Traditional electrical sensor or traditional fiber Bragg grating sensing technology is not applicable to the measurement of nonuniform strain in composite material. Therefore, the distributed nonuniform strain in the lap plate position of composite interlining material is measured using a single fiber with optical frequency domain reflection technology in this study. The experimental results show consistency with the experiment phenomena, and the measurement accuracy could be increased to the submillimeter level.
A fiber Bragg grating (FBG) sensing network with a bus chain typology structure based on time-division multiplexing (TDM) technology has been developed. Each FBG sensor was placed in an isolated branching circuit separated by an optical splitter. By doing this, multiple reflection and spectrum shadow, which are common in a traditional TDM network, were eliminated since incident light reflected by each sensor did not go through the other sensors. Interference among different FBGs was also avoided. The system was experimentally verified by constructing such a network with 17 FBGs involved. Wavelength and position interrogation were successfully realized. Temperature experiment was carried out on four of the FBGs and the sensitivity was 9.87, 9.92, 9.91, and 9.97 pm/°C, respectively. The durability, reliability, and measuring accuracy of the sensing network were effectively improved due to the bus chain typology structure.
Because of the high degree of railway’s electromagnetic radiation and the vulnerability of the traditional electronic sensor, the safety monitoring of railway’s protective net in slopes is an important process. Based on fiber grating’s such advantages as freedom from electromagnet, and the convenient linkage of signal, this thesis formulated tension sensor and acceleration sensor on the basis of protective net’s characteristics. By conducting the impulse test on the protective net that is installed with sensor, we collected the mechanics’ parametric variation during the whole process of impact. The fiber grating sensor used in the test was stable and reliable.
A metal packed fiber Bragg gratings accelerometer has been proposed. Copper was adopted to cover the surface of
optical fiber by magnetron sputtering and electroplating technology, and then tin soldering was used to fix the metalized
fiber on mass and foundation. Because of copper coating and soldering, the elastic coefficient and ductility of FBGs have
been increased, and the problems of aging and creep arising from polymer or adhesive packaged have been avoided.
Experiment result demonstrated that the accelerometer possess of a resonant frequency of 2800Hz, a wide linear
measurement range from 0.5g to 5.3g and a sensitivity of 84mv/g.
The measurement method of strain inside strong pulsed magnet has been demonstrated by using embedded fiber Bragg
gratings. 23 FBGs were embedded in a strong pulsed magnet of 60 Tesla. The strain inside the magnet was successfully
measured in a 10-millisecond pulse time by adopting high speed demodulation devices in normal temperature and liquid
nitrogen temperature, respectively. The research achievement is significant for the further development of strong pulsed
magnetic field technology.
A novel dual chirped fiber Bragg gratings (CFBG) accelerometer is proposed in this paper. The method with CFBG
instead of FBG has two advantages. One is that the matching relation of the dual CFBG can be easily realized, while the
other is that the linear measurement range is larger than that with FBG. The CFBG accelerometer with elastic system
composed of a mass block and a steel tube shows good performances such as higher resonate frequency, larger linear
measurement range, easily to fabricate, lower cost, and so on. The sensor with a resonance frequency of 3812Hz has a
larger measurement range from 0.1g up to 10g.
A novel weigh-in-motion (WIM) system used for high way is developed based on Chirped fiber Bragg gratings (CFBG) in this paper. The WIM system consists of four CFBG pressure sensors, each of which contains a couple of CFBG. The sensor can directly output optical intensity signal, so the postprocessor instrument is simple and cheap
instead of expensive wavelength demodulation apparatus. Theoretical and experimental results indicate that output
optical intensity of the sensor is linearly proportional to the pressure, and the linearity and the repeated error can
respectively reach to 0.9997 and 0.05%FS. We have also exceeded series experiments with several kinds of automobile
with different velocity, and received good results of relative error below 5%.
The photosensitivity of photosensitive fibers is studied experimentally and theoretically. In the experiment, the relationship between the diffractive index increment of photosensitive fibers and UV irradiating dose based on an unbalanced Mach-Zehnder Interferometer (MZI) is measured. The experimental results show that the photo-induced index's increment of photosensitive fibers is multi-decaying-exponentially proportional to the UV irradiating dose and the order of magnitude of the maximum value can reach 10-3. In order to explain the relationship, a new assumption is proposed, which can give a good explanation based on the "color center model" and the "Kramers-Kronig" principal.
We report a new method for writing the dual-wavelength Bragg fiber gratings. The method is based on the photosensitivity of Ge-doped optical fibers. Our double-exposure technique enables a good control over the grating's reflectivity and the separation of the two Bragg wavelengths. A grating with two equal transmission peak of 20.5dB is obtained by using this method and the separation of the two Bragg wavelengths is about 0.8 nm. With the grating, an optical add-drop multiplexer (OADM) that allows dropping two fixed wavelength channels is proposed.