| With the thorough research on fiber optical sensors, some new kinds of fiber optical sensors of phase modulation have drawn more and more attention thanks to its large dynamic measurement range and the high accuracy. The Fiber Mach-Zehnder Interferometer (MZI) is an essential structure of this kind of sensors. The traditional optical fiber MZI, limited by its spatial structure, is likely to be affected by the external environment in service and has a poor anti-interference ability as well as a poor stability. However, the all-fiber MZI has worked out the problem owned by the traditional MZI in the application of sensors by integrating the signal phases on two channels into one fiber, becoming a new research focus in recent years.The Twin-core fiber has the particular refractive index distribution characteristics. Therefore a great variety of devices with all these features of the twin-core fiber have emerged in the field of fiber communication and fiber sensing. In this paper, the stress sensing of a new type of MZI based on the cascaded twin-core fiber is studied. The main achievements of this paper are summarized as follows:1. Introduce the optical fiber sensing technology, the development of the twin-core fiber and the research status of the optical fiber MZI. Summarize all optical fiber stress sensors at both home and abroad and the analysis of the advantages and disadvantages of all kinds of sensors through the literature search.2. Analyse the coupling property of the twin-core fiber based on the coupled mode theory with MATLAB simulation software and the conclusion drawn as follows:the two cores have the same mutual coupling coefficient under the ideal coupling conditions, while the coupling factor can reflect the strength of the cores coupling in the non-ideal coupling conditions.3. Design a MZI sensor based on the twin-core fiber and analyse the effects of the principal parameters on devices as well as the output characteristics deduced from the transmission matrix theory. The acquisition of the spectral characteristics and sensing property of MZI through the MATLAB simulation and the conclusion is as follows: when the transmission phase between the two cores is an even multiple of π, the sensor reaches the maximum transmission power, and when the transmission phase is an odd multiple of π, the sensor reaches the minimum transmission power.4. Design two types of cascaded tapering MZI based on the twin-core fiber and the optimization of the tapered position and the splitting ratio of these two structures through the theoretical analysis. The strain sensing principle of MZI based on the twin-core fiber is derived and the stress sensing characteristics of the device are simulated and analyzed. In the simulation, a 30 cm length is selected as the fiber length, and the wavelength ranges from 1500 nm to 1600 nm. The simulation concludes as follows:the wavelength approaches the shortwave with the increase of stress in the course of measurement. Although the stress sensing characteristics of cascade MZI are not improved, the flattening of the pass band is accomplished. The stress sensitivity of the two structures at wavelengths 1500,1550 and 1600nm are respectively-0.689pm/με,-0.706pm/με,-0.723pm/με. |
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