Study Of Band-pass Filter For The All Fiber Interconnection Between Multimode Fiber And Single Mode Fiber Network

Driven by the swift growth of internet broadband and ultra-high-speed interconnection between computers in global Local Area Network(LAN) for the individual and the enterprises or organizations, optical fiber LAN and Optical Access Network(OAN) become an inevitable trend to replace the traditional unshielded twisted-pair network. FTTH (Fiber To The Home) has become one of the main development directions in the field of all optical network. Based on the present situation of fiber filter and mode converter both at home and abroad, this thesis focuses on a novel all-fiber technology combined with multimode fiber and single mode fiber network. A concatenated all-fiber connector structure of single-mode fiber, a LPFG (long-period fiber grating), core mode blocker and multimode fiber has been proposed in order to realize the single-multiple mode converter and the band-pass filter simultaneously, which has important scientific significance and practical value. The detailed contents are as follows:①Based on the idea of cladding mode extracting, a concatenated all-fiber connector structure of a single mode fiber, a long period fiber grating (LPFG), core mode blocker and multimode fiber is proposed. The distribution of model electric field in cladding mode of SM and core mode of MM are analyzed; the expression of coupling efficiency from the lowest-order cladding mode in LPFG to multimode fiber is derived. To obtain the maximum coupling efficiency, HE13 mode is demonstrated to be the optimal working mode in LPFG.②According to model of simple three-layer step-index structure in the single mode fiber , the refractive index and distribution of model electric field on long-period fiber grating are analyzed based on the theory of mode coupling, The effects on the transmitted spectrum of LPFG caused by period and length of fiber grating and the modulated depth of refractive index are studied. On the basis demands of 1550nm working wavelength and HE13 mode, the long-period fiber grating is designed with the grating period of 450μm.③The concept of core mode blocker is introduced into the analysis of coupling efficiency between cladding mode of SM and core mode of MM. The diameter of core mode blocker is optimized through coupling efficiency and isolation. Combining the technology of microelectromechanial system (MEMS) and the features of optical fiber, core mode blocker is fabricated by self-aligned lithography method, the end surface of fiber is coated by wet chemical coating method and ion beam sputtering method.④Aim of solving the problems of complex lithography and precise control, a new method of MRF-based core mode blocker is proposed. Connecting LPFG ,multimode fiber and CMB together can realize all-fiber band-pass filter and the single to multimode converter with 4nm bandwidth and 25dB isolation.