Investigation On Quasi-leaky Mode Problem And Optical Sensor Based On Leaky-mode

Mode expansion method plays an important role in characteristic analysis of optical devices.Complex mode expansion method,based on equivalence closed model,whose orthonarmal bases are guided modes and quasi-leaky modes,brings convenience to handle radiative related problem,and is verified in fiber grating issues.However,whether quasi-leaky modes can represent radiation modes,is still a puzzle,and affects the application of complex mode expansion method.In this thesis,we study the characteristics of radiation modes,by turning radiation modes to complex modes,reveal the equivalence between radiation modes and quasi-leaky modes eventually by comparing complex modes with quasi-leaky modes.For linear power loss of leaky modes,we pursuit single leaky mode excitation condition,and design sensing scheme based on leaky mode.To simplify the complexity of sensing scheme,we study the sensing potential of W-type fiber.The main achievements are as follows:1.Field distribution and power spectrum of radiation modes in both two-layer and three-layer cross-sectional fibers are investigated.By mapping radiation modes to discrete complex modes,the relationship between radiation modes and quasi-leaky modes obtained in an equivalent numerical model is revealed.Simulation results further show that quasi-leaky modes can accurately and briefly describe packets of radiation modes,from the viewpoint of power distribution and attenuation in the mode propagating.The normalization and orthogonality of quasi-leaky mode,as well as the effect of the numerical model parameters are discussed.2.Single leaky mode excitation in fiber is theoretic studied,then,a new leaky mode based sensing structure is proposed.By simulating the transmission characteristics,we find the scheme is potential to measuring liquid level sensing:when HE₁₆ mode is chosen,the sensing range is about 3cm,and the sensing sensitivity is-23.3dB/cm,when HE₁₉ mode is chosen,the sensitivity can reach to-57.78dB/cm but the sensing range limit to 0.8cm.Our simulation find that the sensing scheme can also sense refractive index higher than silica,reach to 120dB/RIU sensitivity with 10mm leaky fiber segment.The sensor can detect the liquid level or refractive index by monitoring power attenuation,so that it is useful in real-time sensing domain.3.To reduce the demand of optical devices such as fiber grating in sensor design,we take use of W-type to simplify the sensing structure.As classical references pointed out,there is a dip at the cut-off wavelength in the transmission spectrum of depressed inner cladding fiber.It's valuable to verifying the accuracy of the complex mode solver when comes to mode-solving in cut-off condition.Eventually,we propose single-mode-W-type-single-mode structure where modes interference lead to power loss when far from cut-off wavelength.The refractive index sensor get a 200dB/RIU sensitivity when the surrounding refractive index ranges from 1.31 to 1.38.