Fiber Bragg Grating Sensors Interrogation System Using Arrayed Waveguide Gratings Demultiplexer

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光纤Bragg光栅靶式流量传感器设计

光纤Bragg光栅靶式流量传感器设计刘均;李雨泽【摘要】基于工业流体流量测量技术、光纤布拉格光栅（fiber Bragg grating，FBG）传感检测技术与靶式流量计原理，针对单个光纤Bragg光栅传感系统对温度交叉敏感的问题，设计并且制作了一种基于双光纤Bragg光栅流量传感器。

该传感器采用靶盘结构作为光纤Bragg光栅流量传感器的受力元件，对温度起到了补偿作用，并且有效地提高了应变测量灵敏度。

实验表明，该流量传感器的线性误差为0．31％。

%Based on industrial fluid flow measurement technology,fiber Bragg grating (FBG)sensing technology and target type flowmeter for overcoming a single fiber Bragg grating sensing system for temperature sensitive issue cross,we design and produce a fiber Bragg grating based on dual-fiber Bragg grating sensor flow control.The sensor uses the target disk structure as force element for temperature compensation,and effectively improves the strain measurement sensitivity.Experimental results show that the flow sensor linearity error is 0 .3 1%.【期刊名称】《光学仪器》【年(卷),期】2016(038)004【总页数】4页(P368-371)【关键词】光纤Bragg光栅;靶式流量计;温度补偿;传感器【作者】刘均;李雨泽【作者单位】东北石油大学电气信息工程学院，黑龙江大庆 163318;东北石油大学电气信息工程学院，黑龙江大庆 163318【正文语种】中文【中图分类】TP212随着光纤光栅写入技术的不断完善,应用成果日益增多,光纤光栅已成为目前最有发展前途、最具有代表性的光纤无源器件之一。

(完整word版)光纤布拉格光栅(FBG)介绍

光纤布拉格光栅(FBG)介绍1 介绍FBG是Fiber Bragg Grating的缩写，即光纤布拉格光栅。

在纤芯内形成的空间相位周期性分布的光栅，其作用的实质就是在纤芯内形成一个窄带的(透射或反射)滤波器或反射镜。

利用这一特性可制造出许多性能独特的光纤器件。

这些器件具有反射带宽范围大、附加损耗小、体积小，易与光纤耦合，可与其它光器件兼容成一体，不受环境尘埃影响等一系列优异性能。

目前应用主要集中在光纤通信领域(光纤激光器、光纤滤波器)和光纤传感器领域(位移、速度、加速度、温度的测量)。

近年来，随光纤光栅的重要性被人们所认识，各种光纤光栅的制作方法层出不穷，这些方法各有其优缺点，下面分别进行评述。

2光纤光栅制作方法2.1光敏光纤的制备采用适当的光源和光纤增敏技术，可以在几乎所有种类的光纤上不同程度的写人光栅。

所谓光纤中的光折变是指激光通过光敏光纤时，光纤的折射率将随光强的空间分布发生相应的变化，如这种折射率变化呈现周期性分布，并被保存下来，就成为光纤光栅。

光纤中的折射率改变量与许多参数有关，如照射波长、光纤类型、掺杂水平等。

如果不进行其它处理，直接用紫外光照射光纤，折射率增加仅为(10的负4次方)数量级便已经饱和，为了满足高速通信的需要，提高光纤光敏性日益重要，目前光纤增敏方法主要有以下几种：1)掺入光敏性杂质，如：锗、锡、棚等。

2)多种掺杂(主要是B/Ge 共接)。

3)高压低温氢气扩散处理。

4)剧火。

2.2成栅的紫外光源光纤的光致折射率变化的光敏性主要表现在244nm紫外光的错吸收峰附近，因此除驻波法用488nm可见光外，成栅光源都是紫外光。

大部分成栅方法是利用激光束的空间干涉条纹，所以成栅光源的空间相干性特别重要。

目前，主要的成栅光源有准分子激光器、窄线宽准分子激光器、倍频Ar离子激光器、倍频染料激光器、倍频OPO激光器等，根据实验结果，窄线宽准分子激光器是目前用来制作光纤光栅最为适宜的光源。

外文翻译---使用双光纤布拉格光栅传感器和互相关技术的水流量计

A water flowmeter using dual fiber Bragg grating sensors andcross-correlation techniqueAbstractIn this paper, a principle and experimental results of a cross-correlation flowmeter using fiber Bragg grating (FBG) sensors are presented.The flowmeter has no electronics and no mechanical parts in its sensing part and the structure is thus simple and immune to electromagnetic interference (EMI). For water flow measurement, the flowmeter uses the time delay of the vortex signal generated by a bluff body. Karman vortex shedding frequency is also detected and utilized for the flow velocity estimation in the system. In order to realize a low noise and wide bandwidth system, we employed interferometric detection as a FBG wavelength-shift detection method. The noise spectral densityof the FBG sensor with the interferometric detection was 4×10−4 pm/(Hz)1/2 corresponding to 0.33 nε/(Hz)1/2. A water flow experimentshowed that the flowmeter had a linear characteristic at velocity range from 0 to 1.0 m/s and the minimum detectable velocity of 0.05 m/s.1. IntroductionFiber Bragg grating (FBG) sensors have various advantages such as small size, simplicity in sensing principle, electromagnetic interference (EMI) immunity and capability of multiplexing. Because of these advantages, a number of basic researches and applications on FBG sensors have been made [1–3]. In telecommunication systems, FBGs are used as add-drop multiplexers because of their narrow bandwidth (typically 0.1 nm). The FBG application to optical tunable filters is also useful for discrimination of the signals in FBG sensor systems [4]. The applications to smart structures and health monitoring are attractive and have been investigated actively [5,6]. FBGs are embedded in composite materialsand used as strain and temperature sensors in the application. In the field of civil engineering, strain measurements for bridges and buildings are made using FBG sensor arrays with wavelength division multiplexing (WDM) and time division multiplexing(TDM) [7].In the FBG sensor applications, the choice of the wavelength-shift detection method is very important because the noise level and the measurement bandwidth of the system are mainly determined by the detection method. The most commonly used detection method is the tunable filter detection using Fabry–Perot filter. This method is the standard technique and provides static or quasi-static measurement with a strain resolution of 1 _ε. Another promising method is the interferometric detection [8,9]. This method has the capability of dynamic measurement with high strain resolution in the order of nε/(Hz)1/2. There are some reports about the noise estimation of the FBG sensor with interferometric detection [10–12].Our subject of research is the FBG application to a water flowmeter. There are various kinds of flowmeters including turbine flowmeters, vortex flowmeters and differential pressure type flowmeters. Measurands of flowmeters are ranging over various flow including water flow, gas flow and multiphase flow. Cross-correlation flowmeter, which utilizes a time delay of signals by coherent structures including vortices and naturally existing unsteady pressure field, is usually used for pipe flow measurement. The advantage of the cross-correlation flowmeter is its simplicity in sensing principle. The only parameter required to the flowmeter is the distance between two sensors. In thecross-correlation flowmeter, two pair of a ultrasonic transmitter and a receiver are usually used because of their non-intrusiveness to the flow [13]. The flowmeter using the ultrasonic transducers has a good linearity at wide velocity range. The problem with the flowmeter is complexity of the sensing part because the system needs at least four transducers. The cross-correlation flowmeter reported by Dyakowski and Williams [14] uses 16 light rays (eight pairs) to detect flow signals in gas–solid mixture. The velocities are obtained from cross-correlation of the intensity modulated light signals, and the average velocity and the velocity distribution in the pipe are then obtained by combining calculated velocities. This flowmeter is attractive because of EMI immunity and the passive nature. However, the system needs particles, which reflect or scatter the light rays,in the fluid and the application is limited. There are few reports concerning thecross-correlation flowmeter using optical sensors, not light ray or laser beam, suited for water flow measurement.In this paper, we present a water flowmeter using dual FBG sensors andcross-correlation technique. The flowmeter has no electronics and no mechanical parts in its sensing part, and thus the structure is simple. At first, we explain the principle and the schematic diagram of the flowmeter. Next, we present the noise estimation of the FBG sensor with the interferometric detection using a Mach –Zehnder interferometer comprised of a 2 × 2 and a 3 × 3 couplers [9]. Finally, we describe experimental performances of the FBG sensor and the flowmeter.2. A cross-correlation flowmeter using FBG sensorsFig. 1 shows the principle of the flowmeter. The cross-correlation flowmeterpresented here uses FBG strain sensors comprised of FBGs and metal cantilevers. In the flow measurement section, the FBG sensors and a bluff body are used. The bluff body whose shape is a rectangular column generates stable vortices. The time delay between the vortex signals detected by the FBG sensors are estimated using the smoothed coherence transform R SCOT(τ) [15]. The function R SCOT(τ) is expressed as follows:()()()()⎥⎥⎦⎤⎢⎢⎣⎡=-f G f G f G F R yy xx xy RSCOT 1t (1) where Gxx (f ) and Gyy (f ) are the power spectra of the upstream and downstream sensor signals, Gxy (f ) is the cross-spectrum of two signals and F −1 denotes the inverse Fourier transform. The function R SCOT(τ) is a cross-correlation function weighted with thecoherence of the signals and can detect the time delay more precisely and robustly than the simple cross-correlation function. The maximum of R SCOT(τ) is the best estimate _t of the time delay between two FBG sensors. The measured velocity v meas is then calculated from the following simple equation:t ∆=s meas d v (2)where d s is the distance between two sensors.Fig. 2 illustrates the schematic diagram of the whole system. We used an amplified spontaneous emission (ASE) as an optical source of the system. The ASE has output power of 22 dBm and full width at half maximum (FWHM) of 50 nm at C-band. The light from the ASE is separated by an optical 3 dB coupler and then illuminates two FBG sensors installed to the PVC pipe whose inner diameter is 20 mm. The light reflected by the FBG sensors is fed to Mach –Zehnder interferometers, which are comprised of a 2×2 and a 3×3 couplers, with the optical path differences of 1.635 and 3.169 mm. In the 3×3 coupler, three fibers are arranged in a triangular array. These interferometers are used aswavelength-shift detectors for interferometric detection. The incident light isphase-modulated by the Mach –Zehnder interferometer and then converted to voltage signals by photodetectors. Six output signals are simultaneously digitized by an A/D converter with a resolution of 16 bit and sampling frequency of 10 kHz, and the detected signals are then processed to obtain the time delay.The FBG reflects the light wave with a certain wavelength λB called Braggwavelength and the wavelength is then expressed as follows:Λ=n B 2λ，（3）where n is the effective refractive index of the FBG and Λ is the modulation pitch of the refractive index of the FBG. The Bragg wavelength λB changes by longitudinal strain εz applied to the FBG, and the Bragg wavelength-shift δλB is expressed as follows [3]:()Z BB P ελδλ121-= (4) where p 12 is the elasto-optic constant of the optical fiber and is approximately 0.22. This yields the strain sensitiv ity of 1.2 pm/_ε.To obtain the shift δλB, the outputs of the interferometer are used, and the outputs Vm (m = 1, 2, 3) are expressed as follows [9]:Vm = αmV in + Re[Γ(τ)] = αmV in[1 + γ cos (θMZI + θm)], （5）where Γ (τ) is the auto-correlation function of light wave reflected by the FBG sensor, V in is the voltage corresponding to optical power reflected by the FBG sensor, and αm is the coefficient compensating differences of photodetector sensitivities and obtained from preliminary experiments. If the split ratios of the 2 × 2 and 3 × 3 couplers are 1:1 and 1:1:1, respectively, one can obtain θ1 = 0, θ2 = 2π/3 and θ3 = −2π/3, and the outputs V 1, V 2 and V 3 are derived as follows:⎪⎪⎪⎩⎪⎪⎪⎨⎧-+=++=+=)]32cos(1[)]32cos(1[)cos 1(312111πθγαπθγαθγαMZI in MZI in MZI in V V V V V V （6） The signal θMZI can be then calculated using the following equation:1323212)(3tan 2V V V V V LB MZI -+-=-λπθ (7) where L is the optical path difference of the interferometer. The relationship between the signal variation δθMZI and the shift δλB is expressed as follows:()B BB B B B B B B MZI L L L L δλλπδλλδλλπλπδλλπδθ22222-≈+-=-+= (8) where the term (λB + δλB) was assumed to be nearly equal to λB because the shift δλB is significantly smaller than λB. An accidental loss of optical power while measurement, which causes problems in optical intensity modulation type sensors, is admissible in some measure because the wavelength-to-phase sensitivity (=−2πL/λ2B) depends on only thepath difference L .3. Noise estimation of the FBG sensor with interferometric detectionThere are some reports about the noise estimation of FBG sensors with interferometric detection. However the noise estimation reports about the interferometric detection using a 2 × 2 and a 3 × 3 couplers have not been presented.3.1. Noise of the photodetectorFig. 3 shows the circuit diagram of the photodetector. The noise of the photodetector is defined by the noise of a photodiode and a transimpedance amplifier. The noise of the photodetector is determined by thermal noises due to the feedback resistance R f of the transimpedance amplifier R f and shunt resistance R sh of the photodiode and by shot noise due to the output current im (=Vm /R f ) of the photodiode. The equivalent input noise voltage and current of the Op-Amp are neglected because they are a few orders smaller than the other noises. The RMS of the noise voltage V N ,m (m = 1, 2, 3) can be then written as follows:fW B W sh W B f m N R TB B qi R TB R V k 42k 4m ,++= （9） where k B is the Boltzmann constant (1.39 × 10−23 J/K), T is the absolute temperature (300 K), B w is the equivalent noise bandwidth of the photodetector (2.4 kHz) and q is the electronic charge (1.6 × 10−19 C). In order to calculate the shot noise by im , the visibility γshould be known. Although the spectral profile of a FBG is expressed as a function of hyperbolic sine and cosine, for ease of calculation of the visibility γ, we assumed that the FBG had a Gaussian spectral profile S (υ) given as follows [10]:()()⎭⎬⎫-⎩⎨⎧∆∆=222ln 4exp 2ln 2B B B in m v v v v V v S πα(10) where υ is the frequency of the light wave, υB is the Bragg frequency, _υB is FWHM of the FBG in the frequency domain. From Eq. (10) andWiener –Khintchine theorem, the visibility γ can be derived as follows:,ln 4e 222⎩⎨⎧⎭⎬⎫∆-=L v c xp B πγ (11) where c is the speed of the light wave in vacuum. From Eqs. (6) and (11), we can obtain the dc value of im and calculate the shot noise by im .3.2. Quantization noise during A/D conversionQuantization noise should be taken into consideration because the system shown in Fig. 2 uses digitized signals for demodulation of the signal θMZI. We assumed thatquantization noise was a white noise, and the RMS of quantization noise V Q may be then expressed as [16]:12LSBQ V V = (12)where V LSB is the resolution of voltage. The A/D converter with 16 bit and±10V measurement range used in the system yields V LSB = 3.05 × 10−4 V.3.3. Intensity noise of the optical sourceThe fluctuation in the intensity of the ASE output should be considered. The intensity noise arise as a fluctuation in the voltage V in. In the ideal situation, it is found from Eqs. (6) and (7) that the intensity noise is deleted. But in the practical situation, the intensity noise arises due to the existence of the above-mentioned noises. We assumed that the intensity noise was white noise whose RMS was 1% of the output power of the ASE.3.4. Noise estimation of the FBG sensorThree uncorrelated white noises were given for noise estimation of the FBG sensor. The output voltages V est ,m (m = 1, 2, 3) and the sensor signal θMZI are then written as: )(,,m m noise m m est V V V V += （13）,)(3tan 1,3,2,3,2,1est est est est est N MZI V V V V V -+-=+-θθ (14)where V noise ,m (Vm ) is white noise whose RMS is ()2122m ,Q N V V + and θN is thefluctuation of the signal. The noise spectral density is calculated from the RMS of θN divided by 21WB and then converted to the noise in the wavelength domain using Eq. (8). Fig. 4 shows the relationship between FWHM _λB of the FBG in the wavelength domain and the noise spectral density for sufficient optical power. It is found that the noise spectral density is mainly determined by the noise of the photodetector. The increasing noise spectral density with the broad _λB is due to decrease in the visibility γ. At _Λb above 0.4 nm, the noise spectral density for L = 3.169mm is much higher than that for L =1.635 mm. To the contrary, at _λB below 0.4 nm, the noise spectral density for L =3.169mm indicates lower level than that for L = 1.635 mm. This is because the optical path difference L has influences on both the wavelength-to-phase sensitivity (=−2πL/λ2B) andthe visibility γ. The Mach–Zehnder interferometer for L = 3.169mm is expected to yield the lower noise density than that for L = 1.635mm because FBGs in the system normally have _λB of 0.2 nm. The noise spectral densities of the FBG sensor with the interferometric detection were estimated to be 2×10−4 and 10−4 pm/(Hz)1/2 for L = 1.635 and 3.169 mm, respectively. These values can be converted to 1.66 × 10−1 and 8.33 ×10−2 nε/(Hz)1/2, respectively in consideration of the strain sensitivity of the FBG (1.2pm/_ε).4. Laboratory experiments of the cross-correlation flowmeter and discussionFig. 5 illustrates an experimental configuration of the FBG sensors and the bluff body in the flow measurement section. The size d w of the bluff body is 3 mm. From the principle of the cross-correlation function R SCOT(τ), the coherent signal with broad bandwidth is desirable, and we tested several configurations of the sensors and the bluff body for broad bandwidth of signal. The parameters of the configurations are d b, d s and d c, which are the distance between the bluff body and the upstream sensor, the distance between two sensors and the distance between the bluff body and the center of the PVC pipe, respectively.4.1. Performances of the FBG sensorFig. 6 shows a comparison of waveforms detected with a reference strain sensor (KYOWA, KFG-20-120-C1-11) and the FBG sensor. The sensors were attached on a metal cantilever in this experiment. The waveforms of both the FBG sensor and the reference sensor are similar as shown in this figure. The difference between signal amplitudes of two sensors is due to the difference of coupling of the sensors. Fig. 7 shows the noise spectral density of the FBG sensor as a function of _λB. This figure indicates that the experimental noise spectral density is higher than the estimate at narrow _λB. The reason for this difference is considered to be due to the difference of characteristics of the photodetectors and quality (the split ratio) of the couplers used in the Mach–Zehnder interferometers. On one hand, at _λB of 1.2 nm, the experimental noise spectral density is lower than the estimate. This difference is due to assumption of the spectral profile of the FBG. The minimum noise density of 4 × 10−4 pm/(Hz)1/2 was achieved for both L = 1.635 and3.169mm at _λB of 0.2 nm. This value corresponds to 0.33 nε/(Hz)1/2 and is sufficient low to detect vortices in the fluid.4.2. Flow measurement4.2.1. Effect of the bluff body and Karman vortex shedding frequencyTo validate the effect of the bluff body, we compared the signals with/without the bluff body. Fig. 8 shows the measured waveforms, calculated R SCOT(τ) and magnitude squared coherence (MSC) for d s = 20mm and the bulk velocity v b = 1.0 m/s. The bulk velocity is defined as the volumetric flow divided by the cross-sectional area of the pipe. MSC indicates the similarity of two signals in the frequency domain and defined as follows: ()(),)(f )(xy 2xy f G G f G f MSC yy = （15）MSC was used for evaluation of the bandwidth of the detected signal. From Fig. 8a and b , it is clear that the amplitude of the measured waveform with the bluff body is much higher than that without the bluff body. The bluff body influences on both R SCOT(τ) and MSC for the better. MSC with the bluff body has the maximum of 0.9 around frequency of 60 Hz although that without the bluff body does not have such a peak. This coherent signal is due to Karman vortex. As a result, R SCOT(τ) with the bluff body has the sharper and larger peak than that without the bluff body.4.2.2. Influence of the configurations: ds, db and dcFig. 9 shows MSC and R SCOT(τ) obtained by the experiments for various configurations and the bulk velocity v b = 1.0 m/s. At first, influence of the distance d s is discussed. With the shorter distance d s, the bandwidth of the coherent signal becomes broader and the peak of the function R SCOT(τ) becomes larger and sharper. This result means that vortices are attenuated and degraded with its convection. Consequently the distance d s is desirable to be as short as possible in a view point of signal quality. On the other hand, the longer distance d s is desirable for the estimation of velocity if the resolution of the time delay is fixed. The optimum d s depends on the specifications of the flow measurement system, including the minimum and maximum measurement velocities and time resolution (sampling frequency of the A/D converter).Next, we discuss the influence of the distance d b. Shorter distance d b makes the bandwidth of the coherent signal narrow as shown in Fig. 9a. In the region behind the bluff body, the bandwidth of the vortex signal is narrow because the signal is not disturbed. As a result, the upstream sensor detects vortices with narrow bandwidth and the bandwidth of the coherent signal becomes narrow. In the case of the offset of the bluff body d c = 2.5 mm, turbulent flow is generated because flow with different velocities are mixed. The function R SCOT(τ) is expected to have a sharp peak because turbulent flow signal is thought tohave broad bandwidth. From Fig. 9a, it is clear that the bandwidth of the coherent signalfor d c = 2.5mm is broader than that for d c = 0 mm. However, the maximum of MSC for d c = 2.5mm is smaller than that for d c = 0 mm. The function R SCOT(τ) for d c = 2.5mm indicates worse performance on sharpness than that for d c = 0 mm. This may be due to coherence degradation of the signal.4.2.3. Linearity of the flowmeterFig. 10 shows the experimental relationship between the bulk velocity v b and the measured velocity v meas. All results with the bluff body indicate better linearity than that without the bluff body. The result for d s = 20, d b = 20 and d c = 0mm exhibits the best accuracy with the relative error of ±5%.4.2.4. Karman vortexKarman vortex shedding frequency can be also utilized for velocity estimation. The relationship between the bulk velocity v b and Karman vortex shedding frequency f K is expressed as follows:b v Wt K d s f （16） where S t is the Strouhal number and is usually constant at ambient temperature. Fig. 11 displays the relationship between the bulk velocity and Karman vortex shedding frequency for d s = 20, d b = 20 and d c = 0 mm. It shows a good linearity. The Strouhal number is obtained to be 0.168. The problem with flow measurement using Karman vortex shedding frequency is that this method is susceptive to external vibration noise. The flowmeter presented here can measure the flow velocity from both the time delay and Karman vortex shedding frequency. The flowmeter is expected to be reliable in various situations by combining these techniques.5. ConclusionWe presented the principle of the cross-correlation flowmeter using FBG sensors. The interferometric detection was used as a FBG wavelength-shift detection method for dynamic measurement. The noise spectral density of the FBG sensor with theinterferometric detection was discussed and it was estimated to be 2 × 10−4 pm/(Hz)1/2.Experimental noise spectral density of the FBG sensor was 4 × 10−4 pm/(Hz)1/2. The water flow experiments showed that the flowmeter had a good linearity at velocity range from 0 to 1.0 m/s and the minimum detectable velocity of 0.05 m/s. Furthermore, Karman vortex shedding frequency was also detected and indicated to have a linear relationship with the bulk velocity. This can be also utilized to measure the flow velocity, and the flow measurement system is expected to be reliable by combining two measurement techniques. The flowmeter presented in this paper has many advantages including passive nature, explosion-protection, EMI immunity and capability of remote sensing. We try to apply this flowmeter to geophysical use because the advantages are leveraged in the application. By combining the flowmeter with FBG pressure and temperature sensors, FBG sensor system for pressure, temperature and flow measurement in a borehole may be constructed.References[1] G. Meltz, Overview of fiber grating-based sensors, in: Proceedings of the SPIE’s Symposium on Smart Structures and Materials, vol. 2838, 1996, pp. 2–22.[2] Y.J. Rao, In-fiber Bragg grating sensors, Meas. Sci. Technol. 8 (1997) 355–375.[3] A.D. Kersey, M.A. Davis, H.J. Patrick, M. LeBlanc, K.P. Koo, C.G. Askins, M.A. Putnam, E.J. Friebele, Fiber grating sensors, J. Lightwave Technol. 15 (8) (1997)1442–1463.[4] M.A. Davis, A.D. Kersey, Matched-filter interrogation technique for fibre Bragg grating arrays, Electron. Lett. 31 (10) (1995) 822–823.[5] K. Wood, T. Brown, R. Rogowski, B. Jensen, Fiber optic sensors for health monitoring of morphing airframes. I. Bragg grating strain and temperature sensor, Smart Mater. Struct. 9 (2000) 163–169.使用双光纤布拉格光栅传感器和互相关技术的水流量计摘要本文对使用光纤光栅(FBG)传感器的互相关流量计的工作原理和实验结果进行了介绍。

光纤Bragg光栅传感

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7.3 Wavelength Demodulation of Bragg Grating Point Sensors
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7.3.1 Quasi-Static Strain Monitoring
7.3.1.1 Passive Broadband Interrogation 7.3.1.2 Ratio metric Detection with a WDM Fiber Coupler 7.3.1.3 Interrogation via Scanning Optical Filter 7.3.1.4 Bragg Grating Interrogation Using Wavelength Tunable Source
(7.3b)
1 n n2 (1 2v )( 2 p12 p11 ) (7.4) n p 2E
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7.2.2 Dynamic Magnetic Field Sensitivity (DMFS)
Faraday effect - A longitudinal magnetic field applied to the grating changes the refractive index for the two cp (circular polarization ,CP) 2 Bragg conditions:

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7.3.1.1 Passive Broadband Interrogation
IS A{B O } IR
This system offer several advantages - low cost - ease of use - low resolution
(7.6)
For the refractive-pressure coefficients are given by

光纤光栅时分复用传感系统

光纤光栅时分复用传感系统耿淑伟1,余有龙2(1.哈尔滨工业大学土木工程学院,黑龙江哈尔滨150008;2:黑龙江大学光纤技术研究所,黑龙江哈尔滨150080)摘　要:研制了光纤光栅时分复用传感系统,针对脉冲宽带光源的光束注入间隔为51.75m 、波长介于1550～1565nm 的5个光纤光栅组成的光栅串,根据传感元间光纤延时程度的不同,利用模拟电子程控开关对来自不同光栅信号的选择通导,实现传感信号的地址查询;借助非平衡Michelson 扫描干涉仪,待测量引起的来自查询传感元的波长漂移信息得到解调,从而成功地实现了时分复用传感.在[0,550]με的测量范围内,系统传感灵敏度的实验值约为1.67°/με,与理论值非常吻合.关键词:光纤光栅;传感网络;时分复用;干涉解调中图分类号:TM55 文献标识码:A 文章编号:036726234(2002)022*******Fiber bragg grating sensor system using time 2divisionmultiplexing techniqueGEN G Shu 2wei 1,YU Y ou 2long 2(1.School of Civil Engineering ,Harbin Institute of Technology ,Harbin 150008,China ;2.Research Institute of Fiber Technology ,Heilongjiang University ,Harbin 150080,China )Abstract :A five 2element ,time 2division multiplexed ,fiber Bragg grating sensor array operating between wavelengths of 1550and 1565nm was constructed and tested.A pulsed broadband source was employed to illuminate the gratings spaced 51.75meters apart.According to the difference of the delay ,interrogation was demonstrated by using a controlled electronic analog switch to select the returned signal of each grat 2ing.Measurand 2induced changes in the wavelength of the light reflected by the interrogated element were demodulated by introducing an unbalanced Michelson scanning interferometer.Time -division multiplexingoperation was demonstrated successfully.The measured sensitivity of this sensor system is about 1.67°/μεwithin the measured range of [0,550]με,which is in good agreement with the theoretical value.K ey w ords :fiber Bragg grating ;FB G array ;time 2division multiplexing ;interferometric demodulation收稿日期:2001-03-02.基金项目:国家自然科学基金委员会、中国节能投资公司联合研究基金联合资助项目(60177029);黑龙江省01年杰出青年科学基金资助项目.作者简介:耿淑伟(1965-),女,博士生,副教授;余有龙(1965-),男,博士后,教授. 大型施工项目都要考虑防、抗灾能力,这并不意味着会杜绝自然蜕化以及突发灾害的侵袭.若结构的重要部位埋入光纤光栅(FB G ),用来监测温度[1]、应变[2]、压力[3]、位移[4]等量的变化,并在灾难发生时,经控制系统借助自动消防、智能阻尼以及其他响应装置来减小灾难的危害,使原本没有生命、不能感知灾害的工程成为具备预警功能的智能结构工程,因此FB G 传感器在土木工程中的应用,正成为结构健康监测的重要课题之一.FB G 传感器的信息载体是光,无论以波长还是位相为监测对象都具有几乎是极限的传感精度;波长编码保证其传感结果不受系统整体光强和光纤、耦合器连接损耗以及其他器件插入损耗的影响;介电特性使其免受电磁场影响;本征型的该类传感器损耗低、光谱特性好、可靠性高、易于复用和网络化管理,可进行节点(Point )型准分布式传感;光纤相对来说比较柔软,传感回路易于植第34卷第2期哈　尔　滨　工　业　大　学　学　报 Vol.34No.22002年4月 Journalof H arbin Institute of T echnology Apr.,2002入或附着在结构表面,能够实时提供应变、温度以及结构完整性方面的信息,且布置比较灵活.单光栅传感系统价格昂贵,很难推广,若将诸多传感光栅组合起来进行网络式监测,可大幅提高系统的性能价格比,因而引起人们的广泛关注[5～7].波分复用是常见的FB G 网络化信号处理技术;受光源带宽限制,它所允许复用的数目有限.若考虑对不同光栅信号依时序进行分割,则为增加复用数目提供了新的途径.本文利用光栅串中不同位置光栅对同一脉冲信号反射时间的差异,结合非平衡Michelson 干涉仪解调技术,对5个光栅组成的光栅串成功地进行了时分复用传感.1　原　理脉冲宽带信号经m 个光栅反射后进入一臂长差为L 的非平衡Michelson 扫描干涉仪,其输出可以表示为I =∑mi =1I i[1+k icos (ω0t +Δψi)]式中:I i 取决于光栅i 的反射光强,并与光路中的损耗有关;k i 为与条纹可见度有关的系数;ω0为干涉仪扫描角频率.作用于光栅i 上的应变通过波长漂移引起的相移为Δψi =-4πnL λi(1-P e )εi(1)式中:P e 为光纤介质的有效弹光系数.干涉仪输出无法直接读出所需应变信息,假设光栅呈等光程D 分布,栅长为l ,脉冲频率f 和占空比ρ满足f <c/2(m -1)D ρ<2fc(D -nl ) 引入程控选择开关,使得光栅i 的信号通导,其他信号均被阻隔,滤掉载频信号,用相位计观测相位变化便可监测该光栅的应变信息.2　实验结果图1所示的装置中,串接着的光栅G i (i =1,2,…5),彼此间距为51.75m ,其布喇格波长分别为1552.84、1555.54、1557.79、1558.65以及1562.44nm ,各光栅的长度均为1cm ,带宽约为0.2nm.脉冲宽带光源的平均输出功率为0.75mW ,经光栅串反射后由环形器耦合进入端镜反射率均接近90%的非平衡全光纤扫描Michelson 干涉仪,L 值为3.2mm ,M 2所在的短臂缠绕在一PZT 上,条纹的可见度为0.3,经探测器转换为电信号并放大后由电子开关选择通导.锯齿波驱动信号的频率为80Hz ,占空比接近1.M i (i =1,2),反射镜;S i (i =1,2,3,4),延时光纤;C i 环循器图1　试验装置图Fig.1　Experimental set 2up 时钟信号控制系统运作,其频率为100kHz ,占空比为0.025,幅值为4V ,直流电平为-1.5V ;其触发信号通过控制延时信号发生器产生延时信号来控制开关.延时量编程可控,用以确保被测信号到达开关位置时开关处于通导状态.延时信号脉宽为400ns ,与时钟信号同频,设置延时量τ为56、556、1056、1556以及2056nS 时开关分别只对来自G 1、G 2、G 3、G 4以及G 5光栅的信号通导(图2(a )中b 和图2(b )中1均为G 2处于通导时的开关输出).G 1通导对应的延时量不为0,乃信号沿两路径传至开关位置的时程差不为0所至.考察任一光栅(如G 2),对应开关输出为调制了的脉冲信号(见图2(b )中曲线1所示),经[75,85]Hz 带通滤波后见曲线2所示.应变不变时,其位置不变.随着应变的增加曲线向左平移,这表明扫描干涉仪将应力诱致的波长漂移变为相移,从而证实干涉仪的解调功能.图3所示的实验曲线表明两者间呈线性关系,斜率1.6755°/με是本系统传感灵敏度的实验值.所用光纤纤芯的折射率为1.4489,取P e =0.22,式(1)确定的理论值为1.6739°/με.同样操作可以监测作用于G 1、G 3、G 4以及G 5上的应变,测得各自对应的灵敏度为1.6718、1.6691、1.6684以及1.6637°/με,其理论值分别为1.6768、1.6715、1.6706以及1.6665°/με,可见理论与实验结果基本一致.干涉仪两臂间光程差以及相位计的分辨率决定了系统的传感分辨率,所用相位计分辨率为0.01°,它使系统具备6nε的分辨能力.・502・　第2期耿淑伟,等:光纤光栅时分复用传感系统(a )开关输入信号1开关输出信号;2带通滤波器输出信号;3压电陶瓷驱动信号;4时钟信号(b )τ=556ns 时开关输出信号图2　记录不同位置的信号Fig.2　Differentsignals图3　ΔΨ2随ε2变化的实验曲线Fig.3　Experimental plot of ΔΨ2vs ε2 相位计结合峰值计数器时,系统应变测量范围主要取决于光栅的应变承受能力,甚至可达[-4000,4000]με.3　结　论对5个光栅组成的列阵成功地进行了时分复用传感探测,传感灵敏度的实验值与理论值基本一致.结合空分复用技术将可极大地增加光栅复用数目,以植入的方式对复合材料或其他结构的内应力分布进行实时监测,使得皮蒙技术变为现实,在国防和民用工业中具有广阔的应用前景.参考文献:[1]BALL G A ,MOREY W W ,CHEO P K.Fiber lasersource/analyzer for Bragg grating sensor array interroga 2tion[J ].J Lightwave Technol ,1994,12(4):7002703.[2]VOLAN THEN M ,GEIGER H ,COL E M J ,et al.Measurement of arbitrary strain profiles within fiber grat 2ings[J ].Electron Lett ,1996,32(11):102821029.[3]XU M G ,GEIGER H ,DA KIN J P.Fiber grating pres 2sure sensor with enhanced sensitivity using a glass 2bubble housing[J ].Electron Lett ,1996,32(2):1282129.[4]YU Y L ,TAM H Y ,L IU Z G ,et al.Passive tempera 2ture compensation technique for fiber Bragg grating dis 2placement sensor [J ].Electron Lett ,1999,35(25):222422226.[5]DAV IS M A ,BELL EMORE D G ,PU TNAM M A ,etal.Interrogation of 60fiber Bragg grating sensors withmicrostrain resolution capability [J ].Electron Lett ,1996,32(15):139321394.[6]RAO Y J ,JACKSON D A ,ZHAN G L ,et al.Strainsensing of modern composite materials with a spatial/wavelength 2division multiplexed fiber grating network [J ].Opt Lett ,1996,21(9):6832685.[7]BER KOFF T A ,KERSEY A D.Fiber Bragg grating ar 2ray sensor system using a bandpass wavelength division multiplexer and interferometric detection[J ].IEEE Pho 2ton Technol Lett ,1996,8(11):152221524.(编辑　闫　彤)・602・哈　尔　滨　工　业　大　学　学　报第34卷　。

一种光纤Bragg光栅传感智能周界围栏报警系统

一种光纤Bragg光栅传感智能周界围栏报警系统传统的周界安防或围栏报警系统：如主动红外对射、微波对射、泄漏电缆、振动电缆、电子围栏、电网等，虽为安全技术防范做出了一定的贡献，但受一些客观技术条件等因素所限，还存在着一定的缺陷。

而利用光电技术中的新型光纤传感技术做成的周界围栏报警系统具有非常明显的技术优势。

本文介绍传统周界围栏报警系统的缺陷及光纤传感的优势，光纤Bragg光栅传感器的原理、优点，基于光纤Bragg光栅传感智能周界围栏报警系统的组成及工作原理，系统多处侵入定位报警的解决法及其应用与市场前景等。

一、光纤传感技术与周界围栏报警系统比较多年来，传统的周界安防或围栏报警系统：如主动红外对射、微波对射、泄漏电缆、振动电缆、电子围栏、电网等，为安全技术防范做出了一定的贡献。

但是，受一些客观技术条件等因素所限，还存在着一定的缺陷：如主动红外对射的围栏报警系统，易受地形条件的高低、曲折、转弯、折弯等环境限制，而且它们不适合恶劣气候，容易受高温、低温、强光、灰尘、雨、雪、雾、霜等自然气候的影响，易出现误报率；再如泄露电缆、振动电缆、电子围栏、电网等围栏报警系统，均属于有源的电传感，系统功耗很大；且电子围栏、电网等又有一定危害性；它们又易受电磁干扰、信号干扰、串扰等，而使灵敏性下降，误报率、漏报率上升等。

与上述周界安防或围栏报警系统相比，利用光电技术中的新型光纤传感技术做成的周界安防或围栏报警系统具有非常明显的技术优势：(1)抗电磁干扰，电绝缘性好、安全可靠，耐腐蚀、化学性能稳定，因而完全不受雷电影响，能在恶劣的化学环境、野外环境及强电磁干扰等场所下工作；(2)体积小、重量轻，几何形状可塑，传输损耗小，传输容量大，具有非常好的可靠性和稳定性；(3)不仅能发现外界扰动，而且可确定外界扰动的位置，系统具有成本低、结构简单、便于扩展与安装容易；(4)无辐射、无易燃易爆材料，既防水又环保；(5)能源依赖性低，可大大节省供电设备与线路的成本，适合长距离使用；(6)可根据被测对象的情况选择不同的检测方法，再加上其对被测介质影响小，所以它非常有利于在结构检测等具有复杂环境的领域中应用等。

基于单片机的光纤Bragg光栅传感器的解调系统

文章编号　10042924X (2002)0420383205基于单片机的光纤Bragg 光栅传感器的解调系统方福波(佛山市光电器材公司,广东佛山528000)摘要:长期以来,对光纤Bragg 光栅传感器反射波长的检测是采用光谱仪,随着光纤Bragg 光栅传感器应用的不断发展,光谱仪表现出越来越多的局限性。

本文利用调谐法布里-珀罗腔多光束传播,当法布里-珀罗腔满足特定条件时其透射光可达到最强或最弱原理,建立了F -P 腔解调系统模型,通过检测F -P 腔的相关参数而得到所测的参变量;根据光纤Bragg 光栅自身特性,建立了匹配滤波法解调系统模型,应用另一光纤Bragg 光栅跟踪被测光纤Bragg 光栅的变化,从而得到所测量;论文指出了应用时在这两种解调方案中需要解决的关键问题。

为了提高系统测量的准确性和实时性,系统采用单片机系统处理相关数据信息。

本文系统详细阐述了在解调系统中单片机的应用及相关接口模型,给出了详细的软硬件设计。

关　键　词:光纤Bragg 光栅;法布里-珀罗腔;匹配滤波;单片机中图分类号:TP212 文献标识码:A1　引　言自1989年光栅成栅技术取得重大突破以来,众多国家特别是欧美等发达国家投入了大量的财力和物力用于光纤Bragg 光栅的研制和应用研究,在理论上已取得较大成果[1]。

美国及欧洲一些国家已将光纤Bragg 光栅用于对桥梁、建筑体、矿井等结构及状态监测性实验之中[2]。

由于光纤Bragg 光栅独特的光学特性,其应用前景极为广阔。

光纤Bragg 光栅传感器是利用光纤Bragg 光栅受温度和应变的作用时反射波长发生偏移的机理而制成的新型光纤传感器,由于其自身所具有的独特特点,Bragg 光栅传感器特别适合于大坝、桥梁的监测以及各种蒙皮结构的检测。

但目前对Bragg 光栅反射波长的解调还只局限于应用传统的光谱仪。

由于光谱仪本身的因素,限制了Bragg 光栅应用的进一步发展。

Development of fiber Bragg grating sensors for monitoring civil infrastructure_2005

Engineering Structures 27 (2005) 1828–1834/locate/engstructDevelopment of ﬁber Bragg grating sensors for monitoring civilinfrastructureP. Moyo a,∗, J.M.W. Brownjohn b , R. Suresh c , S.C. Tjin ca Department of Civil Engineering, University of Cape Town, Rondebosch 7701, Cape Town, South Africa Department of Civil and Structural Engineering, University of Shefﬁeld, Mappin Street, Shefﬁeld, S1 3JD, UKc School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singaporeb Available online 11 July 2005AbstractThe concept of structural health monitoring has been the subject of research over the last few years, particularly in civil and structuralengineering where ageing infrastructure is of major concern. These studies have led to initiatives towards the development and deployment of new sensing technologies. Owing to the harsh environments found in the construction industry, and the large size of civil engineering structures, such sensors should be robust, rugged, easy to use and economical. Fiber Bragg grating (FBG) sensors offer a viable such sensing approach with a number of advantages over traditional sensors. These include immunity to electromagnetic interference, light weight, small size, multiplexing capabilities, ease of installation and durability. This paper reports some results from a multi-disciplinary research program on FBG sensors involving the School of Civil and Structural Engineering and the School of Electrical and Electronic Engineering at Nanyang Technological University in Singapore. Novel FBG strain sensors have been developed and deployed on highway bridges to measure dynamic strain, static strain, and temperature. Results of these studies indicate that, if properly packaged, FBG sensors can survive the severe conditions associated with the construction environments of civil infrastructure. © 2005 Elsevier Ltd. All rights reserved.Keywords: Fiber optic sensors; Fiber Bragg grating sensors; FBG strain sensors; FBG temperature sensors; Structural health monitoring1. Introductionstructural behaviour and performance. Anomalies include, and are often deﬁned as, deterioration and damage resulting from changes in material properties, geometric properties, boundary conditions, system connectivity and the loading environment of the structure.A typical SHM system comprises an array of sensors, sensor excitation hardware, a host computer and communi- cation hardware and software. Sensors play the important role of providing information about the state of strain, stress and temperature of the structure. Their selection for a par- ticular application is governed by application, sensor sensi- tivity, power requirements, robustness and reliability. Most SHM systems reported in the literature have focused on tra- ditional sensing technologies such as electrical resistance train sensors, vibrating wire strain gauges and piezoelectric accelerometers. While the traditional sensors are robust and strong enough for civil engineering applications they oftenSocio-economic pressure to install structural health monitoring (SHM) systems on civil infrastructure in order to improve safety, and to optimize maintenance strategies, is growing. The demand for SHM systems has largely been driven by deﬁciencies in structural performance arising from ageing phenomena, and the need to increase the load-carrying capacity of structures to enable them to cope with increased loading regimes. By continuously or periodically measuring structural response to live loading and environmental changes such as stress, strain, temperature, and vibration, and applying relevant data analysis procedures, one can detect anomalies in∗Corresponding author. Tel.: +27 21 6502592; fax: +27 21 6897471. E-mail address: pmoyo@ebe.uct.ac.za (P. Moyo).0141-0296/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.engstruct.2005.04.023P.Moyo et al./Engineering Structures27(2005)1828–18341829require many cables to support them,and for long distance monitoring these cables suffer from electromagnetic inter-ference(EMI).Also only sparse populations of electrical resistance(ER)sensors can be installed due to costs.In recent years there have been a number of research initiatives towards the development and deployment ofﬁber Bragg grating sensors(FBG)for sensing applications in civil and structural engineering[1–3].The interest in FBG sensors as a viable sensing approach for civil infrastructure has been motivated by the advantages they offer over traditional sensors.FBG sensors are:immune to electromagnetic inter-ference,lightweight,small in size,easy to install,corrosion resistant,durable and can be multiplexed.The multiplexing capabilities of FBG sensors mean that many sensors can be installed on a structure with minimum wiring.However these sensors are fragile and their packaging should ensure that the sensors are robust and rugged enough to withstand the harsh environment found in the construction industry.A number of FBG based strain monitoring programs have been reported in the technical literature.Todd[4]demon-strated the sensing capabilities of FBG sensors by installing a number of sensors on a steel girder in a bridge.Schulz[5] reported the application of long gauge FBG sensors for mon-itoring civil structures.The performance of FBG sensors has also been demonstrated in the laboratory[6–8].Most of these studies focus on sensing and multiplexing capabil-ities of FBG sensors.Little attention has been paid to the packaging of these sensors for application in civil engineer-ing where conditions are harsh.This paper reports the de-sign and experimental evaluation of FBG sensors carried out at Nanyang Technological University(Singapore),with par-ticular emphasis on packaging of sensors for applications in concrete structures.factors on an FBG are considered,neglecting higher-order cross-sensitivities,then the amount of Bragg wavelength shift is given by[9]λB= Kεε+ K T T+K P P(2) where Kε, K T and K P are cuefﬁcients of wavelength sensitivity to strain,temperature and pressure(force per unit area)for an FBG given byKε=[1−0.5n eff(ρ12−ν(ρ11−ρ12))]λBK=[1+ξ]λ(3)(4) T BK P=−1−2n2ν + 2E(1−2ν)(2ρ12+ρ11) λB(5)Ewhereρ11andρ12are the components of theﬁber optic strain tensor andνis Puissun’s ratio for theﬁber,ξis theﬁbe r thermo-optic cuefﬁcient,E is Yuung’s modulus for theﬁber,n is the refractive index of theﬁber.Most of these properties are known experimentally and remain fairly constant for the differentﬁbers.For a common bare FBG,they are Kε= 1.15pm/µε, K T= 11pm/◦C and K=−3.0pm/MPa,respectively,at1.55µm wavelength.P3.FBG sensing systemsInstallation of bare FBG sensors on components of civil structures may result in a high rate of sensor failure owing to the fragile nature of the glassﬁber and the harsh environment of the construction industry.Also handling problems resulting from the small physical size of theﬁber add to the difﬁculties associated with attaching bare FBG sensors to structural components.As noted above,a bare FBG sensor is sensitive to temperature,strain and pressure. An ideal sensor should be sensitive to only one parameter and be immune to others.Thus there is a need to develop techniques for minimizing cross-effects in FBG sensors. The elimination of cross-sensitivity may be achieved by measurements at two different wavelengths or two different optical modes,for which the strain and temperature responses are different[10–13].Fernández-Valdivielso et al.[14]proposed temperature discrimination based on an FBG sensor and a thermochromic material where the strain is measured through the variations of the FBG wavelength while the temperature is measured using the change in optical power reﬂected by the thermochromic material. However these techniques suffer from system complexity, limited measurement range and limited multiplexing capabilities.A simpler way to correct a strain measurement for the effect of temperature is using physically separated sensors,where the one for temperature compensation is isolated from the strainﬁeld.The following sections describe novel strain FBG sensors and temperature FBG sensors designed for application in civil and structural engineering.The important aspect of the design of these FBG sensors is their packaging for civil engineering applications.2.Principle of FBG sensorsA Bragg grating is a periodic structure fabricated by exposing photosensitizedﬁber core to ultraviolet light (Fig.1).When light from a broadband source interacts with the grating a single wavelength,known as the Bragg wavelength,is reﬂected.The Bragg wavelength is related to the grating period,Λ,and the effective refractive index of theﬁber byλB= 2Λn eff.(1) Both the effective refractive index and the grating period vary with changes in strain,ε,temperature,T,and pressure P,imposed on theﬁber.An applied strain and pressure will shift the Bragg wavelength through expansion or contraction of the grating periodicity and through the photoelastic effect.Temperature affects the Bragg wavelength through thermal expansion and contraction of the grating periodicity and through thermal dependence of the refractive index. These effects are well understood and,when adequately modelled,providea means for predicting strain,pressureand temperature.If only the dominantlinear effects of these three1830P.Moyo et al./Engineering Structures27(2005)1828–1834Fig.1.FBG fabrication process.Fig.4.Embedded FBG strain sensor.3.2.FBG strain sensorThe embedded strain sensor consists of an FBGsandwiched between layers of carbon composite material(Fig.4)and is about50mm long and0.5mm thick.The accuracy and sensitivity of the sensor are dependentupon the optical interrogation system.The function of theoptical interrogating system is to detect the wavelengthshift in relation to external perturbation and to deduce themeasurands using Eq.(2).The interrogating system usedfor strain measurements described here had a sensitivity of1µεand an accuracy of5µε.It should be noted that thestrain sensor is not immune to temperature variations andwould therefore indicate strain due to thermal expansion.The sensor described in Section3.3allows for minimizationof strains related to temperature change.Fig.2.FBG temperature sensor.3.3.Temperature compensated FBG strain sensorThe temperature compensated sensor consists of twoFBGs written in close proximity.One FBG is encased ina metal tube to form a temperature sensor while the otheris sandwiched in carbon composite as described above.Theassembly of the two sensors is then encased in a speciallydesigned dumb-bell(Fig.5).The sandwiched sensor isbonded to the dumb-bell using an epoxy glue and thetemperature sensor is left loose.The design of the dumb-bell is such that the strain sensor is isolated from pressureeffects.This sensor was designed for strain and temperaturemeasurements in mass concrete.Fig.3.Calibration of a temperature sensor.3.1.Temperature sensorThe temperature sensor consists of an FBG sensorpackaged into a35mm long metal tube(Fig.2).Themetal tubing protects the FBG from external stress andincreases the temperature sensing range and sensitivity byenhancing the FBG dilation[15].The Bragg wavelengthshift of the FBG temperature sensor is only related tothe change of ambient temperature.A typical calibrationcurve for the temperature sensor is shown in Fig.3.Thewavelength–temperature coefﬁcient is25pm/◦C,with asensitivity of0.04◦C,and the accuracy is0.2◦C.4.Experimental evaluation of FBG sensorsEmbedding the FBG sensors in carbon compositematerial or encasing the FBG in a metal tube alters itssensitivity coefﬁcients making it necessary to calibrate thesensors.To this end experiments were carried out wherethe embedded FBG strain sensors were installed alongside。

光纤布拉格光栅温度传感技术研究解读

武汉理工大学硕士学位论文光纤布拉格光栅温度传感技术研究姓名:柴伟申请学位级别:硕士专业:机械电子工程指导教师:姜德生20040501摘要光纤Bragg光栅传感器是利用Bragg波长对温度、应力的敏感特性而制成的一种新型的光纤传感器,除具有传统电类传感器的功能外,它还具有分布传感、抗电磁干扰、精度高、长期稳定性好等优点,在大型复合材料和混凝土的结构监测、智能材料的性能监测、电力工业、医药和化工等领域有着广阔的应用前景。

对温度的测量是光纤Bragg光栅传感器的重要应用之一。

对光纤光栅进行温度传感研究不仅满足了对温度检测的需求,而且还为光纤光栅应变传感器的温度补偿提供了必要的基础。

研究表明,光纤Bragg光栅传感特性稳定,是理想的温度传感元件。

但是必须对Bragg光栅进行有效的封装,才能使其成为能满足工程实际要求的传感器。

因此对光纤Bragg光栅传感器封装方法的研究对于其走向实际应用具有重要的意义。

本文对光纤Bragg光栅的温度传感进行了研究,主要工作如下:对光纤Bragg光栅传感技术做了深入的研究和分析。

针对工程实际应用,提出了光纤光栅温度传感器的设计要求。

通过研究目前光纤光栅温度传感器封装的现状,并分析已有封装方法的特点,提出了一种新的光纤光栅温度传感器封装方法。

然后通过实验研究了封装结构及工艺对光纤光栅温度特性的影响,并对实验结果进行了理论分析。

可以得到以下结论:1在封装过程中对光纤光栅旌加一定的预张力可以使光纤光栅温度传感器有很好的重复性。

2封装结构可以提高光纤光栅作为温度传感器的温度灵敏度系数。

3封装后的光纤光栅依然保持着波长与温度良好的线性关系。

因此,采用此种封装结构的光纤光栅温度传感器具备良好的重复性、线性度和灵敏度,可以满足实际应用的要求,具有广阔的应用前景。

此外,本文还介绍了光纤光栅波长解调系统的基本原理,分析比较了几种常用的光纤Bragg光栅波长解调方法。

探讨了基于调谐光纤F.P滤波法的光纤光栅解调器的研制,并组建了比较完整的光纤光栅温度传感检测系统。

Fiber_Bragg_grating(FBG)-光纤光栅,chirp

Figure 1: A Fiber Bragg Grating structure, with refractive index profile and spectral responseFiber Bragg gratingFrom Wikipedia, the free encyclopediaA fiber Bragg grating (FBG ) is a type of distributed Bragg reflector constructed in a short segment of optical fiber that reflects particular wavelengths of light and transmits all others. This is achieved by creating a periodic variation in the refractive index of the fiber core, which generates awavelength specific dielectric mirror. A fiber Bragg grating can therefore be used as an inline optical filter to block certain wavelengths, or as a wavelength-specific reflector.Contents■1History■2Manufacture■2.1Interference■2.2Photomask■2.3Point-by-point■2.4Production■3Theory■4Types of gratings■4.1Standard, ortype I, gratings■4.2Type IAgratings■4.3Type IIA, ortype In, gratings■4.4Regeneratedgratings■4.5Type IIgratings■5Grating structure■5.1Apodizedgratings■5.2Chirped fiberBragg gratings■5.3Tilted fiberBragg gratings■5.4Long-periodgratings■6Applications■6.1Communications■6.2Fiber Bragggrating sensors■6.3Fiber bragggratings used infiber lasers■7See also■8References■9External linksHistoryThe first in-fiber Bragg grating was demonstrated by Ken Hill in 1978.[1]Initially, the gratings were fabricated using a visible laser propagating along the fiber core. In 1989, Gerald Meltz and colleagues demonstrated the much more flexible transverse holographic inscription technique where the laser illumination came from the side of the fiber. This technique uses the interference pattern of ultraviolet laser light[2]to create the periodic structure of the fiber Bragg grating. ManufactureFiber Bragg gratings are created by "inscribing" or "writing" systematic (periodic or aperiodic) variation of refractive index into the core of a special type of optical fiber using an intense ultraviolet (UV) source such as a UV laser. Two main processes are used: interference and masking. The method that is preferable depends on the type of grating to be manufactured. Normally a germanium-doped silica fiber is used in the manufacture of fiber Bragg gratings. The germanium-doped fiber is photosensitive, which means that the refractive index of the core changes with exposure to UV light. The amount of the change depends on the intensity and duration of the exposure as well as the photosensitivity of the fibre. To write a high reflectivity fiber Bragg grating directly in the fiber the level of doping with germanium needs to be high. However, standard fibers can be used if the photosensitivity is enhanced by pre-soaking the fiber in hydrogen. More recently, fiber Bragg gratings have also been written in polymer fibers, this is described in the PHOSFOS entry.[3] InterferenceThis was the first method used widely for the fabrication of fiber Bragg gratings and uses two-beam interference. Here the UV laser is split into two beams which interfere with each other creating a periodic intensity distribution along the interference pattern. The refractive index of the photosensitive fiber changes according to the intensity of light that it is exposed to. This method allows for quick and easy changes to the Bragg wavelength, which is directly related to the interference period and a function of the incident angle of the laser light.PhotomaskA photomask having the intended grating features may also be used in the manufacture of fiber Bragg gratings. The photomask is placed between the UV light source and the photosensitive fiber. The shadow of the photomask then determines the grating structure based on the transmitted intensity of light striking the fiber. Photomasks are specifically used in the manufacture of chirped Fiber Bragg gratings, which cannot be manufactured using an interference pattern.Point-by-pointA single UV laser beam may also be used to 'write' the grating into the fiber point-by-point. Here, the laser has a narrow beam that is equal to the grating period. This method is specifically applicable to the fabrication of long period fiber gratings. Point-by-point is also used in the fabrication of tilted gratings.ProductionFigure 2: FBGs reflected power as a function of wavelength Originally, the manufacture of the photosensitive optical fiber and the 'writing' of the fiber Bragg grating were done separately. Today, production lines typically draw the fiber from the preform and 'write' the grating, all in a single stage. As well as reducing associated costs and time, this also enables the mass production of fiber Bragg gratings. Mass production is in particular facilitating applications in smart structures utilizing large numbers (3000) of embedded fiber Bragg gratings along a single length of fiber.TheoryThe fundamental principlebehind the operation of a FBG, isFresnel reflection. Where lighttraveling between media ofdifferent refractive indices mayboth reflect and refract at theinterface.The refractive index willtypically alternate over a definedlength. The reflected wavelength (), called the Braggwavelength, is defined by therelationship,where is the effectiverefractive index of the grating in the fiber core and is the grating period. The effective refractive index quantifies the velocity of propagating light as compared to its velocity in vacuum. depends not only on the wavelength but also (for multimode waveguides) on the mode in which the light propagates. For this reason, it is also called modal index.The wavelength spacing between the first minima (nulls, see Fig. 2), or the bandwidth (), is (inthe strong grating limit) given by,where is the variation in the refractive index (), and is the fraction of power in the core. Note that this approximation does not apply to weak gratings where the grating length, , is not large compared to \ .The peak reflection () is approximately given by,where is the number of periodic variations. The full equation for the reflected power (), is given by,where,Types of gratingsThe term type in this context refers to the underlying photosensitivity mechanism by which grating fringes are produced in the fiber. The different methods of creating these fringes have a significant effect on physical attributes of the produced grating, particularly the temperature response and ability to withstand elevated temperatures. Thus far, five (or six) types of FBG have been reported with different underlying photosensitivity mechanisms.[4]These are summarized below:Standard, or type I, gratingsWritten in both hydrogenated and non-hydrogenated fiber of all types, type I gratings are usually known as standard gratings and are manufactured in fibers of all types under all hydrogenation conditions. Typically, the reflection spectra of a type I grating is equal to 1-T where T is the transmission spectra. This means that the reflection and transmission spectra are complementary and there is negligible loss of light by reflection into the cladding or by absorption. Type I gratings are the most commonly used of all grating types, and the only types of grating available off-the-shelf at the time of writing.Type IA gratings■Regenerated grating written after erasure of a type I grating in hydrogenated germanosilicate fiber of all typesType IA gratings were first observed in 2001[5]during experiments designed to determine the effects of hydrogen loading on the formation of IIA gratings in germanosilicate fiber. In contrast to the anticipated decrease (or 'blue shift') of the gratings' Bragg wavelength, a large increase (or 'red shift') was observed.Later work showed that the increase in Bragg wavelength began once an initial type I grating had reached peak reflectivity and begun to weaken. For this reason, it was labeled as a regenerated grating.Determination of the type IA gratings' temperature coefficient showed that it was lower than a standard grating written under similar conditions.The key difference between the inscriprion of type IA and IIA gratings is that IA gratings are written in hydrogenated fibres, whereas type IIA gratings are written in non-hydrogenated fibres.[6][7]Type IIA, or type In, gratings■These are gratings that form as the negative part of the induced index change overtakes the positive part. It is usually associated with gradual relaxation of induced stress along the axis and/or at the interface. It has been proposed that these gratings could be relabeled type In (for type 1 gratings with a negative index change; type II label could be reserved for those that are distinctly made above the damage threshold of the glass).[8]Later research by Xie et al. showed the existence of another type of grating with similar thermal stability properties to the type II grating. This grating exhibited a negative change in the mean index of the fiber and was termed type IIA. The gratings were formed in germanosilicate fibers with pulses from a frequency doubled XeCl pumped dye laser. It was shown that initial exposure formed a standard (type I) grating within the fiber which underwent a small red shift before being erased. Further exposure showed that a grating reformed which underwent a steady blue shift whilst growing in strength.[9][10]Regenerated gratingsThese are gratings that are reborn at higher temperatures after erasure of gratings, usually type I gratings and usually, though not always, in the presence of hydrogen. They have been interpreted in different ways including dopant diffusion (oxygen being the most popular current interpretation) and glass structural change. Recent work has shown that there exists a regeneration regime beyond diffusion where gratings can be made to operate at temperatures in excess of 1,295 °C, outperforming even type II femtosecond gratings.[11]These are extremely attractive for ultra high temperature applications.Type II gratings■Damage written gratings inscribed by multiphoton excitation with higher intensity lasers that exceed the damage threshold of the glass. Lasers employed are usually pulsed in order to reach these intensities. They include recent developments in multiphoton excitation usingfemtosecond pulses where the short timescales (commensurate on a timescale similar to local relaxation times) offer unprecedented spatial localization of the induced change. Theamorphous network of the glass is usually transformed via a different ionization and melting pathway to give either higher index changes or create, through micro-explosions, voidssurrounded by more dense glass.Archambault et al. showed that it was possible to inscribe gratings of ~100% (>99.8%) reflectance with a single UV pulse in fibers on the draw tower. The resulting gratings were shown to be stable at temperatures as high as 800 °C (up to 1,000 °C in some cases, and higher with femtosecond laser inscription). The gratings were inscribed using a single 40 mJ pulse from an excimer laser at 248 nm. It was further shown that a sharp threshold was evident at ~30 mJ; above this level the index modulation increased by more than two orders of magnitude, whereas below 30 mJ the index modulation grew linearly with pulse energy. For ease of identification, and in recognition of the distinct differences in thermal stability, they labeled gratings fabricated below the threshold as type I gratings and above the threshold as type II gratings. Microscopic examination of these gratings showed a periodic damage track at the grating’s site within the fiber [10]; hence type II gratings are also known as damage gratings. However, these cracks can be very localized so as to not play a major role in scattering loss if properly prepared [12][13]Grating structureFigure 3: Structure of the refractive index change in a uniform FBG (1), a chirped FBG (2), a tilted FBG(3), and a superstructure FBG (4).Figure 4: Refractive index profile in the core of, 1) a uniform positive-only FBG, 2) a Gaussian-apodized FBG, 3) a raised-cosine-apodized FBG with zero-dc change, and 4) a discrete phase shift FBG.The structure of the FBG can vary via therefractive index, or the grating period. Thegrating period can be uniform or graded, andeither localised or distributed in a superstructure.The refractive index has two primarycharacteristics, the refractive index profile, andthe offset. Typically, the refractive index profilecan be uniform or apodized, and the refractiveindex offset is positive or zero.There are six common structures for FBGs;[14]1.uniform positive-only index change,2.Gaussian apodized,3.raised-cosine apodized,4.chirped,5.discrete phase shift, and 6.superstructure.The first complex grating was made by J.Canning in 1994.[15][citation needed ]This supportedthe development of the first distributed feedback(DFB) fiber lasers, and also laid the groundworkfor most complex gratings that followed,including the sampled gratings first made byPeter Hill and colleagues in Australia.[citation needed ]Apodized gratingsThere are basically two quantities that controlthe properties of the FBG. These are the grating length,, given as and the grating strength, . There are,however, three properties that need to be controlled in a FBG. These are the reflectivity, the bandwidth, and the side-lobe strength. As shown above, in the strong grating limit (i.e., for large ) the bandwidth depends on the gratingstrength, and not the grating length. This meansthe grating strength can be used to set the bandwidth. The grating length, effectively , can then be used to set the peak reflectivity, which depends on both the grating strength and the grating length. The result of this is that the side-lobe strength cannot be controlled, and this simple optimisation results in significant side-lobes. A third quantity can be varied to help with side-lobe suppression. This is apodization of the refractive index change. The term apodization refers to the grading of the refractive index to approach zero at the endFigure 5: Optical add-drop multiplexer.of the grating. Apodized gratings offer significant improvement in side-lobe suppression whilemaintaining reflectivity and a narrow bandwidth. The two functions typically used to apodize a FBG are Gaussian and raised-cosine.Chirped fiber Bragg gratingsThe refractive index profile of the grating may be modified to add other features, such as a linear variation in the grating period, called a chirp. The reflected wavelength changes with the grating period, broadening the reflected spectrum. A grating possessing a chirp has the property of adding dispersion—namely, different wavelengths reflected from the grating will be subject to different delays. This property has been used in the development of phased-array antenna systems and polarization mode dispersion compensation, as well.Tilted fiber Bragg gratingsIn standard FBGs, the grading or variation of the refractive index is along the length of the fiber (the optical axis), and is typically uniform across the width of the fiber. In a tilted FBG (TFBG), the variation of the refractive index is at an angle to the optical axis. The angle of tilt in a TFBG has an effect on the reflected wavelength, and bandwidth.Long-period gratingsTypically the grating period is the same size as the Bragg wavelength, as shown above. For a grating that reflects at 1,500 nm, the grating period is 500 nm, using a refractive index of 1.5. Longerperiods can be used to achieve much broader responses than are possible with a standard FBG. These gratings are called long-period fiber grating. They typically have grating periods on the order of 100 micrometers, to a millimeter, and are therefore much easier to manufacture.ApplicationsCommunicationsThe primary application of fiberBragg gratings is in opticalcommunications systems. Theyare specifically used as notchfilters. They are also used inoptical multiplexers anddemultiplexers with an opticalcirculator, or optical add-dropmultiplexer (OADM). Figure 5 shows 4 channels, depicted as 4colours, impinging onto a FBGvia an optical circulator. The FBG is set to reflect one of the channels, here channel 4. The signal is reflected back to the circulator where it is directed down and dropped out of the system. Since the channel has been dropped, another signal on that channel can be added at the same point in the network.A demultiplexer can be achieved by cascading multiple drop sections of the OADM, where each drop element uses an FBG set to the wavelength to be demultiplexed. Conversely, a multiplexer can be achieved by cascading multiple add sections of the OADM. FBG demultiplexers and OADMs can also be tunable. In a tunable demultiplexer or OADM, the Bragg wavelength of the FBG can be tuned by strain applied by a piezoelectric transducer. The sensitivity of a FBG to strain is discussed below in fiber Bragg grating sensors.Fiber Bragg grating sensorsAs well as being sensitive to strain, the Bragg wavelength is also sensitive to temperature. This means that fiber Bragg gratings can be used as sensing elements in optical fiber sensors. In a FBG sensor, the measurand causes a shift in the Bragg wavelength, . The relative shift in the Bragg wavelength, , due to an applied strain () and a change in temperature () is approximately given by,or,Here, is the coefficient of strain, which is related to the strain optic coefficient. Also, is thecoefficient of temperature, which is made up of the thermal expansion coefficient of the optical fiber, , and the thermo-optic coefficient, .[16]Fiber Bragg gratings can then be used as direct sensing elements for strain and temperature. They can also be used as transduction elements, converting the output of another sensor, which generates a strain or temperature change from the measurand, for example fiber Bragg grating gas sensors use an absorbent coating, which in the presence of a gas expands generating a strain, which is measurable by the grating. Technically, the absorbent material is the sensing element, converting the amount of gas to a strain. The Bragg grating then transduces the strain to the change in wavelength. Specifically, fiber Bragg gratings are finding uses in instrumentation applications such as seismology,[17]pressure sensors for extremely harsh environments, and as downhole sensors in oil and gas wells for measurement of the effects of external pressure, temperature, seismic vibrations and inline flow measurement. As such they offer a significant advantage over traditional electronic gauges used for these applications in that they are less sensitive to vibration or heat and consequently are far more reliable. In the 1990s, investigations were conducted for measuring strain and temperature in composite materials for aircraft and helicopter structures.[18][19]Fiber bragg gratings used in fiber lasersRecently the development of high power fiber lasers has generated a new set of applications for fiber Bragg gratings (FBG’s), operating at power levels that were previously thought impossible. In the case of a simple fiber laser, the FBG’s can be used as the high reflector (HR) and output coupler (OC) to form the laser cavity. The gain for the laser is provided by a length of rare earth dopedoptical fiber, with the most common form using Yb3+ ions as the active lasing ion in the silica fiber. These Yb-doped fiber lasers first operated at the 1 kW CW power level in 2004 [20]based on free space cavities but were not shown to operate with fiber Bragg grating cavities until much later.[21] Such monolithic, all-fiber devices are produced by many companies worldwide and at power levels exceeding 1 kW. The major advantage of these all fiber systems, where the free space mirrors are replaced with a pair of fiber Bragg gratings (FBG’s), is the elimination of realignment during the life of the system, since the FBG is spliced directly to the doped fiber and never needs adjusting. The challenge is to operate these monolithic cavities at the kW CW power level in large mode area (LMA) fibers such as 20/400 (20 um diameter core and 400 um diameter inner cladding) without premature failures at the intra-cavity splice points and the gratings. Once optimized, these monolithic cavities do not need realignment during the life of the device, removing any cleaning and degradation of fiber surface from the maintenance schedule of the laser. However, the packaging and optimization of the splices and FBGs themselves are non-trivial at these power levels as are the matching of the various fibers, since the composition of the Yb-doped fiber and various passive and photosensitive fibers needs to be carefully matched across the entire fiber laser chain. Although the power handling capability of the fiber itself far exceeds this level, and is possibly as high as >30 kW CW, the practical limit is much lower due to component reliability and splice losses.[22]See also■Bragg's law■Diffraction■Diffraction grating■Dielectric mirror■Hydrogen sensor■Long-period fiber grating■Photonic crystal fiber■Distributed temperature sensing by fiber optics■PHOSFOS project -embedding FBGs in flexible skinsReferences1.^Hill, K.O.; Fujii, Y.; Johnson, D. C.; Kawasaki, B. S. (1978). "Photosensitivity in optical fiberwaveguides: application to reflection fiber fabrication". Appl. Phys. Lett.32(10): 647.Bibcode:1978ApPhL..32..647H(/abs/1978ApPhL..32..647H).doi:10.1063/1.89881(/10.1063%2F1.89881).2.^Meltz, G.; et al.(1989). "Formation of Bragg gratings in optical fibers by a transverse holographicmethod". Opt. Lett.14(15): 823. Bibcode:1989OptL...14..823M(/abs/1989OptL...14..823M). doi:10.1364/OL.14.000823(/10.1364%2FOL.14.000823). PMID19752980(///pubmed/19752980).3.^http://www.phosfos.eu/eng/Phosfos/Journals/Bragg-grating-in-polymer-optical-fibre-for-strain-bend-and-temperature-sensing4.^J. Canning, Fiber Gratings and Devices for Sensors and Lasers, Lasers and Photonics Reviews, 2 (4),275-289, Wiley, USA (2008)5.^Liu, Y. (2001). Advanced fiber gratings and their application. Ph.D. Thesis, Aston University.6.^Simpson, A. G. (2005). Optical Fibre Sensors and Their Interrogationdv. Ph.D. Thesis, AstonUniversity.7.^Simpson, A. G.; Kalli, K.; Zhou, K.; Zhang, L.; Bennion, I. (2003). "A method for the fabrication oftemperature compensating IA-I strain sensors". OFS16. Nara, Japan. pp. postdeadline paper PD4.8.^For a contemporary review, see J. Canning, Fiber Gratings and Devices for Sensors and Lasers, Lasersand Photonics Reviews, 2 (4), 275-289, Wiley, USA (2008)9.^Xie, W. X.; Niay, P.; Bernage, P.; Douay, M.; Bayon, J. F.; Georges, T.; Monerie, M.; Poumellec, B.(1993). "Experimental-Evidence of 2 Types of Photorefractive Effects Occurring During Photoinscriptions of Bragg Gratings Within Germanosilicate Fibres". Optics Communications104(1-3): 185–195. Bibcode:1993OptCo.104..185X(/abs/1993OptCo.104..185X).doi:10.1016/0030-4018(93)90127-Q(/10.1016%2F0030-4018%2893%2990127-Q). 10.^Niay, P.; Bernage, P.; Legoubin, S.; Douay, M.; Xie, W. X.; Bayon, J. F.; Georges, T.; Monerie, M.;Poumellec, B. (1994). "Behaviour of Spectral Transmissions of Bragg Gratings Written in Germania-Doped Fibres -Writing and Erasing Experiments Using Pulsed or CW UV Exposure". OpticsCommunications113(1-3): 176–192. Bibcode:1994OptCo.113..176N(/abs/1994OptCo.113..176N). doi:10.1016/0030-4018(94)90606-8(/10.1016%2F0030-4018%2894%2990606-8).11.^J. Canning, M. Stevenson, S. Bandyopadhyay, K. Cook, “Extreme silica optical fibre gratings”,Sensors, 8, pp.1-5, (2008)12.^Dong, L.; Archambault, J. L.; Reekie, L.; Russell, P. S. J.; Payne, D. N. (1993). "Single-Pulse BraggGratings Written During Fibre Drawing". Electronics Letters29(17): 1577–1578.doi:10.1049/el:19931051(/10.1049%2Fel%3A19931051).13.^Archambault, J. L.; Reekie, L.; Russell, P. S. J. (1993). "100-Percent Reflectivity Bragg ReflectorsProduced in Optical Fibres By Single Excimer-Laser Pulses". Electronics Letters29(5): 453–455.doi:10.1049/el:19930303(/10.1049%2Fel%3A19930303).14.^Erdogan, Turan (August 1997). "Fiber Grating Spectra"(/xpl/freeabs_all.jsp?tp=&arnumber=618322&isnumber=13456). Journal of Lightwave Technology15(8): 1277–1294.Bibcode:1997JLwT...15.1277E(/abs/1997JLwT...15.1277E).doi:10.1109/50.618322(/10.1109%2F50.618322).15.^J. Canning, M. G. Sceats, "p-phase-shifted periodic distributed structures in germanosilicate fibre byUV post-processing", Electron. Lett., 30, (16), 1344-1345, (1994)16.^Othonos, Andreas; Kalli, Kyriacos (1999). Fiber Bragg Gratings: Fundamentals and Applications inTelecommunications and Sensing. Artech House. ISBN0-89006-344-3.17.^P. Ferraro; G. De Natale (2002). "On the possible use of optical fiber Bragg gratings as strain sensorsfor geodynamical monitoring". Optics and Lasers in Engineering37(2-3): 115–130.Bibcode:2002OptLE..37..115F(/abs/2002OptLE..37..115F). doi:10.1016/S0143 -8166(01)00141-5(/10.1016%2FS0143-8166%2801%2900141-5).18.^US patent 5493390(/textdoc?DB=EPODOC&IDX=US5493390),"Integrated optical instrumentation for the diagnostics of parts by embedded or surface attached optical sensors", issued Feb. 20, 199619.^US patent 5399854(/textdoc?DB=EPODOC&IDX=US5399854), J.R.Dunphy & et al., "Embedded optical sensor capable of strain and temperature measurement using asingle diffraction grating", issued March 21, 199520.^Jeong, Y., Sahu, J.K., Payne,D.N. and Nilsson., J., “Ytterbium-doped large-core fiber laser with 1kWcontinuous-wave output power”, Electronics Letters, 40: 470-472, 200421.^2. Xiao, Y., Brunet, F., Kanskar, M., Faucher, M., Wetter, A., and Holehouse, N., “1-kilowatt CW all-fiber laser oscillator pumped with wavelength-beam-combined diode stacks”, Optics Express, 20: 3296-3301, 2012.22.^3. Dawson, J.W., Messerly, M.J., Beach, R.J., Shverdin, M.Y., Stappaerts, E.A., Sridharan, A.K., Pax,P.H., Heebner, J.E., Siders, C.W. and Barty, C.J.P.,“Analysis of the scalability of diffraction-limited fiber lasers and amplifiers to high average power” Optics Express, 16: 13240-13260, 2008External linksInternational Optical Sensor Societies■FOSNE(/index.php?option=com_content&view=category&id=11&Itemid=14)-Fibre Optic Sensing NetworkEuropeDevelopment Platforms■TFT (http://www.tft-fos.nl)-Technobis Fibre TechnologiesOther■Bragg grating as hydrogen detector (http://bit.or.at/irca/bbsshow8.php?ref1=OO/INTA/36&vQuelle=&bcc=)■/patent/7133582.html -Fiber -Optic filter with tunable grating Retrieved from "/w/index.php?title=Fiber_Bragg_grating&oldid=554086706"Categories: Optical fiber Diffraction■This page was last modified on 8 May 2013 at 06:43.■Text is available under the Creative Commons Attribution-ShareAlike License;additional terms may apply. By using this site, you agree to the Terms of Use and Privacy Policy.Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a non-profitorganization.页码，11/11Fiber Bragg grating -Wikipedia, the free encyclopedia 2013/5/18/wiki/Fiber_Bragg_grating。

Fiber Bragg Grating

干扰这是广泛用于光纤Bragg光栅的制造第一种方法，使用双光束干涉。

此处所述UV激光被分成两个光束，以彼此创建沿着干涉图案的周期性强度分布干扰。

根据光的，它暴露于强度的光敏光纤的折射率变化。

此方法允许快速和容易变化的布拉格波长，这直接关系到干扰周期和激光光的入射角的函数。

顺序写入复杂的光栅配置文件可以通过暴露在序列有大量的小的，部分重叠的光栅来制造。

先进性能，如相移和不同的调制深度可以通过调整subgratings的相应属性被引入。

[4]在该方法的第一个版本，subgratings通过暴露用UV脉冲形成的，但这种方法有几个缺点，例如作为大的能量波动在脉冲和低平均功率。

一种顺序写入方法与连续的紫外辐射，其克服了这些问题已经证明，现在商业上使用的。

[5]所述的感光性纤维是由一个干涉受控airbearing承担托架翻译。

干扰紫外光束被聚焦到纤维，并作为纤维移动时，条纹沿纤维通过平移反射镜的干涉仪移动。

作为反射镜具有一个有限的范围内，它们必须是重置每一个时期，和条纹移动一个锯齿图案。

所有光栅的参数是在控制软件可访问的，并且因此可以制造任意光栅结构，而不在硬件的任何变化。

光罩还可以在光纤布拉格光栅，用于制造具有预定光栅特征的光罩。

光掩模被放置在UV光源和感光光纤之间。

光掩模的阴影接着确定基于光投射到光纤的透射强度的光栅结构。

光掩模中的啁啾光纤光栅，这是不能被使用的干涉图案制造的制造专门用。

逐点单个UV激光束，也可以使用'写'光栅进入纤维逐点。

这里，激光器具有窄波束等于光栅周期。

该方法的主要区别在于红外激光辐射和介电材料之间的相互作用的机制。

-多光子吸收和隧道电离[6]该方法特别适用于长周期光纤光栅的制造。

逐点也被用在倾斜光栅的制造。

生产本来，光敏光纤的制造和光纤光栅的“写”分别进行。

今天，生产线通常绘制从预制件的纤维和'写'的光栅，在一个单一的级。

以及降低相关的成本和时间，这也使大量生产的光纤光栅。

光纤布拉格光栅传感器

KP
B 1 2 2 P11 / B [ neff ( P12 ) 1] P E 2
(8.3-16)
由于掺杂成分和掺杂浓度的不同，各种光纤光栅的压力灵敏度差别较大。
2019/3/10
8
8.3.2 解调技术
解调方法高折射环形镜边缘滤波法优点可进行静态和动态应变的测量反射方式 :系统结构简单、造价低匹配光纤光栅滤波法可调光纤 F-P 滤波法非平衡扫描迈克尔逊干涉法非平衡 M-Z 光纤干涉法可调谐窄带光源法环形腔光纤激光器激射法廉；透射方式：信号光利用率高，分辨率比前者高 FFP 调谐范围宽，可实现多传感器解调可用于静态或准静态测量具备查询、解调光纤光栅网络传感信号的能力适用于动态参量的高分辨率测量透射方式：跟踪控制复杂，系统非线性误差较大高精度 FFP 价格昂贵，滤波损耗较大动态测量时需要详细分析相位随时间变化的规律静态测量要配合一个固定环境中的参考光栅稳定性和可调谐范围不够理具有较高的信噪比和分辨率想，限制了传感 FBG 的数目和使用范围适合波分复用传感阵列的解调应变分辨率低缺点分辨率较低反射方式：系统信噪比较低；
(8.3-3)
(8.3-4)
这里，ξ 为光纤的热光系数，表示折射率随温度的变化率。式(8.3-3)、(8.3-4)结合(8.3-1)，可知 Bragg 光栅的波长在变化的温度场中的表达式为
B / B ( ) T
(8.3-5)
Bragg 波长的变化与温度之间的变化有良好的线性关系，光栅的温度灵敏度为
z F / E S
式中 E 为光纤的杨氏模量，S 为光纤面积。该拉力引起的 Bragg 波长变化