The various physical parameters which impact the long period fiber grating(LPFG) spectral characteristics are analyzed comprehensively based on the mode coupling theory in this paper.The quantitative or qualitative results of LPFG spectral characteristics,which can provide the theoretical basis for LPFG design and parameter optimization,are obtained.Two long-period fiber gratings with lengths of 3.09 cm and 4.5 cm are fabricated by the amplitude mask method,in which the germanium-doped fiber is ...