Search

Structural health monitoring (SHM) in-service is very important and definitely demanded for the safe working of high performance composite structures in situ. Fiber Bragg grating-based sensors were usually embedded inside a composite to compose a smart structure for a real time monitor. However, the mechanical performance of the materials would be unfavorably influenced. To lower the influence, a smaller diameter optical fiber is preferred. In this paper, a new non-destructive evaluation sensor system based on small diameter fiber Bragg grating (SDFBG) was proposed. The impact results show that the SDFBG sensor could be used in dynamic strain measurement and detect the natural frequency of the structure. Furthermore, the sensor would be applied in SHM. [ABSTRACT FROM AUTHOR]

Aiming at meeting the need of aluminum corrosion monitoring in aerospace field, a pre-load type fiber grating corrosion sensor based on an aluminum thin tube structure is proposed. The corrosion sensor of aluminum alloy structure in-service monitoring mechanism is studied, a theoretical model about the relation of FBG reflection spectral characteristics and aluminum thickness variation is also obtained. Optical fiber grating corrosion monitoring test system based on the capillary structure of aluminum alloy is constructed by acid-base environment. The problem of cross sensitivity of temperature and strain is solved by configuring an optical fiber grating which is not affected by strain and only sensitive to temperature inside the aluminum alloy tube. The results shows that he aluminum tube packaging design not only can sense the effects of corrosion on the mechanical properties, but also can interference shielding effect of corrosion on the tube optical fiber sensing device. With the deepening of the metal tube corrosion and aluminum alloy tube thickness gradually thinning, fiber grating reflective spectrum gradually shift to the short wavelength and the wall thickness and the grating center wavelength offset has a good monotonic relation. These characteristics can provide useful help to further research corrosion online monitoring based on optic fiber sensor.

In the present paper, the theoretical expression of the wavelength change and the axial strain of birefringence fiber loop mirror is developed. The theoretical result shows that the axial strain sensitivity of birefringence photonic crystal fiber loop mirror is much lower than conventional birefringence fiber loop mirror. It is difficult to measure the axial strain by monitoring the wavelength change of birefringence photonic crystal fiber loop mirror, and it is easy to cause the measurement error because the output spectrum is not perfectly smooth. The different strain spectrum of birefringence photonic crystal fiber loop mirror was measured experimentally by an optical spectrum analyzer. The measured spectrum was analysed. The results show that the absolute integral of the monitoring peak decreases with increasing strain and the absolute integral is linear versus strain. Based on the above results, it is proposed that the axial strain can be measured by monitoring the absolute integral of the monitoring peak in this paper. The absolute integral of the monitoring peak is a comprehensive index which can indicate the light intensity of different wavelength. This method of monitoring the absolute integral of the monitoring peak to measure the axial strain can not only overcome the difficulty of monitoring the wavelength change of birefringence photonic crystal fiber loop mirror, but also reduce the measurement error caused by the unsmooth output spectrum.

In the present paper, the theoretical expressions for the axial strain sensitivity of high-birefringence fiber loop mirror (Hi-Bi FLM) was developed. The key influence factors on the axial strain sensitivity are discussed for the same input light source as the high-birefringence fiber material is certain, including the high-birefringence fiber length and the sensing length. The relationship between the high-birefringence fiber material and the axial strain sensitivity is discussed for the same input light source. The results show that the wavelength shift of high-birefringence fiber loop mirror is linear with the axial strain, and the linear sensitivity is constant decided by the high-birefringence fiber material and the center wavelength of input light source, having nothing to do with the high-birefringence fiber length and the sensing length.

As surface plasmon resonance (SPR) spectrum is sensitive to refractive index of the mediums, we explored its sensitivity characteristic of ions composition detection in a solution so as to measure the total dissolved solid value in water. Seven ionic (NaCl, KCl, CaCl2, MgCl2, Na2CO3, MgSO4 and ZnSO4) and 3 non-ionic (glucose, glycerol and sucrose) liquid samples were studied experimentally with a fiber optic SPR sensor. The influence of ion concentration on the resonance wavelength shift in SPR spectrum was investigated and discussed, and with that, the response curves were established to realize the detection of total dissolved solid in water quality analysis. The FO-SPR sensor spectral curve for TDS measurement was in conformity with ionic conductivity in 8 different water samples, and results show that the spectral method was better in high ion concentration detection of water. This research will broaden the SPR application in the field of water quality monitoring.

A new type of humidity sensor based on optical fiber SPR sensor probe was introduced and manufactured. Firstly the sensitivity of optical fiber SPR sensor probe to environment humidity was studied. On this basis, coating with thin PVA film of different thickness and with capacity of adsorbing water molecule on the surface of optical fiber SPR sensor probe, for the purpose of monitoring relative humidity was proposed. The research shows that the SPR sensor probe coated with double layers of thin PVA film gets better detection result, and humidity measurement sensitivity is 1.59%/%RH, being improved notably compared with SPR optical fiber probe. The SPR sensor coated with single layer of thin PVA film has relative humidity measuring sensitivity of 2.411 nm/%RH related to resonance wavelength in high humidity area. In addition, the new type of optical fiber humidity-sensitive probe with PVA film losing efficiency can be reusable after special treatment.

Manufacture of the small diameter FBG was designed. Cross sensitivity of temperature and strain at sensing point was solved. Based on coupled-mode theory, optical properties of the designed FBG were studied. The reflection and transmission spectra of the designed FBG in small diameter were studied A single mode optical fiber, whose cladding diameter is 80 microm, was manufactured to a fiber Bragg grating (phi80FBG). According to spectrum simulation, the grating length and period were chosen as the wavelength was 1528 nm. The connector of the small diameter FBG with demodulation was designed too. In applications, the FBG measures the total deformation including strain due to forces applied to the structures as well as thermal expansion. In order to overcome this inconvenience and to measure both parameters at the same time and location, a novel scheme for simultaneous strain and temperature sensor was presented. Since the uniform strength beam has same deformation at all points, a pair of phi80 FBG was attached on a uniform strength cantilever. One of the FBG was on the upper surface, with the other one on the below. Therefore, the strains at the monitoring points were equal in magnitude but of opposite sign. The strain and temperature in sensing point could be discriminated by matrix equation. The determination of the K is not null and thus matrix inversion is well conditioned, even the values for the K elements are close. Consequently, the cross sensitivity of the FBG with temperature and strain can be experimentally solved. Experiments were carried out to study the strain discriminability of small-diameter FBG sensors. The temperature and strain were calculated and the errors were, respectively, 5% and 6%.

In the present paper, the principle of a long-period fiber grating (LPFG) ammonia-nitrogen degradation monitoring sensor was discussed in detail firstly based on a sensitive characteristic that the resonance spectrum of long-period fiber grating changes with refractive index in external environment. The relationship between the resonance peaking wavelength of long-period fiber grating and the concentration of ammonia-nitrogen solutions was also analyzed detailedly. Then, the long-period fiber grating spectrum measurement technology was selected to obtain long-period fiber grating spectrum curves corresponding to seven different kinds of concentration of ammonia-nitrogen solutions, and the resonance wavelengths increased with the increase in the concentration of ammonia-nitrogen solutions. The variations of the resonance wavelength decreased from 2.707 to 0.068 nm and had a relatively good corresponding relationship with the concentration values of ammonia-nitrogen solutions. The responsivity of this correlation is 52.78 pm x mg(-1) x L. The concentration of ammonia-nitrogen solutions was acquired exactly through the way of monitoring the changes of the spectrum attribute, at the same time, the process and the extent of ammonia-nitrogen wastewater degradation were estimated. This method, which can directly monitor the concentration of ammonia-nitrogen solutions, is simple and easy to operate. The measurement and transmission section of the system are completely composed of optical fiber, which can avoid the electronic interference. There is no necessary to use chemic reagent to sign the solutions, which are going to be degraded. In conclusion, the late-model long-period fiber grating ammonia-nitrogen degradation monitoring system could achieve a real time, rapid, accurate and long distance measurement.

The new scheme of using a long period grating for simultaneous discriminating measurement of temperature and strain is proposed. The scheme is based on the long period grating's spectrum characteristics. The spectrum is composed of multiple and different loss peak. The resonant wavelength of the first and fourth-order loss peak with different temperature and strain sensitivity was selected in this experiment. The wavelength shift resulting from parameter variation can be obtained by observing its corresponding spectrum. The temperature was acquired by a study on the temperature characteristic of a LPG. The strain sensitivity was obtained by the research on the strain characteristic of a LPG. The standard matrix equation was set up and solved according to the corresponding parameter. The authors know by calculating the standard matrix equation that the cross-sensitivity has little influence on the measurement of the parameter. As the effect of cross-sensitivity is lower than the resolution of the LPG in this case, we can eliminate the deviation due to the cross-sensitive by appropriate compensation. The errors of temperature and strain were calculated to be +/- 0.92 degrees C and +/- 22 microepsilon respectively. The cross-sensitivity between temperature and strain was decreased and the measuring precision of the system was improved by using this scheme in which a single long period grating was used to test two measurands. By comparison between the applied and the calculated temperature and strain of a LPG, the experiment results showed that the methods are feasible. The experimental system is small and the cost is relatively low. The experimental device is simple and practical and has good application prospect.

The working principle of the optic fiber SPR sensor was discussed in the present paper at first. The feasibility of using it to monitor the degradation process of the environmental sewage represented by the methyl orange was studied. Finally, the optic fiber SPR sensor was adopted to monitor the change in degradation concentration represented by the original methyl orange solution on the base of 50 mL initial concentration 30 mg x L(-1), and the optic fiber SPR spectrum character of degradation process was analyzed in detail. Meanwhile the UV spectrophotometer was used to measure the change in concentration in the course of the degradation. The measurement data were analyzed and compared at large. The research work indicates that both the methods have consistent results, as the degradation time increases, the absorbance and concentration of the environmental sewage represented by the methyl orange solution decrease by and by, and the resonant wavelength of the optic fiber SPR sensor blue shifts step by step, as compared to the original standardization methyl orange solution resonant spectrum. It was shown that the methyl orange solution was degraded, and the rate of degradation was up to about 73 percent within two hours. The comparative results illustrate that it is feasible to use the optic fiber SPR sensor to monitor the environmental sewage degradation. The research result not only provides a new monitoring method for the degradation process of the environmental sewage, but also promotes the technique of the SPR sensor combined to the environment monitor by a long way.

Spectrum of fiber bragg grating (FBG) sensor modulated by double long period grating (LPFG) is proposed in the paper. Double LPFG consists of two LPFGS whose center wavelengths are the same and reflection spectrum of FBG sensor is located in linear range of double LPFG transmission spectrum. Based on spectral analysis of FBG and double LPFG, reflection spectrum of FBG modulated by double LPFG is obtained and studied by use of band-hider filter characteristics for double LPFG. An FBG sensor is attached on the surface of thin steel beam, which is strained by bending, and the center wavelength of FBG sensor will shift. The spectral peak of FBG sensor modulated by double LPFG is changed correspondingly, and the spectral change will lead to variation in exit light intensity from double LPFG. Experiment demonstrates that the relation of filtering light intensity from double LPFG monitored by optical power meter to center wavelength change of FBG sensor is linear and the minimum strain of material (steel beam) detected by the modulation and demodulation system is 1.05 microepsilon. This solution is used in impact monitoring of optical fibre smart structure, and FBG sensor is applied for impulse response signal monitoring induced by low-velocity impact, when impact pendulum is loaded to carbon fiber-reinforced plastics (CFP). The acquired impact response signal and fast Fourier transform of the signal detected by FBG sensor agree with the measurement results of eddy current displacement meter attached to the FBG sensor. From the results, the present method using FBG sensor is found to be effective for monitoring the impact. The research provides a practical reference in dynamic monitoring of optical fiber smart structure field.

According to the characters of SPR optical fiber sensor spectrum and the requirement of real-time monitoring, a new noise filtering method, mobile lifting wavelet analysis, is presented in this paper. This method is not based on Fourier transform, but on an algorithm of dividing a complicated noise filtering process into a number of reversible simple processes. It is very fast, correct and does not use additional memory. A model of actual SPR optical fiber sensor spectrum is constructed by superposing a certain intensity of Gaussian white noise on the theoretical spectrum, of which the resonant wavelength is 557.70 nm. It applies noise filtering to the simulative spectrum with mobile lifting wavelet analysis based on Haar, CDF(3,1), DD(4,2) and 5/3 mother wavelet respectively and calculates the resonant wavelengths again The results are 556.45, 564.06, 557.27 and 557.91 nm corresponding to each method listed. So a relative error of 0.037 7 percent, obtained after noise filtering with new method based on 5/3 mother wavelet, is the minimum one. It is also lower than 0.430 3 percent obtained after noise filtering with traditional symlet11 wavelet analysis that has been proved to be effective for SPR optical fiber sensor spectrum. At different time gather several spectra of one SPR optical fiber sensor detective system were gathered and mobile lifting wavelet analysis based on 5/3 mother wavelet was done. The result shows that, the standard deviation of resonant wavelengths is reduced to 1.560 8 from 4.186 7 nm, which is calculated before noise filtering. As expected, this result is better than doing the same experiment with traditional symlet11 wavelet analysis, which only reduces the standard deviation to 2.725 3 from 4.186 7 nm. The research shows that mobile lifting wavelet analysis significantly suppresses the system noises, reduces noise influence on the gathering of resonant wavelength information from SPR optical fiber sensor spectrum and gives a guarantee to actual accurate detection with SPR optical fiber sensor.

The consideration on the durability of concrete structures with reinforcement corrosion has become a most urgent problem. A new technique to measure the corrosion of steel in concrete structures was proposed in the present paper. It is based on the microbending characteristic of long period optical grating (LPFG). The temperature spectum character and curvature spectrum character of long period optical fiber grating were studied first. It was shown that the transmission spectrum of long period optical fiber grating shifted right and the transmission of the resonance wavelength was invariable when the temperature increased, while the transmission spectrum of long period optical fiber grating became shallow when the curvature increased, the transmission of the resonance wavelength would increase and it was linear with the curvature. On the basis of the characteristic, a notch shaped pedestal was designed and a long period optical fiber grating was laid on the steel surface. With this method the radial expansion of the steel resulting from the steel corrosion would translate into the curvature of the long period optical fiber grating. The curvature of long period optical fiber grating could be obtained by analyzing the change of spectrum, and then the steel corrosion depth could be measured. This method is simple and immediate and is independent of the variety in temperature, strain and refractive index owing to the inimitable spectrum characteristic of long period optical fiber grating. From the experiment it was found that the precision of the corrosion depth was better than 1.2 microm, and the corrosion depth of 3 mm could be achieved. This measurement could be used to monitor the early to metaphase corrosion of reinforcing steel in concrete structures.

The principle of optic fiber surface plasmon resonance sensor is discussed in detail based on the electromagnetic theory and radial theory in the present paper. The relationship between SPR spectrum resonance wavelength and liquid refractive index is also analyzed. Through relative spectrum measurement technology, SPR spectrum curves, whose resonance wavelength increases with increasing liquid refractive index, were obtained for eight different kinds of chemical liquids. The measurement of concentration of mixed solution including ethanol and glycol was performed by analyzing the spectrum attribute. The experimental results show that the sensing system features simple structure and all light transmitted by fiber. It not only realizes rapid, accurate and real time measurement for target medium, but also can be used in some special situations and easily realize long distance measurement.

The paper discussed the optic-fiber surface-plasmon-wave(SPW) sensor's sensitivity to temperature based on the particle vibration. For the SPW sensor consisting of metal film and dielectric, surface plasmon vibration is essentially the vibration of group electrons. Being irradiated by P light that has a special wavelength, the electrons on the surface of the metal film will absorb the power and change the way of their original movement. When the frequency of the P light is corresponded to the inherent vibration frequency of the group electrons, resonance will occur. Because the different temperature leads to different electron density--the higher the temperature the higher the density on the film surface, and because the vibration of the group electrons is correlative to the electron density closely, the temperature change will influence the inherent vibration frequency of the surface plasmon seriously. We decrease the temperature influence on the SPW by compensating the temperature change of the environment medium according to the effect. On the other hand, the paper discusses that the optic-fiber SPW sensor may be used to measure multi-parameters.