Research Of Multimode Interference Technology In Composite Optical Fiber

ABSTRACT:The SMS (singlemode-multimode-singlemode) optic fiber structure, based on multimode interference and self-imaging phenomenon, has many advantages, such as simple structure and good stability. So it has drawn attention of scholars in recent years and has been successfully applied in optical fiber communications and sensing. According to traditional SMS, the multimode optic fiber is replaced by the combination of coreless fiber (CF) with two different lengths of Graded Index Multimode Fibers (GIMF). Thanks to GIMF’s diverging function, optical field stimulates more clad modes in CF, which can increase environment sensibility. The leading content of this paper is as follows.First, BPM method is adopted to simulate longitudinal optical field distribution, self-imaging location, coupling efficiency of different multimode optic fiber in Matlab, especially aiming at coreless fiber. Besides, the transmission spectra of the whole singlemode-coreless-singlemode optic fiber structure and characteristic wavelength are achieved when the length of coreless fiber is equal to self-imaging location. The temperature and vibration sensing principle is analyzed according to this structure.Second, according to the combination of different lengths of SIMF (step-index multimode fiber) and GIMF, GIMF’s focusing and diverging function is verified. A composite optical fiber structure is proposed, which includes singlemode-graded index multimode-coreless-graded index multimode-singlemode optic fiber. Because clad modes in coreless fiber have high environment sensitivity, this structure can realize high sensitivity sensing.Third, in experiment, SIMF、GIMF、CF and fiber grating are combined in different ways. The tension sensing and vibration sensing are achieved and the feasibility of multi-parameter sensing according to multimode interference and fiber grating is presented.