An analysis of the core mode coupling to the cladding mode in long-period fiber grating

The interaction between a guided core mode and the cladding modes of a long-period fiber grating filters is analyzed theoretically.Based on the modes of a step-index fiber geometry,the coupled-mode equations that describe co-propagating interactions in a long-period grating is simplified by neglecting the longitudinal coupling coefficients,and coupling among cladding modes,as well as coupling to the even cladding modes.It is show that only the interaction of several odd modes is nearly resonant at the certain wavelength.,then a closed-form solution of the coupled-mode equations can be found,and the transmission spectra are also calculated theoretically.Compare with other models,the Numerical calculations agree with the experiment results well and show the advantage of our method.