Bonding layers, serving as the strain transmission mediums, may bring undesirable effects to the sensing properties of fiber Bragg grating (FBG) strain sensors during their fatigue process. To analyze the strain sensitivity and the reflected spectrum of FBG strain sensors in different fatigue stages of bonding layers, their strain sensitivities were derived according to strain transfer models. Resorting to T-matrix formalism, the affected reflected spectra were stimulated. Theoretical analysis results show that there is a gradual decline in the strain sensitivity during the stage of fatigue crack initiation, and that significant distortions emerge in the reflected spectrum during the stage of fatigue crack propagation. In addition, a cyclic loading fatigue test was conducted and the phenomenon observed in the test showed a good agreement with the theoretical prediction.
Steel cable plays an important role in modern infrastructure due to its special characteristics. Because
most of structure load is transformed to the cable tension in cable stayed structures, it is very important to monitor cable tension. Being a slender element, Fiber Grating Strain Sensor is sensitive to axial strain and is regarded as a most prospective way to monitor the cable tension. The paper reviews a
series of problems of FBG when embedded into the cable, and introduced five different embedded FBG strain sensors. Principle, characteristics, and application states of these five sensor has been discussed in details. The prospective of embedded FBG Strain Sensor for cable tension has been forecast.
Metal bonding layer seriously affects the strain transfer performance of Fiber Bragg Grating (FBG). Based on the mode of FBG strain transfer, the influence of the length, the thickness, Poisson’s ratio, elasticity modulus of metal bonding layer on the strain transfer coefficient of FBG is analyzed by numerical simulation. FBG is packaged to steel wire using metal bonding technology of FBG. The tensile tests of different bonding lengths and elasticity modulus are carried out. The result shows the strain transfer coefficient of FBGs are 0.9848,0.962 and their average strain sensitivities are 1.076 pm/<i>με</i>,1.099 pm/<i>με</i> when the metal bonding layer is zinc, whose lengths are 15mm, 20mm, respectively. The strain transfer coefficient of FBG packaged by metal bonding layer raises 8.9 percent compared to epoxy glue package. The preliminary experimental results show that the strain transfer coefficient increases with the length of metal bonding layer, decreases with the thickness of metal bonding layer and the influence of Poisson’s ratio can be ignored. The experiment result is general agreement with the analysis and provides guidance for metal package of FBG.
Thermal injuries are a serious medical problem in the China. The accurate determination of burn degree is difficult for
scatheless diagnose and a precondition of treating burn wounds. Multi-spectral photographic analysis is expected to play
an important role in determining burn wound degree, the Liquid Crystal Tunable Filter has a capability of selecting the
observing wavelength instaneously with high spectral resolution and excellent imaging quality in visible and
near-infrared spectrum band. Taking advantage of this filter, we have developed a LCTF imaging spectrometer prototype
instrument at visible wavelength bands for burn wound diagnose.
In this paper, spectral analysis experiments were first performed on KUNMING mice and burn injury patients to find the
characteristic reflective spectral curves at 400nm-1800nm, the imaging spectrometer prototype instrument using LCTFs
which are sensitive to radiation in 420nm-750nmwavelength bands was built based on spectral analysis results. The
spectral imaging experiments on burn injury patients have verified the excellent properties of the prototype instrument,
including high quality spectral images with spectral resolution of less than 7nm and continuous selection of the output
wavelength. The burn areas of patients were marked with different colors which represents as different burn degree and
the spectral imaging system has thus been proven to have the ability to classify the burn areas through comparing their
reflective spectral curves with characteristic spectrum of the different burn degree in spectral database in the future.
Finally, the application of the LCTF imaging spectrometer to burn wound diagnose are summarized based on the results
of spectral imaging experiments on burn injuries.
Performance degeneration of FBG directly affects the long-term stable work of the whole
FBG sensing network. According to the formula of FBG refelectd spectrum,the impacts of refraction
index modulation depth Δn on refelectd spectrum peak and zero bandwidth are deeply analysed.
Based on the theoretical analysis of energy level transition theory, we can find that the temperature
shock can make electrons in traps which is of certain distribution of energy overflow,and then cause the
reduction of modulation depth Δn , and eventually lead to the performance degeneration of FBG.
Conducted the high-low temperature cycling degenerate experiment of FBGs, experimental results
show temperature cycling alternate shock will produce accelerated aging phenomenon:after 1000
alternating cycles, the reflection spectrum is almost difficult to identify, the sensing ability is basically
lost. Test results prove the correctness of theoretical analysis.
A new type of distributed optic-fiber sensing system based on Sagnac/Mach-Zehnder
has vast prospects in early warning of oil and gas pipeline sabotage, so it is necessary to study the
problem of degradation of polarization state which interrupts the application of the sensing system.
Firstly, a model of the impacts of polarization state on power transmission coefficient is
established in terms of Jones matrix method. With the existence of birefringence, simulations are
respectively made: the influence of the changes of equivalent birefringence element parameters on
the output signals of the Sagnac interference system, and with the different angles of polarized
light, the interruption of Mach-Zehnder interferometer in the response of output to the outside
world. A conclusion is drew out: the existence of birefringence will not only lead sensing system
to misjudge the outside world's behavior, but it will also affect the interference output signal's
phase, and then affect the positioning accuracy. At last, the adjustment of fiber mechanical
polarization controller to the system polarization state is selected and polarization control program
which uses phase modulation method to evaluate the system-polarization is designed.