光纤光栅半导体激光器激射波长与Bragg波长的偏离

利用包含光纤布拉格光栅(FBG)反射率分布的光纤光栅外腔半导体激光器(FGSL)的理 论模型,对FGSL 的激射波长进行了研究。结果表明激射波长并不一定在FBG布拉格反射波长处;布拉格反射波长相对于激射波长的偏移量与FBG的反射率分布、半导体增益介质的增益谱分布及增益峰值波长有关;激射波长可大于或小于布拉格反射波长。     

Strain sensitivity control of fiber Bragg grating structures with fused tapers

We report on the analysis and experimental validation of the strain sensitivity dependences of a fiber Bragg grating written in standard optical fiber when combined with fused tapers. By controlling the difference between the cross sections of the fused taper and the Bragg grating, the strain sensitivity of the Bragg wavelength can be changed by acting on the gauge length. The strain sensing characteristics of an interferometric structure formed by fabricating a fused taper in the middle of a fiber Bragg grating are also reported.     

Distributed measurement of the complex modulation of a photoinduced Bragg grating in an optical fiber

A method of measuring the complex modulation of a Bragg grating is derived from a one-dimensional model of light propagating in an optical fiber. Interference fringes between the Bragg grating and a reference air-gap reflector are measured, and a Fourier transform of the interference fringes generated as a laser is swept through the wavelength is used to compute the complex modulation function of the Bragg grating over a restricted domain. Supporting data, taken by temperature tuning a distributed feedback diode laser, are shown.     

Compact tunable microwave filter using retroreflective acousto-optic filtering and delay controls

Programmable broadband rf filters are demonstrated using a compact retroreflective optical design with an acousto-optic tunable filter and a chirped fiber Bragg grating. This design enables fast 34 micros domain analog-mode control of rf filter time delays and weights. Two proof-of-concept filters are demonstrated including a two-tap notch filter with >35 dB notch depth and a four-tap bandpass filter. Both filters have 2-8 GHz tunability and a 34 micros reset time.     

基于光电缆的分布式温度传感网络的实验研究

本文提出增加一根光纤光栅与光电缆绕制在一起,用于监测电缆中的实时温度.采用有限元分析方法,建立了光电缆温度场模型.使用可调谐脉冲激光为光源,在一根光纤上刻制多个相同中心波长的布拉格光栅,即采用全同光栅作为系统的温度传感器,当光电缆线路中温度发生异常时,反射回来的光栅中心波长发生偏移,通过检测反射光中心波长发生的偏移量可...     

Theory, experiment, and application of optical fiber etching

Based on fiber Bragg grating (FBG), an online monitoring system for the etching process of optical fiber in a hydrofluoric (HF) acid solution has been designed. The variation curves of the wavelength shifts of FBGs with etching time at three different temperatures have been obtained and analyzed theoretically. The results show that the etching process of optical fiber in HF acid solution can be understood by the variation of the wavelength shift of FBG with etching time. Finally, required tapered fiber tips can be made by controlling the etching velocity and the pulling velocity of optical fiber from the etching solution.     

Variations of the growth of harmonic reflections in fiber Bragg gratings fabricated using phase masks

The growth of reflectance peaks from optical fiber Bragg gratings has been studied to determine the relative importance of grating features when writing with the phase-mask technique. Measurements of spectra for two different fiber types using two distinct phase masks allowed the contribution from grating features of half the phase-mask periodicity and of the phase-mask periodicity at the Bragg wavelength to be determined. The dominance of the latter periodicity was ascribed to either the small fiber core diameter that limited the extent of the Talbot diffraction pattern, or the enhanced ±2 diffraction orders of a custom-made phase mask used.     

Multiplexed Fiber Bragg Grating Interrogation System Using a Microelectromechanical Fabry–Perot Tunable Filter

This paper describes a fiber Bragg grating strain sensor interrogation system based on a microelectromechanical systems tunable Fabry–Perot filter. The shift in the Bragg wavelength due to strain applied to a sensor fiber is detected by means of a correlation algorithm which was implemented on an embedded digital signal processor. The instrument has a 70 nm tuning range, allowing multiple strain sensors to be multiplexed on the same fiber. The performance of the interrogator was characterized using an optical fiber containing six grating strain sensors embedded in a fiberglass test specimen. The measured root mean square (RMS) strain error was 1.5 microstrain, corresponding to a 1.2 pm RMS error in the estimated wavelength shift. Strain measurements are produced with an update rate of 39 samples/s.      

Round Robin for Optical Fiber Bragg Grating Metrology

NIST has administered the first round robin of measurements for optical fiber Bragg gratings. We compared the measurement of center wavelength, bandwidth, isolation, minimum relative transmittance, and relative group delay among several grating types in two industry groups, telecommunications and sensors. We found that the state of fiber Bragg grating metrology needs improvement in most areas. Specifically, when tunable lasers are used a filter is needed to remove broadband emissions from the laser. The linear slope of relative group delay measurements is sensitive to drift and systematic bias in the rf-modulation technique. The center wavelength measurement had a range of about 27 pm in the sensors group and is not adequate to support long-term structural monitoring applications.     

Hybrid Wavelength-Time-Domain Interrogation System for Multiplexed Fiber Bragg Sensors Using a Strain-Tuned Erbium-Doped Fiber Laser

A system for the interrogation of fiber Bragg grating (FBG) sensors using a strain-tuned EDF laser with linear cavity is described. An optical switch is spliced to one end of the laser cavity and connects one of two high-strength draw-tower fiber Bragg gratings (DTGs). The gratings are simultaneously tuned by a stretching device and act as the end reflector of the laser cavity. By applying a ramp signal to the actuator synchronized to the optical switch, the laser signal sweeps over two different wavelength intervals, depending on the connected DTG. This approach represents a hybrid wavelength-time-domain interrogation for multiplexed sensors and doubles the number of sensors that may be addressed when compared with single DTG scanning. In addition, the use of the DTG allows a fivefold increase in the strain tuned wavelength interval over standard fiber Bragg gratings. An example application is demonstrated where temperature inside an electrical motor is measured during operation.     

Strain and damage monitoring of CFRP in impact loading using a fiber Bragg grating sensor system

A fiber-optic system featuring strain measurement and ultrasonic detection was constructed with fiber Bragg gratings based on wavelength–light intensity conversion technique. This fiber Bragg grating sensing system consists of a broadband light source, a broadband optical filter for strain measurement and a narrowband tunable filter for ultrasonic detection. The system was applied to strain measurement in impact loading to carbon fiber-reinforced plastics and the subsequent impact damage detection. Experimental results demonstrated that fiber Bragg grating sensors could measure strain with higher resolution compared with conventional strain gauges. Furthermore, ultrasonic inspection in which ultrasonic sensitivity was maximized by controlling the transmissive wavelength of the tunable filter could detect a 6.3 × 9 mm² impact damage.     

Advanced cure monitoring by optoelectronic multifunction sensing system

In this work, a dual functionality fiber optic sensing system for thermoset cure monitoring has been developed and successfully tested. Principle of operation relies on the use of a standard optical fiber in single ended configuration containing a single in-fiber Bragg grating. The change in refractive index directly related to material density at fiber end and the variation in the wavelength reflected by the fiber grating have been monitored simultaneously, throughout an In-Mould curing process of an epoxy resin. A compact and low cost in-fiber dual wavelength demodulation system has been developed and experimental results demonstrate the capability to on line identify the degree of cure, gel point, residual stresses and the glass transition temperature of the cured resin.     

Simple high-resolution wavelength monitor based on a fiber Bragg grating

A compact and low-cost device for monitoring the peak wavelength of single-peak spectral distributions is presented. The system is based on the transmission properties of a fiber Bragg grating when its period is modulated. Different types of optical signal, such as the emission of distributed-feedback lasers and the reflection of a broadband optical source produced by fiber gratings used in sensor systems, can be measured with this device. We demonstrate that a high wavelength resolution of micro 1 pm can be achieved and that our proposal can be used for real-time monitoring.     

Fiber Bragg grating flow sensors powered by in-fiber light

This paper presents an active fiber Bragg grating temperature and flow sensor based on self-heated optical hot wire anemometry. The grating sensors are directly powered by optical energy carried by optical fibers. In-fiber diode laser light at 910 nm was leaked out from the fiber and absorbed by the surrounding metallic coating to raise the temperature and change the background refractive index distribution of the gratings. When the diode laser is turned off, the grating is used as a temperature sensor. When the diode laser is turned on, the resonance wavelength and spectral width change of the self-heated grating sensor is used to measure the gas flow velocity. The grating flow sensors have been experimentally evaluated for different grating length and input laser power. The grating flow sensors have demonstrated a 0.35- m/s sensitivity for nitrogen flow at atmosphere pressure.     

用纤内Bragg光栅进行动态应变测量的研究

描述了用纤内Bragg光栅进行动态应变测量研究的意义，原理和方法，提出了两种用纤内Bragg光栅动态应变测量时进行Bragg波长偏移量的解调方法；光干涉相位检测Bragg光栅反射波长偏移的解调方法和用DWDM密集波分复用器解调Bragg波长的方法，前者适用于高频宽带动态应变测量，后者适用于有限频率或中等频率响应的动态应变测量。     

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.     

Passive temperature-compensating package for optical fiber gratings

We demonstrate a compact, passive temperature-compensating package for fiber gratings. The grating is mounted under tension in a package comprising two materials with different thermal-expansion coefficients. As the temperature rises the strain is progressively released, compensating the temperature dependence of the Bragg wavelength. A fiber grating mounted in a package 50 mm long and 5 mm in diameter exhibited a total variation in Bragg wavelength of 0.07 nm over a 100 °C temperature range, compared with 0.92 nm for an uncompensated grating.     

Response of fiber Bragg gratings to longitudinal ultrasonic waves

In the last years, fiber optic sensors have been widely exploited for several sensing applications, including static and dynamic strain measurements up to acoustic detection. Among these, fiber Bragg grating sensors have been indicated as the ideal candidate for practical structural health monitoring in light of their unique advantages over conventional sensing devices. Although this class of sensors has been successfully tested for static and low-frequency measurements, the identification of sensor performances for high-frequency detection, including acoustic emission and ultrasonic investigations, is required. To this aim, the analysis of feasibilty on the use of fiber Bragg grating sensors as ultrasonic detectors has been carried out. In particular, the response of fiber Bragg gratings subjected to the longitudinal ultrasonic (US) field has been theoretically and numerically investigated. Ultrasonic field interaction has been modeled, taking into account the direct deformation of the grating pitch combined with changes in local refractive index due to the elasto-optic effect. Numerical results, obtained for both uniform and Gaussian-apodized fiber Bragg gratings, show that the grating spectrum is strongly influenced by the US field in terms of shape and central wavelength. In particular, a key parameter affecting the grating response is the ratio between the US wavelength and the grating length. Normal operation characterized by changes in wavelength of undistorted Bragg peak is possible only for US wavelengths longer than the grating length. For US wavelengths approaching the grating length, the wavelength change is accompanied by subpeaks formation and main peak amplitude modulation. This effect can be attributed to the nonuniformity of the US perturbation along the grating length. At very high US frequencies, the grating is not sensitive any longer. The results of this analysis provide useful tools for the design of grating-based ultrasound sensors for meeting specific requirements in terms of field intensity and frequencies.     

Theoretical investigation on the temperature characteristics of liquid-cladding micro/nanofiber Bragg grating

Based on the functions of the effective refractive index of fundamental mode in step-index fiber, a theoretical mode about the Bragg wavelength shift of micro/nanofiber Bragg grating (MNFBG) is presented. The numerical simulation results demonstrate, for a MNFBG with given radius, the Bragg wavelength shifts to short wavelength as ambient temperature increases, and the reason results from the effective index decreasing with the increase of ambient temperature. Moreover, with the reduction of fiber-core radius, as well as the increase of ambient index and its thermo-optic coefficient, the temperature sensitivity, linearity and linear response range of the temperature-dependent Bragg wavelength shift are improved obviously. Especially for a MNFBG with fiber radius smaller than 0.5?μm, the linearity of Bragg wavelength shifting with temperature will be close to the theoretical limit, and the temperature sensitivity is proportional to the thermo-optic coefficient of the ambient liquid. Compared with the temperature properties of conventional fiber Bragg grating (FBG), all the results will provide much theoretical guides for FBG applied in fiber sensing and communication.     

Spectral characterization of integrated acousto-optic tunable filters by means of laser frequency modulation spectroscopy

The spectral characteristics of an integrated acousto-optic tunable filter (AOTF) as well as its responsivity to the rf driving signal and sensitivity to temperature changes are experimentally investigated and quantified using a diode-laser-based interrogation system. A spectroscopic technique, exploiting the rf frequency modulation of the laser beam and the phase-sensitive detection of the AOTF transmission, has been used for this purpose. That allows for the generation of a dispersivelike signal, which serves as a reference for tracking any wavelength change of the filter's peak with high resolution. The possibility of using the integrated AOTF as a spectrum analyzer with this interrogation scheme for fiber Bragg grating (FBG) strain sensing is also discussed.