Research Of Roadbed Settlement Monitoring Using The MZ Optical Fiber Sensing Technology Based On Twin-core Fiber

Roadbed is the foundation of road construction. Since the last century20s, such as Tianjin, Shanghai and other places of China, began to appear various disasters caused by the roadbed settlement. Since1970, many cities in the plains of Hebei province and the Yangtze River Delta area also produce the phenomenon of ground subsidence caused by subgrade settlement, so subgrade settlement has become one of the most common diseases in road construction. As the development of the economy, the pace of the social is accelerating, the EMU (Electric Multiple Unit), HSR (high-speed rail) and other advanced means of transportation have rapid development, at the same time, the pressures of the road becomes heavier than before. So we should not only improve the efficiency of transport, but also pay more attention to the security of transportation. In order to be able to monitor the road safety continuously, many scholars have made the efforts. The article will put forward a new type of Mach-Zehnder (Mach Zehnder, MZ) optical fiber sensor based on dual-core optical fiber, and it will be applied to the subgrade settlement monitor. It has many advantages such as light volume, flexible structure, and strong ability to resist electromagnetic, also it has the objective research value on application.According to the structure of dual-core fiber, this paper first set up double waveguide model and analyzes the coupling theory of that model, and introduces the coupling theory of dual-core fiber. It is concluded that in the ideal case, the two fiber core has equal mutual coupling coefficient. Under the condition of non-ideal coupling, δ2/K12K21can accurately reflect the strength of fiber coupling, δ2is the square of the phase mismatch, K12K21is the product of mutual coupling coefficient.This paper analyses the transfer theory of MZ optical fiber sensor according to the dual-core fiber coupling theory, then get the conclusion that when the transmission phase difference between the two cores is an even multiple of π, the sensor achieves the maximum transmission power, when the transmission phase difference between the two cores is an odd multiple of π, the sensor can reaches the minimum transmission power.The focus of the article is designing a cascading MZ optical fiber sensor and uses it for bending measurement, makes fused taper in the middle of the twin-core fiber, uses OSA (Optical Spectrum Analyzer) to observe and record the sensor outputs in different bending conditions. The experiment selected1500nm-1700nm broadband light as the source of the sensor, the total length of the sensor is235mm and the length of dual-core fiber is230mm, through the experimental analysis, it can be obtained that in the case of measuring bending, with the increasing of bending curvature, the wavelength towards shortwave direction, and the free spectral range is16.44nm, the curvature of the range is0to1.874m-1, the adjustment of the sensor is2.5mm, the bending sensitivity is-8.8253nm/m-1. In the range of the measurable bending curvature, the subsidence peak at1674.12nm can reaches the minimum sensitivity of-6.5433nm/m-1, the maximum sensitivity is-10.1589nm/m-1; the subsidence peak at wavelength of1691.12nm can reaches the minimum sensitivity of-6.6601nm/m-1, the maximum sensitivity is-10.1187/nm/m-1.Through the analysis of the bending characteristics of the MZ optic sensor based on dual-core fiber, the sensor is applied to a preset roadbed subsidence monitoring model, and analyzed its feasibility. The model can obtains the bending curvature according to the subsidence caused by the bending of the sensor measuring in the road settlement, and through the data analysis get the bends of the settlements.