Effects Of Twisting And Bending On LP₂₁ Mode Propagation In Optical Fiber Based On Coupled Mode Theory

Fiber takes an important place in the area of the modern communication and sensors. People insist on the fiber research from the 1960s'.Low-order multimodes (few-mode) in optical fiber have different characteristics. This paper makes the focus on the effects of twisting and bending on LP21 mode propagation in optical fiber which is one of the low-order multimodes. Twisting and bending characteristics of LP21 mode propagation is presented for the first time. Theoretical fiber mode modeling, combining geometrical rotation with opto-elastic effects, demonstrates that the propagation of LP21 mode is bending-effect-immune. Experimental testing verifies that the LP21 mode specklegram rotates 0.9112 of the fiber twist angle in a fused silica fiber, independent of any fiber bending. This characteristic allows for LP21 mode to be highly applicable in fiber specklegram sensors (FSS).Strict theoretical analysis and three experiment setups are discussed in this paper including following parts:Firstly, the fiber mode theory and the coupled mode theory are introduced: common fiber modes with excitation conditions, principle of the coupled mode theory with its application on the polarization optics of twisted single-mode fibers and the fiber specklegram sensors.Secondly, the theory of the twist and bend characteristic of fiber LP21 mode is shown including:the equation of the LP21 mode, the elastic-optic equation, the equation of the coupled mode theory on the LP21 mode and the whole calculation process about the equation in the Maple soft.Thirdly, this paper shows three experimental platforms about the fiber twist, the fiber bend and the mid-fiber twist. Also the data collection method will be discussed.Fourthly, this paper will present the data analysis methods and the software fitting. Further more, the application of the LP21 mode will be put in this part.Lastly, this paper shows the research in the twist and bend characteristic of LP21 mode with important meaning in the fiber mode research and widely application.