Development of fiber specklegram sensor with hetero-core optical fibers.

摘要

Optical fibers have been widely used for transmitting temporal signals. However, the transmission of spatial signals has not been fully exploited. The exit light field from a multimode fiber produces a complicated speckle pattern, resulting from the modal phasing and interference of the fiber. It is difficult to recover the transmitted information from the speckle field. However, the fiber speckle field can be used for fiber sensing with a correlation method. This thesis research exploits the spatial content of fiber speckle for sensing, other than using temporal content. The author proposed to use a hetero-core fiber structure and demonstrated it for fiber specklegram sensing: A computer aided simulation method---Beam Propagation Method was applied to study the mode field and light wave energy distribution within a hetero-core structure. Finite Difference Time Domain (FDTD) method was also applied to investigate the incorporation of photonic crystal fibers (PCF) to a hetero-core fiber sensor. Experimentally, hetero-core fiber structures with different core sizes were fabricated. Their applications to chemical environment (refractive index) change, transversal displacement, and dynamic sensing are also included. The experimental results in chemical environmental sensing have shown a one to one relationship between the intensity of a normalized inner product and the concentration of glycerin. Different linear regions were found with different hetero-core structure: for a 50-9-50 hetero-core structure it was around 50--60% concentration; for a 50-3-50 hetero-core fiber structure sensor it was found to be around 40--50%, with a relatively higher sensitivity than the 50-9-50 one. The displacement experiment shows that the hetero-core FSS offers sensitivity as high as 0.1 mum, with a dynamic range of about 3 mum, which is superior to a straight structure multimode fiber FSS. A faithful response of the displacement sensor setup was found in the dynamic sensing experiments.; In addition, modal phasing caused speckle field phase variation was discussed; and the author proposed a sensor architecture based on electronic fiber speckle pattern interferometry (EFSPI). The concept of vector sensing with EFSPI was analyzed and qualitatively verified with preliminary fiber speckle interferogram results, signaling a significant potential for directional sensing.