Characteristics of the transmission spectrum of the long period fiber gratings based on the coupling of core mode to the higher order cladding modes

The characteristics of the transmission spectrum of the Long-period fiber gratings (LPFGs) based on thecoupling of core mode to a higher order cladding mode (HE mode) are investigated using the coupled modetheory. This kind of LPFGs is different from that based on the coupling of core mode to a lower ordercladding mode because of the effect of the coupling of core mode to EH cladding mode. When the claddingmode order is higher, the coupling coefficients of core mode to HE and EH cladding modes are comparableand both of the propagation constants of HE and EH cladding modes approach, so the spectrum has anadditional loss peak. The bandwidth of LPFG based on the coupling of core mode to different claddingmode differs greatly. With the change of the mode orders from lower to higher, the transmission spectrumchanges from narrow to wide and more narrow.