Design And Characteristic Research Of The Fiber Sensors Based On Interferential Principle

Optical fiber sensing system contains the collecting, transmission and processing of light via fiber. Compared with the electronic sensing system, the sensing system based on optical fiber takes more advantages, such as large capacity information transfer, anti-electromagnetic interference, stronger capacity of resistant to corrosion, more security, and so on. All these excellent properties of fiber sensor lead to the fact that fiber sensing system get to be one important part in the modern sensing field. This thesis focuses on the design of sensors based on fiber interferometer as well as the analysis of their characters, the main research results are listed as follows.1. The fiber up-taper is proposed and fabricated. The radius of the fiber taper is not constant along the fiber, which will inevitably lead to the power exchange between fiber modes. Fiber up-taper is the configuration whose radius is larger than that of the original fiber. Similarly with the traditional down taper, fiber up-taper can exchange power from one mode to another as well. Besides, since the radius of fiber up-taper is larger than the down taper, the mechanical strength get much stronger especially for transverse shear force. The distribution of each mode and the up-tapered coupled mode equation are deduced by applying Maxwell Equation. What’s more, the coupled coefficient between two modes is simplified. At last, the fiber up-taper is produced by using fusion splicer. The fiber is fused by the electrodes of fusion splicer in its manufacture process, and extra force is used for enlarging the fiber radius at the same time.2. Mach-Zehnder interferometer based on fiber up-tapers is proposed. Two up-tapers are produced on a single mode fiber with a certain distance away from each other, so as to form a Mach-Zehnder interferometer. The feasibility of sensing liquid level by the proposed interferometer is verified through both simulation and experiment, and the sensitivity of 0.2250nm/cm has been achieved via the experiment. The power distribution and transmission constant of the fiber modes is related to the angle of bending. With this character, the proposed interferometer has the potential for displacement measurement based on fiber bending. The Mach-Zehnder interferometer can work as a filter in the ring cavity fiber laser which shows a fine linear relationship between the wavelength of the laser peak and displacement, and the sensitivity of the laser for displacement is 4.49nm/mm. Power distributions of the modes are relevant to the external refractive index, especially for thin radius fiber. Thus, by applying thinner fiber between two up-tapers, the dumbbell shaped Mach-Zehnder interferometer is benefit to improving its sensitivity of refractive index sensing. Maximum value of the sensitivity reaches 836nm/RIU.3. Mach-Zehnder interferometer based on fringe space of the spectrum is proposed for micro-displacement sensing. Generally, the fiber Mach-Zehnder interferometer based on cascaded 3-dB couplers is susceptible to interference. Particularly, the transmission spectrum will fluctuate seriously while the fiber lengths of two arms are too long, which is due to the great difference between the interference signal sensed by two arms. In order to ensure the capacity of resisting disturbance of the Mach-Zehnder interferometer, the character of the relationship between fringe space and displacement is used for sensing application. Maximum sensitivity of 66nm/μm is achieved in the experiment.4. The extrinsic Fabry-Perot interferometer based on metallic V-groove is proposed. The temperature sensibilities are dependent on the coefficient of thermal expansion (CTE) of fiber itself for most fiber sensors, which restricts the improvement of the sensitivity. Even under the condition of tiny temperature fluctuation, the cavity length of fiber Fabry-Perot interferometer can be changed drastically by applying metallic V-groove with larger CTE to fix the interferometer, so as to achieve super high sensitivity. Several parameters of the sensor have been analyzed both in theory and experiment, such as original cavity length, original positions of the two fiexed points, material CTE of the V-groove, reflectivities of the fiber ends, and so on. Maximum sensitivity of 260.7nm/℃ has been achieved in the experiment, which is the largest value of the temperature sensitivity for a fiber sensor up to now.