Structure And Characteristic Studies Of Novel Optical Fiber Mach-Zehnder Interferometer

With the rapid economic development and social progress, environmental pollution and engineering quality have become the important factors to restrict the harmonious development of our society. To develop new sensors for monitoring is a hot topic in both academic and engineering areas. Compared to the traditional electrical counterpart, the optical fiber sensors have many advantages:compact structure, electromagnetic and chemical corrosion immunity, multiplex and remote sensing capability and large transmission capacity. Therefore, optical fiber sensors have been increasingly used in various monitoring areas. Especially the phase modulation type optical fiber sensors have attracted more and more attention due to the advantages of fast and accurate measurement of the external parameters in a large dynamic measurement range. Optical fiber Mach-Zehnder interferometer (MZI) is one of the most important and common implementation structures among this type of sensors. The phase difference of signal light and reference light is changed by external parameters, and the information of surrounding physical field is obtained by using the demodulation techniques in real time. As all-fiber Mach-Zehnder interferometer integrates two optical transmission arms into the same fiber, making the device more compact, lower cost and easy to install, and thus arouse extensive attraction in the recent years. In this thesis, we propose some new types of all-fiber Mach-Zehnder interferometers from the perspective of the sensing cell design. The expansion with breakthrough from scalar sensing to one-dimensional vector sensing and further to two-dimensional vector sensing is achieved in the practical application by changing the geometry from symmetry to one-dimensional asymmetry and further to two-dimensional asymmetry. Systematically research on the fabrication method, spectrum characteristics and sensing characteristics is conducted on the designed MZI. The main contents and research achievement are as follows:1. From the point of structural design on common single mode fiber, the impact on the spectral characteristic of optical fiber device is explored by different formation parameters in the same structure and heterogeneous structures. The optimal production parameters in an optimal structure are obtained by simulating and summarizing the law. Subsequently, this type of optical fiber sensor can be fabricated in the experiment.2. Various external parameters are tested in the experiments, including temperature, strain, bending and lateral pressure etc. The sensitivities on different parameters are studied to find the most appropriate sensing application areas. According to the conclusion, the production parameters are optimized again to propose a fabrication program with the highest sensitivity. Finally, optical fiber sensor with the best performance is produced combined with the theoretical results, and have applications in the corresponding sensing fields.3. The optical fiber sensor with the same structure is fabricated on special fibers again, such as photonic crystal fiber, multimode fiber, polarization maintaining fiber and so on. The sensing experiments on external parameters are conducted to analyze the influence on the transmission characteristics and sensing characteristics by production on different types of fibers.4. A novel fiber Mach-Zehnder interferometer is proposed by combination of different types of tapers. The device is based on concatenated down-and up-tapers simply fabricated by arc discharging and excessive fusion splicing. High-quality interference spectra with a maximum attenuation peak depth beyond20dB are obtained. Experimental results show that the RI sensitivity of the MZI reaches82.8nm/RIU for a RI range of1.333-1.3869, which is nearly5times higher than that of the conventional MZI based on two down-tapers, and12times higher than that of MZI based on two up-tapers. The temperature cross sensitivity is discussed and the influence can be eliminated by simultaneous measurement of RI and temperature using different sensitivities of different attenuation wavelength peaks. Such MZIs with low-cost and ease of fabrication will find various sensing applications.5. A bending vector sensor based on all-fiber MZI is presented for the first time. This device consists of a lateral-offset splicing joint and an up-taper formed through excessive fusion splicing method. The lateral-offset splicing breaks the cylindrical symmetry of the device structure and defines a pair of directions along which the bending response of the MZI transmission spectrum is different. That is to say, the interference pattern shift in opposite directions and thus could be used for bending vector measurement in one dimension. Experiment results show that, for a curvature range from-3to3m-1, the bending sensitivities at1463.86nm and1548.41nm reach11.987nm/m-1and8.697nm/m-1, respectively.6. One dimensional bending vector sensing is extended to two dimensions using all-fiber MZI for the first time. The two-dimensional bending vector sensor based on MZI consists of a slight-lateral-offset fusion splicing joint and a strong coupling long period fiber grating (LPFG). The exposure orientation of the LPFG is perpendicular to the lateral-offset orientation, making the geometry of the structure is two-dimensional asymmetric. The combination by strong coupling of LPFG and weak coupling of lateral-offset can generate an interference pattern and keep the resonance dip of the LPFG relatively independent at the same time. The bending directions and amplitude can be identified simultaneously using the different responses of the two resonance dips. Two-dimensional bending vector sensing characteristics, especially in the four orthogonal directions based on the new structured MZI, have been experimentally demonstrated. The device will have great potential in structure monitoring applications due to the advantages of its low cost, ease of fabrication and integration.7. As a research extension on two-dimensional bending vector sensor, another new structure MZI based on common single mode fiber is designed and fabricated. The device consists of two concatenated dislocation junctions with the offset directions perpendicular to each other. The two-dimensional asymmetry of the structure results in its two-dimensional vector sensing capability. Compared to MZI bending vector sensor based on concatenation of lateral-offset splicing and LPFG, this structure is more simple, flexible and easy to mass production applications.