The spectrum characteristic for the coupling between the core mode HE_(11) and high-order cladding modes based on long-period fiber gratings (LPGs) is investigated with the aim of supplying a concrete concept to design narrow-bandwidth optical devices. In contrast to the better-known coupling between the core mode and low-order cladding modes, numerical simulation has shown that a 0.4-nm FWHM can be achieved by properly designing the period of the LPG in choosing the high-order cladding modes. In any case, in order to ensure that the analytic result for high-order cladding modes is meaningful, the validity of two-mode coupled-mode equations is also proved from the three-mode coupled-mode equations. According to this concept, LPG can be used to design not only gain flatteners that have wide bandwidth but also optical communication components that have narrow bandwidth to meet the ITU standard for a DWDM system.
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