Study On The Liquid Level Sensor Based On In-line Mach-zehnder Interferometer Using Polarization Maintaining Fiber

With the improvement of automation in industrial process, the acquisitions of liquid level information become more and more important in petrochemical engineering, aerospace technique and other fields. Optical fiber liquid level sensor receives extensive attention due to its excellent properties. In this thesis, in order to obtain a higher sensing performance, focusing on the in-line Mach-Zehnder optical interferometer based on polarization maintaining fiber(PMF), we mainly completed the following work:1. The background of liquid level sensing technology and the classifications of optical fiber liquid level sensor were introduced. The advantages and disadvantages for each type of reported liquid level sensing technique were briefly commented.2. The polarization characteristics of optical fiber and principle of fiber intermodal interference were analyzed. Based on optical fiber wave theory, the analyses of optical fiber mode were presented. Then, the polarization characteristics of single mode optical fiber and the PMF were introduced. Finally, the mechanism of fiber intermodal interference was deduced through the introductions of the traditional optical fiber Mach-Zehnder interferometer and in-line optical fiber Mach-Zehnder interferometer.3. An in-fiber MZI based on core-offset splicing fusion using PMF for the application of liquid level sensing was proposed. The fabrication process of the interferometer was also introduced. The size and direction of core-offset(fast axis, slow axis and axis alignment of fast and slow axis direction of 45°) were analyzed experimentally by the sandwich structure of SMF-PMF-SMF. The interferometer was made using a fast axis aligned method. The cladding modes were excited through core-offset fusion splicing method and the liquid level information was obtained through the analyses of the interference spectrum. Experimental results showed that the sensitivities of wavelength shifts and intensity variations were-0.4956nm/mm and 0.2204 d B/mm, respectively. The temperature sensitivity of the proposed sensor was about 13pm/℃.4. An in-fiber MZI based on two cascaded butterfly-shape-tapers using PMF for the application of liquid level sensing was proposed. The classification and production process of the optical fiber taper were introduced firstly. The tapers were made by the tapering function produced by an optical fiber fusion splicer. The cladding modes were excited at the beam split point and re-coupled back into the fiber core at the second taper. The butterfly-shape-tapers enhanced the excitation efficiency of cladding modes in some extent. The liquid level changing information was presented in the transmission spectrum. Experimental results showed that the sensitivity of wavelength shifts and intensities variations were-0.501nm/mm and 0.275 d B/mm, respectively. The temperature sensitivity of the proposed sensor was about 12pm/℃.