragg wavelengths to temperature, strain, bending, and transverse-loading. The Bragg wavelength of the FBGs shifts toward longer wavelengths with increasing temperature, tensile strain, and transverse-loading. Moreover, the bending and transverse-loading properties of the FBGs are sensitive to the fiber orientations. The reasonable analyses for these sensing properties also are presented.     

Double-ring cavity configuration of actively mode-locked multi-wavelength fiber laser with equally tunable wavelength spacing

A novel actively mode-locked multi-wavelength tunable fiber laser with equally increased or decreased wavelength spacing is presented. It is constructed using a double-ring cavity combined with an amplitude modulator and cascaded fiber Bragg gratings (FBGs). The cavity lengths for all FBGs are intrinsically identical due to the double-ring configuration, and simultaneous mode locking of multiple wavelengths is therefore achieved by applying only one mode-locking signal to the modulator. The FBGs are mounted on a organic plate with angles between adjacent gratings such that the forces applied to the FBGs have an equal force increment; thus, stretching or compressing the cascaded FBGs can achieve a wavelength tuning with equally increased or decreased wavelength spacing.This revised version was published online in March 2005. In the previous version, the published online date was missing     

A novel method of optimal apodization selection for chirped fiber Bragg gratings

Apodization is a crucial technology for improving the dispersion performance of fiber Bragg gratings (FBGs). In this paper, we focus on how to select an optimal apodization for chirped fiber Bragg gratings and demonstrate a novel apodization selection method capable of choosing the optimal apodization based on the relationship between the bandwidth change and variation in average group delay ripple (GDR). Compared with current approaches, the novel method can select an optimal apodization profile and parameter for FBGs easily and accurately. Two numerical experiments are used to demonstrate the advantages of this method, one exhibits the different performances of five different apodization profiles, the other evaluates the influence of FBG parameters such as grating length, period, chirp, and index change on the apodization performance.     

On Two Parallel Matched Gratings Demodulation Method of Fiber Grating Strain Sensor

The method of demodulation with two parallel matched fiber Bragg gratings( FBGs)system is introduced to enlarge the range of strain that can be detected by the FBG sensor system.     

Chirped fiber Bragg gratings with arbitrary group delay responses using stepped-stretching fiber method

We present the design and development of piece-wise stepped-chirp fiber Bragg gratings (FBGs) with arbitrary group delay responses using a uniform phase mask in a pre-stretched fiber. The method involves writing a series of sub-gratings on a pre-stretched fiber whose length is varied during the UV exposure process. Two motorized stages are employed to control the length of the pre-stretched fiber in order to adjust the grating pitch at each writing step to achieve piece-wise stepped chirping of the FBG. To demonstrate the effectiveness of the method to produce chirped FBGs with arbitrary group delay responses, a linear-chirp FBG and a quadratic-chirp FBG were fabricated, and their measured performances agree well with analytical predictions. PACS 42.81.-i; 42.79.Dj; 42.79.Ci     

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。     

Proposal and realization for a broadband all-fiber non-uniformly spaced multi-channel optical filter

In this paper, we propose a broadband all-fiber non-uniformly spaced multi-channel optical filter, which is based on the utilization of paired multi-channel fiber Bragg gratings (FBGs) located at two arms of an interferometer. Unlike the traditional optical comb filter, the channel spacing may be either linearly or nonlinearly chirped. Serving as an example, two identical linearly-chirped 51-channel FBGs have been successfully employed to demonstrate the proposed filter experimentally.     

Complex fiber grating structures fabricated by sequential writing with polarization control

We propose a flexible method for fabricating complex fiber grating structures based on sequential writing for fiber gratings with polarization control of the UV source beam. Pure apodized as well as arbitrary phase-shifted fiber Bragg gratings (FBGs) can be fabricated in a single scan. Experimental examples of raised-cos2 apodized and dispersionless FBG filters are given to demonstrate the feasibility of this method.     

Differential reflectometry of FBG sensors in the wide spectral range

The paper presents a reflectometric technique for interrogation of multiple fiber Bragg grating (FBG) sensors based on conventional optical time domain reflectometry (OTDR). The method proposed rests on the differential measurement of FBGs’ response to a short probing laser pulse. Implementation of differential measurement principle using several reference FBGs allowed us to eliminate the susceptibility of the system to intensity fluctuations as well as to increase the measurement range as compared to the previous developments. The experimental threshold sensitivity amounted to ∼50 microstrain with the measurement range being defined by the number of reference FBGs and limited only by optical fiber tensile strength. Due to its simplicity, efficiency and usage of conventional OTDR equipment the proposed FBG interrogation technique can find a wide range of applications dealing with strain and temperature measurements.     

用紫外光直接在光纤内写入布拉格反射光栅的实验研究

用紫外光直接在光纤内写入布拉格反射光栅的实验研究杜卫冲谭华耀＊刘颂豪华南师范大学，激光生命科学研究所，广州５１０６３１＊香港理工大学，电机系自１９８９年Ｍｅｌｔａｚ［１］首次提出采用两束波长位于掺Ｇｅ－石英光纤吸收带内的相干紫外激光相干涉，从侧面照射...     

Simulation of amplifying phase-shifted fiber Bragg gratings by the method of single expression

An advanced method of single expression (MSE) is used for simulation of uniform and phase-shifted amplifying fiber Bragg gratings (FBGs) as potential narrowband transmission filters-amplifiers. In MSE the solution of Helmholtz's equation is represented in a special form of a single expression, which enables the exact solution of the corresponding boundary problem by means of numerical integration. Spectral dependences of amplifying uniform and π phase-shifted FBG are obtained. The electric field and power flow density distributions within and outside of amplifying FBGs permit to understand electrodynamically the process of amplification in modulated media. Amplification at transparency points of spectral dependences of uniform and π phase-shifted FBGs indicates essential gain accumulation against other parts of the spectrum. Amplification turns over uniform FBG from the stop-band filter to the narrow pass-band filter-amplifier with two sharp peaks at both edges of the stop-band region of corresponding FBG without gain. In amplifying π phase-shifted FBGs only one amplified narrow transmission peak appears at the center of the spectral dependences that is more acceptable for contemporary DWDM systems.     

Photonic microwave bandpass filter with improved dynamic range

A technique to improve the dynamic range of a photonic microwave bandpass filter is proposed and experimentally demonstrated. The filter is implemented based on phase modulation to intensity modulation conversion using fiber Bragg gratings (FBGs) serving as frequency discriminators, with the optical carriers located at the left or right slopes of the FBGs, to generate positive or negative tap coefficients. The dynamic range of the photonic microwave bandpass filter is increased by reducing the optical-carrier-induced shot noise and relative intensity noise at the photodetector, which is realized by placing the optical carriers at the lower slopes of the FBG reflection spectra. A photonic microwave bandpass filter with an improvement in dynamic range of about 10 dB is demonstrated.     

Dynamic strain monitoring by wavelength locking between two fiber Bragg gratings fiber sensing system

We demonstrated a fiber sensing system using the method of wavelength locking of two identical fibers Bragg gratings (FBG) to interrogate the wavelength shift by directly measuring the intensity of the reflection from the sensing Bragg grating. The light source of the fiber sensing system is an EDFA fiber ring laser pumped by a 980 nm laser diode and a narrow bandwidth fiber Bragg grating for the filter of the ring laser resonator. The wavelength shift is converted to the intensity deviation of the reflection from the sensing FBG under strain variation, and is able to achieve real-time sensing of the dynamic strain sensing in civil engineering. The characteristics and key factors to maintain stability of the dynamic strain sensing system are discussed.     

Customized MCVD fabrication of different application-type silica optical fibers and their FBG inscription characteristics

We report here the experimental realization of Fiber Bragg Gratings (FBG) of desired specifications in different application-types of in-house fabricated silica optical fibers, namely, standard telecom fiber of type SM-28, Er³⁺-doped SiO₂–GeO₂ fiber and photosensitive high GeO₂ fiber. Fibers for this purpose were fabricated by customizing the process variables of the Modified Chemical Vapor Deposition (MCVD) technique used for our laboratory fabrication of optical fibers. To check the performance of the drawn fibers in terms of FBG characteristics, gratings were written by exposing a short length of fibers to interference fringes derived from a frequency-doubled Ar-ion laser using a modified two-beam interferometer system in single phase-mask configuration. Results of inscribing selected wavelength FBGs demonstrate the efficacy of our approach in achieving, reproducibly, high-quality FBGs in all application-fibers.