Tuning effects of micro / nano high refractive index overlay on the transmission spectra of long period fiber gratings( LPFG) with given parameters are theoretically analyzed and numerically simulated by using coupled model theory. The results show that the phenomena of mode transition appear when the cladding mode entering the high refractive index overlay for transmission through properly designing the overlay thickness,overlay refractive index change range,and LPFG coating thickness. With the increase of...