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.     

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.     

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.      

Miniaturized Optical Fiber Sensor Interrogation System Employing Echelle Diffractive Gratings Demultiplexer for Potential Aerospace Applications

To enable the application of optical fiber sensors to aerospace vehicles, the sensor interrogation or data acquisition system has to meet small size and low weight requirements. This paper presents the developmental work of an echelle diffractive gratings (EDGs) demultiplexer-based optical fiber sensor interrogation system. The operation principle of this system and its application to fiber Bragg grating (FBG) sensor interrogation are presented. The experimental results have shown that the developed interrogator (not including the electronic controller) weighs less than 60 g and provides better than 1 pm measurement resolution and better than plusmn10 pm measurement accuracy.     

Multiplexed Long-Base Flexible Optical Fiber Extensometers and Temperature Bragg Sensors Interrogated by Low-Coherence Interferometry

We present a novel concept of quasi-distributed flexible optical fiber extensometers fully compensated from thermal variations. Developed for structural health monitoring applications, the sensors are composed of a combination of intrinsic Fabry-Perot cavities as long-base extensometers, and point-like Bragg gratings inserted along the same fiber used as temperature sensors. This configuration enables a high degree of multiplexing, thus quasi-distributed sensing, and very efficient temperature compensation. Both types of sensors are read by a fiber-optic low-coherence interferometer, used in an original way to measure simultaneously the length variations of the cavities and the wavelength shifts of the Bragg gratings. Finally, we present the experimental validation of the whole measurement system, suitable for concrete structures instrumentation, as it includes an original optical fiber sheath packaging the optical fiber as a flexible sensor.     

Frequency Modulated Continuous Wave System for Optical Fiber Intensity Sensors With Optical Amplification

We report on the use of erbium doped fiber (EDF) amplification to enhance a frequency modulated continuous wave (FMCW) technique for referencing optical intensity sensors located between two Bragg grating structures. The experiment combines the concept of FMCW with the spectrally selective mirror properties of Bragg gratings to interrogate with referencing properties intensity based sensors. The interrogation system without amplification yields a sensor resolution of around 0.078 dB. When the EDF amplifier is introduced into the experimental set up, the sensor sensitivity does not change, but the signal-to-noise ratio is improved, resulting into an enhanced resolution of 0.025 dB. We also obtain a remote sensing operation at a location of 50 km, showing the feasibility of this configuration to be used as a remote sensing application.     

Quadruple-wavelength actively mode-locked fiber laser with polarization-controlled wavelength switching

An actively mode-locked fiber laser with two overlapping cavities is proposed and successfully demonstrated to generate switchable quadruple-wavelength picosecond pulses. The wavelengths are specified by two Bragg gratings in polarization-maintaining and absorption-reducing fiber. By simple adjustment of two polarization controllers, the proposed laser can be made to operate at quadruple wavelength or to switch between wavelengths at room temperature. For wavelength switching, four single-wavelength, six dual-wavelength, and four triple-wavelength operations were performed without changing the repetition rate of the output pulses.     

Fiber grating sensing interrogation based on an InGaAs photodiode linear array

We present a new method of the fiber grating sensing interrogation technique by utilizing an indium gallium arsenide photodiode linear array and blazed fiber Bragg gratings. An interrogation system based on an InGaAs photodiode linear array is designed, and the system performance is analyzed. The interrogation system shows a good prospect for smart sensing.     

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.     

Acousto-optic superlattice modulation in fiber Bragg gratings

The superposition of a long-period grating and a fiber Bragg grating, which we call an optical superlattice, causes high-efficiency narrow-band reflections to be induced on either side of the Bragg wavelength. This effect was recently observed experimentally in a fiber-based acousto-optic superlattice modulator. We develop in detail the theory of optical superlattices in fiber Bragg gratings, treating both the acousto-optic and the fixed-grating cases. Applications include reconfigurable wavelength division multiplexers, fiber lasers and sensors, tunable filters, modulators, and frequency shifters.     

新型分布式布喇格光纤光栅传感系统的研究

提出了一种新型分布式布喇格光纤光栅(FBG)传感系统.为了满足一个大型工程同时监测上百个不同位置的要求,将这一百个点分成若干组,每组构成一个小区域,每个小区域由四个波长相同的光栅组成,这样就满足了在一条光纤上刻一百个光栅的要求.当某点温度发生变化时,该区域的最高温度可测得;若此最高温度超过警戒温度,则系统发出报警信号,进而对这个小区域进行整体降温处理.在理论分析的基础上,对多组不同波长的光纤光栅进行了温控对比实验,在10～110℃的温度范围内,FBG的中心波长随温度变化呈良好的线性,线性度达到99.6%以上.实验表明该系统具有精度高、适用于分布式多点测量的特点.     

Wavelength tuning in optically mode-locked semiconductor fiber ring lasers with linearly chirped fiber Bragg gratings

We demonstrate wavelength tuning in single-wavelength and multiwavelength semiconductor fiber ring lasers that are mode locked with an optically injected control signal. A semiconductor optical amplifier is used to provide gain as well as to function as an optically controlled mode-locking element. Linearly chirped fiber Bragg gratings--single or superimposed--are used to define the lasing wavelengths as well as to provide wavelength tunability and allow for multiwavelength operation. We obtain pulses of tens of picoseconds in duration when we inject a sinusoidal optical control signal into the laser cavity, and we can tune the lasing wavelength(s) over the reflection bandwidth(s) of the grating(s) by simply changing the frequency of the injected control signal.     

Chromatic dispersion measurement using a multiwavelength frequency-shifted feedback fiber laser

In this paper, we present the realization of a fiber laser source emitting simultaneously over 17 wavelengths spread over the whole C-band. An acoustooptic frequency shifter is placed in the laser ring cavity to suppress the cross-gain saturation effects of the erbium-doped fiber. The emitted wavelengths are fixed by a set of fiber Bragg gratings (FBGs). A power uniformity reaching 6 dB is achieved by inscribing the FBGs while monitoring the laser output. We demonstrate the reliability of this laser as a source for characterization of optical components and networks by the measurement of optical fiber chromatic dispersion. The measurement is performed over the whole telecommunication C-band (1530-1560 nm) using the time-of-flight method. We perform the measurement on three different fibers with different levels of dispersion, namely a standard fiber, a nonzero dispersion shifted fiber, and a dispersion compensating fiber. The results are compared with measurements obtained using a standard network analyzer. The agreement between the two methods is better than /spl plusmn/1%, thus proving the suitability of the developed laser source for this application.     

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.     

Acousto-Ultrasonic Optical Fiber Sensors: Overview and State-of-the-Art

This paper gives a review of acoustic and ultrasonic optical fiber sensors (OFSs). The review covers optical fiber sensing methods for detecting dynamic strain signals, including general sound and acoustic signals, high-frequency signals, i.e., ultrasonic/ultrasound, and other signals such as acoustic emissions, and impact induced dynamic strain. Several optical fiber sensing methods are included, in an attempted to summarize the majority of optical fiber sensing methods used to date. The OFS include single fiber sensors and optical fiber devices, fiber-optic interferometers, and fiber Bragg gratings (FBGs). The single fiber and fiber device sensors include optical fiber couplers, microbend sensors, refraction-based sensors, and other extrinsic intensity sensors. The optical fiber interferometers include Michelson, Mach-Zehnder, Fabry-Perot, Sagnac interferometers, as well as polarization and model interference. The specific applications addressed in this review include optical fiber hydrophones, biomedical sensors, and sensors for nondestructive evaluation and structural health monitoring. Future directions are outlined and proposed for acousto-ultrasonic OFS.     

Differential multiplexing of fiber bragg gratings by means of optical time domain refractometry

A differential method of detection and multiplexing of signals from fiber Bragg gratings based on optical time domain reflectometry is developed. The method is based on probing of a system of gratings by means of short laser pulses and measurement of the power of the resulting reflected pulses by means of standard fiber-optic reflectometry. The proposed method may find extensive use in the area of monitoring of deformation and temperature distributions by means of fiber-optic sensors based on Bragg gratings.     

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.     

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。     

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...     

Fiber-optic Fabry-Perot sensors based on a combination of spatial-frequency division multiplexing and wavelength division multiplexing formed by chirped fiber Bragg grating pairs

Effective multiplexing for a very large number of fiber-optic fiber-Bragg-grating-based Fabry-Perot (FBGFP) sensors is proposed that is based on wavelength division multiplexing (WDM) and spatial-frequency division multiplexing (SFDM). For WDM, FBGFP sensors are arranged in different wavelength domains formed by a series of chirped fiber Bragg gratings with different central wavelengths while the sensors with different cavity lengths within the same wavelength domain are multiplexed by use of SFDM because they have different spatial frequencies as a result of their different cavity lengths. In principle, a thousand FBGFP sensors could be multiplexed with such an approach. The experimental results show that a strain accuracy of better than +/-10 microepsilon has been achieved with little cross talk.